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Gibbergunyah Creek Flood Study Draft Report

VOLUME 1: Report and Appendices

Revision 1

December 2012

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Gibbergunyah Creek Flood Study Draft Report

REVISION / REVIEW HISTORY

Revision # Description Prepared by Reviewed by

1 Draft report D. Tetley C. Ryan

DISTRIBUTION

Revision # Distribution List Date Issued Number of Copies

1 Wingecarribee Shire Council 12/12/2012 PDF

Catchment Simulation Solutions

Suite 302

5 Hunter Street

Sydney, NSW, 2000

(02) 6223 0882 [email protected]

(02) 8415 7118 www.csse.com.au

File Reference: gibbergunyah creek flood study (rev #1).docx

The information within this document is and shall remain the property of Catchment Simulation Solutions.

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FOREWORD

The State Government’s Flood Policy is directed towards providing solutions to existing flooding

problems in developed areas and ensuring that new development is compatible with the flood

hazard and does not create additional flooding problems in other areas. The Policy is defined in

the NSW Government’s ‘Floodplain Development Manual’ (NSW Government, 2005).

Under the Policy, the management of flood liable land remains the responsibility of Local

Government. The State Government subsidises flood mitigation works to alleviate existing

problems and provides specialist technical advice to assist Local Government in its floodplain

management responsibilities.

The Policy provides for technical and financial support by the State Government through the

following four sequential stages:

STAGE DESCRIPTION

1 Flood Study Determines the nature and extent of the flood problem.

2 Floodplain Management

Study

Evaluates management options for the floodplain in respect of

both existing and proposed developments.

3 Floodplain Management

Plan

Involves formal adoption by Council of a plan of management

for the floodplain.

4 Implementation of the

Plan

Construction of flood mitigation works to protect existing

development. Use of environmental plans to ensure new

development is compatible with the flood hazard.

The Gibbergunyah Creek Flood Study represents the first of the four stages in the process

outlined above. It has been prepared to assist Council and the community to define and

understand the manner in which floodwaters would be distributed across the Gibbergunyah

Creek catchment and to establish the basis for the assessment of floodplain risk management

measures.

The project was funded by the NSW Government’s ‘Floodplain Management Program’ and

Wingecarribee Shire Council. Technical support for the project was provided by the Office of

Environment and Heritage.

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ACKNOWLEDGEMENTS

Catchments Simulation Solutions would like to acknowledge the valuable contributions of a

number of individuals who assisted with the preparation of this study. In particular, Mr Sha

Prodhan and Mr Dominic Lucas of Wingecarribee Shire Council provided a substantial amount

of information and insights into flooding across the Gibbergunyah Creek catchment. Thanks are

also extended to Mr John Murtagh of the Office of Environment and Heritage for his technical

input and review of the flood study report.

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TABLE OF CONTENTS

1 INTRODUCTION .......................................................................................................... 1

2 METHODOLOGY ......................................................................................................... 2

2.1 General ................................................................................................................ 2

2.2 Objectives ............................................................................................................ 3

2.3 Adopted Approach ............................................................................................. 3

3 REVIEW OF AVAILABLE INFORMATION .............................................................. 4

3.1 Overview .............................................................................................................. 4

3.2 Previous Investigations ..................................................................................... 4

3.2.1 Lot 2 Bessemer Street, Mittagong – Flood Study (2009) ...................... 4

3.2.2 Hydraulic Assessment of Chinamans Creek, Mittagong (2006) .......... 5

3.2.3 Bowral Floodplain Risk Management Study and Plan (2005) .............. 6

3.2.4 Catalogue of Conceptual Models for Groundwater-Stream Interaction in Eastern Australia (2009) ........................................................................ 7

3.3 Hydrologic Data .................................................................................................. 8

3.3.1 Historic Rainfall Data .................................................................................. 8

3.3.2 Historic Streamflow Data .......................................................................... 10

3.4 Topographic Data ............................................................................................. 11

3.4.1 Aerial Laser Survey (ALS)........................................................................ 11

3.4.2 10 Metre Contours ..................................................................................... 12

3.5 GIS Data ............................................................................................................ 12

3.5.1 Aerial Photography .................................................................................... 12

3.5.2 Stormwater Network GIS layer ................................................................ 12

3.5.3 Bridges ........................................................................................................ 13

3.5.4 Building Footprint Polygons ..................................................................... 13

3.6 Community Consultation ................................................................................. 13

3.6.1 Flood Study Website. ................................................................................ 13

3.6.2 Community Information Brochure and Questionnaire ......................... 15

3.7 Cross-Section and Structure Survey ............................................................. 19

4 HYDROLOGY ............................................................................................................. 20

Gibbergunyah Creek Flood Study

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4.1 General .............................................................................................................. 20

4.2 Hydrologic Model Development ..................................................................... 20

4.2.1 Subcatchment Parameterisation ............................................................. 20

4.2.2 Stream Routing .......................................................................................... 21

4.2.3 Rainfall Loss Model ................................................................................... 21

4.2.4 Flood Storage Basins................................................................................ 22

4.3 Hydrologic Model Calibration ......................................................................... 23

4.3.1 General........................................................................................................ 23

4.3.2 Rainfall Data ............................................................................................... 24

4.3.3 Results of Calibration and Verification Simulations ............................. 24

5 HYDRAULICS ............................................................................................................ 26

5.1 General .............................................................................................................. 26

5.2 Hydraulic Model Development ....................................................................... 26

5.2.1 Model Extent .............................................................................................. 26

5.2.2 Model Topography .................................................................................... 26

5.2.3 Material Types / Manning’s ‘n’ Roughness ........................................... 27

5.2.4 Culverts/Bridges ........................................................................................ 27

5.2.5 Pit/Culvert Blockage .................................................................................. 29

5.3 Hydraulic Model Calibration ........................................................................... 30

5.3.1 General........................................................................................................ 30

5.3.2 Calibration/Verification Event Selection ................................................. 30

5.3.3 Model Boundary Conditions ..................................................................... 30

5.3.4 Results of Calibration and Verification Simulations ............................. 31

5.3.5 Additional Model Verification ................................................................... 32

5.3.6 Summary ..................................................................................................... 33

6 DESIGN FLOOD ESTIMATION .............................................................................. 34

6.1 General .............................................................................................................. 34

6.2 Hydrology .......................................................................................................... 34

6.2.1 Design Rainfall ........................................................................................... 34

6.2.2 Probable Maximum Precipitation (PMP) ................................................ 34

6.2.3 Rainfall Loss Model ................................................................................... 35

6.2.4 Baseflow...................................................................................................... 36

6.2.5 Peak Discharges........................................................................................ 37

6.2.6 Verification of Peak Discharges .............................................................. 39


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6.3 Hydraulics .......................................................................................................... 41

6.3.1 General........................................................................................................ 41

6.3.2 Model Boundary Conditions ..................................................................... 41

6.3.3 Design Flood Envelope ............................................................................ 41

6.3.4 Floodwater Depths, Levels and Velocities ............................................ 42

7 SENSITIVITY ANALYSIS ......................................................................................... 46

7.1 General .............................................................................................................. 46

7.2 Hydrologic Model ............................................................................................. 46

7.2.1 Initial Loss ................................................................................................... 46

7.2.2 Continuing Loss Rate................................................................................ 48

7.3 Hydraulic Model ................................................................................................ 49

7.3.1 Pipe/Culvert Blockage .............................................................................. 49

7.3.2 Manning’s ‘n’ .............................................................................................. 51

8 PROVISIONAL FLOOD HAZARD AND HYDRAULIC CATEGORISATION .... 54

8.1 Provisional Flood Hazard Categories ........................................................... 54

8.1.1 Provisional Flood Hazard ......................................................................... 54

8.2 Hydraulic Categories ....................................................................................... 55

8.2.1 Adopted Hydraulic Categories ................................................................. 56

8.3 Flood Risk Precincts ........................................................................................ 58

9 CLIMATE CHANGE ASSESSMENT ...................................................................... 59

9.1 Hydrology .......................................................................................................... 59

9.1.1 General........................................................................................................ 59

9.1.2 Results ........................................................................................................ 59

9.2 Hydraulics .......................................................................................................... 60

9.2.1 Results ........................................................................................................ 60

10 DISCUSSION ............................................................................................................. 63

10.1 Overview ............................................................................................................ 63

10.2 General Description of Flood Behaviour ...................................................... 63

10.3 Flood Liable Areas ........................................................................................... 63

10.4 Emergency Response Infrastructure ............................................................ 64

10.5 Roadways .......................................................................................................... 65

10.6 Railway .............................................................................................................. 66

11 CONCLUSION ............................................................................................................ 67


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12 REFERENCES ........................................................................................................... 69

LIST OF APPENDICES

APPENDIX A Community Consultation

APPENDIX B XP-RAFTS Model Input Parameters

APPENDIX C XP-RAFTS Model Results for Calibration Simulations

APPENDIX D Bridge Loss Calculations

APPENDIX E PMP Calculations

APPENDIX F XP-RAFTS Model Results for Design Flood Simulations

APPENDIX G Probabilistic Rational Method Results

APPENDIX H XP-RAFTS Model Results for Sensitivity Analyses

APPENDIX I TUFLOW Model Results for Sensitivity Analyses

APPENDIX J XP-RAFTS Model Results for Climate Change Assessment

APPENDIX K TUFLOW Model Results for Climate Change Assessment

LIST OF TABLES

Table 1 Peak design discharges extracted from Lot 2 Bessemer Street, Mittagong – Flood Study (2009) ............................................................................................................ 4

Table 2 Available rain gauges in the vicinity of the Gibbergunyah Creek catchment ............. 9

Table 3 Temporal availability and percentage of annual record complete for rain gauges in the vicinity of Mittagong (source: http://www.bom.gov.au/climate/data/) ................. 10

Table 4 Significant Historic Rainfall Events ......................................................................... 11

Table 5 Adopted Impervious Percentage and Manning’s ‘n’ Values for Hydrologic Model ... 21

Table 6 Adopted XP-RAFTS Rainfall Losses for Calibration Simulations ............................ 22

Table 7 TUFLOW Manning's 'n' Roughness Values ............................................................ 28

Table 8 Design Rainfall Intensities ...................................................................................... 35

Table 9 Adopted XP-RAFTS Rainfall Losses for Design Simulations .................................. 36

Table 10 Adopted Baseflow Contributions for Design Simulations ........................................ 36

Table 11 Peak Design Discharges for Existing Conditions .................................................... 37

Table 12 Comparison between Probabilistic Rational Method and XP-RAFTS 100 Year ARI Peak Design Discharges ........................................................................................ 40

Table 13 Comparison between XP-RAFTS Design Discharges and Discharges Documented in Previous Flooding Investigations ........................................................................ 40

Table 14 Peak Design Floodwater Elevations ....................................................................... 43


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Table 15 XP-RAFTS Sensitivity to Variations in Initial Losses ............................................... 47

Table 16 XP-RAFTS Sensitivity to Variations in Initial Losses ............................................... 48

Table 17 TUFLOW Sensitivity to Variations in Culvert/Pipe Blockage ................................... 50

Table 18 TUFLOW Sensitivity to Variations in Adopted Manning’s ‘n’ Roughness ................ 52

Table 19 Qualitative and Quantitative Criteria for Hydraulic Categories ............................... 55

Table 20 Flood Risk Precinct Definitions .............................................................................. 58

Table 21 Predicted Peak Design Discharges with Increases in Rainfall Intensity Associated with Climate Change .............................................................................................. 60

Table 22 TUFLOW Sensitivity to Variations in Culvert/Pipe Blockage ................................... 61

LIST OF PLATES

Plate 1 Chinamans Creek channel upstream of the Old Hume Highway showing dense vegetation in creek channel and overbank areas (i.e., Manning’s ‘n’ = ~0.1) ............ 6

Plate 2 Priestley Street culvert crossing of Chinamans Creek showing significant base flow 8

Plate 3 Conceptual diagram of geology and groundwater movement across the Upper Nepean River catchment (Sydney Catchment Authority, 2006) ................................ 9

Plate 4 Incomplete building polygon in Regent Street, Mittagong ....................................... 14

Plate 5 Missing building polygon in Henderson Avenue, Mittagong .................................... 14

Plate 6 Flooding across southern end of Gibbergunyah Lane during November 2010 event ............................................................................................................................... 16

Plate 7 Flooding across Gibbergunyah Lane during November 2010 event ....................... 16

Plate 8 Flooding in the vicinity of John Street, Mittagong during November 2010 event ..... 17

Plate 9 Flooding across John Street, Mittagong during November 2010 event ................... 17

Plate 10 Flooding across Bowral Lane, Welby during February 2005 event ......................... 18

Plate 11 Flooding across back yard at Lot 8 Bowral Lane, Welby during February 2005 event ............................................................................................................................... 18

Plate 12 Floodwaters along unnamed tributary at rear of properties fronting Bowral Lane, Welby during February 2005 event......................................................................... 19

Plate 13 Main outlet structure for Lake Alexandra ................................................................ 23

Plate 14 Arch culvert with wildlife ledge draining water beneath the railway line .................. 28

Plate 15 Example of vegetation blocking culvert outlet in Gibbergunyah Creek catchment. . 29

Plate 16 Railway Parade culverts showing plates that increase the effective blockage ........ 30

Plate 17 Adopted hydraulic category criteria ........................................................................ 56

Plate 18 Predicted Peak 100 year ARI Flood Levels, Depth and Velocities with Partial Blockage of Floodways (blockage locations highlighted by yellow circles).............. 57

Plate 19 Predicted Change in Peak 100 year ARI Flood Levels with Partial Blockage of Floodways (blockage locations highlighted by yellow circles) ................................. 57


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GLOSSARY

acid sulphate soils are sediments which contain sulfidic mineral pyrite which may

become extremely acid following disturbance or drainage as sulfur

compounds react when exposed to oxygen to form sulfuric acid.

More detailed explanation and definition can be found in the NSW

Government Acid Sulfate Soil Manual published by Acid Sulfate Soil

Management Advisory Committee.

annual exceedance

probability (AEP)

the chance of a flood of a given or larger size occurring in any one

year, usually expressed as a percentage. Eg, if a peak flood discharge

of 500 m3/s has an AEP of 5%, it means that there is a 5% chance

(that is one-in-20 chance) of a 500 m3/s or larger events occurring in

any one year (see ARI).

Australian Height Datum

(AHD)

a common national surface level datum approximately corresponding

to mean sea level.

average annual damage

(AAD)

depending on its size (or severity), each flood will cause a different

amount of flood damage to a flood prone area. AAD is the average

damage per year that would occur in a nominated development

situation from flooding over a very long period of time.

average recurrence interval

(ARI)

the long-term average number of years between the occurrence of a

flood as big as or larger than the selected event. For example, floods

with a discharge as great as or greater than the 20 year ARI flood

event will occur on average once every 20 years. ARI is another way

of expressing the likelihood of occurrence of a flood event.

caravan and moveable home

parks

caravans and moveable dwellings are being increasingly used for

long-term and permanent accommodation purposes. Standards

relating to their siting, design, construction and management can be

found in the Regulations under the Local Governments Act.

catchment the land area draining through the main stream, as well as tributary

streams, to a particular site. It always relates to an area above a

specific location.

consent authority the council, government agency or person having the function to

determine a development application for land use under the EP&A

Act. The consent authority is most often the council, however

legislation or an EPI may specify

a Minister or public authority (other than a council), or the Director

General of DIPNR, as having the function to determine an application.


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development is defined in Part 4 of the Environmental Planning and Assessment

Act (EP&A Act).

infill development: refers to development of vacant blocks of land

that are generally surrounded by developed properties and is

permissible under the current zoning of the land. Conditions such as

minimum floor levels may be imposed on infill development.

new development: refers to development of a completely different

nature to that associated with the former land use. For example, the

urban subdivision of an area previously used for rural purposes. New

developments involve rezoning and typically require major extensions

of existing urban services, such as roads, water supply, sewerage and

electric power.

redevelopment: refers to rebuilding in an area. For example, as

urban areas age, it may become necessary to demolish and

reconstruct buildings on a relatively large scale. Redevelopment

generally does not require either rezoning or major extensions to

urban services.

disaster plan (DISPLAN) a step by step sequence of previously agreed roles, responsibilities,

functions, actions and management arrangements for the conduct of

a single or series of connected emergency operations, with the object

of ensuring the coordinated response by all agencies having

responsibilities and functions in emergencies.

discharge the rate of flow of water measured in terms of volume per unit time,

for example, cubic metres per second (m3/s). Discharge is different

from the speed or velocity of flow, which is a measure of how fast the

water is moving for example, metres per second (m/s).

ESD using, conserving and enhancing natural resources so that ecological

processes, on which life depends, are maintained, and the total

quality of life, now and in the future, can be maintained or increased.

A more detailed definition is included in the Local Government Act,

1993. The use of sustainability and sustainable in this manual relate

to ESD.

effective warning time

The time available after receiving advice of an impending flood and

before floodwaters prevent appropriate flood response actions being

undertaken. The effective warning time is typically used to move

farm equipment, move stock, raise furniture, evacuate people and

transport their possessions.

emergency management a range of measures to manage risks to communities and the

environment. In the flood context it may include measures to

prevent, prepare for, respond to and recover from flooding.

flash flooding flooding which is sudden and unexpected. It is often caused by

sudden local or nearby heavy rainfall. Often defined as flooding which

peaks within six hours of the causative rain.


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flood relatively high stream flow which overtops the natural or artificial

banks in any part of a stream, river, estuary, lake or dam, and/or local

overland flooding associated with major drainage (refer Section C6)

before entering a watercourse, and/or coastal inundation resulting

from super-elevated sea levels and/or waves overtopping coastline

defences excluding tsunami.

flood awareness Awareness is an appreciation of the likely effects of flooding and a

knowledge of the relevant flood warning, response and evacuation

procedures.

flood education flood education seeks to provide information to raise awareness of

the flood problem so as to enable individuals to understand how to

manage themselves and their property in response to flood warnings

and in a flood event. It invokes a state of flood readiness.

flood fringe areas the remaining area of flood prone land after floodway and flood

storage areas have been defined.

flood liable land is synonymous with flood prone land, i.e., land susceptible to flooding

by the PMF event. Note that the term flood liable land covers the

whole floodplain, not just that part below the FPL (see flood planning

area).

flood mitigation standard the average recurrence interval of the flood, selected as part of the

floodplain risk management process that forms the basis for physical

works to modify the impacts of flooding.

floodplain area of land which is subject to inundation by floods up to and

including the probable maximum flood event, that is, flood prone

land.

floodplain risk management

options

the measures that might be feasible for the management of a

particular area of the floodplain. Preparation of a floodplain risk

management plan requires a detailed evaluation of floodplain risk

management options.

floodplain risk management

plan

a management plan developed in accordance with the principles and

guidelines in this manual. Usually includes both written and

diagrammatic information describing how particular areas of flood

prone land are to be used and managed to achieve defined

objectives.

flood plan (local) A sub-plan of a disaster plan that deals specifically with flooding. They

can exist at state, division and local levels. Local flood plans are

prepared under the leadership of the SES.

flood planning area the area of land below the FPL and thus subject to flood related

development controls.

flood planning levels (FPLs) are the combinations of flood levels (derived from significant

historical flood events or floods of specific AEPs) and freeboards

selected for floodplain risk management purposes, as determined in


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management studies and incorporated in management plans.

flood proofing a combination of measures incorporated in the design, construction

and alteration of individual buildings or structures subject to flooding,

to reduce or eliminate flood damages.

flood prone land land susceptible to flooding by the PMF event. Flood prone land is

synonymous with flood liable land.

flood readiness Readiness is an ability to react within the effective warning time.

flood risk potential danger to personal safety and potential damage to property

resulting from flooding. The degree of risk varies with circumstances

across the full range of floods. Flood risk in this manual is divided into

3 types, existing, future and continuing risks. They are described

below.

existing flood risk: the risk a community is exposed to as a result of its

location on the floodplain.

future flood risk: the risk a community may be exposed to as a result

of new development on the floodplain.

continuing flood risk: the risk a community is exposed to after

floodplain risk management measures have been implemented. For a

town protected by levees, the continuing flood risk is the

consequences of the levees being overtopped. For an area without

any floodplain risk management measures, the continuing flood risk is

simply the existence of its flood exposure.

flood storage areas those parts of the floodplain that are important for the temporary

storage of floodwaters during the passage of a flood. The extent and

behaviour of flood storage areas may change with flood severity, and

loss of flood storage can increase the severity of flood impacts by

reducing natural flood attenuation. Hence, it is necessary to

investigate a range of flood sizes before defining flood storage areas.

floodway areas those areas of the floodplain where a significant discharge of water

occurs during floods. They are often aligned with naturally defined

channels. Floodways are areas that, even if only partially blocked,

would cause a significant redistribution of flood flow, or a significant

increase in flood levels.

freeboard provides reasonable certainty that the risk exposure selected in

deciding on a particular flood chosen as the basis for the FPL is

actually provided. It is a factor of safety typically used in relation to

the setting of floor levels, levee crest levels, etc. Freeboard is

included in the flood planning level.

hazard a source of potential harm or a situation with a potential to cause

loss. In relation to this study the hazard is flooding which has the

potential to cause damage to the community.

Definitions of high and low hazard categories are provided in

Appendix L of the Floodplain Development Manual (2005).


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historical flood a flood which has actually occurred.

hydraulics term given to the study of water flow in waterways; in particular, the

evaluation of flow parameters such as water level and velocity.

hydrograph a graph which shows how the discharge or stage/flood level at any

particular location varies with time during a flood.

hydrology term given to the study of the rainfall and runoff process; in

particular, the evaluation of peak flows, flow volumes and the

derivation of hydrographs for a range of floods.

local overland flooding inundation by local runoff rather than overbank discharge from a

stream, river, estuary, lake or dam.

local drainage smaller scale problems in urban areas. They are outside the definition

of major drainage in this glossary.

mainstream flooding inundation of normally dry land occurring when water overflows the

natural or artificial banks of a stream, river, estuary, lake or dam.

major drainage councils have discretion in determining whether urban drainage

problems are associated with major or local drainage. For the

purposes of this manual major drainage involves:

• the floodplains of original watercourses (which may now be

piped, channelised or diverted), or sloping areas where overland

flows develop along alternative paths once system capacity is

exceeded; and/or

• water depths generally in excess of 0.3m (in the major system

design storm as defined in the current version of Australian

Rainfall and Runoff). These conditions may result in danger to

personal safety and property damage to both premises and

vehicles; and/or

• major overland flowpaths through developed areas outside of

defined drainage reserves; and/or

• the potential to affect a number of buildings along the major

flow path.

mathematical / computer

models

the mathematical representation of the physical processes involved

in runoff generation and stream flow. These models are often run on

computers due to the complexity of the mathematical relationships

between runoff, stream flow and the distribution of flows across the

floodplain.


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merit approach the merit approach weighs social, economic, ecological and cultural

impacts of land use options for different flood prone areas together

with flood damage, hazard and behaviour implications, and

environmental protection and well-being of the State’s rivers and

floodplains.

The merit approach operates at two levels. At the strategic level it

allows for the consideration of social, economic, ecological, cultural

and flooding issues to determine strategies for the management of

future flood risk which are formulated into council plans, policy, and

EPIs. At a site specific level, it involves consideration of the best way

of conditioning development allowable under the floodplain risk

management plan, local flood risk management policy and EPIs.

minor, moderate and major

flooding

Both the State Emergency Service and the Bureau of Meteorology use

the following definitions in flood warnings to give a general indication

of the types of problems expected with a flood.

minor flooding: Causes inconvenience such as closing of minor roads

and the submergence of low level bridges. The lower limit of this

class of flooding on the reference gauge is the initial flood level at

which landholders and townspeople begin to be flooded.

moderate flooding: Low lying areas are inundated requiring removal

of stock and/or evacuation of some houses. Main traffic routes may

be covered.

major flooding: Appreciable urban areas are flooded and/or

extensive rural areas are flooded. Properties, villages and towns can

be isolated.

modification measures measures that modify either the flood, the property or the response

to flooding.

peak discharge the maximum discharge occurring during a flood event.

probable maximum flood

(PMF)

the PMF is the largest flood that could conceivably occur at a

particular location, usually estimated from probable maximum

precipitation, and where applicable, snow melt, coupled with the

worst flood producing catchment conditions. Generally, it is not

physically or economically possible to provide complete protection

against this event. The PMF defines the extent of flood prone land,

that is, the floodplain. The extent, nature and potential consequences

of flooding associated with a range of events rarer than the flood

used for designing mitigation works and controlling development, up

to and including the PMF event should be addressed in a floodplain

risk management study.

probable maximum

precipitation (PMP)

the PMP is the greatest depth of precipitation for a given duration

meteorologically possible over a given size storm area at a particular

location at a particular time of the year, with no allowance made for

long-term climatic trends (World Meteorological Organisation, 1986).

It is the primary input to PMF estimation.


vii

probability A statistical measure of the expected chance of flooding (see annual

exceedance probability).

risk chance of something happening that will have an impact. It is

measured in terms of consequences and likelihood. In the context of

the manual it is the likelihood of consequences arising from the

interaction of floods, communities and the environment.

runoff the amount of rainfall which actually ends up as streamflow, also

known as rainfall excess.

stage equivalent to water level (both measured with reference to a

specified datum).

stage hydrograph a graph that shows how the water level at a particular location

changes with time during a flood. It must be referenced to a

particular datum.

survey plan a plan prepared by a registered surveyor.

TUFLOW is a 1-dimensional and 2-dimensional flood simulation software. It

simulates the complex movement of floodwaters across a particular

area of interest using mathematical approximations to derive

information on floodwater depths, velocities and levels.

velocity the speed or rate of motion (distance per unit of time, e.g., metres

per second) in a specific direction at which the flood waters are

moving.

water surface profile a graph showing the flood stage at any given location along a

watercourse at a particular time.

wind fetch the horizontal distance in the direction of wind over which wind

waves are generated.

XP-RAFTS is a non-linear runoff routing software. It incorporates subcatchment

information such as area, slope, roughness and percentage

impervious and is used to simulate the transformation of historic or

design rainfall into runoff (i.e., discharge hydrographs).


viii

1

1 INTRODUCTION

The Gibbergunyah Creek catchment is located in the Southern Highlands of New South Wales

and occupies a total area of 10.5 km2. The extent of the catchment is shown in Figure 1 (refer

Flood Study: Volume 2).

Gibbergunyah Creek originates in the vicinity of Mount Gibraltar and drains in a northerly

direction through the Mittagong urban area, where it is joined by Chinamans Creek and Iron

Mines Creek. It continues to drain in a northerly direction beneath the Hume Highway before

joining the Nattai River. The majority of the catchment is developed comprising a mix of

residential, commercial and industrial land uses, although the downstream sections of the

catchment is undeveloped.

The catchment is drained primarily by natural watercourses and gullies. The urbanised sections

of the catchment are also drained by a stormwater system which carries local catchment runoff

into the natural watercourses via a network of stormwater pipes, pits, open channels and

culverts.

During periods of heavy rainfall, there is potential for the stormwater system to become

overwhelmed and for water to overtop the banks of the natural watercourses and inundate the

adjoining floodplain. Accordingly, there is potential for inundation of properties located in

close proximity to the creeks and drainage lines.

Although several flooding investigations have been completed at isolated locations across the

catchment, a comprehensive flood study of the entire Gibbergunyah Creek catchment has not

previously been prepared. Therefore, with the exception of isolated flooding complaints from

residents, the extent of the existing flooding problem is not well understood.

In recognition of this, Wingecarribee Shire Council decided to prepare a Floodplain Risk

Management Plan for the Gibbergunyah Creek catchment.

The first stage in the development of a Floodplain Risk Management Plan involves the

preparation of a Flood Study. The Flood Study provides a technical assessment of flood

behaviour.

This report forms the Flood Study for the Gibbergunyah Creek catchment. It documents flood

behaviour across the catchment for a range of design floods for existing topographic and

development conditions. This includes information on peak discharges, flood levels, flood

extents, flood depths and flow velocities for a range of design floods. It also provides

provisional estimates of the flood hazard and hydraulic categories across the catchment.

The Flood Study comprises two volumes. Volume 1 (i.e., this document) comprises the report

text and appendices. Volume 2 contains all accompanying report figures.

2

Floodplain Risk

Management

Committee

Data Collection

Flood Study

Floodplain Risk

Management Study

Floodplain Risk

Management Plan

Implementation of

Plan

Established by the local council, must include community groups and state agency specialists

Compilation of existing data and collection of additional data. Usually undertaken by consultants appointed

by the council.

Defines the nature and extent of the flood problem, in technical rather than map form. Usually undertaken by consultants appointed by the council.

Determines options in consideration of social, ecological and economic factors relating to flood risk. Usually undertaken by consultants appointed

by the council.

Preferred options publicly exhibited and subject to revision in light of responses. Formally approved by the council after public exhibition and any necessary revisions due to public comments.

Flood, response and property modification measures including mitigation works, planning controls, flood warnings, flood readiness and response plans, environmental rehabilitation, ongoing data collection and monitoring.

2 METHODOLOGY

2.1 General

The NSW Government’s ‘Floodplain Development Manual’ (NSW Government, 2005) outlines

the steps required to successfully develop a Floodplain Risk Management Plan for flood

affected areas.

The various steps involved in preparing a floodplain risk management plan are also outlined in

the diagram below. As shown, one of the key steps involved in formulating a Floodplain

Management Plan involves the preparation of a Flood Study.

The aim of the Flood Study is to produce information on flood discharges, levels, depths and

velocities, for a range of flood events under existing topographic and development conditions.

This information can then be used as a basis for identifying those areas where the greatest

flood damage is likely to occur, thereby allowing a targeted assessment of where flood

mitigation measures would be best implemented.

Wingecarribee Shire Council initiated the floodplain management process for the Gibbergunyah

Creek catchment by commissioning this Flood Study.


3

2.2 Objectives

The objectives of the Gibbergunyah Creek Flood Study are:

to review available flood-related information for the Gibbergunyah Creek catchment;

to prepare design flow hydrographs describing the spatial and temporal variation in flows

across the catchment using a hydrologic computer model;

to develop a hydraulic computer model to simulate the passage of flood flows across the

Gibbergunyah Creek catchment;

to calibrate the hydrologic and hydraulic computer models to reproduce past floods;

to use the calibrated computer models to define peak discharges, water levels, depths and

velocities for the design 5, 10, 20, 50, 100 and 200 year ARI floods, and the Probable

Maximum Flood (PMF);

to produce maps showing the extent, depth and velocity of floodwaters for the range of

design floods; and,

to produce maps showing provisional flood hazard and hydraulic categories for the range

of design floods.

2.3 Adopted Approach

The general approach and methodology employed to achieve the study objectives involved:

compilation and review of available flood-related information (Chapter 3);

the development and calibration of a computer based hydrologic model to simulate the

transformation of rainfall into runoff (Chapter 4);

the development and calibration of a computer based hydraulic model to simulate the

movement of floodwaters across the Gibbergunyah Creek catchment (Chapter 5);

use of the computer models to determine peak discharges, water levels, depths, flow

velocities and flood extents for the full range of design events up to and including the PMF

(Chapter 6);

testing the sensitivity of the results generated by the computer model to variations in

model input parameters (Chapter 7);

use of the computer model results to generate provisional flood hazard and hydraulic

category mapping using definitions provided in the ‘Floodplain Development Manual’ (NSW

Government, 2005) (Chapter 8); and,

assessment of potential climate change implications on existing flood behaviour

(Chapter 9).

identifying flooding “trouble spots” and key infrastructure and transportation links

impacted by floodwaters (Chapter 10).

4

3 REVIEW OF AVAILABLE INFORMATION

3.1 Overview

A range of data were made available to assist with the preparation of the Gibbergunyah Creek

Flood Study. This included previous reports, hydrologic data and GIS data.

A description of each dataset along with a synopsis of its relevance to the flood study is

summarised below.

3.2 Previous Investigations

3.2.1 Lot 2 Bessemer Street, Mittagong – Flood Study (2009)

The ‘Lot 2 Bessemer Street, Mittagong – Flood Study’ (2009), was prepared by Bewsher

Consulting for Wingecarribee Shire Council. The study was commissioned to define flood

behaviour across the site of a proposed Family Community Centre that is traversed by Iron

Mines Creek (a tributary of Gibbergunyah Creek). The location of the site is shown on Figure 2.

The report incorporates a significant amount of information about the piped and open channel

drainage system in and around the site. This includes the location of stormwater pipes and pits,

pipe and culvert dimensions, pit invert elevations and pit types/lintel lengths.

As part of the investigation, a hydrologic model of the catchment draining to the Community

Centre site was developed using the Drains software. The hydrologic model was used to

simulate a range of design floods ranging from the 1 year ARI flood up to and including the

PMF. The results of the hydrologic modelling determined that the critical storm duration at the

site varied between 25 minutes and 2 hours.

Peak design discharges were extracted from the report at select locations and are reproduced

in Table 1. As shown in Table 1, the peak discharge downstream of Lot 2 is lower than the

corresponding peak discharge upstream of the site during the 1 Year ARI flood. This is most

likely associated with the flood storage that is afforded across the site during smaller storms.

Table 1 Peak design discharges extracted from Lot 2 Bessemer Street, Mittagong – Flood Study (2009)

Location

Peak Discharge (m3/s)

1 Yr ARI 5 Yr ARI 20 Yr ARI 50 Yr ARI 100 Yr ARI PMP

Bessemer Street railway

underpass 1.79 4.99 7.55 8.92 10.2 38.8

Bowral Road 1.08 7.27 11.6 13.7 16.0 63.4

A 2-dimensional hydraulic computer model was also developed as part of the study. The model

was developed using the TUFLOW software and extends from Railway Parade (upstream of the

railway line) to downstream of Bowral Road. The model was developed using a 2 metre grid


5

size to represent the ground surface topography. All stormwater pipes and conduits were also

included as a 1-dimensional network inserted beneath the 2-dimensional domain. Accordingly,

the TUFLOW model provides a good description of the interaction between surface and

subsurface flows and the distribution of flows across the site and along the adjoining roadways.

It was noted, however, that none of the main channels were represented using a 1-dimensional

domain. Accordingly, the conveyance capacity of the relatively narrow channels may be

underestimated using the comparatively course 2 metre grid cells instead of a more detailed 1-

dimensional channel section.

The TUFLOW model was used to produce information on peak design flood levels, floodwater

depths, flow velocities and provisional flood hazard. Additional simulations were also

completed to assess the sensitivity of peak 100 year ARI results to variations in blockage of the

stormwater system.

The report notes that no historic flooding information could be uncovered for the site or the

broader catchment area (i.e., Gibbergunyah Creek catchment). Therefore, neither the

hydrologic or hydraulic models could be calibrated. It also notes that the collection of data

within the railway corridor was difficult due to fencing (i.e., access difficulties) and heavy

vegetation (i.e., poor visibility). Accordingly, it recommends that data within the railway

corridor be verified as part of any future flood study.

Although this report only covers a small part of the overall Gibbergunyah Creek catchment, it is

considered that the peak discharges documented in the report could be used to verify peak

discharges generated as part of this study. The TUFLOW model data could also be used to

assist in the development of the hydraulic computer model for this study.

3.2.2 Hydraulic Assessment of Chinamans Creek, Mittagong (2006)

The ‘Hydraulic Assessment of Chinamans Creek, Mittagong’ (2006) was prepared by RHM

Consulting Engineers as part of a ‘Statement of Environmental Effects’ to support a proposed

bulky goods development adjacent to Chinamans Creek at Mittagong. The report was prepared

to quantify the hydraulic impacts of rehabilitating a section of Chinamans Creek immediately

adjoining the proposed development. The development has since proceeded and is now known

as the Highlands Homemaker Centre.

As part of the investigation, a hydrologic model of the Chinamans Creek catchment was

developed using the Watershed Bounded Network Model (WBNM) hydrologic software. The

WBNM model generated a peak 100 year ARI discharge for Chinamans Creek of 24.6 m3/s at

the site. A rational method calculation was also completed and produced a peak 100 year ARI

discharge of 24 m3/s, although the inputs used in these rational method calculations were not

included in the report.

A steady state HEC-RAS hydraulic model of Chinamans Creek was also developed for the study.

The model extended from upstream of the Old Hume Highway to the downstream end of the

development site and incorporated 18 design cross-sections of the proposed creek channel as

well as the Old Hume Highway culvert crossing. A Manning’s ‘n’ of 0.04 was adopted for all

channel segments and an ‘n’ value of 0.05 was adopted for all overbank areas. It is considered

that these ‘n’ values would not be appropriate for contemporary conditions upstream of the

Old Hume Highway (refer Plate 1).


6

Plate 1 Chinamans Creek channel upstream of the Old Hume Highway showing dense vegetation in creek

channel and overbank areas (i.e., Manning’s ‘n’ = ~0.1)

The outcomes of the HEC-RAS modelling determined that, with the proposed channel

modifications, the peak 100 year ARI flood extent would be fully contained to the creek. It also

determined that the peak 100 year ARI flood level varied between 596.2 mAHD at the

downstream end of the site up to 602.68 mAHD on the downstream side of the Old Hume

Highway.

As discussed, the vegetation density along Chinamans Creek has increased significantly relative

to the ‘design’ conditions documented in the report. Accordingly, contemporary peak 100 ARI

flood levels, depths and extents are likely to be higher than those documented in this report. In

addition the ‘design’ creek cross-sections do not necessarily reflect ‘as built’ conditions.

Nevertheless, the surveyed details of the Old Hume Highway culvert crossing could be

extracted and used in the hydraulic model developed for this study and the peak discharges

extracted from the hydrologic model could be used to assist in the verification of the hydrologic

model developed for this study.

3.2.3 Bowral Floodplain Risk Management Study and Plan (2005)

The ‘Bowral Floodplain Risk Management Study and Plan’ (2005) was prepared by Bewsher

Consulting for Wingecarribee Shire Council. The study was commissioned to investigate a range

of options that could be potentially implemented to reduce flood damages within the


7

Mittagong Creek catchment. The Mittagong Creek catchment is located immediately south of

the Gibbergunyah Creek catchment.

Although this report does not contain any specific flooding information for the Gibbergunyah

Creek catchment, it does incorporate newspaper clippings from ‘The Bowral Free Press’

documenting a large flood that occurred in March 1893. Photos are also provided showing

flooding across different parts of Bowral on:

January, 1915;

March, 1975;

March, 1978;

November, 1985;

August, 1986;

April, 1988;

October, 1999;

February, 2005; and,

June, 2007.

Given the proximity of the Gibbergunyah Creek catchment to the Mittagong Creek catchment,

it is likely that flooding would have also been experienced in the Gibbergunyah Creek

catchment during each of these events.

3.2.4 Catalogue of Conceptual Models for Groundwater-Stream Interaction in Eastern

Australia (2009)

Discussions with Wingecarribee Shire Council staff determined that considerable “base” flow

can originate from the Mount Gibraltar area and enter the various watercourses within the

Gibbergunyah Creek catchment. This anecdotal information appeared to be confirmed based

on the outcomes of a field reconnaissance that was completed on 23rd

May 2012, which

showed flowing water in most tributaries (refer Plate 2) despite less than 0.5 mm of rain falling

in the preceding week.

The presence of base flow within major creeks may reduce the available conveyance (i.e., flow

carrying) capacity of each creek during major storm events, thereby leading to increased

severity of flooding. Accordingly, a review of available literature was completed to identify the

base flow potential across the catchment.

The ‘Catalogue of Conceptual Models for Groundwater-Stream Interaction in Eastern Australia’

(2009), is a technical report prepared by Reid et al for the eWater Cooperative Research Centre.

It was prepared in an effort to estimate exchanges between groundwater and surface water

supplies and predict how these may change with different groundwater and surface water

management techniques across Eastern Australia.

The report incorporates a review of the Upper Nepean catchment, which is located

immediately east of the Gibbergunyah Creek catchment. Therefore, the geologic formations

that extend across the Upper Nepean River also extend into the Gibbergunyah Creek

catchment.


8

Plate 2 Priestley Street culvert crossing of Chinamans Creek showing significant base flow

The report notes that three main geologic units extend across the region. This includes

Hawkesbury Sandstone, which forms a major aquifer within the Sydney Basin. The sandstone

layer generally flows from south-south-west to north-north-east, is typically located between

confining geologic layers and follows the local topography (refer Plate 3). However, the

sandstone can be exposed to the surface in areas of sudden topographic changes, such as

Mount Gibraltar, resulting in the aquifer discharging to surface water systems. The report also

states that base flow from aquifer discharge is evident throughout the year, even during

periods of drought.

Accordingly, the report appears to confirm the potential for groundwater flow contributions

from Mount Gibraltar to the Gibbergunyah Creek catchment. Unfortunately, the report does

not provide any information on the magnitude of the potential base flow contributions.

3.3 Hydrologic Data

3.3.1 Historic Rainfall Data

A number of daily read and continuous (i.e., pluviometer) rainfall gauges are located in close

proximity to the Gibbergunyah Creek catchment. The location of each gauge is shown in

Figure 3. Key information for those gauges located within 10 kilometres of the catchment is

summarised in Table 2 and the temporal availability of rain gauge data is provided in Table 3.


9

Plate 3 Conceptual diagram of geology and groundwater movement across the Upper Nepean River

catchment (Sydney Catchment Authority, 2006)

Table 2 Available rain gauges in the vicinity of the Gibbergunyah Creek catchment

Gauge

Number Gauge Name

Gauge

Type Owner* Period of Record

Percentage

of Record

Complete

Distance

From

Catchment

(km)

68044 Mittagong (Alfred St) Daily BOM 01/01/1886 -> present 86 2.0

68163 Mittagong (Leicester Park) Daily SCA 01/01/1957 -> 31/12/1970 86 4.9

68102 Bowral (Parry Drive) Cont. BOM 30/11/1992 -> present 87 5.1

68102 Bowral (Parry Drive) Daily BOM 08/10/1961 -> present 98 5.1

68184 Bowral Centennial Road Daily BOM 01/01/1967 ->31/12/1977 93 5.1

68255 Bowral (Orchard St) Daily BOM 09/01/2000 -> present 99 5.2

68033 Mittagong (Kia-Ora) Daily SCA 01/01/1902 -> present 67 5.4

68087 Spring Hill (Warana) Daily BOM 01/01/1959->31/12/1967 72 6.2

68005 Bowral Post Office Daily BOM 01/01/1885 -> 01/01/1965 99 6.4

68157 Yarrow (Boural) Daily BOM 01/01/1912 -> 31/12/1930 99 6.4

68092 Berrima (Hillview) Daily BOM 01/01/1959 -> 31/12/1967 98 7.2

68239 Moss Vale AWS Daily BOM 23/02/2001 -> present 98 8.4

NOTE: * BOM = Bureau of Meteorology, SCA = Sydney Catchment Authority


10

Table 3 Temporal availability and percentage of annual record complete for rain gauges in the vicinity of

Mittagong (source: http://www.bom.gov.au/climate/data/)

68044 Mittagong (Alfred Street)

68163 Mittagong (Leicester

Park)

68102 Bowral (Parry Drive)

68184 Bowral Centennial Road

68255 Bowral (Orchard St)

68033 Mittagong (Kia-Ora)

68087 Spring Hill (Warana)

68005 Bowral Post Office

68157 Yarrow (Boural)

68092 Berrima (Hillview)

68239 Moss Vale AWS

The information provided in Tables 2 and 3 indicate that the majority of rain gauges have a

limited record length. Nevertheless, the Mittagong (Alfred St) gauge has over 100 years of daily

rainfall records. The Mittagong (Kia-Ora) gauge also provides over 100 years of daily rainfall

records, however, the record is only 67% complete. Table 2 also shows that no continuous

rainfall data are available prior to November 1992.

A review of the available rainfall data was completed to identify when significant historic

rainfall events have occurred and, consequently, when flooding may have been experienced in

the catchment. The details of the top ten rainfall events are summarised in Table 4.

As shown in Table 4, the most significant rainfall event on record occurred in March 1893,

where nearly 300 mm of rain fell within a 24 hour period. This concurs with a large reported

flood documented in ‘The Bowral Free Press’ (refer Section 2.2.2).

The most significant recent rainfall events occurred in March 1978 and August 1990.

3.3.2 Historic Streamflow Data

There are no stream gauges located within the Gibbergunyah Creek catchment. The closest

stream gauge is operated by the Sydney Catchment Authority and is located downstream of the

Braemer Sewage Treatment (Station 2122791). However, this gauge is located in an adjoining

catchment approximately 5 kilometres away from Gibbergunyah Creek and only has a limited

record length (~10 years, of which, 11% is missing). Accordingly, this gauge was not considered

suitable for use in this study.


11

Table 4 Significant Historic Rainfall Events

Rank Year Day/Month Rainfall in 24 hour

Period (mm)

Rainfall in Preceding 24

Hour Period (mm)

Rainfall in Following 24

Hour Period (mm)

1 1893 5th

March 297 15 3.8

3 1950 18th

January 159 0 5.6

7 1964 11th

June 133 120# 11

5 1966 8th

November 136 27 30

9 1975 20th

June 121 3.4 80

2 1978 19th

March 162 144 109

10 1986 5th

August 118 51 61

4 1990 1st

August 138 73 19

6 1991 10th

June 135 132 58

8 1995 24th

September 123 1.4 26

NOTE: Information in the above table is based upon interrogation of daily rainfall records from Mittagong (Alfred St),

Mittagong (Leicester Park) & Bowral (Parry Drive) rain gauges.

#: this rainfall depth is the accumulated total from the preceding 48 hours

3.4 Topographic Data

3.4.1 Aerial Laser Survey (ALS)

AAM Pty Limited collected Aerial Laser Survey (ALS) across a 124 km2 section of the

Wingecarribee Shire Council Local Government Area on 1st

April 2010. This included a

significant proportion of the Gibbergunyah Creek catchment (refer Figure 3).

The ALS was used as the basis for the development of contours at 0.5 metre intervals as well as

a 1 metre grid-based Digital Elevation Model (DEM). Both of these datasets were provided by

Council for this study.

The ALS has a stated absolute horizontal accuracy of better than 0.55 metres and an absolute

vertical accuracy of better than 0.15 metres (AAM, 2010). Validation of the ALS was completed

using 290 reference points that were surveyed using traditional ground survey techniques. This

included 92 reference points in the Mittagong area. The outcomes of the data validation

determined that the mean difference between ALS and ground surveyed spot heights within

the Mittagong area was less than 2 cm. Therefore, the vertical and horizontal accuracy provided

by the ALS data appears to be suitable for use in this study.

A review of aerial photography indicates that there has been negligible significant developed

across the catchment since the ALS was collected in 2010. As a result, the ALS provides a good

representation of contemporary topography across the catchment and is considered to be

suitable for use in defining topography across the majority of the catchment as part of this

investigation.

However, the ALS does not extend across the entire catchment and the metadata notes that

the ALS data may be less reliable in areas of high vegetation density. In addition, the ALS data

will not pick up the details of topographic and drainage features that are obscured from aerial


12

survey techniques, such as culvert obvert elevations. Accordingly, it was necessary to

supplement the ALS data with additional survey to ensure a reliable representation of the

terrain and drainage structures is provided across all areas of the catchment.

3.4.2 10 Metre Contours

Land and Property Information (LPI) ground surface contours at 10 metres intervals were also

provided by Council. The contours extend across all of the Gibbergunyah Creek catchment. The

horizontal and vertical accuracy of the data is not available. However, it is considered that the

contours are suitable for defining the ground surface topography across the steeper areas of

the catchment not covered by the ALS data. The extent of the area covered by the contours

(not including that area already covered by ALS data) is shown in Figure 2.

The ALS data and the 10 metre contours were combined to form a complete Digital Elevation

Model (DEM) of the study area. The DEM is shown in Figure 2.

3.5 GIS Data

A number of GIS data layers were also provided by Council to assist with the study. This

included:

Aerial Photography;

Stormwater network;

Bridges; and,

Building footprint polygons.

Further detailed information on the GIS layers is provided below.

3.5.1 Aerial Photography

Aerial photography that was captured in 2009 was provided by Council to assist with the

investigation. The imagery is provided at a 0.5 metre pixel size and allowed major features such

as buildings to be identified.

The aerial photography was used to assist with the review of available information (e.g.,

identification of missing buildings). The photography also served as a background layer in the

majority of report figures (refer Flood Study: Volume 2).

3.5.2 Stormwater Network GIS layer

The sub-surface stormwater pipe system plays a significant role in the conveyance of flows

during storm events. Accordingly, it is important to include the trunk stormwater system in any

computer model that is used to define flood behaviour across the Gibbergunyah Creek

catchment.

Two stormwater-related GIS layers were provided by Council. This included:

“Stormwater Nodes”: contains stormwater pit locations. The database was last updated in

October/November 2008 for the Mittagong area. It was noted that only major Council-

owned pits were included (i.e., no pits on privately owned land were incorporated).

“Stormwater Conduits”: contains stormwater pipe / culvert alignments and properties

including conduit type (e.g. pipe, box culvert), material (e.g., concrete), dimensions and

length.


13

The extent of the area covered by the stormwater network GIS layers is shown in Figure 2.

Accordingly, most of the information necessary to include the trunk stormwater drainage

system in the hydraulic model is provided in these GIS layers. Nevertheless, some additional

information including pit depths/invert elevations, pit types (e.g., kerb inlet, gully inlet) and

culvert invert elevations are not available in these layers.

3.5.3 Bridges

A bridges GIS layer was provided by Council showing the location of all bridges within the Local

Government Area. A review of this GIS layer showed that, with the exception of the Hume

Highway crossing of Gibbergunyah Creek, no bridges are located within the catchment.

3.5.4 Building Footprint Polygons

A GIS layer containing building footprint polygons for every significant building within the

Gibbergunyah Creek catchment was provided by Council. The building polygons can be used to

define the impediment to flow afforded by buildings across the catchment.

A review of the building polygons relative to 2009 aerial photography showed the layer suitably

described the majority of buildings across the catchment. However, as shown in Plates 4 and 5,

some buildings were not included in the building polygons layer. Therefore, additional

polygons were digitised by hand based on the 2009 aerial photography to ensure all buildings

were defined.

3.6 Community Consultation

A key component of the flood study involves development and calibration of hydrologic and

hydraulic computer models. Calibration involves using the computer models to replicate floods

that have occurred in the past. Council holds minimal information on historic flooding across

the Gibbergunyah Creek catchment.

However, it is likely that residents and business owners within the Gibbergunyah Creek

catchment may have witnessed past flood events. Accordingly, several community consultation

devices were developed to inform the community about the study and to obtain information

from the community about their past flooding experiences. Further information on each of

these consultation devices is provided below.

3.6.1 Flood Study Website.

A flood study website was established for the duration of the study. The website address is:

http://www.gibbergunyah.floodstudy.com.au/

The website was developed to provide the community with detailed information about the

study and also provide a chance for the community to ask questions and complete an online

questionnaire (this online questionnaire was identical to the questionnaire distributed to

residents and business owners, as discussed in Section 3.6.2).


14

Plate 4 Incomplete building polygon in Regent Street, Mittagong

Plate 5 Missing building polygon in Henderson Avenue, Mittagong


15

During the course of the study (until December 2012), the website was visited 169 times by 79

unique visitors.

3.6.2 Community Information Brochure and Questionnaire

A community information brochure and questionnaire was prepared and distributed to 740

households and businesses within the Gibbergunyah Creek catchment. A copy of the brochure

and questionnaire is included in Appendix A.

The questionnaire sought information from the community regarding whether they had

experienced flooding, the nature of flood behaviour, if roads and houses were inundated and

whether residents could identify any historic flood marks. A total of 135 questionnaire

responses were received, providing a response rate of 18%. A summary of all questionnaire

responses is provided in Appendix A. The spatial distribution of questionnaire respondents is

shown in Figure A1, which is also enclosed in Appendix A.

The following information was gleaned from the responses to the questionnaire:

The majority of respondents have lived in the Gibbergunyah Creek catchment for over 10

years. The average length of residence was about 20 years.

Approximately 20% of respondents had experienced some flooding and/or drainage issues in

the past. Around 15% of respondents have had their front or back yard inundated, 6%

identified traffic as being disrupted by floodwaters, one respondent had their nursery

inundated and another respondent had their garage inundated. The spatial distribution of

respondents that have experienced past flooding problems is shown in Figure A1.

The majority of respondents have not experienced main stream flooding resulting from

floodwaters overtopping creek banks. Most respondents identified inundation from

overland flows caused by poor roadway drainage, insufficient stormwater capacity, excessive

vegetation and/or blocked pipes and culverts as the main causes of flooding.

No specific flood mark information was provided by any of the respondents. However, several

residents provided photographs of historic flooding in the Gibbergunyah Creek catchment. A

selection of photographs provided by the questionnaire respondents are included in Plates 6 to

12.

As shown in Plates 6 to 12, the majority of photographs were provided for a storm that

occurred in early November 2010 (most probably 1st

- 2nd

November, 2010). Several

photographs were also provided for a flood that occurred on 20th

February 2005.

Therefore, although specific flood mark information is not available for any historic floods, it is

considered that the photographs provided for the November 2010 and February 2005 floods

can be used to assist in the verification of the hydrologic and hydraulic computer models.


16

Plate 6 Flooding across southern end of Gibbergunyah Lane during November 2010 event

Plate 7 Flooding across Gibbergunyah Lane during November 2010 event


17

Plate 8 Flooding in the vicinity of John Street, Mittagong during November 2010 event

Plate 9 Flooding across John Street, Mittagong during November 2010 event


18

Plate 10 Flooding across Bowral Lane, Welby during February 2005 event

Plate 11 Flooding across back yard at Lot 8 Bowral Lane, Welby during February 2005 event


19

Plate 12 Floodwaters along unnamed tributary at rear of properties fronting Bowral Lane, Welby during

February 2005 event

3.7 Cross-Section and Structure Survey

To enable development of a hydraulic model capable of providing reliable estimates of flood

behaviour within the study area it was necessary to collect additional survey across the

Gibbergunyah Creek catchment. Consulting surveyors, Lawrence Group, collected the

additional survey information.

The additional data collection comprised the survey of 65 creek cross-sections and 38 hydraulic

structures (i.e., culverts and bridges). The location of cross-sections and structures that were

surveyed is shown in Figure 4.

20

4 HYDROLOGY

4.1 General

The most common method of quantifying flood flows (i.e., discharges) at a particular location in

a catchment is via a hydrologic computer model. A hydrologic model is a mathematical

representation of the various processes that transform rainfall into runoff. The model is

developed so that it incorporates key hydrologic characteristics of the catchment such as area,

slope and roughness. The model can then be used to simulate the transformation of rainfall

into runoff for either historic or statistically derived (i.e., design) rainfall.

The XP-RAFTS software was used to develop a hydrologic computer model of the Gibbergunyah

Creek catchment. XP-RAFTS is a lumped hydrologic computer model that is developed by XP

Software (2009) and is used extensively across Australia for deriving discharge estimates. The

following sections provide a summary of how the model was developed, the adopted input

parameters and the outcomes of the model verification.

4.2 Hydrologic Model Development

4.2.1 Subcatchment Parameterisation

The Gibbergunyah Creek catchment was subdivided into 299 subcatchments based on the

alignment of major flow paths, key topographic divides and the location of stormwater pipes

and pits. The subcatchments were delineated with the assistance of the CatchmentSIM

software (Catchment Simulation Solutions, 2011) using a 2 metre Digital Elevation Model (DEM)

developed using ALS and contour data. The final subcatchment layout is presented in Figure 5.

The Gibbergunyah Creek catchment includes significant urban areas that are relatively

impervious. Urbanisation effectively separates the catchment into two hydrologic systems,

i.e.,:

rapid rainfall response and low infiltration potential across impervious areas; and,

slower rainfall response and high infiltration potential across pervious areas.

In recognition of the differing characteristics of the two hydrologic systems, each XP-RAFTS

subcatchment was subdivided into two sub-areas. The first sub-area was used to represent the

pervious sections of the subcatchment and the second sub-area was used to represent the

impervious sections of the subcatchment. The division of each subcatchment into pervious and

impervious sub-areas allows different loss rates and roughness coefficients to be specified,

thereby providing a more realistic representation of rainfall-runoff processes from the two

different hydrologic systems.

Key hydrologic properties including area and average vectored slope were calculated

automatically for each subcatchment using CatchmentSIM.


21

The catchment was also subdivided into different land use types based on 2009 aerial imagery.

Percentage impervious and Manning’s ‘n’ values were assigned to each land use and are

summarised in Table 5. The percentage impervious and Manning’s ‘n’ values were

subsequently used to calculate weighted average percentage impervious and ‘n’ values for each

subcatchment. The final subcatchment parameters are provided in Appendix B.

Table 5 Adopted Impervious Percentage and Manning’s ‘n’ Values for Hydrologic Model

Land Use Description Manning’s ‘n’ Impervious

(%)

Short grass with isolated trees 0.035 5

Long grass with isolated trees 0.045 2

Light tree coverage 0.050 2

Medium density tree coverage 0.075 2

Dense stand of trees 0.100 2

Rock outcrops 0.040 80

Roadway pavement 0.016 100

Concrete surfaces 0.015 100

Car parks 0.022 100

Water bodies 0.030 100

Railway corridor 0.060 50

Buildings/roof area 0.025 100

4.2.2 Stream Routing

In addition to local subcatchment runoff, most subcatchments will also carry flow from

upstream catchments along the main watercourses. The flow along the watercourses in XP-

RAFTS is represented using a “link” between successive subcatchment “nodes”.

“Routing” type links were used to represent the routing of runoff along the main watercourses

into downstream subcatchments. The routing links employ Muskingum-Cunge routing

procedures and require a representative cross-section, slope, length and Manning’s ‘n’ values

to be defined for each channel reach. Cross-sections were extracted from the available ALS

data and main stream slopes and lengths were calculated automatically by CatchmentSIM.

Manning’s ‘n’ values for the main channel and overbank areas were defined by hand based on

inspection of 2009 aerial photography in conjunction with the Manning’s ‘n’ values listed in

Table 5.

4.2.3 Rainfall Loss Model

During a typically rainfall event, not all of the rain falling on a catchment is converted to runoff.

Some of the rainfall may be intercepted and stored by vegetation, some may be stored in small

depression areas and some may infiltrate into the underlying soils.

To account for rainfall “losses” of this nature, the hydrologic model incorporates a rainfall loss

model. For this study, the “Initial-Continuing” loss model was adopted, which is recommended

in ‘Australian Rainfall and Runoff – A Guide to Flood Estimation’ (Engineers Australia, 1987) for

Eastern NSW.


22

This loss model assumes that a specified amount of rainfall is lost during the initial

saturation/wetting of the catchment (referred to as the ‘Initial Loss’). Further losses are

applied at a constant rate to simulate infiltration/interception once the catchment is saturated

(referred to as the ‘Continuing Loss Rate’). The initial and continuing losses are effectively

deducted from the total rainfall over the catchment, leaving the residual rainfall to be

distributed across the catchment as runoff.

Initial and continuing losses were applied based on standard design values documented in

‘Australian Rainfall and Runoff – A Guide to Flood Estimation’ (Engineers Australia, 1987) and

are summarised in Table 6.

Table 6 Adopted XP-RAFTS Rainfall Losses for Calibration Simulations

Land Use Description

February 2005 Event November 2010 Event

Initial Loss

(mm)

Continuing Loss

(mm/hr)

Initial Loss

(mm)

Continuing Loss

(mm/hr)

Pervious 10 2.5 20 2.5

Impervious 1.5 0 1.5 0

4.2.4 Flood Storage Basins

There are no dedicated flood detention basins located within the Gibbergunyah Creek

catchment. However, Lake Alexandra would likely attenuate downstream flows during

significant storm events by temporarily storing runoff from the upstream catchment. Due to

the potential for the lake to impact on downstream flows, it was incorporated as a flood

storage basin in the XP-RAFTS model.

The representation of flood storage basins in XP-RAFTS requires the outflow and storage

characteristics of the basin to be defined. The outflow characteristics were specified using a

stage-discharge relationship and the storage characteristics were defined using a stage-storage

relationship. The stage-storage relationship was developed using the 2009 ALS data. It was

assumed that no storage was provided below the permanent water level of the lake.

As shown in Plate 13, the Lake Alexandra outlet is quite complex, incorporating 4 separate

outlet structures:

A dual span pedestrian bridge; and,

Three separate pipe outlets (two pipes located adjacent to the pedestrian bridge and a

separate pipe located near the north-eastern corner of the lake).

The stage-discharge relationship for the outlet was developed with the assistance of the HY-8

software (version 7.2), which automates the hydraulic calculations for pipes, culverts and weirs

in accordance with ‘Hydraulic Design Series Number 5 – Hydraulic Design of Highway Culverts’

(U.S. Federal Highway Administration, 2005). The stage-storage and stage-discharge

relationships that were developed for Lake Alexandra are provided in Figure B1, which is

enclosed in Appendix B.


23

Plate 13 Main outlet structure for Lake Alexandra

4.3 Hydrologic Model Calibration

4.3.1 General

Hydrologic computer models are typically developed using parameters that are not known with

a high degree of certainty including imperious proportions, rainfall loss rates and catchment

roughness. Accordingly, the model should be calibrated using rainfall and stream flow data

from historic flood events to ensure the adopted parameters are producing reliable estimates

of rainfall-runoff behaviour.

Recorded stream flow records are required to perform a meaningful hydrologic model

calibration. As discussed in Section 3.3.2, no stream gauges are located within the catchment.

Therefore, the lack of stream flow data means that a comprehensive calibration of the

hydrologic model cannot be completed. Nevertheless, it is possible to complete a ‘pseudo-

calibration’ by routing historic rainfall through the hydrologic model and then routing the

resultant discharge hydrographs through the hydraulic model. Peak flood extents and depths

produced by the hydraulic model can then be compared against recorded flood extents / flood

photographs to verify the combined performance of the hydrologic and hydraulic models.

Calibration is achieved by adjusting hydrologic and/or hydraulic inputs parameters until the


24

recorded flood depths and extents are reproduced by the hydraulic model as closely as

possible.

As discussed in Section 3.6.2, photographs of floods that occurred in February 2005 and

November 2010 were provided by several residents. Accordingly, these events were selected

for the purposes of model calibration / verification.

As a joint calibration was performed using both the hydrologic and hydraulic models, the

hydrologic model calibration should be read in conjunction with the hydraulic model

calibration, which is documented in Section 5.3.

4.3.2 Rainfall Data

Continuous rainfall data are required to define the temporal (i.e., time-varying) distribution of

rainfall in the hydrologic computer model for the nominated calibration / verification event.

There is one continuous rainfall gauge located near the Gibbergunyah Creek catchment that has

records from 1992 onwards (i.e., Bowral (Parry Road) gauge). Accordingly, continuous rainfall

data are available for the February 2005 and November 2010 events.

There are also several daily read rainfall gauges located in close proximity to the catchment.

The daily read rainfall records can be used to provide an indication of the spatial variation in

rainfall during the historic event. Accordingly, these gauges provide sufficient information to

describe the spatial variation in rainfall during both events.

4.3.3 Results of Calibration and Verification Simulations

February 2005 Simulation

The rainfall pluviograph for the Bowral (Parry Road) gauge for the February 2005 event is

enclosed in Appendix C. A review of the rainfall records indicates that approximately 45 mm of

rain fell over a 10 hour period during the 2005 event. A review of intensity-frequency-duration

data indicates that this is slightly less than a 1 year ARI event.

Accumulated daily rainfall totals for each rainfall gauge that was operational during the 2005

event are provided in Figure 6. As shown in Figure 6, four rainfall gauges were operational

during the 2005 event (i.e., 1 pluviometer and 3 daily read gauges).

The accumulated daily rainfall totals were also used to develop a rainfall isohyet map for the

2005 event, which is also included on Figure 6. The isohyet map shows the spatial variation in

daily rainfall depths in the vicinity of the Gibbergunyah Creek catchment for the 2005 event.

The isohyet map indicates that there was minimal spatial variation in rainfall across the

Gibbergunyah Creek catchment during the 2005 event. Total daily rainfall depths across the

Gibbergunyah Creek catchment are estimated to vary between 39 mm and 41 mm.

Accordingly, it was considered that application of a uniform daily rainfall depth of 40mm across

the Gibbergunyah Creek catchment would provide a reasonable estimate of the average depth

of rainfall across the catchment.

The Bowral (Parry Road) pluviometer was used to describe the temporal distribution of rainfall

across the 24 hour period for the 2005 event.


25

A review of the daily read rainfall records indicates the February 2005 event was preceded by

between 5 and 10 mm of rainfall. Therefore, the catchment would have been wet at the start

of the main storm event. Accordingly, an initial loss at the lower end of the suggested

‘Australian Rainfall and Runoff’ range was applied to pervious areas for the 2005 event. A

summary of the adopted initial losses and continuing loss rates is provided in Table 6.

A summary of peak discharges that were generated by the XP-RAFTS model for the February

2005 simulation is provided in Appendix C.

The discharges generated by the XP-RAFTS model were subsequently input into the TUFLOW

hydraulic model to simulate February 2005 flood event. Further information regarding the

TUFLOW model setup and the outcomes of the 2005 flood simulation are provided in Section 5.

November 2010 Simulation

Available rainfall records indicate that approximately 50 mm of rain fell over a 12 hour period

during the November 2010 event. A review of intensity-frequency-duration data indicates that

this rainfall intensity is roughly equivalent to a 1 year ARI event (i.e., slightly more severe than

the 2005 event). The pluviograph for the Bowral (Parry Road) gauge for the November 2010

event is enclosed in Appendix C.

Accumulated daily rainfall totals for the November 2010 event for each rainfall gauge are

provided in Figure 7. The accumulated daily rainfall totals were used to develop a rainfall

isohyet map, which is also included on Figure 7. The isohyet map shows that accumulated daily

rainfall across the catchment varied between 49 mm and 52 mm. Accordingly, there was

negligible spatial variation in rainfall across the catchment. Therefore, a constant accumulated

rainfall depth of 50.5 mm was adopted and applied uniformly across the catchment.

The Bowral (Parry Road) gauge was used to describe the temporal distribution of rainfall across

the 24 hour period for the November 2010 event.

A review of the daily read rainfall records indicates the November 2010 event was preceded by

negligible rainfall. Therefore, the catchment would have been relatively dry at the start of the

main storm event. Accordingly, a higher initial loss of 20 mm was applied to the 2010 event to

reflect additional losses associated with the initial saturation of the catchment. A summary of

the adopted continuing loss rates is provided in Table 6.

A summary of peak discharges that were produced by the XP-RAFTS model for the November

2010 simulation is provided in Appendix C.

The discharges generated by the XP-RAFTS model were subsequently input into the TUFLOW

hydraulic model to simulate the distribution of flows during the November 2011 event. Further

information regarding the TUFLOW model setup and the outcomes of the 2011 flood simulation

are provided in Section 5.

26

5 HYDRAULICS

5.1 General

Hydraulic computer models are the preferred method of simulating flood behaviour through a

particular area of interest. They can be used to predict flood characteristics such as peak flood

level and flow velocity and the results of the modelling can also be used to define the variation

in flood hazard and hydraulic categories across the study area.

The TUFLOW software was used to develop a hydraulic computer model of the Gibbergunyah

Creek catchment. TUFLOW is a fully dynamic, 1D/2D finite difference model developed by BMT

WBM (2012). It is used extensively across Australia to assist in defining flood behaviour.

The following sections describe the model development process as well as the outcomes of the

model calibration and verification.

5.2 Hydraulic Model Development

5.2.1 Model Extent

A linked 1-dimensional/2-dimensional hydraulic model of the creek, floodplain, stormwater

network and overland flow system was developed for the Gibbergunyah Creek catchment using

the TUFLOW software. The model extends across all of the Gibbergunyah Creek catchment

downstream to its confluence with the Nattai River. The extent of the hydraulic model is

shown in Figure 8.

The TUFLOW software uses a uniform grid to define the spatial variation in topography and

hydraulic properties (e.g., Manning’s ‘n’) across the 2D model domain. A 2 metre grid size was

adopted for this study. The 2 metre grid size is considered to provide a reasonable

representation of the variation in terrain, while ensuring simulation times are kept to a

reasonable level and the TUFLOW software is applied appropriately.

A dynamically linked 1-dimensional (1D) network was embedded within the 2D domain to

represent areas that would not be well represented by the 2 metre grid (e.g., narrow creek

channels). The sub-surface piped stormwater system was also represented as a separate 1D

domain. The stormwater pipes were inserted underneath the 2D domain allowing

representation of the conveyance of flows by the stormwater system below ground as well as

simulation of surficial flows in 2D once the capacity of the stormwater system is exceeded. The

extent of the 1D (i.e., channel and stormwater pipe system) and 2D model domains are shown

in Figure 8.

5.2.2 Model Topography

Elevations were assigned to grid cells within the 2D domain based on the Digital Elevation

Model derived from ALS data and contours. As the ALS data was collected in 2009, the terrain

representation in TUFLOW is representative of topographic conditions at that time. That is, any

topographic modifications completed since 2009 will not be reflected in the model. A review of

recent aerial photography indicates there has been negligible large topographic modifications


27

across the catchment since the ALS data was collected. Therefore, the ALS is considered to

provide a reliable representation of contemporary topographic conditions across the majority

of the catchment.

It should be noted that the ALS data does not extend across all of the Gibbergunyah Creek

catchment. Accordingly, the topography across some areas is based on less detailed and less

reliable contour information. However, this typically only impacts on steeper, undeveloped

sections of the catchment.

The elevations assigned to grid cells located within building footprints were elevated by

0.3 metres based on the assumption that the floor level of houses will typically be elevated

above the natural ground surface.

The topography within the surficial 1D domain was defined using surveyed cross-sections. This

was also supplemented with cross-sections extracted from the ALS is areas that were not

obstructed by vegetation. The details of culverts were assigned based on survey information

and the details of the sub-surface piped drainage network were defined based on information

contained within Council’s stormwater asset GIS layer.

5.2.3 Material Types / Manning’s ‘n’ Roughness

The TUFLOW software employs material polygons to define the variation in hydraulic roughness

(i.e., Manning's 'n' values) across a particular study area. 2009 aerial photography was used as

a basis for subdividing the catchment into different material types. Different Manning’s ‘n’

values were assigned to each material to define the resistance to flow afforded by the different

material types.

1D cross-sections, pipes and culverts within the 1D domain of the TUFLOW model also require

the specification of Manning's 'n' values. These values were defined based on field

assessments, survey photography and inspection of 2009 aerial photography.

The adopted materials types and the corresponding Manning's 'n' values are provided in

Table 7.

5.2.4 Culverts/Bridges

Culverts and bridges can have a significant influence on flood behaviour through a particular

study area. For circular or rectangular culverts, the physical dimensions and invert elevations of

the structures were included directly in the TUFLOW model based on the survey information

that was collected. Entry and exit loss coefficients were defined based on default values

provided in the TUFLOW Manual (BMT WBM, 2012). Typically, an entry loss coefficient of 0.5

and an exit loss coefficient 1.0 were adopted for all culverts.

The Gibbergunyah Creek catchment also includes several irregular shaped culvert crossings

(refer Plate 14). The irregular shape of the crossings was defined using a flow height versus

flow width relationship. An entry loss coefficient of 0.5 and an exit loss coefficient of 1.0 was

also adopted for irregular culverts.


28

Table 7 TUFLOW Manning's 'n' Roughness Values

Material Description Manning's 'n'

Open space (i.e., grass) 0.035

Rural grasslands / light brush 0.045

Grass with sparse trees 0.050

Grass with medium density trees 0.075

Dense tree coverage 0.100

Rock outcrops 0.040

Roadway pavement 0.016

Concrete surfaces 0.015

Water bodies 0.030

Railway corridor 0.060

Buildings 3.000

Plate 14 Arch culvert with wildlife ledge draining water beneath the railway line

The catchment also includes several bridge crossings (typically single span pedestrian bridges).

The physical dimensions of the bridge were specified using a surveyed cross-section to define

the variable waterway area beneath the bridge deck. Energy losses were defined using a height


29

versus loss coefficient relationship that was developed based on procedures outlined in

‘Hydraulics of Bridge Waterways’ (Bradley, 1978). The loss coefficient calculations for each

bridge are included in Appendix D.

5.2.5 Pit/Culvert Blockage

As shown in Plate 15, culverts can be subject to partial blockage. Similarly, stormwater inlets

may also become blocked by debris during the course of a flood. As a result, most stormwater

inlets, pipes and culverts will not operate at full efficiency during most floods. This can increase

the severity of flooding across areas located adjacent to this drainage infrastructure.

Plate 15 Example of vegetation blocking culvert outlet in Gibbergunyah Creek catchment.

In recognition of this, blockage factors were applied to all stormwater inlets and culverts. A

blockage factor of 50% was applied to all culvert crossings with a diagonal dimension of less

than 6 metres in accordance with findings documented by Rigby et al (2002) following the 1998

Wollongong floods. A 50% blockage factor was also applied to all sag stormwater inlets and

20% blockage was applied to on-grade stormwater inlets. The impact of alternate blockage

scenarios on flood behaviour was also analysed as part of the TUFLOW model sensitivity

analysis.

A higher blockage factor was applied to a double pipe culvert that drains beneath Railway

Parade (near Huxley Street). As shown in Plate 16, this culvert incorporates plates across both

culvert openings. In recognition of these plates as well as the potential blockage contribution

from vegetation and debris from the upstream catchment, a blockage factor of 80% was

applied to this culvert.


30

Plate 16 Railway Parade culverts showing plates that increase the effective blockage

5.3 Hydraulic Model Calibration

5.3.1 General

A full calibration of the XP-RAFTS hydrologic and TUFLOW hydraulic computer models could not

be completed due to the lack of stream gauging and flood mark data for the Gibbergunyah

Creek catchment. Nevertheless, a joint calibration of the hydrologic and hydraulic models was

attempted to verify the computer models were providing realistic reproductions of past flood

events.

5.3.2 Calibration/Verification Event Selection

As discussed in Section 3.6.2, photographs of floods that occurred in February 2005 and

November 2010 were provided by several residents. Accordingly, these events were selected

for the purposes of model calibration / verification.

As a joint calibration was performed using the hydrologic and hydraulic models, the hydraulic

model calibration should be read in conjunction with the hydrologic model calibration

previously discussed in Section 4.3.

5.3.3 Model Boundary Conditions

Upstream Boundary Conditions

Upstream boundary conditions define the spatial variation in flows with respect to time across

the hydraulic model domain. Inflows to the TUFLOW hydraulic model were defined using 'local'


31

discharge hydrographs (representing flows from the local subcatchments only) extracted from

the XP-RAFTS hydrologic modelling discussed in Section 4.3.

For those XP-RAFTS subcatchments containing stormwater pits, local inflows were distributed

equally to the surface of all pits. For those subcatchments not containing stormwater pits, local

inflows were applied to the lowest point within each sub-catchment.

Downstream Boundary Conditions

Hydraulic computer models also require the adoption of a suitable downstream boundary

condition in order to reliably define flood behaviour throughout the area of interest. The

downstream boundary is typically defined as a known water surface elevation (i.e., stage).

As shown in Figure 8, the downstream boundary of the TUFLOW model is located at the

confluence of Gibbergunyah Creek with the Nattai River. Accordingly, the downstream water

elevation will be governed by the water surface elevation within the Nattai River at the time of

the flood.

Unfortunately, no stream gauges or automatic water level recorders are located along the

Nattai River in the vicinity of Gibbergunyah Creek to provide an indication of water levels during

the 2005 and 2010 floods. In the absence of historic Nattai River water level information, a

“normal depth” boundary was applied at the downstream end of the Gibbergunyah Creek

TUFLOW model. This approach assumes that the downstream water level is influenced only by

the geometry, roughness and slope of the Gibbergunyah Creek channel. Given the relatively

steep slope of the downstream reaches of the Gibbergunyah Creek channel, it is considered

that any uncertainties associated with the downstream boundary condition should not impact

on hydraulic model results across the “built up” sections of the catchment where historic flood

photographs are available.

5.3.4 Results of Calibration and Verification Simulations

February 2005 Simulation

Calibration of the TUFLOW hydraulic model was attempted based on three photographs that

were provided of the Bowral Lane, Welby area during the February 2005 event. The verification

was undertaken by routing the discharge hydrographs generated by the XP-RAFTS model for

the 2005 event through the TUFLOW model and adjusting roughness parameter values until the

model provided a reasonable reproduction of the extent and depth of floodwater depicted in

the photographs.

The roughness values listed in Table 7 were adopted for the 2005 flood simulation. All

roughness parameter values listed in Table 7 are within reasonable limits.

The simulated extent and depth of inundation during the February 2005 event is provided in

Figure 9. Figure 9 also incorporates velocity vector arrows, which show the direction and speed

of movement of the floodwaters, as well as flood level contours, which indicate the maximum

height (i.e., stage) the floodwaters reached during the flood (relative to Australian Height

Datum).

The three photographs that were used as the basis for the model calibration are included in

Figures 9.9 and 9.10. The photographs provided in Figures 9.9 and 9.10, indicate that during

the 2005 event floodwaters travelled along Bowral Lane to a low point in the roadway profile


32

near Lot 8 Bowral Lane. At this location, the water ponded until it started to discharge down

the driveway of Lot 8, surrounding the residence to a depth of 50-100 mm before eventually

draining into an unnamed tributary at the rear of the property.

The flood behaviour shown in Figures 9.9 and 9.10 generally replicates the available

photographic evidence. In particular, it shows water accumulating along Bowral Lane, flowing

south and surrounding some residential dwellings (the depths of inundation are typically

around 50-100 mm). The model results also show the unnamed tributary at the rear of the

property flowing at close to bank-full capacity, which is also consistent with the available

photographs.

Overall, the TUFLOW model is considered to be providing a realistic representation of flood

behaviour across the Bowral Lane, Welby area during the February 2005 flood.

November 2010 Simulation

The TUFLOW hydraulic model was also verified using four photographs for a flood that occurred

in November 2010. The photographs show floodwater depths and extents across

Gibbergunyah Lane and John Street, Mittagong. The verification was completed by routing the

discharge hydrographs generated by the XP-RAFTS model for the 2010 event through the

TUFLOW model while retaining the same roughness parameter values that were adopted for

the 2005 calibration simulation (refer Table 7).

Peak floodwater depths, levels and velocities generated by the TUFLOW model for the 2010

simulation are provided in Figure 10. The four photographs that were used as the basis for

verification are shown on Figures 10.6 and 10.10.

Figure 10.6 shows historic flood photographs of John Street, Mittagong. The photographs

indicate that floodwater in this area originates from undeveloped land to the south of John

Street. Water from this undeveloped area travels down a relatively steep slope onto John

Street, where it ponds at a low point in the roadway profile. The floodwaters are sufficiently

deep to cover the entire roadway at this low point, although the depths of inundation appear

to be less than 100 mm.

Figure 10.10 shows historic flood photographs at the southern end of Gibbergunyah Lane,

Mittagong. They show water ponded behind the small embankment formed by Gibbergunyah

Lane. The depths of inundation appear to be between 100 and 200 mm upstream of the

roadway. Floodwaters overtop the very southern end of Gibbergunyah Lane to a depth of

approximately 50 mm.

The depths and extents of inundation produced by the TUFLOW model appear to provide a

reasonable reproduction of the extents and depths of inundation shown in all four

photographs. Accordingly, the TUFLOW model is considered to be providing realistic estimates

of historic flood behaviour for the November 2010 flood.

5.3.5 Additional Model Verification

Figures 9 and 10 also incorporate points corresponding to locations where community

questionnaire responses were received. These locations were colour-coded based on whether

the resident had reported flooding or drainage problems as part of the questionnaire response.

Accordingly, it was considered that this information could be compared against the extents of


33

inundation for the 2005 and 2010 events to provide an additional means of verifying the model

performance. That is, if the extents of inundation produced by the TUFLOW model coincided

with areas where flooding problems have been reported, it would provide additional

confidence that the model was providing a reasonable reproduction of past floods.

It should be noted that in a lot of instances the exact dates of the flooding problems were not

specified. Accordingly, it is not known whether the reported flooding problems occurred in

2005 or 2010. In addition, the locations are based on the address provided by the respondent

and may not reflect the actual location of the reported flooding “trouble spot”. Nevertheless, it

can provide a general indication of the “reasonableness” of the hydraulic model performance

relative to past flood events.

As shown in Figures 9 and 10, the inundation extents typically coincide with locations where

flooding/drainage problems were reported. There are some areas where inundation is shown

and no flooding problems were reported. However, the depth of inundation across these areas

is typically less than 50 mm and respondents may not have classified these shallow depths of

inundation as flooding.

In general, Figures 9 and 10 indicate that the model is identifying areas were historic flooding

problems have occurred.

5.3.6 Summary

A definitive calibration of the Gibbergunyah Creek XP-RAFTS hydrologic model and TUFLOW

hydraulic model could not be completed due to the lack of historic stream gauging data and

flood marks. Nevertheless, a pseudo calibration and verification of the models was attempted

using historic rainfall data in conjunction with flood photographs provided by members of the

community.

The outcomes of the calibration and verification simulations indicate that the XP-RAFTS

hydrologic model and TUFLOW hydraulic model provides a reliable reproduction of the historic

flood behaviour depicted in the flood photographs. Accordingly, it is considered that the XP-

RAFTS and TUFLOW models are suitable for use in simulating design flood behaviour across the

Gibbergunyah Creek catchment.

34

6 DESIGN FLOOD ESTIMATION

6.1 General

Design floods are hypothetical floods that are commonly used for planning and floodplain

management investigations. Design floods are based on statistical analysis of rainfall and

flood records and are defined by their probability of occurrence. For example, a 100 year

Average Recurrence Interval (ARI) flood is the best estimate of a flood that will likely occur

once every one hundred years, on average.

Design floods can also be expressed by their probability of occurring in a given year. For

example, the 100 year ARI flood can also be expressed as the 1% Annual Exceedance

Probability (AEP) flood. That is, there is a 1% chance of the 100 year ARI flood occurring in

any given year.

It should be noted that there is no guarantee that a 100 year ARI flood will occur just once in

a one hundred year period. It may occur more than once, or at no time at all in the one

hundred year period. This is because design floods are based upon a long-term statistical

average.

The hydrologic and hydraulic computer models were used to derive design flood estimates

for the 5, 10, 20, 50, 100 and 200 year ARI floods, as well as the Probable Maximum Flood

(PMF). The procedures employed in deriving these design flood estimates are outlined in

the following sections.

6.2 Hydrology

6.2.1 Design Rainfall

Design storms for the 5, 10, 20, 50, 100 and 200 year ARI events were derived using

standard procedures outlined in ‘Australian Rainfall and Runoff – A Guide to Flood

Estimation’ (Engineers Australia, 1987). Design rainfall intensities were extracted from

‘Australian Rainfall and Runoff’ and were used in conjunction with design temporal patterns

to describe the temporal variation in rainfall throughout each design storm.

Adopted rainfall intensities for each design storm and duration are summarised in Table 8.

6.2.2 Probable Maximum Precipitation (PMP)

As part of the flood study it was also necessary to define flood characteristics for the

Probable Maximum Flood (PMF). The PMF is estimated by routing the Probable Maximum

Precipitation (PMP) through the hydrologic model. The PMP is defined as the greatest depth

of precipitation that is meteorologically possible for a given duration at a specific location at

a particular time of year.


35

Table 8 Design Rainfall Intensities

DURATION

Return Period (Years) / Rainfall Intensity (mm/hr)

5 10 20 50 100 200 PMP

10 mins 57.5 74.6 97.8 112 130 327.8 N/A

15 mins 41.8 54.2 71.2 81.2 94.4 165.1 560

30 mins 33.9 44 57.8 66 76.8 117.9 420

45 mins N/A N/A N/A N/A N/A 95.0 360

1 hour 23 29.9 39.3 44.8 52.2 81 310

90 mins N/A N/A N/A N/A N/A 63.6 267

2 hours 15.1 19.7 25.9 29.5 34.4 53.3 235

2.5 hours N/A N/A N/A N/A N/A N/A 208

3 hours 11.8 15.3 20.1 23 26.7 41.5 190

4 hours N/A N/A N/A N/A N/A N/A 163

5 hours N/A N/A N/A N/A N/A N/A 142

6 hours 7.61 9.9 13 14.9 17.3 27.0 127

12 hours 4.98 6.47 8.5 9.72 11.3 17.6 N/A

24 hours 3.3 4.28 5.62 6.41 7.44 11.5 N/A

48 hours 2.17 2.81 3.66 4.16 4.82 7.3 N/A

72 hours 1.64 2.13 2.76 3.13 3.63 5.5 N/A

NOTE: N/A indicates a design rainfall is not available for the nominated storm duration

PMP depths were derived for a range of different durations for the Gibbergunyah Creek

catchment based on procedures set out in the Bureau of Meteorology's Generalised Short

Duration Method (GSDM) (Bureau of Meteorology, 2003). The GSDM PMP calculations are

included in Appendix E and the PMP intensities are also summarised in Table 8.

6.2.3 Rainfall Loss Model

As discussed in Section 4.2.3, the Initial-Continuing Loss Model was used to simulated

rainfall losses across the catchments in the XP-RAFTS hydrologic model.

The adopted rainfall losses are summarised in Table 9. The adopted rainfall losses for all

design events up to and including the 100 year ARI were consistent with those adopted in

the calibration simulations. In accordance with recommendations in ‘Australian Rainfall and

Runoff’, no rainfall losses were applied for "extreme" events (i.e., events in excess of the 100

year ARI).


36

Table 9 Adopted XP-RAFTS Rainfall Losses for Design Simulations

Land Use Description

5,10, 20, 50 and 100 Year ARI 200 Year ARI and PMP

Initial Loss

(mm)

Continuing Loss

(mm/hr)

Initial Loss

(mm)

Continuing Loss

(mm/hr)

Pervious 15 2.5 0 0

Impervious 1.5 0 0 0

6.2.4 Baseflow

As discussed in Section 3.2.4, there is potential for baseflow contributions from the Mount

Gibraltar section of the Gibbergunyah Creek catchment. This may reduce the available flow

carrying capacity of major watercourses across the catchment resulting in more severe

flooding.

Therefore, potential baseflow contributions were estimated based on procedures set out in

the Australian Rainfall and Runoff document titled ‘Revision Project 7: Baseflow for

Catchment Simulation’ (Engineers Australia, 2011). This document allows a ratio of

streamflow peak to the baseflow discharge at the time of the streamflow peak to be

calculated. A summary of the calculated ratios for each design flood is provided in Table 10.

Table 10 Adopted Baseflow Contributions for Design Simulations

Design

Event

(ARI)

Baseflow Under

Peak Storm Flow

Ratio

Peak Storm

Discharge*

(m3/s)

Baseflow at Peak

Storm Discharge*

(m3/s)

Baseflow per Unit

Area

(m3/s/km

2)

5 0.084 88.7 7.5 0.69

10 0.070 110 7.7 0.70

20 0.056 142 8.0 0.73

50 0.049 177 8.7 0.80

100 0.042 210 8.8 0.81

200 N/A 250 8.8 0.81

PMF N/A 1100 8.8 0.81

NOTE: * Peak discharge and associated baseflow discharge are calculated at catchment outlet

Once a ratio is derived it can be used in conjunction with the peak stormflow discharge to

estimate the baseflow discharge at the time of peak storm discharge. The resulting

baseflow discharge estimates are provided in Table 10. A complete listing of baseflow

calculations is provided in Appendix B.

For this study, the baseflow discharge was distributed to each subcatchment in the XP-

RAFTS model based on the contributing subcatchment area. The baseflow per unit area that

was applied to each XP-RAFTS subcatchment for each design flood is summarised in

Table 10.

No information is contained in ‘Revision Project 7: Baseflow for Catchment Simulation’

(Engineers Australia, 2011) for estimation of the baseflow discharge during events greater


37

than the 100 year ARI. Therefore, the 100 year ARI baseflow was also applied to the 200

year ARI and PMF events.

6.2.5 Peak Discharges

The XP-RAFTS model was used to simulate the 5, 10, 20, 50, 100, and 200 year ARI design

floods as well as the PMF.

A range of storm durations were modelled for each design storm to establish the critical

storm durations for each Gibbergunyah Creek subcatchment. Peak discharges were

extracted from the XP-RAFTS model for each subcatchment and each storm duration. These

are provided in Appendix F. Peak discharges at key locations throughout the catchment are

also summarised in Table 11.

Table 11 Peak Design Discharges for Existing Conditions

Location

(XP-RAFTS ID)


5 Year

ARI

10 Year

ARI

20 Year

ARI

50 Year

ARI

100

Year

ARI

200

Year

ARI

PMF

Thomas Road

(1.13)

Gibbergunyah

Creek 21.1 26.0 34.0 42.5 50.5 58.5 255

Old Hume Highway

(1.17)

Gibbergunyah

Creek 32.4 39.7 51.5 64.1 76.1 88.6 397

Thomas St

(23.03)

Tributary of

Gibbergunyah

Creek

2.15 2.72 3.51 4.30 5.16 5.99 26.1

Old Bowral Rd at

Railway Underpass

(28.07)

Gibbergunyah

Creek 5.63 7.11 9.08 10.9 12.7 14.5 55.1

Thomas St

(28.10)

Tributary of

Gibbergunyah

Creek

7.93 10.2 13.2 15.9 18.6 21.4 88.1

Cook St

(28.11)

Tributary of

Gibbergunyah

Creek

8.39 10.8 13.9 16.8 19.6 22.6 94.5

Old Hume Highway

(28.13)

Tributary of

Gibbergunyah

Creek

14.0 17.2 22.4 27.6 32.3 37.6 171

US Railway

(52.07) Chinamans Creek 13.7 17.6 22.5 27.4 32.0 36.9 139

Bowral Rd

(52.08) Chinamans Creek 13.4 17.0 21.7 26.5 31.0 36.0 139

Priestley St

(52.11) Chinamans Creek 14.3 18.1 23.0 28.0 32.8 37.8 149


38

Location

(XP-RAFTS ID)


5 Year

ARI

10 Year

ARI

20 Year

ARI

50 Year

ARI

100

Year

ARI

200

Year

ARI

PMF

Old Hume Highway

(52.12) Chinamans Creek 17.6 22.1 27.7 33.8 39.3 45.2 187

Railway Parade

(63.03)

Tributary of

Chinamans Creek 1.96 2.45 3.15 3.86 4.55 5.22 19.0

Brewster St

(63.05)

Tributary of

Chinamans Creek 3.59 4.57 5.9 7.24 8.58 9.95 37.3

US Etheridge St

Retirement Village

(63.06)

Tributary of

Chinamans Creek 4.44 5.63 7.38 9.00 10.6 12.34 46.2

Bessemer St Railway

Underpass

(67.06)

Iron Mines Creek 4.58 5.84 7.51 9.20 10.8 12.36 46.2

Regent Lane

(67.07) Iron Mines Creek 7.69 9.64 12.4 15.1 17.8 20.5 76.6

US RSL Carpark Entry

(67.08) Iron Mines Creek 9.47 11.9 15.2 18.7 22.2 25.6 99.0

DS RSL Carpark Exit

(67.09) Iron Mines Creek 9.91 12.5 15.9 19.6 23.3 26.9 105

Old Hume Highway

(67.09) Iron Mines Creek 9.91 12.5 15.9 19.6 23.3 26.9 105

DS Railway Pde, US

Railway line

(88.04/91.01)

Lake Alexandra 4.22 4.04 5.20 6.20 7.00 8.07 29.3

Main St

(88.05/98.01/97.01) Lake Alexandra 8.69 5.85 7.44 8.78 9.97 11.5 43.9

Edward St

(88.08)

Lake Alexandra 6.60 8.77 11.2 13.5 15.5 17.9 74.6

Alfred St - Upstream

Lake Alexandra entry

(88.09)

Lake Alexandra 7.20 9.56 12.2 14.4 16.7 19.4 83.4

Lake Alexandra Outlet

(88.10) Lake Alexandra 7.77 9.83 12.7 15.5 17.9 20.6 91.6


39

Location

(XP-RAFTS ID)


5 Year

ARI

10 Year

ARI

20 Year

ARI

50 Year

ARI

100

Year

ARI

200

Year

ARI

PMF

Bendooley St – Welby

(48.02/44.04/47.01)

Gibbergunyah

Creek 5.89 7.36 9.30 11.2 13.1 15.1 53.5

Hume Highway (~DS

extent of model)

(1.31)

Gibbergunyah

Creek 95.3 118 149 184 216 254 1164

The results of the design simulations indicate that the critical storm duration for the

majority of subcatchments varies between 1.5 and 4.5 hours. However, during the PMF, the

critical duration is typically between 15 and 45 minutes.

6.2.6 Verification of Peak Discharges

As a comprehensive calibration of the XP-RAFTS model could not be completed due to a lack

of recorded stream flow data. However, the peak 100 year ARI discharges generated by the

XP-RAFTS model were verified against peak discharges calculated using the Probabilistic

Rational Method (PRM). A complete listing of 100 year ARI XP-RAFTS discharges and PRM

discharges at the outlet of each subcatchment is provided in Appendix G. A comparison

between XP-RAFTS and PRM discharges at key locations across the Gibbergunyah Creek

catchment is also provided in Table 12.

In general, the XP-RAFTS and PRM discharges provided in Appendix G and Table 12 show a

good correlation. The PRM typically predicts slightly lower discharges relative to the XP-

RAFTS model. This is not unexpected as the PRM fails to account for the increased runoff

potential across impervious sections of the catchment.

The peak 100 year ARI discharges predicted by XP-RAFTS and the PRM were also compared

against previous flooding investigations within the catchments. This comparison is

summarised in Table 13.

The comparison shows that the peak 100 year ARI discharge generated by the XP-RAFTS

model at the Bessemer Street railway underpass compares favourably with the peak 100

year ARI discharge documented in the ‘Lot 2 Bessemer Street, Mittagong – Flood Study’

(Bewsher Consulting, 2009). Peak discharges at this location agree to within 5%.

However, the comparison in Table 13 also shows that the XP-RAFTS 100 year ARI discharge

is significantly higher than the 100 year ARI discharge documented in the ‘Hydraulic

Assessment of Chinamans Creek, Mittagong’ (RHM Consulting Engineers, 2006). In this

instance, the XP-RAFTS discharge is nearly 40% higher. As outlined in Section 3.2.2,

calibration and verification of the WBNM hydrologic model was not completed as part of

this study. Although the WBNM model was verified against the Rational Method, the

associated rational method calculations are not included in the report. Therefore the

veracity of this verification cannot be confirmed.


40

Table 12 Comparison between Probabilistic Rational Method and XP-RAFTS 100 Year ARI Peak Design

Discharges

Location

(XP-RAFTS ID)

Peak 100 Year ARI Discharge (m3/s)

XP-RAFTS Probabilistic Rational

Method

Old Hume Highway

(1.17) Gibbergunyah Creek 76.1 67.0

Old Bowral Rd at Railway Underpass

(28.07) Gibbergunyah Creek 12.7 11.9

Old Hume Highway

(28.13)

Tributary of

Gibbergunyah Creek 32.3 31.1

Priestley St

(52.11) Chinamans Creek 32.8 30.9

Old Hume Highway

(52.12) Chinamans Creek 39.3 38.5

Brewster St

(63.05)

Tributary of

Chinamans Creek 8.58 8.80

US Etheridge St Retirement Village

(63.06)

Tributary of

Chinamans Creek 10.6 11.6

Bessemer St Railway Underpass

(67.06) Ironmines Creek 10.8 11.4

Old Hume Highway

(67.09) Ironmines Creek 23.3 22.5


(88.10) Lake Alexandra 17.9 23.6

Hume Highway

(1.31) Gibbergunyah Creek 216 171

Table 13 Comparison between XP-RAFTS Design Discharges and Discharges Documented in Previous

Flooding Investigations

Location

Current Study

(XP-RAFTS)

Previous Studies

Hydraulic Assessment of

Chinamans Ck

(RHM, 2006)

Lot 2 Bessemer St, Flood

Study

(Bewsher, 2009)

Old Hume Highway crossing of

Chinamans Creek 39.3 24.6 -


41

Bessemer Street Railway Underpass 10.8 - 10.2

Nevertheless, the outcomes of the calibration and verification and the similarity in

discharges generated by the XP-RAFTS model with the PRM and Bewsher Consulting

discharge estimates indicates that the XP-RAFTS model is producing realistic design

discharge estimates.

6.3 Hydraulics

6.3.1 General

The TUFLOW hydraulic model was used to simulate design flood behaviour across the

Gibbergunyah Creek catchment for the 5, 10, 20, 50, 100, and 200 Year ARI events as well as

the Probable Maximum Flood (PMF).

The procedures employed in developing the design flood estimates are outlined in the

following sections.

6.3.2 Model Boundary Conditions

Flow Boundary Conditions

Flow boundary conditions provide a description of the spatial and time variation of flows

across the hydraulic model domain during each design storm. Inflows to the TUFLOW

hydraulic model were defined using 'local' discharge hydrographs (representing flows from

the local subcatchments only) extracted from the XP-RAFTS hydrologic modelling.

For those XP-RAFTS subcatchments containing stormwater pits, local inflows were

distributed equally to the surface of all pits. For those sub-catchments not containing

stormwater pits, local inflows were applied to the lowest point within each sub-catchment.

Downstream Boundary Conditions

As no definitive design stage information is available for the Nattai River, downstream

boundary conditions for the Gibbergunyah Creek TUFLOW model were defined using a

“normal depth” calculation. That is, the downstream stage was defined based on the

stream geometry and slope as well as the total discharge at the downstream model

boundary.

Given the relatively steep slope of the downstream reaches of the Gibbergunyah Creek

channel, it is considered that any uncertainties associated with the downstream boundary

condition should not impact on hydraulic model results across the “built up” sections of the

catchment.

6.3.3 Design Flood Envelope

The TUFLOW hydraulic model was used to simulate flood behaviour across the

Gibbergunyah Creek catchment for a range of design floods and a range of storm durations.

The model produced information on flood levels, depths and velocities across the

Gibbergunyah Creek catchment for each design flood and each duration.


42

As discussed, a range of critical durations are evident across the study area. Therefore, a

single storm duration will not necessarily produce the “worst case” flooding across all

sections of the catchment.

An important outcome of this study was to ensure that the "worst case" flooding conditions

were defined across the entire Gibbergunyah Creek catchment. Therefore, a design flood

envelope was developed for each ARI based on analysis of each storm duration at each

TUFLOW grid cell. This involved extracting and comparing peak flood levels, depths and

velocities at each TUFLOW model grid cell for each simulated duration and the highest

depth, level and velocity at each grid cell was subsequently adopted. It is this design flood

envelope, comprising the worst case depths, velocities and levels at each grid cell that forms

the basis for the results documented in the following sections.

6.3.4 Floodwater Depths, Levels and Velocities

Peak flood levels, depths and velocities for the 5, 10, 20, 50, 100, and 200 Year ARI events as

well as the Probable Maximum Flood (PMF) were extracted from the results of the TUFLOW

model. Peak floodwater depths and velocity vectors are presented in Figures 11 to 17.

Peak flood levels were also extracted from the results of the modelling and are presented in

Table 14 at key locations throughout the Gibbergunyah Creek catchment. The location

identification (ID) numbers can also be referenced by the yellow points in Figures 11 to 17.


43

Table 14 Peak Design Floodwater Elevations

ID Location Tributary

Peak Stage (mAHD)

5 Year ARI 10 Year

ARI

20 Year

ARI

50 Year

ARI

100 Year

ARI

200 Year

ARI PMF

1 Thomas Road Gibbergunyah Creek 614.78 614.83 614.90 615.66 615.02 615.08 615.78

2 Old Hume Highway Gibbergunyah Creek 600.82 601.03 601.58 602.03 602.10 602.28 603.18

3 Thomas St Tributary of Gibbergunyah Creek 611.88 613.08 613.09 613.08 613.11 613.10 613.24

4 Old Bowral Rd at Railway

Underpass Gibbergunyah Creek 648.81 648.92 649.11 649.20 649.30 649.40 650.63

5 Thomas St Tributary of Gibbergunyah Creek 611.60 611.68 611.78 611.84 611.91 611.96 612.58

6 Cook St Tributary of Gibbergunyah Creek 603.86 603.91 603.96 603.99 604.02 604.05 604.36

7 Old Hume Highway Tributary of Gibbergunyah Creek 600.48 601.04 601.59 601.98 602.11 602.21 603.17

8 U/S Railway Chinamans Creek 633.64 634.38 635.27 636.01 636.59 637.04 643.75

9 Bowral Rd Chinamans Creek 631.06 632.18 633.34 633.87 634.03 634.11 634.96

10 Priestley St Chinamans Creek 613.61 613.65 613.67 613.74 613.80 613.84 614.33

11 Old Hume Highway Chinamans Creek 602.92 603.01 603.09 603.27 603.49 603.65 604.71

12 Railway Parade Tributary of Chinamans Creek 639.13 639.89 640.00 640.03 640.05 640.07 640.29

13 Brewster St Tributary of Chinamans Creek 615.86 615.87 615.90 615.91 615.93 615.94 616.28


44


Peak Stage (mAHD)

5 Year ARI 10 Year

ARI

20 Year

ARI

50 Year

ARI

100 Year

ARI

200 Year

ARI PMF

14 U/S Etheridge St Retirement

Village Tributary of Chinamans Creek 609.20 609.34 609.53 609.64 609.87 609.87 610.30

15 Etheridge St Tributary of Chinamans Creek 607.67 607.69 607.72 607.74 607.76 607.77 608.08

16 Bessemer St Railway Underpass Iron Mines Creek 626.79 626.90 627.06 627.23 627.55 627.93 630.62

17 Regent Lane Iron Mines Creek 621.64 621.68 622.05 622.15 622.18 622.20 622.37

18 U/S RSL Carpark Entry Iron Mines Creek 614.78 614.80 614.77 614.78 614.79 614.83 615.31

19 D/S RSL Carpark Exit Iron Mines Creek 611.79 611.83 611.89 611.89 611.94 612.00 612.37

20 Old Hume Highway Iron Mines Creek 609.74 609.76 609.88 609.89 609.93 609.99 610.57

21 D/S Railway Pde, U/S Railway line Lake Alexandra 630.30 630.39 630.43 630.52 630.63 630.74 631.76

22 Main St Lake Alexandra 627.54 627.56 627.63 627.69 627.74 627.78 628.28

23 Edward St Lake Alexandra 624.27 624.29 624.32 624.35 624.37 624.39 624.68

24 Alfred St - Upstream Lake

Alexandra Lake Alexandra 621.52 621.59 621.66 621.69 621.72 621.74 621.99

25 Lake Alexandra Outlet Lake Alexandra 621.74 621.79 621.86 621.91 621.93 621.98 622.32

26 Lake Alexandra spillway Lake Alexandra 621.52 621.61 621.66 621.68 621.71 621.74 622.04

27 Bendooley St - Welby Gibbergunyah Creek 611.76 611.79 611.83 611.87 611.89 611.92 612.24


45


Peak Stage (mAHD)

5 Year ARI 10 Year

ARI

20 Year

ARI

50 Year

ARI

100 Year

ARI

200 Year

ARI PMF

28 Hume Highway Gibbergunyah Creek 534.41 534.55 534.71 534.90 535.11 535.31 537.66

# refer to Figures 11 to 17 for Location ID

46

7 SENSITIVITY ANALYSIS

7.1 General

Hydrologic and hydraulic computer models require the adoption of several parameters that are

not necessarily known with a high degree of certainty. Each of these parameters can impact on

the results generated by the model.

Typically hydrologic and hydraulic computer models are calibrated using recorded rainfall,

stream flow and/or flood mark information. Calibration is achieved by adjusting the

parameters that are not known with a high degree of certainty until the computer models

reproduce the recorded flood information.

As discussed, the XP-RAFTS hydrologic and TUFLOW hydraulic models developed for this study

could not be comprehensively calibrated as there was insufficient recorded stream flow and

flood mark information. However, the models were verified against floods that occurred in

2005 and 2010 and were found to provide a reasonable description of historic flood behaviour.

Nevertheless, it is important to understand how any uncertainties in model input parameters

may impact on the results produced by the model. Therefore, a sensitivity analysis was

undertaken to establish the sensitivity of the results generated by the computer model to

changes in model input parameter values. The outcomes of the sensitivity analysis are

provided below.

7.2 Hydrologic Model

7.2.1 Initial Loss

An analysis was undertaken for the 100 year ARI storm to assess the sensitivity of the results

generated by the XP-RAFTS model to variations in antecedent wetness conditions (i.e., the

dryness or wetness of the land within the catchment prior to the design storm event). A

catchment that has been saturated prior to a major storm will have less capacity to absorb

rainfall. Therefore, under wet antecedent conditions, there will be less “loss” of rainfall and

consequently more runoff.

The variation in antecedent wetness conditions was represented by modifying the adopted

initial rainfall losses in the XP-RAFTS model. The results of the sensitivity analysis are

summarised in Table 15 at selected locations across the Gibbergunyah Creek catchment for the

100 year ARI event. A complete listing of peak discharges for each XP-RAFTS subcatchment is

provided in Appendix H.


47

Table 15 XP-RAFTS Sensitivity to Variations in Initial Losses

Location

(XP-RAFTS ID)


Adopted Initial

Losses

Pervious = 15mm

Impervious = 1.5mm

Wet Antecedent

Conditions

Pervious = 0mm

Impervious = 0mm

Dry Antecedent

Conditions

Pervious = 30mm

Impervious = 3mm

Old Hume Highway

(1.17)

Gibbergunyah

Creek 76.1 88.7 55.6

Old Bowral Rd at Railway

Underpass

(28.07)

Gibbergunyah

Creek 12.7 13.4 9.49

Old Hume Highway

(28.13)

Tributary of

Gibbergunyah

Creek

32.3 37.1 23.9

Priestley St

(52.11) Chinamans Creek 32.8 36.5 24.4

Old Hume Highway

(52.12) Chinamans Creek 39.3 44.6 30.2

Brewster St

(63.05)

Tributary of

Chinamans Creek 2.92 3.08 2.09

US Etheridge St Retirement

Village

(63.06)

Tributary of


Bessemer St Railway

Underpass

(67.06)

Iron Mines Creek 10.8 11.9 7.78

Old Hume Highway

(67.09) Iron Mines Creek 23.3 25.7 16.9


(88.10) Lake Alexandra 17.9 20.9 14.1

Hume Highway

(1.31)

Gibbergunyah

Creek 216 258 165

The results of the initial loss sensitivity analysis show that decreasing the initial losses would

typically increase the peak discharges generated by the model by up to 20%. However, the

average increase in peak discharge is only predicted to be about 10%.

Increasing the initial loss would decrease peak discharge by 25% (on average). However,

isolated decreases of over 30% are predicted to occur at some isolated locations.


48

Therefore, it can be concluded that the XP-RAFTS model is moderately sensitive to changes in

the adopted initial loss. 'Australian Rainfall & Runoff' (Engineers Australia, 1987) suggests

adopting an initial loss of between 10 mm and 30 mm for design flood estimation. The adopted

initial loss of 15 mm is towards the lower end of the suggested range and would, therefore,

provide reasonably conservative design flood discharge estimates.

7.2.2 Continuing Loss Rate

An analysis was also undertaken to assess the sensitivity of the results generated by the XP-

RAFTS model for the 100 year ARI event to variations in the adopted continuing loss rates. The

results of the sensitivity analysis are summarised in Table 16 at selected locations across the

Gibbergunyah Creek catchment. A complete listing of peak discharges for each XP-RAFTS

subcatchment is provided in Appendix E.

Table 16 XP-RAFTS Sensitivity to Variations in Initial Losses

Location

(XP-RAFTS ID)


Adopted Continuing

Loss Rates

Pervious=2.5mm/hr

Imperv.=0mm/hr

Lower Continuing

Loss Rates

Pervious=1.5mm/hr

Imperv.=0mm/hr

Higher Continuing

Loss Rates

Pervious=3.5mm/hr

Imperv.=1mm/hr

Old Hume Highway

(1.17)

Gibbergunyah

Creek 76.1 76.8 75.3


Underpass

(28.07)

Gibbergunyah

Creek 12.7 12.8 12.6

Old Hume Highway

(28.13)

Tributary of

Gibbergunyah

Creek

32.3 32.6 31.9

Priestley St

(52.11) Chinamans Creek 32.8 33.1 32.4

Old Hume Highway

(52.12) Chinamans Creek 39.3 39.7 38.9

Brewster St

(63.05)

Tributary of



Village

(63.06)

Tributary of


Bessemer St Railway

Underpass

(67.06)

Iron Mines Creek 10.8 10.8 10.6

Old Hume Highway

(67.09) Iron Mines Creek 23.3 23.5 23.1

Lake Alexandra Outlet Lake Alexandra 17.9 18.3 17.6


49

Location

(XP-RAFTS ID)


Adopted Continuing

Loss Rates

Pervious=2.5mm/hr

Imperv.=0mm/hr

Lower Continuing

Loss Rates

Pervious=1.5mm/hr

Imperv.=0mm/hr

Higher Continuing

Loss Rates

Pervious=3.5mm/hr

Imperv.=1mm/hr

(88.10)

Hume Highway

(1.31)

Gibbergunyah

Creek 216 218 213

The results of the sensitivity analysis show that the XP-RAFTS model is relatively insensitive to

changes in continuing loss rates. Increasing the adopted continuing loss values is predicted to

decrease peak 100 year ARI discharges by an average of 1%.

Decreasing the adopted loss rates is predicted to increase peak discharges by an average of

about 1.5%.

Therefore, it can be concluded that any uncertainties associated with the adopted continuing

loss rates are not predicted to have a significant impact on the results generated by the XP-

RAFTS model.

7.3 Hydraulic Model

7.3.1 Pipe/Culvert Blockage

As discussed in Section 5.2.5, a default blockage factor of 50% was applied to all minor culverts

in the TUFLOW hydraulic model. A minor pipe/culvert was defined as having a diameter (for

circular pipes/culverts) or diagonal dimension (for rectangular culverts) less than 6 metres.

Additional TUFLOW simulations were completed to determine the impact that alternate

blockage scenarios would have on simulated flood behaviour. Specifically, additional

simulations with no blockage as well as complete blockage of all minor pipes and culverts were

completed.

The TUFLOW model was updated to represent no blockage of minor pipes and culverts and was

used to re-simulate the 100 year ARI flood. Peak floodwater depths and velocity vectors were

extracted from the results of the modelling and are presented in Figure I1.1 to I1.12, which is

enclosed in Appendix I. The predicted extent of inundation for "baseline" conditions is

superimposed on Figure I1 for comparison.

The TUFLOW model was also updated to include 100% blockage of all minor pipes and culverts

and was used to re-simulate the 100 year ARI flood. Peak floodwater depths and velocity

vectors were extracted from the results of the modelling and are presented in Figure I2.1 to

I2.12, which is enclosed in Appendix I. The predicted extent of inundation for "baseline"

conditions is superimposed on Figure I2 for comparison.

Tabulated flood level comparisons are also provided at select location across the catchment in

Table 17.


50

Table 17 TUFLOW Sensitivity to Variations in Culvert/Pipe Blockage

Location

ID#

Description of Location

Standard 0% Blockage 100% Blockage

Peak Water

Elevation

(mAHD)

Peak Water

Elevation

(mAHD)

Difference

Peak Water

Elevation

(mAHD)

Difference

1 Thomas Road 615.02 615.01 -0.01 615.01 -0.01

2 Old Hume Highway 602.10 602.42 0.32 602.42 0.32

3 Thomas St 613.11 612.82 -0.29 612.82 -0.29

4 Old Bowral Rd at Railway Underpass 649.30 649.37 0.07 649.37 0.07

5 Thomas St 611.91 612.14 0.23 612.14 0.23

6 Cook St 604.02 604.09 0.07 604.09 0.07

7 Old Hume Highway 602.11 602.41 0.30 602.41 0.30

8 U/S Railway 636.59 636.89 0.30 636.89 0.30

9 Bowral Rd 634.03 634.44 0.41 634.44 0.41

10 Priestley St 613.80 613.91 0.11 613.91 0.11

11 Old Hume Highway 603.49 604.56 1.07 604.56 1.07

12 Railway Parade 640.05 640.08 0.04 640.08 0.04

13 Brewster St 615.93 615.90 -0.03 615.90 -0.03

14 U/S Etheridge St Retirement Village 609.87 609.87 0.00 609.87 0.00

15 Etheridge St 607.76 607.76 -0.01 607.76 -0.01

16 Bessemer St Railway Underpass 627.55 629.10 1.55 629.10 1.55

17 Regent Lane 622.18 622.31 0.13 622.31 0.13

18 U/S RSL Carpark Entry 614.79 614.85 0.06 614.85 0.06

19 D/S RSL Carpark Exit 611.94 612.06 0.12 612.06 0.12

20 Old Hume Highway 609.93 609.86 -0.06 609.86 -0.06

21 D/S Railway Pde, U/S Railway line 630.63 631.28 0.65 631.28 0.65

22 Main St 627.74 627.76 0.02 627.76 0.02

23 Edward St 624.37 624.38 0.01 624.38 0.01

24 Alfred St - Upstream Lake Alexandra 621.72 621.68 -0.03 621.68 -0.03

25 Lake Alexandra Outlet 621.93 621.94 0.00 621.94 0.00


51

Location

ID#


Standard 0% Blockage 100% Blockage

Peak Water

Elevation

(mAHD)

Peak Water

Elevation

(mAHD)

Difference

Peak Water

Elevation

(mAHD)

Difference

26 Lake Alexandra spillway 621.71 621.68 -0.03 621.68 -0.03

27 Bendooley St - Welby 611.89 611.90 0.01 611.90 0.01

28 Hume Highway 535.11 535.15 0.04 535.15 0.04

The results documented in Figure I1, Figure I2 and Table 17 show that complete blockage

typically increases the severity of flooding upstream of blocked pipes and culverts, while

decreasing the severity of flooding downstream of blocked pipes and culverts. The average

increase in peak flood level with 100% blockage is predicted to be 200 mm. However, isolated

increases of over 1 metre are predicated upstream of the Old Hume Highway and Bessemer

Street Railway underpass.

The no blockage scenario typically increases the severity of flooding downstream of the

pipes/culverts and decreases the severity of flooding upstream of pipes/culverts. The average

reduction in peak 100 year ARI flood level is about 150mm, although decreases of over

0.5metres are predicted at isolated locations.

The results of the blockage simulations show some significant changes in flood levels at some

locations relative to the “standard” blockage scenario. Accordingly, it is considered that the

hydraulic model is relatively sensitive to variation in culvert and pipe blockage. This outcome

emphasises the need to ensure key drainage infrastructure and bridges/culverts are well

maintained (i.e., debris is removed on a regular basis).

7.3.2 Manning’s ‘n’

Manning’s’ ‘n’ roughness coefficients are one of the primary hydraulic model inputs and

calibration parameters. They are used to describe the resistance to flow afforded by different

land uses / surfaces across the catchment. However, they can be subject to considerable

variability (e.g., vegetation density in the summer would typically be higher than the winter

leading to higher Manning’s ‘n’ values). Therefore, additional analyses were completed to

quality the impact that any uncertainties associated with Manning’s ‘n’ roughness values may

have on predicted design flood behaviour.

The TUFLOW model was updated to reflect an increase and a decrease in “baseline” Manning’s

‘n’ values of 20%. Peak floodwater depths and velocity vectors were extracted from the results

of the modelling and are presented in Figure I3.1 to I3.12 and Figure I4.1 to I4.12, which are

enclosed in Appendix I. The predicted extent of inundation for "baseline" conditions is also

superimposed on Figures I3 and I4 for comparison.

Tabulated flood level comparisons are provided at select location across the catchment in

Table 18.


52

Table 18 TUFLOW Sensitivity to Variations in Adopted Manning’s ‘n’ Roughness

Location

ID#


Standard Manning’s ‘n’ 20% Lower Manning’s ‘n’ 20% Higher

Peak Water

Elevation

(mAHD)

Peak Water

Elevation

(mAHD)

Difference

Peak Water

Elevation

(mAHD)

Difference

1 Thomas Road 615.02 615.01 -0.02 615.05 0.03

2 Old Hume Highway 602.10 602.47 0.37 602.37 0.27

3 Thomas St 613.11 612.68 -0.42 612.55 -0.55

4 Old Bowral Rd at Railway Underpass 649.30 649.30 0.00 649.31 0.01

5 Thomas St 611.91 611.89 -0.03 611.91 0.00

6 Cook St 604.02 603.99 -0.03 604.04 0.02

7 Old Hume Highway 602.11 602.41 0.30 602.36 0.25

8 U/S Railway 636.59 636.60 0.00 636.50 -0.10

9 Bowral Rd 634.03 634.01 -0.02 634.02 -0.01

10 Priestley St 613.80 613.95 0.15 613.97 0.18

11 Old Hume Highway 603.49 604.15 0.66 604.06 0.57

12 Railway Parade 640.05 640.06 0.01 640.09 0.04

13 Brewster St 615.93 615.91 -0.01 615.94 0.01

14 U/S Etheridge St Retirement Village 609.87 609.85 -0.01 609.69 -0.17

15 Etheridge St 607.76 607.75 -0.01 607.76 0.00

16 Bessemer St Railway Underpass 627.55 627.86 0.31 627.51 -0.04

17 Regent Lane 622.18 622.18 0.00 622.19 0.01

18 U/S RSL Carpark Entry 614.79 614.80 0.01 614.78 -0.01

19 D/S RSL Carpark Exit 611.94 611.97 0.03 611.95 0.01

20 Old Hume Highway 609.93 609.91 -0.01 609.96 0.04

21 D/S Railway Pde, U/S Railway line 630.63 630.65 0.02 630.60 -0.03

22 Main St 627.74 627.73 0.00 627.74 0.00

23 Edward St 624.37 624.37 -0.01 624.38 0.01

24 Alfred St - Upstream Lake Alexandra 621.72 621.72 0.01 621.74 0.02

25 Lake Alexandra Outlet 621.93 621.98 0.05 621.88 -0.06


53

Location

ID#


Standard Manning’s ‘n’ 20% Lower Manning’s ‘n’ 20% Higher

Peak Water

Elevation

(mAHD)

Peak Water

Elevation

(mAHD)

Difference

Peak Water

Elevation

(mAHD)

Difference

26 Lake Alexandra spillway 621.71 621.70 -0.01 621.73 0.02

27 Bendooley St - Welby 611.89 611.87 -0.02 611.91 0.01

28 Hume Highway 535.11 535.02 -0.09 535.36 0.25

The results listed in Table 18 show that increasing or decreasing the Manning’s ‘n’ values by

20% will typically alter peak 100 year ARI flood levels by less than 0.1 metres. More significant

changes in peak flood level are predicted in the vicinity of roadway crossing, where the

combined effect of increased/decreased culvert roughness and impediment to flow afforded by

the roadway embankments amplifies the flood level impacts. Most notably, changes in peak

flood level of over 0.5 metres are predicted at Thomas Street and the Old Hume Highway

crossing of Gibbergunyah Creek.

In general, the model is relatively insensitive to changes in Manning’s ‘n’ values. However,

changes in the vicinity of bridges and culverts can be more substantial. Therefore, care should

be taken in interpreting results in the vicinity of hydraulic structures as the combined effect of

increases in Manning’s ‘n’ and blockage of structures (both of which can be caused by debris

accumulation) has the potential to impact on predicted design flood behaviour.

54

8 PROVISIONAL FLOOD HAZARD AND

HYDRAULIC CATEGORISATION

8.1 Provisional Flood Hazard Categories

Flood hazard effectively defines the impact that flooding will have on development and people

across different sections of the floodplain.

The determination of flood hazard at a particular location requires consideration of a number

of factors, including (NSW Government, 2005):

� depth and velocity of floodwaters;

� size of the flood;

� effective warning time;

� flood awareness;

� rate of rise of floodwaters;

� duration of flooding; and

� potential for evacuation.

Consideration of all of the above items is generally

completed as part of the Floodplain Risk Management

Study. The scope of the Flood Study typically only

requires a provisional estimate of the flood hazard to

be determined. The provisional flood hazard is based

solely on the depth and velocity of floodwaters.

The provisional flood hazard at a particular area of a

floodplain can be established from Figure L2 of the

‘Floodplain Development Manual’ (NSW Government,

2005). This figure is reproduced on the right.

As shown in Figure L2, the ‘Floodplain Development

Manual’ (NSW Government, 2005) divides hazard into

two categories, namely high and low. It also includes a

“transition zone” between the low and high hazard categories. Sections of the floodplain

located in the “transition zone” may be classified as either high or low depending on site

conditions or the nature of any proposed development.

8.1.1 Provisional Flood Hazard

The TUFLOW hydraulic software was used to automatically calculate the variation in provisional

flood hazard across the Gibbergunyah Creek catchment based on the criteria shown in

Figure L2. These hazard categories are shown in Figures 18 to 24.


55

It needs to be reinforced that the hazard represented in this mapping is provisional only. This is

because it is based only on an interpretation of the flood hydraulics and does not reflect the

effects of other factors that influence flood hazard.

Accordingly, modification of the hazard categories presented in Figures 18 to 24 may occur as

part of investigations to be carried out during the subsequent Floodplain Risk Management

Study.

8.2 Hydraulic Categories

The NSW Government’s ‘Floodplain Development Manual’ (NSW Government, 2005) also

characterises flood prone areas according to the hydraulic categories presented in Table 19.

The hydraulic categories provide an indication of the potential for development across different

sections of the floodplain to impact on existing flood behaviour and highlights areas that should

be retained for the conveyance of floodwaters.

Table 19 Qualitative and Quantitative Criteria for Hydraulic Categories

Hydraulic Category Qualitative Description Adopted Criteria*

Floodway those areas where a significant volume of water flows

during floods

often aligned with obvious natural channels and

drainage depressions

they are areas that, even if only partially blocked,

would have a significant impact on upstream water

levels and/or would divert water from existing

flowpaths resulting in the development of new

flowpaths.

they are often, but not necessarily, areas with deeper

flow or areas where higher velocities occur.

If not Flood Fringe or Flood

Storage.

Flood Storage those parts of the floodplain that are important for the

temporary storage of floodwaters during the passage

of a flood

if the capacity of a flood storage area is substantially

reduced by, for example, the construction of levees or

by landfill, flood levels in nearby areas may rise and

the peak discharge downstream may be increased.

substantial reduction of the capacity of a flood storage

area can also cause a significant redistribution of flood

flows.

If not Flood Fringe and:

• D <= 2 m AND

V <= (-0.3D +1)

OR

• D >= 2 m AND

V <= 0.4 m/s

Flood Fringe the remaining area of land affected by flooding, after

floodway and flood storage areas have been defined.

development (e.g., filling) in flood fringe areas would

not have any significant effect on the pattern of flood

flows and/or flood levels.

• V <= 2 m/s AND

D <= 0.2

NOTES: V = Velocity, D = Depth

Hydraulic categories were only applied to areas subject to inundation (i.e., D > 0m)

*The adopted criteria were developed specifically for the Gibbergunyah Creek Catchment only and may not

be appropriate for any other areas.


56

8.2.1 Adopted Hydraulic Categories

Unlike provisional hazard categories, the ‘Floodplain Development Manual’ (NSW Government,

2005) does not provide explicit quantitative criteria for defining hydraulic categories. This is

because the extent of floodway, flood storage and flood fringe areas are typically specific to a

particular catchment.

However, the ‘Floodplain Development Manual’ (NSW Government, 2005) does provide

qualitative guidelines to assist in the delineation of hydraulic categories. The 'Floodway

Definition' guideline (Department of Environment and Climate Change, 2007) also provides

additional guidance for the definition of floodway extents. These qualitative guidelines are

summarised in Table 19.

The results of the design flood simulations were interrogated to assess the potential extent of

floodway, flood storage and flood fringe areas based on the qualitative guidelines listed in

Table 19. Preliminary hydraulic category boundaries were delineated by hand across different

areas of the Gibbergunyah Creek catchment. The extent of each preliminary hydraulic category

boundary was superimposed on peak depth, flow velocity and velocity-depth product values to

determine if the hydraulic categories could be defined numerically. The results of this

assessment determined that the depth, velocity and velocity-depth product values listed in the

third column of Table 19 could be used to automate the delineation of hydraulic categories for

the Gibbergunyah Creek catchment. A graphical representation of the numerical criteria listed

in Table 19 is also provided in Plate 17.

Plate 17 Adopted hydraulic category criteria

The resulting hydraulic category maps for the 5, 10, 20, 50, 100, and 200 Year ARI events as well

as the Probable Maximum Flood (PMF) are shown in Figures 25 to 31.

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

2.2

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2 2.4 2.6 2.8 3

Ve

loc

ity (

m/s

)

Depth of Flood at Site (m)

Floodway

Flo

od

Fri

ng

e

Flood Storage


57

In order to verify the suitability of the delineated floodways, additional checks were performed

in accordance with recommendations outlined in the DECC 'Floodway Definition' guideline. This

involved blocking sections of the delineated floodways and quantifying the impact that this

blockage had on peak flood levels as well as the distribution of floodwaters in the vicinity of the

blockage. The outcomes of this assessment are presented in Plates 18 and 19.

Plate 18 Predicted Peak 100 year ARI Flood Levels, Depth and Velocities with Partial Blockage of Floodways

(blockage locations highlighted by yellow circles)

Plate 19 Predicted Change in Peak 100 year ARI Flood Levels with Partial Blockage of Floodways (blockage

locations highlighted by yellow circles)


58

The flood level difference mapping presented in Plate 19 shows that partial blockage of the

delineated floodway extents would increase peak flood level by over 0.3 metres in most

instances. This is considered to be a “significant impact” on upstream water levels.

Plate18 also shows that the blockages would cause a significant redistribution of floodwaters.

That is, a significant proportion of floodwaters would be forced into areas that weren’t

previously conveying a significant amount of the total flow.

The results depicted in Plates 18 and 19 are considered to be consistent with the qualitative

floodway descriptions outlined in the 'Floodway Definition' guideline and indicate that the

delineated floodway extents are reasonable.

8.3 Flood Risk Precincts

Wingecarribee Shire Council’s Develop Control Plan (DCP) No. 34, titled ‘Managing our Flood

Risks’, outlines Council’s requirements for development on all floodplains within the Local

Government Area. This includes the floodplain of Gibbergunyah Creek.

Section 2.3 of the DCP, introduces the concept of “Flood Risk Precincts”, which subdivides the

floodplain accordingly to the potential flood hazard/risk. This flood risk precinct classification,

in turn, determines which development controls are applicable for a particular parcel of land.

The four flood risk precincts that are documented in the DCP are summarised in Table 20.

Table 20 Flood Risk Precinct Definitions

Flood Risk

Precinct

Description

High This Precinct contains that land below the 100 year flood that is either subject to a

high hydraulic hazard or where there are significant evacuation difficulties. The high

flood risk precinct is where high flood damages, potential risk to life, and evacuation

problems would be anticipated or development would significantly and adversely

affect flood behaviour. Most development should be restricted in this precinct. In this

precinct, there would be a significant risk of flood damages without compliance with

flood related building and planning controls.

Medium This Precinct contains that land below the 100 year flood that is not subject to a high

hydraulic hazard and where there are no significant evacuation difficulties. In this

precinct there would still be a significant risk of flood damage, but these damages can

be minimised by the application of appropriate development controls.

Fringe-Low This Precinct contains that land between the extents of the 100 year flood and the

100 year flood plus 0.5m in elevation (being a freeboard). In this precinct there would

still be a significant risk of flood damage, but these damages can be minimised by the

application of appropriate development controls.

Low This Precinct contains that land within the floodplain (i.e. within the extent of the

probable maximum flood) but not identified within any of the above Flood Risk

Precincts. The Low Flood Risk Precinct is where risk of damages is low for most land

uses and most land uses would be unrestricted within this precinct.

To aid Council in defining the spatial variation in flood risk precincts across the Gibbergunyah

Creek catchment, a Flood Risk Precinct map was prepared based on the outcomes of the design

flood simulations and provisional hazard mapping and is shown in Figure 32.

59

9 CLIMATE CHANGE ASSESSMENT

The Office of Environment and Heritage’s (formerly Department of Environment, Climate

Change and Water) 'Practical Consideration of Climate Change' states that climate change is

expected to have adverse impacts on sea levels and rainfall intensities into the future.

Although any future rises in see level are unlikely to have an impact on flood behaviour across

the Gibbergunyah Creek catchment, increases in rainfall intensities have the potential to

increase runoff volumes and peak discharges, thereby increasing the severity of flooding.

This flood study will form the basis for defining flood behaviour for a number of years into the

future. It will also form the basis for the future Floodplain Risk Management Study, where a

range of flood risk mitigation measures will be evaluated. Therefore, it is important that

potential climate change impacts are quantified so that development decisions and the

robustness of flood risk mitigation measures can be assessed in an informed manner.

The following sections describe the process that was employed to quantify climate change

impacts on flooding across the Gibbergunyah Creek catchment.

9.1 Hydrology

9.1.1 General

The 'Practical Consideration of Climate Change' (Department of Environment and Climate

Change, 2007) guideline states that rainfall intensities are predicted to increase in the future.

The NSW Government's 'Climate Change in the Sydney Metropolitan Catchments' (CSIRO, 2007)

elaborates on this further and suggests that annual rainfall is likely to decrease, however,

extreme rainfall events are likely to more intense. It is anticipated that extreme rainfall

intensities could increase by between 2% and 24% by 2070 (Department of Environment and

Climate Change, 2007).

Due to the wide potential variability of future rainfall intensities, the 'Practical Consideration of

Climate Change' (Department of Environment and Climate Change, 2007) provides guidelines

for quantifying the potential impacts of these changes. The guideline states that additional

simulations should be completed with 10%, 20% and 30% increases in rainfall intensities to

quantify the potential impacts associated with climate change.

9.1.2 Results

The XP-RAFTS model was used to perform additional simulations incorporating increases in

rainfall intensity of 10%, 20% and 30% in accordance with the OEH guidelines. The results of

the climate change assessment are summarised in Table 21 at selected locations across the

Gibbergunyah Creek catchment. Complete listings of peak discharges for each subcatchment

are provided in Appendix J.


60

Table 21 Predicted Peak Design Discharges with Increases in Rainfall Intensity Associated with Climate

Change

Location

(XP-RAFTS ID)


Adopted Rainfall

Intensities

10% Increase in

Rainfall Intensities

20% Increase in


30% Increase in


Old Hume Highway

(1.17)

76.1 87.5 98.9 110.2


Underpass

(28.07)

12.7 14.3 15.9 17.6

Old Hume Highway

(28.13)

15.2 17.2 19.2 21.1

Priestley St

(52.11)

32.8 37.4 41.6 46.1

Old Hume Highway

(52.12)

39.3 44.7 49.6 55.3

Brewster St

(63.05)

8.58 9.82 11.00 12.21


Village

(63.06)

10.6 12.2 13.8 15.3

Bessemer St Railway Underpass

(67.06)

10.8 12.2 13.6 15.1

Old Hume Highway

(67.09)

23.3 26.6 29.8 33.1


(88.10)

17.9 20.4 22.9 25.9

Hume Highway

(1.31)

216 251 280 315

The results provided in Table 21 and Appendix J show that a 10% increase in peak 100 year ARI

design rainfall intensities will increase peak 100 year ARI discharges about 14%, on average.

The results also show that a 20% and 30% increase in design rainfall intensities will increase

peak 100 year ARI discharges by about 27% and 40% respectively. Accordingly, increases in

rainfall intensity of this magnitude have the potential to cause significant increases in flood

flows across the entire catchment.

9.2 Hydraulics

9.2.1 Results

In order to verify the impact that increases in rainfall intensity of 10%, 20% and 30% may have

on design floodwater levels, depths and velocities, the flows were extracted from the results of

the updated XP-RAFTS modelling and were routed through the TUFLOW hydraulic model.


61

Peak floodwater depths and velocity vectors were extracted from the results of the modelling

and are presented in Figures K1 to K3, which is enclosed in Appendix K. The predicted extent

of inundation for "baseline" conditions is superimposed on Figures K1 to K3 for comparison.

Tabulated flood level comparisons are also provided at select location across the catchment in

Table 22.

Table 22 TUFLOW Sensitivity to Variations in Culvert/Pipe Blockage

Location

ID#


Peak 100 Year ARI Flood Level (mAHD)

Adopted

Rainfall

Intensities

10% Increase in

Rainfall

Intensities

20% Increase in

Rainfall

Intensities

30% Increase in

Rainfall

Intensities

1 Thomas Road 615.02 615.08 615.12 615.16

2 Old Hume Highway 602.10 602.27 602.32 602.34

3 Thomas St 613.11 613.10 613.11 613.10

4 Old Bowral Rd at Railway

Underpass 649.30 649.38 649.43 649.46

5 Thomas St 611.91 611.96 612.00 612.03

6 Cook St 604.02 604.05 604.06 604.08

7 Old Hume Highway 602.11 602.21 602.28 602.34

8 U/S Railway 636.59 636.99 637.48 637.93

9 Bowral Rd 634.03 634.10 634.17 634.21

10 Priestley St 613.80 613.84 613.86 613.88

11 Old Hume Highway 603.49 603.64 603.76 603.80

12 Railway Parade 640.05 640.07 640.08 640.09

13 Brewster St 615.93 615.94 615.96 615.97

14 U/S Etheridge St Retirement Village 609.87 609.87 609.87 609.88

15 Etheridge St 607.76 607.77 607.77 607.79

16 Bessemer St Railway Underpass 627.55 627.90 628.15 628.38

17 Regent Lane 622.18 622.20 622.21 622.22

18 U/S RSL Carpark Entry 614.79 614.83 614.86 614.90

19 D/S RSL Carpark Exit 611.94 611.99 612.03 612.07

20 Old Hume Highway 609.93 609.98 610.03 610.06

21 D/S Railway Pde, U/S Railway line 630.63 630.73 630.83 630.91


62

Location

ID#


Peak 100 Year ARI Flood Level (mAHD)

Adopted

Rainfall

Intensities

10% Increase in

Rainfall

Intensities

20% Increase in

Rainfall

Intensities

30% Increase in

Rainfall

Intensities

22 Main St 627.74 627.77 627.80 627.83

23 Edward St 624.37 624.39 624.41 624.43

24 Alfred St - Upstream Lake

Alexandra 621.72 621.73 621.76 621.79

25 Lake Alexandra Outlet 621.93 622.04 622.06 622.16

26 Lake Alexandra spillway 621.71 621.75 621.76 621.80

27 Bendooley St - Welby 611.89 611.91 611.93 611.95

28 Hume Highway 535.11 535.29 535.43 535.55

The results provided in Table 22 and Appendix K show that a 10% increase in peak 100 year ARI

design rainfall intensities will increase peak 100 year flood levels by less than 100mm, on

average. The results also show that a 20% and 30% increase in design rainfall intensities will

increase peak 100 year ARI flood levels by about 150 mm and 200 mm respectively.

Accordingly, as floodwater depths across the majority of the floodplain areas are quite shallow,

increases in flood level of this magnitude have the potential to increase the severity of flooding

considerably.

63

10 DISCUSSION

10.1 Overview

The results of the flood modelling were interpolated to identify areas of the Gibbergunyah

Creek catchment that were particularly susceptible to inundation from floodwaters. The results

were also reviewed with respect to key infrastructure and emergency response facilities to

determine if they may be impacted during a flood. A discussion on the outcomes of this review

is presented below.

10.2 General Description of Flood Behaviour

The results of the hydraulic modelling show that:

The Gibbergunyah Creek channel carries the majority of flow during the 100 year ARI flood

and limited inundation of properties occurs as a result of overtopping of the creek banks.

This is consistent with the outcomes of community consultation (refer Section 3.6.2).

Nevertheless, inundation of some properties does occur as a result of overland flows

making their way towards the creek.

Chinaman’s Creek overtops in multiple locations, which results in inundation of a number of

adjoining properties. Most notably, properties in the vicinity of Old Bowral Road and

Priestly Street are subject to inundation.

Floodwaters along Iron Mines Creek are generally contained to the creek channel during

the 100 year ARI flood. However, floodwaters are predicted to overtop the Old Hume

Highway and travel in a westerly direction along the highway toward Chinamans Creek.

Similarly to Gibbergunyah Creek, overland flow is predicted to inundate a number of

properties, however, the depths of inundation are typically shallow (i.e., < 0.1 metres)

Runoff across the Lake Alexandra subcatchment is conveyed via subsurface stormwater

pipes. Accordingly, during large events, water in excess of the capacity of the pipe system

is conveyed overland inundating a number of residential and commercial properties.

10.3 Flood Liable Areas

The results of the modelling show that floodwaters are typically confined to defined

watercourses, drainage lines and roadways. Nevertheless, there are some areas that

experience higher depths of inundation. These areas include:

Commercial/Industrial areas in the vicinity of Cavendish Avenue, most notably at the

intersection with Priestly Street. This same overland flow path is also predicted to cause

inundation of commercial premises on the corner of the Old Hume Highway and Frankland

Street (refer Figure 15.5 and 15.9). A high provisional flood hazard is predicted across

these areas indicating that the floodwaters would pose a significant hazard during the 100

year ARI flood (refer Figures 22.5 and 22.9).

Bessemer Street, between the railway line and the Mittagong RSL Club carries a significant

proportion of flow, especially in the vicinity of the railway underpass. This roadway is


64

predicted to function as a high hazard floodway at the peak of the 100 year ARI flood. (see

Figure 22.4 and 22.8)

A number of properties across West Mittagong are predicted to be subject to inundation.

This includes properties fronting John Street, William Street, Thomas Street, Elizabeth

Street, Cook Street and Hood Street. Flooding across this area appears to be primarily

associated with an inadequate drainage system. In general, this area is predicted to be

exposed to a low provisional flood hazard.

Properties adjoining Main Street, Albert Street, Edward Street/Lane and Alfred Street, are

also predicted to be subject to inundation at the peak of the 100 year ARI flood. This

appears to be primarily associated with a drainage system that is only designed to convey

flows during smaller, more frequent storms.

A significant overland flow path extending between Currockbilly Street and Bendooley

Street at Welby is also predicted to inundate a number of properties (see Figure 15.10).

Additionally, there are numerous ‘pockets’ across Welby that are subject to a high

provisional flood hazard, including a significant portion of Mittagong Street (see Figure

22.10).

10.4 Emergency Response Infrastructure

There is significant infrastructure located within the Gibbergunyah catchment that can play a

key role in emergency response management during floods. As such, it was considered

important to assess the relative safety and accessibility of these buildings during flood events.

Such key infrastructure includes:

Wingecarribee Shire SES/RFS Headquarters (Corner of Priestly St and Etheridge St,

Mittagong). This building is located adjacent to Chinamans Creek, and experiences

inundation across a significant proportion of the site in all design events with depths

ranging from 0.04m in the 5 year ARI event, 0.15m during the 100 year ARI event up to

0.5m during the PMF. Higher depths are experienced adjacent to the buildings; however,

the lack of detailed floor level survey means the flood immunity of the buildings cannot be

defined.

SES station (Regent Lane, Mittagong) is located close to Iron Mines Creek and the Bessemer

St Railway Underpass. However, the main building is located on high ground that does not

experience inundation in any events, including the PMF. Nevertheless, it should be noted

that access to and from the station requires passing across a major overland flow path that

extends across Regent Lane. However, peak depths of inundation during the design 100

year ARI flood are less than 0.2 metres meaning vehicular and pedestrian access should be

achievable. It should also be noted that Bessemer Street is designated as a high hazard

flood hazard and would not be passable during the 100 year ARI flood.

Mittagong Police Station (Corner Regent St and Station St, Mittagong). During all design

events, Station Street becomes an overland flow path with depths ranging from 0.06m

during the 5 year ARI, 0.15m in the 100 year ARI, and 0.30m in the PMF. These depths do

not cause inundation of any buildings along Station Street and are unlikely to cause a

significant impediment to pedestrian and vehicular access during most floods.

Living Care Henley Brae Retirement Village (Etheridge St, Mittagong). This retirement

Village is of special concern due to the probability of a large number of elderly and less

mobile residents being present. This village is also situated in very close proximity to a

tributary of Chinamans Creek (the tributary actually passes under two of the village villas).


65

The majority of the village is predicted to be exposed to a low provisional flood hazard,

however, some areas in close proximity to the watercourse are designated as high hazard

areas. Velocities through the village are generally low (~0.5m/s), however, given the

potential vulnerability of the residents in the area, and the uncertainty of floor levels,

further detailed investigations are recommended and appropriate considerations are made

in emergency planning.

Sewage Pumping Station (Lee St, Mittagong). This pumping station is predicted to be

inundated during every simulated design flood. As such, if any wastewater was located

within pumping station at the time of the flood, there is potential for it to be conveyed into

Iron Mines Creek and then into Gibbergunyah Creek. There is no significant development

downstream of this location, so human exposure is unlikely. However, contamination of

Iron Mines Creek and Gibbergunyah Creek may occur.

10.5 Roadways

There are several major roadways within the Gibbergunyah Creek catchment which may be

required for evacuation or access for emergency vehicles during floods. An investigation was

completed to quantify the impact of flooding on major roadways across the Gibbergunyah

Creek catchment. The outcomes of this investigation indicate that:

Old Hume Highway/ Main St: These roadways link Mittagong to Welby and provide access

to the Hume Highway. They also provide access for residents between the Eastern and

Western portion of Mittagong. During the 100 year ARI flood, there are four major

positions where water crosses these roadways and could potentially cause closure of the

roadway. These locations are :

o Just east of the main Gibbergunyah Creek culvert crossing, where depths generally do

not exceed 0.3m. However, a small section of the roadway is designated as a high

provisional flood hazard area at the peak of the 100 year ARI flood;

o Between Owen St and Franklin St. Depths at this location again generally do not exceed

0.3m;

o Between Iron Mines and Chinamans Creek crossings, where depths can reach in excess

0.7m. As a result, this section of roadway can be subject to a high provisional flood; and

o Between Station St and Fitzroy St, where depths can reach 0.8m in places.

At all four locations, the roadways can be inundated for between 1 and 2 hours.

Bowral Road: This roadway links Mittagong with Bowral/Moss Vale. During the 100 year

ARI flood, the majority of Bowral Road between Tulloona Avenue and the railway overpass

experiences shallow depths. However, more significant depths form at three locations:

o the intersection of Old Bowral Rd and Cavendish St where depths reach up to 0.3m;

o the intersection with Brewster St where depth reach 0.3m; and

o between Iron Mines Creek and Bessemer St, where depths peak at 0.3m.

These depths are generally maintained for less than half an hour. All three locations are

predicted to be exposed to a high provisional flood hazard, and therefore, should not be

traversed for around 30 minutes.


66

10.6 Railway

The Main Southern Railway also passes through Mittagong. The railway is typically elevated

above the floodplain, however, it is subject to inundation at some locations during the 100 year

ARI flood. There are two major locations where the railway is overtopped:

1) Mittagong Station, where a number of overland flow paths converge and cause floodwater

depths to reach up to 0.4m across the station; and,

2) Between the Mount Gibraltar Railway cutting and Old Bowral Road, where a major

overland flow path occurs adjacent to the railway tracks and reaches a depth of 0.6m.

The inundation at these locations is predicted to be maintained for between 1 and 2 hours.

67

11 CONCLUSION

This report documents the outcomes of investigations completed to quantify main stream and

overland flooding across the Gibbergunyah Creek catchment for a full range of design floods up

to and including the PMF. It provides information on design flood discharges, levels, depths,

velocities as well as hydraulic categories and provisional estimates of flood hazard.

Flood behaviour across the study area was defined using a hydrologic computer model of the

Gibbergunyah Creek catchment as well as a two-dimensional hydraulic model incorporating all

major watercourses, stormwater pipes and overland flow paths. The hydrologic computer

model was developed using the XP-RAFTS software and the hydraulic model was developed

using the TUFLOW software.

The computer models were calibrated/verified using rainfall data and photographs for floods

that occurred in 2005 and 2010. The models were subsequently used to simulate a range of

design floods including the 5, 20, 50, 100, 500 and 1000 year ARI floods as well as the PMF. The

following conclusions can be drawn from the results of the investigation:

Flooding across the Gibbergunyah Creek catchment can occur as a result of major

watercourses overtopping their banks as well as overland flooding when the capacity of the

stormwater system is exceeded.

Flooding can occur as a result of a variety of different storm durations. However, a storm

duration of less than 3 hours typically produces the worst case flooding conditions across

most of the study area. That is, relatively short, high intensity rainfall events typically

produce the worst case flooding.

Large areas of the Gibbergunyah Creek catchment are predicted to be inundated during

each of the design floods. The majority of flow during most design floods are predicted to

be contained in designated drainage areas (e.g., waterways, swales). A number of

roadways and properties are also predicted to be inundated, however, the depths of

inundation are typically quite shallow. As a result, most areas are subject to a low

provisional flood hazard.

The catchment incorporates a range of drainage infrastructure to convey storm/flood

waters (e.g., culverts, stormwater pipes). The results of a blockage sensitivity analysis

shows that the severity of flooding upstream of these structures can be increased due to

blockage. Conversely, no blockage typically increases the severity of flooding downstream

of the structures.

A number of properties across the catchment are predicted to be inundated during a range

of design floods. Most notably, a large section of West Mittagong, sections of Welby and

the commercial sections of Easter Mittagong are predicted to be exposed to significant

depths of inundation during the design 100 year ARI flood.

A number of roadway as well as the Main Southern Railway are predicted to be overtopped

at several locations during the 100 year ARI flood. This would typically render the

roadways/railway impassable for up to 2 hours.


68

The Wingecarribee Shire SES/RFS Headquarters located on the corner of Priestly St and

Etheridge St at Mittagong is predicted to be inundated during floods as frequent as the 5

year ARI event. Detailed floor level information should be collected to determine the

susceptibility of the building to over floor flooding. Regardless, it is likely that vehicular

access to the headquarters would be difficult during severe flooding with the catchment.

69

12 REFERENCES

AAM Pty Ltd. (2010). Wingecarribee Flood Study LiDAR Survey. Volume 16563A01NOB:

Prepared for Wingecarribee Shire Council.

Bewsher Consulting. (2005). Bowral Floodplain Risk Management Study and Plan. Prepared

for Wingecarribee Shire Council.

Bewsher Consulting. (2009). Lot 2 Besemer Street, Mittagong - Flood Study (Final Report).

Prepared for Wingecarribee Shire Council.

BMT WBM. (2012). TUFLOW User Manual: GIS Based 1D/2D Hydrodynamic Modelling.

Build 2012-05-AA.

Bradley, J. N. (1978). Hydraulics of Bridge Waterways: HDS 1.

Bureau of Meteorology. (2003). The Estimation of Probable Maximum Precipitation in

Australia: Generalised Short Duration Method.

Catchment Simulation Solutions. (2011). CatchmentSIM User Manual. Version 2.5.

CSIRO. (2007). Climate Change in the Sydney Metropolitan Catchments. Prepared for the

NSW Government.

Department of Environment and Climate Change. (2007). Floodplain Risk Management

Guideline: Floodway Defintion. Version 1.01.

Department of Environment and Climate Change. (2007). Floodplain Risk Management

Guideline: Practical Consideration of Climate Change. Version No. 1.0.

Engineers Australia. (1987). Australian Rainfall and Runoff - A Guide to Flood Estimation.

Edited by D. Pilgrim.

Engineers Australia. (2011). Australian Rainfall and Runoff - Revision Project 7: Baseflow for

Catchment Simulation. ISBN: 978-0-85825-916-4.

NSW Government. (2005). Floodplain Development Manual: the Management of Flood

Liable Land.

Reid, M., Cheng, X., Banks, E., Jankowski, J., Jolly, I., Kumar, P., et al. (2009). Catalogue of

Conceptual Models for Groundwater-Stream Interaction in Eastern Australia. eWater

Technical Report.

RHM Consulting Engineers. (2006). Hydraulic Assessment of Chinamans Creek, Mittagong.

Prepared for BB Retail Capital.

Rigby, E. H., Boyd, M. J., Roso, S., Silveri, P., & David, A. (2002). Causes and Effects of

Culvert Blockage during Large Storms. Eric W. Strecker, Wayne C. Huber, Editors Portland,

Oregon, USA, September 8-13, 2002: 9th International Conference on Urban Drainage.

Wingecarribee Shire Council. (Adopted 28 April 2010, Effective 17 May 2010). Managing

Our Flood Risks - Development Control Plan (DCP) No. 34 (Environemtnal Planning and

Assessment Act, 1979).


70

XP Software. (2009). XP-RAFTS: Urban & Rural Runoff Routing Application. User Manual.

APPENDIX A

COMMUNITY CONSULTATION

Council calls for local data as part of flood studies

Wingecarribee Shire Council is calling on residents living within the Gibbergunyah and Chinaman’s Creek areas to provide information on local flood sites as part of a major flood study designed to help better prepare for future flood events. Flooding causes over $100 million worth of damage each year across the nation and historically causes more damage annually than any other natural disasters in Australia. Council is subsequently preparing three major flood studies across some of the Shire’s key catchment areas in an effort to better plan, predict and manage the risk of flooding across our Shire. The studies will form part of Council’s Floodplain Risk Management Program which aims to reduce the impact of flooding on the community. The Gibbergunyah Creek Flood Study will focus attention on the Gibbergunyah and Chinaman’s Creek areas in Mittagong, which covers an area approximately 10.5 km² in size. As part of the study, computer models will be used to simulate flood behaviour across the catchment. For the study to be as accurate as possible, Council is asking residents to submit any historical information they may have collected. This could include written data or historical photographs. Alternatively, a questionnaire can be completed online at the dedicated website, www.gibbergunyah.floodstudy.com.au. Responses close on the 31st of July 2012 but earlier responses will be appreciated. Two further studies targeting the Wingecarribee River from the Wingecarribee Dam to Berrima and the Burradoo Catchment will be undertaken concurrently with the Gibbergunyah study. For further information about the Gibbergunyah Creek Flood Study or to fill out an online questionnaire visit: www.gibbergunyah.floodstudy.com.au, contact Council’s Floodplain and Stormwater Engineer on phone 4868 0798 or via email at: [email protected], or Council’s consultant - Catchment Simulation

Solutions - David Tetely on (02) 9223 0882 or via email at: [email protected]. The Gibbergunyah Creek Flood Study is jointly funded by Wingecarribee Shire Council and NSW Government’s Office of Environment and Heritage.

END

Your contribution to thisstudy is greatly appreciated!

Fu

rther In

form

ation

:

To obtain further information on the G

ibbergunyah Creek Flood Study or to subm

it any information

that you think may be valuable to the study, please

contact:

David Tetley Catchm

ent Simulation Solutions

Suite 302, 5 Hunter Street Sydney N

SW 2000

(02) 9223 0882

(02) 8415 7118

[email protected]

.au

Sha Prodhan W

ingecaribee Shire Council PO

Box 141 M

oss Vale NSW

2577 (02) 4868 0798

(02) 4869 1203

sha.prodhan@w

sc.nsw.gov.au

Alternatively, you can visit the flood study website:

ww

w.gibbergunyah.floodstudy.com

.au

Brochure prepared by:

Ho

w yo

u can

help

!The flood study w

ill include the development

of computer m

odels to simulate flood

behaviour across the catchment. To ensure

the computer m

odels are providing reliable descriptions of flood behaviour they w

ill be calibrated so they reproduce floods that have occurred in the past.

Enclosed with this brochure is a questionnaire

that aims to collect as m

uch historic flood inform

ation as possible to assist with the m

odel calibration. Anybody w

ith information and/

or historic flood photographs is encouraged to com

plete the questionnaire and return it by 31st July 2012. Alternatively, the questionnaire can be com

pleted online via the Gibbergunyah Creek Flood Study w

ebsite:

ww

w.gibbergunyah.floodstudy.com

.au

Why d

o w

e need

to

prep

are a floo

d stu

dy?

Flooding in Australia is estimated to cause over

$100 million w

orth of damage each year. O

ver 2,300 people have also lost their lives due to floods in Australia over the past 200 years. Accordingly, flooding can im

pose significant financial burdens and place lives at risk.

The preparation of a flood study will help Council

to understand the existing flooding problem w

ithin the G

ibbergunyah Creek catchment. It w

ill also help Council to identify w

ere flood damage reduction

measures m

ay be best implem

ented to reduce the cost of flooding to the com

munity, assist w

ith em

ergency managem

ent / evacuation processes and guide future developm

ent in a way that is

compatible w

ith the flood hazard.

Gibbergunyah

C

reekFlood S

tudyC

om

mu

nity

Info

rmatio

n

Bro

chu

re

Intro

du

ction

The Gibbergunyah Creek catchm

ent covers an area of approxim

ately 10.5 km2 w

ithin the W

ingecarribee Shire Council Local Governm

ent Area. The extent of the G

ibbergunyah Creek catchm

ent is shown below

. It incorporates G

ibbergunyah Creek, Chinamans Creek, Iron M

ines Creek as w

ell as a number of m

inor tributaries, w

hich flow past m

any residential, comm

ercial and industrial properties in M

ittagong. Accordingly, there is potential for significant dam

age, inconvenience and risk to life during large floods w

ithin the catchment.

In recognition of these issues, Wingecarribee Shire

Council has decided to prepare a flood study for the G

ibbergunyah Creek catchment. The flood

study is the first step in assisting Council to better understand, plan and m

anage the risk of flooding across the catchm

ent.

The flood study is being completed as part of

Council’s Floodplain Risk Managem

ent Program,

which aim

s to reduce the impact of flooding on the

comm

unity.

Backg

rou

nd

The NSW

State Governm

ent’s Flood Prone Land Policy is directed tow

ards providing solutions to existing flood problem

s in developed areas and ensuring new

development is com

patible with the

flood hazard and does not create additional flooding problem

s in other areas.

Under the Policy, the m

anagement of flood liable

land is the responsibility of Local Governm

ent with

financial and technical support provided by the State G

overnment. The Policy specifies a staged

approach to the floodplain managem

ent process:

Council has initiated this process by comm

issioning the G

ibbergunyah Creek Flood Study. The main

purpose of the flood study is to determine the

nature and extent of the existing flood problem.

Once the Flood Study is com

pleted, a Floodplain Risk M

anagement Study and Plan can be prepared.

This will quantify the benefits of im

plementing a

range of measures aim

ed at reducing the damage

and inconvenience caused by flooding.

So

wh

at’s a floo

d stu

dy?

A considerable amount of w

ork is involved in the preparation of a flood study. This w

ork typically includes:

Collection and review of all available flood-

related information for the catchm

ent

Consultation with the com

munity to obtain

additional information on flooding

Development of com

puter models to sim

ulate the transform

ation of rainfall into runoff and to sim

ulate how the runoff is distributed across

the catchment

Calibration of the computer m

odels to reproduce historic floods

Use of the com

puter models to sim

ulate a range of hypothetical floods ranging from

relatively frequent storm

s right up to the largest flood that could conceivably occur

Interpretation of the computer m

odel outputs to identify the variation in flood hazard across the catchm

ent and to identify areas that should be preserved in the future for the conveyance of flood flow

s

Preparation of a flood study report and maps

summ

arising the outcomes of all stages of the

investigation

The flood study will ultim

ately provide Council w

ith information on flood flow

s, extents, levels, depths and velocities throughout the catchm

ent.Council has com

missioned specialist flood

consultants, Catchment Sim

ulation Solutions, to prepare the flood study.

Flo

od

Stu

dy

Flo

od

plain

Risk

Man

agem

ent S

tud

y

Flo

od

plain

Risk

Man

agem

ent P

lan

Imp

lemen

tation

o

f Plan

Gib

berg

unyah

Creek F

loo

d S

tud

yC

om

mu

nity Q

uestio

nn

aireW

ingecarribee Shire Council is completing a flood study for the G

ibbergunyah Creek catchment. The flood study is the

first step in assisting Council to better understand, plan and manage the risk of flooding across the catchm

ent.

The information that you provide in the follow

ing questionnaire will prove invaluable in the calibration of com

puter m

odels that are being developed as part of the Gibbergunyah Creek Flood Study. It w

ill also provide Council with

an understanding of existing flooding problems and areas w

here flood damage reduction m

easures should be investigated in the future.

The following questionnaire should only take around 10 m

inutes to complete. Try to answ

er as many questions as

possible and give as much detail as possible (attach additional pages if necessary). O

nce complete, please return the

questionnaires via email or m

ail (no postage stamp required) by 31st July 2012. Alternatively, if you have internet

access, an online version of the questionnaire can be completed at: w

ww

.gibbergunyah.floodstudy.com.au

If you have any questions or require any further information please contact:

David Tetley Sha Prodhan Catchm

ent Simulation Solutions W

ingecaribee Shire Council Suite 302, 5 Hunter Street PO

Box 141 Sydney N

SW 2000 M

oss Vale NSW

2577 (02) 9223 0882 (02) 4868 0798

(02) 8415 7118 (02) 4869 1203

dtetley@

csse.com.au

sha.prodhan@w

sc.nsw.gov.au

QU

ES

TIO

N 1

Please p

rovid

e the fo

llow

ing

con

tact details in

case we n

eed to

con

tact you

for ad

ditio

nal

info

rmatio

n (if yo

u h

ave mo

ved fro

m th

e ‘floo

d zo

ne’, p

lease pro

vide yo

ur p

reviou

s add

ress)N

ote that the information that you provide w

ill become the property of W

ingecarribee Shire Council. However, your personal inform

ation will

remain confidential at all tim

es and will only be used to identify specific locations of flooding problem

s.

Nam

e:_________________________________________________ Phone Num

ber:_________________________________

Address:_______________________________________________ em

ail: ________________________________________

_______________________________________________

_______________________________________________

Are you happy to be contacted in the future to obtain additional inform

ation? Yes No

QU

ES

TIO

N 2

Ho

w lo

ng

have yo

u lived

and

/or w

orked

in th

e area?

At current address: __________ Years ____________ M

onths

In the general area: __________ Years ____________ Months

Fo

ld H

ere

Gib

berg

un

yah C

reek Flo

od

Stu

dy

Fo

ld H

ere

Thank you for taking the time to com

plete this questionnaire! The questionnaire can be returned w

ithout a postage stamp or scanned and em

ailled to [email protected]

.au by 31st July 2012. Flood photos and videos can also be sent to this em

ail address. “Hard copies” of photos or VHS tapes can be posted to:

Catchment Sim

ulation SolutionsSuite 302, 5 Hunter StreetSydney, N

SW 2000

Catchment Sim

ulation Solutions will analyse the com

munity responses and report back to Council. All m

aterials gathered w

ill become the property of Council. How

ever, if you would like to have item

s returned please note this and the item

s will be returned at the conclusion of the study.

How

to send back this questionnaire... Please fold this questionnaire using the ‘Fold Here’ lines as a guide to form

a business sized evelope with the address

on the front and this text box on the back. Seal the folded pages with a piece of tape to help m

aintain privacy (but not so m

uch tape that we can’t open it) and then post it back.

QU

ES

TIO

N 4 - C

ON

TIN

UE

Dc) D

id yo

u keep

any rain

fall record

s du

ring

any p

ast storm

events? If so

can yo

u p

lease inclu

de a co

py o

f th

e record

s or p

rovid

e a descrip

tion

of th

e record

s belo

w?

Yes No

______________________________________________________________________________________________________

______________________________________________________________________________________________________

______________________________________________________________________________________________________

______________________________________________________________________________________________________

QU

ES

TIO

N 5

Are yo

u co

ncern

ed th

at you

r pro

perty co

uld

be flo

od

ed in

the fu

ture?

Yes No

If ‘Yes’, wh

at makes yo

u co

ncern

ed?

_____________________________________________________________________________________________________

_____________________________________________________________________________________________________

_____________________________________________________________________________________________________

______________________________________________________________________________________________________

QU

ES

TIO

N 6

Do

you

have an

y oth

er com

men

ts or in

form

ation

that yo

u th

ink w

ou

ld b

e usefu

l for th

is in

vestigatio

n? (feel free to

inclu

de sketch

es on

add

ition

al pag

es)

______________________________________________________________________________________________________

______________________________________________________________________________________________________

______________________________________________________________________________________________________

______________________________________________________________________________________________________

______________________________________________________________________________________________________

______________________________________________________________________________________________________

______________________________________________________________________________________________________

_____________________________________________________________________________________________________

_____________________________________________________________________________________________________

______________________________________________________________________________________________________

______________________________________________________________________________________________________

QU

ES

TIO

N 3

Have yo

u b

een affected

by flo

od

ing

in th

e past?

Yes No

If ‘Yes’, ho

w h

ave you

been

affected? (Yo

u can

select mo

re than

on

e. Please also

list the lo

cation

/add

ress at w

hich

the flo

od

ing

pro

blem

occu

red an

d th

e app

roxim

ate date o

f the flo

od

)

Traffic was disrupted (please provide a description below

if possible)

My back/front yard w

as flooded (please provide a description below if possible)

My house/business and its contents w

ere flooded (please provide a description below if possible)

Other (please provide a description below

if possible)

Description: ________________________________________________________________________________________________

__________________________________________________________________________________________________________

__________________________________________________________________________________________________________

___________________________________________________________________________________________________________

QU

ES

TIO

N 4

Can

you

pro

vide sp

ecific details o

r eviden

ce of h

ow

hig

h flo

od

waters reach

ed (e.g

., ph

oto

s, hig

h w

ater mark o

n b

uild

ing

, dep

th o

f water acro

ss road

way)?

Yes No

a) If ‘Yes’, please g

ive as mu

ch d

etail as po

ssible (e.g

., dates, tim

es, descrip

tion

of w

ater mo

vemen

t, dep

th o

f w

ater, floo

d m

ark locatio

n, h

igh

water m

ark on

bu

ildin

g, level o

n flo

od

dep

th in

dicato

r)?

___________________________________________________________________________________________________________

___________________________________________________________________________________________________________

__________________________________________________________________________________________________________

___________________________________________________________________________________________________________

b) In

you

r op

inio

n, w

hat w

as the m

ain so

urce/cau

se of th

e floo

din

g (e.g

., water o

vertop

pin

g creek b

anks,

blo

ckage o

f culverts/p

ipes, in

sufficien

t storm

water system

capacity)?

___________________________________________________________________________________________________________

___________________________________________________________________________________________________________

__________________________________________________________________________________________________________

___________________________________________________________________________________________________________

MAP!1

MAP!2

LEGEND!

Suite 302, 5 Hunter St

Sydney, NSW 2000

Notes:

File Name: FigureB1 Spatial Distribution of Questionnaire Responses.wor

Aerial Photograph Date: 2009

Figure A1:Spatial Distribution

of QuestionnaireResponses

LEGEND

Prepared By:

0 1

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Scale 1:16,000 (at A3)

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berg

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Nattai CkNattai CkNattai Ck

Nattai CkNattai Ck

Nattai CkNattai CkNattai CkNattai Ck

BOWRAL RD

BOWRAL RD

BOWRAL RDBOWRAL RD

BOWRAL RDBOWRAL RD

BOWRAL RD

BOWRAL RDBOWRAL RD

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OLD HUME HWY

OLD HUME HWY

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OLD HUME HWY

OLD HUME HWY

HUME HWYHUME HWYHUME HWYHUME HWYHUME HWYHUME HWYHUME HWYHUME HWYHUME HWY

OLD BOW

RAL RD

OLD BOW

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THIRLMERETAHMOOR

BARGO

MITTAGONG

BOWRAL

MOSS VALE

BUNDANOON

Questionnaire Response LocationsHas flooding been experienced?

((((((((((((((((((((((((((((((((((((((((((((((((( Yes

((((((((((((((((((((((((((((((((((((((((((((((((( No

Roads

Railway

Watercourse

Gibbergunyah CreekCatchment

Wingecarribee ShireCouncil LGA

Figure Extent

0.5

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FloodedOther Description Flood Descriptions Source of Flooding Rainfall Records

1 37 years 60 years No No No No No No No No

2 50 years years No Yes No NoBack yard flooded when neighbour at back

loosened back fence to drain their propertyNo

Insufficient stormwater

system capacityNo No

3 23 years 32 years No Yes No No

Council drain running at back of property

overtopped send water across front yard

and driveway on 2 occasions

Water was approximately 15cm deep

but moving very quickly in first week of

Novermber 2010


system capacity & culvert

blockage

No

Yes - drainage channel leading to

Gibbergunyah Creek is full of

debris/branches

There were 2 storms that caused water to overtop drain

and cause damage to Gibbergunyah Lane and driveway.

Photos also provided.

4 10 years 10 years No No No No No No No

Yes - natural debris and dumping of

rubbish blocking drain in front of my

property and needs constant

attention to keep it clean. Family

can no longer do this alone.

A general cleanup every 12 months to remove

vegetation as well as council not leaving the removal of

illegal dumping to residents.





Mittagong/Meranie St has rocks,stones and other debris

after heavy rain making crossing difficult. Often

rainwater over Meranie St itself.


10 24 years 58 years No Yes No NoLack of drainage in the street and no

connection to creekNo No No No Guttering in street would improve the problem


12 13 years 28 years No No No No No No No NoPlease eradicate non native vegetation from creek bank

(Chinamans Creek)



Water runs down the hill slope through our property but

does not flood us. We have good drainage around our

house and drains cleared often.

15 0.25 years 0.25 years No No No No No No No Yes Insurance Cover

16 8 years 82 years No No No No No No No NoHave been in the areas for 82 years and have never seen

flooding problems on Henderson Road.


It is important to keep creeks clean and clear of weeds

and rubbish. Attached map of spill from a tributary,

transit along Thomas St and re-entry to another tributary

of Gibbergunyah Creek.

18 11 years 12 years No No No No

Heavy flow along Meranie St (12 months

ago) from OHH to Welby Cr. Caused by

drain blockage. Neighbours diverted water

saving property.

No No No No

Concern regarding insurance implications of flood prone

status. Concern for Wombats in vicinity of Gibbergunyah

Creek, and for this to be considered if flood mitigation

works occur

19 17 years 18 years No Yes No No

Approx 10 years ago, backyard flooded as

water couldn’t drain fast enough - problem

with backyard drainage

No No No

Yes - neighbours, employer said it

used to flood here, and neighbours

have previously had water inside

The Park in Anne St frequently floods, although it hasn’t

rained since the creek was cleared. The neighbours

house used to have a spring before drought.

20 9 years 9 years Yes

2005, 2008, Mittagong street was a river

and awash with stones/soil 2011/2012.

Bowral Lane awash also

Curb and guttering on stormwater

control up Mittagong St. Streets not

cleared and council verges not

maintained, blocking the few drains

there are in such hilly terrain.

Stormwater pits blocked NoYes-sinkhole may form as spring

runs under 30,32,34 Mittagong St

Lack of curb, gutter and drains in Welby. Better drainage

could prevent flooding to some extent. Maintenance of

verges (grass cutting).

21 13 years 13 years No No No No No No No No High point in area…not building an ark.



Lots of rubbish (incl.shopping trolleys) in Gibbergunyah

Ck especially on the Western side of OHH underpass,

would impact flows during flood event.

24 12 years 70 years No No No No No Blockage No No


Large amount of water run down back

coundary, comed out at no 12, due to old

railway line fence catching debris on

hillview property behind us.

No No No No Map - water collects from gutter

Are you worried your property

could be flooded in the future?Any additional nformation?

Current Address In General Area

Community Questionnaire Responses - Gibbergunyah Creek Flood StudyHow long have your lived in area?

#

Have you been affected by flooding in the past? Can you provide historic flood information?

Questionnaire Response Summary.xlsx Page - 1

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How long have your lived in area?

#


26 8 years 9 years No Yes No No Backyard soden as no way to runoff No No No No


28 36 years 36 years Yes Mittagong St was flood across Meranie St No No No No Keep the drains free of debris!



31 1.5 years 3.5 years No No No No No No No No


33 18.5 years 18.5 years No No No No No No No NoPrevious governor of bushcare group, would like to meet

onsite with Sha


35 14.75 years 14.75 years No Yes No No

Flow from culverts under Gibbergunyah

Lane causing channel through property,

storage in SE corner of property

From western side =

concentrated flow from

culverts under

Gibbergunyah Lane, from

Northern side = overland

flow adjacent property

No Yes

Detiailed information of culverts under Gibbergunyah

Lane, photos of flow through property and

stagnation/storage point



38 11 years 27 years Yes Photos previously provided to Council Photos previously provided to CouncilNot sure- surcrity of

rainfall amount perhaps

Photos previously

provided to CouncilYes - due to previous experience

39 1 years 50 years No No No No No No No No Lived at 8 Cook St for 15 years, and never had a problem



Recalls some flooding around Payten St.

The dome at Welsh St used to flood

although.

No No No No

The last time Gibbergunyah and Chinamans Ck flooded

was in Cobb & Co times. A coach had to wait at the

Hume Horse Inn until water receeded. There was no

bridge along the highway then.


43 34 years 63 years No No No NoGibbergunyah Ck usually 50cm after

heavy rain, never more than 1mNo No No

Gibbergunyah Ck has never surpassed 1m deep and is

usually about 50cm deep after heavy rain. It has never

burst its banks behind Spring St due to the steep

gradient, but not so fast to cause erosion.

44 9.75 years 9.75 years Yes No No NoWater across road (Hood St) near the creek

after heavy rainfall.No

Creek not cleared of

reeds causing spilling of

water

No No

45 17 years 17 years No No No Yes

Water pooling at bottom of driveway,

causing driveway to wash away as flow

from top of Thomas street rushes down

street. No stormwater drain on my side of

street.

No No No NoWater pools across road at bridge in Cook St and also in

Old Bowral Road past Gib Gak school heading south.

46 2.25 years 8 years No Yes No NoWater was just under back step. Still have

water lying under back verandah by 50MLSNo

WSC came to look and

stated pits covered with

debris.

NoYard becomes slush and we have

two small dogs.

47 years years No No No No No No NoYes - knowing that it is in a flood

area (address not supplied)


49 40 years 40 years No No No No No No No NoThe creek flows intermittently, the banks are very step (6

metres)


51 15 years 89.5 years YesDalton St East covered by 20-30cm deep

waterNo No No No

In 2001, water flowed through properties on north side

of Dalton St due to inadequite drainage through 2

culverts under Dalton St. Corner of Dalton St, water from

driveways flows across from south to north. Council

drain on north totally inadequite


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52 20 years 20 years No No No No No

In my experience, water

across roads is due to

blocked drains

No No I will wave to Noah if I flood.


54 52 years 87.75 years No No No No No No No No

55 22.5 years 30.5 years

Drainage creek between no 8&10 Cook St

overtops and flows across the road. Did not

stop traffic however.

blockage of

culverts/pipesEven with pipe cleaning, would still overflow at OHH.



58 17 years 17 years Yes Yes

Water over road in Cook St between

Elizabeth and Anne St. Backyard can flood,

but not from creek, just ponding.

Blocked pipes and

culverts, reeds in channel.

Some incomplete

records.

Following the wet summer, whole backyard is a bog

hole. Is this a backlog of water from the creek system?

Also wonder if main road drainage is working properly.

59 11.5 years 11.5 years Yes

Street became flooded so you couldn’t walk

through it…flow from spring, down cook to

elizabeth st.

Blockage in stormwater

drainage and overgrown

plants in creek




Stormwater from roads and drains have

overflowed and then travelled through

nursery and buildings

blockage of

culverts/pipes

Yes - storm events and high

amounts of water racing through

areas causing erosion and

contamination

The need for drainage on roadways to direct high flows

away from properties




66 4.5 years 15 years Yes

2007, Henley Brae villa 8 - heavy rain water

came under front door & wet carpet - bad

drainage was the cause

Bad drainage at no 10

(complex of 3 - 8 to12)

drain water beside and

under road at this point

into Henley Brae

(Pavilion)

Would like to see stormwater drain covered from

Brewster to Henley Brae

67 4.25 years 4.5 years Yes yesHouse on low side of Bong Bong rd (not in

Gibbergunyah catchment). Early 2008

Shopping centre (Big W and Aldi) are probably subjected

to flooding in carparks. Flash flooding main concern.



Annually, water builds to bottom rail on

pailing fence. Cook St flooded between

Elizabeth & Anne St on any heavy rain

occurrence

Gushing down driveway, running into

garage, pool, lawn, pooling at back

fence to nearly knee height (had to

knock fence pailings off to let water

through)

Blocked drain and creek

overflow (overgrown of

weeds), uncleaned

gutters/drains

No

Yes - backyard becomes unusable

due to large amount of water

running over/into it

Regular cleaning of creek and gutters to stop it running

through properties

70 13.5 years 50 years YesDalton St flooded, rubbish left behind,

locals + council cleaned up

Yes - maybe bottom of yard, not

house

Stormwater drains, but cant do too much when it really

comes down


The last time it flooded was 2002, a little

creek out the back as didn’t drain correctly

between Lyell and Anne Rd but better now

due to deviation of water and piping

Lack of cleaning which

has recently improved

Yes - since 2002, in

diaries so difficult to

copy but will if

absolutely necissary

No - but a good regime of cleaning

pipes should be done as even

cleaning cant stop kids throwing

objects into pipes.

72 16 years 71 years No No No NoI have never experienced floods in Medway,

bowral or MittagongNo No No No






Yes - new development taking place

in area could change drainage

patterns, Climate change increase

rainfall


Yes- map provided with

questionaaire shows where

watercourses run

Outside study area



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81 30 years 32 years No No No No NoBlocked creek with

overgrowthNo No

Upstream land use rezoning on Gibbergunyah Ck for

more residential land


Backyard has flooded several times, not due

to creek but hilly rural land runoff from

between Bowral golf course. Needs a

culvert and re-grading to creek

Western side of John

street needs grading and

culvert put in.

No yes

Gibbergunyah Ck behing #74 Spring St will not flood over

the banks as the erosion has resulted in approx 3m deep

gully

83 8.5 years 24.5 years No No No No No No No NoConcerns for increased insurance if a street is 'coloured'

as flood liable

84 12.75 years 12.75 years YesAt back fence, about 4 inches in depth, runs

in from back of Gib Gate school

Yes - lots of rain with the levee that

was built out the back a couple of

years ago.

Get rid of levee out back and put in drainage.

85 13 years 13 years No No No No No No No No Keep creeks free of vegetation and rubbish

86 24 years 24 years No No No No No No No No Concers for vehicular access via Lyell St

87 17.25 years 37 years No No No No No No No No

88 25 years 52 years Yes Creek crossing over Cook St, opposite park No

All reasons (overtopping,

blockage, insufficient

capacity)

No No


I have never seeen the creeks flood, the overgrowth in

the channels could cause a problem with continuous and

heavy rain


Property not directly impacted, local roads have been

constantly eroded due to runoff and absence of

guttering. Mitigation works should include Kerb &

guttering to maximise effectiveness


Flooding over road in Cook St - near the

park - often water on road after/during rain

due to blockage

No No No NoClear reeds,bushes,etc leads to overflows & Road

Drainage


93 2 years 20 years No No No No No No No NoLots of rubbish in creek and rivulets, should organise

cleanup.


95 52 years 60 years No No No No No NoMeasure but don’t

keep recordsNo

96 1 years 10 years Yes No No No No No No


98 8 years 8 years No No No No No No No No Oily water seeping out of the ground

99 3 years 6.75 years Yes

Water from Thomas St residences flowed

through yard/under house. Water also

flows from Richard/Hood St through front

yard

Insufficient Stormwater

Capacity

100 13.75 years 13.75 years No No No No No No

Feb 2012 -266mm (for

the month) Nov

26,2010 storm, 52mm,

total for month 228mm

Creek in "flood" from this event (2010) but below top of

bank

101 16 years 30 years No No No No No No No NoAbout 2 years ago, creek swelled into a raging torrent,

house was not in danger. No photos taken.

102 3 years 13 years No Yes Yes details sent to Phil MarshallInsufficient Stormwater

Capacity

Yes, because it has flooded in the

past

I have been trying to get something done for 2 years, I

am not keen to invest more time unless it is face to face

103 22 years 22 years Yes YesNumerous flood events - photos and

documents provided

Insufficient

drainage/stormwater on

Bowral lane

No Yes - recent history of flooding See attached documents

104 10.5 years 12 years No No No No No No No

Yes - trees which have fallen into the

creek may cause blockage and

flooding to adjacent properties

105 8 years 8 years No Yes Yes

Water flowing into backyard from

neighbours yards. It took a few days to

soak into the ground.

No No NoNo -not sure, maybe if it rained

heavy for a few days


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107 10.25 years 54.25 years No No No No No No No

108 12.25 years 12.25 years No No No No No No No

109 11.25 years 12 years No No No No No NoYes, I have records

from 2002No


Back shed damaged, driveway had to be

replaced. 2004 approx, Severe rain and

poorly maintained drainage system caused

flooding


capacity, poorly

maintained drainage

Yes - Drainage problems still exist,

William, Thomas and John st subject

to flooding from poor drainage

More effective/modern drainage. It is unsatisfactory that

ratepayers have to endure lack of/no drainage



Blocked drain between Elizabeth and Cook

ST sometimes impacts traffic. Council has

recently removed reeds anc cleared

channel.

No No No No

At no time, even with extreme conditions has

Gibbergunyah Creek not been able to cope. Rubbish

contributes to clockage and a tree blocks creek and will

cause eventual blockage. Council has not removed after

serveral calls.



115 11.25 years 11.25 years Yes Yes

Deep water frequently on Cook St, water

collects on Spring St. Backyard continues to

flood despite small drainage channels with

blue metal/gravel

Photos emailed of John st 2008-2010 of

runoff from Gibbergunyah Estate onto

John St with water marker in backyard.


capacity, residence where

dam was previously,

water table height,

springs.

No

Yes - backyard currently floods,

springs will not dissappear, and

curent drainage inadequite

Emailed photos should be of assistance, neighbour has a

council installed drain, amny residences (including ours)

need this also! *More Drainage Essential*



118 8.75 years 20.75 years Yes Yes

Water for gutters was too much for grate

inlet, council has replaced inlet with drain

entrance. Prior to replacement, had water

throughou 10x4m garage

Stormwater down driveway was 12

inches high, flooding garage at end of

driveway

Stormwater grate

blocked, was wrong sort

of grate, has been

rectified

No No

119 3 years 3 years Yes No No No No No No No

Tennants have never complained about flooding in over

10 years. The soil through the front section get

extremely wet during periods of wet weather.




Yes- fallen trees may cause

obstruction and have not been

removed or attended to.


11 Years ago flooding in area from creek

that runs from Elizabeth to Cook St, roads

closed for 2 days. We were all isolated and

some houses flooded. Lots of water from

Mt Gibraltar. Water diverts across the road

(Dalton St) and through peoples properties.

Lights at Bessemer/Bowral St floods as

insufficient drainage.

Blockage of

culverts/pipes and

insufficient capacity.

Cleaning needs to be

undertaken, especially in

winter when leaves fill

gutters/drains.

Yes - the water that comes from the

embankment on Bowral Rd is

suppose to go directly into

Chinamans Creek and it comes into

my yard and then it washes out

driveway access away



We've had minor flooding in our yard. We

also had a blockage in the drain on the road

on Thomas Street which led to flooding

through our property during a severe

downpour.

the water came up to about 20 cms

through our property caused by a

blockage in the drain on Thomas St.

Blockage of the

stormwater drain on

Thomas St.

We have a small creek running

through our property, so always a

potential issue. We have had

incredibly wet weather this year and

no flooding to cause major damage

to our yard.

We do have considerable issues with water laying in the

south east corner of our block. It appears that the

problem is stemming from a lack of any culverts or

drainage along the western side of Elizabeth St outside

our property.


Traffic

Disrupted

Yard

Flooded

House/Business






#



Cook St used to flood across the road

regularly, works have been done to rectify

this, however bank stabilisation has been

neglected. My neighbour adjusts the height

of the pipe and stormwater entry. There are

kerb and gutter on neighbours but nit my

place. A pothole helps direct water away.

Road surface is eroding on verge.

Probably a combination

of blockages from tree

roots, an insufficient

stormwater capacity and

there being combinations

of areas with and without

cubs and guttering.


A very brief inundation of side yard - water

coming from next door.

About 3 cm of water on two occasions

after very heavy rain.

Neighbours yard built to a

level higher than ours, no

drainage on their side.

The non-existance of

drainage pipes adds to

the problem

Emailed

128 years years



yes -due to changing weather

patterns and increased in-fill

development, run-off will increase.

The block can be frequently sodden (marshy) underfoot,

due to rainfall, run-off from rainfall, and shading from

trees. I have undertaken drainage measures to protect

my built property from being affected


Council land adjacent to property runs off

into our backyard causing it to become a

lake. During prolonged rain, open drain at

front of property overflows and water

comes straight down driveway and into

garage. Ditch drain in crown land is

completely unsuitable for the flow even

when only slightest bit of rain. The ditch

gets blocked with branches, sticks and

debris then overflows. Ditch is extremely

dangerous to children as flows and has a

steep drop into pipe without grate cover.

water sits for weeks and gets smelly. Angle

of culvert is not enough for it to runiff into

drain.


system has overtopped

across John St from

'Kennards land'. Blockage

from insufficient trench

on crown land also causes

overflowing. Culvert in-

front and on crown land

is insufficient.

Yes - in Jan 2011, garage flooded

and all belongings under a foot of

water

132 6 years 18 years No No No No No No NoYes - property has been listed as a 1

in 100 year flood risk

133 38 years 64 years No No No NoChinamans Ck has never flooded in the 38+

years we have lived hereNo No No No

The Flood Study is a total waste of time for the two

creeks nominated.


There has never been any flooding. The map provided is

inaccurate in its depiction of the creek alignment and I

hope the simulated solution is accurate and not based on

this map.

135 21 years 39 years Yes Yes Yes Yes

Water ran from vacant land south of John

St, drain blocked, water overflowed through

yards to my property about 200mm deep.

(1997,1999,2003)

I have photos of all three events if

needed.

Each time was due to

blocked drain in John St

and runoff from land

south of John St

No

Yes, no attention or improvements

to the drainage and water

channeling options have been made

after the three floods.

The entire area appears to be channeled via the drain

and the creek in flood conditions backs up and is blocked

by debris.

136 0.5 years 0.5 years Yes

When heavy rain falls, water flows across

Bowral St, down our driveway, under the

house and makes backyard sodden for

days/weeks afterwards

Photos of March/April 2012, dug a

trench and installed a french drain to

disperse water. We also dug a second

drain further down the garden. Also

dug shallow trnch on Bowral St to

prevent water across Bowral St.

Insufficient drainage on

Bowral St. this

exacerbates the drainage

problems with the clay

soils in the backyard.

No

yes, we wxpect water to collect

under the house and garden unless

drainage along Bowral St is

improved.


APPENDIX B

XP-RAFTS MODEL INPUT PARAMETERS

0.0

10,000.0

20,000.0

30,000.0

40,000.0

50,000.0

60,000.0

70,000.0R

ain

fall

(mm

)

Date/Time

Stage-Discharge Relationship

Stage-Storage Relationship

LEGEND:

Notes:

File Name: Lake Alexandra Stage Storage and Stage Discharge.xls

Figure B1:

Stage-Storage and Stage- Discharge

Relationships for Lake Alexandra

LEGEND:

Notes:

LEGEND:

Notes:

Suite 302, 5 Hunter Street Sydney, NSW, 2000

0

20

40

60

80

100

120

140

0

5,000

10,000

15,000

20,000

25,000

30,000

35,000

40,000

45,000

50,000

620.5 620.7 620.9 621.1 621.3 621.5 621.7 621.9 622.1 622.3 622.5

Dis

ch

arg

e (

m3/s

)

Sto

rag

e (

m3)

Stage (mAHD)

LEGEND:

Notes:

LEGEND:

Notes:

Prepared By:


XP-RAFTS INPUT PARAMETERS - Gibbergunyah Creek

Subcatchment ID Sub-Area Area [ha]Catchment Slope

[%]

Percentage

Impervious [%]Mannings 'n'

1 3.28 51.00 0 0.079

2 0.27 51.00 100 0.015

1 4.19 13.90 0 0.057

2 1.02 13.90 100 0.015

1 2.12 15.62 0 0.072

2 0.29 15.62 100 0.015

1 4.00 16.38 0 0.065

2 0.56 16.38 100 0.015

1 5.74 12.01 0 0.077

2 0.56 12.01 100 0.015

1 5.61 9.80 0 0.093

2 0.61 9.80 100 0.015

1 0.71 6.69 0 0.078

2 0.12 6.69 100 0.015

1 4.50 11.61 0 0.083

2 0.41 11.61 100 0.015

1 4.75 10.05 0 0.072

2 0.25 10.05 100 0.015

1 1.73 13.11 0 0.066

2 0.09 13.11 100 0.015

1 5.71 8.87 0 0.087

2 0.30 8.87 100 0.015

1 1.12 9.96 0 0.074

2 0.06 9.96 100 0.015

1 5.56 4.16 0 0.064

2 0.64 4.16 100 0.015

1 7.02 4.81 0 0.062

2 1.05 4.81 100 0.015

1 2.39 4.30 0 0.069

2 0.50 4.30 100 0.015

1 4.66 4.67 0 0.047

2 1.08 4.67 100 0.015

1 1.02 5.14 0 0.044

2 0.57 5.14 100 0.015

1 4.10 5.86 0 0.060

2 1.15 5.86 100 0.015

1 2.93 4.95 0 0.057

2 0.50 4.95 100 0.015

1 2.68 7.24 0 0.088

2 0.19 7.24 100 0.015

1 2.12 7.15 0 0.041

2 3.17 7.15 100 0.015

1 2.15 7.97 0 0.048

2 1.24 7.97 100 0.015

1 4.11 10.99 0 0.095

2 0.22 10.99 100 0.015

1.19

1.2

1.21

1.22

1.23

1.1

1.11

1.12

1.13

1.14

1.15

1.16

1.17

1.18

1.01

1.02

1.03

1.04

1.05

1.06

1.07

1.08

1.09

Gibbergunyah XP-RAFTS Inputs

XP-RAFTS Inputs Existing.xlsx Page - 1


[%]

Percentage


1 0.67 12.30 0 0.083

2 0.18 12.30 100 0.015

1 6.10 10.97 0 0.092

2 0.91 10.97 100 0.015

1 6.44 10.24 0 0.096

2 0.60 10.24 100 0.015

1 3.43 13.68 0 0.095

2 0.39 13.68 100 0.015

1 2.79 32.90 0 0.094

2 0.37 32.90 100 0.015

1 9.12 26.41 0 0.098

2 0.67 26.41 100 0.015

1 9.52 26.52 0 0.087

2 0.71 26.52 100 0.015

1 3.93 30.41 0 0.076

2 0.68 30.41 100 0.015

1 6.63 28.84 0 0.095

2 0.70 28.84 100 0.015

1 3.16 29.30 0 0.094

2 0.30 29.30 100 0.015

1 3.25 18.93 0 0.066

2 0.36 18.93 100 0.015

1 2.33 30.57 0 0.100

2 0.12 30.57 100 0.015

1 7.15 17.94 0 0.095

2 0.38 17.94 100 0.015

1 1.04 31.36 0 0.100

2 0.05 31.36 100 0.015

1 2.91 29.01 0 0.100

2 0.15 29.01 100 0.015

1 4.86 15.66 0 0.100

2 0.26 15.66 100 0.015

1 7.48 6.28 0 0.061

2 1.05 6.28 100 0.015

1 0.37 14.30 0 0.052

2 0.13 14.30 100 0.015

1 4.92 15.76 0 0.100

2 0.26 15.76 100 0.015

1 3.05 21.93 0 0.100

2 0.16 21.93 100 0.015

1 10.63 15.77 0 0.097

2 1.03 15.77 100 0.015

1 7.05 10.98 0 0.100

2 0.37 10.98 100 0.015

1 6.91 10.49 0 0.100

2 0.36 10.49 100 0.015

1 2.70 22.26 0 0.100

2 0.14 22.26 100 0.015

5.01

5.02

1.28

1.29

1.3

1.31

1.32

1.33

3.01

3.02

4.01

2.01

1.24

1.25

1.26

1.27

9.01

9.02

9.03

9.04

6.01

6.02

7.01

8.01




[%]

Percentage


1 5.07 21.69 0 0.100

2 0.27 21.69 100 0.015

1 0.60 19.26 0 0.100

2 0.03 19.26 100 0.015

1 2.00 6.78 0 0.097

2 0.11 6.78 100 0.015

1 2.18 30.79 0 0.100

2 0.11 30.79 100 0.015

1 7.41 14.69 0 0.096

2 0.83 14.69 100 0.015

1 11.49 14.29 0 0.096

2 1.09 14.29 100 0.015

1 5.88 10.85 0 0.100

2 0.31 10.85 100 0.015

1 12.22 14.27 0 0.098

2 0.90 14.27 100 0.015

1 2.63 13.26 0 0.100

2 0.14 13.26 100 0.015

1 2.87 24.42 0 0.100

2 0.15 24.42 100 0.015

1 5.15 25.44 0 0.100

2 0.27 25.44 100 0.015

1 3.48 17.24 0 0.100

2 0.18 17.24 100 0.015

1 3.43 20.12 0 0.100

2 0.18 20.12 100 0.015

1 4.40 19.87 0 0.100

2 0.23 19.87 100 0.015

1 4.05 10.57 0 0.091

2 0.25 10.57 100 0.015

1 6.50 24.82 0 0.099

2 0.34 24.82 100 0.015

1 9.49 14.49 0 0.091

2 0.50 14.49 100 0.015

1 11.13 8.13 0 0.082

2 0.59 8.13 100 0.015

1 4.66 5.72 0 0.073

2 0.30 5.72 100 0.015

1 9.04 15.17 0 0.100

2 0.48 15.17 100 0.015

1 5.81 13.02 0 0.100

2 0.31 13.02 100 0.015

1 2.30 7.95 0 0.084

2 0.19 7.95 100 0.015

1 7.26 7.83 0 0.052

2 0.38 7.83 100 0.015

1 4.13 5.59 0 0.047

2 0.54 5.59 100 0.015

9.05

9.06

14.01

14.02

15.01

16.01

16.02

16.03

9.07

10.01

11.01

12.01

12.02

13.01

19.02

19.03

20.01

21.01

17.01

18.01

18.02

18.03

18.04

19.01




[%]

Percentage


1 1.93 8.93 0 0.054

2 0.20 8.93 100 0.015

1 5.19 4.36 0 0.046

2 0.75 4.36 100 0.015

1 3.79 5.85 0 0.057

2 0.54 5.85 100 0.015

1 7.16 5.08 0 0.052

2 0.50 5.08 100 0.015

1 2.34 4.26 0 0.030

2 0.98 4.26 100 0.015

1 2.47 4.64 0 0.031

2 1.27 4.64 100 0.015

1 1.99 4.05 0 0.031

2 1.00 4.05 100 0.015

1 3.33 8.93 0 0.053

2 0.18 8.93 100 0.015

1 2.12 8.10 0 0.045

2 0.35 8.10 100 0.015

1 4.15 4.51 0 0.050

2 1.25 4.51 100 0.015

1 2.07 2.45 0 0.041

2 0.37 2.45 100 0.015

1 1.47 7.31 0 0.032

2 0.28 7.31 100 0.015

1 3.43 7.14 0 0.064

2 0.37 7.14 100 0.015

1 2.19 4.48 0 0.048

2 0.12 4.48 100 0.015

1 1.41 69.95 0 0.075

2 0.34 69.95 100 0.015

1 1.86 12.46 0 0.057

2 0.31 12.46 100 0.015

1 1.52 9.88 0 0.064

2 0.19 9.88 100 0.015

1 1.32 7.53 0 0.055

2 0.38 7.53 100 0.015

1 1.58 4.67 0 0.053

2 0.14 4.67 100 0.015

1 0.68 3.40 0 0.056

2 0.12 3.40 100 0.015

1 1.12 5.66 0 0.069

2 0.28 5.66 100 0.015

1 1.50 4.69 0 0.053

2 0.58 4.69 100 0.015

1 3.18 4.22 0 0.053

2 0.17 4.22 100 0.015

1 4.35 4.11 0 0.055

2 1.01 4.11 100 0.015

22.01

22.02

25.01

26.01

26.02

26.03

27.01

27.02

22.03

23.01

23.02

23.03

23.04

24.01

28.07

28.08

28.09

28.1

28.01

28.02

28.03

28.04

28.05

28.06




[%]

Percentage


1 1.78 3.37 0 0.031

2 0.76 3.37 100 0.015

1 0.67 2.21 0 0.047

2 0.14 2.21 100 0.015

1 0.63 1.78 0 0.043

2 0.14 1.78 100 0.015

1 0.76 4.07 0 0.046

2 0.21 4.07 100 0.015

1 2.63 8.04 0 0.054

2 0.58 8.04 100 0.015

1 0.93 81.96 0 0.066

2 0.72 81.96 100 0.015

1 2.35 13.61 0 0.063

2 0.36 13.61 100 0.015

1 3.56 7.77 0 0.057

2 0.44 7.77 100 0.015

1 1.05 3.09 0 0.060

2 0.36 3.09 100 0.015

1 1.47 8.18 0 0.087

2 0.27 8.18 100 0.015

1 3.23 45.15 0 0.081

2 0.95 45.15 100 0.015

1 3.63 11.24 0 0.073

2 0.49 11.24 100 0.015

1 2.18 8.65 0 0.054

2 0.55 8.65 100 0.015

1 3.55 5.39 0 0.053

2 0.59 5.39 100 0.015

1 0.51 3.04 0 0.061

2 0.50 3.04 100 0.015

1 2.82 4.59 0 0.051

2 0.53 4.59 100 0.015

1 3.66 3.51 0 0.048

2 0.93 3.51 100 0.015

1 1.98 6.27 0 0.068

2 0.22 6.27 100 0.015

1 2.79 43.31 0 0.098

2 0.21 43.31 100 0.015

1 1.72 10.37 0 0.063

2 0.27 10.37 100 0.015

1 1.32 9.45 0 0.070

2 0.25 9.45 100 0.015

1 1.71 11.19 0 0.067

2 0.46 11.19 100 0.015

1 2.69 4.13 0 0.057

2 0.70 4.13 100 0.015

1 1.24 2.25 0 0.031

2 3.12 2.25 100 0.015

28.11

28.12

32.01

33.01

33.02

33.03

33.04

34.01

28.13

28.14

29.01

30.01

30.02

31.01

38.03

38.04

38.05

38.06

35.01

35.02

36.01

37.01

38.01

38.02




[%]

Percentage


1 2.55 3.29 0 0.027

2 2.72 3.29 100 0.015

1 2.55 2.88 0 0.029

2 1.58 2.88 100 0.015

1 1.81 33.86 0 0.098

2 0.10 33.86 100 0.015

1 1.36 18.40 0 0.085

2 0.07 18.40 100 0.015

1 1.07 2.47 0 0.023

2 2.25 2.47 100 0.015

1 0.48 3.18 0 0.022

2 1.51 3.18 100 0.015

1 0.45 3.73 0 0.025

2 1.58 3.73 100 0.015

1 1.65 1.57 0 0.027

2 2.41 1.57 100 0.015

1 0.70 2.40 0 0.022

2 2.98 2.40 100 0.015

1 1.86 3.26 0 0.035

2 0.67 3.26 100 0.015

1 2.93 4.13 0 0.031

2 1.41 4.13 100 0.015

1 0.62 2.09 0 0.033

2 0.29 2.09 100 0.015

1 2.67 1.33 0 0.031

2 1.43 1.33 100 0.015

1 4.52 3.76 0 0.032

2 1.74 3.76 100 0.015

1 2.79 5.75 0 0.032

2 1.27 5.75 100 0.015

1 2.91 4.17 0 0.032

2 1.35 4.17 100 0.015

1 6.23 7.09 0 0.062

2 1.80 7.09 100 0.015

1 0.93 1.28 0 0.030

2 0.58 1.28 100 0.015

1 2.58 1.92 0 0.032

2 0.96 1.92 100 0.015

1 3.86 4.66 0 0.036

2 1.71 4.66 100 0.015

1 2.20 9.59 0 0.031

2 1.03 9.59 100 0.015

1 1.98 4.25 0 0.040

2 0.94 4.25 100 0.015

1 2.28 7.89 0 0.065

2 0.56 7.89 100 0.015

1 1.74 4.77 0 0.035

2 0.52 4.77 100 0.015

38.07

38.08

42.04

42.05

43.01

43.02

44.01

44.02

39.01

40.01

41.01

42.01

42.02

42.03

46.01

47.01

48.01

48.02

44.03

44.04

44.05

45.01

45.02

45.03




[%]

Percentage


1 1.56 12.99 0 0.054

2 0.52 12.99 100 0.015

1 3.01 10.32 0 0.054

2 0.89 10.32 100 0.015

1 3.73 13.92 0 0.100

2 0.20 13.92 100 0.015

1 1.85 10.33 0 0.090

2 0.23 10.33 100 0.015

1 3.93 7.68 0 0.052

2 0.51 7.68 100 0.015

1 2.56 11.31 0 0.041

2 0.30 11.31 100 0.015

1 2.89 21.96 0 0.076

2 0.25 21.96 100 0.015

1 3.76 22.86 0 0.095

2 0.34 22.86 100 0.015

1 4.70 15.43 0 0.097

2 0.41 15.43 100 0.015

1 2.89 9.81 0 0.093

2 0.44 9.81 100 0.015

1 4.98 8.58 0 0.084

2 0.89 8.58 100 0.015

1 2.68 0.49 0 0.056

2 1.53 0.49 100 0.015

1 1.66 3.46 0 0.050

2 0.38 3.46 100 0.015

1 4.44 4.23 0 0.043

2 1.54 4.23 100 0.015

1 1.03 3.07 0 0.032

2 1.06 3.07 100 0.015

1 2.11 2.92 0 0.037

2 1.73 2.92 100 0.015

1 1.76 7.25 0 0.040

2 1.73 7.25 100 0.015

1 2.19 15.53 0 0.053

2 0.50 15.53 100 0.015

1 2.57 26.59 0 0.090

2 0.41 26.59 100 0.015

1 2.66 13.18 0 0.067

2 1.72 13.18 100 0.015

1 5.24 18.75 0 0.076

2 0.82 18.75 100 0.015

1 3.12 36.58 0 0.099

2 0.20 36.58 100 0.015

1 4.66 22.37 0 0.096

2 0.25 22.37 100 0.015

1 5.22 19.88 0 0.070

2 1.03 19.88 100 0.015

49.01

50.01

52.05

52.06

52.07

52.08

52.09

52.1

50.02

51.01

52.01

52.02

52.03

52.04

54.02

54.03

54.04

55.01

52.11

52.12

52.13

53.01

53.02

54.01




[%]

Percentage


1 2.41 6.64 0 0.067

2 1.25 6.64 100 0.015

1 3.61 34.07 0 0.094

2 0.61 34.07 100 0.015

1 2.42 14.16 0 0.056

2 0.50 14.16 100 0.015

1 3.08 31.29 0 0.094

2 0.44 31.29 100 0.015

1 1.08 23.73 0 0.099

2 0.06 23.73 100 0.015

1 3.21 24.78 0 0.097

2 0.17 24.78 100 0.015

1 3.55 18.65 0 0.042

2 0.24 18.65 100 0.015

1 8.89 17.99 0 0.079

2 0.63 17.99 100 0.015

1 5.15 10.37 0 0.092

2 0.48 10.37 100 0.015

1 0.54 24.44 0 0.099

2 0.03 24.44 100 0.015

1 3.23 5.52 0 0.080

2 0.89 5.52 100 0.015

1 1.55 4.94 0 0.031

2 0.84 4.94 100 0.015

1 1.05 28.85 0 0.063

2 0.07 28.85 100 0.015

1 6.10 16.55 0 0.067

2 0.87 16.55 100 0.015

1 3.59 9.78 0 0.045

2 1.19 9.78 100 0.015

1 2.03 2.34 0 0.050

2 1.69 2.34 100 0.015

1 2.61 4.56 0 0.036

2 0.93 4.56 100 0.015

1 2.36 3.12 0 0.031

2 1.18 3.12 100 0.015

1 5.81 7.04 0 0.054

2 1.63 7.04 100 0.015

1 1.62 4.73 0 0.028

2 1.38 4.73 100 0.015

1 1.44 2.98 0 0.047

2 0.18 2.98 100 0.015

1 2.06 5.49 0 0.030

2 0.89 5.49 100 0.015

1 1.64 1.65 0 0.030

2 0.79 1.65 100 0.015

1 0.81 29.30 0 0.074

2 0.07 29.30 100 0.015

56.01

56.02

60.03

61.01

61.02

62.01

63.01

63.02

57.01

57.02

58.01

59.01

60.01

60.02

66.01

66.02

66.03

67.01

63.03

63.04

63.05

63.06

64.01

65.01




[%]

Percentage


1 4.99 11.30 0 0.066

2 0.68 11.30 100 0.015

1 2.95 10.27 0 0.043

2 1.09 10.27 100 0.015

1 3.47 3.29 0 0.041

2 1.46 3.29 100 0.015

1 2.31 6.55 0 0.047

2 0.61 6.55 100 0.015

1 2.15 6.38 0 0.044

2 0.66 6.38 100 0.015

1 0.63 3.14 0 0.039

2 0.58 3.14 100 0.015

1 4.30 2.78 0 0.042

2 2.71 2.78 100 0.015

1 2.82 3.02 0 0.040

2 2.34 3.02 100 0.015

1 3.95 4.62 0 0.047

2 0.95 4.62 100 0.015

1 4.05 4.18 0 0.047

2 3.26 4.18 100 0.015

1 4.56 12.60 0 0.046

2 1.04 12.60 100 0.015

1 4.42 11.29 0 0.058

2 0.76 11.29 100 0.015

1 0.94 3.20 0 0.032

2 0.45 3.20 100 0.015

1 2.15 18.25 0 0.049

2 0.68 18.25 100 0.015

1 2.27 7.58 0 0.046

2 0.76 7.58 100 0.015

1 1.51 7.13 0 0.041

2 0.61 7.13 100 0.015

1 2.17 14.24 0 0.048

2 0.51 14.24 100 0.015

1 1.69 4.50 0 0.047

2 0.71 4.50 100 0.015

1 2.35 2.32 0 0.035

2 1.19 2.32 100 0.015

1 0.34 0.08 0 0.044

2 0.26 0.08 100 0.015

1 0.73 3.07 0 0.039

2 1.09 3.07 100 0.015

1 1.35 2.49 0 0.031

2 0.56 2.49 100 0.015

1 0.22 3.81 0 0.041

2 1.10 3.81 100 0.015

1 1.50 4.64 0 0.032

2 1.11 4.64 100 0.015

67.02

67.03

67.1

67.11

68.01

69.01

70.01

71.01

67.04

67.05

67.06

67.07

67.08

67.09

74.03

75.01

76.01

76.02

71.02

71.03

72.01

73.01

74.01

74.02




[%]

Percentage


1 1.61 3.07 0 0.033

2 1.80 3.07 100 0.015

1 1.69 4.40 0 0.036

2 1.17 4.40 100 0.015

1 0.83 4.98 0 0.032

2 0.42 4.98 100 0.015

1 0.53 1.27 0 0.029

2 0.78 1.27 100 0.015

1 0.76 5.05 0 0.029

2 0.56 5.05 100 0.015

1 1.47 6.56 0 0.029

2 1.09 6.56 100 0.015

1 2.55 3.81 0 0.038

2 0.86 3.81 100 0.015

1 5.77 5.32 0 0.062

2 1.19 5.32 100 0.015

1 3.14 3.88 0 0.031

2 1.21 3.88 100 0.015

1 0.17 1.55 0 0.063

2 1.97 1.55 100 0.015

1 0.96 1.90 0 0.034

2 0.32 1.90 100 0.015

1 4.94 16.25 0 0.097

2 0.46 16.25 100 0.015

1 2.37 26.27 0 0.068

2 0.69 26.27 100 0.015

1 2.50 6.72 0 0.087

2 0.49 6.72 100 0.015

1 1.00 8.70 0 0.031

2 0.42 8.70 100 0.015

1 2.73 7.18 0 0.055

2 0.97 7.18 100 0.015

1 1.92 4.83 0 0.041

2 0.68 4.83 100 0.015

1 2.76 1.78 0 0.033

2 1.48 1.78 100 0.015

1 1.41 1.39 0 0.030

2 1.43 1.39 100 0.015

1 0.57 0.62 0 0.036

2 1.18 0.62 100 0.015

1 0.96 2.30 0 0.029

2 0.63 2.30 100 0.015

1 1.18 1.84 0 0.029

2 1.02 1.84 100 0.015

1 2.12 3.35 0 0.031

2 1.01 3.35 100 0.015

1 0.15 0.01 0 0.031

2 1.78 0.01 100 0.015

77.01

78.01

82.01

83.01

84.01

85.01

86.01

87.01

79.01

80.01

80.02

80.03

81.01

81.02

88.07

88.08

88.09

88.1

88.01

88.02

88.03

88.04

88.05

88.06




[%]

Percentage


1 0.62 12.64 0 0.093

2 0.19 12.64 100 0.015

1 5.68 10.29 0 0.086

2 0.84 10.29 100 0.015

1 4.82 16.53 0 0.076

2 0.92 16.53 100 0.015

1 3.53 15.64 0 0.096

2 0.32 15.64 100 0.015

1 1.38 16.76 0 0.097

2 0.12 16.76 100 0.015

1 4.04 18.29 0 0.099

2 0.23 18.29 100 0.015

1 3.15 20.14 0 0.097

2 0.28 20.14 100 0.015

1 0.79 7.16 0 0.031

2 0.45 7.16 100 0.015

1 0.57 2.56 0 0.042

2 0.26 2.56 100 0.015

1 1.10 5.75 0 0.034

2 0.75 5.75 100 0.015

1 3.77 2.59 0 0.034

2 2.20 2.59 100 0.015

1 0.92 1.06 0 0.031

2 0.37 1.06 100 0.015

1 0.94 1.40 0 0.041

2 1.35 1.40 100 0.015

1 1.04 1.18 0 0.033

2 0.95 1.18 100 0.015

1 2.71 6.67 0 0.058

2 0.94 6.67 100 0.015

1 2.35 1.87 0 0.030

2 1.36 1.87 100 0.015

1 0.23 1.83 0 0.029

2 0.26 1.83 100 0.015

1 1.24 2.81 0 0.029

2 0.76 2.81 100 0.015

1 1.53 3.40 0 0.030

2 0.94 3.40 100 0.015

1 1.00 2.46 0 0.028

2 0.81 2.46 100 0.015

1 0.71 1.28 0 0.029

2 0.91 1.28 100 0.015

1 0.26 1.42 0 0.029

2 0.81 1.42 100 0.015

1 0.67 1.87 0 0.029

2 0.99 1.87 100 0.015

1 0.48 6.22 0 0.037

2 0.25 6.22 100 0.015

88.11

88.12

89.02

90.01

91.01

92.01

92.02

93.01

88.13

88.14

88.15

88.16

88.17

89.01

96.03

97.01

98.01

99.01

94.01

94.02

94.03

95.01

96.01

96.02




[%]

Percentage


1 0.61 0.53 0 0.039

2 1.75 0.53 100 0.015

1 0.49 2.88 0 0.032

2 1.31 2.88 100 0.015

1 1.04 2.64 0 0.028

2 1.07 2.64 100 0.015

1 0.56 6.62 0 0.035

2 0.46 6.62 100 0.015

1 0.57 4.37 0 0.030

2 0.33 4.37 100 0.015

1 0.46 4.83 0 0.029

2 0.31 4.83 100 0.015

1 2.22 18.81 0 0.096

2 0.27 18.81 100 0.015

1 1.69 15.83 0 0.065

2 0.40 15.83 100 0.015

1 1.47 8.24 0 0.045

2 0.78 8.24 100 0.015

1 0.89 9.48 0 0.058

2 0.41 9.48 100 0.015

1 1.74 3.91 0 0.028

2 1.34 3.91 100 0.015

1 0.88 12.56 0 0.031

2 0.38 12.56 100 0.015

1 1.70 8.54 0 0.040

2 0.69 8.54 100 0.015

1 2.16 26.76 0 0.100

2 0.11 26.76 100 0.015

1 2.22 25.51 0 0.100

2 0.12 25.51 100 0.015

1 1.99 13.84 0 0.096

2 0.27 13.84 100 0.015

1 3.49 32.38 0 0.100

2 0.18 32.38 100 0.015

1 3.08 31.30 0 0.100

2 0.16 31.30 100 0.015

1 3.14 25.35 0 0.099

2 0.22 25.35 100 0.015

1 1.86 5.73 0 0.100

2 0.10 5.73 100 0.015

1 4.73 34.26 0 0.100

2 0.25 34.26 100 0.015

1 4.84 31.82 0 0.100

2 0.25 31.82 100 0.015

1 2.49 32.94 0 0.100

2 0.13 32.94 100 0.015

1 3.44 18.00 0 0.082

2 0.64 18.00 100 0.015

104.03

104.02

104.01

103.02

103.01

102.01

101.01

100.02

108.02

108.01

107.03

107.02

107.01

106.01

105.02

105.01

104.04

110.01

109.04

109.03

109.02

109.01

108.03

100.01




[%]

Percentage


1 5.84 25.90 0 0.100

2 0.31 25.90 100 0.015

1 3.77 21.17 0 0.100

2 0.20 21.17 100 0.015

1 2.69 31.06 0 0.100

2 0.14 31.06 100 0.015

1 1.68 1.31 0 0.049

2 0.09 1.31 100 0.015

1 4.17 4.70 0 0.081

2 0.67 4.70 100 0.015

1 4.43 10.01 0 0.065

2 0.51 10.01 100 0.015

1 4.70 17.61 0 0.088

2 0.77 17.61 100 0.015

1 4.51 4.60 0 0.092

2 0.65 4.60 100 0.015

1 1.71 4.90 0 0.098

2 0.18 4.90 100 0.015

1 5.15 44.08 0 0.099

2 0.30 44.08 100 0.015

1 1.71 8.64 0 0.074

2 0.24 8.64 100 0.015

1 6.77 35.19 0 0.081

2 0.51 35.19 100 0.015

111.03

111.02

111.01

115.02

115.01

114.01

113.02

113.01

112.04

112.03

112.02

112.01



BASEFLOW AT PEAK STREAMFLOW: CALCULATION SHEET (Calculations based on ‘Revision Project 7: Baseflow for Catchment Simulation’ (Engineers Australia, 2011) )

CATCHMENT INFORMATION

Catchment Gibbergunyah Creek Area 10.89 km2

State New South Wales

10 year ARI Baseflow Peak Factor

(RBPF10yr) (Figure 2 – ARR P7)

0.1

Baseflow Under Peak Streamflow Calculations

ARI

ARI factor for baseflow

peak factor (FARI)

(Table 1 – ARR P7)

Baseflow Peak Factor

RBPFARI

(RBPFARI = RBPF10yr * FARI)

Ratio of baseflow under the

Peak Streamflow RBUPF

RBUPF = 0.7 * RBPFARI

0.5 3.0 0.30 0.210

1 2.2 0.22 0.154

2 1.7 0.17 0.119

5 1.2 0.12 0.084

10 1.0 0.10 0.070

20 0.8 0.08 0.056

50 0.7 0.07 0.049

100 0.6 0.06 0.042

Baseflow Under Peak Streamflow Calculations

ARI Q(m

3/s) Peak

Surface Runoff

Ratio of baseflow

under the Peak

Streamflow RBUPF

Q(m3/s) baseflow at

time of peak surface

runoff

(= QPeak Surface

Runoff * RBUPF)

Q(m3/s) per km

2

(= Q baseflow at

time of peak

surface runoff /

Catchment Area

(km2))

0.5 N/A 0.210 N/A N/A

1 N/A 0.154 N/A N/A

2 N/A 0.119 N/A N/A

5 88.68 0.084 7.50 0.69

10 109.62 0.070 7.67 0.70

20 142.02 0.056 7.95 0.73

50 176.99 0.049 8.67 0.80

100 209.5 0.042 8.80 0.81

200 250 N/A 8.80 0.81

PMF 1100 N/A 8.80 0.81

Prepared By D. Fedcyzna

Date

14/06/2012

Checked By D. Tetley

Date 11/09/2012

APPENDIX C

XP-RAFTS MODEL RESULTS FOR

CALIBRATION SIMULATIONS

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4R

ain

fall

(mm

)

Date/Time

Bowral (Parry Rd)

LEGEND:

Notes:

Figure C1: Pluvio Rainfall Data for

Feburary 2005 Event

Prepared By:


File Name: 2005 Pluvios Chart.xls

0.0

0.5

1.0

1.5

2.0

2.5R

ain

fall

(mm

)

Date/Time

Bowral (Parry Rd)

LEGEND:

Notes:

Prepared By:



Figure C2: Pluvio Rainfall Data for November 2010 Event

LEGEND:

Notes:

LEGEND:

Notes:

Prepared By:


1

10

100

1,000R

ain

fall

Inte

nsi

ty (

mm

/ho

ur)

Duration (hours)

100 Year ARI

50 Year ARI

20 Year ARI

5 Year ARI

1 Year ARI

November 2010 Event

February 2005 Event

LEGEND

Notes:

Figure C4: Intensity-Frequency-Duration Curves for Gibbergunyah Creek

Catchment

Prepared By:



PEAK FLOOD DISCHARGES - Calibration/Verification Events

Feb-05 Nov-10

1.01 0.09 0.15

1.02 0.22 0.35

1.03 0.28 0.43

1.04 0.47 0.74

1.05 0.61 0.94

1.06 0.97 1.48

1.07 1.17 1.78

1.08 1.47 2.21

1.09 1.58 2.37

1.10 1.73 2.59

1.11 1.93 2.86

1.12 3.99 5.87

1.13 4.20 6.17

1.14 5.68 8.25

1.15 6.46 9.35

1.16 6.62 9.55

1.17 6.66 9.58

1.18 7.09 10.15

1.19 9.81 13.94

1.20 10.94 15.58

1.21 11.25 15.95

1.22 11.32 16.03

1.23 11.40 16.11

1.24 15.06 21.17

1.25 17.73 24.72

1.26 18.02 25.09

1.27 18.15 25.24

1.28 20.14 28.40

1.29 20.42 28.73

1.30 20.61 28.94

1.31 20.86 29.21

1.32 20.99 29.34

1.33 21.05 29.39

2.01 0.09 0.14

3.01 0.06 0.09

3.02 0.22 0.33

4.01 0.03 0.04

5.01 0.07 0.11

5.02 0.18 0.28

6.01 0.19 0.26

6.02 0.20 0.28

7.01 0.11 0.17

8.01 0.07 0.11

9.01 0.25 0.34

9.02 0.45 0.63

Subcatchment IDPeak Discharge (m

3/s)

Cal Ver

XP-RAFTS Output.xlsx Page - 1

Feb-05 Nov-10Subcatchment ID


9.03 0.76 1.07

9.04 1.46 2.04

9.05 1.68 2.37

9.06 1.80 2.55

9.07 2.08 2.98

10.01 0.06 0.09

11.01 0.18 0.25

12.01 0.26 0.36

12.02 0.65 0.90

13.01 0.27 0.37

14.01 0.06 0.09

14.02 0.13 0.19

15.01 0.12 0.19

16.01 0.08 0.12

16.02 0.16 0.25

16.03 0.26 0.40

17.01 0.09 0.14

18.01 0.16 0.23

18.02 0.37 0.52

18.03 0.59 0.83

18.04 0.69 0.97

19.01 0.20 0.28

19.02 0.32 0.46

19.03 0.38 0.53

20.01 0.17 0.25

21.01 0.11 0.16

22.01 0.05 0.08

22.02 0.18 0.26

22.03 0.28 0.39

23.01 0.17 0.23

23.02 0.32 0.47

23.03 0.41 0.61

23.04 0.48 0.70

24.01 0.08 0.13

25.01 0.06 0.09

26.01 0.12 0.19

26.02 0.31 0.45

26.03 0.35 0.52

27.01 0.08 0.13

27.02 0.14 0.20

28.01 0.05 0.08

28.02 0.10 0.16

28.03 0.21 0.34

28.04 0.37 0.57

28.05 0.49 0.76

28.06 0.54 0.83

28.07 0.87 1.33

Cal Ver




28.08 0.92 1.40

28.09 1.08 1.63

28.10 1.40 2.08

28.11 1.50 2.22

28.12 2.30 3.29

28.13 2.81 3.92

28.14 2.83 3.95

29.01 0.08 0.12

30.01 0.05 0.08

30.02 0.11 0.18

31.01 0.09 0.14

32.01 0.03 0.05

33.01 0.04 0.05

33.02 0.15 0.23

33.03 0.24 0.37

33.04 0.30 0.47

34.01 0.09 0.14

35.01 0.03 0.04

35.02 0.10 0.15

36.01 0.10 0.15

37.01 0.05 0.07

38.01 0.07 0.12

38.02 0.12 0.19

38.03 0.20 0.31

38.04 0.29 0.44

38.05 0.36 0.54

38.06 0.47 0.66

38.07 0.60 0.82

38.08 0.79 1.08

39.01 0.05 0.07

40.01 0.03 0.05

41.01 0.10 0.16

42.01 0.06 0.10

42.02 0.13 0.20

42.03 0.23 0.36

42.04 0.34 0.52

42.05 0.41 0.61

43.01 0.11 0.17

43.02 0.14 0.20

44.01 0.09 0.14

44.02 0.24 0.37

44.03 0.33 0.53

44.04 0.76 1.18

44.05 1.18 1.76

45.01 0.04 0.05

45.02 0.12 0.17

45.03 0.33 0.51

Cal Ver




46.01 0.09 0.14

47.01 0.07 0.11

48.01 0.07 0.10

48.02 0.12 0.18

49.01 0.05 0.08

50.01 0.10 0.15

50.02 0.22 0.34

51.01 0.05 0.07

52.01 0.11 0.16

52.02 0.17 0.27

52.03 0.25 0.39

52.04 0.48 0.75

52.05 1.64 2.51

52.06 1.71 2.61

52.07 2.33 3.56

52.08 2.37 3.57

52.09 2.41 3.62

52.10 2.54 3.75

52.11 2.64 3.85

52.12 3.44 4.80

52.13 3.66 5.06

53.01 0.07 0.11

53.02 0.14 0.22

54.01 0.12 0.17

54.02 0.42 0.62

54.03 0.68 1.03

54.04 0.98 1.49

55.01 0.16 0.23

56.01 0.09 0.13

56.02 0.19 0.29

57.01 0.07 0.12

57.02 0.16 0.25

58.01 0.03 0.04

59.01 0.08 0.12

60.01 0.10 0.16

60.02 0.30 0.46

60.03 0.42 0.64

61.01 0.01 0.02

61.02 0.10 0.15

62.01 0.07 0.10

63.01 0.03 0.05

63.02 0.20 0.29

63.03 0.32 0.47

63.04 0.57 0.84

63.05 0.65 0.96

63.06 0.82 1.20

64.01 0.17 0.26

Cal Ver




65.01 0.08 0.13

66.01 0.04 0.05

66.02 0.11 0.17

66.03 0.16 0.24

67.01 0.02 0.04

67.02 0.15 0.23

67.03 0.25 0.38

67.04 0.50 0.76

67.05 0.69 1.05

67.06 0.80 1.18

67.07 1.32 1.95

67.08 1.77 2.55

67.09 1.89 2.70

67.10 2.13 3.05

67.11 2.70 3.83

68.01 0.14 0.22

69.01 0.13 0.19

70.01 0.04 0.05

71.01 0.07 0.12

71.02 0.15 0.23

71.03 0.33 0.50

72.01 0.07 0.11

73.01 0.06 0.09

74.01 0.08 0.12

74.02 0.14 0.20

74.03 0.18 0.27

75.01 0.05 0.07

76.01 0.04 0.07

76.02 0.11 0.17

77.01 0.10 0.14

78.01 0.08 0.11

79.01 0.03 0.05

80.01 0.04 0.06

80.02 0.07 0.11

80.03 0.15 0.22

81.01 0.08 0.12

81.02 0.34 0.50

82.01 0.11 0.17

83.01 0.07 0.11

84.01 0.03 0.04

85.01 0.12 0.18

86.01 0.08 0.13

87.01 0.06 0.09

88.01 0.04 0.06

88.02 0.13 0.20

88.03 0.19 0.29

88.04 0.38 0.58

Cal Ver




88.05 0.72 1.08

88.06 1.21 1.75

88.07 1.26 1.81

88.08 1.40 1.98

88.09 1.57 2.20

88.10 1.75 2.28

88.11 1.52 1.85

88.12 1.65 1.92

88.13 1.80 2.00

88.14 1.84 2.02

88.15 2.00 2.12

88.16 2.08 2.35

88.17 2.24 2.88

89.01 0.03 0.05

89.02 0.05 0.08

90.01 0.05 0.08

91.01 0.14 0.22

92.01 0.03 0.04

92.02 0.09 0.13

93.01 0.05 0.08

94.01 0.09 0.13

94.02 0.17 0.25

94.03 0.23 0.34

95.01 0.05 0.08

96.01 0.07 0.10

96.02 0.11 0.16

96.03 0.15 0.22

97.01 0.03 0.05

98.01 0.05 0.07

99.01 0.02 0.03

100.01 0.06 0.11

100.02 0.11 0.19

101.01 0.06 0.09

102.01 0.03 0.05

103.01 0.03 0.04

103.02 0.05 0.07

104.01 0.06 0.09

104.02 0.11 0.16

104.03 0.16 0.25

104.04 0.20 0.29

105.01 0.09 0.12

105.02 0.12 0.18

106.01 0.06 0.10

107.01 0.05 0.08

107.02 0.11 0.16

107.03 0.16 0.24

108.01 0.09 0.14

Cal Ver




108.02 0.16 0.26

108.03 0.24 0.38

109.01 0.04 0.05

109.02 0.14 0.23

109.03 0.26 0.41

109.04 0.32 0.50

110.01 0.10 0.15

111.01 0.14 0.21

111.02 0.23 0.34

111.03 0.29 0.44

112.01 0.03 0.04

112.02 0.13 0.18

112.03 0.35 0.53

112.04 0.45 0.70

113.01 0.10 0.14

113.02 0.13 0.19

114.01 0.13 0.21

115.01 0.04 0.06

115.02 0.21 0.33

Cal Ver


APPENDIX D

BRIDGE LOSS CALCULATIONS

Representation of Bridges in TUFLOW

TUFLOW does not explicitly allow inclusion of bridge structure details, such as abutments or piers like

other hydraulic software, such as HEC-RAS. Therefore, the variation in energy losses that can be expected

through a bridge opening must be defined using a height varying loss coefficent.

This requires calculation of suitable loss coefficient values from the channel invert up to the elevation of

the underside of the bridge deck.

The following pages present the calculations that were completed to determine appropriate bridge loss

coefficients.

All calculations were completed in accordance with procedures detailed in the 'TUFLOW User Manual'

(BMT WBM, 2010) and 'Hydraulics of Bridge Waterways' (Bradley, 1978).

Introduction

Appendix - Bridge Loss Calculations_GibbergunyahTimber.xlsx 1 of 13

Prepared by: Date:Checked by: Date:

Reference: 'Hydraulics of Bridge Waterways: HDS 1' (Bradley, March 1978)

The total backwater (i.e., energy loss) coefficient is calculated as:

K* = K b + K p + K e + K s

First need to calculate the Bridge Opening Ratio (M) All flow contained in channel

M = Unimpeded Flow / Total Flow Unimpeded Flow = 3.1 m3/s

M = Qb / (Qa + Qb + Qc) Total Flow= 3.1 m3/s

M = 1.00

John Street Crossing of Gibbergunyah Creek

Kb (base coefficient)

D. Tetley 3/09/2012

GibbTimber Low Flow


Abutment Type = 90o Wingwall

Kb = 0.00

Ratio of gross waterway area to pier area

J = Ap / An3 Ap = 0 m2

J = 0 An2 = 1.5 m2

Pier Type: Multi I-Beam

s = 1.00

DK = 0.00

Kp = sDK

Kp = 0.00

Kp (Pier Coefficient)

GibbTimber Low Flow


Qc = 1 m3/s

Qa = 1 m3/s

e = 0.00

Ke = 0.00

f = 0

Ke (Eccentricity Coefficient)

Ks (Skew Coefficient)

GibbTimber Low Flow


Abutment Type = (A)-Angled

theta

0

15

20

30

40

45

theta

0

15

30

45

A

B

Ks = 0.00

K* = Kb + Kp + Ke + Ks

K* = 0.00

(K*) Total Backwater Coefficient

Notes

No obstructions in low flow channel. Therefore, K* = 0.0 between 616.52 & 617.02

GibbTimber Low Flow





K* = K b + K p + K e + K s




M = 1.00



D. Tetley 3/09/2012

GibbTimber HalfFlow



Kb = 0.00

Assume fences abstruct flow similar to pier


J = Ap / An3 Ap = 0 m2

J = 0 An2 = 5 m2

Pier Type: Single Rectangular Pier

s = 1.00

DK = 0.00

Kp = sDK

Kp = 0.00


GibbTimber HalfFlow


Qc = 1 m3/s

Qa = 1 m3/s

e = 0.00

0.9

0.85

0.8

0

0.8

0.85

0.9

0.95

1

Ke = 0.00

f = 0



GibbTimber HalfFlow



40

45

theta

0

15

30

45

A

B

Ks = 0.00


K* = 0.00

Notes

No obstructions in channel. Therefore, K* = 0.0 between 617.02 & 618.02


GibbTimber HalfFlow





K* = K b + K p + K e + K s

First need to calculate the Bridge Opening Ratio (M) Some obstruction from abutments:



M = 0.98



D. Tetley 3/09/2012

GibbTimber Invert Deck



Kb = 0.03



J = Ap / An3 Ap = 0 m2

J = 0 An2 = 8.5 m2


s = 1.00

DK = 0.00

Kp = sDK

Kp = 0.00




Qc = 1 m3/s

Qa = 1 m3/s

e = 0.00

0.9

0.85

0.8

0

0.8

0.85

0.9

0.95

1

Ke = 0.00

f = 0






40

45

theta

0

15

30

45

A

B

Ks = 0.00


K* = 0.03

Notes

Minor obstruction by abutment. Therefore, K* = 0.03 between 618.02 to 618.47







K* = K b + K p + K e + K s




M = 1.00


D. Tetley 3/09/2012

Lake Alexandra Pedestrian Bridge


LakeAFootbridge LowFlow

Appendix - Bridge Loss Calculations_LakeAFootbridge.xlsx 1 of 8

Kb = 0.00


J = Ap / An3 Ap = 0 m2

J = 0 An2 = 1.88 m2


s = 1.00

DK = 0.00

Kp = sDK

Kp = 0.00





Qc = 1 m3/s

Qa = 1 m3/s

e = 0.00

Ke = 0.00

f = 0





theta

0

15

20

30

40

45

theta

0

15

30

45

Ks = 0.00


K* = 0.00

Notes








K* = K b + K p + K e + K s

First need to calculate the Bridge Opening Ratio (M) Some flow obstructed by abutments



M = 0.78



Lake Alexandra Pedestrian Bridge

D. Tetley 3/09/2012

LakeAFootbridge HalfFlow


Kb = 0.41



J = Ap / An3 Ap = 0 m2

J = 0 An2 = 6.6 m2


s = 0.94

DK = 0.00

Kp = sDK

Kp = 0.00





Qc = 1 m3/s

Qa = 1 m3/s

e = 0.00

Ke = 0.00

f = 0





theta

0

15

20

30

40

45

theta

0

15

30

45

Ks = 0.00


K* = 0.41

Notes

K* = 0.41 between 620.57 and 621.13







K* = K b + K p + K e + K s




M = 1.00

Mittagong RSL Pedestrian Bridge Crossing or Iron Mines Creek


D. Tetley 3/09/2012

RSLFoorbridge LowFlow

Appendix - Bridge Loss Calculations_RSLFootbridge.xlsx 1 of 16


Kb = 0.00


J = Ap / An3 Ap = 0 m2

J = 0 An2 = 1.88 m2


s = 1.00

DK = 0.00

Kp = sDK

Kp = 0.00




Qc = 1 m3/s

Qa = 1 m3/s

e = 0.00

Ke = 0.00

f = 0






theta

0

15

20

30

40

45

theta

0

15

30

45

A

B

Ks = 0.00


K* = 0.00


Notes







K* = K b + K p + K e + K s




M = 1.00


Mittagong RSL Pedestrian Bridge Crossing or Iron Mines Creek

D. Tetley 3/09/2012

RSLFoorbridge HalfFlow



Kb = 0.00



J = Ap / An3 Ap = 0 m2

J = 0 An2 = 6.6 m2


s = 1.00

DK = 0.00

Kp = sDK

Kp = 0.00




Qc = 1 m3/s

Qa = 1 m3/s

e = 0.00

Ke = 0.00

f = 0






theta

0

15

20

30

40

45

theta

0

15

30

45

A

B

Ks = 0.00


K* = 0.00

Notes

No obstructions in channel. Therefore, K* = 0.0 between 612.60 and 613.37




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