RANDWICK CITY COUNCIL Maroubra Bay Flood Study Draft For Public Exhibition | February 2011

Level 2, 160 Clarence Street Sydney, NSW, 2000

Tel: 9299 2855 Fax: 9262 6208 Email: [email protected] Web: www.wmawater.com.au

MAROUBRA BAY CATCHMENT FLOOD STUDY

Project Project Number Maroubra Bay Catchment Flood Study 27033

Client Client’s Representative Randwick City Council Terry Kefalianos

Authors Prepared by Ben Baker Matt Chadwick

Verified by

Status Description Date

Draft Approved Draft for Public Exhibition 02/02/2011 Draft Amended Draft for Public Exhibition 03/08/2010 Draft Draft for Public Exhibition 11/01/2010 Amended Draft For RCC and DECC review 13/01/2009 Initial Draft For RCC and DECC review 04/09/2008

MAROUBRA BAY CATCHMENT FLOOD STUDY

TABLE OF CONTENTS

PAGE

FOREWORD...... i EXECUTIVE SUMMARY ...... 1 1. INTRODUCTION ...... 4

1.1. General ...... 4 1.2. Objectives ...... 4 2. BACKGROUND ...... 6

2.1. Catchment Description ...... 6 2.2. Causes of Flooding ...... 8 3. DATA...... 9

3.1. Drainage Information ...... 9 3.2. Topographic Survey ...... 9 3.2.1. Aerial LIDAR Scanning (ALS) Survey ...... 9 3.2.2. Detail Ground Survey ...... 9 3.3. Rainfall Data ...... 10 3.3.1. Overview ...... 10 3.3.2. Available Rainfall Data ...... 11 3.3.3. Analysis of Daily Read Data ...... 12 3.3.4. Analysis of Pluviometer Data ...... 14 3.4. Design Rainfall ...... 15 3.5. Maroubra Bay Water Level Data ...... 16 3.6. Historical Flood Information ...... 17 3.6.1. Overview ...... 17 3.6.2. Previous Studies ...... 17 3.6.3. Council Records ...... 17 3.6.4. Community and Local Resident Survey ...... 18 3.6.5. January 1999 Flood Event ...... 19 3.6.6. 30th October 1959 Event ...... 19 4. APPROACH ADOPTED ...... 21

4.1. General ...... 21

4.2. Hydrologic Modelling ...... 21 4.3. Hydraulic Modelling ...... 22 4.3.1. Overview ...... 22 4.3.2. SOBEK Modelling Software ...... 22 5. HYDROLOGIC (MIKE-STORM) MODEL CONFIGURATION ...... 24

5.1. Sub-catchment Layout ...... 24 5.2. Model Parameters ...... 24 5.2.1. Impervious Fraction ...... 24 5.2.2. Rainfall Losses & Soil Type (MIKE-Storm Hydrologic Component) ...... 24 5.2.3. Time of Concentration (MIKE-Storm Hydrologic Component) ...... 25 6. SOBEK MODEL CONFIGURATION ...... 26

6.1. Model Extents ...... 26 6.2. Sub-surface Drainage Network ...... 26 6.3. Overland Flow Paths ...... 27 6.3.1. Model Representation ...... 27 6.3.2. Manning‟s Roughness for Overland Flowpaths (SOBEK Model) ...... 28 7. MODEL VALIDATION ...... 29

7.1. Approach ...... 29 7.2. Results ...... 29 7.3. Discussion ...... 30 8. DESIGN EVENT MODELLING ...... 32

8.1. Approach ...... 32 8.2. Boundary Conditions ...... 32 8.2.1. Hydrologic (MIKE-Storm) Model ...... 32 8.2.2. Hydraulic (SOBEK) Model ...... 32 8.3. Results ...... 32 8.3.1. Overview ...... 32 8.3.2. Critical Storm Duration ...... 33 8.4. Flooding within Trapped Low Points behind Maroubra Beach ...... 34 9. COMPARISON OF RESULTS WITH PREVIOUS STUDIES ...... 36

9.1. Design Flood Behaviour...... 36 10. SENSITIVITY ANALYSES ...... 38 10.1. Overview ...... 38 10.2. Modelled Scenarios and Assumptions ...... 39 10.3. Results ...... 39

10.4. Accuracy of Estimated Design Flood Levels ...... 42 11. ACKNOWLEDGEMENTS...... 44 12. REFERENCES ...... 45

LIST OF APPENDICES

Appendix A Glossary of Flood Terms

LIST OF TABLES

Table 1 Rainfall Stations within a 10km Radius of Catchment ...... 11 Table 2 Ranked Daily Rainfalls Depths at Selected Stations (greater than 150 mm) ...... 13 Table 3 October 1992, September 1995, August 1998 and January 1999 Maximum Recorded Storm Depths (in mm) ...... 14 Table 4 October 1992, September 1995, August 1998 and January 1999 Total Recorded Storm Depths (in mm) ...... 14 Table 5 Malabar STP Pluviometer Storm Intensities (mm/h) ...... 15 Table 6 Design Rainfall Data ...... 15 Table 7 Design Rainfall Parameters ...... 16 Table 8 Adopted Design Water Levels at Fort Denison ...... 16 Table 9 Summary of Community Survey Responses ...... 18 Table 10 Observed Flood Levels – January 1999 Event (Source: Ref 5) ...... 19 Table 11 Initial Assumed Land Use Paved Percentage ...... 24 Table 12 Adopted Hydrologic Model Parameters ...... 25 Table 13 Model Validation Results – January 1999 Event ...... 30 Table 14 Adopted Model Parameters – January 1999 Event ...... 30 Table 15 Model Results – October 1959 Flood ...... 30 Table 16 Design Flows and Levels ...... 35 Table 17 Peak Water Levels – Comparison of Results ...... 36 Table 18 Sensitivity Analyses – Change in Peak Flows for the 1% AEP (1 in 100 year) Event 41 Table 19 Sensitivity Analyses – Change in Peak Flood Height for the 1% AEP (1 in 100 year) Event 41

LIST OF PHOTOGRAPHS

Photo 1 Marine Parade lowpoint looking west from main Maroubra Beach ...... 6 Photo 2 Stormwater outlet on Maroubra Beach ...... 6 Photo 3 Haig Street lowpoint ...... 6 Photo 4 French Street lowpoint ...... 6 Photo 5 Lowpoint along Anzac Parade, upper catchment ...... 6 Photo 6 View from within Coral Sea Park (looking east). Note embankment at edge of park near fenceline of adjacent properties...... 7 Photo 8 View from within Marine Parade lowpoint looking east towards the dune line along Maroubra Beach...... 7 Photo 7 Looking east along Chapman Avenue, towards intersection of Fenton Avenue...... 7 Photo 9 January 1999-Flooding in Chapman Avenue ...... 19 Photo 10 January 1999 - View along Chapman Avenue looking west ...... 19 Photo 11 October 1959 - Photo taken at intersection of Chapman and Fenton Avenues (looking south west along Fenton Ave) ...... 20 Photo 12 October 1959 - Photo from Chapman Avenue looking east towards intersection of Fenton Avenue...... 20 Photo 13 October 1959 - Flooding in Chapman Avenue (view looking west) ...... 20 Photo 14 October 1959 - Flooding in Chapman Avenue – note flood mark on car door ...... 20

LIST OF FIGURES

Figure 1 Study Area Figure 2 Drainage Network Layout Figure 3 Available ALS Data Figure 4 Available Hydrological Data Figure 5 24 Jan 1999 Rainfall Figure 6 Hydrologic Model – Sub-catchment Layout Figure 7 Layout of Overland Flow Model Figure 8 Key Locations Figure 9 1D Overland Flows – 1% AEP Event Figure 10 Peak Flood Levels and Depths – 1% AEP Event Figure 11 Design Flood Information – Selected Trapped Low Points Figure 12 Peak Levels – 1% AEP Event + 0.5 m Figure 13 Flood Extents – Design Events Figure 14 Provisional Hydraulic Hazard Categories – 1% AEP Event Figure 15 Potential for Overland Flow Through Properties In Upper Reaches

Maroubra Bay Catchment Flood Study

FOREWORD

The NSW State Government‟s Flood Policy provides a framework to ensure the sustainable use of floodplain environments. The Policy is specifically structured to provide solutions to existing flooding problems in rural and urban areas. In addition, the Policy provides a means of ensuring that any new development is compatible with the flood hazard and does not create additional flooding problems in other areas.

Under the Policy, the management of flood liable land remains the responsibility of local government. The State Government subsidises flood mitigation works to alleviate existing problems and provides specialist technical advice to assist Councils in the discharge of their floodplain management responsibilities.

The Policy provides for technical and financial support by the Government through four sequential stages:

1. Flood Study  Determine the nature and extent of the flood problem. 2. Floodplain Risk Management  Evaluates management options for the floodplain in respect of both existing and proposed development. 3. Floodplain Risk Management Plan  Involves formal adoption by Council of a plan of management for the floodplain. 4. Implementation of the Plan  Construction of flood mitigation works to protect existing development.  Use of Local Environmental Plans to ensure new development is compatible with the flood hazard.

The Maroubra Bay Catchment Flood Study constitutes the first stage of the management process for the Maroubra Bay Catchment. WMAwater (formerly known as Webb, McKeown & Associates) were commissioned by Randwick City Council to prepare this flood study on behalf of Council‟s Floodplain Risk Management Committee. Funding for this study was provided from the State Governments Flood Risk Management Program and Randwick City Council. Specialist technical assistance was also provided by the NSW Department of Environment, Climate Change and Water (DECCW). The following report documents the work undertaken and presents outcomes that define design flood behaviour for existing catchment conditions.

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EXECUTIVE SUMMARY

The NSW Government‟s Flood Policy provides:  a framework to ensure the sustainable use of floodplain environments,  opportunities for the development of solutions to flooding problems, and  a means of ensuring that new development is compatible with the flood hazard and does not adversely affect existing flood risk.

Implementation of the Policy requires a four stage approach, the first of which is the preparation of a Flood Study to determine the nature and extent of the flood problem.

The Maroubra Bay Flood Study was initiated by Randwick City Council and the Department of the Environment, Climate Change and Water and prepared by WMAwater (formerly Webb McKeown and Associates).

The specific objectives of the Maroubra Bay Flood Study are to:  define flood behaviour within the study catchment,  prepare mapping showing the nature and extent of flooding within specified areas,  prepare suitable models of the catchment and floodplain suitable for use in subsequent Floodplain Risk Management Studies and Plans.

Description of Catchment: The Maroubra Bay catchment (approx. 2.5 km2) between Anzac Parade and the Pacific Ocean is located entirely within the Randwick City Council Local Government Area and drains a large proportion of the suburb of Maroubra. The topography varies significantly across the catchment with ground elevations ranging between 0 mAHD to in excess of 50 mAHD. The steep upper reaches of the catchment drain to a flat, low-lying area behind Maroubra beach which is susceptible to inundation.

The trunk drainage system within the upper catchment consists of a network of underground concrete pipes and cast in-situ concrete box culverts that were largely installed by Randwick City Council in the 1950's. These conduits flow into larger concrete box culverts and out through the main stormwater outlet to Maroubra Bay. According to Council records the main stormwater outlet to the Bay was constructed in 1950.

Land usage within the study area is predominantly low to medium density urban residential development, comprising a mixture of single residential building and medium size apartment blocks. Non-residential development in the catchment includes several schools, parks, churches and community buildings. Significant areas of open space exist in Coral Sea Park, Arthur Byrne Reserve, Broadarrow Reserve and John Shore Park. There are pockets of commercial development located along the Marine Parade precinct, Maroubra Road and Lexington Place.

Flooding problems have been experienced at a number of locations during periods of heavy

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Collation and Review of Available Data: A review of past reports, Council records and photographs was undertaken. A comprehensive range of datasets was also compiled including topographic survey information from multiple sources, details of the drainage network and historical rainfall and flood level information.

Preparation of Computer Models: A rainfall-runoff modelling approach was adopted due to the absence of long term historical flood records. This approach involved the setting up of two modelling platforms – a hydrologic model to convert rainfall to runoff and a hydraulic model to then determine flow distributions, flood levels and velocities throughout the floodplain. A 1D hydraulic model was used in the upper catchment and a 2D hydraulic model in the lower catchment. The performance of the models was validated against available data from the October 1959 and January 1999 storm events.

Determination of Design Flood Behaviour: Design rainfall data from Australian Rainfall and Runoff (1987) was obtained for design floods ranging from the 50% AEP (1 in 2 year) flood to the 1% AEP (1 in 100 year) flood and the Probable Maximum Flood (PMF) event. This information was input into the hydrologic/hydraulic models to determine design flood behaviour in terms of flood levels, flows and velocities throughout the floodplain.

Based on the outcomes of the model validation and sensitivity analyses, the accuracy of the results within the lower reaches of the catchment (i.e. in and around the Marine Parade and Fenton Avenue low points) is estimated to be within ±0.3 m.

Elsewhere in the catchment, the model is effectively uncalibrated and the corresponding accuracy of flood level estimates is likely to be within ±0.5 m.

Note that when interpreting the model results to derive flood level estimates care should be taken to review both the estimated level and depth results together with detail survey to confirm an appropriate flood level, particularly where the estimated depths are reasonably shallow (e.g. less than 0.3 m for the 1% AEP event). In these instances, the depths approach the limit of accuracy of the underlying survey data.

Flood Problem Areas: Urbanisation has dramatically altered the nature of available drainage within the catchment. Consideration of the natural drainage systems present prior to development provides the context for many of the flood problems known to exist in the area today.

Flood problems within the upper portion (modelled in 1D) of the catchment typically result from ponding in trapped low-points and mapping is provided at eight of these locations. Elsewhere in the upper catchment design peak flows are provided.

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In the lower catchment (modelled in 2D) design flood details (levels, depths, provisional hazard categorisation) are provided. This indicates an extensive area of ponding behind the main dune of Maroubra Beach.

Outcomes: The main outcomes of this Flood Study include:  full documentation of the methodology and results,  preparation of flood maps defining the nature and extent of flooding for the majority of the floodplain, and  modelling tools and information suitable for use in the preparation of a Floodplain Risk Management Study and Plan.

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1. INTRODUCTION

1.1. General

The Maroubra Bay catchment between Anzac Parade and the Pacific Ocean is located entirely within the Randwick City Council Local Government Area (LGA). The catchment drains a large proportion of the suburb of Maroubra (refer Figure 1).

The topography of the catchment varies significantly across the catchment with ground elevations ranging between 0 mAHD to in excess of 50 mAHD. The steep upper reaches of the catchment drain to a flat, low-lying area behind Maroubra beach which is susceptible to inundation.

Flooding problems within the catchment have been experienced at a number of locations during periods of heavy rainfall. Randwick City Council has undertaken to address this issue by commissioning this comprehensive Flood Study of the Maroubra Bay catchment.

The trunk drainage system within the upper catchment consists of a network of underground concrete pipes and cast in-situ concrete box culverts that were largely installed by Randwick City Council in the 1950's. These conduits flow into larger concrete box culverts and out through the main stormwater outlet to Maroubra Bay. According to Council records the main stormwater outlet to the Bay was constructed in 1950.

Land usage within the study area is predominantly low to medium density urban residential development, comprising a mixture of single residential building and medium size apartment blocks. Non-residential development in the catchment includes several schools, parks, churches and community buildings. Significant areas of open space exist in Coral Sea Park, Arthur Byrne Reserve, Broadarrow Reserve and John Shore Park. There are pockets of commercial development located along the Marine Parade precinct, Maroubra Road and Lexington Place.

1.2. Objectives

Randwick City Council engaged WMAwater (formerly known as Webb, McKeown & Associates) to undertake the Maroubra Bay Catchment Flood Study utilising current technology and data. The information and results obtained from the study are to provide the basis for the development of targeted stormwater management strategies, and a subsequent Floodplain Risk Management Study and Plan.

In accordance with the Floodplain Development Manual (Reference 1), this Flood Study has been prepared to define the nature and extent of flooding throughout the catchment for a range of design storm events including the 1% AEP (1 in 100 year) flood and the Probable Maximum Flood (PMF).

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The Flood Study also provides information to:  to assess the adequacy and capacity of Council‟s existing pipe network and quantify overland flows,  identify overland flow paths and determine design flood levels along major overland flow paths.

This report details the methodology and outcomes of the Flood Study investigations. The key elements include:  a summary of available historical flood related data,  details of hydrologic and hydraulic models established specifically for this study (including model calibration and validation),  definition of the design flood behaviour for existing conditions,  documentation of the assumptions made to derive the information and conclusions presented herein.

A glossary of flood related terms is provided in Appendix A.

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2. BACKGROUND

2.1. Catchment Description

Typically, the Maroubra Bay catchment consists of residential and commercial development although there are a number of open space areas (parks and reserves) throughout the area. The only portion of natural bushland remaining is located on the sand dune between Arthur Byrne reserve and Maroubra Beach (Figure 1).

In terms of local drainage, the roads have been formed with kerbs and gutters, the trunk drainage system ultimately drains to the ocean at several points along Maroubra Beach (refer Photo 1). There are two main trunk drainage lines which discharge to the stormwater outlet on Maroubra Bay beach. The larger trunk line runs along Fitzgerald Avenue from Malabar Road and along Marine Parade up to McKeon Street. This line drains a large portion of the catchment. The other trunk line runs along McKeon Street to Marine Parade. The stormwater outlet is located on the beach in front of the McKeon Street/Marine Parade intersection.

Photo 1 Marine Parade lowpoint looking west from Photo 2 Stormwater outlet on Maroubra Beach main Maroubra Beach

Within the study area, there is a portion of the South Maroubra catchment (having an area of approximately 7.6 ha.) that is drained via separate pipe networks to south of the Arthur Byrne reserve. Overland flows from a portion of this catchment, drain to the main Maroubra Bay catchment at via Fitzgerald Avenue.

The steeper upper reaches to the north and south of the catchment are predominantly established residential. Due to the steep terrain there are many instances of trapped low points examples of which include Haig Street, French Street and Anzac Parade.

Photo 3 Haig Street lowpoint Photo 4 French Street lowpoint Photo 5 Lowpoint along Anzac Parade, upper catchment

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Much of the western portion of the catchment drains to Coral Sea Park. The park itself is around 5.5 ha. in size and is located within a residential area. A small commercial strip is located just east of Coral Sea Park in Lexington Avenue. The main trunk drainage line from Coral Sea Park runs towards Maroubra Bay along Yorketown Parade, then along Fitzgerald Avenue to the south of Broadarrow Reserve. In large floods overland flows would tend to follow the road network in this area, generally following an easterly direction to the lower reaches of the catchment. There is a major trapped low point adjacent to the Maroubra Bowling Club greens along Malabar Road.

Photo 6 View from within Coral Sea Park (looking east). Note embankment at edge of park near fenceline of adjacent properties.

Overland flow from the upper reaches (and from local sources) has been known to inundate significant portions of the lower catchment (e.g. west of Malabar Road) where the terrain is generally less steep. Some areas of the lower catchment in the vicinity of Marine Parade are lower than the remnant dune behind Maroubra beach, causing water to pond until drained by the pipe network. The main trapped low points within this area are located at the intersection of McKeon Street and Marine Parade and at the intersection of Fenton Avenue and Chapman Avenue. There is a major commercial centre along Marine Parade generally located between McKeon Street and Mons Avenue.

Photo 7 Looking east along Chapman Avenue, Photo 8 View from within Marine Parade lowpoint towards intersection of Fenton Avenue. looking east towards the dune line along Maroubra Beach.

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2.2. Causes of Flooding

Based on the available information flooding within the Maroubra Bay catchment may occur as a result of a combination of factors including:

 Elevated water levels along roads and through private property as a result of intense rain over the upper part of the Maroubra Bay catchment. Runoff in excess of the capacity of the trunk drainage system will also result in overland flow. Water levels may also be affected by localised features that constrict or block runoff from entering the drainage network, or diverts the path of overland flows.

 In larger events where the amount of runoff that exceeds the capacity of the main trunk drainage system, the lower reaches of the catchment are flooded as a result of overland flow from the upper catchment (and local runoff to a lesser extent). Ponding then occurs in the various low points located behind the main dune of Maroubra Beach.

The factors causing flooding may occur in isolation or in combination with each other.

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3. DATA

The first stage in the investigation of flooding matters is to establish the nature, size and frequency of the problem. On large river systems such as the Hawkesbury River there are generally stream height and historical records dating back to the early 1900's, or in some cases even further. However, in small urban catchments such as Maroubra Bay there are typically no stream gauges or official historical records available. An understanding of flooding must therefore be obtained from an examination of rainfall records and local knowledge in combination with available datasets describing the catchment topography and layout of the drainage system. For this reason, a comprehensive data collation exercise was undertaken.

3.1. Drainage Information

A comprehensive drainage assets database for the Maroubra Bay catchment was provided by Randwick City Council. This data was originally collected by AWT Survey in 2005 and included details of all drainage inlet pits and pipes/conduits for the catchment (refer to Figure 2).

3.2. Topographic Survey

3.2.1. Aerial LIDAR Scanning (ALS) Survey

Randwick City Council commissioned AAMHATCH Pty. Ltd. to undertake an Aerial Laser Scanning (ALS) survey within the extents of the Randwick LGA including the Maroubra Bay study catchment (refer Figure 3). The survey was flown in December 2005 at a 1:2000 scale flying height. The resultant mapping was provided to Council in March 2006. In terms of ground level information the ALS survey provides numerous ground level spot heights, from which a Digital Terrain Model (DTM) can be constructed.

For well defined points mapped in clear areas, the expected nominal point accuracies (based on a 68% confidence interval) are in the order of:  Vertical Accuracy: ±0.15 m  Horizontal Accuracy: ±0.57 m

When interpreting the above, it should be noted that the accuracy of the ground definition can be adversely affected by the nature and density of vegetation and/or the presence of steeply varying terrain.

3.2.2. Detail Ground Survey

Ground survey plans for localised areas were provided by Randwick City Council during the course of the study including:  Maroubra Beach Detail Survey, Randwick City Council, Plan No. 5139, April 1997.  Coral Sea Park Topographic Survey, Randwick City Council, Plan No. 5562, May 2006.

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3.3. Rainfall Data

3.3.1. Overview

Rainfall data is recorded either daily (24hr rainfall totals to 9:00am) or continuously (pluviometers measuring rainfall in 0.5 mm rainfall increments). Daily rainfall data have been recorded for over 100 years at many locations within the Sydney basin, including at Observatory Hill since 1858. In general, pluviometers have only been installed since the 1970's. Together these records provide a picture of when and how often large rainfall events have occurred in the past.

However, care must be taken when interpreting historical rainfall measurements. Rainfall records may not provide an accurate representation of past events due to a combination of factors including local site conditions, human error or limitations inherent to the type of recording instrument used. Examples of limitations that may impact the quality of data used for the present study are highlighted in the following:

1. Rainfall gauges frequently fail to accurately record the total amount of rainfall. This can occur for a range of reasons including operator error, instrument failure, overtopping and vandalism. In particular, many gauges fail during periods of heavy rainfall and records of large events are often lost or misrepresented.

2. Daily read information is usually obtained at 9:00am in the morning. Thus if the storm encompasses this period it becomes “split” between two days of record and a large single day total cannot be identified.

3. In the past, rainfall over weekends was often erroneously accumulated and recorded as a combined Monday 9:00am reading.

4. The duration of intense rainfall required to produce flooding in the Maroubra Bay catchment is typically less than two hours (this aspect is discussed further in later sections). This is termed the “critical storm duration”. For a much larger catchment (such as the Parramatta River) the critical storm duration may be from 24 to 36 hours. For the Maroubra Bay catchment a short intense period of rainfall can produce flooding but if the rain stops quickly (as would be typical of a thunderstorm), the daily rainfall total may not necessarily reflect the magnitude of the intensity and subsequent flooding. Alternatively the rainfall may be relatively consistent throughout the day, producing a large total but only minor flooding.

5. Rainfall records can frequently have “gaps” ranging from a few days to several weeks or even years.

6. Pluviometer (continuous) records provide a much greater insight into the intensity (depth vs time) of rainfall events and have the advantage that the data can generally be analysed electronically. These data have much fewer limitations than daily read data. The main drawback is that many of the relevant gauges have only been installed since 1990 and

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hence have a very short period of record compared to the daily read data. The Sydney Observatory and Sydney Water Board Head Office gauges were installed in 1970 but unfortunately are outside of the Maroubra Bay catchment. Pluviometers can also fail during storm events due to the extreme weather conditions.

7. Rainfall events which cause flooding in the Maroubra Bay catchment are usually very localised and as such are only accurately “registered” by a nearby gauge. Gauges sited only a few kilometres away can show very different intensities and total rainfall depths.

3.3.2. Available Rainfall Data

Table 1 presents a summary of the official rainfall gauges (provided by the Bureau of Meteorology and Sydney Water) located within 10km of the catchment. These gauges are operated either by Sydney Water (SW) or the Bureau of Meteorology (BOM). There may also be other private gauges in the area (bowling clubs, schools) but data from these has not been collected as there is no public record of their existence. Of the 49 gauges listed in Table 1 over 49% (24) have now closed. The gauge with the longest record is Observatory Hill, operating from 1858 to the present.

Table 1 Rainfall Stations within a 10km Radius of Catchment

No Owner Station Elevation Distance Date Date Type (mAHD) from Opened Closed Maroubra (km) 66122 BOM Maroubra RSL Bowling Club ? 0.6 1/11/1964 31/12/1974 Daily 566123 SWB Maroubra Bowling Club ? 0.6 21/11/1995 9/10/1998 Continuous 566034 SWB(NSW) Pagewood 20 1.9 12/01/1959 5/10/1973 Continuous 566034 SWB(NSW) Pagewood 20 1.9 12/01/1959 5/10/1973 Daily 566088 SWB(NSW) Malabar STP 15 2.8 17/12/1990 Continuous 66007 BOM(NSW) Botany No.1 Dam 6.1 3.0 1/01/1870 1/01/1978 Daily 566043 SWB Randwick (Army) 30 3.4 12/12/1956 2/09/1970 Continuous 66051 BOM(NSW) Little Bay (The Coast Golf Club) 22 3.9 1/01/1925 Daily 66051 BOM Little Bay (The Coast Golf Club) 22 3.9 21/04/1997 Operationa 66051 BOM Little Bay (The Coast Golf Club) 22 3.9 1/01/1925 Synopl 566016 SWB Botany Water Reserve 10 3.9 1/01/1870 6/02/1979 Daily 566099 SWB(NSW) Randwick Racecourse 30 4.0 29/11/1991 Continuous 66052 BOM(NSW) Randwick Bowling Club 75 4.3 1/01/1888 Daily 566028 SWB Mascot Bowling Club 5 4.4 28/08/1973 Continuous 566028 SWB(NSW) Mascot Bowling Club 5 4.4 28/08/1973 Daily 66073 BOM(NSW) Randwick Racecourse 25 4.8 1/01/1937 Daily 66066 BOM Waverley Shire Council ? 5.4 1/09/1932 31/12/1964 Daily 66021 BOM Erskineville 6 5.4 1/05/1904 31/12/1973 Daily 66179 BOM Bronte Surf Club 15 5.4 1/01/1918 1/01/1922 Daily 66179 BOM Bronte Surf Club 15 5.4 7/09/2001 Daily 566114 SWB Waverley Bowling Club 0 5.6 5/01/1995 Continuous 66187 BOM(NSW) Tamarama (Carlisle St) 30 5.8 1/07/1991 21/03/1999 Daily 66160 BOM Centennial Park 38 5.9 1/06/1900 Daily 66160 BOM Centennial Park 38 5.9 1/06/1900 Synop 566077 SWB Bondi (Dickson Park) 60 6.4 21/12/1989 5/02/2001 Continuous 66037 BOM(NSW) Sydney Airport Amo 6 6.7 1/01/1960 Continuous 66192 BOM Sydney Airport Tbrg 3 6.7 1/06/1997 Continuous 66037 BOM Sydney Airport Amo 6 6.7 1/07/1994 Synop 66097 BOM Randwick Bunnerong Rd ? 6.7 1/01/1904 1/01/1924 Daily 566015 SWB Alexandria 5 6.7 1/05/1904 1/08/1989 Daily (NSW)

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Table 1 Rainfall Stations within a 10km Radius of Catchment (cont‟d)

No Owner Station Elevation Distance Date Date Type (mAHD) from Opened Closed Maroubra (km) 566015 SWB Alexandria 5 6.7 1/05/1904 1/08/1989 Daily 566032 SWB(NSW) Paddington (Composite Site) 45 6.8 10/04/1961 Continuous 566032 SWB(NSW) Paddington (Composite Site) 45 6.8 10/04/1961 Daily 566110 SWB(NSW) Erskineville Bowling Club 10 7.1 2/06/1993 8/02/2001 Continuous 66033 BOM(NSW) Alexandria (Henderson Rd) 15 7.2 1/05/1962 31/12/1963 Daily 66033 BOM Alexandria (Henderson Rd) 15 7.2 1/04/1999 14/03/2002 Daily 566115 SWB Bondi Golf Club 0 7.4 24/08/1994 15/11/1995 Continuous 66005 BOM(NSW) Bondi Bowling Club 15 7.5 1/07/1939 31/12/1982 Daily 66139 BOM Paddington 4.6 7.7 1/01/1968 1/01/1976 Daily 66098 BOM Royal Sydney Golf Club 8 7.7 1/03/1928 Daily 566091 SWB Kyeemagh Bowling Club 5 7.9 19/09/1991 Continuous 66101 BOM(NSW) Fernbank ? 8.0 1/01/1889 1/01/1913 Daily 566009 SWB Rushcutters Bay Tennis Club 0 8.1 26/05/1998 Continuous 66072 BOM(NSW) Kurnell (Caltex Oil Refinery) 3 8.1 1/03/1956 Daily 566011 SWB Victoria Park @ Camperdown 0 8.2 27/05/1998 Continuous 566006 BOM(NSW) Bondi (Sydney Water ) 10 8.3 18/06/1997 Operationa 566042 SWB Sydney H.O. Pitt St 15 8.3 11/08/1949 26/02/1965 Continuousl 66015 BOM(NSW) Crown St Reservoir ? 8.3 1/02/1882 31/12/1960 Daily 566041 SWB Crown St Reservoir 40 8.4 1/02/1882 31/12/1960 Daily 566010 SWB(NSW) Cranbrook School @ Bellevue 0 8.5 25/05/1998 Continuous 566026 SWB(NSW) MarrickvilleHill SPS 5 8.6 1/05/1904 Continuous 566026 SWB(NSW) Marrickville SPS 5 8.6 1/05/1904 Daily 66006 BOM(NSW) Sydney Botanic Gardens 15 9.5 1/01/1885 Daily

3.3.3. Analysis of Daily Read Data

For the purposes of this investigation, an analysis of longer term daily rainfall data (in proximity of the catchment) was undertaken to identify and place past storm events in some context.

The stations used for this analysis included Botany No. 1 Dam (108 years of record), Randwick Bowling Club (119 years of record) and Randwick Racecourse (70 years of record). For each of these stations the recorded depths greater than 150 mm have been ranked as shown in Table 2.

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Table 2 Ranked Daily Rainfalls Depths at Selected Stations (greater than 150 mm)

Botany No. 1 Dam (66007) Randwick Bowling Club Randwick Racecourse (66073) (66052) Jan 1870 - Jan 1978 Jan 1888 - Jan 1937 - Rank Date Rainfall Rank Date Rainfall Rank Date Rainfall (mm) (mm) (mm) 1 28/05/1889 252 1 06/08/1986 297 1 10/02/1992 294 2 10/02/1956 221 2 29/10/1959 265 2 20/11/1961 270 3 10/02/1958 220 3 28/03/1942 243 3 30/10/1959 267 4 14/05/1962 206 4 03/02/1990 225 4 06/08/1986 263 5 23/06/1975 189 5 10/02/1956 213 5 11/03/1975 261 6 06/04/1882 173 6 31/01/1938 213 6 14/05/1962 258 7 20/03/1978 169 7 11/03/1975 201 7 10/02/1958 256 8 19/11/1900 168 8 17/01/1988 178 8 05/02/1990 248 9 13/01/1911 166 9 12/10/1902 178 9 03/02/1990 244 10 28/07/1952 163 10 28/04/1966 177 10 09/11/1984 240 11 20/03/1892 161 11 04/02/1990 175 11 20/03/1978 237 12 10/01/1949 155 12 19/11/1900 164 12 06/11/1984 223 13 16/06/1952 155 13 09/02/1992 162 13 28/03/1942 213 14 23/06/1885 153 14 28/07/1908 161 14 31/01/1938 211 15 30/10/1959 153 15 09/02/1958 158 15 10/02/1956 195 16 28/04/1966 151 16 29/05/1906 155 16 30/04/1988 175 17 30/08/1963 152 17 30/08/1963 174 18 27/04/1901 150 18 07/08/1967 171 19 10/01/1949 170 20 14/11/1969 160 21 05/02/2002 157 22 16/06/1952 156 23 04/03/1977 155 24 03/05/1948 154 25 04/04/1988 152 26 28/04/1966 151 27 05/03/1979 151

The main points regarding these data are: 1. Common large events in all three gauges include 10/02/1956, 29-30/10/1959, 9-10/02/1958. The more recent events of 11/03/1975 and 6/08/1986 occur in both Randwick gauges still operating. 2. The records indicate a significant difference between the Randwick gauges (~265 mm) and the Botany Dam station (153 mm) for the October 1959 event. 3. These records are based on 24 hour totals (to 9:00am) are showing rainfall throughout the entire day, whereas the critical duration of the Maroubra catchment is likely to be under two hours. Hence a large daily rainfall of greater than 200 mm may not necessarily produce severe flooding.

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3.3.4. Analysis of Pluviometer Data

In comparison to daily rainfall data, pluviometer records provide a more detailed description of temporal variations in rainfall. Table 3 lists the maximum storm intensities for the four largest recent rainfall events identified from rainfall records in proximity of the catchment. The storms identified include events in October 1992, September 1985, August 1998 and January 1999. The total recorded depths are shown in Table 4.

Table 3 October 1992, September 1995, August 1998 and January 1999 Maximum Recorded Storm Depths (in mm)

Station No. 27 Oct 1992 25 Sept 1995 18 Aug 1998 24 Jan 1999 20 min 30 min 20 min 30 min 20 min 30 min 20 min 30 min Malabar STP 566088 34.0 44.0 32.5 34.5 20.5 25.5 29.0 40.0 Randwick Racecourse 566099 14.5 18.5 14.0 15.5 13.5 15.5 16.0 23.5 Mascot Bowling Club 566028 6.5 8.5 23.0 28.0 5.5 8.0 23.0 30.5 Note: Station locations shown on Figure 4.

Comparison with Australian Rainfall and Runoff 1987 (AR&R87) design rainfall intensities (Table 6) indicate that the October 1992, September and January 1991 events were less than a 5 year ARI design intensity for the 20 minute and 30 minute durations.

The February 1992 event was the most severe of these storms, with a maximum 30 minute burst of 44 mm. The Malabar STP pluviometer registered significantly higher rainfalls for all events, suggesting these were localised storm events. As shown in Table 1, Malabar STP gauge is closest to the catchment, and hence flood events in the Maroubra area are likely to be best represented by this gauge.

Table 4 October 1992, September 1995, August 1998 and January 1999 Total Recorded Storm Depths (in mm)

Station No. 27 Oct 1992 25 Sept 1995 18 Aug 1998 24 Jan 1999 Malabar STP 566088 102 75.5 34.5 96.5 Randwick Racecourse 566099 49 65.5 16 57 Mascot Bowling Club 566028 19.5 53 29.5 75

Table 5 indicates that the peak bursts for the 1992, 1995, and 1998 storms were part of a much longer rainfall event, as the total rainfall is significantly higher. The August 1998 storm had a smaller total rainfall but a similar peak burst rainfall, indicating it was a small duration but reasonably intense storm. In all of these storms, Malabar STP received either more intense rainfall or similar intensity rainfall over a longer duration.

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Table 5 Malabar STP Pluviometer Storm Intensities (mm/h)

Event 6min 10min 20min 30 min 60min 120min 25/10/92 120 117 102 88 58 32 (approx ARI) (2) (5) (10) (10) (10) (5) 23/09/95 80 78 62 51 32 21 (approx ARI) (1) (1) (1) (1) (1) (1) 16/08/98 135 114 98 69 35 18 (approx ARI) (2) (2) (5) (2) (10) (1) 22/01/99 105 99 87 80 71 46 (approx ARI) (1) (2) (5) (5) (20) (20)

For durations of between 60 to 120 minutes, the January 1999 storm was the most severe.

Figure 5 shows the cumulative mass curves for the 8th January 1999 event for the Malabar, Randwick Racecourse and Mascot pluviometers. This figure illustrates the spatial variability of the storm, all gauges are located within 5km of the catchment.

3.4. Design Rainfall

Table 6 shows the design rainfall depths used for the hydrologic analysis.

Table 6 Design Rainfall Data

Duration Average Recurrence Interval PMP 5y 10y 20y 50y 100y 30 minutes intensity (mm/h) 78 88 100 117 130 480 depth (mm) 39 46 53 62 69 240 1 hour intensity (mm/h) 42 54 61 71 83 350 depth (mm) 42 54 61 71 83 350 1.5 hours intensity (mm/h) 42 48 55 65 72 273 depth (mm) 63 72 83 98 108 410 2 hours intensity (mm/h) 35 40 46 54 60 230 depth (mm) 70 80 23 108 120 460 3 hours intensity (mm/h) 27 31 36 42 47 177 depth (mm) 81 93 12 126 141 531

The design rainfall depths and temporal patterns were obtained from Australian Rainfall and Runoff 1987 (Reference 2). The ARR87 parameters adopted for the catchment are shown in Table 7. The adopted ARR87 Estimates of Probable Maximum Precipitation (PMP) were obtained using BoM guidelines (Reference 3).

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Table 7 Design Rainfall Parameters

Parameter (ARR87) Value 2y, 1hr 42 mm/h 2y, 12hr 8.5 mm/h 2y, 72hr 2.82 mm/h 50y, 1hr 83 mm/h 50y, 12hr 17.3 mm/h 50y, 72hr 5.2 mm/h Skew 0 F2 Geographical Factor 4.29 F50 Geographical Factor 15.85

3.5. Maroubra Bay Water Level Data

Water level variations in Maroubra Bay will impact on flood levels in the lower reaches of the catchment and may affect the outflow from the trunk drainage system. The variations are largely as a function of astronomic tides but may also be influenced by:  wind set up and the increased barometric effect,  wave set up,  wave runup,  sea level rise due to estimated climate change.

The adopted design water levels for Sydney Harbour are shown in Table 8 and assume no wave set up, wave run up or sea level rise component.

Table 8 Adopted Design Water Levels at Fort Denison

ARI (years) Water Level (mAHD) 20 1.38 50 1.42 100 1.45 Events >100 year ARI Not known but assumed as 1.50 Source: Reference 4 (Rushcutters Bay Flood Study)

Elevated water levels are associated with major storm events (low pressures, strong onshore winds and large waves) but the peak level is determined by the height and timing of the high tide. As a result, peak levels are unlikely to occur in conjunction with a flood over the Maroubra Bay catchment which is more likely to be generated by an intense, short duration rainfall event (say less than 2 hours). The coincidence of rainfall and ocean level events has been assessed in many similar studies, including Reference 5. A somewhat conservative approach of using a static water level of 1.0 mAHD in conjunction with flooding due to runoff in the local catchment was adopted for this present study. This level approximates to a tide that is only exceeded a few times in a year. The sensitivity of the study outcomes to this particular assumption is discussed in Section 10 and indicates that an increase in the ocean tailwater by 0.91m produces no significant changes in flows or flood levels. For this reason further detailed review of ocean tailwater conditions was not undertaken.

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3.6. Historical Flood Information

3.6.1. Overview

A data search was carried out to identify the dates and magnitudes of historical floods. The search concentrated on the period since approximately 1970 as data prior to this date would generally be of insufficient quality and quantity for model calibration. Unfortunately there were no stream height gauges in the catchment or any other means of reliably determining the level of past flood events so the following sources were used:  Randwick City Council records,  previous reports,  questionnaire issued in August 2007,  local residents.

As discussed previously, a detailed review of rainfall records was also undertaken to establish the likely dates of floods (refer to Section 3.1). The outcomes of this stage provided a limited amount of flood level information for storm events in January 1999 and October 1959. In both cases, the information was available within the lower most reaches in and around Fenton Avenue/Chapman Avenue and McKeon Street and Marine Parade.

3.6.2. Previous Studies

A review of Council information indicates that there are very few formal flood studies available for this catchment. The only report held by Council that assesses design flood behaviour for the overall catchment draining to the main Maroubra Beach is titled “Maroubra Beach Flood Investigation”, December 1999 (Reference 5).

This 1999 study undertook an overview assessment of design flood behaviour for that portion of the catchment draining to the main Maroubra Beach. The investigation assessed flood behaviour in both the Fenton Avenue/Chapman Avenue and McKeon Street/Marine Parade trapped low points.

The hydrologic/hydraulic analysis was undertaken using an ILSAX model calibrated to ponding observed in the lower reaches of the catchment during the storm of 24th January 1999. Observed flood levels for this storm are reported. The model was then used to analyse flood behaviour for a range of design storms and produce flood level estimates at select locations.

The model was then used to assess the feasibility of various flood mitigation works.

3.6.3. Council Records

During the course of this investigation historical flood information was provided by Randwick City Council including:  copies of historical flood photographs (for events in January 1999 and in October 1959),

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 documentation describing instances of flooding noted to have occurred in the past.

3.6.4. Community and Local Resident Survey

As part of this study a community survey was carried out by Council including:  advertisements in local papers,  a notice on Council‟s web-site,  distribution of a community survey/questionnaire to residents throughout the study catchment,  provision of information and questionnaire at a stall held at a community open day,  receipt of additional responses and follow-ups via telephone and email.

A total of fifty (50) survey responses were received during the course of the project, the general outcomes of which are provided in Table 9.

Table 9 Summary of Community Survey Responses

Type of Flooding None Flooding above Flooding of garage/non- Flooding within property Reported habitable floor level habitable area (or neighbouring property) Number of 35 1 6 7 Respondents Note: One other response was received for a location outside the study area.

The responses reflected a number of areas within the catchment that have been known to experience overland flooding in the past including:  a number of trapped low points throughout the catchment (e.g. Fenton Avenue, Chapman Avenue, Maxwell Avenue, McKeon Street, Marine Parade, French Street and Byrne Crescent),  within Coral Sea Park and some areas of Chester Avenue,  flooding of car park and surrounding parklands at Arthur Byrne Reserve. The southern end of Arthur Byrne reserve was reported as being a „lake‟ between 1940-1960.

Several respondents provided reports of pit/drain blockages during past storms due to rubbish and debris. Evidence of overland flow along the kerb/gutter system being diverted onto property due to the presence of parked cars was also reported.

Although it is beyond the scope of the present study, the community survey results also identified concerns relating to stormwater quality and pollutant loadings in and around the southern end of Maroubra Beach. The potential for additional impacts in this area resulting from erosion and leachate was also mentioned. These issues were brought to the attention of Council.

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3.6.5. January 1999 Flood Event

Several observed flood levels for this event were established as part of Reference 5. These levels were determined using historical photographs, data from Council and verbal information provided by residents and property owners. A summary is provided in Table 10. Selected photographs of flooding during this event provided by Council are shown in Photos 9 & 10.

Table 10 Observed Flood Levels – January 1999 Event (Source: Ref 5)

Location Estimated Flood Level Comments (mAHD) Fenton Ave. /Chapman Ave. 5.88 Based on verbal information provided to Council. 5.45 Based on reported 0.8 m depth of flooding at lowpoint in Fenton Avenue. McKeon St/Marine Pde 5.65 - 5.85 Based on reported depths of 0.8 m – 1.0 m at lowpoint in Marine Parade. 5.63 Based upon depths above floor level in various shops along McKeon Street.

Photo 9 January 1999-Flooding in Chapman Avenue Photo 10 January 1999 - View along Chapman Avenue looking west

3.6.6. 30th October 1959 Event

Records for this event indicate that a large amount of rainfall was experienced within the broader region. Rainfall totals measured at Randwick Bowling Club and Randwick Racecourse on and around 30/10/1959 were 265 mm and 267 mm respectively. There was no pluviograph record available for this event. However, information provided by local residents indicates that 11½ inches (295 mm) of rain fell within four hours on the morning of the 30/10/1959. A number of historical photographs showing the extent of flooding within the Fenton Avenue/Chapman Avenue lowpoint were also provided (see Photos 11-14).

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Photo 11 October 1959 - Photo taken at intersection Photo 12 October 1959 - Photo from Chapman Avenue of Chapman and Fenton Avenues (looking south looking east towards intersection of Fenton Avenue west along Fenton Ave)

Photo 13 October 1959 - Flooding in Chapman Photo 14 October 1959 - Flooding in Chapman Avenue Avenue (view looking west) – note flood mark on car door

No recorded flood heights are available for the October 1959 event. However, the various photographs showing the depth of inundation together with Council‟s ALS data were used to estimate indicative levels in the Fenton Avenue/Chapman Avenue area. On this basis, the flood level was estimated to be between 5.6 and 5.7mAHD.

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4. APPROACH ADOPTED

4.1. General

The analysis approach adopted has been influenced by the study objectives and the quality and quantity of available data. The urbanised nature of the catchment with its underlying soil characteristics, mixture of pervious/impervious surfaces and the development of a piped drainage system has created a complex hydrologic-hydraulic flow system. The analysis is further complicated by:  a lack of recorded data describing past floods within the catchment,  interactions between the overland flows and the sub-surface drainage system.

In an urban drainage catchment such as Maroubra Bay there is rarely a historical flood record available and the use of a flood frequency approach for the estimation of design floods is not possible.

In view of the above, the approach adopted for this study was to use a widely regarded hydrologic model (for urban situations) in conjunction with a hydraulic model. The models were then calibrated using the historical flood information from the June 1999 event. A limited validation was undertaken based upon the 1959 event. The sensitivity of the model results to the adopted model parameters was also assessed for the 1% AEP (1 in 100y) design storm event. An outline of the overall process is shown in Diagram 1.

4.2. Hydrologic Modelling

Techniques suitable for design flood estimation in an urban environment are described in ARR87 (Reference 2). These techniques range from simple procedures for peak flow estimation (e.g. Probabilistic Rational Method calculations), to more complex rainfall-runoff routing models that estimate time-varying flow hydrographs and can be calibrated to recorded flow data.

For the present study, the DHI software package MIKE-Storm has been used to estimate the catchment hydrology (Reference 6). The MIKE-Storm model has been configured to utilise a runoff routing formulation that is based on methodology contained in ILSAX/DRAINS models (References 7 and 8). The ILSAX/DRAINS type method has been widely adopted in Australia for use in urban catchments, similar to that of the present study. Furthermore, the use of ILSAX/DRAINS style hydrology is consistent with the approaches taken in previous studies (e.g. Reference 5).

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4.3. Hydraulic Modelling

4.3.1. Overview

A key objective of the present Flood Study is to produce estimates of design flood behaviour throughout the catchment .The outcomes are to facilitate the detailed analysis of potential flood management options. Dynamic hydraulic modelling integrating the sub-surface drainage system and overland flow paths has been employed for this study. In part, this has been facilitated by the acquisition of more detailed survey data describing the catchment topography and the sub-surface drainage network.

The sub-surface drainage system was represented using a one-dimensional model that was linked to overland flow paths. The majority of potential overland flow paths within the upper catchment are steep and are reasonably well defined consisting mainly of formal drainage easements/reserves and roadways). In view of this, the hydraulic modelling of overland flow paths in the upper reaches of the Maroubra Bay catchment was undertaken using a one- dimensional (1D) modelling approach.

For the lower portions of the catchment the ground topography within the flood prone area contains significant localised variations. A combination of field inspections and a review of the corresponding topographic survey indicates that potential overland flow paths in the lower parts of the catchment are ill-defined and would reflect localised controls formed by the ground topography and existing building footprints. In order to better represent the complexity of the overland flow behaviour in this area, a 2D hydraulic modelling approach was employed. This 2D component was dynamically coupled with the 1D model components. The integrated 1D/2D hydraulic model was established using the SOBEK modelling package (Reference 9).

The hydrologic (MIKE-Storm) and hydraulic (SOBEK) models were linked (via appropriate boundary conditions) to provide an integrated and consistent set of model results to describe the design flood behaviour of the overall study area. Additional details of the SOBEK hydraulic modelling package used for the present study are provided in the following sections.

4.3.2. SOBEK Modelling Software

The SOBEK modelling package includes a finite difference numerical model for the solution of the depth averaged shallow water flow equations in two dimensions. The SOBEK software is produced by WL/Delft Hydraulics (Reference 9). SOBEK has been widely used for a range of similar projects both internationally and within Australia. The model is capable of dynamically simulating complex overland flow regimes and interactions with sub-surface drainage systems. It is especially applicable to the hydraulic analysis of flooding in urban areas which is typically characterised by short-duration events and a combination of supercritical and sub-critical flow behaviour.

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For the hydraulic analysis of complex overland flow paths (such as those identified in the present study downstream of Coral Sea Park, a combined 1D/2D model such as SOBEK provides several key advantages when compared to a traditional 1D only model. For example, in comparison to a purely 1D approach, a combined 1D/2D approach can:  provide localised detail of any topographic and/or structural features that may influence flood behaviour,  better facilitate the identification of the potential overland flow paths and flood problem areas,  dynamically model the interaction between the drainage system and complex overland flow paths, including surcharging effects, and  inherently represent the available flood storage within the 2D model geometry.

Importantly, a 2D hydraulic model can better define the spatial variations in flood behaviour across the study area. Information such as flow velocity, flood levels and hydraulic hazard can be readily mapped across the model extent. This information can then be easily integrated into a GIS based environment enabling the outcomes to be readily incorporated into Council‟s planning activities. Furthermore, the model developed for the resent study provides a more flexible modelling platform to properly assess the impacts of any overland flow management strategies within the floodplain (compared to those models established as part of previous investigations).

4.3.3 Supercritical Flow

Flow can be classified as either subcritical or supercritical by comparing the ratio of inertial and gravitational forces at a location.

The ratio of these two forces is called the Froude Number where the Froude Number is >1 the flow is supercritical and the channel velocity is high and the depth is low. Their condition is generally associated with steep slopes. For the majority of stream conditions the Froude Number is <1 and the flow is subcritical. Supercritical flow is generally not sustainable over a long river reach as obstructions etc cause it to revert to the subcritical conditions. However, it will occur over short lengths (say 1m to 2m) throughout the catchment. The change between the two conditions is termed a hydraulic jump and is associated with high turbulence.

SOBEK is able to account for the two conditions but no checks have been made to determine where supercritical conditions occur within the study area.

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5. HYDROLOGIC (MIKE-STORM) MODEL CONFIGURATION

5.1. Sub-catchment Layout

A hydrological model of the study catchment was established using the MIKE-Storm model software package (refer to Figure 6). A total catchment area of 244ha comprising 567 sub-catchments was represented in the model.

A sub-catchment area was specified at each pit or node accepting inflow into the system. Sub-catchment boundaries were manually delineated based on interpretation of the available topographic data, aerial photography and drainage information.

5.2. Model Parameters

5.2.1. Impervious Fraction

For each sub-catchment, the portion of impervious area for each sub-catchment was determined from an inspection of aerial photographs and land use types from GIS information supplied by Council. The impervious/pervious fraction defined for each sub-area was initially based on typical industry standard values for different landuse types (refer to Table 11). As will be discussed in later sections these values were refined as part of the model validation process (refer to Section 7.2). It should also be noted the values tabulated are general only and were sometimes varied for particular sub-catchments where appropriate.

Table 11 Initial Assumed Land Use Paved Percentage

Land Use Percentage Paved Percentage of Catchment Area (%) (%) General Residential 75 85 Parkland and Open Space 10 11 Commercial and Industrial 90 2 Medium to High Density Residential 95 2 NOTE: refer to Section 5.1 when interpreting the information in the above table

5.2.2. Rainfall Losses & Soil Type (MIKE-Storm Hydrologic Component)

Losses from paved areas are considered to comprise only of an initial loss i.e., an amount sufficient to wet the pavement and fill minor surface depressions. Losses from grassed areas are more complex. They are made up of both an initial loss and a continuing loss. The continuing loss was calculated within the model using an initial loss-continuing loss model, in accordance with previous studies (refer to Table 12).

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Table 12 Adopted Hydrologic Model Parameters

RAINFALL LOSSES Paved Area Depression Storage (Initial Loss) 1 mm Grassed Area Depression Storage (Initial Loss) 10 mm

5.2.3. Time of Concentration (MIKE-Storm Hydrologic Component)

Overland travel times for surface runoff within a sub-catchment were calculated using the kinematic wave equation. This relationship is based on the nature of the sub-catchment and accounts for different travel times with varying rainfall intensities.

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6. SOBEK MODEL CONFIGURATION

6.1. Model Extents

An integrated 1D/2D SOBEK overland flow model of the lower reaches of study area was established based upon a digital terrain model (DTM) compiled from the available detail survey information (refer to Section 3.2). The extents of the SOBEK model are shown in Figure 7. Model cross-sections for the 1D overland flow reaches, were extracted from the DTM. Within the 2D model domain, the topography was defined using a regular grid of 2 m x 2 m cells. This very fine spatial resolution was adopted to properly define significant localised ground details and other features expected to function as hydraulic controls.

Large buildings and other significant features likely to act as flow obstructions were also incorporated into the model network based on surveyed building footprints and available aerial photography. These types of features were modelled as impermeable obstructions to the floodwaters.

The inflow boundary conditions for the SOBEK model were based on the results obtained from the hydrologic (MIKE-Storm) model of the catchment. Key model parameters and further details of the boundary conditions adopted for the SOBEK model are presented in Section 7. Details of the sub-surface drainage network and overland flow paths are presented in the following sections.

6.2. Sub-surface Drainage Network

Figure 2 shows the location and extent of drainage lines within the study catchment that have been included in the SOBEK model. The drainage system defined in the model comprises:  1042 pits and nodes, including surface inlets, junctions, headwall inlets and outlets,  1046 links representing underground drainage lines (e.g. circular pipes or box culverts) or channel lengths between nodes.

Within the drainage network the invert levels of certain pits and pipes could not be surveyed. In these instances an estimation of the pit/pipe invert level was made based on an assumption of a cover of 400 mm to the top of the pipe. Where necessary additional refinements along the major lines were made to ensure that the pipe reach graded downstream, assisting model stability (invert levels were adjusted where necessary). For locations where there was missing pipe information, an appropriate size was defined based on adjacent pipes (minimum of 300 mm diameter).

The pits and nodes (inlets, bends and junctions) modelled in SOBEK can be classified as being surface inlet pits (on-grade or sag) or otherwise (junctions and outlets). Surface inlets located at low points are termed sag inlets. The inlet capacities for all pits (sag or on-grade) were determined based on a free overflow weir control with an effective weir length based on the inlet dimensions. For on-grade inlets, the effective weir length was reduced to account for the momentum of flow travelling past the pit (based on a 30% factor). The potential for pit

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Direct private property connections into Council's pipe system were not taken into account due to the lack of appropriate information. Hence, the present model configuration only allows runoff to enter the drainage system via street surface inlets. For the purposes of design flood estimation, this assumption is considered to be conservative given that a proportion of the runoff would enter the system via direct pipe connections from private properties, particularly for some of the larger industrial/commercial buildings.

The SOBEK model does not implicitly calculate energy losses at pits in the pipe drainage network. Typically these types of losses result from changes in flow direction, changes in elevation and losses associated with the expansion and contraction of flow as it passes through the pit. It was therefore necessary to modify the SOBEK model configuration to account for these types of losses by incorporating an orifice at each exit from a pit. The geometry of the orifice was based upon the cross-section of the pipe immediately downstream of the pit. Representative orifice coefficients were selected based on calibration against corresponding pit losses calculated in similar studies using MIKE-Storm and DRAINS.

6.3. Overland Flow Paths

6.3.1. Model Representation

The overland flow paths defined in the SOBEK model are shown in Figure 7. The definition of these overland flow paths was based on the locations of pits and the layout of roads, drainage reserves and other potential flow paths identified from site inspections, topographic information and available survey data. Overland flow paths in the steeper upper reaches of the catchment were defined using 1D cross-sections. Downstream of Coral Sea Park the hydraulic behaviour of overland flow was assessed using a 2D grid comprising 2m x 2m cells. Both the 1D cross-sections and the 2D grid were extracted directly from the DTM based on Council‟s ALS dataset.

For most 1D reaches, the particular flow path was modelled as a single branch (extending from pit to pit) that was linked into the overall network. However, the individual streets and roads were often represented in the model by two separate branches (left and right) with a sufficient number of interconnections to ensure that the flood behaviour along the roads was properly reproduced. These interconnections were defined as broad-crested weirs with a crest.

The length and grade of each branch was based on the topographic data available in each location.

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6.3.2. Manning’s Roughness for Overland Flowpaths (SOBEK Model)

The hydraulic efficiency of flowpaths within the SOBEK model is represented in part by the hydraulic roughness or friction factor formulated as Manning‟s „n‟. This factor describes the net influence of bed roughness and incorporates the effects of vegetation and other features that may affect the hydraulic efficiency of the particular flow path.

Much of the ground surface within the hydraulic model extents is cleared and/or paved. However there are numerous instances of localised features and pockets of vegetation adjacent to the main roadways and within individual sites. In view of this a Manning‟s „n‟ of 0.015 was adopted within the road reserve defined by Council‟s cadastre. A higher value of 0.03 was typically adopted across the grassed areas and parks. Certain private properties located within trapped low points in the lower reaches were also included in the model extents to account for potential floodplain storage. The Manning‟s „n‟ within these property boundaries was set to an artificially high value of 0.9 to minimise the amount of active flow area represented in these areas.

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7. MODEL VALIDATION

7.1. Approach

The January 1999 storm is the largest of recent events for which there is rainfall and flood height data available. Although there are only a limited number of level observations, the 1999 flood height data provides a reasonable indication of the runoff potential of the catchment in terms of ponding in the lower reaches. The Marine Parade and Fenton Avenue low points and surrounds are known to be one of the major flood problem areas within the overall study area. For calibration the model parameters were adjusted such that the model performance reproduced the observed behaviour for the 1999 event.

The adjusted models were then validated against estimated ponding levels for the October 1959 event. In the absence of a temporal rainfall pattern for this event, a number of rainfall patterns were analysed to validate the model performance.

It is acknowledged that this process of model calibration/validation is limited to the lower reaches of the catchment (where flood heights were available). However, the outcomes are still useful as they provide an indication of the ability of the model to perform within reasonable limits.

When flooding occurs within the catchment in future, it is recommended that Council undertake to collect any available information (rainfall data, flood heights, etc.) as soon as practicable after the event (including after smaller, more frequent flooding such as would be expected in the 50% AEP event).

7.2. Results

Preliminary runs using industry-standard values for hydrological parameters (e.g. fraction impervious, rainfall losses) were found to significantly over-estimate the level of ponding in the lower reaches of the catchment. The rainfall loss values for pervious portions of residential areas were increased iteratively until a continuing loss of 25 mm/hr was reached. This rate is considered to be the maximum that could be reasonably justified for design flood estimation in the absence of detailed rainfall/runoff data for the catchment. Following this, the model was still found to over-estimate the level of ponding occurring in the lower reaches of the catchment i.e. the volume of runoff estimated by the model exceeded that actually generated during the 1999 event.

To address this, the effective fraction of impervious surfaces within the catchment was reduced until the January 1999 ponding levels were reasonably estimated by the models. The calibrated model results for the January 1999 event are compared to observed flood levels in Table 13. The corresponding parameters required to achieve this calibration are provided in Table 14. For residential areas within the study catchment (i.e. 85% of the total catchment area), the fraction impervious ultimately adopted for calibration was 10%.

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Table 13 Model Validation Results – January 1999 Event

Location Observed Water Level Modelled Water Level (mAHD) (mAHD) Fenton Ave & Chapman Ave 5.45 – 5.88 5.98 Marine Pde & McKeon St 5.63 – 5.85 5.97

Table 14 Adopted Model Parameters – January 1999 Event

Land Use Type Fraction Initial Loss Continuing % of Total Impervious Loss Catchment Area (%) (mm) (mm/hr) General Residential 10 20 25 85 Parkland and Open Space 2.5 50 50 11 Commercial and Industrial 75 20 25 2 Medium to High Density Residential 80 20 25 2

The calibrated models were then used to assess the October 1959 event. It is worth noting that the reported 24 hour totals in excess of 265 mm are equivalent to greater than a 1 in 50 year ARI rainfall (for a 24 hour duration storm). The 1959 event would represent greater than a 1 in 500 year rainfall across the reported duration of around four hours. In the absence of any temporal rainfall pattern for the October 1959 event, various alternatives based on design storm temporal patterns were used for validation purposes. The corresponding results in terms of ponding in the lower reaches are indicated in Table 15.

Table 15 Model Results – October 1959 Flood

Assumed Rainfall Peak Flood Level (mAHD) Temporal Pattern Fenton Ave & Chapman Ave McKeon St & Marine Pde Modelled Observed Modelled Observed AR&R87 24 hour 5.98 5.6 – 5.7 5.98 n/a AR&R87 4.5 hour peak burst 6.09 6.09 embedded within a 24 hour duration event Uniform Rainfall over 4.5 6.49 6.49 hours

7.3. Discussion

The model validation results for the January 1999 and October 1959 events indicate that „industry-standard‟ values typically adopted for hydrological modelling in urban areas are not appropriate for the Maroubra Bay catchment. To better reflect the runoff response of the catchment significant changes to the loss rates and the fraction impervious were required. In addition to the results presented above for two historical events, supporting evidence for this approach is also provided in Reference 10. This latter reference documents the outcomes of detailed rainfall-runoff modelling undertaken on a 57ha gauged catchment located immediately north of the Maroubra Bay catchment.

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In terms of land use types, the gauged catchment studied in Reference 10 is similar to the Maroubra Bay catchment with 81% of the area being residential. Previous rainfall/runoff investigations found….”recorded runoffs being much smaller than the high impervious area of the catchment would indicate” (Reference 10).

Detailed comparison of model results against observed data undertaken as part of Reference 5 indicates that a combination of high infiltration rates (between 25 mm/hr and 300 mm/hr) and reduced fraction impervious (ranging between 7% and 17% applied to residential areas) was needed to calibrate the rainfall to observed runoff behaviour.

The findings of Reference 10 are consistent with the outcomes of the model validation undertaken for the Maroubra Bay catchment. The adopted model parameters and resulting model validations are reasonable and the models are considered suitable for design flood estimation.

Given the limited amount of data used in the validation and the atypical rainfall-runoff behaviour for this catchment it is recommended that the model performance be re-assessed against data obtained following future floods within the catchment.

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8. DESIGN EVENT MODELLING

8.1. Approach

The validated models were used to estimate design flood behaviour for existing conditions. A number of design storms were analysed ranging from smaller events (e.g. 1 in 2 year event) through to large and extreme events such as the 1% AEP (1 in 100 year) flood and the Probable Maximum Flood (PMF).

The boundary conditions adopted and the corresponding model results are presented in the following sections.

8.2. Boundary Conditions

8.2.1. Hydrologic (MIKE-Storm) Model

Design rainfall depths and temporal patterns across different storm durations for the study catchment were obtained in accordance with AR&R87 and BOM guidelines (refer also to Section 3.4 previous).

The resulting rainfall hyetographs were converted by the MIKE-Storm hydrologic model into paved area and pervious area runoff hydrographs. These hydrographs are then super-imposed for each sub-catchment to give total flow hydrographs. These were then used to provide inflow boundary conditions to the SOBEK hydraulic model.

8.2.2. Hydraulic (SOBEK) Model

The runoff hydrographs for each sub-area were defined as point source inflow boundaries defined at the corresponding pit in the SOBEK model.

Tailwater boundaries were used at the downstream limits of the SOBEK model representing the water level in Maroubra Bay. A static water level of 1.0 mAHD was adopted for the various design events. The sensitivity of the model results to changed tailwater assumptions is provided in Section 10.

8.3. Results

8.3.1. Overview

The results from the design event modelling provide a description of the design flood behaviour of the study area. Information such as peak flood levels, flows and depths were extracted and have been documented as part of this report. In addition, the model results have also been produced in a digital format that can be readily imported into Council‟s GIS systems.

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The results of the hydraulic modelling have been analysed and presented in accordance with the Brief. Table 16 provides a summary of design flood levels and flows at key locations for each event. A schematic showing the overland flow distribution for the 1% AEP event is presented on Figure 9. Corresponding flood level and depth information is provided on Figure 10a and Figure 10b for that portion of the floodplain represented in the 2D model. Similar information for significant trapped low points in the upper catchment is provided on Figure 11a to Figure 11d.

At Council‟s request, information showing the 1% AEP design flood level +0.5 m has also been provided within the 2D model domain (refer to Figure 12a and Figure 12b) and for significant trapped low points in the upper reaches.

Flood extents for the full range of design events have been determined within the 2D model domain (refer to Figure 13). These extents have been estimated on the basis of Council‟s ALS dataset and should be confirmed via detail survey at an individual lot level.

The provisional hydraulic hazard for the 1% AEP event within the 2D model domain is shown on Figure 14. The provisional hydraulic hazard has been calculated as the product of peak depth and peak velocity in accordance with Figure L2 of Reference 1 (FDM). Areas where the peak depth is greater than or equal to 1m have been assigned as high hazard.

Following a review of Council‟s ALS dataset in conjunction with the design flood results, areas within the catchment where there may be potential for overland flow to occur through properties were broadly identified (Figure 15). More definitive information regarding this aspect is difficult to quantify due to the influence of localised features within each property including fences, local ground levels and the layout of buildings and structures.

8.3.2. Critical Storm Duration

The determination of the critical storm duration for an urban catchment is more complex than for a rural catchment. Consideration must be taken of:  the peak flow from the subcatchment surface,  the peak flow arriving at a surface inlet pit from upstream (conduit and overland flows),  the peak flow in the pit,  the volume temporarily collected in ponding areas,  the location within the catchment.

Standard AR&R storm durations ranging from 30 minutes to 3 hours were run for the 1% AEP event. The corresponding peak flow and water level estimates were then compared. The critical duration was found to vary across the catchment ranging from 60 minutes to 120 minutes. However, a detailed review of the results showed that the relative differences between these storm durations were only minor. In addition, the 120 minute storm was found to be the critical storm duration in terms of peak outflows and levels from the trapped low point behind Marine Parade. The 120 minute storm was therefore adopted as the representative critical duration for the study area to ensure consistency in results and reporting. However, it is recommended

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8.4. Flooding within Trapped Low Points behind Maroubra Beach

As identified previously the trapped low points of Fenton Avenue/Chapman Avenue and McKeon Street/Marine Parade are known to be susceptible to inundation. Modelling undertaken for the present study indicates that the trunk system has sufficient capacity to prevent significant ponding in these areas for events having an ARI of 5 years or less. In larger events significant ponding in these areas occurred as capacity of the trunk system was exceeded. Modelling results for the larger events such as the 5% AEP (1 in 20 year) and 1% AEP (1 in 100 year) event indicates that the main stormwater outlet onto the beach has a capacity of around 13-14 m3/s.

Overflow from Marine Parade across the dune line onto Maroubra Beach did not occur in the 1% AEP (1 in 100 year) event although it was shown to overtop in the PMF event. The 1% AEP flood level of 6.2 mAHD was slightly lower than the 6.3 m crest level along the beach frontage.

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Table 16 Design Flows and Levels

ARI

1y 2y 5y 20y 100y PMF Location Overland Pipe Water Overland Pipe Water Overland Pipe Water Overland Pipe Water Overland Pipe Water Overland Pipe Water Flows Flows Level Flows Flows Level Flows Flows Level Flows Flows Level Flows Flows Level Flows Flows Level (m3/s) (m3/s) (mAHD) (m3/s) (m3/s) (mAHD) (m3/s) (m3/s) (mAHD) (m3/s) (m3/s) (mAHD) (m3/s) (m3/s) (mAHD) (m3/s) (m3/s) (mAHD)

1D HYDRAULIC MODEL (PIPE AND OVERLAND FLOW):

Bottom of Byrne Crescent <0.1 <0.1 16.2 <0.1 1.9 16.2 0.1 3.3 16.4 2.1 3.4 17.1 3.3 3.4 17.3 7.9 3.2 17.6

French Street <0.1 <0.1 38.4 <0.1 0.3 38.4 0.2 0.3 38.6 0.5 0.3 38.7 1.0 0.3 38.7 2.9 0.4 38.8

Astoria Circuit <0.1 <0.1 17.7 <0.1 0.6 17.8 0.0 0.7 18.2 0.6 0.8 18.4 1.0 0.8 18.5 2.3 0.8 18.6

Near corner of Anzac Pde & Fitzgerald Ave <0.1 <0.1 20.0 0.3 0.2 20.0 0.3 0.3 20.1 0.8 0.4 20.6 1.5 0.3 21.0 6.9 0.4 22.7

Beatty Lane <0.1 <0.1 30.3 <0.1 0.0 30.4 0.0 0.1 30.4 0.1 0.1 30.4 0.1 0.1 30.5 0.1 0.2 30.9

U/S of Retirement Village, Curtin Cr <0.1 <0.1 23.9 0.1 1.1 24.0 0.1 1.7 24.0 0.2 1.9 24.3 0.4 1.9 24.5 1.2 1.9 25.0

Haig Street <0.1 <0.1 46.5 0.1 0.4 46.6 0.2 0.4 46.7 0.5 0.4 47.0 1.4 0.4 47.6 4.3 0.4 48.1

Main Stormwater Outlet n/a 2.3 n/a n/a 6.1 n/a n/a 11.1 n/a n/a 13.1 n/a n/a 14.2 n/a n/a 16.3 n/a

2D HYDRAULIC MODEL (OVERLAND FLOW ONLY):

Malabar rd U/S of Fitzgerald Avenue - - Not Wet - - Not Wet No Flow - 10.9 2.8 - 11.1 7.6 - 11.3 63.0 - 12.1

Coral Sea Park n/a - Not Wet - - 13.8 - - 14.8 - - 14.3 - - 14.5 - - 15.5

Yorketown Pde U/S of New Orleans Cr - - Not Wet - - 13.0 No Flow - 13.0 0.8 - 13.1 2.6 - 13.2 31.0 - 13.9 Fitzgerald Avenue - - Not Wet - - 7.0 No Flow - 7.1 4.6 - 7.3 10.7 - 7.4 69.0 - 8.0 (St Mary & St Josephs school) D/S End of Bowling Club - - 7.0 - - 7.6 No Flow - 7.8 3.5 - 7.0 2.4 - 8.1 2.9 - 8.3

Fenton Ave & Chapman Ave Intersection - - Not Wet - - Not Wet - - 5.5 - - 5.9 - - 6.3 - - 7.3

Marine Parade & McKeon St Intersection - - 5.0 - - 5.0 - - 5.1 - - 5.9 - - 6.3 - - 7.3

Overflow to Beach No Flow - - No Flow - - No Flow - - No Flow - - No Flow - - 110.0 - - NOTES: Results based on 120 min storm duration for all events except the PMF Overland flows within trapped ponding areas not shown (e.g. Coral Sea Park, Fenton Avenue/Chapman Avenue).

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9. COMPARISON OF RESULTS WITH PREVIOUS STUDIES

9.1. Design Flood Behaviour

The following outlines the key differences between the design flood results of the current study and those published previously (Reference 5) for the trapped low points at Fenton Avenue/Chapman Avenue and McKeon Street and Marine Parade. A comparison of estimated flood levels for various design events is shown in Table 17.

Table 17 Peak Water Levels – Comparison of Results

Event Fenton Ave/Chapman Ave McKeon St/Marine Pde Current Study Reference 5 Current Study Reference 5 99.9% AEP Not wet 5.4 5.0 5.2 50% AEP Not wet 5.6 5.0 5.5 20% AEP 5.5 6.0 5.1 6.2 5% AEP 5.9 6.0 5.9 6.3 1% AEP 6.3 6.3 6.3 6.3

The comparison shows that the 1 in 100 year estimates obtained in the two studies compare well. However, there are notable discrepancies for the smaller, more frequent events. For example, the 20% AEP flood level at the Marine Parade low point estimated by the current study is 5.4 mAHD. The corresponding estimate from Reference 5 is 6.2 mAHD. For reference, the crest level onto the main Maroubra Beach is at 6.3 mAHD.

In the context of known flood behaviour and the significant magnitude of property inundation that would occur in the Marine Parade area should a flood level of 6.2 mAHD be recorded, it would appear that the previous estimates may be conservative.

As a comparison, the available flood observations from as far back as 1959 would suggest flood heights have not exceeded 6 mAHD. Although larger floods may have occurred during this time from which no records were taken it is considered unlikely that any of these events would reach the previously reported 20% AEP year level of 6.2 mAHD as there would have been significant levels of property inundation should this have occurred.

The differences in flood level estimates for the range of design events between the two studies can be attributed to a number of factors including:  differences in catchment hydrology and assumed model parameters,  representation of flow paths and floodplain storage.

It is difficult to provide comment in terms of assumed model parameters as the values adopted for the earlier study are not reported. However, the parameters used for the current study have been validated based on historical flood events and are consistent with those found in independent studies in adjacent catchments (e.g. Reference 10).

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In terms of the representation of floodplain behaviour, the quality and coverage of base data available for the current study far exceeds that available previously. The current study incorporates a detailed representation of the overland flow paths and sub-surface drainage network throughout the catchment. The current model also accounts for the available floodplain storage in greater detail, particularly in the lower reaches where the ground topography is directly represented in the 2D model grid. Hence in comparison to the previous study, the current approach provides a detailed model of the drainage network and overland flow paths throughout the catchment. It is therefore better able to take into account the movement of surface water through the floodplain and the corresponding effects on flooding in the lower reaches.

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10. SENSITIVITY ANALYSES

10.1. Overview

The models established for the present study rely on a number of assumed parameters, the values of which are considered to be the most appropriate for the Maroubra Bay catchment based on limited calibration/validation and published studies of similar catchments. Although a limited model validation has been performed, a range of sensitivity analyses were also undertaken to quantify the potential variation in the model results due to different assumptions in the key modelling parameters adopted.

The 2005 Floodplain Development Manual also requires that Flood Studies and Floodplain Risk Management Studies consider the impacts of climate change on flood behaviour. Hence the sensitivity of the model results to various Climate Change scenarios was assessed as part of this Study.

Within the last twelve months current best practice for considering the impacts of climate change (in terms of ocean level rise and rainfall increase) have been evolving rapidly. Key developments have included:  the release of the Fourth Assessment Report by the Inter-governmental Panel on Climate Change (IPCC) in February 2007 (Climate Change 2007), which updated the Third IPCC Assessment Report of 2001;  the preparation of Climate Change Adaptation Actions for Local Government by SMEC Australia for the Australian Greenhouse Office in mid 2007;  the preparation of Climate Change in Australia by CSIRO in late 2007, which provides an Australian focus on Climate Change 2007;  the release of the Floodplain Risk Management Guideline Practical Consideration of Climate Change by the NSW Department of Environment and Climate Change in October 2007 (referred to herein as the DECC Guideline 2007 – Reference 11).

In accordance with the DECC Guideline 2007, the following climate change scenarios (by the year 2100) are considered: • ocean level rise: ‒ medium level ocean rise = 0.55 m, ‒ high level ocean rise = 0.91 m.

• increase in peak rainfall and storm volume: ‒ low level rainfall increase = 10%, ‒ medium level rainfall increase = 20%.

A high degree of uncertainty surrounds the likely impact of climate change upon rainfall. Hence, a range of increased rainfalls have been assessed for this study. It is understood that work currently being undertaken by CSIRO and the Sydney Catchment Authority may provide better direction on the possible impacts to rainfall.

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10.2. Modelled Scenarios and Assumptions

The following scenarios were considered to represent the envelope of likely parameter values:  increasing the impervious fraction for the established residential areas of the catchment (20%, 40%, 60%),  +10% and 20% change in design rainfall,  +0.55 m,+0.91 m increase in ocean tailwater levels,  20% change in Manning=s >n= value for overland flow paths.

To assess the effects of ocean level rise the static 1.0 mAHD tailwater boundary was increased by the nominated DECC 2007 values (for both the mid-level and high-level sea level rise scenarios). It should be noted that an ocean level rise of 0.55 m or 0.91 m represents a significant increase in the design ocean levels. By comparison, at Fort Denison the 1% AEP ocean level is only 1.5 mAHD and the 5% AEP approximately 1.4 mAHD. To assess the effects of an increase in peak rainfall and storm volume each ordinate design rainfall hyetograph was increased by the nominated DECC 2007 value.

When interpreting the results, it should also be noted that the sensitivity analysis for the drainage system may not always result in a change in peak flow attained downstream if (for instance) the size of the pipe or pit is the control and there is no change in the flow conveyed in the pipe. There may be a change in the overland flow but the effect further downstream will depend on the particular characteristics of the pit and pipe network. At some locations the change in upstream flow is not reflected downstream due to the effects of ponding at sag pits or the relative timing of overland flows.

For each of the above scenarios, the models were run for the 1% AEP 120 minute duration design storm using the calibrated model parameters shown previously in Table 14. A relative comparison of the resultant changes in peak overland flows and flood heights at various locations is provided in Table 18 and Table 19.

10.3. Results

The results from the sensitivity analyses can be summarised as follows:  a 20% change in the rainfall produces a corresponding 30% to 70% (approximately) change in peak overland flow,  the model is very sensitive to changes in impervious area above the calibrated 10%, with some flows increased by 70-90% when 60% impervious area is used. This is due to the adopted high loss ratio for previous areas which means the majority of the runoff is from the impervious areas.  increases up to +0.91 m in the downstream ocean tailwater produced no significant changes in flows or flood levels,  increasing the Manning‟s „n‟ value for overland flow paths caused a greater attenuation of flows and generally resulted in a reduction in peak flows of less than 10%. However, there were some locations where the peak flows were increased. This could be attributed to the relative timing of overland flows from contributing subcatchments. The

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converse of these observations holds true for the effect of decreasing Manning‟s „n‟ values by a similar amount.

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Table 18 Sensitivity Analyses – Change in Peak Flows for the 1% AEP (1 in 100 year) Event

20% IMP 20% 40% IMP 40% 60% IMP 60% +20% +20% -20% -20% +10% +10% +20% +20% -20% -20% 1.55m AHD 1.55m AHD 1.91m AHD 1.91m AHD Overland IMP Overland IMP Overland IMP Manning's Manning's Manning's Manning's Rain Rain Rain Rain Rain Rain TW Overland TW Pipe TW Overland TW Pipe Pipe Pipe Pipe Overland Pipe Overland Pipe Overland Pipe Overland Pipe Overland Pipe 1D Part of SOBEK Model Bottom of Byrne Crescent 5% 0% 17% 0% 28% 0% -1% 0% 0% -1% 16% 0% 31% 0% -37% 0% negligible negligible negligible negligible French Street 9% 1% 25% 2% 41% 3% -2% 0% 2% 0% 22% 1% 41% 2% -42% -1% negligible negligible negligible negligible Astoria Circuit 9% 0% 30% 0% 49% 0% 0% 0% 0% 0% 16% 0% 33% 0% -34% 0% negligible negligible negligible negligible Near corner of Anzac Pde & Fitzgerald Ave 11% -4% 33% 1% 58% -2% 0% 1% 1% -2% 22% -3% 45% 0% -44% 0% negligible negligible negligible negligible Beatty Lane 5% 3% 15% 7% 23% 12% -3% 1% 3% -1% 18% 9% 35% 17% -37% -23% negligible negligible negligible negligible U/S of Retirement Village, Curtin Crescent 7% 1% 26% 1% 41% 2% 2% 1% 2% 0% 22% 0% 40% 2% -48% -1% negligible negligible negligible negligible Haig Street 8% -1% 26% -1% 42% -1% -5% 0% 6% -1% 19% -2% 38% -1% -35% 0% negligible negligible negligible negligible Main Stormwater Pipe Outflow 0% 2% 3% 0% 0% 2% 4% -8% negligible 2% 2D Part of SOBEK Model Malabar Road u/s of Fitzgerald Avenue 16% - 48% - 78% - -3% - 4% - 38% - 76% - -60% - negligible - negligible - Yorketown Pde u/s of New Orleans Cres 18% - 56% - 97% - -3% - 2% - 35% - 77% - -62% - negligible - negligible - Fitzgerald Avenue 13% - 41% - 66% - -3% - 2% - 31% - 64% - -52% - negligible - negligible - (St Marys & St Josephs Schools) D/s end of Bowling Club 2% - 6% - 8% - -1% - 1% - 3% - 5% - -10% - negligible - negligible - Fenton Ave & Chapman Ave Intersection 6% - 16% - 20% - 0% - 4% - 16% - 27% - -26% - negligible - negligible -

Table 19 Sensitivity Analyses – Change in Peak Flood Height for the 1% AEP (1 in 100 year) Event

20% IMP 40% IMP 60% IMP +20% Manning's -20% Manning's +10% Rain +20% Rain -20% Rain 1.55m AHD TW 1.91m AHD TW 1D Part of SOBEK Model Bottom of Byrne Crescent negligible 0.07 0.09 negligible negligible 0.06 0.10 -0.21 negligible negligible French Street negligible 0.03 0.05 negligible negligible 0.02 0.03 -0.03 negligible negligible Astoria Circuit negligible 0.03 0.05 negligible negligible negligible 0.03 -0.03 negligible negligible Near corner of Anzac Pde & Fitzgerald Ave 0.07 0.21 0.32 negligible negligible 0.16 0.29 -0.41 negligible negligible Beatty Lane negligible negligible 0.02 negligible negligible negligible 0.03 -0.03 negligible negligible U/S of Retirement Village, Curtin Crescent 0.03 0.10 0.15 negligible negligible 0.10 0.18 -0.22 negligible negligible Haig Street 0.04 0.11 0.20 -0.03 0.03 0.10 0.22 -0.36 negligible negligible 2D Part of SOBEK Model Malabar Road u/s of Fitzgerald Avenue 0.04 0.10 0.16 negligible negligible 0.08 0.15 -0.19 negligible negligible Coral Sea Park 0.05 0.15 0.24 negligible negligible 0.12 0.22 -0.25 negligible negligible Yorketown Pde u/s of New Orleans Circuit 0.02 0.05 0.08 negligible -0.02 0.03 0.06 -0.07 negligible negligible Fitzgerald Avenue 0.03 0.08 0.12 negligible -0.02 0.06 0.12 -0.13 negligible negligible (St Marys & St Josephs Schools) D/s end of Bowling Club 0.02 0.04 0.06 negligible negligible 0.02 0.04 -0.08 negligible negligible Fenton Ave & Chapman Ave Intersection 0.06 0.17 0.26 negligible negligible 0.15 0.26 -0.36 negligible negligible Marine Parade & McKeon St Intersection 0.06 0.17 0.26 negligible negligible 0.15 0.26 -0.37 negligible negligible

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In terms of flood height estimates, the greatest changes were found to result from variations in estimated runoff due to either increased rainfall or changes in the assumed impervious fraction for the catchment. For example, an increase of +20% in rainfall or an impervious fraction of 60% was found to increase flood levels by between 0.1 m to 0.3 m, depending upon the location within the catchment.

By contrast the flood level estimates were much less sensitive to variations in hydraulic model parameters such as Manning‟s „n‟. For ±20% variations in Manning‟s „n‟ the corresponding variations in flood levels was found to be within ±0.05 m, the variation in flows typically being within ±5%.

The outcomes indicate that the estimated 1% AEP flood levels behind the main dune are not adversely impacted by changes in sea level rise (up to an increase of 0.91 m). A review of model results for the various sea level scenarios suggests that this outcome reflects the significant hydraulic gradient in the main trunk system between the Marine Parade low point (where the invert of the trunk system is at 3.0 mAHD) and the main system outlet (where the inverts of the 3.06 m W x 1.61 m H outlet is at 0.41 mAHD). Under the high sea level rise scenario, the peak velocities within the main outlet on the beach was found to be in the order of 3 m/s (for the 1% AEP event). It is considered unlikely that blockage of the outlet by sand would be sustained during a flood as the system discharges.

The outcomes of the sensitivity analyses highlight the influence of the assumed fraction impervious adopted for residential areas. Given the relatively sandy nature of the upper soils found in this and adjacent catchments, it is recommended that opportunities for monitoring runoff behaviour be pursued wherever possible in the future. It should also be noted that the ponding levels in trapped low points have been estimated in line with the blockage assumptions discussed previously in Section 6.2. Flood levels in these locations may be affected should the inlets be blocked to a greater degree than that assumed in this study.

10.4. Accuracy of Estimated Design Flood Levels

Based on the outcomes of the model validation and sensitivity analyses, the accuracy of the results within the lower reaches of the catchment (i.e. in and around the Marine Parade and Fenton Avenue low points) is estimated to be within ±0.3 m.

Elsewhere in the catchment, the model is effectively uncalibrated and the corresponding accuracy of flood level estimates is likely to be within ±0.5 m.

Note that when interpreting the model results to derive flood level estimates care should be taken to review both the estimated level and depth results together with detail survey to confirm an appropriate flood level, particularly where the estimated depths are reasonably shallow (e.g. less than 0.3 m for the 1% AEP event). In these instances, the depths approach the limit of accuracy of the underlying survey data.

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Care should also be taken when interpolating flood levels between cross-sections particularly where there are steep and variable road gradients and the depth of flow is shallow. In these areas, the hydraulic gradient may follow the longitudinal road gradient. Hence in areas where the road gradient between adjacent cross-sections is not linear, a linear interpolation of flood levels between sections may lead to an erroneous flood level estimate. As noted previously it is recommended that a suitable flood level estimate in these areas be confirmed based on the estimated depth of flow and a review of available survey.

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11. ACKNOWLEDGEMENTS

This study was carried out by WMAwater and funded by Randwick City Council and the NSW State Government. The assistance of the following in providing data and guidance to the study is gratefully acknowledged:  Maroubra Bay Floodplain Management Committee,  Randwick City Council,  NSW Department of Environment, Climate Change and Water,  the Maroubra Bay Floodplain Management Committee,  residents of the Maroubra Bay catchment.

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12. REFERENCES

1. New South Wales Government Floodplain Development Manual April 2005.

2. Pilgrim, D H (Editor in Chief) Australian Rainfall and Runoff – A Guide to Flood Estimation Institute of Engineers, Australia, 1987.

3. Bureau of Meteorology The Estimation of Probable Maximum Precipitation in Australia : Generalised Short-Duration Method Melbourne, Australia, June 2003.

4. Woollahra Municipal Council Rushcutters Bay Catchment Flood Study Webb, McKeown & Associates, October 2007.

5. Randwick City Council Maroubra Beach Flood Investigation Willings & Partners, December 1999.

6. DHI Water & Environment MIKE-Storm User & Reference Guides 2004.

7. O‟Loughlin, G G The ILSAX Program for Urban Drainage Design and Analysis School of Civil Engineering, NSW Institute of Technology, 1986.

8. Watercom Pty Ltd and Dr G G O‟Loughlin DRAINS Software Manual, V2006.02 2006.

9. DELFT Hydraulics SOBEK Reference Guide,V2.10.003

10. International Hydrology & Water Resources Symposium Authors: O‟Loughlin, G G, Haig, R C, Altwater, R B and Clare, G R Calibration of Stormwater Rainfall-Runoff Models Perth, October 1991.

11. Department of Environment & Climate Change NSW DRAFT Floodplain Risk Management Guideline – Practical Consideration of Climate Change October 2007.

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Maroubra Bay Catchment Flood Study

APPENDIX A: GLOSSARY OF FLOOD TERMS

Taken from the Floodplain Development Manual (April 2005 edition)

acid sulfate soils Are sediments which contain sulfidic mineral pyrite which may become extremely acid following disturbance or drainage as sulfur compounds react when exposed to oxygen to form sulfuric acid. More detailed explanation and definition can be found in the NSW Government Acid Sulfate Soil Manual published by Acid Sulfate Soil Management Advisory Committee.

Annual Exceedance The chance of a flood of a given or larger size occurring in any one year, usually Probability (AEP) expressed as a percentage. For example, if a peak flood discharge of 500 m3/s has an AEP of 5%, it means that there is a 5% chance (that is one-in-20 chance) of a 500 m3/s or larger event occurring in any one year (see ARI).

Australian Height Datum A common national surface level datum approximately corresponding to mean (AHD) sea level.

Average Annual Damage Depending on its size (or severity), each flood will cause a different amount of (AAD) flood damage to a flood prone area. AAD is the average damage per year that would occur in a nominated development situation from flooding over a very long period of time.

Average Recurrence The long term average number of years between the occurrence of a flood as big Interval (ARI) as, or larger than, the selected event. For example, floods with a discharge as great as, or greater than, the 20 year ARI flood event will occur on average once every 20 years. ARI is another way of expressing the likelihood of occurrence of a flood event.

caravan and moveable Caravans and moveable dwellings are being increasingly used for long-term and home parks permanent accommodation purposes. Standards relating to their siting, design, construction and management can be found in the Regulations under the LG Act.

catchment The land area draining through the main stream, as well as tributary streams, to a particular site. It always relates to an area above a specific location.

consent authority The Council, government agency or person having the function to determine a development application for land use under the EP&A Act. The consent authority is most often the Council, however legislation or an EPI may specify a Minister or public authority (other than a Council), or the Director General of DIPNR, as having the function to determine an application.

development Is defined in Part 4 of the Environmental Planning and Assessment Act (EP&A Act).

infill development: refers to the development of vacant blocks of land that are generally surrounded by developed properties and is permissible under the current zoning of the land. Conditions such as minimum floor levels may be imposed on infill development.

new development: refers to development of a completely different nature to that associated with the former land use. For example, the urban subdivision of an area previously used for rural purposes. New developments involve rezoning and typically require major extensions of existing urban services, such as roads, water supply, sewerage and electric power.

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redevelopment: refers to rebuilding in an area. For example, as urban areas age, it may become necessary to demolish and reconstruct buildings on a relatively large scale. Redevelopment generally does not require either rezoning or major extensions to urban services.

disaster plan (DISPLAN) A step by step sequence of previously agreed roles, responsibilities, functions, actions and management arrangements for the conduct of a single or series of connected emergency operations, with the object of ensuring the coordinated response by all agencies having responsibilities and functions in emergencies.

discharge The rate of flow of water measured in terms of volume per unit time, for example, cubic metres per second (m3/s). Discharge is different from the speed or velocity of flow, which is a measure of how fast the water is moving for example, metres per second (m/s).

ecologically sustainable Using, conserving and enhancing natural resources so that ecological processes, development (ESD) on which life depends, are maintained, and the total quality of life, now and in the future, can be maintained or increased. A more detailed definition is included in the Local Government Act 1993. The use of sustainability and sustainable in this manual relate to ESD.

effective warning time The time available after receiving advice of an impending flood and before the floodwaters prevent appropriate flood response actions being undertaken. The effective warning time is typically used to move farm equipment, move stock, raise furniture, evacuate people and transport their possessions.

emergency management A range of measures to manage risks to communities and the environment. In the flood context it may include measures to prevent, prepare for, respond to and recover from flooding.

flash flooding Flooding which is sudden and unexpected. It is often caused by sudden local or nearby heavy rainfall. Often defined as flooding which peaks within six hours of the causative rain.

flood Relatively high stream flow which overtops the natural or artificial banks in any part of a stream, river, estuary, lake or dam, and/or local overland flooding associated with major drainage before entering a watercourse, and/or coastal inundation resulting from super-elevated sea levels and/or waves overtopping coastline defences excluding tsunami.

flood awareness Flood awareness is an appreciation of the likely effects of flooding and a knowledge of the relevant flood warning, response and evacuation procedures.

flood education Flood education seeks to provide information to raise awareness of the flood problem so as to enable individuals to understand how to manage themselves an their property in response to flood warnings and in a flood event. It invokes a state of flood readiness.

flood fringe areas The remaining area of flood prone land after floodway and flood storage areas have been defined.

flood liable land Is synonymous with flood prone land (i.e. land susceptible to flooding by the probable maximum flood (PMF) event). Note that the term flood liable land covers the whole of the floodplain, not just that part below the flood planning level (see flood planning area).

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flood mitigation standard The average recurrence interval of the flood, selected as part of the floodplain risk management process that forms the basis for physical works to modify the impacts of flooding.

floodplain Area of land which is subject to inundation by floods up to and including the probable maximum flood event, that is, flood prone land.

floodplain risk The measures that might be feasible for the management of a particular area of management options the floodplain. Preparation of a floodplain risk management plan requires a detailed evaluation of floodplain risk management options.

floodplain risk A management plan developed in accordance with the principles and guidelines management plan in this manual. Usually includes both written and diagrammetic information describing how particular areas of flood prone land are to be used and managed to achieve defined objectives.

flood plan (local) A sub-plan of a disaster plan that deals specifically with flooding. They can exist at State, Division and local levels. Local flood plans are prepared under the leadership of the State Emergency Service.

flood planning area The area of land below the flood planning level and thus subject to flood related development controls. The concept of flood planning area generally supersedes the Aflood liable land@ concept in the 1986 Manual.

Flood Planning Levels FPL=s are the combinations of flood levels (derived from significant historical flood (FPLs) events or floods of specific AEPs) and freeboards selected for floodplain risk management purposes, as determined in management studies and incorporated in management plans. FPLs supersede the Astandard flood event@ in the 1986 manual.

flood proofing A combination of measures incorporated in the design, construction and alteration of individual buildings or structures subject to flooding, to reduce or eliminate flood damages.

flood prone land Is land susceptible to flooding by the Probable Maximum Flood (PMF) event. Flood prone land is synonymous with flood liable land.

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

flood risk Potential danger to personal safety and potential damage to property resulting from flooding. The degree of risk varies with circumstances across the full range of floods. Flood risk in this manual is divided into 3 types, existing, future and continuing risks. They are described below.

existing flood risk: the risk a community is exposed to as a result of its location on the floodplain.

future flood risk: the risk a community may be exposed to as a result of new development on the floodplain.

continuing flood risk: the risk a community is exposed to after floodplain risk management measures have been implemented. For a town protected by levees, the continuing flood risk is the consequences of the levees being overtopped. For an area without any floodplain risk management measures, the continuing flood risk is simply the existence of its flood exposure.

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flood storage areas Those parts of the floodplain that are important for the temporary storage of floodwaters during the passage of a flood. The extent and behaviour of flood storage areas may change with flood severity, and loss of flood storage can increase the severity of flood impacts by reducing natural flood attenuation. Hence, it is necessary to investigate a range of flood sizes before defining flood storage areas.

floodway areas Those areas of the floodplain where a significant discharge of water occurs during floods. They are often aligned with naturally defined channels. Floodways are areas that, even if only partially blocked, would cause a significant redistribution of flood flows, or a significant increase in flood levels.

freeboard Freeboard provides reasonable certainty that the risk exposure selected in deciding on a particular flood chosen as the basis for the FPL is actually provided. It is a factor of safety typically used in relation to the setting of floor levels, levee crest levels, etc. Freeboard is included in the flood planning level.

habitable room in a residential situation: a living or working area, such as a lounge room, dining room, rumpus room, kitchen, bedroom or workroom.

in an industrial or commercial situation: an area used for offices or to store valuable possessions susceptible to flood damage in the event of a flood.

hazard A source of potential harm or a situation with a potential to cause loss. In relation to this manual the hazard is flooding which has the potential to cause damage to the community. Definitions of high and low hazard categories are provided in the Manual.

hydraulics Term given to the study of water flow in waterways; in particular, the evaluation of flow parameters such as water level and velocity.

hydrograph A graph which shows how the discharge or stage/flood level at any particular location varies with time during a flood.

hydrology Term given to the study of the rainfall and runoff process; in particular, the evaluation of peak flows, flow volumes and the derivation of hydrographs for a range of floods.

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

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

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

major drainage Councils have discretion in determining whether urban drainage problems are associated with major or local drainage. For the purpose of this manual major drainage involves: $ the floodplains of original watercourses (which may now be piped, channelised or diverted), or sloping areas where overland flows develop along alternative paths once system capacity is exceeded; and/or

$ water depths generally in excess of 0.3 m (in the major system design storm as defined in the current version of Australian Rainfall and Runoff). These conditions may result in danger to personal safety and property damage to both premises and vehicles; and/or

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$ major overland flow paths through developed areas outside of defined drainage reserves; and/or

$ the potential to affect a number of buildings along the major flow path.

mathematical/computer The mathematical representation of the physical processes involved in runoff models generation and stream flow. These models are often run on computers due to the complexity of the mathematical relationships between runoff, stream flow and the distribution of flows across the floodplain.

merit approach The merit approach weighs social, economic, ecological and cultural impacts of land use options for different flood prone areas together with flood damage, hazard and behaviour implications, and environmental protection and well being of the State=s rivers and floodplains.

The merit approach operates at two levels. At the strategic level it allows for the consideration of social, economic, ecological, cultural and flooding issues to determine strategies for the management of future flood risk which are formulated into Council plans, policy and EPIs. At a site specific level, it involves consideration of the best way of conditioning development allowable under the floodplain risk management plan, local floodplain risk management policy and EPIs.

minor, moderate and major Both the State Emergency Service and the Bureau of Meteorology use the flooding following definitions in flood warnings to give a general indication of the types of problems expected with a flood:

minor flooding: causes inconvenience such as closing of minor roads and the submergence of low level bridges. The lower limit of this class of flooding on the reference gauge is the initial flood level at which landholders and townspeople begin to be flooded.

moderate flooding: low-lying areas are inundated requiring removal of stock and/or evacuation of some houses. Main traffic routes may be covered.

major flooding: appreciable urban areas are flooded and/or extensive rural areas are flooded. Properties, villages and towns can be isolated.

modification measures Measures that modify either the flood, the property or the response to flooding. Examples are indicated in Table 2.1 with further discussion in the Manual.

peak discharge The maximum discharge occurring during a flood event.

Probable Maximum Flood The PMF is the largest flood that could conceivably occur at a particular location, (PMF) usually estimated from probable maximum precipitation, and where applicable, snow melt, coupled with the worst flood producing catchment conditions. Generally, it is not physically or economically possible to provide complete protection against this event. The PMF defines the extent of flood prone land, that is, the floodplain. The extent, nature and potential consequences of flooding associated with a range of events rarer than the flood used for designing mitigation works and controlling development, up to and including the PMF event should be addressed in a floodplain risk management study.

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Probable Maximum The PMP is the greatest depth of precipitation for a given duration Precipitation (PMP) meteorologically possible over a given size storm area at a particular location at a particular time of the year, with no allowance made for long-term climatic trends (World Meteorological Organisation, 1986). It is the primary input to PMF estimation.

probability A statistical measure of the expected chance of flooding (see AEP).

risk Chance of something happening that will have an impact. It is measured in terms of consequences and likelihood. In the context of the manual it is the likelihood of consequences arising from the interaction of floods, communities and the environment.

runoff The amount of rainfall which actually ends up as streamflow, also known as rainfall excess.

stage Equivalent to Awater level@. Both are measured with reference to a specified datum.

stage hydrograph A graph that shows how the water level at a particular location changes with time during a flood. It must be referenced to a particular datum.

survey plan A plan prepared by a registered surveyor.

water surface profile A graph showing the flood stage at any given location along a watercourse at a particular time.

wind fetch The horizontal distance in the direction of wind over which wind waves are generated.

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