bunbury flood management strategy

Bunbury Flood ManagementStrategy

APPENDIX A

FLOOD MODELLING REPORT Final 20 September 2004

165163221 165163221:165106543:Local

Bunbury Flood Management Strategy

APPENDIX A

FLOOD MODELLING REPORT Final 20 September 2004

Sinclair Knight MerzABN 37 001 024 095369 Ann Street, Brisbane 4000PO Box 246Spring Hill QLD 4004 AustraliaTel: +61 7 3244 7100Fax: +61 7 3244 7307Web: www.skmconsulting.com

COPYRIGHT: The concepts and information contained in this document are the property of SinclairKnight Merz Pty Ltd. Use or copying of this document in whole or in part without the writtenpermission of Sinclair Knight Merz constitutes an infringement of copyright.

Flood Modelling Report

SINCLAIR KNIGHT MERZ 165163221 165163221:165106543:Local

I:\QENV\Projects\QE04516\Reports\FINAL_QE04516_FloodModelling.doc PAGE 1

Contents

1. Introduction 41.1 Purpose 41.2 Study Area Description 41.3 Study Area Features 61.4 Description of Flooding 9

2. Previous Studies 10

3. Hydraulic Model Development 123.1 Catchment Hydrology 123.2 Hydraulic Model Description 133.3 Model Establishment and Parameterisation 133.3.1 Digital Terrain Model 133.3.2 Hydraulic Model Boundaries 153.4 Roughness Coefficients 163.5 Model Sensitivity Analysis 173.5.1 Sensitivity of Surface Roughness 173.5.2 Sensitivity of Downstream Boundary Conditions 183.6 Direct Rainfall Application 20

4. Hydraulic Modelling Results 244.1 No Levee Breach 244.2 Left Levee Breach 254.3 Right Levee Breach 274.4 Combined Breach Impact ("Base Conditions") 274.5 Comparison to Existing Flood Management Strategy 274.6 Hydraulic Model Limitations 314.7 500 Year Annual Recurrence Interval Flooding 314.8 Direct Rainfall Flood Inundation Map 334.9 Combined Breach Scenario and Direct Rainfall Application 33

5. Development Impact Assessment 365.1 Glen Iris Floodway Assessment 365.2 Harbour Development Assessment 425.3 "Moorlands" Development Assessment 465.4 Australind Bypass Bridge Upgrade 485.5 Combined Developments and Direct Rainfall Application 49

6. References 52




Document history and statusRevision Date issued Reviewed by Approved by Date approved Revision type

Final 20/9/04 A. Howard D. Sheehan 20/9/04 Final

Distribution of copiesRevision Copy no Quantity Issued to

Final 1 5 hard, 1 electronic City of Bunbury

Printed: 18 January 2005

Last saved: 21 September 2004 02:16 PM

File name: I:\QENV\Projects\QE04516\Reports\FINAL_QE04516_FloodModelling.doc

Author: Adrian Howard & Francesco Vitalini

Project manager: Adrian Howard

Name of organisation: City of Bunbury

Name of project: Bunbury Flood Management Strategy

Name of document: Flood Modelling Report

Document version: Final

Project number: QE04516




Executive SummarySinclair Knight Merz performed detailed hydraulic modelling of the Preston River and floodplainto give technical background to the Bunbury Flood Management Strategy. This report documentsthe technical processes of the hydraulic modelling and its findings.

The key findings of the hydraulic modelling were:

In the event of a Preston River levee breach, East Bunbury will be provided flood protectionby Robertson Drive;

The flood protection afforded by Robertson Drive does not have adequate freeboard to beconsidered to give 100 year annual recurrence interval flood immunity to East Bunbury;

The flood protection afforded by Robertson Road does not provide flood immunity in the 500year flood event;

Future development on the eastern floodplain, including the Inner Harbour extension and the"Moorlands" development, will alter the characteristics of the Preston River and floodplain andalter flood levels and minimum floor level requirements;

The Glen Iris floodway proposal will decrease flooding of East Bunbury in the 500 year ARIflood event;

Raising Robertson Drive to have freeboard in the 500 year ARI flood event may be a morecost-effective flood mitigation measure than constructing the Glen Iris floodway;

100 year ARI flood levels and recommended minimum building floor levels have been revisedfrom the levels reported in the Preston River Flood Study (Drawing BF43-3-1, 1989); and

Flooding cannot be considered in isolation in the context of floodplain management.




1. Introduction

1.1 PurposeSinclair Knight Merz was commissioned by City of Bunbury to review the City of Bunbury FloodManagement Strategy. The original purpose of this review was to:

1) Review the extent of the Preston River floodplain;

2) Review the floodplain management strategy for the Bunbury area;

3) Review the Council's Policy "Development in Flood Affected Areas" and provide a draftrevised policy;

4) Review the concept of the proposed Glen Iris floodway and evaluate any other possible floodmanagement strategies;

5) Identify the need for any additional flood protection measures;

6) Perform a flood damage assessment;

7) Assess the regional flooding impacts of development of the Glen Iris area, in particular the"Moorlands" development; and

8) Assess the regional flooding impacts of the proposed extension of the Bunbury Inner Harbourbasin and associated Preston River diversion.

This report addresses the technical aspects of the hydraulic modelling that was undertaken for eachof the points listed above. The results of the hydraulic modelling, reported here, have beenincluded in the Master document, "City of Bunbury Flood Management Strategy".

The study has involved extensive hydraulic modelling. The purpose of the modelling was todevelop a tool that can be used for assessing flooding on the Preston River floodplain. Thehydraulic model was developed in a flexible manner to allow it to be used for assessing existingcase flooding characteristics as well as assessing the flooding impacts of future floodplaindevelopment. The model was also established to assess the impacts of a breach of the PrestonRiver levees.

1.2 Study Area DescriptionThe study area is the Preston River floodplain from Picton Road Bridge to the Preston Rivermouth.

The catchment of the study area catchment includes the Preston and Ferguson Rivers' catchmentsand includes other minor tributaries including Crooked, Thompson and Booyup Brooks. Thecatchment is bounded in the south east by the Darling Range and the study catchment area isapproximately 1190km² (GHD, 1990). Figure 1-1 shows the Preston River catchment. The




Preston River flows generally north west through the study area to Vittoria Bay and the IndianOcean west of the town of Bunbury.

Figure 1-1 Preston River Catchment

Source: Department of Environment Internet site (www.wrc.wa.gov.au)

The study area commences downstream of the confluence of Ferguson and Preston Rivers at thePicton Road bridge and includes approximately four kilometres of the Preston River. Refer toFigure 1-2 for the extent of the hydraulic model study area.

The Preston River is flanked by levees on its left and right banks throughout the length of the studyarea. These levees are designed to contain the 100 year ARI flood event. The river channeldownstream of the Australind Bypass has been subject to river training works. The originalchannel was a series of oxbows and meanders which is replaced by a wide, excavated andstreamlined channel.

West of the Preston River is East Bunbury, which is the older area of Bunbury. This area has ahistory of flooding. To the east is Glen Iris which is largely rural with areas of developmentlimited to the floodplain fringe. The final kilometre of the River is flanked by the harbourdevelopment on its left bank. This land has been filled to approximately 3.6m AHD to betheoretically immune to 500 year ARI flooding.




1.3 Study Area FeaturesThe hydraulic model study area is described in Figure 1-2. The significant features of the studyare detailed below.

Picton Road Bridge is the upstream (southern) extent of the study area. Picton Road Bridgeforms a hydraulic control for flow into the study area. Hydrology supplied by DOE was inputat this location.

Preston River Levees flank the river on both the east (right) and west (left) banks and areaimed to give 100 year ARI flood immunity to the city of Bunbury. The impact of a leveebreach is the subject of this investigation.

Robertson Drive runs parallel to the Preston River from Picton Road to the Eelup Rotary.The road was recently duplicated and elevated. It provides additional flood protection toBunbury as it was constructed to have a flood immunity.

Eelup Rotary is a large round-a-bout at the intersection of Robertson Drive and the AustralindBypass. The traffic control structure was upgraded and realigned in the recent past,subsequent to the 1964 Preston River flood.

Australind Bypass Bridge is a bridge structure over the Preston River. The structure wasupgraded in the recent past and subsequent to the 1964 Preston River flood.

Vittoria Bay is the bay at the mouth of the Preston River, in the Leschenault Estuary.

Leschenault Estuary is the downstream end of both the Preston and Collie Rivers. Theestuary is large and shallow and has been the subject of numerous studies over the years. Theestuary is artificially drained at the "Cut".

The "Cut" is a man made opening for the drainage of the Leschenault Estuary. The "Cut"allows floods from the Preston River and/or the Collie River to pass out to sea with majortailwater impacts. The "Cut" acts to attenuate the impacts of storm surges and large tides intothe estuary.

Bunbury Inner Harbour is man-made harbour that is economic and social importance to thecity of Bunbury. The harbour owns land downstream of the Australind Bypass Bridge toVittoria Bay with plans of extending the harbour inland and filling large areas of floodplain inthe future.

Leschenault Inlet was previously the mouth of the Preston River. The inlet is a controlledestuary with high value residential property on its banks and significant recreational and socialimportance to the city of Bunbury.

The "Plug" is a gated opening of the Leschenault Inlet to Koombana Bay and the IndianOcean. The navigable structure acts to protect East Bunbury from storm surge.




Glen Iris and Moorlands are the land areas of low lying floodplain east of the Preston River.These ares have a history of flooding and the presence of ox-bows and flood runners showsigns of the area being an active part of the river and floodplain in the recent past.

East Bunbury is the suburb east of Bunbury City. The suburb is generally low-lying and hasa history of flooding. East Bunbury suffered significant damage during the 1964 Preston Riverflood.




Figure 1-2 Study Area

Picton Road Bridge

Bunbury Inner Harbour

Leschenault Inlet

EastBunbury

Glen Iris

Australind Bypass

Robertson Road

Vittoria Bay

Preston R Levees




1.4 Description of FloodingThe City of Bunbury has a history of flooding due to each of river flooding, king tides, stormsurges and drainage problems. This study relates to river flooding and in particular, breach of thePreston River levees. Such a flood has occurred in recent history when the Preston River leveesbreached in August 1964. It is understood that sections of the eastern levees were breachedmechanically and sections of the western levee overtopped despite this.

The August 1964 event was of approximately 60 hour duration and created a flood discharge ofapproximately 430m³/s (GHD, 1990). The levee was breached at its upstream end allowing watersto enter Moorlands and Glen Iris. The flood waters also overtopped the levees upstream of theAustralind Bridge inundating land out to Vittoria Bay and connecting with flood waters from theCollie River.

The majority of property damage occurred when the left levee bank overtopped upstream of theAustralind Bypass bridge and the flood flowed into East Bunbury.

The extent of the 1964 flood was approximated by the Public Works Department (1981) and isshown in figures in subsequent sections.

The City of Bunbury provided a summary of newspaper and other records of flooding. Exertsrelevant to the hydraulic modelling are included in Table 1-1.

Table 1-1 Historical Flooding Events

Date Source Description21 June 1945 Unknown Moorlands and Glen Iris flooded28 June 1945 South Western Times Moorlands and Glen Iris flooded again with many properties

isolated. Roof of verandah ripped off by flood water oncorner of Victoria and Wellington Streets. Harvey Weiroverflowed, little damage to the Bunbury district

18 July 1946 South Western Times 617 points (157mm) recorded in 7 days. Picton Road Bridgesubmerged for 100 yards (91m) with flood waters lapping atfloorboards of the homestead. Total rainfall for the first6 months 2913 points (740mm)

August 1964 Worst recorded flooding in Bunbury. Preston River levees breached Estuary levels 1.24AHD at McLeod Point 1.14mAHD at power station 1.92m AHD at Australind Bridge




2. Previous StudiesThis section reviews the previous studies of the Preston River, Leschenault Estuary and VittoriaBay that were made available for this investigation.

Department of Environment Hydrologic Review - December 2002The Department of Environment (DOE) reviewed the previous hydrology of the Preston River andfound:

The 100 year ARI flow in the Preston River downstream of Picton Bridge is 360m³/s whichwas revised upwards by 10% compared to previous estimates;

The increased flow increases the water level in the Preston River by approximately 0.25m;

The increased water levels in the Preston River reduce the freeboard available in the PrestonRiver levees;

The Glen Iris Relief Floodway will be promoted as part of the review of the floodplainmanagement strategies for the Bunbury area.

Preston River - Picton Bridge to Bunbury Harbour (GHD 1996)This report was prepared by GHD for a floodplain landholder. It investigated water levels in thePreston River downstream of the Picton Bridge and considered the changes in water levels due tothe realignment of the Preston River for the Bunbury Inner Harbour expansion.

The alignment of the Preston River diversion in this study is not consistent with the current DOEPreston River Floodplain Management Strategy. Subsequently, the findings of the study are notrelevant to this hydraulic modelling.

An Assessment of the 1964 Flood Event in the Preston an Collie River Catchments(GHD 1990)This report reviewed the hydrology of the Preston River and the rainfall and streamflow records ofthe 1964 Preston River flood. The aim was to quantify the recurrence interval of the flood eventusing the latest stochastic methodology of the day as described in Australian Rainfall and Runoff(1987).

The report found that the areal distribution of the 1964 event was confined. The rainfall thatcaused the storm had a high probability of 10% annual recurrence interval (or 10 year annualrecurrence interval). However, this translated to significant runoff as the antecedent moisturecontent of the catchment was high following earlier rainfall events. The study qualified the 1964flood as having a 0.6% annual excedance probability (or a 167 year annual recurrence interval) atthe Picton Road bridge.




The concurrent flooding in the Collie River was qualified as 2.5% AEP (40 year ARI). Thiselevated river condition contributed to flooding in the Preston River as it increased the tailwaterlevel in the Leschenault Estuary.

Leschenault Estuary, Collie River, Preston River Regional Flood Study (PWD, 1981)The former Public Works Department (Engineering Division) undertook this substantial study.The study provides a sound basis for the current investigation and includes a significant amount ofhydrologic and hydraulic modelling along with flood mitigation proposals.

The PWD study aimed to:

Identify areas inundated in the 100 year ARI flood event;

Investigate flood mitigation options, both structural and non-structural; and

Define limits for future development on the floodplains of the Preston and Collie Rivers andalong the foreshore of Leschenault Estuary

The PWD study included a simple analysis of routing the flow of the Collie and Preston Riversthrough the volume of Leschenault Estuary with the downstream constriction of "The Cut". Theanalysis provided an elevation versus time hydrograph for the Estuary and this has been used forthis current hydraulic study. The predicted peak estuary level was 1.92m AHD with concurrent100 year ARI floods occurring in the Preston and Collie Rivers and a 10 year ARI tidal surgeoccurring outside the estuary.

The PWD study also provided a hydraulic backwater analysis and predicted flood levels in thePreston and Collie Rivers using the "IRWASP" hydraulic model. Flood levels for the 50 and 100year ARI floods were predicted.

It was observed in the PWD study that the Preston River floodplain is naturally extensive in thelower sections of East Bunbury and Glen Iris. This water is contained within the Preston Riverlevee system. The report noted a number of inadequacies in the levee system and recommendedboth raising and strengthening works. It also recommends the construction of the Glen Irisfloodway as discussed in detail in this hydraulic report.




3. Hydraulic Model DevelopmentA hydraulic model was developed to assess the extent of inundation in the Preston River floodplaingiven the updated 100 year ARI hydrology provided by DOE and assuming that the Preston Riverlevee system failed during an extreme event. This section describes the modelling process andpresents the results.

3.1 Catchment HydrologyDepartment of Environment provided the catchment hydrology for the hydraulic model. Thehydrology was the subject of a review in 2002. The supplied Preston River hydrographs werereported at Picton Bridge and are shown as Figure 3-1.

Figure 3-1 Preston River Hydrology at Picton Bridge

0

50

100

150

200

250

300

350

400

0 12 24 36 48 60 72 84 96 108 120 132 144

Time (hours)

Dis

char

ge (m

³/s)

The study was focussed on the impacts of river flooding. Therefore, the DOE hydrology wasintroduced to the hydraulic model as a single inflow hydrograph at Picton Road Bridge. It wasassumed that the local catchments in the study area were insignificant contributors to flooding andthey were not modelled.

100 year ARI

50 year ARI

10 year ARI




3.2 Hydraulic Model DescriptionThe hydraulic model was developed using the MIKE21 suite of software. MIKE21 is a modellingsystem for two-dimensional free-surface flows. MIKE 21 is commonly used to model thehydraulics of systems where flooding does not generally follow the direction of the main riverchannels. This makes it ideal for floodplain scenarios where flow patterns can be chaotic anddependent on numerous terrain obstructions such as roads, levees, railways and diversion channelsrunning at obscure angles to the direction of river flow. MIKE21 is also ideal for floodingscenarios where floodplain storage is a key issue.

MIKE21 is interactive with geographical information systems (GIS) making the output andinterpretation of results straightforward.

3.3 Model Establishment and ParameterisationThe primary inputs to a MIKE21 model are a high quality digital terrain model (DTM) and theflood hydrology. Other inputs include surface roughness and tidal boundaries. Theparameterisation of the model is discussed below.

3.3.1 Digital Terrain ModelA DTM of the study area was produced. The basis of the DTM was one metre contour informationsupplied by Department of Land Administration (DOLA). The DOLA data was interpreted into aDTM using the ARCVIEW GIS. The representation of the terrain model of the study area is shownin Figure 3-2.




Figure 3-2 Digital Terrain Model

The DOLA contour information was dated 1986 and was based on 1:2,000 scale aerialphotography. The age of the base data was a concern. Initial model runs identified the followingdisparities between the supplied terrain data and the terrain as observed in its current state:

Robertson Road was shown as a single carriageway road. The current road is actually dualcarriageway and the new duplication may have been raised,

The configuration of the Eelup Rotary to Estuary Drive has changed and possibly elevated,

The configuration of the railways to Bunbury Inner Harbour changed with new lines and railbridges included since the date of survey,

The land surrounding the Bunbury Inner Harbour has been filled in places to 3.6mAHD. Onelocation at the Estuary Drive, Koombana Drive intersection was considered to be significant tothe flooding characteristics in East Bunbury.

Additional terrain data was requested subsequent to initial model runs. The extent of the additionalsurvey is shown in Figure 3-3.




Figure 3-3 Additional Terrain Survey

The additional survey information was incorporated into the DTM for use in the MIKE21 model.

3.3.2 Hydraulic Model BoundariesThe hydraulic model boundaries for the MIKE21 model were constrained to a single inflowhydrograph at Picton Bridge and tailwater boundaries at Vittoria Bay at the "Plug", the downstreamcontrol of Leschenault Inlet.

As discussed previously, the inflow hydrograph for the hydraulic model was supplied by DOE.The modelling involved the simulation of the 100 year ARI flood. The model was run over thepeak of the inflow hydrograph and two days into its recession limb to ensure that the full effects ofthe 100 year ARI flood were simulated.

A tailwater boundary was applied at Vittoria Bay to mimic the water level anticipated in theLeschenault Estuary if both the Preston River and the Collie River were experiencing 100 year ARIflood simultaneously. This is consistent with the logic applied in previous studies. The tailwaterhydrograph was taken from PWD (1981) and is reproduced in Figure 3-4.




Figure 3-4 Leschenault Estuary Tailwater Level

A tailwater hydrograph was applied to the ocean side of the "The Plug" (Point MacLeod). For thepurpose of this modelling, it was assumed that the "Plug" would be opened during a 100 year floodevent. A tidal cycle equivalent to the 100 year ARI storm surge was applied to the ocean side ofthe "Plug" and the opening width was left to dictate the variation in the storage levels withinLeschenault Inlet.

The downstream boundary conditions were subject to a sensitivity analysis, which is discussed in asubsequent section.

3.4 Roughness CoefficientsSurface roughness is a key parameter in defining the hydraulic characteristics of rivers andfloodplains. MIKE21 represents surface roughness as a two dimensional surface.

Cadastral data and aerial photography were used to map land uses to the model area. The definedland uses included rivers, roads, urban areas, open grassed paddocks, treed paddocks etc. Eachland use was initially assigned a roughness parameter value using the Mannings 'm' roughnesscoefficient. The roughness coefficients became subject to a sensitivity analysis discussed in asubsequent section. The ranges of roughness values are listed in Table 3-1.




Table 3-1 Mannings 'm' Roughness Coefficients By Landuse

Land Use RoughnessRiver 0.020 - 0.030Urban Areas 0.080 - 0.10Open Grassed Paddocks 0.025 - 0.035Treed Paddocks 0.060 - 0.080

3.5 Model Sensitivity AnalysisThe hydraulic model for a Preston River levees breach could not be calibrated against recordedflood events, as the Preston River levees have not breached since the flood event of 1964. Sincethis event, the terrain of the Preston River floodplain has changed significantly with new roads,railways and residential developments. Subsequently, the model could only be validated through aprocess of testing the sensitivity of model results against input terrain and parameters.

Two sensitivity analyses were run being:

Sensitivity of surface roughness;

Sensitivity of tidal tailwater levels.

3.5.1 Sensitivity of Surface RoughnessThe hydraulic model was found to be generally insensitive to surface roughness. Althoughincreases in surface roughness did change the 100 year ARI flood water surface levels, themaximum change was found to be approximately +0.10m with a 30% increase in surfaceroughness. Figure 3-5 shows the change in flood levels. Areas shown in shades of red are areasexperiencing degrees of increase in flood levels. Areas shown blue are experiencing degrees ofdecrease in flood levels.

The sensitivity analysis was performed on the model prior to the terrain modifications discussed inSection 3.3.1.




Figure 3-5 Sensitivity Analysis - Increased Surface Roughness

This result shows that the hydraulics of the floodplain is more likely to be sensitive tomodifications to the floodplain storage (ie. land development) rather than modifications to theexisting vegetation cover.

The adopted Mannings roughness parameters are as shown in Table 3-2.

Table 3-2 Adopted Mannings 'm' Roughness Coefficients By Landuse

Land Use Roughness (Manning's 'n')River 0.025Urban Areas 0.100Open Grassed Paddocks 0.035Treed Paddocks 0.080

3.5.2 Sensitivity of Downstream Boundary ConditionsThe model was tested for sensitivity to downstream boundary conditions. The model was run witha dynamic tidal cycle in Leschenault Estuary as presented in Figure 3-4. This was compared to amodel run with a static downstream water level of 0.8mAHD




Figure 3-6 shows the impact that the changed downstream boundary conditions have on 100 yearARI flood levels.

Figure 3-6 Sensitivity Analysis – Dynamic Boundary Conditions

This result demonstrates that the floodplain behaviour is not influenced by downstream boundaryconditions. The only increases in water levels in the Preston River occurs within a range of 50-60m upstream the railway bridge. No other significant changes in water levels can be observed onthe overall floodplain.

This is explained by the presence of the railway bridge immediately upstream of Vittoria Bay. Thehydraulic structure forms a control that slows and dissipates the propagation of the estuary levelsurge that is dominated by Collie River flow filling Vittoria Bay.

The constricted flow area of the "Plug" imposes a similar effect. This hydraulic control ensuresthat the water level in the Leschenault Inlet is not greatly impacted by tidal storm surge.

This sensitivity test concluded that a conservative estimate for a downstream boundary would be afixed water level equal to the peak of the cycle shown in Figure 3-4.




The sensitivity analysis was performed on the model prior to the terrain modifications discussed inSection 3.3.1.

3.6 Direct Rainfall ApplicationHydraulic modelling found that it is unlikely that river flooding will occur in the eastern floodplainand East Bunbury during the 100 year ARI flood event (refer Section 4). However, flooding willstill occur due to local rainfall runoff or storm surge.

The MIKE21 model was modified so that rainfall was directly applied to East Bunbury to provide100 year ARI flood levels over the area. This new technology allowed the simulation of directrainfall hyetographs to the existing digital terrain model. The model dynamically simulated flowacross the surface following the natural morphology and filled existing depressions and pondageareas. Potential localised stormwater flooded areas were be clearly identified and then includedinto the overall regional flood management strategy.

The 100 year ARI rainfall event data (IFD and temporal patterns) was extracted from AustralianRainfall and Runoff (AR&R, 1987). A series of eight different storm duration hyetographs weregenerated, with a precipitation rate specified in mm/day, as shown in Figure 3-7. No evaporationlosses were considered during this application.

Figure 3-7 Rainfall Hyetographs - 1 in 100 year ARI event

Rainfall Hyetographs

0

500

1000

1500

2000

2500

3000

3500

01/01/2004 0:00 01/01/2004 1:12 01/01/2004 2:24 01/01/2004 3:36 01/01/2004 4:48 01/01/2004 6:00 01/01/2004 7:12

Time (min)

Prec

ipita

tion

Rat

e (m

m/d

ay)

30 min 45 min

60 min 90 min

120 min 180 min

270 min 360 min




As the purpose of this application focused exclusively on the East Bunbury drainage area, theinitial extents of the MIKE21 hydraulic model were reduced to the study area shown Figure 3-8.

Figure 3-8 Direct Rainfall Application on East Bunbury

In order to identify the likely limitations of this application, it is important to note the followingconsiderations:

The MIKE21 terrain modelling is based on 1m contour lines. The accuracy of the results islimited by this resolution, as active conveyance in roads and table drains is approximated.However results can be considered representative as roads and all houses present in EastBunbury at the time of the aerial photography were “forced” into the terrain model. It is alsoimportant to note that the terrain modelling is quite aged and many houses in newer areas oftown are currently missing;

Initial and continuing losses representing differences in pervious and impervious effectiverainfall were not removed from the initial rainfall hyetographs. Different pervious andimpervious area runoff characteristics were modelled using appropriate roughness coefficientsfor each area.

MIKE21 boundary conditions were implemented on the following assumptions:

- pumping stations discharging to the Leschenault Inlet were not operating;




- the pipe drainage network is overwhelmed and is not contributing to local drainage;

- "The Plug" is open and a 1 in 100 year ARI storm surge is creating a high tailwater levelin Leschenault Inlet, and the Indian Ocean at the Big Swamp outlet; and

- Five Mile Brook is discharging a 1 in 100 year ARI flood event into the "Big Swamp". Adischarge hydrograph (with adopted peak discharge of 7.8 m3/s) was extracted from theFive Mile Brook Flood Study (Water Authority, 1995).

The series of eight rainfall events were run through the MIKE21 model. Results were comparedand the 360 minutes storm duration was selected as the critical rainfall event for East Bunbury.The flood inundation map showing flood depths and water surface levels contours is presented inSection 4.

The model was tested for sensitivity to loss of storage due to the volume occupied by the houseslocated on the floodplain. The model was run with a modified terrain model simulating naturalconditions (no houses) for the 360 minutes critical storm duration. Results were compared with theprevious scenario (with houses) and are presented Figure 3-9 below.

Results show the differences in water levels between the two scenarios, with and without houses onthe floodplain. Areas shown red show where an increase in flood depth was modelled. Floodwaters are forced to flow along existing major roads rather than following natural drainage lines,causing increases in water levels in confined urban areas rather than in natural depressions.




Figure 3-9 Sensitivity Analysis – Direct Rainfall Application




4. Hydraulic Modelling ResultsThe aim of the hydraulic modelling was to provide the inundation extents of a number of PrestonRiver Levee breach scenarios for the 100 year ARI flood event. The hydraulic model was initiallyused to assess the hydraulics of the river if the levees do not breach. The terrain was then modifiedto assess the hydraulics of the floodplain if both the left and right levees breached simultaneously.The modelling did not account for breach mechanisms nor breach timing.

There is a low probability that both the right and left bank levees will breach simultaneously.Failure is likely to be concentrated in a single area of weakness in a length of levee and propagateoutwards. This will flood the area of floodplain adjacent of the breach, reducing flood levels in theriver and subsequently reducing pressure on the levees on the opposite side of the river.

It is difficult to predict the exact location and extent of levee failure but the location of the failurewill dictate the resulting inundation. For example, a right levee breach will give greater inundationon the right floodplain compared to a simultaneous left and right breach. On the request of Counciland DOE, two scenarios were tested. The first looked at the inundation if just the left leveebreached. The second looked at a right levee failure. The separate results were then combined toform a map of the worst case extent of inundation irrespective of breach location. This is referredto as "Combined Breach Impact" scenario.

4.1 No Levee BreachAn initial assessment of the Preston River hydraulics during a 1 in 100 year ARI flood event wasundertaken by using the two dimensional hydraulic model to reproduce a no levee breach scenario.Results were compared with the water surface profile extracted from the previous Preston RiverFlood Study – Leschenault Inlet to Picton Bridge, Water Authority of Western Australia, 1989(Report No. WS36), as shown in Figure 4-1.

The water surface levels predicted by the MIKE21 model are generally similar to the previousresults (WA, 1989), with maximum differences within the range of 0.20 – 0.25m upstream of theAustralind Bypass Bridge. In the downstream section of the Preston River the water surface profilepredicted by MIKE21 is generally 0.40 – 0.50m higher than the previous one dimensionalhydraulic model results. These differences can be attributed to a combination of several factorsincluding the reviewed Preston River catchment hydrology (DOE, 2002), different tailwater levelboundary conditions and finally the different nature of the two hydraulic models used in the twodifferent flood studies.

Generally, within the constraints imposed by the model limitation described in Section 4.6, it canbe considered that the MIKE21 model provides a reasonable representation of the flooding patternsthroughout the modelled area.




The purpose of the study was to represent flooding in the floodplain so further calibration of the in-bank hydraulics was not undertaken.

Figure 4-1 No Levee Breach 100 Year ARI Flood Profile

1 in 100 Year ARI Flood Profile - No Levee Breach

-4.000

-2.000

0.000

2.000

4.000

6.000

8.000

10.000

0.0 500.0 1000.0 1500.0 2000.0 2500.0 3000.0 3500.0 4000.0

Cross Section Chainage (m)

Elev

atio

n (m

AH

D)

Preston River Invert LevelWater Surface Profile (Mike21)Water Surface Profile (WA, 1989)Right Levee BankLeft Levee Bank

Railw

ay Bridge

Australind Bypass Bridge

Estuary Drive Bridge

Picton Road Bridge

4.2 Left Levee BreachThe terrain model was modified by removing the left (west) levee and leaving the right levee inplace. This scenario gives a conservative estimate of the inundation due to a levee breach along theleft bank. It was found that Robertson Road intercepted flood flow into East Bunbury. The fillingof land around the Bunbury Inner Harbour intercepted flow that would naturally flow to theLeschenault Inlet.

The resulting inundation is show as Figure 4-2. Flooding is generally contained within the riverupstream the Australind Bypass bridge. Flood levels at the Eelup Rotary is 3.7m AHD which is0.17m below the crest of the road.




Figure 4-2 Inundation - 100 Year ARI Flood Left Levee Breach




4.3 Right Levee BreachThe terrain model was modified by removing the right (east) levee, leaving the left levee in place.This scenario gives a conservative estimate of the inundation of the eastern floodplain due to alevee breach along the right bank. Extensive flooding was found through the Moorlands and GlenIris areas downstream to Vittoria Bay.

The resulting inundation is shown as Figure 4-3. Flooding in the river is lower than the left leveebreach scenario for its length from the first breakout of the river into the eastern floodplain.

4.4 Combined Breach Impact ("Base Conditions")The results of the left "levee breech scenario" and the "right levee breach" scenario were combinedin a process in GIS. A query across the two results returned the greater flood level from the resultsets into a single flood surface. The result favoured the right levee breach scenario for the westernfloodplain and the right levee breach scenario for the eastern floodplain. The resulting inundationis shown as Figure 4-4.

4.5 Comparison to Existing Flood Management StrategyThe hydraulic modelling from the existing strategy could not be used for model calibration andvalidation purposes. Most importantly, the modelling that was undertaken for the previous strategywas based different terrain and hydrology. However, the comparison must be made to quantify theeffects of these differences.

From initial model runs, it became apparent that terrain modifications had resulted in significantchanges to the floodplain hydraulics. The most significant change was discovered at RobertsonRoad. Robertson Road has recently been upgraded and duplicated. The Eelup Rotary andAustralind Bypass Bridge have undergone a re-configuration. Road levels have been raised toabove 3.8m AHD, which is higher than the flood levels predicted in the existing FloodManagement Strategy. This resulted in an inundation extent that was confined to within the riverand the east floodplain in the event of levee failure.

The extent of inundation on the east floodplain was similar to the existing strategy in the event oflevee failure. Inundation extended generally north east, taking in significant areas of Glen Iris toVittoria Bay.

An assessment of the hydraulic capacity of the Preston River levees found that the 100 year ARIflood event can be contained within the banks of the levees if the levees remain in tact.

Figure 4-5 shows a comparison between the extent of inundation of the 100 year flood event fromthe previous strategy and the modelled for this study.




Figure 4-3 Inundation - 100 Year ARI Flood Left Levee Breach




Figure 4-4 Inundation - 100 Year ARI Flood Combined Levees Breach (Base Case)




Figure 4-5 Comparison of Inundation Extents of Previous and Current Strategies




4.6 Hydraulic Model LimitationsGreat care has been taken to compile a thorough representation of the Preston River Floodplain.The accuracy of the model is limited by the accuracy of the input terrain modelling and inputhydrology. The terrain modelling that was used as the basis of the hydraulic model wasconstructed from 18 year old aerial photography. Additional terrain was added to the hydraulicmodel where shortcomings were identified. This will not have identified all developments that willimpact on regional flood levels including floodplain filling being undertaken on the easternfloodplain by the Bunbury Ports Authority. Developments in this area should be assessed on aregional basis rather than an individual basis.

The resolution of the hydraulic model is 10m. This resolution is suitable for the development ofinput into the regional flood management strategy. It is suitable for providing regional flood watersurface levels, indicative flood water depths and flood extents based on the input terrain model. Itis not suitable for assessing the impact that small-scale floodplain development (less than 25 modelgrid squares or 2,500m²) will have on local flood levels. However, it does provide suitablebackground input to such assessments.

The hydraulic assessment of breach scenarios was achieved by removing the levees from the terrainmodel. Such an instantaneous failure is highly improbable and it will not occur from the start of aflood event. This results in a conservative estimate of inundation depths and extent.

The timing and mechanism of a levee failure will determine the hydraulics of the floodplainadjacent to the breach. The modelling undertaken does not attempt to quantify the velocity of flowthrough a breach. Velocities that are presented assume that the levees do not exist at the time of aflood and are therefore gross underestimates of the flow velocity adjacent to a levee breach. Theflood modelling should not be used to make an assessment of the hazard related to a levee failure inclose proximity to levees.

4.7 500 Year Annual Recurrence Interval FloodingThe 500 year ARI flood event was run through the hydraulic model for the purpose of assessingflood mitigation measures discussed in subsequent sections. The peak flow in the 500 year ARIevent was supplied by DOE. The 500 year ARI inflow hydrograph was then approximated bylinearly increasing the 100 year ARI hydrograph to meet the 500 year peak. This was done on theinstruction of DOE.

A combined levee breach assessment was performed. The resulting inundation showed thatRobertson Road does not afford 500 year ARI protection to East Bunbury. Floodwatersovertopped the road at the Eelup Rotary as did occur in the Bunbury flood of 1964. Figure 4-6shows the flooding inundation as modelled for this extreme rainfall event.




Figure 4-6 Inundation - 500 Year ARI Flood Combined Levees Breach




4.8 Direct Rainfall Flood Inundation MapThe application of direct precipitation to the existing MIKE21 hydraulic model was developed toprovide 100 year ARI local flood levels over the East Bunbury area. The 360 minutes stormduration was selected as the critical rainfall event. The corresponding flood inundation mapshowing flood depths is presented in Figure 4-7.

This scenario represents the flooding characteristics of the East Bunbury area during a local rainfalland runoff event. Potential localised stormwater flooded areas can be clearly identified and thencombined with proposed river flooding mitigation measures into the overall regional floodplain riskmitigation strategy, as presented in subsequent sections.

Figure 4-7 Inundation Map - 100 year ARI Local Rainfall Event

4.9 Combined Breach Scenario and Direct Rainfall ApplicationThe results of the “Combined Breach Scenario" (Figure 4-4) and the "Direct Rainfall Application"were combined into a single flood surface, using GIS capabilities. The resulting inundation andflood contour maps are shown as Figure 4-8 and Figure 4-9.




Figure 4-8 Inundation 100 Year ARI Flood - Combined Breach Scenario and Direct Rainfall Application - Existing Conditions




Figure 4-9 Contours 100 Year ARI Flood - Combined Breach Scenario and Direct Rainfall Application - Existing Conditions




5. Development Impact AssessmentA number of questions arose during the course of the study. SKM undertook additional modellingto establish the impacts of a number of floodplain development proposals current at the time of thestudy. These were the:

Glen Iris Floodway proposal for flood mitigation in extreme flood events;

An impact assessment of the Bunbury Inner Harbour extension;

An impact assessment of Stage 1 of the proposed "Moorlands" residential development; and

An impact assessment of the complete "Moorlands" residential development and the BunburyInner Harbour extension.

5.1 Glen Iris Floodway AssessmentThe Glen Iris floodway is a flood mitigation option proposed by DOE as part of the Preston RiverFloodplain Management Strategy. The floodway would remove a section of higher terrain to theeast of the Preston River to promote flow in the eastern floodplain and reduce the pressure on thePreston River levees.

The proposed terrain for the floodway was supplied by DOE and it was included in the hydraulicmodel. The surface roughness of the channel was assumed to be grazed grass, with an adoptedManning coefficient set at 0.035. An opening was added to the Australind Bypass. It was assumedthat the opening would be equivalent to the width between the edges of the channel. The updateddigital terrain model including the proposed Glen Iris floodway layout is shown in Figure 5-1.

Both left and right levee breach scenarios were run separately for the 100 year and 500 year ARIflood events. Results were then combined into a single flood surface in order to reproduce acombined breach scenario. It was found that the proposed floodway design did not flow in the 100year event and it provided limited benefit for flood levels in the 500 year ARI event.

Figure 5-2 shows the flood extent and inundation depths for the 1 in 500 year ARI flood event. Itcan be observed that the proposed floodway can divert a portion of the floodwaters towards theeastern floodplain, but it can not prevent the overtopping of Robertson Road and the consequentflooding of East Bunbury. As presented in Figure 5-3, the benefits of this proposed floodplaindevelopment are very limited, with maximum decreases of floodwaters in the order of 0.1m or lessin the East Bunbury area.

Due to this limited performance, the design of the Glen Iris floodway was altered to improve itseffectiveness. The base of the channel was lowered by approximately 0.5m, thus lowering thetrigger level that the channel begins to flow and eventually increasing discharges through the




channel. The corresponding flood inundation map and afflux plots are presented in Figure 5-4 andFigure 5-5 respectively.

The new proposed design floodway provided some improved benefits as the extent of flooding inthe East Bunbury floodplain decreased and flood levels reduced by approximately 0.3 – 0.4m.However, the new floodway design did not modify the overall hydraulic behaviour of the entirefloodplain by stopping flows to overtop Robertson Road, thus questioning the effectiveness of theproposed floodplain development during extreme events

Figure 5-1 Glen Iris Floodway General Arrangement




Figure 5-2 Inundation - 500 Year ARI With Glen Iris Floodway




Figure 5-3 Afflux - 500 Year ARI With Glen Iris Floodway




Figure 5-4 Inundation - 500 Year ARI With Revised Glen Iris Floodway




Figure 5-5 Afflux - 500 Year ARI With Revised Glen Iris Floodway




5.2 Harbour Development AssessmentAn important element of the future development of the Preston River floodplain is the extension ofthe Bunbury Inner Harbour. The proposed extension will fill the majority of the floodplaindownstream of the Australind Bypass Bridge, incise a new harbour into the floodplain and divertthe River off its current course and around the harbour. The proposed general arrangement isshown in Figure 5-6.

The proposed Preston River diversion has a large capacity relative to the existing river channel.The channel is broad and creates a lower water level downstream of the Australind Bypass Bridge.This effect is propagated upstream of the bridge with levels reduced by more than 200m in theriver. The effect extends and gradually dissipates for over a kilometre upstream and inside theriver zone.

The River diversion makes a 90° bend directly downstream of the Australind Bypass Bridge andthen runs parallel to the Bypass. Water that would previously overtop the road and flow across thefloodplain to the north-east is blocked by the diversion channel and water backs up behind theAustralind Bypass. This increases flood levels upstream of the Bypass by more than 200mm. Theflood is diverted further to the north east than before and increased flood levels are experiencednorth east of the channel's second bend. These increases are in excess of 300mm in their worstlocations.

The flood inundation with the harbour fully developed is shown as Figure 5-7. The impact that theharbour development has on flood levels is shown as Figure 5-8.




Figure 5-6 Bunbury Inner Harbour Extension General Arrangement




Figure 5-7 Inundation - 100 Year ARI With Inner Harbour Extension




Figure 5-8 Afflux - 100 Year ARI With Inner Harbour Extension




5.3 "Moorlands" Development AssessmentThe "Moorlands" development is a residential development proposed for the east floodplain of thePreston River at the time of the report. The proposal is that the development will be constructed ona very low area of the floodplain in an area that currently acts as active flood conveyance andstorage. The development will fill the floodplain to above 100 year ARI flood levels.

The development is to be completed in two distinct phases. When it is complete, the extent of thedevelopment is further south (upstream) than the first of the Preston River levees. Therefore, abreach of the Preston River levees will not be able to flood the east floodplain and will becontained by the Moorlands fill.

On the request of Council, the impact of the Moorlands development and the Inner HarbourExtension were considered concurrently. A separate investigation was undertaken of the impactsof the Moorlands Development Stage 1. That investigation was authored as part of a review of thedevelopment application for the site.

When considering impacts of the Moorlands development and the Inner Harbour extension areconsidered in conjunction:

Flooding is trapped in the zone of the river and away from the eastern floodplain by theMoorlands development, and

The increased conveyance of the Preston River Diversion conveys the flow away to the rivermouth without allowing it to enter the eastern floodplain.

The result is that water levels in the Preston River are higher than the concurrent breach scenario asfloodplain flow is forced down the river channel. This effect occurs adjacent to the Moorlandsdevelopment but does not extend far upstream. The effect also registers downstream of theMoorlands development and downstream of the Australind bypass, almost reaching Vittoria Bay.

The increased river levels do not overtop Robertson Road to enter East Bunbury in the event of theleft levee breaching. However, the increased river levels reduce the freeboard of the existinglevees and increase the probability of their failure. Subsequently, the protection of East Bunburywill become more dependent on the protection afforded by Robertson Road.

The converse of the impacts described above is that the eastern floodplain becomes largely floodimmune. Vast areas of flood prone Glen Iris and beyond will not be affected by river floodingleaving only issues of local drainage.

The flood inundation with the harbour fully developed and Moorlands development is shown asFigure 5-9.




Figure 5-9 Inundation - 100 Year ARI With Inner Harbour Extension and "Moorlands" Developed




5.4 Australind Bypass Bridge UpgradeA final modification of the future development of the Preston River floodplain is a proposedupgrade of the existing Australind Bypass Bridge opening. The existing structure configuration hasthe potential to block water levels behind the bridge, due to a relatively limited conveyance,causing increases in water levels and propagation of floodwater around the proposed Moorlandsdevelopment area.

An extension of the existing opening of approximately 60m was proposed and included into thefinal development strategy. The 1 in 100 year ARI flood event was modelled and a comparisonwith the previous scenario (refer to Figure 5-9) is presented in Figure 5-10.

Figure 5-10 Afflux Plot - Australind Bridge Upgrade

The proposed Australind Bridge upgrade provides improved benefits reducing flood levels byapproximately 0.15 – 0.20m upstream the existing structure and causing a general reduction of theoverflows towards the Moorlands development area.




5.5 Combined Developments and Direct Rainfall ApplicationThe results of the combined previous developments (Inner Harbour Extension – MoorlandsDevelopment – Australind Bypass Bridge Upgrade) and the "Direct Rainfall Application" weremerged into a single flood surface, showing the resulting inundation extent (Figure 5-11) and floodcontours (Figure 5-12) during the 100 year ARI river and rainfall flooding event.




Figure 5-11 Inundation 100 Year ARI Flood - Combined Developments and Direct Rainfall Application - Developed Conditions




Figure 5-12 Contours 100 Year ARI Flood - Combined Developments and Direct Rainfall Application - Developed Conditions




6. ReferencesGutteridge Haskins and Davey Pty Ltd (1990); An Assessment of the 1964 Flood Event in thePreston and Collie River Catchments; Report to the Water Authority of Western Australia

Gutteridge Haskins and Davey Pty Ltd (1996); Preston River - Picton Bridge to Bunbury Harbour

Public Works Department of Western Australia (1981); Leschenault Estuary, Collie River, PrestonRiver Regional Flood Study

bunbury flood management strategy

Documents