LAKE MACQUARIE WATERWAY FLOOD STUDY LAKE MACQUARIE CITY COUNCIL JUNE 2012
LAKE MACQUARIE
WATERWAY
FLOOD STUDY
LAKE MACQUARIE CITY COUNCIL
JUNE 2012
Level 2, 160 Clarence Street Sydney, NSW, 2000 Tel: 9299 2855 Fax: 9262 6208 Email: [email protected] Web: www.wmawater.com.au
LAKE MACQUARIE WATERWAY FLOOD STUDY
JUNE, 2012
Project
Lake Macquarie Waterway Flood Study
Project Number 29076
Client Lake Macquarie City Council
Client’s Representative Greg D Jones Greg Giles
Author P Wongpaibool R W Dewar
Prepared by
Date 26 June 2012
Verified by
Revision Description Date
1 1st Draft Report 24th November 2010
2 2nd
Draft Report 3rd
August 2011
3 3rd
Draft Report 8th September 2011
4 Public Exhibition 14th September 2011
5 Final Draft 24th January 2012
6 FINAL 26 June 2012
LAKE MACQUARIE WATERWAY FLOOD STUDY
TABLE OF CONTENTS PAGE
FOREWORD ............................................................................................................................... i
EXECUTIVE SUMMARY ............................................................................................................ ii
1. INTRODUCTION ........................................................................................................ 1
1.1. Background ................................................................................................ 1
1.2. Objectives ................................................................................................... 2
1.3. The Flood Problem ..................................................................................... 3
1.4. Causes of Flooding ..................................................................................... 4
1.5. Previous Studies ......................................................................................... 5
1.5.1. Lake Macquarie Flood Study - 1998 ........................................................... 5
1.5.2. Tidal Prism Modelling of Lake Macquarie - 2010 ........................................ 7
1.5.3. Lake Macquarie Adaptive Response of Estuarine Shores to Sea Level Rise
– 2010 ..................................................................................................... 8
1.6. Land Use .................................................................................................... 8
1.7. The Entrance Channel ................................................................................ 9
2. AVAILABLE DATA .................................................................................................. 11
2.1. Flood Levels ............................................................................................. 11
2.1.1. Water Level Recorders ............................................................................. 11
2.1.2. Flood Levels from Debris or Other Marks ................................................. 11
2.2. Rainfall Stations ........................................................................................ 12
2.3. Flow Measurements ................................................................................. 13
2.4. Survey ...................................................................................................... 15
2.5. Flood Photographs ................................................................................... 16
2.6. Ocean Levels ............................................................................................ 16
3. APPROACH ............................................................................................................. 17
3.1. Hydrologic Model ...................................................................................... 18
3.2. Hydraulic Model ........................................................................................ 18
3.3. Calibration Events..................................................................................... 19
3.4. Design Flood Modelling ............................................................................ 19
3.5. Climate Change ........................................................................................ 19
3.6. Wind Wave Assessment ........................................................................... 19
4. OCEAN WATER ASSESSMENT ............................................................................. 22
4.1. Approach Adopted in the 1998 Lake Macquarie Flood Study -Reference 1
................................................................................................................. 22
4.2. Adopted Approach .................................................................................... 23
4.2.1. Available Tidal Data .................................................................................. 23
4.2.2. Astronomic Tides ...................................................................................... 24
4.2.3. Ocean Tidal Anomaly ............................................................................... 24
4.2.4. Wave Setup .............................................................................................. 25
4.2.5. Tidal Anomaly Analysis ............................................................................. 27
4.2.6. Summary .................................................................................................. 28
5. HYDROLOGIC MODELLING ................................................................................... 30
5.1. Watershed Bounded Network Model (WBNM) .......................................... 30
5.2. Calibration ................................................................................................ 30
6. HYDRAULIC MODELLING ...................................................................................... 32
6.1. TUFLOW .................................................................................................. 32
6.2. Calibration ................................................................................................ 32
6.3. Design ...................................................................................................... 33
6.3.1. Critical Duration Analysis .......................................................................... 33
6.3.2. Approach for Coincidence of Rainfall and Ocean Levels .......................... 34
6.3.3. Sensitivity Analysis - Varying Ocean Levels.............................................. 35
6.3.4. Variation in Starting Water Level for Design Analysis ............................... 36
6.3.5. Probable Maximum Flood ......................................................................... 37
7. CLIMATE CHANGE ASSESSMENT ........................................................................ 38
7.1. Background .............................................................................................. 38
7.2. Rainfall and Ocean Dominated Flooding ................................................... 39
7.3. Increase in Average Lake Water Level ..................................................... 40
7.4. Flood Extent Mapping ............................................................................... 41
8. REVIEW OF STORM SURGE, WAVE SETUP AND WAVE RUNUP ....................... 42
8.1. Effect of Climate Change on Storm Surge ................................................ 42
8.1.1. Ocean Storm Surge .................................................................................. 42
8.1.2. Lake Storm Surge ..................................................................................... 43
8.2. Local Wind Wave Runup .......................................................................... 44
8.2.1. Design Wind Speeds ................................................................................ 44
8.2.2. Design Wave Climate ............................................................................... 46
8.2.3. Foreshore Profiles and Wave Runup ........................................................ 46
8.2.4. Design Wave Runup and Water Level ...................................................... 47
8.2.5. Results from the 1998 Lake Macquarie Flood Study - Reference 2 .......... 47
9. ACKNOWLEDGEMENTS ...................................................................................... 49
10. REFERENCES ...................................................................................................... 50
LIST OF APPENDICES APPENDIX A Glossary of Terms APPENDIX B Flood Extent Mapping
LIST OF FIGURES Figure 1: Study Area Figure 2: Water Levels Marmong Point Figure 3: Water Levels Belmont Figure 4: Water Levels Swansea Figure 5: Jigadee Creek Gauge Figure 6: Water Levels February 1990 Figure 7: Water Levels June 2007 Figure 8: Rainfall Data 2-4 February 1990 Figure 9: Rainfall Data 7-9 June 2007 Figure 10: Cumulative Rainfall 2-4 February 1990 Figure 11: Cumulative Rainfall 7-9 June 2007 Figure 12: Flood Photographs Figure 13a: Port Stephens Tidal Analysis Figure 13b: Port Stephens Tidal Anomalies Figure 14: Tides and Critical Duration Analysis Figure 15: Model Calibration February 1990 Figure 16: Model Calibration June 2007 Figure 17: Sensitivity Analysis Tides Figure 18: Summary of Design Lake Levels Figure 19a: Climate Change: Hydrographs Figure 19b: Climate Change: Estuary Profiles Figure 20: Assessment of Climate Change: Increased Rainfall Figure 21: Assessment of Climate Change: Sea Level Rise Figure 22: Assessment of Climate Change: Combination of Sea Level Rise and Rainfall
Increase Figure 23: Assessment of Climate Change: Ocean Dominated with Sea Level Rise Figure 24: Wave Runup Levels 100 year ARI Figure 25: Wave Runup Levels- Lake Macquarie Flood Study - Part 2
LIST OF ACRONYMS
AEP Annual Exceedance Probability AHD Australian Height Datum ARI Average Recurrence Interval ALS Airborne Laser Scanning BOM Bureau of Meteorology CSIRO Commonwealth Scientific and Industrial Research Organisation HW Hunter Water Corporation IFD Intensity, Frequency and Duration of rainfall IPCC Inter-governmental Panel on Climate Change LIDAR Light Detecting and Ranging (ALS and LIDAR refer to exactly the same process of
obtaining survey) m metre m3/s cubic metres per second PMF Probable Maximum Flood TUFLOW one-dimensional (1D) and two-dimensional (2D) flood and tide simulation software
program (hydraulic computer model) WBNM Watershed Bounded Network Model (hydrologic computer model) WTP Water Treatment Plant 1D One Dimensional hydraulic computer model 2D Two Dimensional hydraulic computer model
LIST OF TABLES
Table i: Design Event Scenarios (year 2011 conditions) iii Table ii: Summary of Still Water Flood Levels iv Table iii: Summary of Design Flood Levels in Lake Macquarie vi Table 1: Lake Macquarie Waterway: Main Features.......................................................... 1 Table 2: Flood Events ...................................................................................................... 3 Table 3: Factors Affecting the Peak Lake Level ................................................................ 4 Table 4: Peak Design Levels from the 1998 Lake Macquarie Flood Study (Reference 1) ... 6 Table 5: History of the Swansea Channel ......................................................................... 9 Table 6: Water Level Recorders in the Lake Macquarie waterway ................................... 11 Table 7: Availability of Rainfall Data for each Flood Event ............................................... 12 Table 8: Continuously Read (Pluviometer) Rainfall Stations ............................................ 12 Table 9: BOM Daily Read Rainfall Stations ..................................................................... 13 Table 10: Jigadee Creek Gauge – Peak Annual Peak Water Levels and Flows ................. 14 Table 11: Factors Influencing Wave Runup Effects ........................................................... 20 Table 12: Wave Runup Effects – 100 year ARI Flood and 1 year ARI Event ...................... 21 Table 13: Peak Design Ocean Levels (1998 Lake Macquarie Flood Study - Reference 1) . 22 Table 14: Tidal Anomalies and Wave Setup (m) during March 2005 Large Wave Energy
Event ............................................................................................................... 27 Table 15: Estimated Design Ocean Peak Levels .............................................................. 29 Table 16: Adopted Manning’s “n” Values – TUFLOW model ............................................. 33 Table 17: 48 Hour Design Rainfall Intensities (mm/h)........................................................ 34 Table 18: 100 year ARI Wind Data (m/s) .......................................................................... 45
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FOREWORD
The NSW State Government’s Flood Policy provides a framework to ensure the sustainable use
of floodplain environments. The Policy is specifically structured to provide solutions to existing
flooding problems in rural and urban areas. In addition, the Policy provides a means of ensuring
that any new development is compatible with the flood hazard and does not create additional
flooding problems in other areas.
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 Councils in the discharge of their
floodplain management responsibilities.
The Policy provides for technical and financial support by the Government through four
sequential stages:
1. Flood Study
Determine the nature and extent of the flood problem.
2. Floodplain Risk Management Study
Evaluates management options for the floodplain in respect of both existing and
proposed development.
3. Floodplain Risk Management Plan
Involves formal adoption by Council of a plan of management for the floodplain.
4. Implementation of the Plan
Use of Local Environmental Plans and Development Control Plans to ensure new
development is compatible with the flood hazard. Construction of foreshore
protection works and other measures to protect existing developments.
This Lake Macquarie Waterway Flood Study constitutes a review of the first stage of the
management process, namely to update the 1998 Lake Macquarie Flood Study Part 1 and Part
2. This current study has been prepared by WMAwater for Lake Macquarie City Council and
was undertaken to include the June 2007 long weekend storm/flood event and to incorporate the
implications of predicted climate change.
The results provide the basis for the future management of flood liable lands adjacent to the
foreshores of Lake Macquarie. The study concentrates on those areas of the lake foreshore
within the boundaries of Lake Macquarie City local government area, with little emphasis on land
within the Wyong local government area.
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EXECUTIVE SUMMARY
The catchment area of the Lake Macquarie waterway to the Pacific Ocean is approximately 700
square kilometres, of which approximately 95% is within the City of Lake Macquarie. Of this
approximately 110 square kilometres (16%) is the area of the lake. The lake is approximately
22 kilometres in length and up to 8 kilometres wide, with a perimeter of approximately
170 kilometres.
The lake is surrounded by extensive rural and residential developments that value its scenic
quality as well as its commercial and recreational value. The entrance to the Pacific Ocean is by
the narrow and shallow 4 kilometre long Swansea Channel. The lake level is normally at 0.1
mAHD and average tidal fluctuations are ± 0.05m. Elevated ocean levels (high tides and storm
surge) as well as intense rainfall over the catchment cause the lake level to rise. The highest
recorded level is 1.25 mAHD in 1949, with 1.05 mAHD reached in the June 2007 long weekend
storm/flood event, and 1.00 mAHD recorded in February 1990. The June 2007 long weekend
storm/flood event and the February 1990 event were of the order of a 30 year ARI design event.
Flooding causes significant hardship (tangible and intangible damages) to the community and
for this reason Lake Macquarie City Council has undertaken a program of studies to manage
flood risks.
The present study was initiated by Lake Macquarie City Council to research and to update the
1998 Flood Study, to incorporate predicted impacts of climate change and catchment
modifications. The primary objective of the Study is to assess scenarios that would arise due to
climate change, such as an increase in rainfall and sea level rise. The study builds on the 1998
Lake Macquarie Flood Study - Parts 1 and 2, which defined design flood levels for the foreshore
area. In addition, this present study incorporates modelling of the June 2007 long weekend
storm/flood event which occurred subsequent to publication of the 1998 Flood Study.
The outcomes of this Study provide an indication of the impacts likely to occur on lake levels
due to flooding and climate change.
This report does not consider the effects of flooding due to a tsunami.
Reasons for Updating the Hydraulic Modelling Approach
The main reasons for updating the hydraulic modelling approach are as follows:
availability of a two dimensional (2D) hydraulic model,
availability of detailed bathymetric data to better describe the bed of the Swansea
channel rather than the use of cross sections used previously,
availability of Airborne Laser Scanning (ALS) or LiDAR (Light Detecting and
Ranging) survey data that provides a very accurate definition of the topography of
the floodplain,
a more detailed appraisal of design ocean level conditions,
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incorporation of data for the June 2007 long weekend storm/flood event in the
calibration process,
incorporation of predicted sea level rises in ocean boundary conditions and lake
still water conditions, and
incorporation of an approach based on the maximum of an ocean dominated
event and a runoff dominated event.
Adopted Hydraulic Modelling Approach
The adopted approach was to establish a TUFLOW hydraulic model based on the available
bathymetric and ALS survey with inflows from a WBNM hydrologic model. A
calibration/verification was undertaken to the February 1990 and the June 2007 long weekend
storm/flood events.
The model was then used for design flood estimation with sensitivity analysis undertaken to
determine the impacts of various model parameters.
Coincidence of Ocean Levels and Runoff
Flood levels on the Lake Macquarie foreshore are affected by runoff from the surrounding
catchments into the lake as well as inflows from the Pacific Ocean, via Swansea Channel during
elevated ocean levels. Elevated ocean levels occur due to a combination of tides (the ocean’s
high tide varies from approximately 0.5 m to 1.1 mAHD during the year) and what are known as
ocean anomalies. The main components of ocean anomalies (difference between the predicted
or astronomical tide and the recorded tide) are storm surge and wave setup. Together these
components can raise ocean levels by up to 1 metre.
As part of the Study, ocean anomalies were investigated and two runoff/ocean design scenarios
were adopted. A design ocean event in conjunction with a similar or smaller magnitude rainfall
event (termed an ocean-dominated event) and a design rainfall event in conjunction with a
similar or smaller magnitude ocean event (termed a rainfall dominated event). A summary of
the design scenarios is provided in Table i).
Table i: Design Event Scenarios (Year 2011 conditions)
OCEAN DOMINATED DESIGN
EVENT
(ARI)
RAINFALL DOMINATED
Peak Design Ocean
Level + Wave Setup
(mAHD)
Co incident Design
Rainfall Event
(ARI)
Co incident Design
Ocean Event
(ARI)
Co incident Design
Ocean Level + Wave
Setup (mAHD)
2.18 100 year PMF 100 year 1.70
1.80 100 year 500 year 100 year 1.70
1.75 100 year 200 year 100 year 1.70
1.70 20 year 100 year 20 year 1.63
1.67 20 year 50 year 20 year 1.63
1.63 20 year 20 year 20 year 1.63
1.41 10 year 10 year 10 year 1.41
1.38 5 year 5 year 5 year 1.38
1.30 2 year 2 year 2 year 1.30
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The following conditions were adopted for the design flood analysis:
0.1 mAHD initial water level in the Lake Macquarie waterway, which is the average Lake
water level,
48 hour critical rainfall storm duration inflows (for all design events except the probable
maximum flood {PMF}) in conjunction with the respective ocean tides as shown in Table
i),
design ocean levels based on the design levels in Fort Denison/Sydney Harbour plus a
wave setup component (0.2 m assumed for the 100 year ARI event),
all design tides assume the “shape” of the tidal hydrograph of the May 21st to 27th 1974
East Coast Low event (approximately 160 hours with the peak at 110 hours) as
recorded at Fort Denison in Sydney Harbour. This tidal hydrograph approximates the
100 year ARI design ocean event,
the wave setup component was assumed as 0 m at time zero and was increased
linearly to peak at the same time as the ocean peak (time 110 hours). Thereafter it
decreased linearly to 0 m at time 160 hours,
the peak ocean level was coincided with the peak rainfall burst in the 48 hour duration
event.
Design Flood Approach
A approach was adopted which assumed the maximum of an ocean dominated event and a
rainfall dominated event. The results indicated that, downstream of the Swansea Bridge the
ocean dominated event produces the higher water level but, upstream the runoff dominated
event produces the higher water level. The adopted design flood levels for the lake are provided
in Table ii below, together with a comparison with those adopted previously.
Table ii: Summary of Still Water Flood Levels
Event
(ARI) Still Water Level on the Lake
Macquarie foreshore
(excludes wave runup in the lake)
Still Water Level downstream of
Swansea Bridge
Year 2011 OLD
(mAHD)
Year 2011 NEW
(mAHD)
Year 2011 Difference
(m)
Year 2011 OLD
(mAHD)
Year 2011 NEW
(mAHD)
Year 2011 Difference
(m)
PMF/extreme 2.63 2.45 -0.18 2.01 2.06 +0.05
500 year n/c 1.87 n/c 1.69
200 year n/c 1.69 n/c 1.64
100 year 1.38 1.50 +0.12 1.67 1.57 -0.10
50 year 1.24 1.38 +0.14 1.64 1.54 -0.10
20 year 0.97 1.23 +0.26 1.49 1.50 +0.01
10 year n/c 0.94 n/c 1.27
5 year n/c 0.82 n/c 1.24
2 year n/c 0.65 n/c 1.15
Notes: n/c = not calculated previously
The previous maximum estimated flood level was termed “extreme” rather than a PMF as a rigorous PMF analysis was not undertaken.
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The main reason that the levels have changed is because of different assumptions regarding the
peak ocean levels and the joint co-incidence of ocean and rainfall events. Changes to the 100
year, 50 year and 20 year ARI levels range from increases of 0.12 m to 0.26 m but in the PMF
there is a reduction of 0.18 m. Re-modelling of design events will always produce minor
changes to flood levels due to the different approaches and models employed.
In addition to inundation due to runoff/ocean levels there is another form of inundation resulting
from waves running up the foreshore (wave runup). This is where waves (caused by wind
acting on the water surface of the lake) break and “runup” the foreshore. Part 2 of the 1998
Lake Macquarie Flood Study investigated the effects of wave runup at 48 locations. The results
indicate that wave runup may increase the still water design lake levels by up to 1 m (average of
0.3 m for the 100 year ARI event). Wave runup heights are highly site specific as they are
affected by local wind speed and fetch (the length of lake across which the wind is blowing) and
local bathymetry and topography.
Climate Change
A worldwide anthropomorphic climate change is projected to raise sea levels and increase
rainfall intensities. The NSW Government has introduced a set of guidelines for the assessment
of raised sea levels and increases in rainfall intensities. As a result, the following climate
change scenarios were analysed for the 5 year, 20 year and 100 year ARI events (results can
be interpolated for intermediate events).
Rainfall Dominated flooding: increase in design rainfall intensities of 10%, 20% and
30%,
Rainfall Dominated flooding: increase in design sea levels of 0.4 m and 0.9 m. Sea
level rise scenarios assume that the initial water level in the lake rises by a similar amount
to the sea level rise, thus for a 0.4 m sea level rise the initial water level in the lake
increases from 0.1 m to 0.5 mAHD,
Rainfall Dominated flooding: combination of increase in design rainfall (10%, 20% and
30%) and increase in design sea levels (0.4 m and 0.9 m),
Ocean Dominated flooding: increase in design sea levels of 0.4 m and 0.9 m.
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Table iii) provides a summary of the design flood levels due to projected sea level and rainfall
increases.
Table iii: Summary of Design Flood Levels in Lake Macquarie
Peak Lake Level (mAHD)
Sea Level Rise Rainfall Increase
Event (ARI)
Existing + 0.4m + 0.9m 10% 20% 30%
2 year 0.65 1.04 1.54 0.71 0.77 0.83
5 year 0.82 1.21 1.71 0.88 0.94 1.00
10 year 0.94 1.32 1.81 1.03 1.11 1.19
20 year 1.23 1.61 2.10 1.32 1.40 1.49
50 year 1.38 1.74 2.20 1.50 1.61 1.72
100 year 1.50 1.86 2.32 1.62 1.73 1.84
200 year 1.69 2.05 2.51 1.81 1.92 2.03
500 year 1.87 2.23 2.69 1.99 2.10 2.21
PMF 2.45 2.81 3.27 2.57 2.68 2.79
Note: Underlined levels have been derived by interpolation from model results rather than actual modelling
A summary of the results are:
The effect of rainfall increase varies depending upon the size of the event. At the 5 year
ARI level a 10% rainfall increase approximates a 0.06 m increase in the peak lake water
level while at the 100 year ARI level the increase in rainfall intensity approximates a 0.12
m increase in the peak lake water level.
The effect of a sea level rise varies depending upon the size of the event. At the 5 year
ARI level a 0.4 m sea level rise approximates a 0.39 m increase in the peak water level
while at the 100 year ARI level the increase approximates a 0.36 m increase.
Results for a combined sea level and rainfall increase for the rainfall dominated scenario
generally reflects the addition of the rainfall and sea level increases.
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1. INTRODUCTION
1.1. Background
The Lake Macquarie waterway is a saline tidal lake, with a permanently open entrance, located
in the Hunter Region of New South Wales, 95 kilometres north of Sydney and 20 kilometres
south of Newcastle (Figure 1). The main features of the lake are provided in Table 1.
Table 1: Lake Macquarie Waterway: Main Features
Total Catchment Area 700 km2
Area of Lake 110 km2 (16% of the total catchment area)
Length of Lake 22 km in a north-south direction
Width of Lake varying from 2 km to 8 km in an east-west direction
Perimeter Length 170 kilometres
Average Water Depth 8 to 9 metres
Maximum Water Depth 11 metres (near Pulbah Island)
Contributing Catchments Subcatchments (refer Figure 1) Area (km2)
Dora Creek 19, 20, 21, 22, 23,24, 25, 26, 27, 28,
29, 30, 31,32 231.2
Stony Creek 9, 10 35.7
L T Creek 11 16.4
Cockle Creek 18, 17, 15, 14, 16, 13 111.1
Mangrove Gully Creek 2 19.5
Pourmalong Creek 6 33.7
Wyee Creek 4 28.6
Lake itself 1 110.8
Subcatchment 3 3 21.1
Subcatchment 5 5 25.1
Subcatchment 7 7 21.2
Subcatchment 8 8 18.5
Subcatchment 12 12 24.8
The Lake Macquarie waterway is the largest coastal lake in eastern Australia and is surrounded
by extensive residential, commercial and industrial developments. The lake is a valuable natural
resource for the region providing commercial and recreational usage as well as being of high
scenic value. The outlet of the Lake Macquarie waterway to the Pacific Ocean is by the narrow
and shallow entrance channel at Swansea (Swansea Channel). Today it has a permanently
open entrance which has been extensively modified by man made structures (filling of the
northern embankment, dredging, sea walls).
The water level in the lake is typically at 0.1 mAHD but can rise to 0.4 mAHD following a period
of high ocean levels. Australian Height Datum (AHD) is the common national geodetic plane
approximating to mean sea level. Under average circumstances, the ocean tide (±0.5 m) has
little impact on the water level (±0.05 m) in the lake. Intense rainfall over the catchment
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combined with elevated ocean levels can raise the water level in less than 24 hours causing
significant flooding of the foreshore areas and hardship to the community.
1.2. Objectives
The key objective of this Flood Study is to develop a suitable hydrologic/hydraulic model that
can project flood and permanent inundation water levels in Lake Macquarie from rainfall, sea
level rise and storm surge. These results will be used by Lake Macquarie City Council, in
consultation with the community of Lake Macquarie City, to manage flood and permanent
inundation risks to low lying land around the Lake Macquarie waterway.
The key stages in the process are:
Undertaken a comprehensive review of the 1998 Lake Macquarie Flood Study (Part 1 –
Reference 1) and develop suitable hydrologic/hydraulic models to define flood behaviour
over the full range of design events for existing catchment conditions,
Use the hydrologic/hydraulic models to assess various climate change scenarios,
including application of the NSW Government’s sea level rise benchmarks,
Assess the potential increase in storm surge as a result of climate change and its impact
on elevated ocean levels,
Review the potential impact of climate change on the local wind/wave climate as this
affects the extent of wave runup on the foreshore,
Assess the hydraulic and hazard categories for existing and climate change conditions.
This report details the results and findings of the above investigations. The key elements
include:
a summary of available historical flood related data,
establishment of the hydrologic and hydraulic models,
calibration of the hydrologic and hydraulic models,
definition of the design flood behaviour for existing catchment conditions,
sensitivity analysis of the design flood behaviour,
assessment of the impacts of climate change on the still water and wave runup water
levels
re-definition of the flood extent and hydraulic and hazard categories mapping for existing
and climate change conditions.
A Flood Study is a technical document and not easily understood by the general public. A
glossary of flood related terms is provided in Appendix A to assist. If more explanation of terms
or a better understanding of the approach is required, type “NSW Government Floodplain
Development Manual” into an internet search engine and you will be directed to the NSW
Government web site which provides a copy of this manual and further explanation.
Flood levels given in this report relate only to the water level with the lake itself. Design water
levels in the creek systems entering the lake (Cockle Creek, Dora Creek etc.) will be higher than
those shown for the lake.
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1.3. The Flood Problem
Historical records (started in 1927) show that periodically the level of the lake has risen in
response to heavy rainfall over the catchment and/or elevated ocean levels. This has resulted
in inundation of land and occasionally of building floors. The records show that the highest
recorded level was 1.25 mAHD in 1949 (observed at Marks Point) with the most recent major
events occurring over the June 2007 long weekend (1.05 mAHD) and in February 1990 (1.00
mAHD). The June 2007 long weekend and the February 1990 events were of the order of a 30
year ARI design event according to the design levels in the 1998 Lake Macquarie Flood Study
(Reference 1). Accurate recording of lake levels has only been available since installation of the
NSW state government operated gauges at Marmong Point and Belmont in 1986 (Figures 2 and
3). A further water level recorder was installed at Swansea in 1995 (Figure 4) and due to the
closeness to the ocean, this gauge shows considerable tidal fluctuations in the water level.
The dates and approximate peak lake levels of all known significant floods are shown in Table 2.
The February 1990 and June 2007 events were both greater than a 20 year ARI event (Table 4)
and smaller than a 50 year ARI event (thus both approximately a 30 year ARI event).
Table 2: Flood Events
Date (in order of severity)
Approximate Peak Lake Level (mAHD)
18 June 1949 1.25
Easter 1946 1.20
11 June 1930 1.10
9 June 2007 1.05
2 May 1964 1.00
4 February 1990 1.00
1953 0.90
1926/27 0.80
25 February 1981 0.80
May 1974 0.80
4 March 1977 0.70
Notes: Data obtained from the 1998 Lake Macquarie Flood Study - Reference 1.
Levels are an average of several recorded heights. It is likely that several floods prior to 1970 may not have been recorded.
Water levels are also available on Jigadee Creek, a tributary to Dora Creek (Figure 5). Water
levels for the available recorders in Lake Macquarie and the tide gauge at Port Stephens for the
February 1990 and the June 2007 long weekend storm/flood events are shown on Figures 6 and
7.
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1.4. Causes of Flooding
Flooding on the Lake Macquarie foreshore may occur as a result of a combination of factors
(Table 3) including:
an elevated ocean level due to an ocean storm surge, wave setup at the entrance
and/or a high astronomic tide,
rainfall over the lake and the tributaries entering the Lake Macquarie waterway,
wind wave action causing wind setup and wave runup on the foreshore within the
lake,
a permanent rise in ocean and lake levels due to climate change.
Table 3: Factors Affecting the Peak Lake Level
Major Factors Comment
Volume of Rainfall It is the volume of runoff entering the lake and not the peak flows from the various tributaries that cause the lake level to rise. Generally this significant volume of rainfall can only be obtained from rainfall over a period of 3 to 7 days. However in the June 2007 long weekend storm/flood event the rainfall was for a period of less than 12 hours.
Size of the Entrance Channel at Swansea
The size (width and depth) of the channel controls how much water is released from the lake, as well as how much enters from the ocean.
Ocean Water Level / Sea level rise
An elevated ocean level can result from a high tide, a storm surge and an ocean wave setup, or a combination thereof. It can also alter as a result of a climate change induced sea level rise.
Local Wave Runup caused by Wind Waves on the Lake
The flood level may be raised in a local area as a result of wave runup. The amount of runup depends upon the local wind/wave climate and the foreshore profile. Little is known about this effect. The main factors affecting the wave climate are the intensity of the wind and the fetch (horizontal distance in the direction of wind over which wind waves are generated). This issue is further discussed in Section 8.
Minor Factors Comment
Initial Water Level There is little variation in the normal water level (Figure 3) except in Swansea Channel and areas close to its entry into the lake.
Antecedent Catchment Moisture Conditions
The “wetness” of the catchment prior to the rainfall event determines the volume of runoff. Generally if the catchment is “very dry” prior to the event it will “soak” up a lot of the rainfall and produce less runoff than from a “wet” catchment.
Volume of Temporary Floodplain Storage (includes the area of the lake)
As the surface area of the lake is very large (110 km2), a minor reduction
in the volume of temporary storage (filling of the floodplain) will have no significant impact upon the peak lake level.
Intensity/duration of Rainfall It is the volume of rainfall rather than the peak intensity of rainfall which is more important. Thus a longer duration of rainfall (say > 12 hours) is more likely to produce flooding rather than an intense burst for say 4 hours.
Level of Catchment Development Sealing of pervious areas (houses, roads, factories, etc.) will increase the volume of runoff. However it is considered that the present extent of development has had only a minor impact, as it represents only a small percentage of the total catchment area.
Catchment Deforestation or Other Agricultural Changes
These activities will tend to increase the volume of runoff. It is considered that these changes have had only a minor impact upon runoff volumes during floods.
Evapo-transpiration Any change in the amount of evapo-transpiration will produce only a minor change in the total runoff volume.
Wind Setup within the Lake The 1998 Lake Macquarie Flood Study (Reference 1) concluded that under average conditions a maximum increase in level of only 0.04 m would occur. This issue is further discussed in Section 8.
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One of the key considerations in modelling coastal systems is the probability of occurrence of a
combined ocean and rainfall event and the relative magnitude of both. It is considered to be
overly conservative to assume a 100 year ARI ocean event will occur concurrently with a 100
year ARI rainfall event, however there are no data available to accurately define a suitable
approach.
For this reason two scenarios were analysed: a Rainfall Dominated scenario which assumes
the design rainfall over the catchment in conjunction with a design ocean event of equal or
smaller magnitude and an Ocean Dominated scenario which assumes the design ocean event
in conjunction with the design rainfall of equal or smaller magnitude. More details on this
approach are discussed in Section 4.2.
1.5. Previous Studies
1.5.1. Lake Macquarie Flood Study - 1998
The Flood Study completed in January 1998 (References 1 and 2) by Manly Hydraulic
Laboratory was undertaken to determine flood behaviour for the 100 year, 50 year and 20 year
ARI design floods and an extreme flood event. This study determined design flood levels using
two approaches:
Still Water Design Lake Levels (Reference 1): These were obtained using a combination of
hydrologic and hydraulic computer models. The hydrologic model converts rainfall over the
catchment into stream flows. These are input into the hydraulic model which determines the
design still water lake level. The hydraulic model takes account of:
the bathymetry of the lake,
the dimensions of the Swansea entrance channel,
the complex interaction between ocean levels and outflow from the lake,
wind setup across the lake.
The models were calibrated to historical data (November 1983 gauging and May 1974, February
1990 and March 1990 floods) and the critical design storm duration was found to be 6 days (144
hours).
The adopted design levels are shown in Table 4.
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Table 4: Peak Design Levels from the 1998 Lake Macquarie Flood Study (Reference 1)
Event
(ARI)
Still Water Level
in the Lake
(excludes wave
runup effects)
(mAHD)
Still Water Level in the Swansea
Channel (approximately midway
between the bridge and the ocean)
(excludes wave runup effects)
(mAHD)
Ocean Still Water
Level (includes
storm surge only) at
the Entrance
(mAHD)
Ocean Level
(includes storm
surge and ocean
wave setup)
(mAHD)
Extreme 2.63 2.12 1.78 2.18
500 year 1.75 * 1.80 * n/c n/c
200 year 1.55 * 1.75 * n/c n/c
100 year 1.38 1.70 1.50 1.80
50 year 1.24 1.67 1.47 1.77
20 year 0.97 1.52 1.43 1.63
10 year 0.80 * 1.45 * n/c n/c
5 year 0.65 * 1.4 * 1.38 n/c
2 year 0.45 * 1.3 * n/c n/c
NOTES: * Estimated as part of this present study.
n/c not calculated. The peak levels at each location are not coincident.
These levels are referred to as still water levels in the lake as they exclude the effect of wind
and wave set up in the lake itself. However, an ocean wave set up component is included in
determination of the design ocean level. The impact of lake wind and wave set up is discussed
below.
The “1% AEP” or “100 year ARI" flood has a 1 in 100 chance of being equalled or exceeded in
any year. On a long-term average it will happen once every 100 years, but it is wrong to think it
can only happen once in a century. Because floods are random events, there is still a 1 in 100
chance of the flood occurring next year no matter what happens this year. One of the key
features of the above results is that for all design events, except the extreme flood, the peak
ocean level (i.e. includes ocean wave setup) is higher than the peak still water level in the lake.
The difference is 0.42 m in a 100 year ARI event and 0.66 m in a 20 year ARI event.
Wave Runup: The flood level at a particular location depends upon a combination of the still
water design lake levels and the effects of local wind/wave action (wave runup). The 1998 Lake
Macquarie Flood Study included a separate study (Reference 2) to examine the effects of wave
runup at 48 locations around the lake. The results indicate that wave runup may increase the
local still water design lake levels by up to 1 m (average of 0.3 m for the 100 year ARI event).
Sensitivity analyses were undertaken to determine the impacts of the following parameters on
lake water levels:
volume of rainfall runoff,
ocean level,
coincidence of rainfall and ocean high tide,
bed friction in the Swansea Channel,
bathymetry of the Swansea Channel,
tide sequence,
wind setup.
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The results indicated that the critical factor affecting the peak lake water level is the peak ocean
level and altering this level by (say) ±0.2 m results in a similar order of change in the peak lake
water level.
1.5.2. Tidal Prism Modelling of Lake Macquarie - 2010
This study (Reference 3) modelled tidal behaviour of the Lake Macquarie waterway and
examined predicted impacts due to climate change. The study focussed on the Swansea
Channel and the entrance area. The study assessed impacts likely to occur from rising sea
levels, storm surge and changes to channel morphology. A two dimensional MIKE-21
hydrodynamic model was established based on detailed current hydrographic surveys.
The model describes tidal water levels, tidal prism and the flow patterns associated with the
coast and estuary.
Morphological modelling was also included in this study. The aim of the morphological
modelling was to determine the new channel configuration that would be likely due to rising sea
levels. It is believed that sea level rise is likely to increase the rate of scouring within the
channel as a result of increased tidal velocities. The results from the study indicate that the
morphological response is tending toward erosion and this would result in a deeper channel.
The study also looked at storm surge which is predicted to increase due to climate change. The
study used existing extreme water level information to simulate storm surge events. These
synthetic storm surge levels were combined with ocean wave processes and sea level rise
levels to capture the effects of wave set up on the entrance.
Key results from the study are:
Tidal analysis indicates an increase in tidal range over the last 22 years of records,
The entire channel has experienced a net loss of sediment and that scoured sediments
have likely been deposited at either end of the channel,
Channel scour may potentially be slightly accelerated with sea level rise,
With a sea level rise of 0.91 m (year 2100 conditions) the spring tidal range is expected
to more than double with half due to sea level rise and half due to ongoing channel
scour and flushing times will be reduced from 270+ to about 170 days,
the wind setup effects in the lake were estimated as only ±0.05 m.
The study also analysed the impact of sea level rise on the 100 year ARI design ocean event.
The 100 year ARI design event assumed a peak ocean level of 1.5 mAHD based on the May
1974 event (the same as in the present study – refer Section 4) and used the hydraulic model to
estimate wave setup at the entrance.
The results produced a peak level in the lake of 1.25 mAHD but this assumed no catchment
inflows and thus is not comparable to the results in this present study. The study also analysed
the effects of climate change by the year 2100.
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These effects were assumed as:
increase in sea level of 0.91 m,
an 8% increase in storm surge (the storm surge component was increased from 0.63
m to 0.66 m),
the maximum significant wave height was increased by 15% from 9.3 m to 10.7 m,
the wind speed component was increased by 5%.
The results indicated that the 100 year ARI design ocean storm in the year 2100 (assuming the
above effects) would produce a water level of 2.35 mAHD in the lake (an increase of 1.1 m).
Again this result is not comparable to those provided in this present study as the latter has not
quantified the effects of climate change on storm surge, wave height or wind speed.
1.5.3. Lake Macquarie Adaptive Response of Estuarine Shores to Sea
Level Rise – 2010
The objective of this report (Reference 4) was to gain an appreciation of how the foreshores of
the Lake Macquarie waterway might respond to rising sea levels Ten case study locations were
examined in terms of sediment/rock material, vegetation, back beach form and profile. The
study also examined wind and wave set up. The outcome was to develop a methodology to
investigate foreshore changes to sea level rise that can be re-applied at other sites.
The study established a hydrodynamic model to investigate shoreline erosion and recession.
The model simulated the effect of larger storms on the foreshore profile and was able to look at
seabed forces generated by storm waves at each location. The model results indicate the
factors affecting the shoreline response around the Lake Macquarie waterway are:
Wave climate and near shore depth,
Vegetation,
Sediment type, and
Sediment sources and sinks.
The study concluded that it was likely that the existing Lake Macquarie shoreline would shift in
as a result of sea level rise and shoreline erosion. The extent of the inundation is dependant on
the topography, while the extent of erosion is dependant largely on the sediment type and wave
energy. Mapping potential risk from erosion is site specific and as such was not incorporated
into this plan.
1.6. Land Use
The majority of the lake perimeter is within the Lake Macquarie local government area, with
approximately 15% within Wyong Shire. Wyong Shire includes land to the south around Point
Wollstonecraft.
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The land use (within the Lake Macquarie local government area) surrounding the lake (assumed
as land below 4 mAHD) comprises the full range of planning zones provided in Local
Environmental Plan 2004 namely:
• Rural (1),
• Residential (2.1 and 2.2),
• Business (3),
• Industrial (4),
• Infrastructure (5),
• Open Space (6),
• Environmental Protection (7),
• National Park (8),
• Natural Resources (9),
• Investigation (10).
1.7. The Entrance Channel
Water levels in the Lake Macquarie waterway are dependent on ocean levels but are controlled
by the entrance channel (Swansea Channel) which connects the Lake Macquarie waterway to
the ocean. The channel is approximately 4 kilometres long and is characterised by numerous
shoals and scoured deeper areas. The entrance at Blacksmiths Point is approximately 350 m
wide (between the breakwaters). As the volume of the Lake Macquarie waterway is so large,
less than one percent is exchanged in each tidal cycle.
A brief summary of the history of the channel is provided in Table 5.
Table 5: History of the Swansea Channel
Year Event
1878 Construction of the Swansea breakwaters commence
1884 Construction of the 1st Swansea Bridge
1887 Construction of the Swansea breakwater is completed
1939 to 1996 Dredging works begin in 1939 and continue in order to improve the
navigability of the channel
1980 to 2001 Salts Bay foreshore is stabilised after a long period of recession
1996 to 2008 Dredging removes 210,000m3 from the upper reaches of the
Swansea Channel
The channel has been extensively altered by human activities notably:
ocean entrance training works (late 1800's) which removed the shoals at the
entrance, producing an increased tidal range in the lake,
construction of the 1st Swansea bridge in 1884 and reclamation of the northern
approach (late 1800's) producing a significant hydraulic restriction at this point,
construction of the 2nd Swansea bridge in 1909,
in the 1950's various dredging and reclamation activities were undertaken in the
vicinity of Elizabeth and Pelican Islands near Marks Point,
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construction of the 3rd Swansea bridge in 1955,
construction of the 4th Swansea bridge in 1980 (a duplicate bridge structure was
constructed),
more recently in 2006, stabilisation works involving placing ballast around the piers
was undertaken,
dredging of the ocean entrance channel around 1981 and more recently.
The channel has responded to natural and man-made effects through changes in the pattern of
erosion and sedimentation. These are natural phenomena which will always occur, but the
pattern and rate and change is affected by human modifications such as breakwalls, dredging,
and seawalls. Changes to the entrances to coastal lakes such as Lake Macquarie can disrupt
the natural estuarine processes and consequently cause ecological changes in the lake.
Solving one problem with man-made works tends to impact upon other areas. Management of
the estuary and lake environs must therefore consider the broader implications of any works and
their inter-relationships.
Sedimentation in the channel can potentially restrict access for deeper draft vessels. Typically
the depth at low water in the channel is around 2.5 m (requirement for larger vessels) but in
places due to shoaling it may reduce to 1.5 m restricting access. Many of the vessels that are
moored within the Lake Macquarie waterway are capable of ocean sailing as opposed to lake
sailing. According to the Roads and Traffic Authority’s web site the bridge opens about
2000 times each year, and around five to six times per day, allowing up to 4500 boat
movements annually.
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2. AVAILABLE DATA
2.1. Flood Levels
2.1.1. Water Level Recorders
The main sources of flood level data relevant to this study are the water levels recorders (Figure
1) at Marmong Point, Belmont, Swansea Channel and Jigadee Creek at Avondale, as indicated
in Table 6. The complete historical records for these gauges are shown on Figures 2 to 5 and
the record for the February 1990 and the June 2007 long weekend storm/flood events shown on
Figures 6 and 7.
The Kalang Road gauge on Dora Creek and the Stockton Creek gauge at Morisset are water
level only gauges and have not been used in this study as the hydraulic model of the Lake
Macquarie waterway does not include estimation of the water levels in the various tributary
creeks upstream of the Lake Macquarie waterway itself.
The Jigadee Creek gauge is also excluded from the hydraulic model extent of this study but is of
value as it has been “gauged” and thus a rating curve (relationship between water level and
flow) is available. Design flood levels in these tributary creeks are provided in separate studies
(such as the 1986 Dora Creek Flood Study - Reference 5).
Table 6: Water Level Recorders in the Lake Macquarie waterway
Data Available
NAME Opened February
1990 June 2007
Belmont 1986 Y Y
Marmong Point 1986 Y Y
Swansea 1996 N Y
Jigadee Creek at Avondale 1986 Y Y
Kalang Road at Dora Creek 1993 N Y
Stockton Creek at Morisset unknown N Y
Notes: Opening dates are approximate and records may be available outside those periods
2.1.2. Flood Levels from Debris or Other Marks
Apart from the water level recorders the other source of flood peaks are the surveying of flood
marks recorded during/after the flood. The accuracy of these levels will vary depending upon
the nature of the mark. A “tide mark” on a building wall or fence is probably accurate to within a
few centimetres but surveying of a “vegetative debris” mark is probably only accurate to ±0.3 m
depending upon the exact nature of the mark. These levels have not been considered in this
study due to the availability of the high quality water level gauges for both February 1990 and
the June 2007 long weekend storm/flood events.
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2.2. Rainfall Stations
Rainfall data from past flood events is required for the calibration of hydrologic models. For this
reason rainfall data has been collected from the relevant rainfall stations (Figure 1) within or
near to the catchment of the Lake Macquarie waterway for the two largest flood events (greater
than 1 mAHD) in the last 30+ years, namely:
4th February 1990,
10th June 2007.
Table 7 indicates the total number of rainfall stations (where data are available) for each flood
event and Table 8 lists the continuously read (pluviometer) stations that have data available.
The pluviometers are operated by either the Bureau of Meteorology (BOM) or Hunter Water
(HW - gauges not operating in 1990).
Table 7: Availability of Rainfall Data for each Flood Event
Type Total February 1990 June 2007
Daily 17 10 7
Continuous 16 2 14
Table 8: Continuously Read (Pluviometer) Rainfall Stations
Station Operator Station Name Data Available
February 1990
June 2007
Bureau of Meteorology Barnsley Y Y
Bureau of Meteorology Martinsville N Y
Bureau of Meteorology Mandalong N Y
Bureau of Meteorology Whitemans Ridge Y Y
Hunter Water R11-Swansea N Y
Hunter Water R14-Wallsend N Y
Hunter Water R32-Morriset/Dora Creek N Y
Hunter Water R33-Wangi Wangi N Y
Hunter Water R38-Hamilton N Y
Hunter Water R39-Kotara N Y
Hunter Water TR100-Eleebana N Y
Hunter Water TR101-Valentine N Y
Hunter Water TR102-Belmont N Y
Hunter Water TR103-Belmont N Y
Hunter Water TR104-Redhead N Y
Hunter Water TR105-Tooltaba N Y
Hunter Water TR106-Dudley N Y
Hunter Water TR107-Swansea N Y
N = data NOT available, Y = data available
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Table 9 lists the BOM daily read stations that have data available for at least one flood event.
Table 9: BOM Daily Read Rainfall Stations
Number Station Name Opened Closed
61299 Belmont WTP 1990 -
61133 Bolton Point (The Ridge Way) 1962 -
61011 Cockle Ck (Pasminco Metals) 1900 2003
61393 Edgeworth WTP 1990 -
61359 Mt Hutton (Auklet Road) 1986 2005
61377 Swansea (Catherine Street) 1987 -
61322 Toronto WTP 1972 -
61357 Mandalong 1986 -
61323 Dora Creek 1972 1993
61012 Cooranbong 1903 -
61041 Balcolyn (Bay Street) 1999 -
61406 Blacksmiths 2003 -
61376 Eraring (Payten Street) 1993 -
61382 Wyong 1993 -
The location of available rainfall gauges and cumulative rainfall totals for the February 1990 and
the June 2007 long weekend storm/flood events are shown on Figures 8 to 11.
2.3. Flow Measurements
An automatic water level recorder is located on Jigadee Creek (Gauge No: 211008) immediately
downstream of the Newport Road bridge (Figure 1). The gauge was installed in 1969 and is
operated by the Office of Environment and Heritage.
The recorded water levels for the period December 1973 to February 2010 were obtained from
Pinneena (NSW Government’s database of surface water records) and are shown on Figure 5.
The overall quality of the data is reported as being good but the data set is not complete as
some readings are missing for various reasons. The maximum annual water levels were
extracted and are provided in Table 10.
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Table 10: Jigadee Creek Gauge – Peak Annual Peak Water Levels and Flows
Year Peak Recorded Water
Level (mAHD)(1)
Peak Flow based on the Pinneena Rating Curve
(m3/s)
1974 5.40* 121
1975 5.37 118
1976 5.14 97
1977 5.49* 130
1978 5.56 138
1979 4.16 35
1980 2.92 4
1981 5.8 166
1982 4.75 67
1983 4.39 46
1984 5 86
1985 4.78 69
1986 4.24 38
1987 4.52 53
1988 4.94* 81
1989 5.03 88
1990 5.11 95
1991 3.11 19
1992 4.62* 59
1993 3.24* 8
1994 3.13 6
1995 3.19* 7
1996 3.08* 6
1997 3.59* 20
1998 4.03 27
1999 3.84 25
2000 4.03 27
2001 4.4 50
2002 4.86 73
2003 3.16 10
2004 3.88 26
2005 3.89 27
2006 5.04 84
2007 5.59* 144
2008 4.65* 60
2009 2.72* 5
Notes: Gauge zero at Jigadee Creek gauge is at 2.03 mAHD * gauge records missing in the year so possibly the annual peak was never recorded
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Figure 3 of the 1986 Dora Creek Flood Study (Reference 5) includes peak water levels for the
1977 and 1981 floods (it should be noted that the level of 6.5 mAHD quoted thereon for the
1981 event at the Jigadee Creek gauge appears to be a typographic error because Table 6 of
Reference 5 lists the peak water level as 5.81 mAHD). The available gauge records (refer to
Table 10) indicate the highest water level occurred in 1981 (February) with five other annual
peaks within 0.4 m of the 1981 peak. The June 2007 long weekend storm/flood event was the
2nd highest on record.
For calibration of a hydrologic model and to a lesser extent a hydraulic model, a recorded flow
(in m3/s) in the tributary river/creek is required. The estimated flow at a given water level is
obtained from a rating curve which provides a relationship between the known water level and
estimated flow. This relationship is derived from velocity readings (obtained from a current
meter) at a known water level and cross sectional water area (obtained by survey). Many of
these velocity readings are taken over a period of years at different water levels (termed
gaugings) and in this way, a rating curve is developed as a “line of best fit” between the
gaugings.
It is relatively easy to obtain “low flow” gaugings as small rises in water level occur frequently
and the gauging party has therefore ample opportunity to undertake them. It is much harder to
obtain “high flow” gaugings as they can only be obtained during large floods (which occur
infrequently) and it may be that the gauging party cannot get access to the site or are otherwise
engaged. Thus all rating curves have few “high flow” gaugings and there is therefore
considerable uncertainty about the flow estimates at high water levels. A graph of the gaugings
indicates how many “high flow” gaugings were undertaken and the height at which they were
taken, from this an estimate of the accuracy of the high flows can be made. Generally there are
no gaugings taken at the peak of a flood and thus the highest gaugings may be several metres
below the peak and the rating curve must be extrapolated.
Gaugings are usually taken from a bridge over the river with several velocity measurements at
various depths and distances across the river. These velocity measurements are averaged and
the flow calculated (flow {m3/s} = mean velocity {m/s}*waterway area {m2}).
The Pinneena rating curves (flow versus height relationship) including the actual gaugings for
two periods are provided as Figure 5. The differences between the two curves should not be
interpreted as there being a difference in the channel morphology at the changeover date.
Rather, rating curves are derived based on the gaugings available at the time, thus the later
rating curve is based on a different dataset to the former or different interpretation.
2.4. Survey
Airborne Laser Scanning (ALS) or LiDAR of the surrounding topography was provided by Lake
Macquarie City Council as part of this study. The ALS did not pick up “below water levels” and
the bathymetry survey was obtained from the 2010 Tidal Prism Modelling Study of Lake
Macquarie (Reference 3). These two datasets were merged together to obtain a grid of the
below water and above water topography.
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2.5. Flood Photographs
A number of flood photographs taken in the June 1949, February 1990 and June 2007 long
weekend storm/flood events are available and selected photographs are provided on Figure 12.
In the absence of the automatic water level recorders in the lake, these photographs would be a
valuable source of flood height data.
2.6. Ocean Levels
Ocean water level data are available from the Port Stephens gauge. Port Stephens is the
nearest “ocean” gauge and historical records for this gauge were obtained and used to
represent ocean conditions at the entrance to the Lake Macquarie waterway for the historical
events. The highest level recorded since 1986 at the Port Stephens gauge is 1.34 mAHD in
June 1999. The design ocean levels at Fort Denison in Sydney Harbour based on 80+ years of
record (these values are the same as used in the 1998 Lake Macquarie Flood Study -
Reference 1) are:
100 year ARI 1.50 mAHD,
50 year ARI 1.47 mAHD,
20 year ARI 1.43 mAHD,
10 year ARI 1.39 mAHD,
5 year ARI 1.38 mAHD,
1 year ARI 1.26 mAHD.
It is interesting to note that there is less than a 0.3 m difference between the 1 year ARI event
and the 100 year ARI event and the highest astronomic tide in a year reaches approximately
1.1 mAHD. No accurate estimates of ocean levels for events greater than the 100 year ARI are
available. However an indicative estimate for an extreme event is of the order of 1.8 m to 1.9
mAHD. These levels are applicable along the NSW coast where there is no wave setup
component.
The May 1974 Fort Denison (Sydney Harbour) tide is the highest on record (1.48 mAHD) and
approximates the 100 year ARI ocean level. This tide encompasses a storm surge component
of 0.5+ m and a high tide of 0.9 mAHD. Due to the gauge location in Sydney Harbour this
record does not include any wave setup component.
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3. APPROACH
The approach adopted in flood studies to determine design flood levels largely depends upon
the objectives of the study and the quantity and quality of the data (survey, flood, rainfall, flow
etc.). In the absence of an extensive historical flood record, a flood frequency approach cannot
be undertaken for the Lake Macquarie waterway and must rely on the use of design rainfalls and
establishment of a hydrologic/hydraulic modelling system. A diagrammatic representation of the
flood study process is shown below.
CA TCHMENT INFORMA TION
sub-areas
land-use
stream length
observed runoff volumes or rates
RA INFA LL DA TA
historical or design storm events
rainfall depths (Isohyets)
temporal patterns (intensity v
time)
MODEL BOUNDARY CONDITIONS
downstream ocean/tide levels
upstream inflow hydrographs
direct rainfall - lateral inflows
CA LIBRATION/VERIFICATION
Computational Modelling Software
HYDROLOGIC ANALYSIS
QUANTIFY CA TCHMENT RUNOFF
estimated flow hydrographs
HYDRAULIC
CHARACTERISTICS
topographic data
bridge/culvert details
ov erflow weir structures
define flow paths
stream roughness values
OBSERVED FLOOD
BEHAVIOUR
peak heights
stage or flow hydrographs
relative timing of ev ents
velocity estimates
general observations
COMPUTER MODEL PARAMETERS
storage-routing coefficient
rainfall losses
CA LIBRA TION/VERIFICATION
Computational Modelling
Software
QUA NTIFY FLOOD
BEHA VIOUR
flood levels
flows
velocities
HYDRA ULIC ANALYSIS
REVIEW
Diagram 1 - Flood Study Process
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3.1. Hydrologic Model
Inflow hydrographs to the Lake Macquarie waterway are required as input to the hydraulic
model. Typically in flood studies a rainfall-runoff hydrologic model (converts rainfall to runoff) is
used to provide these inflows. A range of runoff routing hydrologic models is available as
described in the 1987 edition of Australian Rainfall and Runoff (Reference 6). These models
allow the rainfall depth to vary both spatially and temporarily over the catchment and readily lend
themselves to calibration against recorded data. The Watershed Bounded Network Model
(WBNM) was adopted for the reason that it was used in the 1998 Lake Macquarie Flood Study
(Reference 1) and the 2004 Jigadee Creek Flood Study (Reference 7) and this allowed a
comparison between the results from these studies.
For the historical events (used to calibrate the TUFLOW hydraulic model) historical rainfall data
was input to the WBNM model to obtain the inflows and the WBNM model could be calibrated to
the flow data from the Jigadee Creek stream flow gauge.
3.2. Hydraulic Model
Originally it was intended to use the hydraulic model established for the previous 1998 Lake
Macquarie Flood Study (Reference 1) or an updated hydraulic model used for the 2010 Tidal
Prism Modelling Study of Lake Macquarie (Reference 3). However the former model did not
include the current bathymetry and needed to be updated and the latter model could not be
obtained for licence reasons. Thus a new hydraulic model had to be established. The
availability of high quality ALS and aerial photographic data means that the study area is
suitable for two dimensional (2D) hydraulic modelling. Various 2D software packages are
available (SOBEK, TUFLOW) and the TUFLOW package was adopted as it is widely used in
Australia.
In TUFLOW the ground topography is represented as a uniform grid with a ground elevation and
Manning’s “n” roughness value assigned to each grid cell. The size of grid is determined as a
balance between the model result definition required, the dimensions of the river channel (as a
rough guide the channel should have over 4 cells widths in order to accurately define it) and the
computer run time (depends on the number of grid cells).
The adopted approach was to establish a 40m by 40m grid TUFLOW model. The model
extends from the ocean to the upstream limit of Swansea Channel (Figure 1) with the lake itself
represented as a storage node.
By modelling historical flood events and matching the model versus the recorded data the
TUFLOW model can be “calibrated” or tuned to replicate actual flood events. This process is
critical to the success of the approach and comprises the majority of the effort in the study.
Water level recorders (Marmong Point, Belmont and Swansea) provide a continuous record of
the water level for the duration of the historical flood events and these data are used to compare
to the results from the TUFLOW model.
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3.3. Calibration Events
The choice of calibration events for flood modelling depends on a combination of the magnitude
of the flood event and the quality and quantity of available flood height data. Clearly it is
preferable to use recent events as generally they have a higher quality and quantity of data
(February 1990 and the June 2007 long weekend storm/flood events). Calibration to earlier
events (say 1949) is not possible due to the lack of pluviometer and water level data.
The quality and quantity of available data for each flood event has varied considerably over the
years. For the February 1990 flood event, pluviometers at Barnsley and Whitemans Ridge were
the only gauges in existence. For the June 2007 long weekend storm/flood event, pluviometer
data were available at 18 locations within or around the catchment (Table 7).
For February 1990 water level data was available from the Belmont and Marmong Point gauges,
whilst for the June 2007 long weekend storm/flood event additional data was available from the
later installed Swansea gauge.
3.4. Design Flood Modelling
Following model establishment and calibration the following steps were undertaken:
Design tributary inflows were obtained from the WBNM hydrologic model and included
in the TUFLOW hydraulic model,
Sensitivity analysis was undertaken to assess the effect of changing model
parameters and the assumed ocean boundary conditions.
3.5. Climate Change
The calibrated hydrologic/hydraulic models were used to assess the effects of climate change
induced sea level rise (+0.4 m and +0.9 m) as well as rainfall volume increase (10%, 20% and
30%). The basis for these climate change impacts are provided in Section 7. For the sea level
rise scenarios it was assumed that the design ocean hydrograph was elevated by the magnitude
of the assumed sea level rise (i.e no change to the assumed storm surge or wave setup
component in the derivation of the design ocean hydrograph). For the rainfall increase
scenarios the design rainfalls over the catchment were increased by the nominated percentage
(10%, 20%, 30%) within the hydrologic model (i.e an increase in volume and intensity but no
change to the adopted temporal pattern or loss rates).
3.6. Wind Wave Assessment
The actual flood level at a site depends upon a combination of the still water level (determined
from the hydrologic and hydraulic modelling approach) and the effect of local wind/wave action
(wave runup). The wave runup effect at Lake Macquarie depends upon a number of interrelated
factors summarised in Table 11.
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Table 11: Factors Influencing Wave Runup Effects
General Factors
Comment
Maximum Fetch across Lake Macquarie
The length of open water used to determine the wind wave
condition (varies from 1.5 km to 9 km). Direction of Maximum Fetch
Design wind data vary depending upon the direction (by up to
20%). Approximate Offshore Water Depth
Can vary from 1 m to 5 m. This influences the breaking of the
waves.
Local Factors
Comment Offshore Beach Profile
The slope of the lake bed can vary significantly.
Foreshore Beach Profile
The slope and vegetation type influence the extent of wave
activity. Embankment or Seawall
Many locations have stone or earthen embankments. The
height, slope and location of these structures relative to the
shoreline and buildings influences the breaking waves. Location of Nearest Building
Some buildings are located on the shoreline whilst others are
over 50 m away.
The 1998 Lake Macquarie Flood Study Part 2 – Foreshore Flooding (Reference 2) uses a
“Guideline” method to combine wind setup and catchment runoff water levels to determine the
100 year ARI (and the 20 year ARI) design runup levels at the 48 locations around the foreshore
of Lake Macquarie.
The guideline method for the 100 year ARI event was to adopt the highest of either:
100 year ARI design lake level (taken as 1.38 mAHD) with an approximate 1 year ARI
wind velocity,
the 100 year ARI wind velocity with an approximate 1 year ARI design lake level of 0.4
mAHD).
The results showed that there were no locations where the second scenario (the higher wave
runup condition) produced the highest runup levels and only one location (Site 4, Bolton Point)
where the difference was less than 0.3 m and at that location there was no development likely to
be affected.
The results from the 1998 Lake Macquarie Flood Study (Reference 2) for the nominated 48 sites
around the lake indicate (for the 100 year ARI event) that the maximum increase as a result of
wave runup is 1.0 m, the minimum is 0.1 m and the average is 0.3 m, as summarised in Table
12. This table also shows the increase in local water level due to wave runup above the 1 year
ARI design lake level scenario.
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Table 12: Wave Runup Effects – 100 year ARI Flood and 1 year ARI Event
% of Sites with Runup Level below
Runup Level above 100 year ARI
(m)
Runup Level above 1 year ARI
(m)
10% 0.2 0.4
20% 0.2 0.6
30% 0.2 0.6
40% 0.2 0.6
50% 0.3 0.7
60% 0.3 0.7
70% 0.4 0.8
80% 0.5 0.9
90% 0.5 1.2
100% 1.0 1.4
The key points regarding the use of wave runup data are summarised below:
• Wave runup effects produce an increase in the design flood level (Table 12) and also
require that the structural integrity of any proposed structure be more closely examined.
• Council has adopted a 0.5 m freeboard (for setting floor levels of residential buildings)
above the 100 year ARI flood level. A significant component of this freeboard
allowance is to cater for the effects of wave runup.
• 90% of the 48 sites analysed have a wave runup effect of 0.52 m or less for the 100
year ARI scenario.
• Of the remaining 10% for the 100 year ARI scenario, at four out of the five sites the
high level is due to the effect of an existing building or structure on the foreshore. The
remaining site is at Marmong Point where the level is attributable to the particular
beach profile.
• Wave runup effects will generally only occur over a small percentage of the lake
foreshore in a given event (in the prevailing wind direction).
• The effects will vary in time and space as a result of changing foreshore profiles. This
may occur naturally (sedimentation, erosion, vegetation growth) or as a result of human
activities (construction).
• Buildings located close to the foreshore will experience the greatest wave runup impact
(increased design flood level and increased potential for structural damage). Further
away from the foreshore the impacts reduce significantly. The zone of influence of the
wave runup effect varies considerably depending upon the topography of the area. In a
relatively flat area (Swansea) the impact may be up to 50 m whilst in a steeply rising
foreshore area the impact may be 5 m or less.
• Of the factors influencing wave runup (Table 11) only three, foreshore beach profile,
embankment or seawall and location of nearest building, can possibly be modified to
reduce the impact.
Further discussion on wave runup and the likely impacts of climate change on wave runup are
provided in Section 8.
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4. OCEAN WATER ASSESSMENT
4.1. Approach Adopted in the 1998 Lake Macquarie Flood Study -
Reference 1
The 1998 Lake Macquarie Flood Study (Reference 1) used ocean entrance design hydrographs
as the downstream boundary conditions for the hydraulic model of the lake. The likely maximum
ocean entrance levels during the design flood events were determined by examining the ocean
level component parts, namely:
astronomic tide,
storm surge (barometric and wind stress effects),
wave setup at the entrance.
Table 13 lists the “still water” (tide and storm surge levels but no ocean wave setup component)
ocean levels adopted. The adopted temporal pattern of the ocean levels was the May 1974
event which is generally assumed as approximating a 100 year ARI ocean event in Sydney
Harbour.
The 1998 Lake Macquarie Flood Study (Reference 1) undertook a review of the ocean wave
setup component at the entrance to the Lake Macquarie waterway and concluded that the likely
maximum wave setup is of the order of 0.4 m in an extreme event. The adopted peak ocean
levels for design are shown in Table 13.
Table 13: Peak Design Ocean Levels (1998 Lake Macquarie Flood Study - Reference 1)
Event (ARI)
“Still Water” Tide and
Storm Surge Level
(mAHD)
Wave
Setup
(m)
Peak Ocean
Level
(mAHD)
20 year 1.43 0.2 1.63
50 year 1.47 0.3 1.77
100 year 1.50 0.3 1.80
Extreme 1.78 0.4 2.18
The adopted approach in Reference 1 for determining design flood levels in the lake assumed:
a 6 day (144 hours) critical storm duration,
an ocean hydrograph based on the temporal pattern of the May 1974 event,
the time of the peak ocean level approximates the time of peak rainfall intensity,
joint coincidence of the same design ocean and rainfall event (i.e a 100 year ARI
rainfall and 100 year ARI ocean level are coincident and a 50 year ARI rainfall and 50
year ARI ocean level are coincident etc.).
The study examined the effects of wind setup on the lake and concluded that the maximum was
less than 0.05 m in the lake. The report states “Because wind setup is small during flooding
events, and requires winds from specific directions, the impacts of winds on lake water levels
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were not included in the modelling of design flood water levels”.
4.2. Adopted Approach
The procedures and assumptions used to determine the maximum design levels in the 1998
Lake Macquarie Flood Study (Reference 1) were “standard” at the time of the assessment.
However, since 1998 further investigations and long term ocean and entrance water level data
collection and analyses have provided a much better understanding of the processes operating
at estuary entrances during storms. This is particularly the case for wave setup and the impacts
on flood levels inside entrances. As a result, the assumptions and procedures used for the 1998
Lake Macquarie Flood Study (Reference 1) are now considered to be conservative.
The following sections examine relevant available data and determine design ocean
hydrographs that better reflect the conditions applying at the entrance to the Lake Macquarie
waterway during design rainfall events.
The basic methodology for this Flood Study Review is similar to that used for the 1998 Lake
Macquarie Flood Study (Reference 1) in that the individual component parts that make up
elevated ocean levels at the Lake Macquarie waterway entrance were examined, and summed
to produce design ocean levels. An allowance for sea level rise due to climate change was
added to produce levels for the years 2050 and 2100.
The significant water level components affecting the entrance to the Lake Macquarie waterway
are:
astronomic tide,
tidal anomaly:
o storm surge (barometric and wind stress effects),
o oceanographic effects (shelf waves, ocean currents, temperature variations),
wave setup.
The latter two components are likely to be affected by predicted future climate change.
4.2.1. Available Tidal Data
The tidal record for Fort Denison in Sydney Harbour is over 125 years long. Since completion of
the 1998 Lake Macquarie Flood Study (Reference 1) the almost continuous record from 1914
has been digitised and analysed to accurately determine its astronomic and anomaly
components (1995 Harmonic Analysis of NSW Gauge Network - Reference 8). Since around
1984 there has also been accurate tidal data recorded at a number of ocean and estuary
entrance locations along the NSW coast. These data sets have been analysed by numerous
studies and provide a much improved understanding of tidal conditions and influences along the
NSW coast and inside estuary entrances (for example 1992 Mid NSW Coastal Region Storm-
Tide Surge Analysis – Reference 9).
The Fort Denison gauge, although within Sydney Harbour, is considered to be a “deep still
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water” gauge location. This basically means that the gauge records the ocean astronomic tide
plus ocean tidal anomaly components (such as storm surge and oceanographic effects) without
significant interference from non-ocean effects such as breaking or broken waves, catchment
runoff, shallow water effects, local wind shear, etc. Other “deep still water” gauge sites along
the NSW coast include Coffs Harbour, Crowdy Head, Port Stephens (Tomaree), Jervis Bay and
Batemans Bay.
In addition to the “deep still water” sites, there are also gauges located just inside estuary
entrances that respond closely to ocean conditions but are also influenced to some (varying)
extent by non-ocean effects. These gauges record the ocean astronomic tide and the ocean
tidal anomaly components, but also some wave and/or estuary effects. The Lake Macquarie
waterway (Swansea) gauge installed in 1995 is an example, as are the Hastings River (Port
Macquarie), Manning River (Harrington) and Wallis Lake (Forster) gauges.
4.2.2. Astronomic Tides
Astronomic tides are caused by the gravitational and centripedal forces between the earth and
moon, and to a lesser extent the sun and other planets. They can be predicted with accuracy
based on the harmonic movements of these bodies. Along the NSW open coast, astronomic
tides are very similar in terms of their levels and timing. There are two high and two low tides
per day, with a range of up to around 2.0 m during the summer and winter “King” tides.
Analysis of the long term tidal harmonics for Fort Denison shows that the highest astronomical
tide level is approximately 1.1 mAHD, and that a level of 0.6 mAHD is exceeded around 10% of
the time. The 0.6 mAHD level is also approximately the Mean High Water Springs tide level (the
average of two highest new moon and full moon tides).
Harmonic analyses for the other “deep still water” gauge locations along the NSW coast, as well
as many of the entrance gauge locations give very similar harmonic constituents to Fort Denison
(1995 Harmonic Analysis of NSW Gauge Network - Reference 8). This similarity shows that the
maximum astronomic tide level at these locations (including the entrance to the Lake Macquarie
waterway) is also less than 1.1 mAHD and that an astronomic tide level of 0.6 mAHD would be
exceeded around 10% of the time.
4.2.3. Ocean Tidal Anomaly
As mentioned, the ocean tidal anomaly component recorded at a “deep still water” gauge
location is made up of storm surge and oceanographic effects. This anomaly is recorded as a
variation from the predicted astronomic tide level.
The storm surge component is the increase in ocean water level that occurs during storms as a
result of inverse barometric pressure and wind stress. Barometric pressure causes a localised
rise in ocean water levels of about 0.1 m for each 10hPA drop in pressure and strong onshore
winds produce surface currents that cause a build up of water against the coastline.
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The oceanographic component of the tidal anomaly covers a range of other factors that can
affect ocean water levels. The most important of these are the shelf waves generated by large
storms remote from the NSW coast. These waves are long and low, with heights of up to 0.2 m
and periods of many days. When these waves reach the eastern continental shelf they are
confined and migrate along the coast producing elevated ocean water levels.
The size and occurrence of oceanographic effects is hard to determine accurately. However, for
the purposes of determining an ocean hydrograph for the Lake Macquarie waterway this is not
necessary, as statistically these effects are accounted for in the overall “deep still water” tidal
anomaly analysis.
An analysis of “deep still water” anomalies along the NSW coast (1992 Mid NSW Coastal
Region Storm-Tide Surge Analysis – Reference 9) found very good correlation between
anomaly levels and occurrence north and south of the Lake Macquarie waterway between
Crowdy Head and Batemans Bay. This correlation reflected the size and similarity of the
weather systems along the coast despite the more localised nature of the effects, and the
remote nature of shelf waves. As a result of the correlation it is reasonable to assume that the
tidal anomaly conditions near the entrance to the Lake Macquarie waterway would be similar to
those at Fort Denison.
Analysis of the tidal anomalies recorded at Fort Denison since 1914 shows that the maximum
“deep still water” increase is around 0.6 m (as occurred in May 1974) and that a 0.2 m level
occurs for around 5% of the time, but a 0.4 m level occurs for less than 0.1% of the time (1992
Mid NSW Coastal Region Storm-Tide Surge Analysis – Reference 9). However, there is a
correlation between a storm event capable of producing major flooding in a large catchment
such as the Lake Macquarie waterway catchment and a storm event likely to produce a large
storm surge tidal anomaly.
A major flood producing storm event is likely to last several days and be associated with very
low barometric pressure and strong onshore winds (as well as very heavy rain). Based on the
above, it is reasonable to assume that the maximum tidal anomaly (storm surge plus
oceanographic effects) would be less than 0.6 m. However, because of the strong correlation
between the flood/rainfall event and the conditions likely to produce a high storm surge, an
anomaly level of greater than 0.4 m could be expected.
4.2.4. Wave Setup
Wave setup occurs in the surf zone where the shoreward kinetic energy of the breaking and
broken waves is converted to gravitational potential energy in the form of increased water levels.
Wave setup is largely confined to the nearshore area and is highly dependent on factors such as
the wave height, wave length, water depth and embayment geometry.
Wave setup along exposed NSW beaches can be of the order of 1.5 m during very large energy
wave climate conditions, but this setup is only maintained if the wave energy remains high for a
sustained period (in excess) of an hour. Wave setup can be relieved by a lull in wave energy,
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by alongshore rips and currents and at estuary entrances. The extent of the relief is highly
dependent on the specific site conditions and the method used to calculate setup in the 1998
Lake Macquarie Flood Study (Reference 1) is still valid.
“Deep still water” locations not in the breaker zone, such as Fort Denison, Coffs Harbour,
Crowdy Head and Port Stephens (Tomaree) gauge locations have negligible wave setup
because there is no significant capacity for the waves to break and convert shoreward kinetic
energy into increased water levels. This is reflected in the correlation between the astronomic
tide predictions at these sites. However, most estuary entrance locations are exposed to ocean
waves and have shallow foreshore conditions capable of producing breaking waves under some
high energy wave climate conditions. These locations are inside the breaker zone and under
these conditions will be affected by wave setup to some extent.
The degree to which estuary entrance locations are affected by wave setup depends on the
exposure of the site and the capacity of the waves to break and produce setup. It also depends
on how quickly any setup can be relieved by flow into the estuary.
Some locations with relatively high exposure and shallow bed conditions such as the entrance to
the Manning River experience significant wave setup. Other locations with some protection but
with shallow bed conditions such as the entrance to the Lake Macquarie waterway (Swansea) or
the Hastings River (Port Macquarie) have significant setup during larger wave climate
conditions, but negligible setup during low conditions. Other, semi-protected and deep
entrances, such as the entrance to Wallis Lake, have very little wave setup under most
conditions.
Analysis was undertaken of the 22nd and 23rd March 2005 large energy wave event. The low
pressure system causing that event was centred off the coast of NSW between Sydney and
Coffs Harbour moving south to north. The central pressure dropped to 996 hectopascals and
winds were south easterly at around 35 knots. Under these conditions, a storm surge anomaly
of between 0.3 m and 0.4 m could be expected at “deep still water” gauge locations.
Table 14 sets out the tidal anomalies recorded at a number of “deep still water” gauges as well
as the tidal anomaly plus wave setup at a number of estuary entrance gauges during the event.
The table also shows an approximation of the tidal anomaly component at the estuary entrance
locations based on the adjoining “deep still water” locations, and by subtraction the resultant
wave setup component at the estuary entrances.
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Table 14: Tidal Anomalies and Wave Setup (m) during March 2005 Large Wave Energy Event
Gauge Location Gauge
Type
Maximum
Anomaly +
Setup
High Tide
Anomaly + Setup
Estimated
Storm Surge
Resultant
Wave Setup
Coffs Harbour Deep S W 0.34 0.31 0.35 0.0
Hastings River Estuary 0.65 0.33 0.40 0.25
Manning River Estuary 0.77 0.65 0.40 0.37
Wallis Lake Estuary 0.44 0.22 0.40 0.04
Port Stephens Deep S W 0.45 0.43 0.45 0.0
Hunter River Estuary 0.46 0.36 0.45 0.0
Lake Macquarie Estuary 0.50 0.16 0.40 0.10
Port Jackson Deep S W 0.29 0.28 0.30 0.0
Shoalhaven
River
Estuary 0.26 0.11 0.25 0.0
Batemans Bay Deep S W 0.27 0.12 0.25 0.0
The analysis shows that significant wave setup occurred at the Hastings River and Manning
River entrances of 0.25 m and 0.37 m respectively. Such a response is in keeping with the
wave exposure and shallow nature of the entrances. Similarly, the smaller 0.1 m results for the
Lake Macquarie waterway entrance, which is well sheltered from south easterly waves, and the
even smaller 0.04 m setup for the Wallis Lake entrance, which is both well sheltered and deep,
are as expected.
These wave setup differences were also reflected in the analysis of tidal anomalies for the years
between 1987 and 1991 (1992 Mid NSW Coastal Region Storm-Tide Surge Analysis –
Reference 9). All the estuary entrance sites show good correlation with Port Jackson during low
wave climate conditions, but the Hastings River and Lake Macquarie deviate significantly during
larger wave climate conditions.
Assuming sustained large energy wave breaking occurs across the Lake Macquarie waterway
entrance during a major storm event, there should be some wave setup at the entrance. The
level of setup would initially be partially relieved by flows into the estuary, and later by the bed
scour and the entrance rip formed by catchment outflows. However, provided the wave energy
is sufficiently large and sustained wave setup would occur. Based on the available information,
the maximum wave setup during a major flood event is unlikely to be greater than 0.2 m.
4.2.5. Tidal Anomaly Analysis
A limited tidal anomaly analysis was undertaken to determine the magnitude of the recorded
anomalies at Lake Macquarie and whether this accords with the storm surge component
assumed for the May 1974 event.
Water level data in the Swansea Channel at the mouth of the Lake Macquarie waterway have
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been recorded continuously since late 1995. It should be noted that this gauge is located at
Swansea near the bridge and thus is influenced by entrance conditions within the Swansea
Channel and is not representative of the ocean level. These data can be compared with the
“predicted” tidal data to estimate the difference in water levels resulting from any tidal anomaly.
To some extent the Swansea gauge will be influenced by elevated water levels in the Lake
Macquarie waterway, resulting from runoff from the catchment (as occurred in February 1990
and the June 2007 long weekend storm/flood events). The value of this record is limited due to
its relatively short period of operation. A much longer record is available at the Port Stephens
gauge (25 years as opposed to 15 years at Swansea).
Manly Hydraulics Laboratory (MHL) undertook a comparison of the recorded versus predicted
water levels at the Port Stephens tidal gauge to obtain the residual or anomaly (Figure 13). The
results indicated that the maximum water level recorded at Port Stephens (datum conversion of
-0.944) is 1.34 mAHD with the maximum predicted level of 1.23 mAHD approximately 0.1 m
lower than the maximum recorded levels. 75 incidences of an anomaly greater than 0.3 m were
recorded with the largest being 0.56 m (May 1997).
In conclusion, based on the limited data and analysis undertaken, the storm surge component
assumed for the May 1974 event (0.5+ m) is comparable with the maximum value recorded at
the Port Stephens gauge.
However, the highest two anomalies at Port Stephens were approximately 0.1 m greater than
the third largest and further investigation of the record for the largest anomaly (May 1997) was
undertaken. The record shows that the anomaly is not a smooth line, rather it consists of peaks
and troughs which can vary by over 0.1 m in an hour. The peak anomaly of 0.56 m is one such
peak and a more representative anomaly value for this period is 0.5 m.
4.2.6. Summary
Based on the above assessment and in conjunction with advice from the NSW Office of
Environment and Heritage the maximum ocean boundary levels as set out in Table 15 have
been determined for the Lake Macquarie waterway entrance for current and year 2050 and 2100
conditions (refer Section 7).
The levels are based on the design ocean levels at Fort Denison (Section 2.6) with the addition
of a wave setup component (the adopted amount is less than assumed in Table 12 of the 1998
Lake Macquarie Flood Study - Reference 1).
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Table 15: Estimated Design Ocean Peak Levels
Design Event
(ARI)
Ft Denison
Design
Ocean Level
(mAHD)
Wave
Setup
(m)
Peak Ocean Level
(mAHD)
(year 2011)
Peak Ocean Level
(mAHD) with 0.4m
Sea Level Rise
(year 2050)
Peak Ocean Level
(mAHD) with 0.9m
Sea Level Rise
(year 2100)
PMF/Extreme 1.78 0.4 2.18 2.58 3.08
500 year 1.60 0.2 1.80 2.20 2.70
200 year 1.55 0.2 1.75 2.15 2.65
100 year 1.50 0.2 1.70 2.10 2.60
50 year 1.47 0.2 1.67 2.07 2.57
20 year 1.43 0.2 1.63 2.03 2.53
10 year 1.41 0.0 1.41 1.81 2.31
5 year 1.38 0.0 1.38 1.78 2.28
2 year 1.30 0.0 1.30 1.70 2.20
Selected historical and design tides are shown on Figure 14.
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5. HYDROLOGIC MODELLING
5.1. Watershed Bounded Network Model (WBNM)
The WBNM hydrologic runoff-routing model was used to determine inflows from the local
catchments to the Lake Macquarie waterway. This model is widely used throughout NSW and
the model layout for the Lake Macquarie waterway is shown as Figure 1. The model input
parameters are a storage lag factor (termed C which accounts for the attenuation in the peak
flow as floodwaters travel downstream) and the rainfall initial and continuing loss.
If data are available the model can be “calibrated” to historical flow records by including the
historical rainfall data and adjusting the model parameters until a good match to the recorded
data is achieved. The main issue with this approach is the limited amount of pluviometer
records available. Pluviometer data are required to provide a temporal pattern to be applied to
the daily rainfall records. It is known that the rainfall temporal patterns can vary greatly across
even a small area and thus over these relatively large catchments the availability of only a few
pluviometers means that the resulting “accuracy” of the calibrated model is low.
5.2. Calibration
The only flow gauging records available are for Jigadee Creek (Table 9 and Figures 1 and 5).
The accuracy of the flow gauging has been investigated in the 2004 Jigadee Creek Flood Study
(Reference 7) which concluded that it was likely that the recorded flow gauging was
underestimating the flows at high flood levels. The 2004 Jigadee Creek Flood Study (Reference
7) established a WBNM and a 1D hydraulic model (MIKE-11) and undertook a joint
hydrologic/hydraulic model calibration to the recorded stage hydrographs for the February 1981
(largest on record), June 1989 and February 1990 events. This approach is the most accurate
that is possible as it relies on a hydraulic model, using surveyed cross sections, to match to the
recorded heights through adjustment of the model parameters in WBNM to determine the
necessary inflows. The adopted design model parameters in the 2004 Jigadee Creek Flood
Study (Reference 7) were:
C value = 2.3,
Initial Loss = 20 mm,
Continuing Loss = 2.5 mm/h.
These parameters were identical to those adopted in the 1998 Lake Macquarie Flood Study
(Reference 1) with the exception of an urban initial loss of 10 mm and a rural initial loss of 25
mm. For the present study the following parameters were adopted for the calibration (February
1990 and the June 2007 long weekend storm/flood events) and design events:
C value = 2.4,
Initial Loss = 10 mm,
Continuing Loss = 2.5 mm/h,
On the Lake Macquarie waterway no losses assumed.
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A comparison between the WBNM flows and those based on the Pinneena record and in the
2004 Jigadee Creek Flood Study (Reference 7) for February 1990 and the June 2007 long
weekend storm/flood event is shown on Figures 15 and 16. As noted above the WBNM flows
are much higher than the Pinneena record. The results provide a reasonable match to the data
using the rating curve obtained in the 2004 Jigadee Creek Flood Study (Reference 7). The
adopted parameters were very similar to Reference 7 and thus the model hydrographs in each
study are very similar. A slight change in the C value was adopted in this study to more closely
align the peaks and a slight reduction in the initial losses was required.
Sensitivity analysis was not undertaken to assess the impacts of varying the above hydrologic
model parameters for the following reasons:
the hydrologic\hydraulic modelling approach was calibrated in tandem to recorded
levels, thus any significant change to any of the parameters would require an
adjustment of other parameters (say Manning’s “n”) to achieve the same calibration,
the effect of varying the hydrologic model parameters (within an acceptable range) is
small given the large surface area of the lake and the significant tidal/oceanic
influence on flood levels.
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6. HYDRAULIC MODELLING
6.1. TUFLOW
A TUFLOW 2D hydraulic model was established, calibrated to historical events (February 1990
and the June 2007 long weekend storm/flood events) and used for design flood estimation.
The TUFLOW model extended from upstream of the Swansea Channel to the Pacific Ocean
(Figure 1) and was based on the same bathymetry as used in the 2010 Tidal Prism Modelling of
Lake Macquarie (Reference 3). The model covered an area of approximately 21 km2 using a
40m by 40m grid. The lake itself was modelled as a 1D storage node which had an
area/elevation relationship based on the ALS and connected to the 2D domain. Each grid cell is
assigned a ground level and a Manning’s “n” value which reflects the hydraulic roughness of the
topography.
The only hydraulic structure included in the model was the Swansea Bridge. This bridge
represents a significant hydraulic restriction due to the width of the piers, the presence of eddies
around the piers and because there are two sets of piers (see photographs below).
Aerial view of Swansea Bridge looking upstream View between bridge structures indicating
piers and other restrictions
6.2. Calibration
The calibration process was based on matching the TUFLOW results to produce the best fit to
the recorded water level data for the most recent flood events in February 1990 and the June
2007 long weekend.
The inflows from the calibrated WBNM hydrologic model were included into TUFLOW and the
model run for both events. The Manning’s “n” within the Swansea Channel was adjusted so that
the modelled stage hydrograph at the water level gauges matched the recorded hydrograph.
This was an iterative procedure and the adopted Manning’s “n” values are provided in Table 16.
However it should be noted that other combinations of hydrologic and hydraulic parameters
could produce similar results. The February 1990 event, which had two peaks, was sensitive to
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a change in Manning’s “n” value whereas the June 2007 long weekend storm/flood event which
only had one peak was less sensitive.
It was considered that there was no justification for varying the Manning’s “n” value within the 2D
area except between the channel and the floodplain.
Table 16: Adopted Manning’s “n” Values – TUFLOW model
Description February 1990, June 2007
and Design Events
Swansea Channel 0.025
General floodplain 0.080
The calibration results are provided in Figures 15 and 16.
It should be noted that the emphasis in calibration / verification of the computer models was to
find the optimal balance of model parameters (such as roughness) that gave the overall best
match to observed historic flood behaviour. This set of parameters could then be used to
estimate design flood behaviour.
For this study, there was only a very limited amount of historic flood data but it was of high
quality (from a gauge). It is likely that the channel bed has varied between 1990 and 2007 due
to erosion/sedimentation and/or dredging. Unfortunately there is no detailed information
available that allows for these changes to be included in the calibration process.
The quality of match to the peak was better for the June 2007 long weekend storm/flood event
than February 1990 with the match to the shape of the hydrographs good, particularly for the
June 2007 long weekend storm/flood event. Overall it was considered preferable to determine a
consistent set of modelling parameters and assumptions, rather than modifying the parameters
for each event as this would provide the best estimate of design flood behaviour under present
conditions.
Sensitivity analysis could be undertaken into the effects of changing the Manning’s “n” values (or
any other parameter) but as the hydraulic model is calibrated to recorded data this approach is
of limited value, as any significant change would require a re-calibration.
6.3. Design
6.3.1. Critical Duration Analysis
The Lake Macquarie waterway design inflows to the TUFLOW model were determined using the
calibrated WBNM model. Four evenly spaced design Intensity, Frequency, Duration (IFD)
locations were chosen and the closest sub catchments to these locations were assigned to that
IFD. WBNM automatically assumes an areal reduction factor based on the catchment size
(approximately 0.95).
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A range of storm durations for the 100 year ARI event were input into TUFLOW and the duration
which produced the highest lake level determined as the critical duration (a static ocean level of
0.1 mAHD was taken to eliminate issues with the coincidence of the peak ocean level and peak
flow). The results are shown on Figure 14 which indicates that the critical storm duration is the
48 hour event (design rainfalls shown on Table 17). The 36 hour event produces a similar but
slightly lower peak lake level, the 48 hour duration was adopted as it has a greater volume. This
duration is significantly shorter than the assumed 144 hour (6 day) duration adopted in the 1998
Lake Macquarie Flood Study - Reference 1 (Section 4.1). A 48 hour duration appears more
reasonable and also is compatible with the relatively rapid response (24 hours) in the June 2007
long weekend event.
Table 17: 48 Hour Design Rainfall Intensities (mm/h)
IFD Locations 2 year
ARI
5 year
ARI
10 year
ARI
20 year
ARI
50 year
ARI
100 year
ARI
1 (N) 3.24 4.25 4.86 5.64 6.70 7.51
2 (SE) 3.24 4.34 5.01 5.86 7.02 7.90
3 (S) 3.43 4.56 5.24 6.12 7.30 8.21
4 (SW) 3.50 4.78 5.58 6.61 8.00 9.09
Note: Locations represent the north, south east, south and south west parts of the catchment
The 100 year ARI 48 hour design inflow hydrographs are provided in Figure 14. These show
the peak inflows occur approximately 20 hours after the commencement of the design rainfall
event.
6.3.2. Approach for Coincidence of Rainfall and Ocean Levels
Peak water levels in the Lake Macquarie waterway result from a combination of rainfall over the
catchment and elevated ocean levels. Thus the assumed design ocean level in conjunction with
the design rainfall event over the catchment will affect the resulting design flood level in the Lake
Macquarie waterway. The approach adopted in the 1998 Lake Macquarie Flood Study
(Reference 1) was to combine the design rainfall event with the same design ocean level as
shown in Table 4 (i.e. a 100 year ARI rainfall event occurs in conjunction with a 100 year ARI
ocean event, a 50 year ARI rainfall event occurs in conjunction with a 50 year ARI ocean event
etc.).
There is no definitive combination of rainfall and ocean levels that has been universally adopted
in NSW. The former NSW Department of Environment, Climate Change and Water (now Office
of Environment and Heritage) produced the Flood Risk Management Guide, incorporating sea
level rise benchmarks in flood risk assessments in August 2010 (Reference 10) which
supersedes the approach adopted in the 1998 Lake Macquarie Flood Study (Reference 1).
According to the 2010 Flood Risk Management Guide (Reference 10) the entrance to Lake
Macquarie is a Class 2 entrance (catchments that drain directly to the ocean via trained or
otherwise stable entrances). The “default” ocean condition for this type of entrance is to assume
a 100 year design ARI ocean hydrograph with a peak of 2.6 mAHD (Figure 7.1 of Reference
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10). However, this is superseded if a site specific analysis of elevated water levels at the ocean
boundary is undertaken. The latter approach has been undertaken as part of this present study,
as reported in Section 4.
The 2010 Flood Risk Management Guide (Reference 10) recommends an approach that
combines a design ocean event with a design rainfall event and suggests the following
scenarios:
100 year ARI ocean flooding with 20 year ARI catchment flooding with coincident peaks,
20 year ARI ocean flooding with 100 year ARI catchment flooding with coincident peaks,
neap tide cycle with 100 year ARI catchment flooding with coincident peaks.
In conjunction with advice from the NSW Government Office of Environment and Heritage, the
combination of ocean and rainfall for design flood analysis as shown in Table i) in the summary
was developed. The design scenarios are defined as either Rainfall Dominated (design inflow
event) or Ocean Dominated (design ocean event) mechanisms.
The following conditions were adopted for the year 2011 design flood analysis:
0.1 mAHD initial water level in the Lake Macquarie waterway,
48 hour critical rainfall storm duration inflows (for all design events except the PMF)
in conjunction with the respective ocean tides as shown in the table above,
design ocean levels based on the design levels in Fort Denison/Sydney Harbour
plus a wave setup component,
all design tides assume the “shape” of the tidal hydrograph of the May 21st to 27th
1974 event (approximately 160 hours with the peak at 110 hours) as recorded at
Fort Denison in Sydney Harbour. This tidal hydrograph approximates the 100 year
ARI design ocean event,
the wave setup component was assumed as 0 m at time zero and was increased
linearly to peak at the same time as the ocean peak (time 110 hours). Thereafter it
decreased linearly to 0 m at time 160 hours,
the peak ocean level was coincided with the peak rainfall burst in the 48 hour
duration event.
6.3.3. Sensitivity Analysis - Varying Ocean Levels
Figure 17A indicates the effect on lake water levels of different ocean levels in combination with
the 100 year ARI 48 hour design rainfall event:
In combination with the 20 year ARI ocean event produces an increase in the peak lake
water level of approximately 0.55 m compared to with a 0.1 m static ocean water level
(this represents the “normal” water level in the lake and by assuming a static water
level excludes the influence of ocean tides),
In combination with the 20 year ARI ocean event produces an increase in the peak
water level of approximately 0.35 m compared to with the June 2007 tide,
Varying the coincidence of the peak outflow and the peak ocean level for the 100 year
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ARI 48 hour event and the June 2007 tide makes less than 0.05 m difference in the
peak water level in the lake,
The peak of the June 2007 long weekend storm/flood event was only 0.1 m lower than
the 100 year ARI 48 hour event combined with the June 2007 tide. This indicates that
the June 2007 long weekend rainfall was approaching the 100 year ARI rainfall
intensity, though a direct comparison is not possible due different storm durations and
areal distributions.
Varying the timing of the peak rainfall and ocean hydrograph can change the peak level
in the lake by approximately ± 0.05 m.
Figure 17B indicates that:
The 100 year ARI 48 hour event and the 20 year ARI ocean event produces a peak
level in the lake of 1.5 mAHD. This scenario is in accordance with the 2010 Flood Risk
Management Guide (Reference 10) and constitutes the Rainfall Dominated flood
scenario,
The 20 year ARI 48 hour event and the 100 year ARI ocean event produces a peak
level in the lake of 1.3 mAHD. This scenario is in accordance with the 2010 Flood Risk
Management Guide (Reference 10) and constitutes the Ocean Dominated flood
scenario,
Comparison of the above two flooding scenarios indicates that the Rainfall Dominated
flood scenario produces the greater flood level in the lake (1.5 mAHD) and should
therefore be adopted as the 100 year ARI design flood scenario for the lake.
Downstream of the bridge the Ocean Dominated flood scenario produces the higher
peak levels. This scenario is not considered in detail in this study but is evaluated for
climate change (Section 7),
The Rainfall Dominated peak lake level of 1.5 mAHD is 0.12 m higher than that
derived in the 1998 Flood Study (refer Table 4 - derived using the design rainfall in
combination with the 100 year ARI ocean event),
Various runs were undertaken to assess the variation in lake level for different design
rainfall and ocean event scenarios (Figure 17). The 100 year ARI 48 hour rainfall event
in combination with a 100 year, rather than a 20 year ocean event increases the lake
level by 0.05 m (approximately the difference in ocean level peak). The 100 year ARI
48 hour rainfall event in combination with the May 1974 tide results in a lake level of
1.4 mAHD. Increasing the design ocean event from the 20 year ARI to the 100 year
ARI in combination with the 20 year ARI design rainfall increases the lake level by
approximately 0.05 m.
The resulting design flood levels in the Lake Macquarie waterway, including flood levels that
incorporate sea level rises by 2050 and 2100, are shown in Table ii) and Table iii) in the
summary and in Figure 18.
6.3.4. Variation in Starting Water Level for Design Analysis
The 2011 initial water level in the lake for all design runs was taken as 0.1 mAHD and this level
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was derived from the long term average water level as shown on Figures 2 and 3. If the design
rainfall event is preceded by more rain, or a period of elevated ocean levels, the initial water
level in the lake could be greater than 0.1 mAHD. Changing the initial water level in the lake for
all the design events makes no difference to the peak level as the design events are run for over
100 hours prior to the peak rainfall burst, in this time the effect of the initial water level
dissipates. Sensitivity analysis was undertaken into the effect of varying the initial water level for
the 100 year ARI 48 hour rainfall event in combination with a much shorter duration tidal
hydrograph (the initial 2 day period prior to the start of the 48 hour design rainfall was omitted, it
is during this period that the elevated ocean levels “pump up” the lake levels). The results vary
depending upon the run duration but for a 40 hour run the results are shown below:
Peak with 0.1 mAHD initial water level = 1.30 mAHD,
Peak with 0.5 mAHD initial water level = 1.38 mAHD,
Peak with 0.7 mAHD initial water level = 1.42 mAHD.
In conclusion varying the initial water level in the Lake Macquarie waterway for the design flood
analysis makes only a slight difference to the resulting peak water level.
6.3.5. Probable Maximum Flood
The Probable Maximum Flood (PMF) was determined using the methodology provided in the
Bureau of Meteorology’s 2003 Estimation of Probable Maximum Precipitation in Australia –
(Reference 11) which indicated a critical storm duration of 6 hours. The peak outflows from the
WBNM hydrologic model (i.e not routed through the lake) are 3,220 m3/s in the 100 year ARI (48
hour duration) event and 10,250 m3/s in the PMF (6 hour) event.
The results shown in Table ii) in the Executive Summary for the Rainfall Dominated flood
scenario indicate a lower PMF level than that given in the 1998 Lake Macquarie Flood Study
(Reference 1) by approximately 0.18 m. The key reasons for this difference are due to different
ocean level assumptions, the use of a 2D hydraulic model that encompasses the entire
floodplain across the Pacific Highway and approaches for estimation of the PMF inflows (the
1998 Lake Macquarie Flood Study - Reference 1 adopted a runoff hydrograph three times the
100 year ARI event whereas this present study adopted Reference 11).
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7. CLIMATE CHANGE ASSESSMENT
7.1. Background
The 2005 NSW Government Floodplain Development Manual (Reference 12) requires that
Flood Studies and Floodplain Risk Management Studies consider the impacts of climate change
on flood behaviour.
Since completion of the 1998 Lake Macquarie Flood Study (Reference 1), current best practice
for considering the impacts of climate change (sea level rise and rainfall increase) have been
evolving rapidly. Key developments in the last four years have included:
release of the Fourth Assessment Report by the Inter-governmental Panel on Climate
Change (IPCC) in February 2007 (Reference 13), which updated the Third IPCC
Assessment Report of 2001 (Reference 14);
preparation of Climate Change Adaptation Actions for Local Government by SMEC
Australia for the Australian Greenhouse Office in mid 2007 (Reference 15);
preparation of Climate Change in Australia by CSIRO in late 2007 (Reference 16), which
provides an Australian focus on Reference 13;
release of the Floodplain Risk Management Guideline Practical Consideration of Climate
Change by the NSW Department of Environment and Climate Change in October 2007
(Reference 17 - referred to as the DECC Guideline 2007);
adoption by Lake Macquarie City Council of the Lake Macquarie Sea Level Rise
Preparedness and Adaptation Policy in August 2008 and the preparation of interim
development assessment procedures in areas vulnerable to sea level rise;
Hunter, Central and Lower North Coast Regional Climate Change Project — Report 3:
Climate Change Impact for the Hunter, Lower North Coast and Central Coast Region of
NSW (Hunter and Central Coast Regional Environmental Strategy, 2009 (Reference 18).
In October 2009 the NSW Government issued its Policy Statement on Sea Level Rise
(Reference 19) which states: “Over the period 1870-2001, global sea levels rose by 20 cm, with
a current global average rate of increase approximately twice the historical average. Sea levels
are expected to continue rising throughout the twenty-first century and there is no scientific
evidence to suggest that sea levels will stop rising beyond 2100 or that the current trends will be
reversed.
Sea level rise is an incremental process and will have medium to long-term impacts. The best
national and international projections of sea level rise along the NSW coast are for a rise relative
to 1990 mean sea levels of 40 cm by 2050 and 90 cm by 2100. However, the 4th
Intergovernmental Panel on Climate Change in 2007 also acknowledged that higher rates of sea
level rise are possible”;
In August 2010, the former NSW Department of Environment, Climate Change and Water
issued the following:
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Flood Risk Management Guide (Reference 10): Incorporating sea level rise benchmarks
in flood risk assessments,
Coastal Risk Management Guide (Reference 20): Incorporating sea level rise
benchmarks in coastal risk assessments.
In addition an accompanying document Derivation of the NSW Government’s sea level rise
planning benchmarks – October 2009 (Reference 21) provided technical details on how the sea
level rise assessment was undertaken, using peer reviewed scientific research from the IPCC,
CSIRO, BOM and other scientific agencies.
As a result of the information provided in the above and other documents, and to keep up-to-
date with current best practice, this study incorporates an assessment of climate change.
Although there are some minor variations in the sea levels predicted in these studies, policies,
and guides, they all agree on an ocean level rise on the NSW coast of around 0.9 m by the year
2100 relative to 1990 levels.
The most recent guideline, the NSW Sea Level Rise Policy Statement (2009) (Reference 19)
and associated guides, indicates a 0.9 m sea level rise by the year 2100 and a 0.4 m rise by the
year 2050. It should be noted that climate change and the associated rise in sea levels will
continue beyond 2100.
The climate change scenarios in the earlier DECC Guideline 2007 (Reference 17) suggested for
undertaking rainfall sensitivity analysis in flood studies are indicated below.
increase in peak rainfall and storm volume:
low level rainfall increase = 10%,
medium level rainfall increase = 20%,
high level rainfall increase = 30%.
A high level rainfall increase of up to 30% is recommended for consideration in the DECC
Guideline 2007 (Reference 17) due to the uncertainties associated with this aspect of climate
change and to apply the “precautionary principle”. A 30% rainfall increase is probably overly
conservative. The Hunter & Central Coast Regional Environmental Management Strategy 2009
(Reference 18) climate change study of the Hunter, for example, predicted an increase of spring
rainfall of about 15% by 2080, and a drop in the other three seasons, although this does not
predict the intensity of individual design events. A timeframe for the provision of definitive
predictions of the actual increase is unknown. The DECC Guideline 2007 (Reference 17) is
currently the only NSW reference providing guidelines for rainfall increases for design flood
analysis due to climate change.
7.2. Rainfall and Ocean Dominated Flooding
The following scenarios were modelled for the 5 year, 20 year and 100 year ARI events (results
have been interpolated for intermediate events and are provided in Table iii) in the Executive
Summary). It should be noted that the same 48 hour critical duration rainfall event adopted for
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the scenarios described in Section 6 has been adopted for all the climate change scenarios
(except for the PMF) as the next longer duration (72 hours) design event produces much lower
results for these scenario (Figure 14).
Rainfall Dominated flooding: increase in design rainfall volume of 10%, 20% and 30%,
Rainfall Dominated flooding: increase in sea level of 0.4 m and 0.9 m for the design
ocean event. All sea level rise scenarios assume that the initial water level in the lake
rises by a similar amount to the sea level rise, thus for a 0.4 m sea level rise the initial
water level increases from 0.1 m to 0.5 mAHD,
Rainfall Dominated flooding: combination of increase in design rainfall volume (10%,
20% and 30%) and increase in sea level (0.4 m and 0.9 m) for the design ocean event,
Ocean Dominated flooding: increase in the design ocean event of 0.4 m and 0.9 m.
A summary of the results shown on Figures 18 to 23 are as follows.
Figures 18 and 19: These graphs summarises the results for the 100 year ARI event
including the increase in design rainfall of 10%, 20% and 30% and the increase in
design ocean level of 0.4 m and 0.9 m.
Figure 20: This graph shows the results for the 5 year, 20 year and 100 year ARI events
with a 10%, 20% and 30% increase in design rainfall. The effect of rainfall increase
varies depending upon the size of the event. At the 5 year ARI level a 10% rainfall
increase approximates to a 0.06 m increase in peak water level while at the 100 year
ARI level the increase approximates to a 0.12 m increase.
Figure 21: This graph shows the results for the 5 year, 20 year and 100 year ARI events
with an increase in sea level of 0.4 m and 0.9 m (Rainfall dominated). The effect of a
sea level rise varies depending upon the size of the event. At the 5 year ARI level a 0.4
m sea level rise approximates a 0.4 m increase in peak water level while at the 100 year
ARI level the increase approximates a 0.35 m increase.
Figure 22: This graph shows the results for the 5 year, 20 year and 100 year ARI events
for a combined sea level and rainfall increase for the rainfall dominated scenario. In
summary the results reflect the addition of the rainfall and sea level increases.
Figure 23: This graph (as well as Figure 19) shows the results for the 5 year, 20 year
and 100 year ARI events for the ocean dominated scenario. It can be seen that this
scenario produces greater flood levels downstream of the bridge than in the lake
(because the elevated ocean level is not at its peak for long enough to elevate the entire
lake. This statement does not agree with the outcomes of the 2010 Tidal Prism
Modelling of Lake Macquarie - Reference 3 – refer Section 1.5.2). The results indicate
that flood levels downstream of the bridge are increased by a similar magnitude to the
sea level increase.
7.3. Increase in Average Lake Water Level
Sea level rise will increase the “average” water level in Lake Macquarie by a similar amount to
the sea level increase. The 2010 Tidal Prism Modelling of Lake Macquarie (Reference 3)
indicates that the tidal range may also change. The “average” water level in Lake Macquarie is
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0.1 mAHD and this will rise to 0.5 mAHD (+0.4 m) in the year 2050 and to 1.0 mAHD (+0.9 m) in
the year 2100. The available survey data is not accurate enough to map the current average
water level of 0.1 mAHD as this relies on an accurate definition of the ground/water interface
which cannot be accurately defined by ALS. Appendix B provides maps showing the extent of
inundation to the 1.0 mAHD contour (year 2100 average water level).
7.4. Flood Extent Mapping
Appendix B provides maps showing the extent of inundation in the following events:
Year 2011: existing 100 year ARI water level of 1.5 mAHD,
Year 2050: 100 year ARI water level of 1.86 mAHD (assumes sea level rise of 0.4 m),
Year 2100: 100 year ARI water level of 2.32 mAHD (assumes sea level rise of 0.9 m),
Year 2100: the “average” lake water level of 1.0 mAHD (assumes sea level rise of 0.9 m).
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8. REVIEW OF STORM SURGE, WAVE SETUP AND WAVE RUNUP
8.1. Effect of Climate Change on Storm Surge
Storm surge is the increase in ocean water level that occurs during storms as a result of the
inverse barometric pressure effect and wind stress. Together with wave setup at the mouth of
the estuary these effects cause a local raising in the ocean level. These effects have been
investigated as part of this study (Section 4.2.3) however they may be impacted by climate
change as implied in the IPCC 2007 report (Reference 13).
Storm surge affects estuaries in two ways, as an elevated ocean surge that is translated into the
estuary through the entrance and as an internally generated estuary wind setup and barometric
pressure effect.
8.1.1. Ocean Storm Surge
Ocean storm surge impacts on design water levels in Lake Macquarie were also considered as
part of the 2010 Tidal Prism Modelling Study of Lake Macquarie (Reference 3). Both this
reference and the present study adopted a year 2011 design ocean storm surge component of
0.63 m for the 100 year ARI storm event (the same as the highest recorded May 1974 storm
residual peak). Based on recommendations made by the CSIRO in their 2007 Projected
Changes in Climatological Forcing for Coastal Erosion in NSW (Reference 22), the 2010 Tidal
Prism Modelling Study of Lake Macquarie (Reference 3) increased this component by 8% to
produce a year 2100 climate change affected ocean storm surge component of 0.68 m and also
increased the assumed wave height and local wind speed (wave setup effects).
To determine the year 2100 design Lake Macquarie water level (100 year ARI level due to
ocean influence in the absence of catchment runoff effects), the 2010 Tidal Prism Modelling
Study of Lake Macquarie (Reference 3) added the 0.68 m ocean storm surge to a climate
change sea level rise of 0.91 m (a total increase of 0.96 m) and translated it into the lake using
their hydraulic model, producing a design lake level of 2.35 mAHD. This level is 1.1 m higher
than their modelled year 2011 design lake level of 1.25 mAHD. The 0.14 m additional difference
between the year 2100 level and the year 2011 level is presumed to be due to increased depth
and hence conveyance along the entrance channel resulting from climate change effects over
time in the estuary.
The present study (Table iii in the Summary) indicated that under year 2011 conditions the 100
year ARI rainfall plus 20 year ARI ocean levels produced a lake level of 1.5 mAHD, but when the
year 2100 climate change sea level rise of 0.9 m was included, the lake level reached only 2.32
mAHD. The -0.08 m difference is explained by increased conveyance in the channel but this
time the greater out flows have reduced the peak lake level.
Based on the above, it is concluded that climate change induced increases in storm intensity
could increase ocean storm surge levels by around 0.05 m. This increase in ocean level would
then increase the entrance conveyance producing a reduction in the height of rainfall dominated
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flooding in the lake and/or increase the height of ocean storm surge flooding in the lake by up to
0.15 m.
8.1.2. Lake Storm Surge
Although theoretically storm surge is caused by wind setup and inverse barometric pressure
effects, estuaries along the NSW coast are much smaller than the weather systems and so
barometric pressure gradients across an estuary are small. As a result, pressure effects cannot
develop and barometric pressure can be disregarded as part of an internal NSW estuary storm
surge assessment. However, wind setup can develop locally within the lake and needs to be
considered.
Wind setup is caused by wind drag on the water surface (wind stress) creating surface currents
that convert to water level increases against a land mass (1984 Shore Protection Manual -
Reference 23). Wind stress is proportional to the square of wind speed, thus any significant
increase in wind speed has the potential to increase wind setup. However, conversion to setup
or an increase in the local lake level requires quite restricted conditions related to shallow depths
and topography and will not develop where relieving flow paths (back flows) can form.
An investigation into design water levels and wave climate as part of the 1997 Port Stephens
Flood Study – Stage 2 (Reference 24) found that internal wind setup in that estuary was up to
around 0.5 m at some restricted bay heads but generally, wind setup levels were less than 0.05
m. The 2010 Tidal Prism Modelling Study of Lake Macquarie (Reference 3) also found that wind
setup was less than 0.05 m. It should be noted that the 1997 Port Stephens Flood Study
(Reference 24) included winds from all directions while the 2010 Tidal Prism Modelling Study of
Lake Macquarie (Reference 3) considered only wind from the southeast and associated with two
major coastal storm events, the May 1974 “Sygna” storm and the June 2007 “Pasha Bulker”
storm.
An investigation into wind setup within Lake Macquarie was also undertaken as part of the 1998
Lake Macquarie Flood Study Part 1 – Design Lake Water Levels and Wave Climate (Reference
2). However, this investigation used hourly exceedance wind speeds with occurrences
measured in days or months. As a result, the wind speeds (refer Table 18) and hence the
assessed wind setup from the 1998 Lake Macquarie Flood Study (Reference 2) is much smaller
than would be determined for say a 20 or 100 year ARI design storm event. As a result the
outcomes from the 1998 Lake Macquarie Flood Study (Reference 2) were disregarded.
The implication from the above is that although wind setup in estuaries can be substantial, the
conditions required are not usual and that this is particularly the case for Lake Macquarie where
for southeast storm conditions the foreshore morphology and water depths are generally quite
conducive to the formation of relieving flow paths and hence setup levels are low (<0.05 m).
Further, in relation to the impacts of climate change induced ocean (and lake) water level
increases, although the 1997 Port Stephens Flood Study (Reference 24) did not examine the
effects of a climate change sea level rise, it did model wind setup at different water levels. The
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modelling showed that the predominant effect of increasing water levels was to reduce wind
setup. This finding is consistent with the fact that significant local setup is difficult to generate
and that relieving flow paths are usually facilitated by increasing foreshore water depths.
In shallow restricted bays exposed to winds from the south to east (i.e southern secondary lows
and east coast lows) there theoretically could be potential for climate change increased wind
speeds to increase wind setup by the difference in the squares of the velocities. Therefore, an
increase in wind speed of say 20% could potentially increase wind setup by 44%. At a restricted
bay head this could increase setup from say 0.5 m to 0.7 m. However, generally where setup is
less than 0.05 m any increase would be minimal and even in a restricted bay any general
climate change sea level rise would increase the relieving flow path and hence reduce any
possible increase.
The critical direction to achieve maximum wind setup is likely to be from the south resulting in
increased water levels in the north and north east (Speers Point and Warners Bay). At Speers
Point the foreshore land is largely parks and at Warners Bay (North Creek catchment) the
buildings are on the landward side of The Esplanade and thus largely protected. Investigation of
the water level gauges for the June 2007 long weekend event (Figure 7) indicates that there
may have been some wind setup (as the Belmont record is lower than the Marmong Point
record).
In conclusion, wind setup may be higher than estimated in References 2 and 3 and although
climate change induced storm intensity could increase wind speeds and hence the potential for
increased wind setup in the lake it is likely that these would be small (<0.05 m) and would be
compensated for by increased flow relief. Any potential change within shallow restricted bays
with exposure to winds from the south to east may need to be assessed on a site by site basis.
8.2. Local Wind Wave Runup
The heights and periods (wave climate) of local wind waves are largely related to wind speed,
duration, fetch and water depth (1984 Shore Protection Manual - Reference 23). When waves
reach a shoaling foreshore they break and runup, potentially increasing the inundation level.
Increasing the height and/or period of a wind wave could therefore increase the runup and
inundation level. Increases in storm intensity (and hence wind speed) due to climate change
and/or sea level rise therefore have the potential to increase wave climate and inundation levels.
8.2.1. Design Wind Speeds
Table 18 provides three sources of wind data that could be used to determine wave runup levels
on the foreshore of Lake Macquarie, namely:
The 1998 Lake Macquarie Flood Study Part 1 (Reference 1) used AS 1170.2, 1989
Loading Code Part 2 Wind Loads to source 20, 50 and 100 year ARI 3 second gust wind
speeds and the 1984 Shore Protection Manual (Reference 23) to modify this data to 10
minute average gusts at 10 m height to estimate the resultant wave climate at 48
locations around the foreshore of Lake Macquarie.
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For comparison purposes the same data as used in the 1998 Lake Macquarie Flood
Study Part 1 (Reference 1) was further modified as part of this present Flood Study and
30 minute 100 year ARI wind gusts were determined.
As part of the 1997 Port Stephens Flood Study – Stage 2 (Reference 24) design water
levels and wave climate were estimated using 38 years of wind data from Williamtown
Airport (20 km north of Newcastle) and the 30 minute average maximum gust at 10 m
height for a 100 year ARI event is provided.
The final column (Lake Macquarie – Munmorah) shows the 1 hour, 1% exceedance wind speed
(this value indicates the wind speed that is exceeded 1% of the time during the year – i.e 3-4
days a year) used to determine wave setup in the 1998 Lake Macquarie Flood Study (Reference
2).
Table 18: 100 year ARI Wind Data (m/s)
Lake Macquarie Port Stephens Lake Macquarie
Direction (Ref 1)*
AS 1170.2,
10 min, 10m
(Present Study)**
AS 1170.2,
30 min, 10m
(Ref 25)
Williamtown,
30 min, 10m
Munmorah
1 hour, 1%
Exceedance
N 24 23 16 9
NE 25 23 14 10
E 24 23 16.5 11
SE 29 27 16.5 10
S 28 26 21 12
SW 28 27 25 12
W 30 29 37 10
NW 28 27 36 11
Note: * using AS 1170.2, 1989
** using AS 1170.2, 2002
Comparison of the wind data shows that there are substantial differences between the AS
1170.2 data and the Williamtown data. Reconciliation of these differences is beyond the scope
of this study, but it is relevant to note that the AS 1170.2 data used for Lake Macquarie is
generally higher and therefore more conservative than the Williamtown data used for the 1997
Port Stephens Flood Study (Reference 24). It is also relevant to note that it is only winds from
the west and northwest that are not more conservative and these are the least relevant in terms
of assessing wave runup levels as these directions have a relatively short fetch.
Based on the AS 1170.2, 1989 wind speeds adopted in the 1998 Lake Macquarie Flood Study
(Reference 1) suggest a reasonable but potentially conservative wind speeds were applied.
Analysis of Mascot (Sydney Airport) or the now longer term Williamtown wind data would
provide a more rigorous outcome.
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8.2.2. Design Wave Climate
The 1998 Lake Macquarie Flood Study Part 1 – (Reference 1) used the “Simplified Wave
Prediction Model” as set out in the 1984 Shore Protection Manual (Reference 23) to estimate the
resultant wave heights and periods at the 48 sites around the foreshore of Lake Macquarie.
However, since 1984 this approach has been significantly modified and a revised method of
“Wave Hindcasting and Forecasting” was developed in the 2002 Coastal Engineering Manual
(Reference 25). As a result of the changes (and even when using the 1984 Shore Protection
Manual - Reference 23) WMAwater were not able to fully reproduce the wave climate data
presented in the 1998 Lake Macquarie Flood Study (Reference 1). As a result, five sites
covering a range of fetch lengths and directions were examined to test the sensitivity of the
system to climate change increases in ocean/lake levels and storm intensity/wind speeds.
One of the main differences between the approaches in the 1984 Shore Protection Manual
(Reference 23) and the 2002 Coastal Engineering Manual (Reference 25) was that the influence
of shallow water effects on wind wave development was found to be far less than initially
assumed. As a result, the 2002 Coastal Engineering Manual (Reference 25) recommended that
all wind wave estimations be made on the basis of deep water wave growth.
This change in approach has significant ramifications for the wave climate calculations in Lake
Macquarie for two reasons:
it increases the likely wave heights and periods, and
it makes consideration of fetch depths irrelevant.
In relation to the first point, as mentioned above, this present study was not able to fully match
the wave climate analysis undertaken for the 1998 Lake Macquarie Flood Study (Reference 1).
The reason for this is unclear but could be because Reference 1 used shallow water
calculations, did not adjust wind speed for fetch related wind duration, contains
typographical/translational errors, etc. These have been assumed in the present analysis,
irrespectively the size of the “differences” was small (<0.2 m and <0.3 seconds).
In relation to the second point, the fact that water depth is no longer considered significant when
estimating the wave climate means that any climate change increase in ocean/lake levels would
not affect the wave heights or periods used to calculate wave runup. Note however, this does
not mean that an increase in lake level would not affect the foreshore profile and hence runup.
This is examined in the following Section.
8.2.3. Foreshore Profiles and Wave Runup
The 1998 Lake Macquarie Flood Study Part 2 – Foreshore Flooding (Reference 2) used seven
different “typical” foreshore profiles to calculate possible wave runup. Increasing the lake still
water level in response to sea level rise has the potential to change the assessment
requirements for some cross sections. Based on the available information, it was determined
that there would be no change in assessment methods for the year 2050 increase (sea level rise
of 0.4 m) but that a year 2100 increase (sea level rise of 0.9 m) would change Type 3 foreshores
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to Type 4 and Type 2 foreshores could change to Type 3, depending on the height of the
foreshore wall.
The implications of the above changes depends on the specific site conditions, but generally a
Type 3 to 4 change would marginally increase runup, while a Type 2 to a Type 3 change should
reduce runup. There are only two locations identified with Type 3 foreshores occurring at the
100 year ARI lake level and none at the 1 year ARI level. As a result it is unlikely that changes
in the foreshore type would significantly affect the wave runup levels.
8.2.4. Design Wave Runup and Water Level
As part of this present 2011 Flood Study, the height of the 100 year ARI design lake level has
increased from 1.38 mAHD to 1.50 mAHD, further reinforcing the first scenario as the design
condition. As a result, irrespective of the effect climate change increased storm intensity/wind
speeds may have on the lake wind wave climate, the only runup level that needs to be re-
examined would be the first scenario (1 year ARI runup level).
Examining the results from the 1998 Lake Macquarie Flood Study (Reference 2) for the 1 year
ARI wind wave assessment showed that the maximum significant wave height at any location
was 0.5 m and that the maximum runup was 0.5 m except at one location (Site 3, Marmong
Point) where it was 0.8 m. Four other sites indicated maximum runup levels greater than 0.5 m
but these were only where the waves would break against a building, without the building the
runup levels were within 0.5 m. As part of the present study, the wind speeds were increased by
10% and 20% to determine the sensitivity of the calculated wind wave heights and periods and
to indicate the implications of a possible climate change induced increase in wind speed. The
likely maximum affect on design runup levels for a 20% increase to the 1 year ARI 10 m/s wind
speed, resulted in an increase in the estimated significant wave height of less than 0.1 m and
the change in runup of less than 0.1 m. The only exception was at Site 3 where it was
marginally greater.
Further, as an initial check a 20% increase was applied to the 100 year ARI design wind speed
at some of the sites covering a range of site conditions. The resultant increase in wave height
was from around 15% or 0.1 m for fetch lengths around 2 km, up to around 25% or 0.3 m for
fetch lengths around 6 km. The resultant increase in period was from around 15% for fetch
lengths around 2 km and 20% for fetch lengths around 6 km. These changes would not be
sufficient for the lake plus runup levels calculated by second scenario (high wind speed) to
exceed the first scenario (high lake level).
8.2.5. Results from the 1998 Lake Macquarie Flood Study - Reference 2
The estimated increase in the 100 year ARI water level due to wave runup taken from the 1998
Lake Macquarie Flood Study (Reference 2) is shown on Figure 24. Figure 25 graphs the wave
runup levels for the two scenarios of:
the 100 year ARI design lake level with an approximate 1 year ARI runup level,
the 100 year ARI runup level with an approximate 1 year ARI design lake level.
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The results on Figure 25 indicate that the 100 year ARI runup levels are significantly higher than
those assumed for the 1 year ARI. Of issue therefore is the joint probability of the lake water
level and the wave runup level. It is generally accepted that a 100 year ARI water level in
combination with the 100 year wave runup level would imply an event of great ARI than the 100
year ARI. However there is no data available to suggest what the joint probability of these two
conditions should be for the range of design flood events.
There is obviously some joint co-incidence of these conditions, as indicated in the June 2007
long weekend event where the “Pasha Bulker” tanker was swept onto Nobby’s Beach at
Newcastle by strong winds and there was intense rainfall causing flooding. However at the time
of the peak water level in the Lake Macquarie (approximately 6am on 9th June 2007) the rainfall
and high winds had largely ceased (refer Figures 7 and 11).
The joint co-incidence of these two conditions can only be clarified as further data becomes
available. As there has only been two events for which data have been available (February
1990 and June 2007) this issue may take a considerable time to resolve and it may be prudent
to consider comparable data for say Wallis Lake and Tuggerah Lakes.
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9. ACKNOWLEDGEMENTS
This study was carried out by WMAwater and funded by Lake Macquarie City Council and the
NSW State Government. The assistance of the following in providing data and guidance to the
study is gratefully acknowledged:
Lake Macquarie City Council,
NSW Office of Environment and Heritage,
Ministry for Police and Emergency Services Department of Attorney General and
Justice,
Council’s Floodplain Management Committee,
Residents surrounding the foreshores of the Lake Macquarie waterway.
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10. REFERENCES
1. Lake Macquarie City Council
Lake Macquarie Flood Study Part 1 – Design Lake Water Levels and Wave
Climate Report
Manly Hydraulics Laboratory, Report MHL 682, January 1998
2. Lake Macquarie City Council
Lake Macquarie Flood Study Part 2 – Foreshore Flooding
Manly Hydraulics Laboratory, Report MHL 715, April 1998
3 Lake Macquarie City Council
Tidal Prism Modelling of Lake Macquarie, Volumes 1 and 2
Worley Parsons Report No. 301020- 02167- 01, September 2010
4 Lake Macquarie City Council
Lake Macquarie Adaptive Response of Estuarine Shores to Sea Level Rise
Cardno Lawson Treloar, LJ2857/R2629, June 2010
5 Public Works Department
Dora Creek Flood Study
Report No. 85019, May 1986
6 Pilgrim H (Editor in Chief)
Australian Rainfall and Runoff – A Guide to Flood Estimation
Institution of Engineers, Australia, 1987
7 Chase, Burke & Harvey
Jigadee Creek Flood Study for Proposed Development at Lot 2 DP778019
& Lot 15 DP129150
Webb McKeown & Associates Pty Ltd, July 2004
8 Department of Commerce
Harmonic Analysis of NSW Gauge Network
Manly Hydraulics Laboratory, MHL604, 1995
9 Department of Commerce
Mid NSW Coastal Region Storm-Tide Surge Analysis
Manly Hydraulics Laboratory, MHL621, 1992
10 Department of Environment, Climate Change and Water
Flood Risk Management Guide
August 2010
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11 Bureau of Meteorology
The Estimation of Probable Maximum Precipitation in Australia:
Generalised Short-Duration Method
Australian Government, 2003
12 NSW Government
Floodplain Development Manual
April 2005
13 Fourth Assessment Report “Climate Change 2007” - Synthesis Report
Intergovernmental Panel on Climate Change, 2007
14 Third Assessment Report “Climate Change 2001” - Synthesis Report
Intergovernmental Panel on Climate Change, 2001
15 Australian Greenhouse Office – Department of the Environment and Water Resources
Climate Change Adaptation Actions for Local Government
SMEC, 2007
16 Climate Change in Australia – Technical Report 2007
CSIRO, 2007
17 Floodplain Risk Management Guideline - Practical Consideration of Climate
Change
NSW Department of Environment and Climate Change (DECC), October 2007
18 Hunter, Central and Lower North Coast Regional Climate Change Project 2009 –
Report 3, Climatic Change Impact for the Hunter, Lower North Coast and Central
Coast Region of NSW
Hunter & Central Coast Regional Environmental Management Strategy, 2009
19 NSW Sea Level Rise Policy Statement
New South Wales Government, October 2009
20 Coastal Risk Management Guide
Department of Environment, Climate Change and Water NSW, August 2010
21 Department of Environment, Climate Change and Water
Derivation of the NSW Government’s Sea Level Rise Planning Benchmarks
October 2009
22 McInnes K L, Abbs D J, O’Farrell S P, Macadam I, O’Grady J, & Ranasinghe R
Projected Changes in Climatological Forcing for Coastal Erosion in NSW
DECC NSW, CSIRO Marine & Atmospheric Research, 2007
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23 Coastal Engineering Research Center
Shore Protection Manual
US Department of Army, 1984
24 Manly Hydraulics Laboratory
Port Stephens Flood Study – Stage 2
Port Stephens and Great Lakes Councils, 1997
25 US Army Corp of Engineers
Coastal Engineering Manual
US Dept of Army, 2002
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APPENDIX A: GLOSSARY of TERMS
Taken from the Floodplain Development Manual (April 2005 edition)
acid sulfate 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. For example, 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 event 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 LG 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.
development Is defined in Part 4 of the Environmental Planning and Assessment Act (EP&A
Act).
infill development: refers to the 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,
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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).
ecologically sustainable
development (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 the
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.
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 before entering a watercourse, and/or coastal
inundation resulting from super-elevated sea levels and/or waves overtopping
coastline defences excluding tsunami.
flood awareness Flood 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 an
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
probable maximum flood (PMF) event). Note that the term flood liable land covers
the whole of the floodplain, not just that part below the flood planning level (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.
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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 State Emergency Service.
flood planning area The area of land below the flood planning level and thus subject to flood related
development controls. The concept of flood planning area generally supersedes
the Aflood liable land@ concept in the 1986 Manual.
Flood Planning Levels
(FPLs)
FPL=s 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 management studies and incorporated
in management plans. FPLs supersede the Astandard flood event@ in the 1986
manual.
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 Is land susceptible to flooding by the Probable Maximum Flood (PMF) event.
Flood prone land is synonymous with flood liable land.
flood readiness Flood 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 flows, or a significant increase in flood levels.
freeboard 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.
habitable room in a residential situation: a living or working area, such as a lounge room, dining
room, rumpus room, kitchen, bedroom or workroom.
in an industrial or commercial situation: an area used for offices or to store
valuable possessions susceptible to flood damage in the event of a flood.
hazard A source of potential harm or a situation with a potential to cause loss. In relation
to this manual the hazard is flooding which has the potential to cause damage to
the community. Definitions of high and low hazard categories are provided in the
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Manual.
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 Are 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 purpose 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 0.3 m (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 flow paths 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.
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 floodplain 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
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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.
Examples are indicated in Table 2.1 with further discussion in the Manual.
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.
probability A statistical measure of the expected chance of flooding (see AEP).
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 Awater level@. Both are 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.
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.