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Fairy and Cabbage Tree Creeks Flood Study

A part of BMT in Energy and Environment

R.B14106.002.09.doc October 2009

G:\ADMIN\B14106.G.GJR\R.B14106.002.09.DOC

Fairy and Cabbage Tree Creeks Flood Study

Prepared For: Bewsher Consulting

Prepared By: BMT WBM Pty Ltd (Member of the BMT group of companies)

Offices

Brisbane Denver

Karratha Melbourne

Morwell Newcastle

Perth Sydney

Vancouver

FOREWORD I


FOREWORD

The NSW State Government’s Flood Policy is directed towards providing solutions to existing flooding problems in developed areas and ensuring that new development is compatible with the flood hazard and does not create additional flooding problems in other areas. Policy and practice are defined in the NSW State Government’s Floodplain Development Manual (FDM, 2005).

Under the Policy the management of flood prone 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 State Government through the following four sequential stages:

Stages of Floodplain Management

Stage Description

1 Flood Study Determines 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 developments.

3 Floodplain Risk Management Plan

Involves formal adoption by Council of a plan of management for the floodplain.

4 Implementation of the Plan Construction of flood mitigation works to protect existing development. Use of environmental plans to ensure new development is compatible with the flood hazard.

This study represents the first of the four stages for the Fairy Creek and Cabbage Tree Creek area in Wollongong. It has been prepared for Wollongong City Council to describe and define the existing flood behaviour and establish the basis for floodplain risk management activities in the future.

Council has prepared this document with financial assistance from the NSW Government through the Department of Environment, Climate Change and Water (DECCW). This document does not necessarily represent the opinions of the NSW Government or the Department of Environment, Climate Change and Water.

CONTENTS II


CONTENTS

1 INTRODUCTION 1-1

1.1 General 1-1 1.2 Catchment Description 1-1 1.3 Purpose of this Report 1-2 1.4 Previous Studies 1-2

1.4.1 Cabbage Tree Creek Flood Study 1-2 1.4.2 Fairy Creek Floodplain Management Study 1996 1-3 1.4.3 Cabbage Tree Creek Floodplain Management Study 1997 1-3 1.4.4 An Interim Report on Flooding and Floodplain Management in the

South Fairy Meadow Area Incorporating the August 1998 Event (DRAFT) 1-3

1.4.5 Fairy Creek and Cabbage Tree Creek Floodplain Risk Management Plan – Photographs Taken During Catchment Tour on 27 October 2001 1-4

1.4.6 Summary 1-4

2 STUDY METHODOLOGY 2-1

3 DATA COLLATION 3-1

3.1 Rainfall Data 3-1 3.2 Topographic Data 3-1 3.3 Flood Level Data 3-2

3.3.1 Peak Flood Level Data 3-2 3.3.2 Continuous Stage Recorders 3-2

4 HYDROLOGIC AND HYDRAULIC MODEL DEVELOPMENT 4-1

4.1 Introduction 4-1 4.2 Hydrologic Model 4-1

4.2.1 Model Selection and Development 4-1 4.2.2 Routing of Flows 4-1 4.2.3 WBNM Lag Parameter 4-2

4.3 Hydraulic Model 4-2 4.3.1 Main 2D Model Domain 4-2 4.3.2 Wellington Drive 2D Model Domain 4-3 4.3.3 1D Network Upstream of Main 2D Domain 4-3

CONTENTS III


4.3.4 1D/2D Network Within 2D Domain 4-4 4.3.5 Structures 4-4 4.3.6 Representation of Handrails 4-4 4.3.7 Pipe Drainage Network Representation 4-5 4.3.8 Mt Ousley Road 4-5 4.3.9 Downstream Ocean Levels 4-6 4.3.10 Entrance Conditions 4-6

5 MODEL CALIBRATION 5-1

5.1 Hydraulic Model Calibration: August 1998 Flood 5-1 5.1.1 Model Calibration Procedure 5-1 5.1.2 Description of Rainfall Event 5-1 5.1.3 Assumptions Regarding Rainfall Distribution 5-2 5.1.4 Assumptions Regarding Rainfall Losses 5-3 5.1.5 Culvert and Bridge Blockage Assumptions 5-3 5.1.6 Manning’s “n” for Calibrated Model 5-4 5.1.7 Flood Model Replication of Flood Levels at Gauges 5-6 5.1.8 Flood Model Replication of Peak Flood Levels 5-8 5.1.9 Conclusions on Model Calibration to August 1998 Flood 5-8

5.2 Hydraulic Model Verification: October 1999 Flood 5-14 5.2.1 Description of Rainfall Event 5-14 5.2.2 Assumptions Regarding Rainfall Distribution 5-14 5.2.3 Assumptions Regarding Rainfall Losses 5-15 5.2.4 Culvert and Bridge Blockage Assumptions 5-15 5.2.5 Manning’s “n” for October 1999 Model 5-15 5.2.6 Flood Model Replication of Recorded Flood Behaviour 5-15 5.2.7 Conclusions on Model Verification to October 1999 Flood 5-17

5.3 Sensitivity Analyses of C Value 5-17

6 DESIGN EVENT MODELLING 6-1

6.1 Introduction 6-1 6.2 Design Flood Hydrology 6-1

6.2.1 Design Rainfall Totals and Temporal Patterns 6-1 6.2.2 Design Event Rainfall Losses 6-2 6.2.3 Resulting Inflows to Hydraulic Model 6-3

6.3 Design Flood Hydraulics 6-3 6.3.1 Updating of Model Geometry 6-3

LIST OF FIGURES IV


6.3.2 Manning’s “n” 6-3 6.3.3 Downstream Ocean Levels 6-4 6.3.4 Beach Bar Conditions 6-4 6.3.5 Critical Duration Rainfall Events 6-5

6.4 Culvert and Bridge Blockage Assumptions 6-5 6.4.1 WCC Blockage Policy 6-5

6.5 Design Flood Behaviour 6-7 6.5.1 Interpretation of Results 6-7 6.5.2 Flood Mapping of Design Flood Behaviour 6-7 6.5.3 Mapping of 1D Flood Levels Near Road Crossings 6-8 6.5.4 General Discussion on Design Flood Behaviour 6-8 6.5.5 Discussion on Design Flood Behaviour at Specific Areas of Interest 6-9 6.5.6 Assessment of Peak Flows 6-11

6.6 Provisional Flood Hazard 6-12 6.7 Conclusions on Design Flood Events 6-12

7 REFERENCES 7-1

APPENDIX A: DIPNR HISTORICAL SURVEY AND ANALYSIS OF THE FAIRY CREEK BEACH BAR A-1

APPENDIX B: ASSESSMENT OF CRITICAL DURATIONS B-1

LIST OF FIGURES

Figure 1-1 Locality Map 1-5 Figure 1-2 Fairy Creek and Cabbage Tree Creeks 1D and 2D Model Extents 1-6 Figure 3-1 Fairy Creek Continuous Stage Recorder 3-2 Figure 3-2 Cabbage Tree Creek Continuous Stage Recorder 3-3 Figure 4-1 Comparison of Fairy Creek Gauge and Ocean: August 1998 4-6 Figure 4-2 Comparison of Fairy Creek Gauge and Ocean: October 1999 4-7 Figure 5-1 Cumulative Rainfall for 17/8/1998 5-2 Figure 5-2 Blockage of Montague Street Bridge over Cabbage Tree Creek 5-4 Figure 5-3 Blockage in Concrete Open Channels 5-6 Figure 5-4 Comparison of Gauge Record with Unblocked Model Results:

August 1998 5-7 Figure 5-5 Comparison of Gauge Record with Blocked Model Results:

August 1998 5-8

LIST OF TABLES V


Figure 5-6 Cumulative Rainfall for 24/10/1999 5-14 Figure 5-7 Comparison of Gauge Record with Model Results: October 1999 5-16 Figure 5-8 Comparison of Gauge Record with Model Results: October 1999

(Manning’s n sensitivity) 5-17 Figure 5-9 Rainfall Stations 5-19 Figure 5-10 Areal Rainfall Distribution Assumed for August 1998 Flood Event 5-20 Figure 5-11 Areal Rainfall Distribution Assumed for October 1999 Flood Event 5-21 Figure 6-1 Design Rainfall Locations 6-13 Figure 6-2 Culvert and Bridge Blockages – Case 1 6-14 Figure 6-3 Culvert and Bridge Blockages – Case 2 6-15 Figure 6-4 Culvert and Bridge Blockages – Case 3 6-16 Figure 6-5 Culvert and Bridge Blockages – Case 4 6-17 Figure 6-6 Culvert and Bridge Blockages – Case 5 6-18 Figure 6-7 F6 Freeway – Flood Behaviour 1%AEP (9 Hour, Case 1 Event) 6-19 Figure 6-8 Northern Distributor Area – Flood Behaviour 1% AEP

(9 Hour Case 2 Event) 6-20 Figure 6-9 Montague St Area – Flood Behaviour 1%AEP (2 Hour Case 3 Event) 6-21 Figure 6-10 Anama St Area – Flood Behaviour 1% AEP (2 Hour, Case 2 Event) 6-22 Figure 6-11 Exeter Avenue Area – Flood Behaviour 1% AEP

(9 Hour Case 5 Event) 6-23 Figure 6-12 Nyrang Park Area – Flood Behaviour 1% AEP

(2 Hour, Case 2 Event) 6-24 Figure 6-13 Selected Peak Flow Locations 6-25 Figure 6-14 Provisional Hydraulic Hazard Categories (NSW, 2005) 6-26

LIST OF TABLES

Table 3-1 Topographic Information 3-1 Table 5-1 Roughness Values Used in Calibrated Model 5-5 Table 5-2 Discussion of Peak Flood Levels for August 1998 Flood 5-10 Table 6-1 Design Rainfall Parameters 6-1 Table 6-2 Design Rainfall Event Totals (mm) 6-2 Table 6-3 Roughness Values Used in Design Conditions Model 6-4 Table 6-4 Detention Basin Performance 6-10 Table 6-5 Peak Flows at Selected Locations 6-12

GLOSSARY VI


GLOSSARY

Annual Exceedance Probability (AEP)

The chance of a flood of a given size (or larger) 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 (i.e. a 1 in 20 chance) of a peak discharge of 500 m3/s (or larger) occurring in any one year. (see also average recurrence interval)

Australian Height Datum (AHD)

National survey datum corresponding approximately to mean sea level.

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 20yr ARI design flood will occur on average once every 20 years. ARI is another way of expressing the likelihood of occurrence of a flood event. (see also annual exceedance probability)

catchment The catchment at a particular point is the area of land that drains to that point.

design floor level The minimum (lowest) floor level specified for a building.

design flood A hypothetical flood representing a specific likelihood of occurrence (for example the 100 year or 1% probability flood). The design flood may comprise two or more single source dominated floods.

development Existing or proposed works that may or may not impact upon flooding. Typical works are filling of land, and the construction of roads, floodways and buildings.

discharge The rate of flow of water measured in terms of volume over time (i.e. the amount of water moving past a point). Discharge and flow are interchangeable.

DTM Digital Terrain Model – a three-dimensional model of the ground surface.

DEM Digital Elevation Model – a three-dimensional model of the ground surface. Often used interchangeably with DTM.

effective warning time The available time that a community has from receiving a flood warning to when the flood reaches them.

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 river or creek flows, which overtop the natural or artificial banks, and inundate floodplains and/or coastal inundation resulting from super elevated sea levels and/or waves overtopping coastline defences.

flood awareness An appreciation of the likely threats and consequences of flooding and an understanding of any flood warning and evacuation procedures. Communities with a high degree of flood awareness respond to flood warnings promptly and efficiently, greatly reducing the potential for damage and loss of life and limb. Communities with a low degree of flood awareness may not fully appreciate the importance of flood warnings and flood preparedness and consequently suffer greater personal and economic losses.

flood education Flood education seeks to provide information to raise awareness of the flood problem so as to enable individuals

GLOSSARY VII


flood damage The tangible and intangible costs of flooding.

flood behaviour The pattern / characteristics / nature of a flood.

flood fringe Land that may be affected by flooding but is not designated as floodway or flood storage.

flood hazard The potential risk to life and limb and potential damage to property resulting from flooding. The degree of flood hazard varies with circumstances across the full range of floods.

flood level The height or elevation of floodwaters relative to a datum (typically the Australian Height Datum). Also referred to as “stage”.

flood liable land see flood prone land

floodplain Land adjacent to a river or creek that is periodically inundated due to floods. The floodplain includes all land that is susceptible to inundation by the probable maximum flood (PMF) event.

floodplain management The co-ordinated management of activities that occur on the floodplain.

floodplain management measures

A range of techniques that are aimed at reducing the impact of flooding. This can involve reduction of: flood damages, disruption and psychological trauma.

floodplain risk management plan

A document outlining a range of actions aimed at improving floodplain management. The plan is the principal means of managing the risks associated with the use of the floodplain. A floodplain risk management plan needs to be developed in accordance with the principles and guidelines contained in the NSW Floodplain Development Manual. The plan will usually contain both written and diagrammatic information describing how particular areas of the floodplain are to be used and managed to achieve defined objectives.

floodplain management scheme

A floodplain management scheme comprises a combination of floodplain management measures. In general, one scheme is selected by the floodplain management committee and is incorporated into the plan.

Flood Planning Levels (FPL)

Flood planning levels selected for planning purposes are derived from a combination of the adopted flood level plus freeboard, as determined in floodplain management studies and incorporated in floodplain risk management plans. Selection should be based on an understanding of the full range of flood behaviour and the associated flood risk. It should also take into account the social, economic and ecological consequences associated with floods of different severities. Different FPLs may be appropriate for different categories of landuse and for different flood plans. The concept of FPLs supersedes the “standard flood event”. As FPLs do not necessarily extend to the limits of flood prone land, floodplain risk management plans may apply to flood prone land beyond that defined by the FPLs.

flood prone land Land susceptible to inundation by the probable maximum flood (PMF) event. Under the merit policy, the flood prone definition should not be seen as necessarily precluding development. Floodplain Management Plans should encompass all flood prone land (i.e. the entire floodplain)

flood proofing Measures taken to improve or modify the design, construction and alteration of buildings to minimise or eliminate flood damages and threats to life and limb.

flood source The source of the floodwaters. In this study, the Richmond River is the primary source of floodwaters, in conjunction with its tributary, Fawcetts Creek.

flood storages Floodplain areas that are important for the temporary storage of floodwaters during a flood.

floodway A flow path (sometimes artificial) that carries significant volumes of

GLOSSARY VIII


floodwaters during a flood.

freeboard A factor of safety usually expressed as a height above the adopted flood level thus determining the flood planning level. Freeboard tends to compensate for factors such as wave action, localised hydraulic effects and uncertainties in the design flood levels.

historical flood A flood that has actually occurred.

hydraulic The term given to the study of water flow in rivers, estuaries and coastal systems.

hydrograph A graph showing how a river or creek’s discharge changes with time.

hydrology The term given to the study of the rainfall-runoff process in catchments.

local overland flooding Inundation by local runoff rather than overbank discharge from a stream, river, estuary, lake or dam.

local drainage Smaller scale problems in urban areas. They are outside the definition of major drainage in this glossary.

mainstream flooding Inundation of normally dry land occurring when water overflows the natural or artificial banks of a stream, river, estuary, lake or dam.

peak flood level, flow or velocity

The maximum flood level, flow or velocity occurring during a flood event.

Probable Maximum Flood (PMF)

An extreme flood deemed to be the maximum flood likely to occur.

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 likely frequency or occurrence of flooding.

runoff The amount of rainfall from a catchment that actually ends up as flowing water in the river or creek.

stage See flood level.

stage hydrograph A graph of water level over time.

TUFLOW Hydrodynamic modelling software package developed by BMT WBM

velocity The speed at which the floodwaters are moving. Typically in 2D model studies, modelled velocities in a river or creek are quoted as the depth averaged velocity, i.e. the average velocity over the depth. In other situations, such as for 1D components of the model, velocities can be quoted as depth and width averaged velocities i.e. the average velocity across the whole river or creek section.

water level See flood level.

LIST OF ABBREVIATIONS IX


LIST OF ABBREVIATIONS

1D / 2D/ 3D One dimensional / Two dimensional / Three dimensional

AAD Annual Average Damages

AEP Annual Exceedance Probability

AHD Australian Height Datum

ARI Average Recurrence Interval

AR&R Australian Rainfall and Runoff

CBD central business district

cm centimetre

cumecs cubic metres per second

DA Development Application

DCP Development Control Plan

DECC Department of Environment and Climate Change (now DECCW)

DECCW Department of Environment, Climate Change and Water

DIPNR Department of Infrastructure, Planning and Natural Resources (now DECCW)

DNR Department of Natural Resources (now DECCW)

DLWC Department of Land and Water Conservation

DEM Digital Elevation Model

DTM Digital Terrain Model

EIS Environmental Impact Study

FPL Flood Planning Level

FRMS&P Floodplain Risk Management Study and Plan

GIS Geographic Information System

km kilometre

LGA Local Government Area

LEP Local Environmental Plan

m metre

m3/s cubic metres per second

m AHD Elevation in metres relative to the Australian Height Datum

MLHW Mean lower high water (tide)

PMF Probable Maximum Flood

PWD NSW Public Works (or Public Works Department) (now Department of Public Works and Services)

REP Regional Environmental Plan

RTA Roads and Traffic Authority of NSW

SES NSW State Emergency Services

WCC Wollongong City Council

INTRODUCTION 1-1


1 INTRODUCTION

1.1 General

Hydrologic and hydraulic flood models of Fairy Creek and Cabbage Tree Creek in Wollongong have been developed for Wollongong City Council (WCC) by Bewsher Consulting and BMT WBM. A locality map is provided in Figure 1-1.

The flood models define existing flood behaviour and provide a means for assessing floodplain management measures. The flood models are comprised of a hydrologic model and a hydraulic model and these are discussed separately below.

The hydrologic model determines the runoff resulting from a particular rainfall event. The hydrologic model covers the entire catchment. The primary outputs from the hydrologic model are hydrographs at varying locations along the waterways to describe the quantity, rate and timing of stream flow that results from rainfall events. These hydrographs then become a key input into the hydraulic model.

The hydraulic model simulates the movement of floodwaters through waterway reaches, storage elements, and hydraulic structures. The hydraulic model calculates flood levels and flow patterns and also models the complex effects of backwater, roughness, overtopping of embankments, waterway confluences, bridge constrictions and other hydraulic structure behaviour across the study area.

The hydrologic model used is WBNM. The hydraulic model developed is a hydro-dynamically linked two-dimensional / one-dimensional (2D / 1D) hydraulic model of the study area using the software TUFLOW. The model extents are shown in Figure 1-2.

The hydrologic and hydraulic models were calibrated to historical flood events to demonstrate the validity of the models. To calibrate the models, it was first necessary to obtain information such as flood heights, flooding patterns and velocities during historical flood events. The historical event used in the calibration process was the August 1998 with the smaller October 1999 flood event used as a verification event.

1.2 Catchment Description

Fairy and Cabbage Tree Creeks drain a relatively small catchment of 2074 ha or 20.74 km2.

The catchments of the tributary streams rise from the beach area (near sea level) to levels of between 300mAHD and 500mAHD. The width of the catchment (high western areas to coastal eastern areas) is only in the order of 4km. These steep catchment conditions provide for dynamic flooding and geomorphic conditions in the catchment.

Figure 1-1 shows the catchments, which can be generally classified into three relatively distinct bands:

• The upper areas are forested and very steep with some slopes approaching 65%. There are numerous parallel streams / gullies draining these areas.

INTRODUCTION 1-2


• The mid-catchment areas have elevations from about 100mAHD down to 10mAHD and are typically urban areas. The slopes are in the order of 2% to 4%. The watercourses are beginning to combine into less and larger watercourses.

• The lower catchment areas have elevations from about 10mAHD down to 1mAHD and are a mixture of urban and commercial areas. The slopes are in the order of 2% or flatter. The watercourses have combined into two major watercourses (Fairy Creek and Cabbage Tree Creek) prior to joining and forming a single creek at the outlet (Para Creek).

Given this small and steep catchment, the propensity for very heavy rain and progressive urbanisation, flooding is frequent and floods tend to be very flashy, with rapid rises and falls.

1.3 Purpose of this Report

A number of previous flood studies have been carried out in the Fairy and Cabbage Tree Creeks catchments (see Section 1.4). These studies used different flood models and provided different levels of detail in terms of their flood assessments. Most of the studies were carried out in isolation without proper recognition of the hydraulic inter-relation of Fairy Creek and Cabbage Tree Creek. The studies were also completed prior to the August 1998 flood, which provided a large amount of data that could be used as inputs to a flood model. In addition, the 1998 flood demonstrated problems with blockage of culverts causing flow diversions and increased water levels, and led to the establishment of WCC’s conduit blockage policy. Several flood mitigation works have also been undertaken since the earlier flood studies, which should be included in a new model.

For these reasons, WCC commissioned a new Flood Study and Floodplain Risk Management Study and Plan (FRMS&P) for Fairy and Cabbage Tree Creeks.

This Flood Study report defines flood problems in the study area. It is a technical document describing the development and simulation of the hydrologic and hydraulic models and presenting the simulation results for the calibration and design events with existing conditions.

The FRMS&P uses the flood model results presented in this Flood Study report to evaluate options for managing the flood problem. It includes a climate change sensitivity test and considers the implications of existing and future flood behaviour for setting Flood Planning Levels (FPLs).

This main volume of this Flood Study report is accompanied by an A3 Drawing Addendum.

1.4 Previous Studies

Many studies have previously been undertaken relating to flooding issues and impacts within the Fairy Creek and Cabbage Tree Creek catchments.

1.4.1 Cabbage Tree Creek Flood Study

Reference: PWD (1991)

Prepared For: Wollongong City Council

By: Public Works Department

INTRODUCTION 1-3


Date: February 1991

• Details the calibration and verification of the hydrologic (RORB) and hydraulic (MIKE 11) models to the March 1975 and March 1983 events.

• Design flood behaviour was predicted using the calibrated models for a number of design events.

• The study involved a compilation of rainfall and flood records for the March 1975 and March 1983 events.

1.4.2 Fairy Creek Floodplain Management Study 1996

Reference: WCC (1996)


By: Kinhill Engineers Pty Ltd

Date: 1996

• Investigates flood behaviour within the lower reaches of the Fairy Creek and Cabbage Tree Creek Catchments.

• Investigates the hydraulic impacts of F6 freeway.

• Reviews Wollongong City Council’s adopted Floodplain Management Strategy for Fairy Creek.

• Assesses flood mitigation opportunities and prepare viable options for consideration.

1.4.3 Cabbage Tree Creek Floodplain Management Study 1997

Reference: WP (1997)


By: Willing and Partners Pty Ltd

Date: 1997

• Using hydrologic and hydraulic models to define nature and extent of flooding.

• Use of models to estimate flood damages and assesses management options for reducing flood damages.

• Recommended both structural and non-structural measures to mitigate the impact of flooding within the Cabbage Tree Creek catchment.

1.4.4 An Interim Report on Flooding and Floodplain Management in the South Fairy Meadow Area Incorporating the August 1998 Event (DRAFT)

Reference: FR (1999)


INTRODUCTION 1-4


By: Forbes Rigby Pty Ltd

Date: August 1999

• Development and calibration of hydrologic and hydraulic models to the August 1998 event. The calibrated models were used to define 1% AEP and PMF flooding.

• Collation of available rainfall and flooding data for the rainfall event and subsequent flooding during August 1998. Rainfall records detailed in this report were used as part of the current study.

1.4.5 Fairy Creek and Cabbage Tree Creek Floodplain Risk Management Plan – Photographs Taken During Catchment Tour on 27 October 2001

Reference: Bewsher (2002)


By: Bewsher Consulting Pty Ltd

Date: May 2002

• A compilation of photographs taken of the catchment during a Committee Tour.

1.4.6 Summary

Flood levels estimated in the current Flood Study supersede those estimated in these earlier studies because of:

• more advanced computer modelling;

• new topographical information;

• an abundance of flood data from the August 1998 flood, used to calibrate the flood model;

• application of Council's (post-1998) conduit blockage policy to the design flood levels; and

• flood mitigation works implemented since the earlier models.

Note that the design flood levels presented in this report, plus freeboard, may not translate precisely to the FPLs to be determined in the FRMS&P. Other factors including climate change sensitivity and the potential for additional development of the catchment will be considered in the FRMS&P.

STUDY METHODOLOGY 2-1


2 STUDY METHODOLOGY

The methodology employed in undertaking this study is summarised as follows:

• All data necessary to develop the hydrologic and hydraulic models and against which the models were to be calibrated was collected, reviewed and placed in a format suitable for use. This process is detailed in Section 3.

• The hydrologic and hydraulic models were then developed. Topography, structures, land uses (impervious area) and initial roughness values were input. Details on the model development and a technical description of the models are contained in Section 4.

• The 2D/1D hydraulic model was then adapted to meet the need of the calibration/verification process. Historical changes to the topography and/or structures were integrated into the relevant model. This calibration/verification process is documented in Section 5.

• The August 1998 flood event was used as the calibration event. October 1999 was used as a verification event. Details are provided in Section 5.

• Following acceptance of the calibration by Council, the model was used to simulate the design flood events. Design event modelling and results are presented in Section 6.

• It is noted that Section 6 presents the design flood results under existing conditions. A climate change sensitivity test is presented in the FRMS&P. The important matter of setting FPLs used for development control is also considered in the FRMS&P. Relevant factors include existing flood behaviour, climate change flood risk and the potential for additional development of the catchment.

DATA COLLATION 3-1


3 DATA COLLATION

3.1 Rainfall Data

Wollongong City Council (WCC) provided digital data of rainfall records for a number of pluviometers in and around the catchment.

Rainfall data in the study area is collected by various agencies for their own specific purposes. The rainfall data for the stations within Wollongong and its vicinity were obtained from:

• Commonwealth Bureau of Meteorology (CBM);

• Roads and Traffic Authority (RTA);

• Manly Hydraulics Laboratory (MHL);

• University of Wollongong (UOW);

• Wollongong City Council (WCC); and

• Sydney Water.

3.2 Topographic Data

Several sources of topographic data were required for hydraulic model development. These sources, along with their use in modelling, are detailed in Table 3-1.

Table 3-1 Topographic Information

Source Description Model Application

Photogrammetry Developed from 1:4,000 aerial photography. Flown in 2001 and provided by HATCH as part of a larger data set

Topography base (DEM) for 2D / 1D hydraulic model

Airborne Laser Scanning Data

Flown in 2001 and provided by HATCH as part of a larger data set

Topography base (DEM) for 2D / 1D hydraulic model

Ground survey Additional data for DEM Topography base (DEM) for 2D / 1D hydraulic model

Structure Ground survey

Inverts and sizes of structures provided by HATCH and WCC

Development of culverts / bridges in the 2D / 1D model

The Digital Elevation Model (DEM) was developed by combining a number of digital data sets using 12D and Vertical Mapper software. The 12D software creates a Triangulated Irregular Network (TIN) and the Vertical Mapper software creates a raster representation of this TIN. The raster grid was developed with a resolution of 0.5m. The Digital Elevation Model is shown in Drawing 1 in the A3 addendum.

DATA COLLATION 3-2


3.3 Flood Level Data

3.3.1 Peak Flood Level Data

For the August 1998 flood, Council supplied flood levels for the entire Wollongong area. From this data, flood levels relevant to this study were derived resulting in 83 flood levels in the 2D flood model domain and 82 flood levels in the area represented by the 1D model network (i.e. a total of 165 points) for the August 1998 flood.

In addition, Council supplied a spreadsheet containing data associated with affected properties from numerous flood events but primarily the August 1998 flood. This information was input into a GIS format by linking the information with the property polygon from the digital cadastre information. While this information did not contain many flood levels, the qualitative information in the form of comments provided by residents has proved very valuable in the calibration phase.

For the October 1999 flood, there was no peak flood level information supplied by Council. This is most probably due to the relatively small size of the flood and the lack of any significant out-of-bank flooding.

3.3.2 Continuous Stage Recorders

There are two Continuous Stage Recorders located in the study area, one on Fairy Creek (Figure 3-1) and one on Cabbage Tree Creek (Figure 3-2). The gauge on Fairy Creek is located immediately downstream of the Princes Highway (Flinders Street) bridge near the corner of Bourke Street. The gauge on Cabbage Tree Creek is also located immediately downstream of the Princes Highway bridge.

Figure 3-1 Fairy Creek Continuous Stage Recorder

DATA COLLATION 3-3


Figure 3-2 Cabbage Tree Creek Continuous Stage Recorder

Levels for the August 1998 and October 1999 events were available from both of the recorders at an interval of every 15 minutes. The time histories of these gauges are presented as part of the discussion on the model calibration (Section 5).

HYDROLOGIC AND HYDRAULIC MODEL DEVELOPMENT 4-1


4 HYDROLOGIC AND HYDRAULIC MODEL DEVELOPMENT

4.1 Introduction

A flood model of Fairy and Cabbage Tree Creeks (Wollongong) was developed to define the flood behaviour in this area and assess flood hazard. The flood model was developed to define the existing flood behaviour and to provide a base case against which flood management measures are assessed. The flood model was comprised of a hydrologic model and a hydraulic model.

The hydrologic model determines the runoff resulting from a particular rainfall event. The primary output from the hydrologic model are hydrographs at varying locations along the waterways to describe the quantity, rate and timing of stream flow that results from rainfall events. These hydrographs then become a key input into the hydraulic model.

The hydraulic model simulates the movement of floodwaters through waterway reaches, storage elements, and hydraulic structures. The hydraulic model calculates flood levels and flow patterns and also models the complex effects of backwater, overtopping of embankments, waterway confluences, bridge constrictions and other hydraulic structure behaviour.

A WBNM (Watershed Boundary Network Model) hydrologic model of the catchment was developed. WBM then developed a hydro-dynamically linked two-dimensional / one-dimensional (2D / 1D) hydraulic model of the study area using the software TUFLOW.

The hydrologic and hydraulic models were calibrated to historical flood events to demonstrate the validity of the models. To calibrate the models, it was first necessary to obtain information such as flood heights, flooding patterns and velocities during historical flood events.

4.2 Hydrologic Model

4.2.1 Model Selection and Development

WBNM was the hydrologic model chosen to represent catchment rainfall-runoff relationships. It was chosen as a number of WBNM models have been developed in the Wollongong area including one for the Cabbage Tree Creek catchment.

A Uniform Continuing Loss Model was used for this study. An initial loss of rainfall occurs before any rainfall becomes effective as runoff. A continuing loss rate (millimetres per hour) was then applied to the rainfall to derive excess rainfall to be converted to runoff.

A total of 170 sub-areas were used to represent Fairy Creek and Cabbage Tree Creek catchments, as shown in Drawing 1 (refer Drawing Addendum). The total area of all sub-areas is 2074 ha.

4.2.2 Routing of Flows

The WBNM model was developed with the view to its ultimate use of flow input to TUFLOW. Therefore, in the area that is covered by the 2D / 1D hydraulic model, flow paths (i.e. connections in



WBNM from one sub-area to the next) are only as long as was needed to provide an appropriate entry point into the hydraulic model.

Most of the 170 sub-areas are linked in the WBNM model to a dummy node (9999) to allow direct input of the sub-area flows into the 2D / 1D hydraulic model for subsequent hydrodynamic routing. Hence, there are very few sub-areas that require routing of flows through a downstream sub-area prior to input into the 2D / 1D hydraulic model.

The routing of flows from one sub-area to the next is primarily a process carried out in the hydraulic model using more complex processes (such as simulating varying stage-storage relationships). Therefore, the use of the WBNM processes for routing based on a channel lag parameter is minimised.

Rainfall losses for the calibration, verification and design events are presented in Sections 5 and 6.

4.2.3 WBNM Lag Parameter

As part of this study, a previous calibration of the model used a varying C value (i.e. WBNM lag parameter) based on slope. Following discussion with WCC and DIPNR, the models (both hydrological and hydraulic) were re-calibrated with a constant C value of 1.3.

Following a review of the calibration with a constant C value, it was decided that a calibration with a constant C value of 1.3 was preferable to the calibration with the varying C value.

It needs to be noted that, in accordance with recommended WBNM modelling practice, all impervious fractions of sub-areas areas will necessarily have a C value of 0.13 as the C values for these areas are 10% of the C value for that sub-area.

4.3 Hydraulic Model

The complicated nature of the floodplain flow patterns and importance of obtaining community confidence in the process required that state-of-the-art modelling techniques be adopted. Hence, TUFLOW, a fully 2D / 1D dynamic hydraulic modelling system, was used to model the floodplain within the area of interest. This model required the development of two 2D domains (i.e. main area and Wellington Drive) linked by a complex 1D network.

4.3.1 Main 2D Model Domain

The 2D hydraulic model domain covers an area of almost 5.61 km2 (561 ha) as shown in Drawing 1. The model is based on a 5 m square grid, resulting in approximately 225,000 grid cells.

Each square grid element contains information on ground topography sampled from the DEM at 0.5m spacing, surface resistance to flow (Manning’s n value) and initial water level. A total of 13 areas of different land-use type based on aerial photography and site inspections were identified for setting Manning’s n values.



4.3.2 Wellington Drive 2D Model Domain

A 2D hydraulic model domain for Wellington Drive was developed in order to better represent the flood behaviour of this area. This 2D domain covers an area of 0.07 km2 (7 ha) as shown in Drawing 1. The model is based on a 1.5 m square grid, resulting in approximately 230,000 grid cells.

Each square grid element contains information on ground topography sampled from the DEM at 0.5m spacing, surface resistance to flow (Manning’s n value) and initial water level.

4.3.3 1D Network Upstream of Main 2D Domain

1D networks were also used upstream of the 2D model domain to simulate flood behaviour in the steeper parts of the catchment. The 1D model upstream of the 2D domain represents numerous individual tributaries of both Fairy Creek and Cabbage Tree Creek. Where it was obvious that there are complex flow interactions between tributaries (e.g. Helen Brae Avenue) the 2D domain was extended to include these areas.

The 1D network is comprised of the following elements:

• Nodes, which represent all the storage in the 1D network system. There are approximately 440 nodes in the model representing storage as a table of elevation versus area;

• Channels, which represent the conveyance between nodes. There are approximately 530 channels in the model and these take the form of:

Open channels defined by a cross-section with varying Manning’s n across the section;

Culverts (circular and box), which are defined by the shape and inlet / outlet losses;

Bridges, which are a closed section with a height varying loss coefficient

Weirs, which are defined by a cross-section.

Each channel is defined by a length, a cross-sectional shape (offset vs elevation and roughness integer) and an upstream and downstream invert level (i.e. slope). As for the entire model, all channel data is stored and managed in a GIS.

Originally, it was envisaged that the DEM would be of sufficient accuracy to allow derivation of the channel cross-sections. However, a number of checks were made on the DEM accuracy against ground-surveyed cross-sections, which indicated that the accuracy was not sufficient for the purposes of hydraulic modelling. Hence, approximately 125 cross-sections were surveyed by HATCH using ground survey methods in areas where it was expected that the survey accuracy of the DEM was insufficient. The ground-survey was provided in areas with dense riparian vegetation and supplemented with data from the DEM in areas away from the creek. The DEM was used to derive the remaining 230 cross-sections where it was expected that the accuracy of the DEM would be adequate. Another 10 cross-sections were derived from the 1996 MIKE-11 model created as part of the Fairy Creek Floodplain Management Study (Kinhill, 1996), mainly in the lower reaches of Fairy Creek.

Upstream and downstream inverts were generally derived by interrogation of the DEM or by interpolation from adjacent ground-surveyed cross-sections (where available).



The roughness integer varies across the cross-section allowing representation of the changes in vegetation and land-use across the section representing the channel. The 1D channels use the same relationship between roughness integer and Manning’s “n” as the 2D model cells (see Section 4.3.1).

All open channels were set as steep channels, which enable TUFLOW to check for super-critical (i.e. upstream controlled) flow.

The majority of the channels are used to derive the elevation versus area relationships for the nodes (to allow representation of storage). A typical node storage table is derived from half the length of the upstream channel multiplied by the cross-sectional width and the same for half the length of the downstream channel. The exceptions to this are the detention basins, which derive a stage-area relationship from drawings supplied by Council or interrogation of the DEM in the absence of design data.

4.3.4 1D/2D Network Within 2D Domain

While the main floodplain is aptly represented using 5m grid cells, the narrow width of the creek system and the pipe / culvert network within the 2D domain is better modelled as a 1D network. The width of creek modelled using 1D was removed from the 2D calculations for the length of the creeks to prevent ‘doubling up’ of creek conveyance. Surveyed cross-sections or those derived from interrogation of the DEM were used to determine 1D model node and channel characteristics.

The 1D and 2D components of the hydraulic model were dynamically linked, allowing water to flow out of the 1D model into the 2D floodplain once the water level reached bank height, and vice versa. The 1D/2D linkage is further explained in the TUFLOW manual.

4.3.5 Structures

Within the 2D model area, bridge structures were represented as a dynamically nested 1D bridge channel based on a cross-section through under the bridge deck. Bridge decks were modelled as dynamically nested 1D broad-crested weirs to allow flow over the bridge. The exception to this is Squires Way Bridge, which is modelled as a 1D weir. Culverts were modelled as dynamically nested 1D culvert structures.

For more explanation on the mechanisms for representing the hydraulic structures in TUFLOW, the reader is referred to the TUFLOW User Manual (WBM, 2008).

Survey of the structure sizes and inverts was provided by HATCH and Council surveyors. The data was provided to WBM in a variety of forms included GIS data and field sketches.

4.3.6 Representation of Handrails

Hand-rails and traffic guard-rails represent an obstruction to overland flow when a culvert or bridge capacity is exceeded (possibly due to blockage of the structure). Council provided details of all hand-rails and guard-rails in the study area in a GIS format with photographs for most features and details of the height of the hand-rail above road crest level. These features are represented in the model in two ways.



Firstly, if the structure is in the upper part of the study area covered by the 1D model elements only (i.e. outside the 2D domain), then the hand-rails are simulated as part of the “overflow weir” from the upstream side of the culvert to the downstream side. The hand-rails were assumed to block all flow up to the height of the rail. Flow is also able to flow around the hand-rail.

Secondly, if the structure is in the lower part of the study area covered by the 2D domain, the hand-rails are simulated as edges of cells raised by the height (usually about 1.0m). This resulted in a blockage of flow for these cells up to the height of the rail. Due to the resolution of the 2D grid, the length of the hand-rail was rounded to the nearest 5m.

4.3.7 Pipe Drainage Network Representation

TUFLOW has the ability to represent pipe network of underground drainage systems that are linked to either a 1D network or 2D domain of the overland flowpaths. Limited data was available on the location, size and invert levels of Council’s underground drainage system. A number of assumptions were required regarding the location of pipes and the invert levels. However, some data was supplied as part of the structure survey.

4.3.8 Mt Ousley Road

It was a requirement of the model that it represent the flow through the culverts under Mt Ousley Road as well as the ability to represent the overflow of runoff that cannot be carried by these culverts. It was assumed that this overflow would flow onto Mt Ousley Road and flow down the road towards the floodplain. The importance of this flowpath is that it has the ability to convey flow from one sub-catchment to another via a lateral link.

This arrangement was simulated in the 1D network by including the culverts under the road as channels in the network. A total of 21 culverts under Mt Ousley Road are represented in the model as individual flowpaths.

In conjunction with the culverts, the overflow to Mt Ousley Road is represented by weirs at the level of the western road crest at the culvert location (derived from the DEM). The weirs transfer flow only when the head level at the culverts has risen above the edge of the road. The excess flow is then transferred into steep slope channels representing the flow along Mt Ousley Road. The shape of the cross-section for the Mt Ousley Road channels was derived from the DEM and included representation of the New Jersey Rail in the median strip.

At the southern end of Mt Ousley Road, the road heads east-west and the camber of the road changes from a west-east slope to a north-south slope. At this location (i.e. immediately north of Dallas Street), in 1998 there was a potential for the Mt Ousley Road flow to weir into the back yards of Dallas Street residences. The 1D network represents this flowpath as a series of four weirs, which transfer flow down into the Dallas Street flowpath. After the 1998 flood, the RTA sealed the soundwalls, which is expected to prevent flow escaping from Mt Ousley Road into Dallas Street in subsequent events, and as such, this flowpath was excluded from the design model.



4.3.9 Downstream Ocean Levels

The downstream boundary for the hydraulic 2D / 1D model was assumed to be tidal recordings at Port Hacking which is the closest tidal recorder to the site. Data on a 15 minute interval was obtained from Manly Hydraulics Laboratory for the months of August 1998 and October 1999.

4.3.10 Entrance Conditions

The entrance of Fairy / Cabbage Tree Creek to the ocean is a beach bar that changes in geometry over time (WBM, 2005). The 1996 Fairy Creek Floodplain Management Study found that the bar builds up over time due to coastal processes and erodes quickly during a flood event (or with artificial opening). It was found that periods of strong tidal influence were recorded after significant rainfall events indicating that greater erosion occurred during high flows.

In August 1998, there was a minor flood event in the early part of the month, which would appear to have opened the entrance to an unknown degree. This is demonstrated in Figure 4-1 in a comparison between the gauge level at Fairy Creek and the ocean level at Port Hacking (assumed to be similar to the ocean level at Wollongong).

Hence, in simulating the August 1998 flood event, it was assumed that the entrance was already open and the bathymetry from the late August 1998 survey (after the flood event) was assumed to be the entrance condition throughout the entire flood.

For the month of October 1999, it is apparent from a similar comparison (see Figure 4-2) that the entrance was closed and that runoff during the month was constrained by the creek mouth. More importantly, it is apparent from this plot that the entrance was open after the flood event as the Fairy Creek gauge level responds to the ocean tides.

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Figure 4-1 Comparison of Fairy Creek Gauge and Ocean: August 1998



Hence, for the simulation of the October 1999 flood event, the “variable geometry” feature of TUFLOW was employed1. It was assumed that the beach bar was at an initial level of approximately 1.3 mAHD (with a limited outflow capacity) as indicated by the water level in the Fairy Creek gauge in the few days prior to the flood event on 24th October 1999. The model used a bar at this height for the initial geometry in the creek mouth. Model parameters were such that the bar scoured down to the same level and width as that indicated by the survey following the August 1998 flood event.

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Figure 4-2 Comparison of Fairy Creek Gauge and Ocean: October 1999

1 This feature allows TUFLOW to calculate changes to the bed bathymetry over time.

MODEL CALIBRATION 5-1


5 MODEL CALIBRATION

5.1 Hydraulic Model Calibration: August 1998 Flood

5.1.1 Model Calibration Procedure

The general steps of the process of calibrating the 2D / 1D model to the August 1998 flood were:

• review available historical data to establish appropriate calibration events;

• process data for the selected events and set up boundary conditions for the hydrologic model;

• carry out initial calibration and verification of the 2D / 1D model with parameters set at best estimate based on experience;

• continue calibration and verification of both hydrologic and hydraulic models using an iterative process which seeks to find the optimum combination of hydrologic and hydraulic parameters;

• present calibration to Council for review and feedback on flood extent and flooding patterns;

• finalise calibration based on feedback.

5.1.2 Description of Rainfall Event

The August 1998 flood was a somewhat rare flood event in the Wollongong area. The following are selected excerpts from the DLWC/WCC report “17 August 1998 Storms in Wollongong - Flood Data Report” (June 2002):

“Severe rainfall from the storm caused flash flooding with extensive damage to property. Some 1000 houses were estimated to have experienced above floor flooding caused by that storm and one fatality resulted. The main areas affected were the more populated central and northern suburbs of Wollongong City. These areas are located mainly within the Hewitts, Slacky, Bellambi, Towradgi, Cabbage Tree and Fairy Creek catchments. The event was of sufficient significance to be classified as a natural disaster.”

“During the period 15 August to 19 August a low pressure system in the northern Tasman Sea and a high pressure system in the southern Tasman Sea, were directing a moist unstable easterly airstream towards the New South Wales coast. Rainfall was recorded along most of the New South Wales coast during this time. In the Wollongong area, the most significant rainfall was recorded during the afternoon and evening of Monday 17 August 1998 (1700 to 2000 Eastern Standard Time). Severe flooding with significant damage occurred as a result of this rainfall.”

“The 17 August 1998 storm was characterised by a strip of very severe rainfall along the escarpment particularly over the central and northern catchments of Wollongong. The most significant recorded rainfall intensities occurred over a 3 to 6 hour time period with some rainfall stations recording intensity with an Average Recurrence Interval (ARI) in the order of 100 years. However, there was a significant decrease in rainfall intensities to the south of the CBD and to the north of Thirroul. The highest 24 hour total up to 9 am on Tuesday 18 August 1998 of 445 mm was recorded at Mt Ousley.”



5.1.3 Assumptions Regarding Rainfall Distribution

The rainfall for the August 1998 event was somewhat varied over the catchment. However, a plot of the cumulative rainfall totals for the day of 17th August 1998 presented here in Figure 5-1 indicates that there were three distinct rainfall patterns.

The pattern of the higher areas is represented by the Mount Ousley gauge and the Rixons Pass gauge. These gauges recorded nearly 400mm of rainfall for the day. The data for the Mount Ousley gauge (operated by the RTA) is considered to be some of the most important data for the flood simulation as it represents the highest rainfall on the steepest part of the catchment. However, this gauge only recorded rainfall every 15 minutes and does not adequately represent the intensity of the rainfall at the peak of the event. In order to improve the representation of this rainfall in short bursts, it was adjusted with rainfall from the Rixons Pass gauge.

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Figure 5-1 Cumulative Rainfall for 17/8/1998

There are three gauges (Glenifer Brae, Russel Vale and Corrimal) at mid-elevations in the vicinity of or inside the catchment, which received approximately 60% of the rainfall of stations in the higher areas. Unfortunately, the rainfall gauge at Balgownie did not record data for the August 1998 flood after 8:00pm. This incomplete data for Balgownie was not provided to WBM.

There are two stations near the Central Business District of Wollongong, which received considerably less rainfall in the order of 35% of the rainfall of stations in the higher areas. It is unknown whether this lower rainfall intensity is representative of the north-eastern parts of the study area (e.g. Towradgi Arm) or whether this area received rainfall similar to that of the Corrimal station further to the north of the catchment.



Recognising the variability of the rainfall over the catchment, and the sharp variations in catchment slope (i.e. steeply rising hills in the west), the method of triangulation by WBNM to derive rainfall weightings of nearby stations was not considered sufficient for the Cabbage Tree Creek catchment. Instead, rainfall-weighting factors were derived by manually drawn rainfall contours based on the total rainfall for the day of nearby stations. The rainfall stations in vicinity of the catchment are shown in Figure 5-9. The distribution of the rainfall assumed is presented in Figure 5-10.

For the Fairy Creek catchment, it was assumed that the triangulation by WBNM of the five gauges that surround the catchment (including one inside the catchment) would be sufficient to define the rainfall variability.

5.1.4 Assumptions Regarding Rainfall Losses

It is apparent from rainfall and runoff records for the month of August 1998 preceding the 17th August event that the catchment was well saturated. The Forbes Rigby report stated:

“The 17 August 1998 event was the second of two significant rain events affecting the Illawarra coast in August 1998. The first occurred during the period 6-8 August, when an intense low off the coast also brought heavy falls…When these totals are compared with medians for August, it is evident that even before 15 August the rainfall totals were already well above average for the month, in some cases exceeding over three times the average. As a result, the initial loss of rainfall events in the period 15 to 19 August in the Illawarra region would have been negligible. This would have exacerbated the runoff compared to the catchments if they had drier antecedent conditions.”

Hence, initial losses of 0 mm were assumed for this event for both impervious and pervious areas. As well, a value of 2 mm/h for continuing losses was assumed for the pervious areas. It needs to be recognised that a doubling of the continuing loss to 4mm would result in only a 5% reduction in net runoff volume for the upper parts of the catchment (where the majority of the pervious areas are located).

5.1.5 Culvert and Bridge Blockage Assumptions

The severity of this flood event was such that a significant amount of debris blocked many culverts and bridges to varying degrees throughout the study area. There was little information available on which culverts were blocked and to what degree. Hence, assumptions on the degree of blockage were made based on the need to match the recorded flood levels. As well, there were some records of residents reporting blockage of culverts and bridges in the data supplied by Council.

A feature in TUFLOW, which allows for the blockage of culverts, was employed to easily represent and track culvert blockages. The blocked or partially blocked culverts were included in a separate 1D network, which superseded the original network in the model-building phase of the simulation. The blocked or partially blocked culverts are presented on Drawing 2 as red lines.

For bridges, a range of blockage fractions were used depending on the afflux across the structure as indicated by the recorded flood levels. The bridge blockage was represented in two ways. Firstly, the width of the cross-section was reduced by either 25% or 50% (depending on the bridge). Secondly, it was recognised that the blockage by debris represents not only a loss of flow area but also a



significant increase in the energy loss through the structure due to the hydraulically inefficient shape of the debris. This is demonstrated in the figure below, which shows a car lodged against the piers of Montague Street Bridge across Cabbage Tree Creek following the August 1998 flood and other debris. As well, there is a significant amount of debris in the form of uprooted vegetation and fence material that contributed to both the reduction in available flow area and the hydraulic inefficiency of the opening.

For the major bridges across the creeks, loss factors of 0.2 v2/2g were used for levels up to 0.2m from the bridge deck. At this level, the loss factor varied linearly to 3.0 v2/2g to represent the blockage of the opening by debris that may have been floating through prior to the obvert being inundated. This value of 3.0 was derived from calculations of the head loss required across some of the bridges to match the recorded flood levels upstream and downstream. It is recognised that the reduction in width and area is also a factor in deriving the loss across the structure.

Figure 5-2 Blockage of Montague Street Bridge over Cabbage Tree Creek

5.1.6 Manning’s “n” for Calibrated Model

As discussed in Section 4.3, a total of 13 different roughness areas were identified in the 2D / 1D model. The Manning’s “n” values used to represent the roughness of each land-use type are presented in Table 5-1. The areal distribution of the differing roughness types was carried out using a GIS by inspection of a geo-referenced aerial photograph of the study area.

The change of the dense land vegetation and the concrete lined channels downstream of Guest Park stem from a changed approach to defining the effects of blocked open channels. The previous calibration used high Manning’s n values to account for the blockage of open channels by debris. The re-calibration used an approach of using a more expected value of Manning’s n with a multiplying factor to account for the blockage by debris (see below).



Table 5-1 Roughness Values Used in Calibrated Model

Roughness Type Manning’s “n” for C=1.3 Calibration Sand bed of creek 0.030

Mangroves and vegetation around creek mouth 0.150

Grass (maintained) 0.035

Grass (un-maintained) 0.045

Road Surface 0.025

Concrete Areas 0.026

Dense Riparian Vegetation 0.080

Dense Land Vegetation 0.040

Creek Bed and Middle of Creek 0.060

Building 3.000

Urban Residential Block (including fences) 1.000

Concrete Lined Channels downstream of Guest Park 0.025

Tidal Creek Bed and Banks 0.031

Fenced Areas 1.000

Dense Creek Vegetation 0.080

Mesh fences 0.200

The process of calibration involved consideration of the factors influencing flood behaviour. This was based largely on the consideration of photographs and comments by the community.

It was apparent in this process that a large length of Cabbage Tree Creek was partially obstructed during the peak of the flood by debris washed into the creek. Some of this debris resulted in the partial or complete blockage of structures. This was simulated in the model in a similar manner to that for the previous calibration and the previous design flood assessments.

In order to represent the partial obstruction caused by this debris in the creek, it was decided that a higher Manning’s n value should be applied. A multiplier of 1.5 was found to provide the best results.



Figure 5-3 Blockage in Concrete Open Channels

5.1.7 Flood Model Replication of Flood Levels at Gauges

It needs to be noted that the calibration of the 2D/1D model relies heavily upon assumptions regarding blockages of major structures and debris in parts of the creek system. These changes to the model tend to mask any impact of changing C values.

Further, the blockages most probably occurred after 7:00pm (19:00hrs) on the day of the flood when the flows generally doubled in response to a sharp, intense burst of rainfall.

Hence, any assessment of the model’s performance in matching the shape and timing of recorded flood level hydrographs at the two gauges should be made in the absence of blockages. This would allow an assessment of the model performance up to 7:00pm during which two peaks occurred.

To assist in this assessment, the model was run without any blockages or increases in n values along some reaches (to account for debris). The results of this simulation are presented in Figure 5-4.

Also shown in this figure is the performance of the previously calibrated model with a varying C value (and assuming blockage as per the previous reported calibration). By way of further explanation on this previous calibration, the hydraulic model was originally calibrated using inflows from a WBNM model with a C value varying with slope. Following discussion with WCC and DIPNR, the models (both hydrological and hydraulic) were re-calibrated with a constant C value of 1.3.

Following a review of the calibration with a constant C value, it was decided that a calibration with a constant C value of 1.3 was preferable to the calibration with the varying C value.



This figure shows that the timing of the model up to 7:00pm (19:00 on the X-axis) is quite good. In particular in Cabbage Tree Creek, the replication of the smaller middle peak (14:30 to 17:00) and the rise of the large peak (17:00 to 19:00) show a better pattern than that of the varying C (with blockages).

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Fairy Creek Gauge RecFairy Ck Gauge: TUFLOW with C = 1.3 UnblockedFairy Ck Gauge: TUFLOW with Variable CCabbage Tree Creek Gauge RecCabbage Tree Ck Gauge: TUFLOW with C = 1.3 UnblockedCabbage Tree Ck Gauge: TUFLOW with Variable C

Figure 5-4 Comparison of Gauge Record with Unblocked Model Results: August 1998

Figure 5-5 shows the model comparison for the blocked case. Also shown in this figure is the performance of the previously calibrated model with a varying C value (and the same degree of blockage).

Here, the influence of the assumed blockages can be seen. Specifically, in Cabbage Tree Creek, the blockages of the three bridges immediately downstream of the gauge were re-assessed. Following re-consideration of the photographic evidence and consultation with Council staff, it was concluded that the ship container was lodged against the railway bridge (and not the Freeway bridge as assumed in the previous calibration).



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Figure 5-5 Comparison of Gauge Record with Blocked Model Results: August 1998

5.1.8 Flood Model Replication of Peak Flood Levels

The 159 peak flood levels recorded throughout the study area and the flood level simulated by the 2D / 1D flood model at the same location are presented in Drawing 2. These levels are discussed below in detail and listed in Table 5-2.

Each of the 159 points has been numbered (up to 165 as there are some gaps in the sequence) and labelled to allow referencing in the discussion.

In the discussion, any point that the model matched within 0.2m was considered a good representation of recorded level. Of the 159 points assessed:

• 88 points are matched by the model to within 0.2m (previously 73); and

• 117 points are matched by the model to within 0.3m (previously 86).

This level of accuracy needs to be considered in conjunction with the variables of this type of modelling which include issues such as debris, local blockages and errors in recorded flood levels. As well, there is a degree of inaccuracy with the DEM, which has been relied upon to provide much of the cross-sectional data and creek inverts.

5.1.9 Conclusions on Model Calibration to August 1998 Flood

It needs to be recognised that the exercise of calibrating the model to the August 1998 flood is focussed on ensuring that the model accurately represents the flooding behaviour of the study area



during a large flood event. If there are parts of the study area where the model flood levels are not close to the recorded flood levels due to reasons such as an unknown degree of culvert blockage, then the consequences of this are not considered significant in the context of design flood runs in which nearly all culverts will be blocked.

The quality of the calibration of the model to the August 1998 flood should be considered on the following issues:

• Confidence that the hydrological model with a constant C value is representing the runoff characteristics of the catchment in terms of volume and response to rainfall (i.e. shape of hydrograph) as demonstrated in Figure 5-4;

• Confidence that the hydraulic model represents the timing of the catchment response to rainfall and runoff;

• Confidence that the hydraulic model represents peak flood levels to a suitable accuracy recognising the possible inaccuracies in the measurement of these levels and the unknown aspects of the flood event (e.g. debris blockage).

On the basis of the above issues, it is concluded that the models (hydrological and hydraulic) accurately represent the flooding characteristics of the catchment sufficient for defining flooding behaviour with design storm events.



Table 5-2 Discussion of Peak Flood Levels for August 1998 Flood

Point No.

Rec. Level (mAHD)

Model Level

(mAHD)

Error (m) Comments

1 38.12 37.55 -0.57 Poor representation of recorded level 2 32.45 31.90 -0.55 Difficult to represent geometry prior to Brokers Road Basin 1 construction 3 40.06 39.71 -0.35 Good representation of recorded level 4 40.19 40.10 -0.09 Represented well by Wellington Drive 2D model 5 40.16 39.87 -0.29 Represented OK by Wellington Drive 2D model - fences playing large part in flood levels6 40.30 39.92 -0.38 Represented OK by Wellington Drive 2D model - fences playing large part in flood levels7 28.29 27.94 -0.35 Culvert blockage assumed to 30% - maybe too high 8 27.94 27.92 -0.02 Good representation of recorded level 9 28.13 27.92 -0.21 Good representation of recorded level 10 27.16 27.92 0.76 Model too high - level taken from node U/S of node - possibly blockage to high 11 24.28 24.09 -0.19 Good representation of recorded level 12 23.60 23.60 0.00 Good representation of recorded level 13 23.03 22.80 -0.23 Good representation of recorded level 14 22.24 22.00 -0.24 Good representation of recorded level 15 21.33 20.98 -0.35 Fair representation of recorded level 16 21.33 20.83 -0.50 Suspect that this is same as Point 15 17 20.56 20.65 0.09 Good representation of recorded level 18 20.57 20.76 0.19 Good representation of recorded level 19 19.96 19.85 -0.11 Good representation of recorded level 20 19.81 19.81 0.00 Good representation of recorded level 21 19.85 19.60 -0.25 Fair representation of recorded level 22 17.20 17.45 0.25 Fair representation of recorded level 23 17.31 17.45 0.14 Good representation of recorded level - level of node U/S of road used 24 17.21 17.45 0.24 Good representation of recorded level - level of node U/S of road used 25 17.40 17.45 0.05 Good representation of recorded level - level of node U/S of road used

26 16.85 16.49 -0.36 This is level will always be difficult to reproduce as it is probably influenced by fence and gradient from road overflow back into creek.

27 16.85 16.49 -0.36 Suspect that this is same as Point 26 28 15.08 15.20 0.12 Good representation of recorded level 29 15.24 15.31 0.07 Good representation of recorded level

30 15.00 14.26 -0.74 Hard to believe this flood level as it is only 0.08m lower than Point 26 which is 120m upstream. Gradient not possible to replicate.

31 12.47 12.66 0.19 Good representation of recorded level

32 11.92 11.05 -0.87 This is level will always be difficult to reproduce as it is probably influenced by steep gradient across bridge deck

33 10.53 10.42 -0.11 Good representation of recorded level 34 9.97 9.73 -0.24 Good representation of recorded level 35 24.14 24.20 0.06 Good representation of recorded level 36 17.96 18.19 0.23 Model too high – recorded level conflicts with those D/S 37 21.76 21.75 -0.01 Good representation of recorded level 38 18.29 18.24 -0.05 Good representation of recorded level 39 18.16 17.77 -0.39 Fair representation of recorded level 40 11.32 11.39 0.06 Good representation of recorded level 41 11.32 11.29 -0.03 Good representation of recorded level 42 10.12 10.40 0.28 Fair representation of recorded level 43 10.22 10.39 0.17 Good representation of recorded level 44 10.32 10.34 0.02 Good representation of recorded level 46 10.00 9.70 -0.30 Fair representation of recorded level



Point No.

Rec. Level (mAHD)

Model Level

(mAHD)

Error (m) Comments

47 10.09 9.59 -0.50 Poor representation of recorded level

49 26.02 0.00 -26.02 Photo evidence indicates detention basin did not overtop. Only other possible source of flooding is local road flooding (from Foothills Rd)

50 23.72 23.77 0.05 Good representation of recorded level 51 22.16 21.96 -0.19 Good representation of recorded level 52 22.11 21.77 -0.34 Fair representation of recorded level. 53 15.17 14.97 -0.20 Good representation of recorded level 54 15.20 14.93 -0.26 Good representation of recorded level 55 15.30 14.95 -0.35 Fair representation of recorded level 56 13.79 13.27 -0.52 Fair representation of recorded level 57 13.70 13.25 -0.45 Fair representation of recorded level - level taken from node U/S of weir 58 12.17 12.04 -0.12 Good representation of recorded level 59 11.37 11.37 0.00 Good representation of recorded level 60 27.42 27.79 0.37 Model to high 61 22.98 23.86 0.88 Model too high - detail of local overland flowpath not in model. 62 47.48 47.06 -0.42 Fair representation of recorded level 63 24.67 24.38 -0.29 Fair representation of recorded level 64 16.79 Not wet N/A No overland flow here but could be achieved if higher blockage value used. 65 51.93 51.98 0.04 Good representation of recorded level 66 47.62 47.42 -0.20 Good representation of recorded level 67 18.63 18.41 -0.22 Good representation of recorded level 68 52.90 52.68 -0.22 Good representation of recorded level 69 52.46 50.38 -2.08 Good representation of recorded level. Level taken from node U/S of road 71 48.26 47.85 -0.41 Fair representation of recorded level 73 42.04 41.34 -0.70 Poor representation of recorded level Difficult urban overland flowpath. 74 47.90 38.73 -9.17 Recorded level possibly incorrect given surrounding levels 75 30.82 30.30 -0.52 Model too low – possibly more blockage in urban overland flowpath 76 30.10 30.22 0.12 Good representation of recorded level 77 30.25 30.22 -0.03 Good representation of recorded level 79 24.91 25.06 0.15 Good representation of recorded level 80 24.76 24.69 -0.07 Good representation of recorded level

81 20.21 20.58 0.37 Fair representation of recorded level. Highly sensitive to blockage assumption for Nyrang St basin culvert outlet.

82 20.40 20.10 -0.30 Fair representation of recorded level. 83 19.66 19.44 -0.22 Fair representation of recorded level. 84 19.52 19.39 -0.13 Good representation of recorded level 85 19.69 19.40 -0.29 Model too low - complex flowpaths out of ret village through fences 87 57.71 Not wet N/A Urban overland shallow flooding – difficult to accurately represent 88 57.57 Not wet N/A Urban overland shallow flooding – difficult to accurately represent 89 57.57 Not wet N/A Point disregarded as it is a repeat of Point 88 90 52.88 Not wet N/A Urban overland shallow flooding – difficult to accurately represent 91 52.88 Not wet N/A Point disregarded as it is a repeat of Point 90 92 52.31 Not wet N/A Urban overland shallow flooding – difficult to accurately represent 93 52.31 Not wet N/A Point disregarded as it is a repeat of Point 92 94 30.99 Not wet N/A Point disregarded as it is a repeat of Point 95 95 30.99 30.66 -0.33 Fair representation of recorded level 96 30.36 30.65 0.29 Fair representation of recorded level - complex weir flow across road 97 27.30 29.61 2.31 Suspect recorded level at same level as creek bed 98 24.86 24.73 -0.13 Good representation of recorded level



Point No.

Rec. Level (mAHD)

Model Level

(mAHD)

Error (m) Comments

99 24.16 24.73 0.57 Difficult to model this flowpath 100 9.35 8.99 -0.36 Fair representation of recorded level 101 8.66 8.37 -0.29 Fair representation of recorded level 102 8.66 8.84 0.18 Fair representation of recorded level 103 9.05 8.85 -0.20 Good representation of recorded level 104 8.96 8.52 -0.44 Good representation of flood levels - level taken from node u\s of road 105 9.12 8.72 -0.40 Model too low 106 8.28 8.27 -0.01 Good representation of recorded level 107 8.35 8.10 -0.25 Fair representation of recorded level 108 8.11 8.00 -0.11 Good representation of recorded level 109 6.33 6.52 0.19 Model too high – maybe blockage too high 110 5.20 5.73 0.53 Model too high – maybe blockage too high 111 6.09 6.14 0.05 Good representation of recorded level 112 7.07 7.38 0.31 Fair representation of recorded level 113 7.13 7.41 0.28 Fair representation of recorded level 114 4.34 4.41 0.07 Good representation of recorded level 115 4.16 4.26 0.10 Good representation of recorded level 116 3.98 4.20 0.22 Fair representation of recorded level 117 6.06 6.43 0.37 Model too high – maybe blockage too high 118 4.00 4.11 0.11 Good representation of recorded level 119 3.95 4.12 0.17 Good representation of recorded level 120 3.88 4.09 0.21 Fair representation of recorded level 121 3.86 4.02 0.16 Good representation of recorded level 122 3.94 4.16 0.22 Fair representation of recorded level 123 10.58 10.58 0.00 Good representation of recorded level 124 10.17 10.13 -0.04 Good representation of recorded level 125 10.07 9.93 -0.14 Good representation of recorded level 126 10.05 9.67 -0.38 Model too high – level taken from node U/S of road 127 6.96 6.63 -0.33 Model too low 128 6.96 6.82 -0.14 Good representation of recorded level 129 6.20 6.73 0.53 Model too high 130 6.03 6.11 0.08 Good representation of recorded level 131 6.04 6.09 0.05 Good representation of recorded level 132 5.98 6.09 0.11 Good representation of recorded level 133 4.08 4.13 0.05 Good representation of recorded level 134 3.71 4.01 0.30 Fair representation of recorded level 135 3.74 4.04 0.30 Fair representation of recorded level 136 4.01 3.93 -0.09 Good representation of recorded level 137 3.90 3.96 0.06 Good representation of recorded level 138 3.39 3.47 0.08 Good representation of recorded level 139 3.41 3.46 0.05 Good representation of recorded level 140 3.47 3.48 0.01 Good representation of recorded level 141 3.47 3.47 0.01 Good representation of recorded level 142 11.23 10.93 -0.30 Fair representation of recorded level 143 6.14 6.16 0.02 Good representation of recorded level 144 3.91 4.01 0.10 Good representation of recorded level 145 3.98 4.02 0.04 Good representation of recorded level 146 3.94 4.01 0.07 Good representation of recorded level



Point No.

Rec. Level (mAHD)

Model Level

(mAHD)

Error (m) Comments

147 3.84 3.80 -0.04 Good representation of recorded level 148 3.89 3.78 -0.11 Good representation of recorded level 149 3.89 3.75 -0.14 Good representation of recorded level 150 3.89 3.75 -0.14 Good representation of recorded level 151 3.86 3.75 -0.11 Good representation of recorded level 152 3.92 3.74 -0.18 Good representation of recorded level 153 11.69 11.68 -0.01 Good representation of recorded level 154 10.46 10.26 -0.21 Good representation of recorded level 155 6.15 6.02 -0.13 Good representation of recorded level 156 4.42 4.39 -0.03 Good representation of recorded level 157 4.39 4.37 -0.03 Good representation of recorded level 158 4.36 4.33 -0.02 Good representation of recorded level

159 3.34 4.34 1.00 Recorded level suspect – seems to be debris on pipe and possibly not at peak of flood as it is lower than those downstream.

160 4.27 4.02 -0.26 Fair representation of recorded level 161 3.99 3.82 -0.17 Good representation of recorded level 162 6.82 6.56 -0.25 Fair representation of recorded level 163 5.49 5.66 0.17 Good representation of recorded level 164 4.75 5.59 0.84 Recorded level conflicts with point 165 D/S 165 5.14 5.08 -0.06 Good representation of recorded level



5.2 Hydraulic Model Verification: October 1999 Flood

5.2.1 Description of Rainfall Event

The October 1999 rainfall event was a short rainfall burst in the late part of the morning of 24th October 1999. The rainfall preceding the event in the month was relatively low and, hence, there was some degree of absorption of initial rainfall in the pervious area of the catchment.

5.2.2 Assumptions Regarding Rainfall Distribution

The rainfall for the October 1999 event was very varied over the catchment. A plot of the cumulative rainfall totals for the day of 24th October 1999 presented here in Figure 5-6 indicates that the rainfall in the western (i.e. Mount Pleasant) and south-western (i.e. Figtree) parts of the study area received the highest rainfalls. However, the coastal areas (i.e. Wollongong) received very little rainfall. The data for the Corrimal gauge was not available for this event. In the absence of this data, it was assumed that the Wollongong station represented that rainfall along the south-western part of the catchment.

0

20

40

60

80

100

120

140

160

180

200

220

8:00 AM 8:30 AM 9:00 AM 9:30 AM 10:00 AM 10:30 AMTime on 24/10/1999

Rai

nfal

l for

24/

10/1

999

(mm

)

Russel Vale

Mount Pleasant

Rixons Pass

Keiraville

Wollongong

Balgownie

Figtree

Figure 5-6 Cumulative Rainfall for 24/10/1999

Recognising that the wide variation in rainfall totals across the catchment, the standard triangulation by WBNM of rainfall totals was not employed for this event. Isohyets of rainfall depths were derived from the totals recorded at the rainfall gauges. These isohyets were drawn in a similar manner to those derived by Reinfelds (2003) for the entire area. The resulting rainfall isohyets and point rainfall totals used are presented in Figure 5-11.



Four of the seven of the rainfall gauges (i.e. Wollongong, Figtree, Balgownie and Keiraville) recorded rainfall exactly one hour after the other two gauges. More importantly, this rainfall was recorded as falling after the peak of the flow recorded at the two stream gauges. Recognising that daylight savings started at midnight on the 23rd October 1999 (i.e. the night prior to the flood event), it is likely that these three gauges were set on Eastern Summer Time (i.e. daylight savings time) while the three other rainfall gauges, the two stream gauges and the tide gauge remained on Eastern Standard Time. Adjustments to the timings of these rainfall gauges were made to account for this assumption.

5.2.3 Assumptions Regarding Rainfall Losses

As discussed above, the rainfall preceding the event was relatively low. The initial losses were assumed to be 50mm for the pervious areas. Continuing losses were set at 2 mm/h.

5.2.4 Culvert and Bridge Blockage Assumptions

Due to the relatively small magnitude of the flood event in comparison with the August 1998 flood event, there was not any indication of significant culvert or bridge blockage. Hence, blockages were not simulated in the model.

5.2.5 Manning’s “n” for October 1999 Model

The same Manning’s “n” values were used for the October 1999 flood event as those used for the August 1998 flood event. There is some sense in reducing Manning’s “n” values for the in-stream areas due to the expected cleaning of the creeks following such a large event in August 1998. However, the degree of clearing is difficult to quantify. This issue is discussed further below in Section 5.2.6.

5.2.6 Flood Model Replication of Recorded Flood Behaviour

The model’s representation of the October 1999 flood event is presented in Figure 5-7 showing time histories for the two gauges. It needs to be recognised that the October 1999 flood peak at Cabbage Tree Creek gauge is lower than the middle and lowest peak of the August 1998 flood event. Hence, it is a small flood event that could be overly influenced by minor features related to channel conveyance.

It is apparent from Figure 5-7 that the model generally represents the timing and correct height of the Fairy Creek gauge. The timing of the flood peak and the general shape are considered adequate at the Fairy Creek gauge.

However, the model produces higher flood levels at the Cabbage Tree Creek gauge than those recorded by about 0.4m.

The reason for the model producing higher flood levels than those recorded at the Cabbage Tree Creek gauge could be due to the following:

• Possible lower Manning’s “n” values for this flood compared with the much larger August 1998 flood. (Either the August 1998 flood may have removed some of the creek vegetation or it was removed following the flood by Council recognising that it possibly contributed to the severity of the flood impacts).



• Under-estimation of rainfall losses;

• Poor definition of rainfall distribution in the northern part of the catchment.

Of the above reasons, it is thought that the third reason may be the primary contribution to the discrepancy. There was a wide variation in rainfall across the catchment as shown in Figure 5-6. It is possible that there was very little rainfall in the north-western parts of the catchment.

0

1

2

3

4

5

6

7

8

9

10

8.00 8.50 9.00 9.50 10.00 10.50 11.00 11.50 12.00 12.50 13.00Time (h)

Leve

l (m

AH

D)

TUFLOW at Cabbage Tree Ck GaugeCabbage Tree Ck Gauge RecTUFLOW at Fairy Ck GaugeFairy Ck Gauge Rec

Figure 5-7 Comparison of Gauge Record with Model Results: October 1999

In order to assess the validity of the first reason and its possible effect on flood levels at the gauges, a simulation with reduced Manning’s “n” values for the “Dense Riparian Vegetation” (reduced from 0.08 to 0.06) and “Creek Bed and Middle of Creek” (reduced from 0.06 to 0.03) was carried out. This simulation resulted in the model representation of the levels at the flood gauges as shown in Figure 5-8.



0

1

2

3

4

5

6

7

8

9

10

8.00 8.50 9.00 9.50 10.00 10.50 11.00 11.50 12.00 12.50 13.00

Time (h)

Leve

l (m

AH

D)

TUFLOW at Cabbage Tree Ck GaugeC abbage T ree Ck Gauge RecTUFLOW at Fairy Ck GaugeFairy Ck Gauge Rec

Figure 5-8 Comparison of Gauge Record with Model Results: October 1999 (Manning’s n sensitivity)

5.2.7 Conclusions on Model Verification to October 1999 Flood

The verification of the model to the October 1999 flood event highlights the following issues:

• The model may be quite sensitive to the state of vegetation in the creeks

• The model adequately represents the timing of small in-bank flood events.

• If the rainfall is adequately represented in the vicinity of the catchment, such as is the case with Fairy Creek, then the model accurately represents the flood levels at the gauge.

5.3 Sensitivity Analyses of C Value

The calibrated models (for 1998 and 1999) were re-run with a range of constant C values. The agreed values were 0.8, 1.1 and 1.7. The resulting peak levels were compared with the results of using a constant C value of 1.3. All other parameters were kept equal.

Drawing 3 shows the comparison of peak levels for these four cases at selected locations. As well, peak flows are presented for a number of locations (as identified by WCC).

In the upper parts of the study area (i.e. the area modelled only in 1D), the differences are in the order of 0.03m although some differences are up to 0.2m. In the lower parts of the study area (i.e. the area modelled 2D and 1D), the differences are all less than 0.1m, with most in the order of 0.02m. In general, the impact of the C value decreases with more routing in the 1D/2D model.

As expected, the results for the C value of 0.8 provide the highest results and the results for the C value of 1.7 provide the lowest results.



The peak flows at selected locations show a similar trend. As an example, the total flow in Cabbage Tree Creek just upstream of Guest Avenue varies 3% up (C=0.8) and 2% down (C=1.7).

The October 1999 flood results show a similar trend (see Drawing 4). However, the differences in some places are larger than the August 1998 flood as it was largely in-bank. Hence, the relationship between increased flow and increased level is more linear.

Based on this sensitivity analysis and the considerations regarding a varying C value, it was decided to adopt a constant C value of 1.3 for this study.

DESIGN EVENT MODELLING 6-1


6 DESIGN EVENT MODELLING

6.1 Introduction

The previous section described the development of the hydrologic and hydraulic models developed for the Fairy and Cabbage Tree Creek Flood Study. The calibration and verification of these models to actual flood events was also discussed. This section describes the next phase in the study; the development of design floods. The design hydrologic and hydraulic models are described and preliminary flood maps of the design floods are presented.

6.2 Design Flood Hydrology

6.2.1 Design Rainfall Totals and Temporal Patterns

The previous chapter described the WBNM hydrologic model that was developed for the study. The 20%, 5%, 2% and 1% Annual Exceedance Probability (AEP) rainfall events were simulated in the WBNM model using temporal patterns (variation of rainfall over time) and rainfall intensities for the catchment as described in Australian Rainfall and Runoff (2000) (AR&R). This is the standard approach for the development of design hydrographs in Australia.

Design rainfall totals were derived at five locations (shown in Figure 6-1) in and around the catchment in order to reflect the varying rainfall intensities over the catchment. The parameters derived from AR&R for these five locations are presented in Table 6-1.

Table 6-1 Design Rainfall Parameters

Parameter Location 1 Location 2 Location 3 Location 4 Location 5

50 year ARI, 1 hour intensity 46.5 49.0 50.9 47.5 46.0



2 year ARI, 1 hour intensity 90 99 111 102 95



F2 factor 4.28 4.28 4.28 4.28 4.28

F50 factor 15.85 15.85 15.85 15.85 15.85

Skew 0 0 0 0 0

Note: Locations shown in Figure 6-1



Based on the approach outlined in AR&R, the 1% AEP storm was simulated for the 1, 1.5, 2, 3, 4.5, 6, 9, 12-hour durations. The design rainfall totals for the design events are presented in Table 6-2.

The peak levels across the catchment were compared for all of the above durations. From this comparison the 2 hour and 9 hour durations were chosen to best define the critical durations for the study area. That is, the 2 hour and 9 hour durations produced the highest 1% AEP flood levels across the majority of the study area. The 2%, 5%, and 20% storm events were then simulated using the 2 hour and 9 hour durations. This is discussed further in Section 6.3.5.

Table 6-2 Design Rainfall Event Totals (mm)

Rainfall Event Location 1 Location 2 Location 3 Location 4 Location 5 20% AEP, 2 hour 79 85 93 86 81

20% AEP, 9 hour 144 155 177 163 150

5% AEP, 2 hour 104 114 127 118 109

5% AEP, 9 hour 196 214 248 229 206

2% AEP, 2 hour 123 135 153 142 130

2% AEP, 9 hour 235 259 303 279 249

1% AEP, 1 hour 100 110 125 115 106

1% AEP, 1.5 hour 120 133 152 140 128

1% AEP, 2 hour 137 151 173 160 146

1% AEP, 3 hour 164 181 209 193 174

1% AEP, 4.5 hour 196 217 251 232 208

1% AEP, 6 hour 222 246 287 264 236

1% AEP, 9 hour 266 294 345 318 282

1% AEP, 12 hour 302 334 394 363 320

Note: Locations shown in Figure 6-1

The probable maximum flood (PMF) is the largest flood that could reasonably be expected to occur on a catchment. For the PMF event, the rainfall patterns and totals were derived from the Bureau of Meteorology’s Bulletin 53 “The Estimation of Probable Maximum Precipitation in Australia: Generalised Short-Duration Method”. This approach was used because the critical duration for the catchment is generally less than 6 hours. This approach is recommended by Chapter 3.4.2 of Book 6 of Australian Rainfall and Runoff (IEAust, 2001). The 2 hour duration PMP rainfall total was determined to be 372 mm.

6.2.2 Design Event Rainfall Losses

An initial loss of 0 mm was assumed for this catchment. This assumption was made due to the likelihood that the short duration rainfall events that result in the critical durations for this catchment will not occur in isolation but part of a longer rainfall event with preceding rainfall to wet the catchment. This is a conservative assumption in line with the objectives of the study.



Continuing losses of 2 mm/h were used for all pervious surfaces in the catchment. Continuing losses of 0 mm/h were used for all impervious surfaces.

6.2.3 Resulting Inflows to Hydraulic Model

As described in Section 4.2.2, the hydrological model does not represent a full hydrological model of the catchment, but rather a network of individual sub-catchments that provide inflows to discrete points in the hydraulic model. Hence, it is not possible to discuss peak flows from the hydrological model at the catchment outlet.

6.3 Design Flood Hydraulics

6.3.1 Updating of Model Geometry

The calibrated TUFLOW hydraulic model described in Section 6 was slightly different for each of the calibration events, representing differences in the conditions at 1998 and 1999 (eg basins, culvert upgrades, creek changes). The model was updated to represent current conditions and included the following changes from the 1998/1999 model:

• Wollongong High School detention basin construction, including blockage of three of the six culverts under the F8 Freeway;

• Construction of the Brokers Road Detention Basin No.1 in 1999;

• Cabbage Tree Creek channel upgrades from Foothills Road Bridge to Church Street;

• Purchase and demolition of Properties in Anama Street;

• Reconstruction of footbridge between Rae Crescent and Church Street.

• Crawford Avenue / Bodes Bridge. Changes to four sections between the railway line and Bodes Bridge.

• Inclusion of the drainage path (piped and overland) at Williams Street in the 1D model.

6.3.2 Manning’s “n”

The Manning’s “n” values use for the design conditions flood model were the same as those used for the calibrated flood model. These values are reproduced below in Table 6-3.



Table 6-3 Roughness Values Used in Design Conditions Model

Roughness Type Manning’s “n” Sand bed of creek 0.030

Mangroves and vegetation around creek mouth 0.150

Grass (maintained) 0.035

Grass (un-maintained) 0.045

Road Surface 0.025

Concrete Areas 0.026

Dense Riparian Vegetation 0.080

Dense Land Vegetation 0.040

Creek Bed and Middle of Creek 0.060

Building 3.000

Urban Residential Block (including fences) 1.000

Concrete Lined Channels downstream of Guest Park 0.025

Tidal Creek Bed and Banks 0.031

Fenced Areas 1.000

Dense Creek Vegetation 0.080

Mesh fences 0.200

6.3.3 Downstream Ocean Levels

The downstream boundary for the hydraulic 2D / 1D model was assumed to be 1.0m AHD. This level was derived during discussion with WCC and is based on the findings of the Hewitt’s Creek Flood Study (WCC, 2002). The Hewitts Creek Flood Study concluded that an ocean level of 1.0m AHD would provide a conservatively high but ‘likely’ tidal/ocean condition at the peak of a design flood (MLHW+0.7m). This level was applied as a constant water level to the downstream boundary. Varying the water level over time was shown to result in only minor changes in peak flood levels for the very lower parts of the model (i.e. downstream of Squires Way Bridge)..

The 1% AEP and PMF flood maps represent a combination of the modelled flood events and a peak ocean level of 2.7m AHD and 3.7m AHD, respectively. The elevated ocean levels were chosen for consistency with the Towradgi Creek Flood Study Addendum (Bewsher Consulting, 2003).

6.3.4 Beach Bar Conditions

The entrance of Fairy / Cabbage Tree Creek to the ocean is a beach bar that changes in geometry over time (WBM, 2005). DIPNR carried out a review of the beach bar conditions. The results of this review are reproduced in Appendix A of this report (prepared by DIPNR). It was concluded that a fixed bar at 1.6mAHD should be used for all design storm events.

Based on an analysis of the beach bar, it was estimated that an average length of the bar would be 80m (i.e. in the direction of flow). The width of this bar was estimated to be 170m wide (i.e. perpendicular to the direction of flow).



6.3.5 Critical Duration Rainfall Events

In order to determine the critical duration flood event for the study area, a number of different durations (i.e. 1, 1.5, 2, 3, 4.5, 6, 9, 12 hours) were simulated for the 1% AEP flood event. The aim of this exercise was to determine which of these flood events provides the highest peak flood levels. For simplicity, the case with all culverts blocked (except Mount Ousley Road culverts) was used for this assessment.

The critical duration was determined from this assessment to be the 2 hour event in some parts of the study area and the 9 hour event in other parts of the study area. A full list of peak flood levels at all 470 node locations throughout the hydraulic model is presented in Appendix B. Also presented is the duration that results in the highest flood level at each node as well as the difference between the highest flood level and the 2 hour or 9 hour flood level.

It is apparent from this comparison that the 2 hour or 9 hour flood events represent the majority of peak flood levels. The 2 hour duration represents the critical duration at 307 of the 470 locations while the 9 hour duration represents the critical duration at 11 of the 470 locations. The 1.5 hour duration storm represents the critical duration at 85 of the 470 locations. However, it is expected that the behaviour of the 1.5 hour storm will be very similar to that of the 2 hour storm. Furthermore, when considering the impacts of fully blocked cases, the longer duration events (with more volume of rainfall) are expected to be more significantly influenced by blockage than the short duration events. Hence, the choice of the 2 hour and 9 hour duration events for the critical durations will provide a good representation of critical conditions over the entire study area for a range of blocked and unblocked scenarios.

Of those locations that do not have a critical duration of 2 hours or 9 hours, the peak flood level is within 100mm of either the 2 hour or 9 hour flood level.

6.4 Culvert and Bridge Blockage Assumptions

6.4.1 WCC Blockage Policy

Wollongong City Council developed a policy for assumptions regarding blockage of culverts and bridges. This policy is presented below:

The following blockage factors are to be applied to structures across all watercourses when calculating design flood level:

(i) 100% blockage for structures with a major diagonal opening width of less than 6m;

(ii) 25% bottom up blockage for structures with a major diagonal opening width of greater than 6m. For bridge structures involving piers or bracing, the major diagonal length is defined as the clear diagonal opening between piers/bracing, not the width of the channel at the cross-section;

(iii) 100% blockage for handrails over structures covered in (i) and for structures covered in (ii) when overtopping occurs.

This policy is open to interpretation in relation to the treatment of blocking bridge openings described in (ii) above. If the major diagonal length is less than 6m in a single span, the question remains if that



requires a 25% blockage of that particular span or the entire bridge. It was decided in conjunction with Council that if a single span is greater than 6m then a 25% blockage would be applied to all spans of the bridge.

One of the key outcomes from this study is the definition of peak flood levels resulting from the worst combination of blockages under Council’s conduit blockage policy. In conjunction with Council officers, six cases were derived that were considered to cover the worst possible case for all parts of the study area. These six cases are discussed below and shown in Figure 6-2 to Figure 6-6.

• Case 0: All culverts and bridges fully open.

• Case 1: All culverts and bridges fully blocked (according to the policy) except:

Mount Ousley Road culverts;



Culvert number W209_0130_C, which runs under the F8 at Graham Avenue; and

Culvert number W309_0110_C, which passes under the F8 at University Avenue.

By modelling these two culverts as unblocked, flooding is maximised downstream of the F8.


Culvert number W211_0020_C, which runs parallel to the F8 from Graham Avenue to University Avenue.

With this culvert unblocked upstream flow from this watercourse is forced into the northern watercourse. The blockage of the Mount Ousley Road culverts forces flow down Mount Ousley Road. The combination maximises flooding to this area.



Culvert number W211_0020_C, which runs parallel to the F8 from Graham Avenue to University Avenue and

Culvert number WC20_070_C, the Wollongong High School detention basin outlet under the F8.

This combination of modelling W211_0020_C (described above) and the detention basin outlet as unblocked maximises flooding downstream of the F8 in the areas of Montague Street and Ralph Black Drive.


Culvert number W211_0020_C, which runs parallel to the F8 from Graham Avenue to University Avenue;

Culvert number W309_0110_C, which passes under the F8 at University Avenue, FC10_0130_C, which runs under the F6 at Gilmore Park, and



Culvert number CC40_0020_C, which runs under the F6 at the University. This Case evaluated several different conditions.

By modelling W211_0020_C and W309_0110_C as unblocked flooding is maximised downstream of the F8 at University Avenue. Modelling FC10_0130_C as unblocked maximises the flow through Gilmore park detention basin and down Fairy Creek. The blockage of the Mount Ousley Road culverts combined with modelling culvert CC40_0020_C as unblocked maximises the flow downstream of the culvert through Wollongong High School and Jobsons Avenue.

It should be noted in the details presented above that the expression “All culverts and bridges fully blocked” also means that the high Manning’s n values associated with expected debris in open channels (see Section 5.1.7) were also simulated with the same Manning’s n multipliers as used in the 1998 calibration event on the same reaches of open channel. As well, in all design cases listed above (except for Case 0), all handrails were assumed to be fully blocked.

6.5 Design Flood Behaviour

6.5.1 Interpretation of Results

The interpretation of the drawings, maps and other data presented in this report should include an appreciation of the limitations of the accuracy. While the points below highlight these limitations, it is important to note that results presented provide an up-to-date, prediction of design flood behaviour using the best modelling techniques currently available. Points to remember are:

• Recognition that no two floods behave in exactly the same manner;

• Design floods are a best estimate of an “average” flood for their probability of occurrence;

• The topography datasets (photogrammetry, ALS, ground survey) used to generate the DEM all have varying uncertainties associated. Flood depths and flood extents, which are determined using this DEM, should be interpreted accordingly.

All design floods are based on statistical analyses of recorded data such as rainfall and flood levels. Statistical analysis is used in the creation of design isopleths (AR&R, 1987). The longer the period of recordings that the statistical analysis is based on, the greater the certainty. For example, derivation of the 1% AEP rainfall from 20 years of recordings would have a much greater error margin than from 100 years of recordings.

Similarly, the accuracy of the hydrological and hydraulic computer models is dependent on the amount and range of reliable rainfall and flood level recordings for model calibration.

6.5.2 Flood Mapping of Design Flood Behaviour

For the 1%, 2%, 5%, 20% AEP and PMF flood events, there are 12 events for each AEP in all, as there are six blockage cases (including one un-blocked case) for two durations (i.e. 2 hour and 9 hour events). An envelope of peak flood levels was derived for each AEP from the 12 flood events. The 1% AEP model results were then enveloped with an elevated 1% AEP ocean boundary of 2.7m AHD (refer Section 6.3.3). The PMF model results were enveloped with an elevated PMF ocean boundary of 3.7m AHD (refer Section 6.3.3). The envelope of the peak flood levels are mapped in Drawings 5 to 9 for the 1%, 2%, 5%, 20% AEP and PMF floods.



For areas outside the 2D domain, the flood level surface is calculated by subtracting the ground elevation (based on the DEM) from a surface of flood levels (as calculated by the model). The derivation of this surface of flood levels is based on a linear interpolation from one 1D node to the next along the 1D channel path. However, in reality, this may not be the case.

As well, there are discrepancies associated with the DEM ground levels. Where the DEM is indicating ground levels higher than those surveyed as part of the ground-survey cross-sections, the depth surface is generally too thin or non-existent as the DEM ground levels are higher than the model flood levels.

Outside of the 2D model the predicted water levels were trimmed to a DEM of various sources. A composite DEM was used to overcome areas in which the flood study DEM was higher than surveyed ground sections in the watercourses. This composite DEM was derived from the following process:

1 DEM created from the lower of the flood study DEM (See Section 3.2) and an ALS DEM (2005) provided by WCC;

2 Created a DEM from the TUFLOW processed data;

3 The DEM of TUFLOW data was trimmed to areas where combined DEM in step 1 was above ground survey;

4 A composite DEM from those described in steps 1 and 3 (minimum).

The 2D model areas are considered “high confidence” with an expected flood level accuracy of ±300mm. The 1D model areas that are included in the mapping are considered “medium confidence” with an expected flood level accuracy of ±500mm.

Areas upstream of the limits of mapping shown on Drawings 5 to 10 are those areas either not modelled or where the confidence in the flood modelling is not sufficiently high enough to rely upon for flood management. One reason that areas may be outside the limit of mapping is that overland flow paths may occur that are not represented in the hydraulic model.

6.5.3 Mapping of 1D Flood Levels Near Road Crossings

In areas upstream of the 2D model where flood levels are derived from the 1D model, flood levels are assigned based on a linear interpolation between the upstream and downstream nodes. At road crossings this creates a flood gradient that produces levels significantly lower downstream of the road than on the upstream side. In reality, the flood gradient across these roads is relatively flat across the road and then very steep in the area / house block immediately downstream of the road crossing.

In order to represent this flood behaviour adequately, the road crossings and the upstream edge of the residential block immediately downstream of the road crossing were mapped with the flood level derived from the node upstream of the road.

6.5.4 General Discussion on Design Flood Behaviour

The flooding behaviour represented by the hydraulic model for the design flood events is similar to that experienced in the August 1998 flood. However, the six blockage scenarios provide differing degrees of flooding in various parts of the study area. It should be noted that in some cases the



mapping of the design events show areas where the flows decrease moving downstream. The flows presented are from the 1D model and, therefore, do not take into account breakout points where the flooding spreads to the main 2D model domain.

The following general comments are made about the resulting peak 1% AEP flood level in relation to the 18 events simulated:

• The 2 hour duration event with Case 0 or Case 1 blockages results in the highest peak flood levels for the steeper parts of the study area.

• The 9 hour duration event with Case 3 blockages results in the highest peak flood levels between Puckey Avenue and Squires Way Bridge in the downstream parts of the Cabbage Tree Creek catchment. This is primarily due to the concentration of flow down Mount Ousley Road from blocking of all the culverts under this road.

• The 2 hour duration event with Case 3 blockages results in the highest peak flood levels for the areas along and downstream of Mount Ousley Road. This is primarily due to the concentration of flow down Mount Ousley Road from blocking of all the culverts under this road resulting in approximately 100 m3/s flowing down the road.

6.5.5 Discussion on Design Flood Behaviour at Specific Areas of Interest

Council has highlighted the following areas as being of particular interest. The flood behaviour from the 1% AEP flood envelope for each area is discussed below.

• Mt Ousley Road: At several locations along Mount Ousley Road culverts under Mt Ousley Rd reach full capacity and the road is overtopped. Due to the Jersey rail in the centre of the road, flow is then trapped in the northbound lane and flows down the road. Flow is contained on Mount Ousley Road northbound lane until it reaches a low point at Binda St. At this point flows on Mount Ousley Road are 2 m3/s and 96 m3/s for culverts fully open and fully blocked on Mount Ousley Road respectively, for the 2 hour event. For the 9 hour event, flows are 59 m3/s for the blocked cases. There are no flows on Mount Ousley Road in the 9 hour unblocked case. Flows break out to the south at the junction of the F6 and Mount Ousley Road.

• Cassian Street: The dendritic pipe network behind houses in Cassian Street is modelled as fully open for all cases. The pipe accommodates a peak flow of 6 m3/s. The overland flows in this area are up to 8 m3/s for the 2 hour event and 0.5 m3/s for the 9 hour event. Peak envelope depths are up to 2m.

• Andrew Avenue, Keiraville: With the culverts operating at full capacity, 4 m3/s flow out of the detention basin upstream of Andrew Avenue and 9 m3/s flow over the weir in the 2 hour event and 4.3 m3/s in the 9 hour event. When the culvert is fully blocked, 13 m3/s flows over the weir in the 2 hour event and 8 m3/s in the 9 hour event. The capacity of the culvert under Andrew Avenue is 3.6 m3/s when fully open, with 14 m3/s flowing over the road in the 2 hour event and 6 m3/s in the 9 hour event. With the culvert fully blocked 17 m3/s flow over the road in the 2 hour event and 10 m3/s in the 9 hour event. Overland flows continue across Murphy’s Avenue and spread across the residences bounded by Murphy’s Ave, Robsons Rd and Harkness Ave.

• The F6 Freeway: There are several road crossings and areas of inundation along the F6 Freeway. For the crossing at the northern end of the F6 Freeway, inundation is highest when the



culverts on Mount Ousley Road are blocked. Flow over the road at this point is 102 m3/s in the Dallas Street Branch. During the 1% AEP event (with all culverts blocked), a diversion occurs at the crossing on the F6 Freeway at the University Branch. Flows move south along the F6 Freeway into the Botanic Gardens Branch via culverts with velocities of up to 3 m/s. Diversions also occur in the Nyrang Park Branch and move north into the Botanic Gardens Branch with velocities of up to 1.2m/s. This diversion occurs in the Nyrang Park Branch at the Gipps Rd overpass. This flow moves north along the Northern Distributor to the crossing of the Nyrang Park Branch under the Northern Distributor. Flood levels and flow vectors for the 1% AEP (Case 1) event in the F6 Freeway area are presented in Figure 6-7.

• The Northern Distributor and Railway Line: Overtopping of the Northern Distributor occurs at several locations. To the north of the study area, diversions occur due to blockage of the culverts under the Northern Distributor. At the junction of the Northern Distributor and Chapman Street, flow overtops the road and flows south to the next crossing. There is no overtopping of the railway in this section. South to the next crossing, flows divert on to the Northern Distributor due to culvert blockages and move south. There is also flow south on the railway at this point. This flow moves south and then turns east through a low section, through properties onto Elliotts Road. Flood levels and flow vectors for the 1% AEP event in this section area are presented in Figure 6-8. Inundation of the Northern Distributor and Railway Line is predicted in the fully blocked case at approximately Woodhill St due to two sets of culverts being blocked. At this point flows from the south (from overtopping at the Botanic Gardens Branch crossing) re-enter the creek system. Flood levels and flow vectors for the 1% AEP event in this area are presented in Figure 6-9.

• Performance of Existing Detention Basins: Table 6-4 shows the flow upstream and downstream of each detention basin structure, including the culvert and weir immediately downstream of the basin for the 1% AEP 2 hour duration event with no blockages. Nyrang Park and Wollongong High School Detention basins are not shown as they are modelled in the fully 2D domain. Flow lines would be required to quantify the inflow and outflow to each basin.

Table 6-4 Detention Basin Performance

Maximum Outflow (m3/s)

Detention Basin

(Inflow m3/s) Culvert Weir Combined

Gilmore Park 26.5 14.7 11.8 26.2 Foothills Rd 29.4 18.1 0.0 18.1* Wiseman Park 36.3 32.1 0.0 32.1 Gunyah Pk 5.9** 2.5 0.0 2.5 Brokers Rd 2 9.1 3.1 0.4 3.5 Brokers Rd 1 127.6 86.8 35.6 122.4 Robsons Rd 12.0 8.2 0.0 8.2 Fisher St 31.0 25.0 0.0 25.0 Andrews Ave 20.1 4.8 8.9 12.9 G:\Admin\B14106.g.gjr\[Detention_basin_performance_130.xls]Tables

*Enters 2D domain at this point **WBMN inflow

• Montague Street: Early in a large flood event, water flowing down Cabbage Tree Creek and the Dallas Street Branch breaks out and flows down Montague Street. Shortly after, increasing flows



cause waters from Fairy Creek to inundate areas at the southern end of Montague Street. At the peak of the flood, velocities down Montague Street range from 0.6 m/s to 1.2 m/s. Depths range up to 1.6m on Montague Street and private properties. Flood levels and flow vectors for the 1% AEP event in the Montague St area are presented in Figure 6-9.

• Anama Street: Flow from Cabbage Tree Creek breaks out of the main channel and flows down Anama Street when levels in the creek reach approximately 8.0m AHD. Velocities down Anama Street at the peak of the flood envelope reach 1.6 m/s. Flow continues down Anama Street and across the Princes Highway and then re-enters Cabbage Tree Creek. Flood levels and flow vectors for the 1% AEP event in the Anama St area are presented in Figure 6-10.

• Exeter Avenue and Achilles Avenue: Flows enter both these roads through breakouts upstream. With all culverts fully open, the main source of flows is The Nyrang Park Branch, a tributary of Fairy Creek. When culvert blockages are in place, flows also enter from the Botanic Gardens Branch caused by blockage of Flinders Street Culverts and the Railway line culverts. Velocities are up to 1.8 m/s in Exeter Ave and 0.6 m/s in Achilles Avenue. Flood levels and flow vectors for the 1% AEP event in the Exeter/Achilles Avenue area are presented in Figure 6-11.

• Nyrang Park, Keiraville: In a large event, or when blockages are in place, flows exceed the capacity in the pipe network in the Nyrang Park area. The pipe network connects the open channel at Southern end of Rosedale Avenue and the Nyrang Park Detention Basin. When this pipe capacity is exceeded a diversion occurs and water flows through private properties and over Braeside Avenue. Flow continues along Anne St and into John St before crossing Gipps St and flowing through the retirement village on Gipp St. The water then re-enters the Creek. Flood levels and flow vectors for the 1% AEP event in the Nyrang Park area are presented in Figure 6-12.

6.5.6 Assessment of Peak Flows

Peak flows are presented in Table 6-5 for several locations in the study area (shown in Figure 6-13) for the 1%, 5% and 20% AEP flood events for a 2 hour duration. The “Case 0” (i.e. unblocked) scenario was used for this analysis.



Table 6-5 Peak Flows at Selected Locations

Location Peak Flow 20% AEP (m3/s)

Peak Flow 5% AEP (m3/s)

Peak Flow 1% AEP (m3/s)

Peak Flow PMF (m3/s)

Crawford Ave Overland Flow 1 5 13 43

Crawford Ave Culverts 26 29 30 31

Irvine Street Overland Flow 0 6 14 41

Irvine Street Culverts 9 9 9 9

Nyrang Park Branch - Hillview Ave Overland 0 0 0 3

Nyrang Park Branch - Hillview Ave Culvert 25 31 36 46

Gilmore Park Branch - Gipps St Overland Flow 1 9 20 149

Gilmore Park Branch - Gipps St Culvert 31 34 35 38

Towradgi Arm (near Para Ck junction) 18 18 19 19

Over Squires Way Bridge South (main bridge) 32 72 129 508

Through Squires Way Bridge 155 170 181 209

Downstream of Squires Way Bridge 184 241 313 813

Entrance 186 249 318 877

6.6 Provisional Flood Hazard

The 1% AEP hydraulic modelling was used to calculate provisional hydraulic hazard areas using Figure L2 of the NSW Government’s Floodplain Development Manual (FDM, 2005). The Manual’s figure is reproduced in this report as Figure 6-14. The provisional hazard areas are presented in Drawing 10. It is noted that the Manual distinguishes the provisional hazard from the true hazard. The provisional hazard also differs from the high, medium and low Flood Risk Precincts defined in the FRMS&P and utilized in Council’s DCP.

6.7 Conclusions on Design Flood Events

The calibrated hydrologic and hydraulic models were used to simulate the 20%, 5%, 2% and 1% AEP design flood events, along with the PMF event under present-day conditions.

The dominant critical duration was found to be the 2 hour flood event. The 9 hour flood event was also chosen for critical duration assessments as it represented the largest of the longer duration flood events.

Six culvert blockage scenarios were modelled. A peak flood envelope was produced for each of the five flood design events. The envelopes for the 1% AEP and PMF events included an allowance for elevated ocean levels.

The FRMS&P uses the flood model results presented in this Flood Study report to evaluate options for managing the flood problem. It includes a climate change sensitivity test and considers the implications of existing and future flood behaviour for setting FPLs.

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