Aire River Estuary Hydraulic Study Final Report

June 2009

Aire River Estuary Hydraulic Study

Final Report

ISO 9001 QEC22878

SAI Global

Corangamite Catchment Management Authority

Aire River Hydraulic Study

J1145 / R01v02 ii

DOCUMENT STATUS

Version Doc type Reviewed by Approved by Date issued

v01 Draft Ben Tate Warwick Bishop 25/06/09

v02 Draft Ben Tate Warwick Bishop 10/07/09

PROJECT DETAILS

Project Name Aire River Hydraulic Study

Client Corangamite Catchment Management Authority

Client project manager Rhys Collins

Water Technology project manager Steve Duggan

Report authors Steve Duggan/Ben Tate

Job number J1145

Report number R01

Document Name Rpt_J1145_Draft_Report_01.doc

Cover Photo: Aire River looking upstream from the Great Ocean Road bridge.

Copyright

Water Technology Pty Ltd has produced this document in accordance with instructions from Corangamite Catchment

Management Authority for their use only. The concepts and information contained in this document are the copyright of

Water Technology Pty Ltd. Use or copying of this document in whole or in part without written permission of Water

Technology Pty Ltd constitutes an infringement of copyright.

Water Technology Pty Ltd does not warrant this document is definitive nor free from error and does not accept liability for

any loss caused, or arising from, reliance upon the information provided herein.

15 Business Park Drive

Notting Hill VIC 3168

Telephone (03) 9558 9366

Fax (03) 9558 9365

ABN No. 60 093 377 283

ACN No. 093 377 283



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TABLE OF CONTENTS

1. Introduction .................................................................................................................. 1

2. Available Information .................................................................................................... 2

2.1 Topographic and Cadastral Survey Data ................................................................................. 2

2.1.1 Aerial Survey ............................................................................................................ 2

2.1.2 Terrestrial and Bathymetric Survey ......................................................................... 5

2.2 Hydrologic Data ....................................................................................................................... 6

2.3 Historic Estuary Level Data ...................................................................................................... 8

3. Hydrologic Analysis ........................................................................................................ 9

3.1 Overview .................................................................................................................................. 9

3.2 March/April 2002 Event .......................................................................................................... 9

3.3 February 2005 Event ............................................................................................................. 10

3.4 Discussion .............................................................................................................................. 13

4. Hydraulic Analysis ........................................................................................................ 14

4.1 Overview ................................................................................................................................ 14

4.2 Hydraulic Model Software ..................................................................................................... 14

4.3 One Dimensional Model Structure ........................................................................................ 14

4.4 Two-dimensional Model Structure ........................................................................................ 15

4.5 Linked Model Setup ............................................................................................................... 16

4.6 Results and Mapping ............................................................................................................. 17

4.6.1 March/April 2002 ................................................................................................... 17

4.6.2 February 2005 ........................................................................................................ 18

5. Conclusions and Recommendations ............................................................................... 5

APPENDIX A – MARCH/APRIL 2002 EVENT MAP SERIES ................................................................. 6



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LIST OF FIGURES

Figure 2-1 ALS Data: 1 m DEM ........................................................................................................ 2

Figure 2-2 ALS Data: 10 m DEM ...................................................................................................... 2

Figure 2-3 Statistical method results with comparison to base data ............................................. 3

Figure 2-4 Cross-section method results with comparison to base data ....................................... 4

Figure 2-5 Entire DEM Comparison ................................................................................................ 5

Figure 2-6 Terrestrial and Bathymetric Survey Components ......................................................... 6

Figure 2-7 2005 Event Pluviographic Data ..................................................................................... 7

Figure 2-8 Available Hydrologic Data ............................................................................................. 8

Figure 3-1 2002 Hydraulic Model Boundaries .............................................................................. 10

Figure 3-2 2005 Event Isohyets .................................................................................................... 11

Figure 3-3 2005 RORB model Calibration ..................................................................................... 12

Figure 3-4 2005 Hydraulic Model Boundaries .............................................................................. 12

Figure 4-1 1-Dimensional Hydraulic Model Structure .................................................................. 15

Figure 4-2 2-Dimensional Hydraulic Model Structure .................................................................. 16

Figure 4-3 1D/2D Linked Hydraulic Model Structure ................................................................... 17

Figure 4-4 March/April 2002 Estuary Filling Event: Example Depth of Inundation ....................... 1

Figure 4-5 3rd

February 2005 2:00 PM: Depth of Inundation ......................................................... 2

Figure 4-6 3rd


Figure 4-7 4th


LIST OF TABLES

Table 2-1 Streamflow Gauge Summary ........................................................................................ 6

Table 2-2 Daily Rainfall Gauge Summary ...................................................................................... 7

Table 2-3 Parks Victoria Estuary Level Data – 2005 Event ............................................................ 8

Table 3-1 2002 Event Hydrograph scaling results ......................................................................... 9

Table 3-2 2005 Event Loss Model Formulation Comparison (lower interstation area only) ...... 11

Table 4-1 Hydraulic Roughness Parameters ............................................................................... 16

Table 4-2 Hydraulic Model Validation – 2005 Event ................................................................... 18



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1. INTRODUCTION

Water Technology was commissioned by Corangamite Catchment Management Authority (CCMA) to

undertake a hydraulic assessment of the Aire River Estuary. The primary outcome of the study will

be the development of a detailed understanding of flow characteristics for the drainage lines across

the river flats and the interactions between the three lakes, the estuary mouth and the levels of

inundation within the lake system during estuary entrance closures and floods. This information will

feed into the Estuary Entrance Management Support System (EEMSS) decision support tool as a

sequence of GIS inundation layers. The improved understanding of hydraulic behaviour of the river,

floodplain, wetlands and lakes will also guide the CMA’s actions in the future long-term

management of the system.

In order to gain the necessary detailed understanding of system hydraulics, a combination of

Airborne Laser Scanning (ALS) and bathymetric survey was incorporated into a detailed two-

dimensional hydrodynamic model and dynamically linked with a one-dimensional model of in-

channel geometry. This report describes the data, methodologies, and results of the hydraulic

assessment of the Aire River estuary and floodplain.



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2. AVAILABLE INFORMATION

2.1 Topographic and Cadastral Survey Data

2.1.1 Aerial Survey

ALS datasets were supplied by CCMA for review as text files in xyz format. The study team

triangulated the data and then created digital elevation models (DEMs) at 1 m and 10 m resolutions.

Figure 2-1 and Figure 2-2 illustrate the DEMs for the 1m and 10m resolution respectively.

Figure 2-1 ALS Data: 1 m DEM

Figure 2-2 ALS Data: 10 m DEM

The grids clearly illustrate a distinctly non-uniform distribution of elevation points. The DEM in this

condition was deemed unacceptable for hydraulic modelling purposes. The study team attributes

this to insufficient thinning of non-ground survey strikes, i.e. where an elevation has been returned

that represents the top of some vegetation.

The study team undertook a thinning process and applied it only to problem areas with severely

impacted ALS data. This process was limited to areas crucial to the successful inundation mapping of

the Aire River estuary and floodplain. The study team attempted to ensure that the thinning process

applied was objective and rigorous. However due to the density of erroneous points it was found

that this was not easily achieved. As such, two different approaches were applied.

The first was a statistical analysis of raw ALS strikes allowing vegetation strikes to be automatically

identified and removed from problem areas. The statistical justification for removing points for



J1145 / R01v02 2

selected areas was the Mean elevation plus half of the Standard Deviation of elevations. All points

above this threshold were deemed to be vegetation strikes and removed.

The second methodology applied only to problem areas which were so heavily influenced by

vegetation that a statistical approach would be unsuccessful as the vast majority of points were

vegetation strikes. Numerous cross-sections were extracted from the problem areas at various

intervals and alignments. These sections were then visually inspected to assess the points which

could be identified as ground strikes. This was done for all sections and an average value across all

sections identified which represented the highest ground strike. All elevation data above this

elevation was deemed to be vegetation strikes and removed.

On completion of reprocessing of raw data, the study team combined the revised data set and re-

triangulated the ALS points to form a new TIN from which a new hydraulic model DEM would be

constructed.

The thinning process was successful in minimising the effect of vegetation in areas most critical to

the success of the hydraulic analysis. The study team consider the DEM generated to be adequate

for the purposes of this study but recommends CCMA pursue the data supplier for further

refinement of the data set. This will be particularly important if this ALS dataset is to be used in any

future flood studies, where it is critical that the topography truly reflects floodplain geometry.

Figure 2-3 and Figure 2-4 depict sample before-and-after images of the statistical and cross-section

methodologies. Figure 2-5 depicts the before-and-after image of the entire ALS dataset.



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Figure 2-3 Statistical method results with comparison to base data



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Figure 2-4 Cross-section method results with comparison to base data



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Figure 2-5 Entire DEM Comparison

The manual thinning of vegetation strikes was contained to areas where it was identified that the

erroneous data would have severe impact on the hydraulic modelling results. It does not cover the

entire dataset. This thinning has not been validated with field survey, and as such only provides an

approximation of the ground surface under the dense vegetation canopy.

The approach as outlined in this document provides a dataset suitable for the purposes of this study.

However it is strongly emphasised that this approach is unsuitable for use in preparing the

remainder of the dataset for use in a flood study. Instead it is recommended that ALS be thinned

using the complex algorithms commonly used by aerial survey suppliers.

The methodology adopted to manually thin the ALS data of vegetation strikes has yielded a much

more suitable dataset for assessing the hydraulic characteristics of the Aire River estuary and

floodplain. This dataset was further improved by field survey, allowing the waterway bathymetry to

be included in the final terrain model.

2.1.2 Terrestrial and Bathymetric Survey

Water Technology commissioned Redborough Mapping Services to collect the bathymetric survey

and cross-section data. The cross-section and bathymetric data was used to supplement the ALS

data in order to provide a better representation of in channel geometry where the aerial survey

accuracy is poor. Figure 2-6 depicts the extent of the terrestrial and bathymetric survey collected.



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Figure 2-6 Terrestrial and Bathymetric Survey Components

The budget available for additional survey was limited; however sufficient survey was collected to

successfully compliment the ALS data. The cross-section density is approximately 400 – 500 m along

all major waterways and drains and is thought to be adequate for the purposes of this investigation.

Bathymetric data was collected from an amphibious vehicle fitted with a Ceeman Data

Measurement and Recording System. The bathymetric survey covered the Aire River from the

entrance to the Great Ocean Road, the Ford and Calder Rivers where access was achievable, and

Lakes Craven, Costin and Hordern.

Whilst a better representation of channel geometry would be available through acquisition of more

survey data, the study team consider the resolution of terrestrial survey to be adequate for the

purposes of this study.

2.2 Hydrologic Data

Streamflow was obtained from Thiess Environmental for all streamflow gauges within the Aire River

catchment. Table 2-1 provides a summary of gauge data available.

Table 2-1 Streamflow Gauge Summary

Site Code Gauge Name Start Finish # of Records

235204 Little Aire Creek @ Beech Forest 12/05/1976 12:10 PM Current 86948

235209 Aire River @ Beech Forest 8/03/1991 9:30 AM Current 26449

235219 Aire River @ Wyelangta 6/11/1974 12:50 PM Current 81567



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Daily rainfall data was collected from the Bureau of Meteorology (BoM) for all stations located

within or in the vicinity of the Aire River catchment. Table 2-2 provides a summary of gauge data

available.

Table 2-2 Daily Rainfall Gauge Summary

Site Code Gauge Name Start Finish 2005 Depth (mm)

90001 APOLLO BAY 01/06/1898 Current 100.4

90015 CAPE OTWAY LIGHTHOUSE 01/01/1868 Current 57.8

90042 GELLIBRAND RIVER WEST 01/01/1915 Current 115.8

90076 TANYBRYN 14/02/1950 Current 151.0

90083 WEEAOPROINAH 01/08/1901 Current 185.2

90087 WYELANGTA 01/02/1936 Current 201.4

90093 HORDERN VALE 01/10/1953 Current 124.2

90183 HAINES JUNCTION 01/12/2000 Current 232

Pluviographic data was obtained for the BoM gauge at Wyelangta (90087). Figure 2-7 provides a

summary of the pluviographic trace for the 2005 flood event.

Figure 2-7 2005 Event Pluviographic Data

A RORB model of the Aire River catchment was supplied by Tony Jones of the CCMA. The study team

briefly reviewed the catchment delineation, network, and calibration and found the model to be

suitable for generating inflow boundaries for the hydraulic modelling. Figure 2-8 depicts the RORB

model setup, rainfall and streamflow gauges used in this assessment.

3 6 9 12 15 18 21 24 27 30



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Figure 2-8 Available Hydrologic Data

2.3 Historic Estuary Level Data

Parks Victoria supplied the study team with an Excel Spreadsheet of Aire River estuary observations.

The spreadsheet contained information pertaining to estuary water levels and mouth condition, with

some additional annotations regarding other environmental factors worthy of note. This data along

with anecdotal evidence was the primary source for model validation. Three entries of particular

relevance to this study are presented in Table 2-3.

Table 2-3 Parks Victoria Estuary Level Data – 2005 Event

Date and Time Observed Gauge Board Level

(at Campground Bridge)

3rd

February 2005 14:00 1.65 m AHD

3rd


4th




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3. HYDROLOGIC ANALYSIS

3.1 Overview

In order to perform the required hydraulic analysis of flow and inundation over the floodplain,

catchment inflows are required. Hydrographs were required for two events for the Aire River, Ford

River, Big Calder River, Little Calder River and Duck Creek. This section describes the works

undertaken to develop these inflow hydrographs.

3.2 March/April 2002 Event

The period between the 1st of March and the 30

th of April 2002 typifies an autumn flow sequence

and contains two artificial berm openings. This preliminary work has been completed without

considering the artificial opening of the berm. As such, the 2002 hydraulic simulation will represent a

natural filling event over a long period, clearly outlining the progression of inundation through the

Aire River estuary complex.

A simplified scaling approach was adopted for estimating the boundary conditions for the 2002

event. Instantaneous streamflow gauging from Wyelangta station was scaled by catchment area to

each hydraulic model inflow boundary. Table 3-1 presents the scaling factors and results. Figure 3-1

depicts the resultant scaled hydrographs.

Table 3-1 2002 Event Hydrograph scaling results

Catchment Area Scaling Factor Peak Flow Event Volume

Aire River @ GOR 138.1 km2 1.463 162.4 ML/d 3730.6 ML

Ford River @ GOR 60.5 km2 0.636 70.5 ML/d 1620.4 ML

Calder River @ GOR 29.9 km2 0.317 35.2 ML/d 808.6 ML

Duck Creek 14.8 km2 0.157 17.5 ML/d 400.8 ML



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Figure 3-1 2002 Hydraulic Model Boundaries

3.3 February 2005 Event

The February 2005 flood event was driven by a large east coast low which swept over the south and

east coast of Victoria producing significant rainfall totals across the state. The 2005 event in the Aire

River catchment produced a flood event large enough to culminate in a natural opening of the

estuary berm. This event is of particular interest due to the relatively short time for the estuary to fill

and the inundation of floodplain areas not typically flooded due to artificial berm openings.

The RORB model supplied by CCMA was utilised to develop boundary conditions for the February

2005 flood event. An isohyetal surface was created in a GIS environment by interpolating the daily

rainfall totals from stations within and surrounding the study area. Figure 3-2 depicts the results of

the interpolation and station depths for the 2005 event.



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Figure 3-2 2005 Event Isohyets

A RORB *.STM file was created for the 2005 event and run through the model utilising parameters

supplied with the model. It was not included in this projects scope to drastically change the existing

RORB model. The results from the existing model as shown below, do not justify further significant

changes to the model for the purposes of this investigation.

RORB has been developed such that multiple loss formulation options are available to the user when

modelling observed flood events. The two options typically most useful when modelling observed

flood events are FIT and DESIGN run modes. During FIT mode operation, RORB estimates Continuing

Loss parameters ensuring continuity between the rainfall excess and hydrograph volumes. During

DESIGN run mode, the user can specify the Continuing Loss parameters to optimise the fit of the

modelled hydrograph to observed, with continuity of volume an optional concern.

Using a FIT run the model provided a good fit with some issues matching 2005 hydrograph volumes.

The study team re-ran the model in the DESIGN mode such that the fit with the observed 2005

hydrograph was optimised. Further discussion regarding the implications of loss formulation mode

selection is presented later in this document. Table 3-2 outlines the parameters adopted for both

the FIT and DESIGN modes. Figure 3-3 and Figure 3-4 depict the calibration results and the hydraulic

model boundaries respectively, for the adopted design model run.

Table 3-2 2005 Event Loss Model Formulation Comparison (lower interstation area only)

Mode Kc m IL CL Aire River @ Glenaire (modelled/observed)

FIT 45 0.7 40 mm 10.04 mm/hr 98.6/122.7 m3/s

DESIGN 45 0.7 40 mm 8.55 mm/hr 120.7/122.7 m3/s



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Gauging station at: Aire River at Beech Forest

0

20

40

60

80

100

120

140

160

Dis

charg

e (m

³/s)

0 10 20 30 40 50 60 70

Time (hr)

Calculated

Actual

02468

1012141618202224

Rai

nfal

l (m

m) Gross rainfall

Rainfall excess

Gauging station at: Little Aire at Beech Forest

0

20

40

60

80

100

120

140

160

Dis

charg

e (m

³/s)

0 10 20 30 40 50 60 70

Time (hr)

Calculated

Actual

02468

1012141618202224

Rai

nfal

l (m

m) Gross rainfall

Rainfall excess

Gauging station at: Aire River at Wyelangta

0

20

40

60

80

100

120

140

160

Dis

charg

e (m

³/s)

0 10 20 30 40 50 60 70

Time (hr)

Calculated

Actual

02468

1012141618202224

Rai

nfal

l (m

m) Gross rainfall

Rainfall excess

Calculated hydrograph, Aire River Outflow to Sea

0

20

40

60

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100

120

140

160

Dis

charg

e (m

³/s)

0 10 20 30 40 50 60 70

Time (hr)

Calculated02468

1012141618202224

Rai

nfal

l (m

m) Gross rainfall

Rainfall excess

Figure 3-3 2005 RORB model Calibration

Calculated hydrograph, Ford River at Great Ocean Ro

0

20

40

60

80

100

120

140

160

Dis

charg

e (m

³/s)

0 10 20 30 40 50 60 70

Time (hr)

Calculated02468

1012141618202224

Rai

nfal

l (m

m) Gross rainfall

Rainfall excess

Calculated hydrograph, Aire River at Great Ocean Rd

0

20

40

60

80

100

120

140

160

Dis

charg

e (m

³/s)

0 10 20 30 40 50 60 70

Time (hr)

Calculated02468

1012141618202224

Rai

nfal

l (m

m) Gross rainfall

Rainfall excess

Calculated hydrograph, Calder River at Great Ocean

0

20

40

60

80

100

120

140

160

Dis

charg

e (m

³/s)

0 10 20 30 40 50 60 70

Time (hr)

Calculated02468

1012141618202224

Rai

nfal

l (m

m) Gross rainfall

Rainfall excess

Calculated hydrograph, Duck Creek Inflow

0

20

40

60

80

100

120

140

160

Dis

charg

e (m

³/s)

0 10 20 30 40 50 60 70

Time (hr)

Calculated02468

1012141618202224

Rai

nfal

l (m

m) Gross rainfall

Rainfall excess

Figure 3-4 2005 Hydraulic Model Boundaries



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3.4 Discussion

As with many aspects of hydrology, estimating catchment response is highly uncertain. This is

particularly pertinent when considering the simplified approach to estimating the inflow boundaries

for the 2002 event. Scaling by area does not consider catchment characteristics which affect routing

of flows and thus leads to inaccuracies in timings of flood peaks. However the study team deem the

method to be fit-for-purpose for the study, producing inflow hydrographs typical of an autumn flow

sequence.

Significant uncertainty also surrounds the RORB modelling of the 2005 event. Parameters adopted to

provide the best fit to the observed hydrographs are outside the range of values typical of such

calibrations. Numerous factors could be attributed to this and further understanding would require

detailed analysis which is outside of the current project scope. The study team would recommend

further investigation into additional refinement of the Aire River RORB model, if it is to be used for

more detailed studies.



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4. HYDRAULIC ANALYSIS

4.1 Overview

The complicated interaction of the estuary and floodplain geometry, flood flows and ocean water

levels requires detailed hydraulic modelling analysis to determine appropriate flood inundation

extents and levels within the study area. The following sections describe the works undertaken to

develop the hydrodynamic model and the results of the preliminary hydraulic mapping.

4.2 Hydraulic Model Software

Hydraulic modelling of the study area has been undertaken utilising the Danish Hydraulic Institute’s

(DHI) MIKE FLOOD modelling software. MIKEFLOOD is a state of the art tool for floodplain modelling

that has been formed by the dynamic coupling of DHI’s well proven MIKE 11 river modelling and

MIKE 21 fully two-dimensional modelling systems. Through this coupling it is possible to extend the

capability of the 2D MIKE 21 model to include:

• a comprehensive range of hydraulic structures (including weirs, culverts, bridges, etc);

• ability to accurately model sub-grid scale channels; and

• ability to accurately model dam break or levee failures.

For the present study, a two-dimensional (2D) MIKE 21 model has been set up to model the overall

floodplain flows. A coupled one dimensional (1D) MIKE 11 model has also been utilised to explicitly

model in-channel flows, waterway bridge and culvert crossings within the study area. Further details

can be found at: http://www.dhigroup.com/Software/WaterResources/MIKEFLOOD.aspx

4.3 One Dimensional Model Structure

The basis of the 1D model is the terrestrial cross-sectional survey data. The 1D model network

encompasses all major waterways in addition to some of the minor drainage networks within the

study area, as presented in Figure 4-1. Waterways selected to be modelled in 1D included the

following:

• Aire River;

• Ford River;

• Calder River (including Big and Little Calder branches);

• Duck Creek; and

• Ford River Floodplain Drains.

The study area contained numerous minor drains which were modelled in the 2D domain. Model

boundaries were situated at the following locations:

• Aire River upstream of the Great Ocean Road;

• Ford River upstream of the Great Ocean Road;

• Little Calder River at the Great Ocean Road;

• Big Calder River at model domain boundary;

• Duck Creek upstream of Horden Vale Bridge; and

• Ford River floodplain drains.



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Roughness values for each waterway were estimated from site visits and professional experience in

modelling similar systems. A Manning’s ‘n’ value of 0.035 was adopted uniformly across all streams

to represent in-bank channel roughness.

Figure 4-1 depicts the 1D model structure.

Figure 4-1 1-Dimensional Hydraulic Model Structure

4.4 Two-dimensional Model Structure

The basis of the 2D model is the topographic grid which is based on the ALS, bathymetric data and

field survey. A 10 m grid was interpolated from the detailed topography for use in the hydraulic

model. Figure 4-2 displays the two-dimensional hydraulic model topography.

The variation in hydraulic roughness within the study area has been schematised as a hydraulic

roughness grid, representing various hydraulic roughness (e.g. open grassland, reeds, thick

vegetation). The hydraulic roughness grid was based principally on the aerial orthophoto provided

by CCMA and visual inspection undertaken during field visits. Hydraulic roughness values adopted

for the two-dimensional hydraulic model are summarised in Table 4-1.



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Table 4-1 Hydraulic Roughness Parameters

Topography Class Manning’s “n”

Open Floodplain (Pasture) 0.04

Vegetated 0.05

Estuary 0.035

Roads 0.02

Thick ground-cover (Reeds) 0.06

Figure 4-2 2-Dimensional Hydraulic Model Structure

4.5 Linked Model Setup

MIKE FLOOD is a state-of-the-art tool for floodplain modelling that combines the dynamic coupling

of the one-dimensional MIKE 11 river model and MIKE 21 fully two-dimensional model systems.

The one-dimensional MIKE11 model was dynamically linked to the two-dimensional MIKE21 model

using lateral links along the entire length of the MIKE11 network. This allows for free exchange of

water between the two models once the water level exceeds the MIKE21 topographic level. Figure

4-3 displays the two-dimensional hydraulic model structure, the location of one-dimensional

hydraulic structure elements and the model boundary conditions.



J1145 / R01v02 17

Figure 4-3 1D/2D Linked Hydraulic Model Structure

Of note is the use of the 2D hydraulic model to approximate the hydrodynamics of each of the larger lakes.

This provides for the best approximation of bathymetric geometry and better simulates any two-dimensional

flow behaviour that may be present.

4.6 Results and Mapping

The 2002 and 2005 events were run through the hydraulic model and the results compared with the

observed values provided by Parks Victoria. Minor adjustments to the hydraulic roughness values

were made to optimise the reconciliation of model results with observed values. Figure 4-4 depicts

an example of the 2002 event simulation results. The full suite of 2002 event mapping is provided in

Appendix A. Figure 4-5, Figure 4-6 and Figure 4-7 depict the results of the 2005 event simulation.

4.6.1 March/April 2002

The March/April 2002 flow sequence was simulated and several maps developed depicting various

cumulative flow volumes. This series of maps essentially depicts the estuary filling process with no

opening, artificial or natural, of the berm. It should be noted that this is an example of a slow estuary

filling event, but is in no way definitive of all filling events. Significant variability in the filling process

is likely to occur between events, with the magnitude and timing of tributary flows in addition to

other environmental factors influencing the flood inundation and flow distribution across the

estuary floodplain.

No validation of the 2002 model was attempted due to the lack of observed data and the significant

uncertainty surrounding the inflow hydrology.



J1145 / R01v02 18

4.6.2 February 2005

Modelled water levels for the 2005 event were compared with Parks Victoria monitoring data to

validate model performance. The model was found to perform adequately given the relative

uncertainties in base-data with good agreement with observed levels. Table 4-2 summarises the

model validation data for the 2005 flood event simulation.

Table 4-2 Hydraulic Model Validation – 2005 Event

Date and Time Observed Gauge Board Level/Simulated Level

3rd

February 2005 14:00 1.68/1.86 m AHD

3rd

February 2005 16:00 2.01/2.02 m AHD

4th

February 2005 14:00 1.27/1.37 m AHD

The first data point indicates an artificially increased rate of filling of the estuary complex when

compared with observed values. This could be due to higher tributary inflows or higher initial

storage volume in the floodplain. The second point, immediately prior to the estuary berm opening

indicates good agreement between the observed and simulated values. The third data point

indicates the simulated berm opening provided insufficient conveyance or that the Lorne tide gauge

utilised as the downstream boundary was higher than the actual sea level at the time, restricting the

recession of the estuary water level.

Whilst the study team consider these values to reflect adequate model performance given project

objectives, it should be noted that there is significant uncertainty in the simulated flood levels.

Added confidence in model performance could be achieved through additional survey, additional

investigations into catchment hydrology, and higher-level simulation of berm dynamics.



J1145 / R01v02 1

Figure 4-4 March/April 2002 Estuary Filling Event: Example Depth of Inundation



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Figure 4-5 3rd

February 2005 2:00 PM: Depth of Inundation



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Figure 4-6 3rd




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Figure 4-7 4th




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5. CONCLUSIONS AND RECOMMENDATIONS

A hydraulic model of the Aire River estuary floodplain was developed from the Great Ocean Road

downstream to the estuary entrance. The model was utilised to simulate a typical low flow filling

event (March/April 2002), and a large flood event (February 2005). Observed levels in the estuary

were used to successfully validate the model for the February 2005 event. The 2005 event included a

basic estuary berm scour to simulate the opening of the estuary entrance.

Field survey of the rivers and major drain in the study area was collected to supplement the ALS

survey of the floodplain. The accuracy of the model could be improved by additional field survey of

areas covered by dense vegetation and in-channel bathymetry. The accuracy of the current ALS

limits the accuracy of the model, as such care must be taken in interpreting the results. For the

purposes of this study the model is deemed to be suitable, however for higher level requirements

such as flood mapping for planning purposes the ALS and the model may require refinement.

The model could also be improved by incorporating more sophisticated modelling techniques of

estuary berm openings. Scour modelling of the mobile bed could be undertaken to determine a

series of conditions that would induce natural openings and also natural estuary closing. The model

could also be utilised to investigate the impacts of storm-surge, climate change and salinity within

the estuary. The model could also be utilised to investigate the dynamics of the floodplain on the

recession of a flood.



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APPENDIX A – MARCH/APRIL 2002 EVENT MAP SERIES



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Aire River Estuary Hydraulic Study Final Report

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