BIRDS GULLY & BUNNERONG ROAD FLOOD STUDY
FINAL REPORT
JUNE 2018
Level 2, 160 Clarence Street Sydney, NSW, 2000
Tel: (02) 9299 2855 Fax: (02) 9262 6208 Email: [email protected] Web: www.wmawater.com.au
BIRDS GULLY & BUNNERONG ROAD FLOOD STUDY
FINAL REPORT JUNE 2018
Project Project Number Birds Gully & Bunnerong Road Flood Study 116083
Client Client’s Representative Randwick City Council and Bayside Council Sebastien Le Coustumer
Authors Prepared by Fabien Joly Dan Morgan Rhys Hardwick Jones
Date Verified by 21/06/2018
Revision Description Distribution Date 1 Draft Report RCC, BC, OEH, SES Feb 2018 2 Draft Report for Public Exhibition Public March 2018 3 Final Report RCC, BC, OEH, SES June 2018
BIRDS GULLY & BUNNERONG ROAD FLOOD STUDY
TABLE OF CONTENTS
PAGE 1. INTRODUCTION ...... 1 1.1. Study Area ...... 1 1.2. Objectives ...... 2 1.3. Description of the Catchment and Flood History Overview ...... 2 1.4. Community Consultation ...... 4
2. AVAILABLE DATA ...... 7 2.1. Overview ...... 7 2.2. Data Sources ...... 7 2.3. Topographic Data ...... 7 2.4. Pit and Pipe Data ...... 8 2.5. Historical Flood Level Data ...... 9 2.6. Historical Rainfall Data ...... 10 2.6.1. Overview ...... 10 2.6.2. Rainfall Stations ...... 11 2.6.3. Analysis of Daily Read Rainfall Data ...... 13 2.6.4. Pluviometer Rainfall Data ...... 14 2.6.5. Design Rainfall Data ...... 15 2.7. Temporal Patterns ...... 15 2.7.1. ARR1987 Temporal Patterns ...... 16 2.7.2. ARR2016 Temporal Patterns ...... 16 2.8. Previous Studies ...... 17 2.8.1. SWC 10 & SWC 12 Capacity Assessment (Reference 4) ...... 18 2.8.2. SWC 11AMP Capacity Assessment (Reference 5) ...... 18 2.8.3. SWC 11 Capacity Assessment (Reference 6)...... 19
3. MODELLING METHODOLOGY OVERVIEW ...... 20 3.1. Hydrologic Model ...... 21 3.2. Hydraulic Model ...... 22 3.3. Calibration to Historical Events ...... 22
4. HYDROLOGIC MODEL SETUP ...... 23 4.1. Sub-catchment Delineation ...... 23 4.2. Impervious Surface Area ...... 23 4.3. Sub-catchment Slope ...... 24 4.4. Losses ...... 24
5. HYDRAULIC MODEL SETUP ...... 25 5.1. Digital Elevation Model ...... 25 5.2. Boundary Locations ...... 25 5.2.1. Inflows ...... 25 5.2.2. Downstream Boundary ...... 25 5.2.3. Roughness Co-efficient ...... 27 5.3. Continuous Infiltration Rate ...... 27
5.4. Hydraulic Structures ...... 28 5.4.1. Buildings ...... 28 5.4.2. Fencing and Obstructions ...... 28 5.4.3. Sub-surface Drainage Network ...... 28 5.4.4. Open Channels, Bridges and Culverts ...... 29 5.4.5. Road Kerbs and Gutters ...... 30 5.5. Blockage Assumptions ...... 32 5.5.1. Background ...... 32 5.5.2. Blockage for Calibration Events ...... 33 5.5.3. Blockage for Design Events ...... 33
6. MODEL VALIDATION ...... 34 6.1. Overview ...... 34 6.2. Summary of Historical Event Rainfall Data ...... 34 6.3. Recorded Flood Levels and Observed Behaviour ...... 35 6.4. Hydraulic Model Validation ...... 38 6.4.1. Validation to Observed Depths – December 2015 ...... 38 6.4.2. Validation to Observed Depths – March 2014 ...... 40 6.4.3. Validation to Qualitative Flood Behaviour ...... 40 6.4.4. Validation Conclusions ...... 42
7. DESIGN EVENT MODELLING ...... 43 7.1. Overview ...... 43 7.2. Critical Duration ...... 43 7.3. Design Results ...... 44 7.3.1. Summary of Results ...... 45 7.3.2. Provisional Flood Hazard Categorisation ...... 50 7.3.3. Provisional Hydraulic Categorisation ...... 52 7.3.4. Preliminary Flood Emergency Response Classification ...... 53 7.4. Road Inundation ...... 54 7.4.1. Preliminary “True” Flood Hazard Categorisation ...... 55 7.4.2. Pipe Capacity Assessment ...... 56
8. SENSITIVITY ANALYSIS ...... 57 8.1. Overview ...... 57 8.2. Climate Change Background ...... 57 8.2.1. Rainfall Increase ...... 58 8.2.2. Sea Level Rise ...... 59 8.3. Sensitivity Analysis Results ...... 60 8.3.1. Roughness and Infiltration Variations ...... 60 8.3.2. Blockage Variations ...... 61 8.3.3. Climate Variations ...... 66
9. FLOODING HOT SPOTS ...... 68 9.1. Harold Street and Perry Street Low Point – Matraville ...... 68 9.2. Paton Street and Byrd Avenue Low Point – Kingsford ...... 69 9.3. Holmes Street and Benvenue Street Low Point – Maroubra ...... 69 9.4. Alma Road Low Point – Maroubra ...... 69 9.5. Glanfield Street Low Point – Maroubra ...... 70
9.6. Jersey Road Low Point – Matraville ...... 71 9.7. Flack Avenue Low Point – Matraville ...... 71 9.8. Denison Street and Nilson Avenue Low Point – Hillsdale ...... 72 9.9. Boonah Avenue Low Point – Eastgardens ...... 72 9.10. Parer Street Low Point – Maroubra ...... 73 9.11. Glanfield Street / Maroubra Road Low Point – Maroubra ...... 73 9.12. Edward Circuit Low Point – Pagewood ...... 73 9.13. Bunnerong Road Low Point – Hillsdale ...... 73 9.14. Irvine Street Low Point – Kingsford ...... 74 9.15. Hincks Street Low Point – Kingsford ...... 75 9.16. Harbourne Road Low Point – Kingsford ...... 75 9.17. Hayward Street / Jacques Street Low Point – Kingsford ...... 76
10. PRELIMINARY FLOOD PLANNING AREA ...... 77 10.1. Background ...... 77 10.2. Identification of Flood Control Lots ...... 77
11. PUBLIC EXHIBITION ...... 80 12. REFERENCES ...... 81
APPENDICES Appendix A: Glossary Appendix B: Calibration Results Appendix C: Design Flood Results Appendix D: Emergency Response Planning Information Appendix E: Community Consultation Materials Appendix F: Public Exhibition Submissions
LIST OF PHOTOGRAPHS
Photo 1: Army truck in Garrett Street during the October 1959 Flood Event ...... 2 Photo 2: Corner of Garrett Street and Storey Street during the October 1959 Flood Event ...... 3 Photo 3: Flood photographs along Flack Street, Hillsdale ...... 5 Photo 4: Flood photographs at the laneway connecting Apsley Avenue to Lancaster Crescent, Kingsford ...... 5 Photo 5: Flood photographs outside 141 Bunnerong Road, Kingsford ...... 6 Photo 6: Cars in a culvert inlet – Newcastle (Reference 14) ...... 32 Photo 7: Urban debris in Wollongong (Reference 14) ...... 32 Photo 8: Collection of sample model validation photographs...... 37 Photo 9: Harold Street Low Point ...... 68 Photo 10: Alma Road Residential Dwellings ...... 70 Photo 11: Glanfield Road Low Point ...... 70 Photo 12: Flack Avenue Low Point...... 71 Photo 13: Boonah Avenue Low Point ...... 72 Photo 14: Reported Flooding Bunnerong Road Low Point ...... 74 Photo 15: Irvine Street Low Point ...... 75 Photo 16: Hayward / Jacques Street Low Point...... 76
LIST OF DIAGRAMS
Diagram 1: Temporal Pattern Ranges 17 Diagram 2: Flood Study Process 20 Diagram 3: Hazard Classifications 51 Diagram 4: Provisional Hydraulic Hazard Categories 52 Diagram 5: Stages for Identification of Flood Control Lots 78
LIST OF TABLES
Table 1: Data Sources ...... 7 Table 2: Historic water levels obtained from community consultation ...... 9 Table 3: Nearby daily rainfall sations ...... 12 Table 4: Nearby pluviometer gauges ...... 12 Table 5: Daily rainfall measurements (mm) for past significant flooding events ...... 13 Table 6: Daily rainfall measurements (mm) for past significant flooding events ...... 13 Table 7: Peak Burst Intensities of Significant Rainfall Events (mm/h) ...... 14 Table 8: Approximate AEP of Pluviometer Storm Bursts ...... 14 Table 9: ARR2016 IFD data ...... 15 Table 10: Burst Loading distribution for the East Coast South region ...... 17 Table 11: Number of gauges and events within the temporal pattern region ...... 17 Table 12: Capacity ratings from Bunnerong to Botany Bay (SWC 11) Capacity Assessment .... 19 Table 13: Impervious Percentage per Land-use ...... 23 Table 14: Adopted rainfall loss parameters ...... 24 Table 15: HQ Boundary Locations and Values Adopted ...... 26 Table 16: Assumed Lurline and Botany Bay Tailwater Levels ...... 26 Table 17: Assumed Astrolabe Park Tailwater Levels...... 26 Table 18: Manning’s ‘n’ roughness values adopted in TUFLOW ...... 27 Table 19: Most Likely Blockage Levels – BDES (Table 6 in Reference 14) ...... 33 Table 20: Blockage Factors for Sag and On Grade Pits ...... 33 Table 21: Data Available for Calibration Storm Events ...... 35 Table 22: Estimated Flood Depths – Historical Events ...... 36 Table 23: Historic Flood Observations – Unknown Events ...... 36 Table 24: Comparison of Modelled and Observed Peak Flood Depths – December 2015 ...... 38 Table 25: Comparison of Modelled and Observed Peak Flood Depths – March 2014 ...... 40 Table 26: Validation to Observed Flood Behaviour ...... 41 Table 27: Design event critical duration ...... 44 Table 28: Temporal Pattern Selected ...... 44 Table 29: Peak Flood Levels (m AHD) and Depths (m) at Key Locations ...... 45 Table 30: Peak Flows (m3/s) at Key Locations ...... 47 Table 31: Weightings for Assessment of True Hazard ...... 55 Table 32: Overview of Sensitivity Analyses ...... 57 Table 33: 24hr Duration and Climate Change comparison ...... 58 Table 34: Results of Roughness and Infiltration Sensitivity Analysis – 1% AEP Levels (m) ...... 60 Table 35: Bunnerong Open Channel Blockage Sensitivity Analysis – 1% AEP Depths (m) ...... 62 Table 36: Results of Pit Inlet Blockage Sensitivity Analysis – 1% AEP Depths (m) ...... 64 Table 37: Results of Climate Change Analysis – 1% AEP Depths (m) ...... 66 Table 38: Ground truthing classifications for flood control lot identifications process ...... 79
LIST OF FIGURES
Figure 1: Catchments and Previous Studies Figure 2: Birds Gully and Bunnerong Road Study Area Figure 3: Places of Interest Figure 4: Community Consultation – Flooding Hotspots Figure 5: Community Consultation Responses Figure 6: LiDAR data – Digital Elevation Model Figure 7: Rainfall Gauges Figure 8: Burst Intensities and Frequencies April 1998 Flood Event Figure 9: Burst Intensities and Frequencies January 1999 Flood Event Figure 10: Burst Intensities and Frequencies March 2014 Flood Event Figure 11: Burst Intensities and Frequencies December 2015 Flood Event Figure 12: Trunk Drain Assets Figure 13: Hydrological Model Subcatchments Figure 14: Hydraulic Model Boundary and Drainage Figure 15: Hydraulic Model Manning’s ‘n’ Figure 16: Hydraulic Model Null Buildings and Gutters Figure 17: Hydraulic Model Drainage Data Figure 18: Historical Events Rainfall Hyetographs Figure 19: December 2015 Rainfal Isoheyts Figure 20: March 2014 Rainfal Isoheyts Figure 21: Flood Level Report Locations Figure 22: Peak Flow Report Locations Figure 23: Pipe Capacity Assessment
Appendix B Figure B1: Locations of Observed Flood Depths and Observed Flood Behaviour Figure B2: December 2015 Event – Randwick Pluviometer 566099 Figure B3: December 2015 Event – Randwick Pluviometer 566099 – Photos BG016 Figure B4: December 2015 Event – Randwick Pluviometer 566099 – Photos BG031 Figure B5: December 2015 Event – Randwick Pluviometer 566099 – Photos BG139 Figure B6: December 2015 Event – Randwick Pluviometer 566099 – Photos BG010 Figure B7: December 2015 Event – Randwick Pluviometer 566099 Observed Flood Behaviour Figure B8: December 2015 Event – Randwick Pluviometer 566099 Observed Flood Behaviour Figure B9: December 2015 Event – Randwick Pluviometer 566099 Observed Flood Behaviour Figure B10: December 2015 Event – Randwick Pluviometer 566099 Observed Flood Behaviour Figure B11: March 2014 Event – Malabar Pluviometer 566088 Figure B12: March 2014 Event – Malabar Pluviometer 566088 – Photos BG065 Figure B13: March 2014 Event – Malabar Pluviometer 566088 Observed Flood Behaviour Figure B14: March 2014 Event – Malabar Pluviometer 566088 Observed Flood Behaviour Figure B15: March 2014 Event – Malabar Pluviometer 566088 Observed Flood Behaviour Figure B16: March 2014 Event – Malabar Pluviometer 566088 Observed Flood Behaviour
Appendix C Figure C1: Peak Flood Levels and Depths – 1 EY Figure C2: Peak Flood Levels and Depths – 0.5 EY Figure C3: Peak Flood Levels and Depths – 20% AEP Figure C4: Peak Flood Levels and Depths – 10% AEP Figure C5: Peak Flood Levels and Depths – 5% AEP Figure C6: Peak Flood Levels and Depths – 2% AEP Figure C7: Peak Flood Levels and Depths – 1% AEP Figure C8: Peak Flood Levels and Depths – 0.5% AEP Figure C9: Peak Flood Levels and Depths – PMF Figure C10: Peak Flood Velocities – 1 EY Figure C11: Peak Flood Velocities – 0.5 EY Figure C12: Peak Flood Velocities – 20% AEP Figure C13: Peak Flood Velocities – 10% AEP Figure C14: Peak Flood Velocities – 5% AEP Figure C15: Peak Flood Velocities – 2% AEP Figure C16: Peak Flood Velocities – 1% AEP Figure C17: Peak Flood Velocities – 0.5% AEP Figure C18: Peak Flood Velocities – PMF Figure C19: Provisional Hydraulic Hazard – 20% AEP Figure C20: Provisional Hydraulic Hazard – 5% AEP Figure C21: Provisional Hydraulic Hazard – 1% AEP Figure C22: Provisional Hydraulic Hazard – PMF Figure C23: Hydraulic Categorisation – 1% AEP / PMF Figure C24: HotSpot Overview Figure C25: Harold Street and Perry Street Low Point - Matraville Figure C26: Paton Street and Bryd Street Low Point - Kingsford Figure C27: Holmes Street and Benevue Street Low Point - Maroubra Figure C28: Garden Street Low Point - Maroubra Figure C29: Glanfield Street Low Point – Glanfield Street Figure C30: Jersey Road Low Point - Matraville Figure C31: Flack Avenue Low Point - Matraville Figure C32: Denison Street and Nilson Avenue Low Point - Hillsdale Figure C33: Boonah Avenue Low Point - Eastgardens Figure C34: Parer Street Low Point - Maroubra Figure C35: Glanfield Street And Maroubra Road Low Point - Maroubra Figure C36: Edward Circuit Low Point - Pagewood Figure C37: Bunnerong Road Low Point - Hillsdale Figure C38: Irvine Street Low Point - Kingsford Figure C39: Hinck Street Low Point - Kingsford Figure C40: Harbourne Road Low Point – Kingsford Figure C41: Jacques Street / Hayward Street Low Point - Kingsford
Appendix D Figure D1: SES Emergency Response Classification Figure D2: SES Flood Level Hydrograph Location Figure D3: SES Flood Level Hydrograph – Perry Street - Matraville Figure D4: SES Flood Level Hydrograph – Beauchamp Road - Matraville Figure D5: SES Flood Level Hydrograph – Bunnerong Road - Dacyville Figure D6: SES Flood Level Hydrograph – Anzac parade - Kingsford Figure D7: SES Flood Level Hydrograph – Avoca Street - Maroubra Figure D8: SES Flood Level Hydrograph – Anzac Parade - Maroubra Figure D9: SES Flood Level Hydrograph – Bunnerong Road - Eastgardens Figure D10: SES Flood Level Hydrograph – Maroubra Road - Maroubra Figure D11: SES Flood Level Hydrograph – Cook Avenue - Dacyville Figure D12: SES Flood Level Hydrograph – Botany Road - Matraville Figure D13: SES Flood Level Hydrograph – Rainbow Street - Randwick Figure D14: SES Flood Level Hydrograph – Wentworth Avenue - Banksmeadow
LIST OF ACRONYMS
AEP Annual Exceedance Probability ARI Average Recurrence Interval ALS Airborne Laser Scanning ARR Australian Rainfall and Runoff BOM Bureau of Meteorology DECC Department of Environment and Climate Change (now OEH) DNR Department of Natural Resources (now OEH) DRM Direct Rainfall Method DTM Digital Terrain Model GIS Geographic Information System GPS Global Positioning System IFD Intensity, Frequency and Duration (Rainfall) mAHD meters above Australian Height Datum OEH Office of Environment and Heritage PMF Probable Maximum Flood SRMT Shuttle Radar Mission Topography TUFLOW one-dimensional (1D) and two-dimensional (2D) flood and tide simulation software (hydraulic model) WBNM Watershed Bounded Network Model (hydrologic model)
ADOPTED TERMINOLOGY
Australian Rainfall and Runoff (ARR, Ball et al, 2016) recommends terminology that is not misleading to the public and stakeholders. Therefore the use of terms such as “recurrence interval” and “return period” are no longer recommended as they imply that a given event magnitude is only exceeded at regular intervals such as every 100 years. However, rare events may occur in clusters. For example there are several instances of an event with a 1% chance of occurring within a short period, for example the 1949 and 1950 events at Kempsey. Historically the term Average Recurrence Interval (ARI) has been used.
ARR 2016 recommends the use of Annual Exceedance Probability (AEP). Annual Exceedance Probability (AEP) is the probability of an event being equalled or exceeded within a year. AEP may be expressed as either a percentage (%) or 1 in X. Floodplain management typically uses the percentage form of terminology. Therefore a 1% AEP event or 1 in 100 AEP has a 1% chance of being equalled or exceeded in any year.
ARI and AEP are often mistaken as being interchangeable for events equal to or more frequent than 10% AEP. The table below describes how they are subtly different.
For events more frequent than 50% AEP, expressing frequency in terms of Annual Exceedance Probability is not meaningful and misleading particularly in areas with strong seasonality. Statistically a 0.5 EY event is not the same as a 50% AEP event, and likewise an event with a 20% AEP is not the same as a 0.2 EY event. For example an event of 0.5 EY is an event which
would, on average, occur every two years. A 2 EY event is equivalent to a design event with a 6 month Average Recurrence Interval where there is no seasonality, or an event that is likely to occur twice in one year.
The Probable Maximum Flood is the largest flood that could possibly occur on a catchment. It is related to the Probable Maximum Precipitation (PMP). The PMP has an approximate probability. Due to the conservativeness applied to other factors influencing flooding a PMP does not translate to a PMF of the same AEP. Therefore an AEP is not assigned to the PMF>
This report has adopted the approach recommended by ARR and uses % AEP for all events rarer than the 50 % AEP and EY for all events more frequent than this.
Birds Gully & Bunnerong Road 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 Study • Evaluates management options for the floodplain in respect of both existing and proposed development. 3. Floodplain Risk Management Plan • Involves formal adoption by Council of a plan of management for the floodplain. 4. Implementation of the Plan • Construction of flood mitigation works to protect existing development, use of Local Environmental Plans to ensure new development is compatible with the flood hazard.
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EXECUTIVE SUMMARY
BACKGROUND The Birds Gully and Bunnerong Road catchment covers a total area of 9.9 km2 and is located in the eastern suburbs of Sydney, within Randwick City Council and City of Bayside Council local government areas (LGA). The study area encompasses the suburbs of Kingsford, South Coogee, Daceyville, Pagewood, Maroubra, Eastgardens, Hillsdale, Banksmeadow, Matraville and Port Botany. The study components are to: • collate available historical flood related data; • undertake a community consultation program; • prepare suitable models for use in a subsequent Floodplain Risk Management Study; • validate the models against historical events; • undertake design flood estimation utilising the ARR2016 techniques; • provide design flood levels, depths, velocities, flows and flood extents; • provide provisional hydraulic hazard and hydraulic categories mapping; • assess sensitivity to potential climate change effects; • Undertake “hotspot” analysis.
COMMUNITY CONSULTATION Approximately 8,798 questionnaires were distributed to residents within the catchment and 208 responses were received, 26 of which were completed using the online survey. This equates to a 2.36% return rate and therefore it should be recognised that the findings from this sample may not accurately represent the total population within the catchment. Of these responses, 44 have reported their property has previously been affected from flooding and of these 23 have experienced above floor flooding. The information above relates to the combined responses from both the Randwick and Bayside government areas, although relatively few responses were received from Bayside.
MODELLING SUMMARY The study used hydrologic and hydraulic modelling techniques in order to define flood behaviour in the study area. The modelling programs used in the study are: • DRAINS Hydrologic model converts rainfall to runoff for input into the TUFLOW model. • TUFLOW 2D Hydraulic model was established to analyse the flooding behaviour.
MODEL VALIDATION In order to provide robust design flood data the models should be calibrated to historical flood data but typically in an urban catchment there is insufficient high quality data available. The March 2014 and December 2015 events were chosen for model validation but the process was limited by the quality and quantity of the available rainfall and flood data.
DESIGN FLOOD MODELLING The ARR 2016 methodology was adopted for design flood estimation which utilises an ensemble of 10 temporal patterns that are applicable across four AEP ranges for durations ranging from 15 mins to 7 days within each region.
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The technique for the critical duration analysis of the temporal pattern ensembles is outlined in Section 7. It was determined that the upper reaches of the catchments where overland flow was the dominant flood mechanism had a shorter critical duration and the downstream region of the catchment where mainstream flooding was the dominant flood mechanism had a longer critical duration. For each AEP, design flood behaviour was based on a shorter duration event of either 30 minutes or 60 minutes, and a longer duration event of either 90 minutes, 180 minutes, or 120 minutes.
The study results have been provided to RCC and BCC in digital format and mapped in Appendix C:
The results from this study are presented as: • Peak flood levels and depths in Figure C1 to Figure C9 • Peak flood velocities in Figure C10 to Figure C18 • Provisional hydraulic hazard in Figure C19 to Figure C22; and • Provisional hydraulic categorisation in Figure C23 to Figure C24
HOTSPOT ANALYSIS The following areas were identified for potential further investigation in and are displayed in Figure C25 to Figure C40. • Harold Street and Perry Street – Matraville • Paton Street and Byrd Street – Kingsford • Holmes Street and Benevue Street - Maroubra • Alma Road - Maroubra • Glanfield Street - Maroubra • Jersey Road - Matraville • Flack Avenue - Matraville • Denison Street and Nilson Avenue - Hillsdale • Boonah Avenue - Eastgardens • Parer Street - Maroubra • Glanfield Street And Maroubra Road - Maroubra • Edward Circuit - Pagewood • Bunnerong Road - Hillsdale • Irvine Street - Kingsford • Hincks Street - Kingsford • Harbourne Road - Kingsford
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1. INTRODUCTION
This flood study was prepared by WMAwater on behalf of the Randwick City Council and Bayside Council. The study was commissioned by the Randwick City Council and Bayside Council with funding from the NSW Office of Environment and Heritage (OEH) under the Floodplain Management Program. The main objective of the study is to define existing flood behaviour within the Birds Gully and Bunnerong Road catchment. The study examined past flood events that have occurred, in addition to undertaking a flood assessment for a range of design storms.
There have been a number of previous studies undertaken for Randwick City Council and Bayside Council adjacent to this study area. These studies and their locations are shown in Figure 1.
1.1. Study Area
The Birds Gully and Bunnerong Road catchment covers a total area of 9.9 km2 and is located in the eastern suburbs of Sydney, within Randwick City Council and City of Bayside Council local government areas (LGA). The study area is shown in Figure 2 and encompasses the suburbs of Kingsford, South Coogee, Daceyville, Pagewood, Maroubra, Eastgardens, Hillsdale, Banksmeadow, Matraville and Port Botany.
The catchment can be divided into two separate catchments; the Birds Gully catchment and the Bunnerong Road catchment. The Birds Gully catchment is 1.7 km2 and is located in the north western section of the catchment. The Bunnerong Road catchment comprises the remaining 8.2 km2, with the catchment boundaries shown in Figure 12. The majority of the watercourses within the catchment have been replaced with trunk drainage with the Birds Gully catchment draining to the Botany Bay wetlands at the Eastlakes Golf Course, and the Bunnerong Road catchment discharges to both Botany Bay and Lurline Bay.
The study area is highly urbanised and consists of a combination of residential, commercial and industrial properties. There are some areas of open spaces within the catchment, including recreational parks, sporting fields and the Randwick Environmental Park in the north- east section of the catchment. The Randwick Army Barracks are located in the upper parts of the Bunnerong Road catchment. The Birds Gully and Bunnerong Road catchment consists of steeper topography in the upstream sections of the catchment, ranging from up to 80 mAHD along the north-eastern boundary of the study area before becoming flatter, reaching close to 0 mAHD in the downstream areas nearing closer to Port Botany.
Recently, the study area has undergone significant development and urbanisation, which may impact the flood behaviour of the catchment. There is a significant history of flooding within the catchment, with both councils receiving frequent complaints of flooding.
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1.2. Objectives
The scope of this flood study is to develop a robust hydrologic and hydraulic modelling package with the capability to accurately simulate existing and historic flood behaviour. Given a history of flooding within the catchment, there is a strong need to define and map flood behaviour in the catchment in order to provide Council with the planning tools necessary to mitigate flood risk for current and future development. The information and results obtained from this study will provide the basis for the development of a subsequent Floodplain Risk Management Study and Plan (FRMS&P) which will explore flood modification works, various planning instruments and flood response measures.
Flood study elements undertaken as part of this study include: • Undertake a community consultation program, • Develop a hydrologic and hydraulic modelling package to appropriately represent the catchment and floodplain, • Validate the hydrologic and hydraulic models against historical events, • Determine the sensitivity of the model outcomes to modelling parameters and assumptions, • Define flood characteristics including flood extent, levels, depths, velocities and flows, • Determine floodplain planning categories including, hydraulic categories, hazard categories and the flood emergency response classification, • Define the capacity of the existing drainage network and determine potential upgrades, and • Undertake a climate change assessment including assessing the effects of an increase in different rainfall intensities.
A glossary of flood related terms is provided in Appendix A.
1.3. Description of the Catchment and Flood History Overview
Photo 1: Army truck in Garrett Street during the October 1959 Flood Event a. b.
Source: Randwick City Council Website, donated by Mrs Dorothy Stafford The Birds Gully and Bunnerong Road catchment has largely been developed, with the majority
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of the waterways within the catchment having been replaced with urban drainage networks such as concrete lined channels and pipes. The drainage network within the study area was primarily constructed in the 1960s, however some of the oldest assets date back to the 1860s. Council receives frequent complaints of flooding from events exceeding the capacity of the drainage network, and flooding due to overland flow. Historic newspaper articles, SES reports, the Bureau of Meteorology (BoM) and council websites indicate that there has been previous flooding in 1959, 1984, 1989, 1998, 1999, 2009, 2014 and 2015, with the October 1959 event possibly being the largest event on record (see Photo 1 and Photo 2).
Photo 2: Corner of Garrett Street and Storey Street during the October 1959 Flood Event
Source: Randwick City Council Website, donated by Mrs Dorothy Stafford
The trunk drainage networks within both the Birds Gully catchment and the Bunnerong Road catchment are primarily owned by Sydney Water and comprise of three main trunk drainage lines. The Birds Gully trunk drainage line drains the Birds Gully catchment and discharges to the Botany Bay wetlands at Eastlakes Golf Course in Daceyville. The Bunnerong to Tasman Sea Trunk Drainage line partially drains the northern Bunnerong catchment, which leaves the remaining section of the Bunnerong catchment draining via the Bunnerong to Botany Bay line (see Figure 12).
In addition to the catchment comprising a large proportion of residential properties, there are a number of notable institutional facilities, including educational and medical facilities. Figure 3 details the locations of these educational and medical facilities within the catchment. There are a number of major hospitals within the catchment that should be noted including: • Sydney Children’s Hospital, • Prince of Wales Hospital, • Royal Hospital for Women, • Prince of Wales Private Hospital,
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• Part of the university of New South Wales, and • Randwick Army Barracks.
1.4. Community Consultation
Community consultation is an important element of the floodplain risk management process and is important in the development of a flood study as it facilitates community engagement and acceptance of the overall project. A newsletter and questionnaire was prepared and distributed to the residents within the Birds Gully and Bunnerong Road catchments to assess the flood experiences of the community and gather additional data. In addition, an online version of the questionnaire was also made available. The newsletter and questionnaire is attached in Appendix E.
The newsletter described the purpose of the Flood Study and requested information residents may have of historical flooding in the catchment. 8798 questionnaires were distributed to residents within the catchment and 208 responses were received, 26 of which were completed using the online survey. This equates to a 2.36% return rate and therefore it should be recognised that the findings from this sample may not accurately represent the total population within the catchment. Of these responses, 44 have reported their property has previously been affected from flooding and of these 23 have experienced above floor flooding. Figure 4 and Figure 5 detail the location of all properties that have reported previous flooding and some statistics from the returned questionnaire. The information above relates to the combined responses from both the Randwick and Bayside government areas, although relatively few responses were received from Bayside.
The responses to the community questionnaire highlighted specific problems related to flooding that residents are particularly concerned about. These concerns include: • Inadequate drainage, including undersized pits and pipes and ineffective gutter systems, • Debris blocking drains and gutter systems, • Flooding due to overland flow, • Algae build up in water drains, • Standing water in trapped low points unable to drain and remaining for time periods of up to a week, • Flood damages to garages (in some locations properties are affected on roughly an annual basis); and, • Some residents have employed their own flood mitigation measures; including building drains on the side of their properties and flood barrier gates along the front of their property.
The community consultation responses were used to identify potential flooding “hotspot” locations (see Figure 4). In addition to responding to the questionnaire, some residents have provided photographs of past rainfall events, as shown below in Photo 3 to Photo 5.
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Photo 3: Flood photographs along Flack Street, Hillsdale
Photo 4: Flood photographs at the laneway connecting Apsley Avenue to Lancaster Crescent, Kingsford
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Photo 5: Flood photographs outside 141 Bunnerong Road, Kingsford
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2. AVAILABLE DATA
2.1. Overview
Data collection is the first stage in the floodplain risk management process and is essential to gain an understanding of the flooding characteristics within the catchment, including the nature, size and frequency of the problem. To determine an accurate understanding of the flooding problem within the catchment, it is preferable to have an extensive period of historical records including stream flow records and stream water level records. In some creek systems there are permanent water level gauges, maximum height records or stream flow gauges, which assist in the hydrologic and hydraulic model calibration and give insight into the size and frequency of the flooding problem. However, in urban catchments like Birds Gully and Bunnerong Road catchment, which are relatively small compared to major river or creek systems, there are generally no stream gauges or official historical records available.
2.2. Data Sources
The available data sets for this study are summarised in the following sections. Table 1 provides a summary of the type of data sources, the supplier, and its application in the study.
Table 1: Data Sources Type of Data Format Provided (Source) Application Ground Levels from ALS data DEM (LPI) Hydrologic and hydraulic models (2013) Pits, Pipes and Hydraulic GIS (RCC and BCC) Hydraulic model Structures Trunk Drainage and Hydraulic GIS and WAE plans (SWC) Hydrologic and hydraulic model Structures GIS Information (Cadastre) GIS (RCC and BCC) Hydraulic model ARR Design Rainfalls Tabulated (BoM) Hydrologic model Rainfall Gauge (Daily) Spreadsheet (BoM) Hydrologic model Spreadsheet (SWC) Pluviometer (Continuous) Hydrologic model Spreadsheet (BoM)
2.3. Topographic Data
The digital elevation model (DEM), which forms the basis of the two-dimensional hydraulic modelling for this study, was obtained from the Sydney North 1m dataset from the Department of Land and Property Information (LPI). The DEM was produced using Triangular Irregular Network (TIN) method to formulate a regular grid from the Airborne Laser Scanner (ALS). The Sydney North 1m DEM dataset was collected in June 2013. For areas of clear, hard ground is has an accuracy in the order of: • ± 0.8m in the horizontal direction (95% CI); and • ± 0.3m in the vertical direction (95% CI).
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The accuracy of ALS data can be influenced by the presence of open water or vegetation (tree or shrub canopy) at the time of survey, which means in some areas data is missing or the points are of lesser quality than the stated accuracy. Figure 6 shows the DEM for the Birds Gully and Bunnerong Road catchment.
2.4. Pit and Pipe Data
Randwick City Council and Bayside Council provided a GIS database of pit and pipe data. In both cases, the pit and pipe data was missing some relevant information, and not appropriate for direct input into the hydraulic model. An initial desktop review was undertaken of both pit and pipe datasets to determine sections of missing data and sections of inaccurate data. It was determined the missing data was not extensive enough to require a comprehensive detailed survey of the drainage pits and pipes within the catchment area. However, WMAwater undertook a site visit to verify pit and pipe locations and obtain a more accurate understanding of the drainage network within the catchment. The site visit also included the inspection of other hydraulic controls within the catchment, such as detention basins and their outlet embankments, swales, bridges and open channels.
SWC provided both GIS data and Work-As-Executed (WAE) survey plans. The GIS trunk drainage database included major pipes and other hydraulic controls including open channel drainage structures. Although the GIS database was not complete for direct input into the hydraulic model, the missing data was verified by referring back to the WAE survey plans. The data from Sydney Water is relatively high quality and gives a high level of confidence about the geometry and levels of the trunk drainage systems through the catchment.
The GIS dataset provided from RCC was partly incomplete, containing sections where invert and geometry details were missing and other sections where the details appeared to be inaccurate. It was generally possible to infer the pit and pipe invert and geometry details in sections where there was adequate data upstream and downstream for comparison. There is a reasonably high level of confidence in the stormwater drainage network data within the RCC area.
The BCC GIS dataset was mostly incomplete with small sections of the pipe network missing and incomplete geometry details. A combination of the SWC trunk drainage data and the site inspection was useful in estimating pit and pipe locations. Where geometry and invert details were missing this data was assumed based on the ground level and upstream and downstream pipe details. The data was also supplemented by survey collected by Council across Rowland Park and Prince Edward Circuit. The confidence in the stormwater drainage network data in the BCC area is therefore relatively low. However for larger design storm events such as the 1% AEP, where a relatively small proportion of runoff is conveyed by the pipe network, it is not anticipated to significantly affect the study outcomes.
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2.5. Historical Flood Level Data
Historical flood level data is important for hydrologic and hydraulic model calibration and validation. However, the Birds Gully and Bunnerong Road study area is lacking any stream flow or water level gauges. Therefore, an understanding of historical flooding within the study area must be sourced from a combination of previous flood assessment records, rainfall records, Council records and local knowledge of flooding obtained from the community consultation questionnaire. Table 2 details all estimated water levels and their corresponding locations that were obtained from the community consultation (see Figure B1 for locations).
Table 2: Historic water levels obtained from community consultation ID Location Suburb Date Report BG140 147 Bunnerong Road Kingsford 25-04-15 25cm at front step of property. BG009 1 Flack Avenue Hillsdale - 93cm at garages of Unit 6 and Unit 14. BG016 1 Flack Avenue Hillsdale 24-03-14 1m depth at garages. BG010 143 Bunnerong Road Kingsford - 6-8inches at brick fence. BG011 80 Perry Street Matraville - Up to 1m. Drains back up into the property. BG014 11 Snape Street Maroubra 20cm deep in the street in front of property. 1m deep water at stormwater drain in front of BG012 13 Snape Street Maroubra - property Hastings Avenue BG019 between Macquarie Chifley - 4 – 6 inches in street. Street and Hall Street Land adjacent 52 Eyre Up to 30cm deep at approximately 10m west BG025 Chifley - Street of border with 52 Eyre Street. Intersection of Haig Ave BG027 Daceyville - About 22cm within intersection and Gwea Ave 16-12-15 Up to 500mm deep in garage. BG031 21 Beulah Street Kingsford Midday 15-12- 500mm at rear land access. 15 40-50cm in basement and backyard of BG033 267 Botany Street Kingsford - property. 50 Irvine Street and 650mm above floor, 700mm in street and BG039 Kingsford - Marville Lane laneway. BG041 105 Rainbow Street Kingsford - 60 cm above drain 16-12-15, BG043 8 Snape Street Kingsford 1-2 inches in driveway and garages. 22-12-15 BG044 82 Sturt Street Kingsford 01-1999 55cm above floor. Corner of Avoca Street BG048 Maroubra 1 foot at footpath and Holmes Street 10:30am Up to air vents in 1st row of bricks above BG049 8 Benvenue Street Maroubra 29-01-99 ground. BG051 102 Gale Road Maroubra - Water up to step at the front door. BG052 107 Garden Street Maroubra 11-59 6 inches above floor. BG054 55 Hannan Street Maroubra Millimetres from flooding inside. BG055 8 Holden Street Maroubra 25/04/14 2 feet at garage. BG059 Corner of Royal St and Maroubra - 400-500mm above Royal Street.
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ID Location Suburb Date Report Maroubra Rd Walsh Avenue and Wild BG062 Maroubra - 5-30cm Street BG063 Australia Avenue Matraville - Several inches at the footpath. BG063 56 Australia Avenue Matraville - Ankle deep water in backyard. BG064 60 Australia Avenue Matraville 02-16 2 foot water in garage. BG065 20 Harold Street Matraville 400mm in garage. BG066 7 Harold Street Matraville 21-12-16 20 inches above floor level. 1998 18cm BG135 42-56 Harbourne Road Kingsford 1999 67-78cm (Flooding above floor level) 02-10 54-63cm 1 foot deep front yard entry from Botany BG136 307 Botany Street Kingsford Street. 22-04-15 and BG139 1-5 Apsley Avenue Kingsford Around 45cm from the footpath. 22-12-15 4:40pm 25-04- BG140 147 Bunnerong Road Kingsford 25cm at front step of property. 15 Corner of Marville and BG143 Kingsford - Half a car wheel. Irvine Street South side of Waratah BG144 Randwick - 2-3 inches. Avenue BG145 1 Apsley Avenue Kingsford - 20cm at front door. Laneway at 1 Apsley BG145 Kingsford - 33cm Avenue 505/438-448 Anzac BG148 Kingsford - 10cm Parade Corner of Walsh and BG149 Maroubra - 5-10cm Donovan Avenue Belongings in garage BG174 - 50cm and garden damaged
2.6. Historical Rainfall Data
2.6.1. Overview
Rainfall data is recorded either daily (24-hour rainfall totals to 9:00 am) or continuously (pluviometers measuring rainfall in small increments – less than 1 mm). Daily rainfall data has been recorded for over 100 years at many locations within the Sydney basin. However, pluviometers have only been installed for widespread use since the 1970s.
Care must be taken when interpreting historical rainfall measurements. Rainfall records may not provide an accurate representation of past flooding 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: • 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
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and vandalism. In particular, many gauges fail during periods of heavy rainfall and records of very intense events are often lost or misrepresented. • Daily read information is usually obtained at 9:00 am in the morning. Thus if a single storm is experienced both before and after 9:00 am, then the rainfall is “split” between two days of record and a large single day total cannot be identified. • In the past, rainfall over weekends was often erroneously accumulated and recorded as a combined Monday 9:00 am reading. • The duration of intense rainfall required to produce overland flooding in the study area is typically less than 4 hours (though this rainfall may be contained within a longer period of rainfall). This is termed the “critical storm duration”. For the study area a short intense period of rainfall can produce more severe flooding than sustained rainfall with a higher total depth. If the rain occurs quickly (e.g. a thunder storm), the daily rainfall total may not necessarily reflect the severity of the storm and the subsequent flooding. Alternatively, the rainfall may be relatively consistent throughout the day, producing a large total but only minor flooding. • Rainfall records can frequently have “gaps” ranging from a few days to several weeks or even years. • Pluviometer (continuous) records provide a much greater insight into the intensity (depth vs. time) of rainfall events. This data has much fewer limitations than daily read data, but there are far fewer pluviometers available in the vicinity of the catchment. • Pluviometers have moving parts and automated recording mechanisms, which can fail during intense storm events due to the extreme weather conditions.
Intense rainfall events which cause overland flooding in highly urbanised catchments are usually localised and as such are only accurately represented by a nearby gauge, preferably within the catchment. Gauges sited just a kilometre apart can show very different intensities and total rainfall depths.
2.6.2. Rainfall Stations
Table 3 and Table 4 present a summary of the official rainfall gauges operated by the BoM and pluviometer gauges operated by SWC and BoM located either within the catchment or nearby. The locations of these rainfall gauges are displayed on Figure 7 and show that no gauges are located within the catchment extent.
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Table 3: Nearby daily rainfall sations Distance from Station Operating Elevation Date Date Station Name catchment Number Authority (mAHD) Opened Closed centroid (km) 66073 Randwick Racecourse BoM 3.48 25 1937 Open 66052 Randwick (Randwick St) BoM 3.5 74 1917 Open 66160 Centennial Park BoM 4.88 38 1990 Open 66051 Little Bay (The Coast Golf Club) BoM 4.95 22 1925 Open 66037 Sydney Airport AMO BoM 5.97 6 1929 Open 66098 Rose Bay (Royal Sydney Golf Club) BoM 7.05 8 1928 Open 66006 Sydney Botanic Gardens BoM 8.45 15 1885 Open 66209 Dover Heights (Portland St) BoM 8.53 70.5 2007 Open 66036 Marrickville Golf Club BoM 9.26 6 1904 Open 66062 Sydney (Observatory Hill) BoM 9.28 39 1858 Open 66000 Ashfield Bowling Club BoM 11.28 25 1894 Open 66058 Sans Souci (Public School) BoM 11.66 9 1899 Open 66184 Mosmon Council BoM 11.8 85 1984 Open 66194 Canterbury Racecourse AWS BoM 12.04 3 1995 Open
Table 4: Nearby pluviometer gauges Distance from Station Operating Date Date Station Name catchment Number Authority Opened Closed centroid (km) 566028 Eastlakes SW Depot SWC 2.22 1973 Open 566088 Malabar STP SWC 2.61 1990 Open 566099 Randwick Racecourse SWC 3.22 1991 Open 066037 Sydney Airport Amo BoM 5.91 1998 Open 566032 Paddington (composite site) SWC 5.96 1979 Open 566110 Erskineville Bowling Club SWC 6.19 1993 Open 566091 Kyeemagh Bowling Club SWC 7.09 1991 Open 566026 Marrickville Bowling Club SWC 7.52 1979 Open 066062 Sydney (Observatory Hill) BoM 9.22 1998 Open 566065 Lilyfield Bowling Club SWC 9.95 1989 Open 566112 Ashfield (Ashfield Park Bowling Club) SWC 11.24 1993 Open 566113 Canterbury Racecourse SWC 11.63 1993 Open 566062 Bexley Bowling Club SWC 11.73 1987 Open 566066 Five Dock SPS065 SWC 12.8 1989 Open 566020 Enfield (Composite Site) SWC 14.45 1983 Open 566047 Mortdale Bowling Club SWC 15.21 1977 Open 566064 Concord Greenlees BC (formerly Wests Rugby Club) SWC 15.23 1988 Open 566078 South Cronulla BC (formerly South Cronulla PS) SWC 16.5 1990 Open 566022 Homebush SPS041 (formerly Homebush BC) SWC 17.09 1969 Open 566036 Potts Hill Reservoir SWC 19.33 1981 Open 566031 Revesby Bowling Club (formerly Padstow) SWC 20.13 2005 Open
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2.6.3. Analysis of Daily Read Rainfall Data
An analysis of daily read data was undertaken to place past storm events in some context. The daily read rainfall data was analysed for past flooding events that had either been reported by the BoM, City Council websites, the SES or online news sources. The daily rainfall stations included in this analysis were chosen by proximity to the catchment centroid (see Figure 7 for locations). Typically, during major storm events, it is common for daily read gauges to remain unread for several days and the resulting record being an accumulated total over several preceding days.
Table 5 and Table 6 provide rainfall measurements for some past flooding events. From this data, it can be seen that the October 1959 event was by far the largest rainfall event recorded within the catchment. The April 1998 and January 1999 storm events were also significant rainfall events but of much lesser total rainfall in a single day. However, as described in Section 2.6.1 daily read rainfall data does not provide a clear indication of an events severity or rainfall intensity.
Table 5: Daily rainfall measurements (mm) for past significant flooding events
October 1959 April 1989 April 1998 Event Station Name 29th 30th 31st 1st 2nd 3rd 10th 11th 12th 13th Randwick Racecourse 11.9 266.7 14 57.5 96.02* 118 105 0 0 Randwick (Randwick St) 265.4 29.2 4.2 47 52.2 38 70.8 87.8 0 0 Centennial Park - 0 47.4 54 29.4 109.4 68 0 0 Little Bay - 12.8 92.03* 30.4 52.03 (The Coast Golf Club) Sydney Airport AMO 8.1 112.3 11 42.2 42.2 35 75.2 70.6 0 0 * Rainfall was measured over a time period greater than 24 hours
Table 6: Daily rainfall measurements (mm) for past significant flooding events December January 1999 May 2009 March 2014 Station Name 2015 22nd 23rd 24th 25th 1st 2nd 3rd 25th 26th 27th 28th 17th Randwick 57 34 0 69 85.03* 25.4 0.8 26 8.4 38.4 Racecourse Randwick 54.6 34 73.8 0 11 0 76.6 27.2 0.6 29 7.8 58 (Randwick St) Centennial Park 113* 0 10 0 67 36.2 3 29.6 5.5 - Little Bay (The 34.2 167.43* 29 0 50 - - Coast Golf Club) Sydney Airport 84.6 32.8 61.6 1.4 43.4 4 0 40 0.2 22.8 6.4 5.6 AMO * Rainfall was measured over a time period greater than 24 hours
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2.6.4. Pluviometer Rainfall Data
Continuous pluviometer records provide a more detailed description of temporal variations in rainfall. While the October 1959 event has been noted as the worst flooding event in recorded history within the catchment, pluviometer data was not available for this event. The Eastlakes SW Depot and Malabar STP gauges were analysed to determine the peak burst intensities for the historical flooding events and are shown in Table 7 and Table 8. These stations were chosen based on their proximity to the catchment centroid (see Figure 7 for locations).
Table 7: Peak Burst Intensities of Significant Rainfall Events (mm/h) Eastlakes SW Depot Malabar STP Rainfall Event 30 min 1 hour 2 hour 30 min 1 hour 2 hour April 1998 46 39 28.75 27 17.5 14 Jan 1999 - 80 71 45.75 March 2014 89 59 30 107 58 29.75 Dec 2015 63 35 17.75 106 64 32.25
Table 8: Approximate AEP of Pluviometer Storm Bursts Eastlakes SW Depot Malabar STP Rainfall Event 30 min 1 hour 2 hour 30 min 1 hour 2 hour April 1998 > 1EY 20% AEP 20% AEP > 1EY > 1EY > 1EY Jan 1999 - 10% AEP 2% AEP 2% AEP March 2014 5% AEP 5% AEP 20% AEP 2% AEP 5% AEP 20% AEP Dec 2015 20% AEP 50% AEP > 1EY 2% AEP 5% AEP 20% AEP “> 1EY” indicates the intensity occurs roughly once a year or more frequently than this (i.e. not particularly intense)
Rainfall intensities at the gauges were assessed for the 30 minute, 1 hour and 2 hour storm burst durations and compared to intensities from the updated 2016 IFD. These durations were selected for analysis based upon experience that these types of storm durations would be critical (i.e. produce the highest flood levels) for the size of the Birds Gully and Bunnerong Road catchment. It can be seen that the March 2014 and January 1999 event produced more widespread high intensities for three storm bursts at the two gauges. A comparison of significant rainfall events and the design rainfall intensities from AR&R 2016 IFDs are shown in Figure 8 to Figure 11.
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2.6.5. Design Rainfall Data
The BoM recently released new design rainfalls in 2016 (Reference 1) to be used in conjunction with the updated Australian Rainfall and Runoff (ARR) (Reference 2). These new design rainfalls are based on larger datasets, produce more accurate estimates and provide better estimates of the 2% and 1% AEP events. The rainfall intensities presented in Table 9 were extracted from BoM for the Birds Gully and Bunnerong Road catchment.
Table 9: ARR2016 IFD data Design Rainfall (mm) From Bureau of Meteorology Duration EY Annual Exceedance Probability (AEP)
1 EY 50% 20% 10% 5% 2% 1%
5 min 8.5 9.4 12.4 14.4 16.4 19 21
10 min 13.3 14.8 19.5 22.7 25.8 29.8 32.9
15 min 16.5 18.4 24.3 28.3 32.2 37.2 41
30 min 22.7 25.3 33.3 38.8 44.1 51 56.3
1 hour 29.7 33.1 43.5 50.7 57.7 67.1 74.5
2 hour 38.2 42.5 56.1 65.6 75.1 88 98.2
3 hour 44.3 49.3 65.5 76.8 88.3 104 116.5
6 hour 57.6 64.4 86.7 102.6 119 141.4 159.2
12 hour 75.6 85.1 116.6 139.5 163.1 195.2 220.8
24 hour 98.6 112 156.2 188.3 221.4 266.1 301.3
48 hour 125.2 143.2 202 244 286.8 344.3 389
72 hour 141 161.7 228.5 275.4 322.5 385.6 434.1
96 hour 152 174.4 245.8 295.3 344.5 409.9 459.6
The guidelines to using the 2016 IFDs (Reference 2) recommend that the 2016 IFD data should not be used with the probabilistic rational method, any other regional flood techniques based on the 1987 IFDs and should not be used in conjunction with the 1987 temporal patters.
2.7. Temporal Patterns
Temporal patterns are a hydrologic tool that describe how rainfall falls over time and are often used in hydrograph estimation. Previously, a single burst temporal pattern has been adopted for each rainfall event duration. However ARR2016 (Reference 2) discusses the potential inaccuracies with adopting a single temporal pattern, and recommends an approach where an ensemble of different temporal patterns are investigated.
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2.7.1. ARR1987 Temporal Patterns
The 1987 temporal patterns can be obtained from ARR87 (Reference 3) and were developed using the Average Variability Method (AVM). The 1987 method divides Australia into 8 zones and provides two temporal patterns for 20 storm durations for ARI ≤ 30 years and ARI > 30years.
The AVM provides a pattern that describes the rainfall pattern of the most intense burst within a storm event and should not be considered representative of a typical rainfall pattern. A limitation with the AVM, as discussed in ARR2016 (Reference 2), is that it assumes that the variability of the pattern is of less importance than the central tendency, that is the central value of the probability distribution of rainfall volume. In reality, the runoff response can be very catchment-specific and therefore it is recognised that a representative pattern will not necessarily produce the median response from an ensemble of patterns. In addition to these concerns, it is not recommended using design rainfall bursts on catchments with significant natural or man-made storages. The 1987 temporal patterns should only be used in conjunction with the 1987 IFD tables.
2.7.2. ARR2016 Temporal Patterns
Temporal patterns for this study were obtained from ARR2016 (Reference 2). The revised 2016 temporal patterns attempt to address the key concerns practitioners found with the 1987 temporal patterns. It is widely accepted that there are a wide variety of temporal patterns possible for rainfall events of similar magnitude. This variation in temporal pattern can result in significant effects on the estimated peak flow. As such, the revised temporal patterns have adopted a different method to the 1987 AVM and provide an ensemble of design rainfall events. Given the rainfall-runoff response can be quite catchment specific, using an ensemble of temporal patterns attempts to produce the median catchment response.
As hydrologic modelling has advanced, it is becoming increasingly important to use realistic temporal patterns. The 1987 temporal patterns only provided a pattern of the most intense burst within a storm, whereas the 2016 temporal patterns look at the entirety of the storm including pre-burst rainfall, the burst and post-burst rainfall. There can be significant variability in the burst loading distribution (i.e. depending on where 50% of the burst rainfall occurs an event can be defined as front, middle or back loaded). The 2016 method divides Australia into 12 temporal pattern regions, with the Birds Gully and Bunnerong Road catchment falling within the East Coast South region. Each region was analysed to determine the proportion of front, middle and back loaded events and was separated into events shorter and longer than 6 hours. Table 10 provides the burst loading distribution for the East Coast South region. Table 11 details the gauge and event information used to derive the temporal patterns for the East Coast South region.
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Table 10: Burst Loading distribution for the East Coast South region
Region Duration Front Loaded (%) Middle Loaded (%) Back Loaded (%)
≤ 6hr 26.5 57.1 16.4 East Coast South > 6hr 17.1 58.6 24.3
Table 11: Number of gauges and events within the temporal pattern region
Number of station Number of Average number of Region Number of gauges years events events per station
East Coast 331 8067 19856 2.46 South
An ensemble of 10 temporal patterns are applicable across four AEP ranges for durations ranging from 15 mins to 7 days within each region. The four AEP categories are as follows with a diagram of the temporal pattern ranges shown in Diagram 1: • Frequent - more frequent than 14.4% AEP, • Intermediate - between 3.2% AEP and 14.4% AEP, • Rare - rarer than 3.2% AEP, and • Very Rare – rarest 10 within the region.
Diagram 1: Temporal Pattern Ranges
The ARR 2016 Temporal Patterns were used in this study for design storm modelling. Details of the methodology used to derive the critical duration are discussed in greater detail in Section 7.2.
2.8. Previous Studies
A number of previous studies have been undertaken in the vicinity of the Birds Gully and Bunnerong Road study area. These studies are detailed in Figure 1 and listed below: • Daceyville Flood Study, 2011, • Centennial Park Flood Study, 2013, • Draft Mascot, Roseberry and Eastlakes Flood Study, 2014, • Botany Wetlands – Draft MRE Flood Study, 2014, • Draft Kensington – Centennial Park FRMS, 2014,
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• Coogee Bay FRMS&P, 2016, • Maroubra Bay FRMS&P, 2016, • Springvale and Floodvale Drain FRMS&P Current; and • Bay Street Catchment Flood Study, 2016.
Additionally, Sydney Water has previously undertaken three capacity assessments of the pipe networks within the Birds Gully & Bunnerong Road catchment area. These capacity assessments were produced using a similar methodology for all three. The networks that the assessment reports relate to are the Birds Gully SWC 10 and Banks to Cook Avenue SWC 12, Bunnerong to Tasman Sea SWC 11AMP and Bunnerong to Botany Bay (SWC 11) and can be seen in Figure 12.
2.8.1. SWC 10 & SWC 12 Capacity Assessment (Reference 4)
The 1997 Birds Gully SWC 10, Banks to Cook Ave SWC 12 Capacity Assessment was undertaken by Sydney Water to assess the performance of the pipe network and determine impacts of future development on their performance. The systems discharge to the eastern corner of Astrolabe Park and eventually to Botany Bay. The catchment areas for the Birds Gully SWC 10 and Banks to Cook Avenue SWC 12 are 2.01 km2 and 0.68 km2 respectively.
The purpose of the capacity assessment was to determine the Storm Event Capacity (SEC) of each pipe section. The SEC represents the intensity of rainfall the pipe network can withstand before there is flooding outside of the drainage path or overland flow.
Hydrologic and hydraulic modelling was done using a spreadsheet approach. Peak flow rates were estimated using the Rational Method in conjunction with the 1987 IFD tables as per the methodology detailed in ARR 1987. The hydraulic capacity of each pipe section was determined using the Manning Formula and the ARR 1987 method for composite roughness and compound sections. From this, the SEC was determined by finding the corresponding storm event that resulted in a peak flow equal to the hydraulic capacity.
The report indicates that the pipe system in this area has relatively low capacity with: • only a small component of the network with capacity for a 20% AEP rainfall event, • one-fifth of the system with capacity for a 50% AEP rainfall event, and • two-thirds of the network with capacity for a 1 EY rainfall event.
2.8.2. SWC 11AMP Capacity Assessment (Reference 5)
The 2002 Bunnerong to Tasman Sea SWC 11 Amp Capacity Assessment was prepared by Sydney Water to determine the capacity of this section of pipe network. The Bunnerong to Tasman Sea pipe network, shown in Figure 12, has a catchment area of 2.37 km2 and drains to Lurline Bay.
This capacity assessment follows a similar methodology to the Reference 4 study. The peak
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flow estimates were calculated using a combination of the 1987 IFD tables and the Rational Method and the hydraulic capacity of each pipe section was estimated from the Manning’s Formula.
Results from this capacity assessment show that the capacity of the pipe system ranges from 1 EY to 1% AEP, with the majority of pipe reaches rated with a 1 EY capacity.
2.8.3. SWC 11 Capacity Assessment (Reference 6)
Sydney Water completed the Bunnerong to Botany Bay (SWC 11) Capacity Assessment in 2002 and follows the same methodology adopted within the Reference 4 and Reference 5 capacity assessments. The Bunnerong to Botany Bay pipe network has a catchment area of 4.37 km2 and discharges to Botany Bay. Figure 12 shows the location of the Bunnerong to Botany Bay pipe system. Results show that the pipe system ratings range from 1 EY to 1% AEP. Table 12 details a summary of the findings from Reference 6.
Table 12: Capacity ratings from Bunnerong to Botany Bay (SWC 11) Capacity Assessment
Pipe Network Branch Capacity
Less than two-thirds 20% AEP or larger capacity Main Channel Approximately 40% with 10% AEP capacity
Maroubra Bay Road Branch 50% less than 20% AEP capacity Jersey Road Branch 50% less than 20% AEP capacity Robey Street Branch 100% less than 20% AEP capacity Fitzgerald Avenue Branch Approximately 20% AEP to 10% AEP capacity Holden Street Branch Less than 20% AEP capacity Taylor Street Branch Less than 50% AEP capacity North East Sub-branch Approximately 20% AEP capacity Snape Park Branch Generally less than 20% AEP capacity
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3. MODELLING METHODOLOGY OVERVIEW
The urbanised nature of the study area with its mix of pervious and impervious surfaces, and existing piped and overland flow drainage systems, creates a complex hydrologic and hydraulic flow regime. A diagrammatic representation of the Flood Study process to address the issues is shown in Diagram 2.
Diagram 2: Flood Study Process
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For this study, the estimation of flood behaviour in a catchment was undertaken as a two- stage process, consisting of: 1. hydrologic modelling to convert rainfall estimates to overland flow runoff; and 2. hydraulic modelling to estimate overland flow distributions, flood levels and velocities.
The broad approach adopted for this study was to use DRAINS, a widely utilised and well- regarded hydrologic model for urban catchments, to conceptually model the rainfall concentration phase (including runoff from roof drainage systems, gutters, etc.). Design rainfall depths and patterns specified in AR&R (Reference 2) were input into the model and the runoff hydrographs were then used in a hydraulic model to estimate flood depths, velocities and hazard in the study area. Hydraulic modelling will be carried out using TUFLOW on a fixed 2 m grid.
The sub-catchments in the hydrologic model were kept small (on average approximately 2.5 ha) such that the overland flow behaviour for the study area was generally defined by the hydraulic model. This approach allows the concentration phase of the runoff to be modelled in a conceptual manner, since the scale of these concentration processes is too small to be modelled adequately by the hydraulic model (which has a grid cell size of 2 m). The concentration phase refers to runoff from roof/gutter/downpipe systems, intra-lot drainage, and other small scale flow paths in the most upstream parts of the catchment. WMAwater have previously used this method for similar overland flow catchment flood studies, and verified its suitability through comparisons with other commonly used hydrologic approaches.
The DRAINS hydrologic model software (Reference 7) was used to create the flow boundary conditions for input into a 2D unsteady flow (estimates the full storm hydrograph rather than just the peak flow as occurs with a steady state hydraulic model) hydraulic model using the TUFLOW software (Reference 8).
There are no stream-flow records in the catchment, so the use of a flood frequency approach for the estimation of design floods or calibration of the hydrologic model (independently from the hydraulic model) was not possible.
3.1. Hydrologic Model
DRAINS (Reference 7) is a widely used hydrologic and hydraulic modelling package built for the purpose of designing and analysing urban catchments and urban stormwater networks. It is capable of describing the flow behaviour of a catchment and pipe system for real storm events, as well as statistically based design storms. DRAINS models the conversion of rainfall to runoff and offers the option of routing these runoff hydrographs through a network of pipes, channels and streams.
For this study, DRAINS was used solely for the hydrological model and the hydraulic component of the modelling package was not utilised. The ILSAX hydrological model was adopted, as it has seen wide usage and acceptance throughout Australia. ILSAX adopts the
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time-area calculations and Horton infiltration procedures to determine flow hydrographs. The hydrologic outputs for each sub-catchment were used as inputs into the hydraulic model.
3.2. Hydraulic Model
The hydrodynamic modelling package TUFLOW (Reference 8), was used to assess the flooding behaviour of the Birds Gully and Bunnerong Creek catchments. TUFLOW is a widely used and accepted modelling package within Australia and internationally and was developed by BMT WBM in conjunction with the University of Queensland. An advantage of TUFLOW is its capability of dynamically simulating complex overland flow regimes. TUFLOW is particularly applicable to the hydraulic analysis of flooding in urban areas, which are typically characterised by short duration events, a combination of supercritical and subcritical flow behaviour and interactions between overland flow and a sub-surface drainage network.
This hydraulic modelling package utilises a grid based solution of the two-dimensional depth averaged, momentum and continuity equations for free surface flows. In addition to modelling of two-dimensional overland flow, TUFLOW incorporates one-dimensional elements within the model, including 1D open channels and sub-surface one-dimensional elements, such as pit and pipe networks. This component of the modelling packaged solves the full one-dimensional free-surface St Venant flow equation. The 1D and 2D components of the model can be dynamically linked during the simulation.
3.3. Calibration to Historical Events
When available, historical flood data can be used to calibrate the models and increases confidence in the estimates. The calibration process involves modifying the initial model parameter values to produce modelled results that concur with observed data. If records are available from multiple storms, validation can be undertaken to ensure that the calibration model parameter values are acceptable in other storm events with no additional alteration of values. Recorded rainfall and stream-flow data are required for calibration of the hydrologic model, while historic records of flood levels, velocities and inundation extents can be used for the calibration of hydraulic model parameters. In the absence of such data, model validation using limited historical data is the only option and a detailed sensitivity analysis of the different model input parameters constitutes current best practice.
Recent historical storms of significance are the April 1998, January 1999, March 2014 and December 2015. Sub-hourly rainfall data is available for these events to be modelled. Validation of the modelling package in comparison to the reported flood levels and flood behaviour is outlined in Section 6.
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4. HYDROLOGIC MODEL SETUP
4.1. Sub-catchment Delineation
The total catchment represented by the DRAINS model is 9.9 km2. This area was represented by a total of 395 sub-catchments shown in Figure 13, giving an average sub-catchment size of approximately 2.5 ha (approximately the size of two football fields). This relatively small sub-catchment delineation ensures that where significant overland flow paths exist that these are accounted for and able to be appropriately incorporated into hydraulic routing in the TUFLOW model. The sub-catchment layout is shown in Figure 13.
4.2. Impervious Surface Area
Runoff from connected impervious surfaces such as roads, gutters, roofs or concrete surfaces occurs significantly faster than from vegetated surfaces. This results in a faster concentration of flow within the downstream area of the catchment, and increased peak flow in some situations. It is therefore necessary to estimate the proportion of the catchment area that is covered by such surfaces.
DRAINS categorises these surface areas as either: • paved areas (impervious areas directly connected to the drainage system); • supplementary areas (impervious areas not directly connected to the drainage system, instead connected to the drainage system via the pervious areas); and • grassed areas (pervious areas).
Within all sub-catchments, a uniform 5% was adopted as a supplementary area across the catchment. The remaining 95% was attributed to impervious (paved) and pervious surface areas, as estimated for each individual sub-catchment. The percentage of pervious surface was estimated by determining the proportion of the sub-catchment area covered by different land zoning classifications. The estimated impervious percentage of the chosen zoning classifications as summarised in Table 13. Sensitivity analysis was conducted on these assumptions.
Table 13: Impervious Percentage per Land-use
Land-use Impervious Percentage Urban Residential 70% Open Space 5% Roads 100% Industrial 95% Infrastructure 70% Barracks 30%
The proportion of each zone within a sub-catchment was determined based GIS zoning files
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provided by RCC and BC.
4.3. Sub-catchment Slope
The slope of each sub-catchment was determined using an automated algorithm based on the following characteristics of each area: • Minimum and maximum elevations based on LiDAR • The ratio of the catchment area to its perimeter, used to estimate an indicative length
The typical slopes used for each sub-catchment were in the range of 1% to 6%, with an average of 3.5%. The minimum sub-catchment slope was 0.3% and the maximum was 17%.
4.4. Losses
Methods for modelling the proportion of rainfall that is “lost” to infiltration are outlined in AR&R (Reference 2). The methods are of varying degrees of complexity, with the more complex options only suitable if sufficient data are available. The method most typically used for design flood estimation is to apply an initial and continuing loss to the rainfall. The initial loss represents the wetting of the catchment prior to runoff starting to occur and the continuing loss represents the ongoing infiltration of water into the saturated soils while rainfall continues.
Rainfall losses from a paved or impervious area are considered to consist of only an initial loss (an amount sufficient to wet the pavement and fill minor surface depressions). Losses from grassed areas are comprised of an initial loss and a continuing loss. The continuing loss is calculated from an infiltration equation curve incorporated into the model and is based on the selected representative soil type and antecedent moisture condition.
The adopted loss parameters are summarised in Table 14. These are generally consistent with the parameters adopted flood studies in similar catchments within the Sydney metropolitan area.
Table 14: Adopted rainfall loss parameters
Rainfall Losses Paved Area Depression Storage (Initial Loss) 1.0 mm Grassed Area Depression Storage (Initial Loss) 5.0 mm SOIL TYPE 1
Slow infiltration rates (may have layers that impede downward movement of water). This parameter, in conjunction with the AMC, determines the continuing loss
ANTECEDENT MOISTURE CONDITONS (AMC) (mm) 3 Description Rather wet Total Rainfall in 5 Days Preceding the Storm 12.5 to 25 mm
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5. HYDRAULIC MODEL SETUP
5.1. Digital Elevation Model
A regularly spaced computational grid with a cell size 2 m by 2 m was utilised. This resolution was adopted as it was fine enough to accurately model roads and overland flow paths and did not result in excessive computational run-times. The model grid was established by sampling from a triangulation of filtered ground points from the 2011 LiDAR dataset.
The study area included in the 2D model encompassed an area of 9.9 km2 as shown in Figure 14.
5.2. Boundary Locations
5.2.1. Inflows
For local sub-catchments within the TUFLOW model domain, local runoff hydrographs were extracted from the DRAINS model (see Section 4). These were applied to the receiving area of the sub-catchments within the 2D domain of the hydraulic model. These inflow locations typically correspond with gutter lines and inlet pits on the roadway, or specific drainage reserves. These features have typically been constructed to receive intra-lot drainage and sheet runoff flows in upstream catchment areas.
Utilising this method, the DRAINS model is essentially used to approximate the concentration phase of runoff, used a lumped conceptual approach to model features such as roofs, gutters, downpipes, gardens and other features of intra-lot drainage that are too complex or small to be accurately modelled by the TUFLOW hydraulic model. It is assumed that intra-lot drainage is effectively conveyed to the receiving street gutter, pipe system or overland flow path.
5.2.2. Downstream Boundary
There are multiple downstream boundaries built into the model. The boundaries fall into two separate categories: • HQ Boundary – The outflow from this boundary is dependent on water level, using a rating curve in which the topographic gradient is assumed to equal the water level gradient (i.e. uniform flow); and • HT Boundary – The water level at the boundary set, and can be a static or varying water level over time.
The boundary locations are shown in Figure 14 and the HQ boundaries are identified below in Table 15 with their adopted values.
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Table 15: HQ Boundary Locations and Values Adopted Slope Adopted HQ Boundary (m/m) Belmore Road Randwick 0.01 Borrowdale Road Kingsford 0.04 Meares Avenue Randwick 0.06 Belmore Road Randwick 0.022 Anzac parade Maroubra 0.017 Bunnerong Road Matraville 0.005 Cornish Circuit Eastgardens 0.001 Heffron Road Eastgardens 0.008
These locations correspond to areas where cross-catchment flow occurs from the study area catchment into adjacent urban catchment areas.
HT Boundary • Botany Bay • Lurline Bay • Astrolabe Park (Botany Wetlands)
The design tailwater levels for Botany Bay and Lurline Bay area shown in Table 16 and the design tailwater levels for Astrolabe Park are shown in Table 17. The design tailwater levels for Botany Bay were adopted from Reference 23 and the design tailwater levels for Astrolabe Park were taken from Reference 24.
Table 16: Assumed Lurline and Botany Bay Tailwater Levels Design Event (AEP) Design Tidal Level Botany Bay 1 EY 1.2 50% AEP 1.2 20% AEP 1.2 10% AEP 1.2 5% AEP 1.4 2% AEP 1.42 1% AEP 1.43 PMF 1.45
Table 17: Assumed Astrolabe Park Tailwater Levels Design Event (AEP) Astrolabe North Astrolabe South 1 EY 16.84 14.95 50% AEP 16.84 14.95 20% AEP 16.98 15.11
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Design Event (AEP) Astrolabe North Astrolabe South 10% AEP 17.01 15.27 5% AEP 17.12 15.45 2% AEP 17.29 15.67 1% AEP 17.39 15.81 PMF 17.97 16.73
5.2.3. Roughness Co-efficient
The hydraulic efficiency of the flow paths within the TUFLOW model is represented in part by the hydraulic roughness or friction factor formulated as Manning’s “n” values. This factor describes the net influence of bed roughness and incorporates the effects of vegetation and other features which may affect the hydraulic performance of the particular flow path.
The Manning’s “n” values adopted for the study area, including flow paths (overland, pipe and in-channel), are shown in Table 18). These values have been adopted based on site inspection and past experience in similar floodplain environments. The values are consistent with typical values given in Chow, 1959 (Reference 9) and Henderson, 1966 (Reference 10). The spatial variation in Manning’s ‘n’ is shown in Figure 15
Table 18: Manning’s ‘n’ roughness values adopted in TUFLOW Surface Manning’s “n” Adopted Urban Residential 0.05 Open Space 0.03 Roads 0.02 Industrial 0.07 Infrastructure 0.06 Barracks 0.06 Concrete Channel 0.015
5.3. Continuous Infiltration Rate
The study area catchment is located over the Botany Aquifer, and is renowned for relatively fast infiltration of runoff into the ground. Reports of flooding often indicate that ponded floodwaters dissipate relatively quickly, even in the absence of pipe drainage in some areas. It was found during the model calibration process that unless infiltration losses were applied to areas of ponded water in the hydraulic model, the modelling significantly overestimated observed flood levels for a given rainfall, particularly in localised depression storage areas.
An infiltration rate of 118 mm/h from Reference 11 was utilised to represent the sandy soils, and for different land-use zones, the loss was adjusted based on the percentage of impervious surface assumed for each zone. This rate was chosen as part of the validation process and is discussed further Section 6.
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5.4. Hydraulic Structures
5.4.1. Buildings
Buildings and other significant features likely to act as flow obstructions were incorporated into the model network based on building footprints, defined using aerial photography. These types of features were modelled as impermeable obstructions to flow and are shown in Figure 16. Thus there is no assumed flood storage capacity within the buildings. Building delineation was based on aerial photographs, and validated for key overland flow areas by site inspection and use of Google “Streetview” photographs.
Buildings were “blocked out” from the 2D model grid, in line with research undertaken for the AR&R revision (Reference 2). The research project found that “Numerical model trials showed that on the basis of the available data sets, the best performing method when representing buildings in a numerical model was to either remove the computational points under the building footprint completely from the solution or to increase the elevation of the building footprint to be above the maximum expected flood height.” The project also found that “Analysis of flood volumes on the floodplain has shown that in a floodplain with flows passing through the floodplain, achieving peak levels due to peak flow rate rather than peak stored volume, the influence of the flow volume stored inside buildings is not significant to the presented flood levels in the prototype floodplain.”
5.4.2. Fencing and Obstructions
Smaller localised obstructions, such as fences, can be explicitly represented in TUFLOW in a number of ways including as an impermeable obstruction, a percentage blockage or as an energy loss. Often these obstructions are relatively transient, non-permanent structures, which do not require Council approval for modification. During site inspections for the study, WMAwater did not identify major fences or similar obstructions requiring specific modelling in TUFLOW. Instead, these obstructions were allowed for in a general sense by adopting a slightly increased Mannings “n” roughness value for residential and commercial land use areas, to represent the typical type of fencing used in such areas.
5.4.3. Sub-surface Drainage Network
The stormwater drainage network was modelled in TUFLOW as a 1D network dynamically linked to the 2D overland flow domain. This stormwater network includes conduits such as pipes and box culverts that were greater than or equal to 0.3 m in diameter or width, and stormwater pits, including inlet pits and junction manholes and is shown in Figure 17. The schematisation of the stormwater network was undertaken using the detail “pit and pipe” database supplied by RCC and BC as well as Works-as-Executed plans from SWC (Reference 4, 5 and 6) to validate the information where appropriate. This validation was particularly necessary for some of the larger trunk drainage pipes, which in many instances pass for long distances through private property, and where junction pits are no longer
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accessible due to development over time.
Details of the 1D solution scheme for the pit and pipe network are provided in the TUFLOW user manual (Reference 8). For modelling of inlet pits the “R” pit channel type was utilised, which requires a width and height dimension for the inlet in the vertical plane. The width dimension represents the effective length inlet exposed to the flow, and the vertical dimension reflects the depth of flow where the inlet becomes submerged, and the flow regime transitions from the weir equation to the orifice equation. For lintel inlets, the width was based on the length of the opening. For inlet grates, the width was based on the perimeter of the grate. For combined lintel and grate inlets, the inlet width was the combination of the lintel and grate edge lengths, minus the portion of the grate adjacent to the lintel (to avoid double counting). This method applies to both sag and on-grade pits. Figure 17 shows the location and dimensions of drainage lines within the study catchment that have been included in the TUFLOW model.
5.4.4. Open Channels, Bridges and Culverts
Detailed schematisation of key hydraulic structures was included in the hydraulic model. Major open channel’s culverts and bridges were generally modelled as 1D elements within the 2D domain, based on the scale of the structure and the key flow characteristics in comparison with the 2D cell size of 2 m. The decision on whether to model a structure in 1D or 2D was based primarily on the findings of Reference 12.
The modelling parameter values for the culverts and bridges were based on the geometrical properties of the structures, which were obtained from detailed SWC database and design plans, photographs taken during site inspections, and previous experience modelling similar structures.
The major hydraulic structures in the catchment are: • The Bunnerong to Tasman Sea Trunk Drainage (also called Lurline Bay Trunk Drain Diversion) • The Bunnerong to Botany Bay Trunk Drainage • The Birds Gully Trunk Drainage
The Bunnerong to Tasman Sea Trunk Drainage
The trunk drainage system consists of circular culverts upstream of Storey Street and a box culvert with dimension of 2.7 m x 2.4 m between Storey Street, Cooper Street and the ocean. The box culvert was modelled using the design cross section from Reference 5. The box culvert was modelled as an enclosed 1D open channel, since this gives a better solution for the super-critical flow regime caused by the steep gradient of the system.
The trunk drainage system includes a highly complex inflow structure at a location where the tunnel is located approximately 40 m below the ground surface. The structure includes an
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underground basin and weir to prevent the inflow disturbing flow in the culvert. This complex structure is not supported by the standard solution methods available in TUFLOW. Interpretation of the flow conditions was required to determine an appropriate method to schematise the structure in the model. It was determined that under the flood conditions being investigated, the key hydraulic control from this structure is the capacity of the incoming pipes, which is adequately resolved by the TUFLOW solution.
Bunnerong to Botany Bay Trunk Drainage
The system has been designed with circular culverts upstream of the major concrete channel between Paine Street and Port Botany. Sections of the channel have been enclosed by development since the channels were originally constructed in the 1970’s. The channel was modelled using the SWC cross section from plans provided in Reference 6. The enclosed sections of the channel were modelled as a 1D rectangular culvert.
Several bridges traverse the open channel. The major bridges are located at Perry Street, Donovan Avenue and Wild Street. These major bridges were modelled in 2D. The pedestrian bridge at Rhodes St reserve and all the bridges within the downstream industrial area were modelled in 1D.
The 1D bridges were modelled in two sections: • The section below the deck was modelled using the SWC cross-section. A loss versus depth relationship is applied, where as soon as the water reaches the bridge obvert, flow is affected by a reasonably high loss coefficient (K = 1.5). • The section above the deck is modelled as a weir with a level taken from the LiDAR survey with the deck depth have been assumed to be 0.3m.
The Birds Gully Trunk Drainage
The trunk drainage system has been modelled using rectangular and circular culverts using conventional methods. A CCTV survey was undertaken by BC to investigate the network below Rowland Park. The results of that survey have been applied to the model.
5.4.5. Road Kerbs and Gutters
LIDAR typically does not have sufficient resolution to adequately define the kerb/gutter system within roadways. The density of the aerial survey points is in the order of one per square metre, and the kerb/gutter feature is generally of a smaller scale than this, so the LIDAR does not pick up a continuous line of low points defining the drainage line along the edge of the kerb.
To deal with this issue, Reference 13 provides the following guidance:
Stamping a preferred flow path into a model grid/mesh (at the location of the physical kerb/gutter system) may produce more realistic model results, particularly
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with respect to smaller flood events that are of similar magnitude to the design capacity of the kerb and gutter. Stamping of the kerb/gutter alignment begins by digitising the kerb and gutter interval in a GIS environment. This interval is then used to select the model grid/mesh elements that it overlays in such a way that a connected flow path is selected (i.e. element linkage is orthogonal). These selected elements may then be lowered relative to the remaining grid/mesh.
The road gutter network plays a key role for overland flow in the Birds Gully and Bunnerong Road catchment. Preliminary modelling indicated that a significant portion of the catchment flows were within the roadways, which often traversed perpendicular to the land slope, and the flow depths were in the order of the depth of a typical kerb/gutter system (i.e. 0.1 m to 0.15 m), but using the raw LIDAR data resulted in multiple breakouts of flow over the kerb lines that did not appear to be realistic.
It was determined that in order to resolve these systems effectively, the gutters would be stamped into the mesh using the method described above, the locations of the gutters and are shown in Figure 16. The method used was to digitise breaklines along the gutter lines, and reduce the ground levels along those model cells by 0.15 m, creating a continuous flow path in the model.
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5.5. Blockage Assumptions
5.5.1. Background
In order to determine design flood behaviour the likelihood and consequences of blockage needs to be considered. Guidance on the application of blockage can be found in AR&R Revision Project 11: Blockage of Hydraulic Structures, 2014 (Reference 14).
Blockage of hydraulic structures can occur with the transportation of a number of materials by flood waters. This includes vegetation, garbage bins, building materials and cars, the latter of which has been seen in the June 2007 Newcastle and August 1998 Wollongong Floods (Photo 6 and Photo 7).
Photo 6: Cars in a culvert inlet – Newcastle Photo 7: Urban debris in Wollongong (Reference 14) (Reference 14)
The potential quantity and type of debris reaching a structure from a contributing source area depends on several factors. AR&R guidelines suggest adopted design blockage factors are based upon consideration of: • the availability of debris; • the ability for it to mobilise, and • the ability for it to be transported to the structure.
The availability of debris is dependent on factors such as the potential for soil erosion, local geology, the source area, the amount and type of vegetative cover, the degree of urbanisation, land clearing and preceding wind and rainfall. However, the type of materials that can be mobilised can vary greatly between catchments and individual flood events.
Observations of debris conveyed in streams strongly suggest a correlation between event magnitude and debris potential at a site. Rarer events produce deeper and faster floodwater able to transport large quantities and larger sizes of debris, smaller events may not be able to transport larger blockage material at all. Debris potential is adjusted as required for greater or lesser probabilities to establish the most likely and severe blockage levels for that event.
The likelihood of blockage at a particular structure depends on whether or not debris is able
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to bridge across the structure inlet or become trapped within the structure. Research into culvert blockage in Wollongong showed a correlation with blockage and opening width. The most likely blockage to occur at a structure is determined by considering the potential quantity and type of debris and the structure opening size as in Table 19.
Table 19: Most Likely Blockage Levels – BDES (Table 6 in Reference 14) At-Site Debris Potential Control Dimension High Medium Low
W < L10 100% 50% 25%
W ≥ L10 ≤ 3 x L10 20% 10% 0%
W > 3 x L10 10% 0% 0% Notes: W refers to the opening diameter / width L10 refers to the 10% percentile length of debris that could arrive at the site
A severe blockage level is proposed where the consequences are very high and Reference 14 suggests a severe blockage of twice the most likely blockage criteria. At structures where the consequence of blockage is very low, a 0% blockage is suggested.
5.5.2. Blockage for Calibration Events
There was no indication of blockage identified for historical floods. Therefore no blockage factors were applied for the calibration events.
5.5.3. Blockage for Design Events
For design flood modelling, a blockage factor of 10% was applied to bridges and culverts along the open channel reaches of the Bunnerong line. This value was selected based on consideration of the ARR guidance summarised above. Sensitivity analysis was undertaken for this blockage assumption.
Blockage factors for stormwater inlets were based on whether the pit had a sag and or on- grade inlet, as shown in Table 20.
Table 20: Blockage Factors for Sag and On Grade Pits Pit Type Blockage Adopted On Grade Pit 20% Sag Pit 50%
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6. MODEL VALIDATION
6.1. Overview
Prior to defining design flood behaviour it is important that the performance of the overall modelling system be substantiated. Calibration involves modifying the initial model parameter values to produce modelled results that concur with observed data. Validation is undertaken to ensure that the calibration model parameter values are acceptable in other storm events with no additional alteration of values. Ideally the modelling system should be calibrated and validated to multiple events, but this requires adequate historical flood observations and sufficient pluviometer rainfall data.
Typically in urban areas such information is lacking. Issues which may prevent a thorough calibration of hydrologic and hydraulic models are: • There is only a limited amount of historical flood information available for the study area. For example, in the Sydney metropolitan area there are only a few water level recorders in urban catchments, and none in this study area; and • Rainfall records for past floods are limited and there is a lack of temporal information describing historical rainfall patterns (pluviometers) within the catchment.
In the event that a calibration and validation of the models is not possible or limited in scope, it is best practice to undertake a validation of the models based on what data is available, along with a detailed sensitivity analysis. This was the approach adopted for this study.
6.2. Summary of Historical Event Rainfall Data
The choice of calibration or validation events for flood modelling depends on a combination of the severity of the flood event and the quality of the available data. As is the case with most urban studies there was limited quantitative data available either in the form of flood marks or surveyed flood levels for the study area. There was qualitative information provided by residents through the community consultation process with regard to their properties being flood affected, and whether they had been flooded in their yard, garage or above floor level. In some cases this was used to estimate a depth of flooding or an extent of the flow path.
The majority of available flood observations were from the December 2015 storm. December 2015 was a relatively recent event that was identified through the community consultation as having caused significant flooding problems in the study area. Additional storms from March 2014 and January 1999 were also modelled for validation purposes, as there were some anecdotal reports of flooding issues, and these were known to be relatively intense rainfall events over the catchment. However most residents could not recall which event specifically had caused prior flood issues.
Figure 18 shows rainfall hyetographs adopted for the three above mentioned historical calibration events. Rainfall isohyets for two of the historical events were produced by gridding
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the recorded daily gauges in and adjacent to the catchment. The results were as follows:
• Figure 19 – 17th December 2015. Displays a rainfall gradient from south-east to north- east grading from approximately 100 mm to 50 mm • Figure 20 – 25th March 2014. Displays a rainfall gradient from south to north grading from approximately 60 mm to 30 mm
Comparisons of the rainfall data for the historical / calibration events with design rainfall intensities from ARR 2016 (Reference 2) are shown in and summarised in Table 21.
Table 21: Data Available for Calibration Storm Events
Storm Approximate AEP of Rainfall Duration Pluviometer Stations Event recorded rainfall (mm) (minutes)
2% to 1% 98.5 45 Malabar STP (566088) January Erskineville Bowling Club 1999 20% to 10% 58.5 30 (566110) 1% 64 30 Malabar STP (566088) March Eastlakes SW Depot 2014 2% 62 30 (566028) 2% to 1% 104 30 Malabar STP (566088) December Randwick Racecourse 2015 10% 45.5 15 (566099)
The calibration and validation process was limited by incompleteness of the available rainfall data. The nearest pluviometers were outside the catchment, and it is likely they do not accurately reflect the actual rainfall that fell within the catchment during the historical events. In particular, the rainfalls at the Malabar STP site are likely to be more intense than those within the catchment, due to the proximity of the gauge to the coast. Given this level of uncertainty, and that results are typically more sensitive to the input rainfall than other model parameters, it was considered inappropriate to deviate significantly from typical modelling parameters used in similar urban catchments from the Sydney metropolitan area.
6.3. Recorded Flood Levels and Observed Behaviour
As part of the community consultation process data was received in regard to historical flooding in the catchment. This data ranged from residents qualitative descriptions of flood behaviour in and around their property to estimated flood depths for specific historical events. The locations for which data or observations were provided by the community are displayed on Figure B1.
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Table 22: Estimated Flood Depths – Historical Events ID Resident Description Depth (m) Year March 2014, BG016 Garages in block. 1 m 2015 0.4 m BG031 Rear Lane to Property 2015 approx. BG033 Basement and backyard only 0.4 – 0.5 m 2012 0.02 – 0.05 BG043 Driveway and garages 2015 m BG044 Backyard 0.55 m 2015 0.18 m 1998 0.67 – 0.78 1999 m 0.54 – 0.63 Feb 2010 BG037 Building 2 lifts, resident car damage m 0.11 m June 2010 0.18 m May 2011 0.37 m Dec 2015
Table 23: Historic Flood Observations – Unknown Events
ID Resident Description Depth (m) BG009 Flooding of the garages 0.93 m BG010 Flooding at rear granny flat 0.15 – 0.2 m BG011 Drains back up 1 m BG012 House front 1 m BG019 Shallow, Garages and driveway 0.1 – 0.15m BG039 Water into property and home 0.65 m BG040 Above kerb level - BG045 Cannot walk onto footpath property flooded high BG049 Up to air vents in 1st row of bricks above ground - BG051 Water came up to the step at the front door - BG052 - 0.12 m BG054 Millimetres inside the house BG063 None to house, some to contents of garage 0.1 – 0.15m BG064 Backyard, garage 0.6m BG065 Water runs down from street into backyard 0.4m House cracked inside specially above windows BG066 0.5m and doors BG136 Front yard entry 0.3 - 0.6m BG144 Water floods footpath on south side of road 0.02 – 0.05 m BG145 Water on floorboards in entry to the house 0.2 m BG148 Basement level 2 carpark 0.1 m BG150 Floods garage near the water drain 0.01m BG151 Driveway and garage 0.07 – 0.1m BG173 Driveway to back of the house 0.25m BG174 Belongings in garage and garden messed up 0.5m
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The data points that provide an estimated flood depth that correspond to a specific historical event are shown in Table 22. The data points that provide a description of flood behaviour and typical flow behaviour but don’t correspond to any specific historical flood depth are shown in Table 23. Example photographs of flood behaviour where flood depths have been recorded are shown in Photo 8.
Photo 8: Collection of sample model validation photographs
ID BG016 ID BG016
ID BG031 ID BG139
ID BG139 ID BG139
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6.4. Hydraulic Model Validation
Validation of the hydraulic model was undertaken using two techniques: • A comparison of the observed depths from the community consultation for the December 2015 event with the modelled depths for the same event. • A qualitative assessment of all the locations that had reported flooding was undertaken for the January 1999, March 2014 and December 2015 events to determine if the TUFLOW model could replicate this reported flood behaviour at these locations.
6.4.1. Validation to Observed Depths – December 2015
The December 2015 event was modelled using the temporal pattern from Randwick Racecourse pluviometer. Multiple sets of parameters were used to obtain the best fit to the recorded flood levels based on catchment topography and historical conditions. The observed depths as well as the differences in modelled depths for all the validation scenarios is shown in Table 24. The results for the Randwick pluviometer and the final parameters chosen are shown in Figure B2 to Figure B10.
Table 24: Comparison of Modelled and Observed Peak Flood Depths – December 2015
Modelled Depth minus Observed Depth (m) Using Using Randwick Using Using Randwick Using Racecourse Randwick Randwick Racecourse Randwick (566099) Racecourse Racecourse (566099) Racecourse pluviometer (566099) (566099) pluviometer (566099) pattern with pluviometer pluviometer pattern with pluviometer Dry initial pattern with pattern with attenuated pattern Condition Infiltration Final Malabar Observed and porous modelled parameters gauges ID Depth (m) soil BG016 0.20 0.46 0.37 0.39 0.43 0.21
BG031 0.40 0.09 0.01 0.03 0.03 -0.10
BG043 0.05 -0.05 -0.05 -0.05 -0.05 -0.05 BG037 0.37 0.18 0.13 0.09 0.09 -0.03 BG139 0.20 0.00 0.00 0.00 0.07 0.07
BG145 0.30 0.22 0.20 0.20 0.18 0.12 BG010 0.20 0.32 0.17 0.24 0.17 0.01 BG010 0.10 0.24 0.09 0.16 0.09 -0.01 Average Error 0.19 0.13 0.14 0.14 0.07
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Overall the model replicates the observed flooding behaviour quite well utilising the Randwick Racecourse pluviometer and the final calibration parameters. The average is 0.07 m which is a reasonable match with the data available.
A sensitivity to the models parameters shows: • The Malabar rainfall gauge recorded two peaks of same intensity during the 2015 events. The Randwick Racecourse gauge shows only the second rainfall peak. An attenuation of the Malabar first peak of rainfall depth gives a flood depth closer to the observed. The average error is 0.13 m by reducing the rainfall recorded at Malabar gauge. • The lack of rainfall recorded prior the 2015 event indicates a relatively dry antecedent condition may have occurred. A dryer antecedent moister condition combined with a more porous soil type in the hydrologic model gives a flood depth closer to observed. The average error is 0.14 m by modelling the drier antecedent condition. • The high infiltration rates of the Botany Aquifer sandy soils are also potentially influential on peak flood levels. The use of infiltration as discussed in Section 5.3 reduces the flood depth. The average error is 0.14 m by modelling infiltration. • The combination of all the above changes gives a flood depth close to the observed depth. The average error is 0.07 m. This scenario was adopted for the final calibration results.
Sources of uncertainty to be considered include: • The variation in rainfall depth that results from the two temporal patterns recorded at the Randwick and Malabar pluviometers suggests that the rainfall behaviour in the catchment was not uniform. Due to the pluviometers being located outside the catchment, the available rainfall data does not give an accurate record across the entire catchment. • The recorded flood depths are estimations by the residents, rather than accurately surveyed depths or levels at a specific location. This can result in an observation errors by the resident as well as errors when sampling the modelled depth grid at the wrong location. • The observed depth of inundation may not have been observed at the peak of the flood.
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6.4.2. Validation to Observed Depths – March 2014
The March 2014 event was modelled using the temporal patterns from Malabar and East Lakes pluviometer’s. The set of parameters use for the March 2014 event are the same than the final parameters of December 2015. It includes infiltration and dry antecedent condition. The Malabar pluviometer produced the best fit to observed flood depths with results shown in Figure B11 to Figure B16. The observed depths as well as the differences in modelled depths is shown in Table 25.
Table 25: Comparison of Modelled and Observed Peak Flood Depths – March 2014 Modelled Depth minus Observed Depth (m)
Using East Lakes SW Depot Using Malabar (566088) Observed (566028) pluviometer pattern pluviometer pattern with ID Depth (m) with final parameters final parameters BG016 0.20 0.34 0.11
BG033 0.40 0.23 -0.14
BG010 0.05 0.17 0.03 BG010 0.37 -0.18 -0.28 BG065 0.20 0.18 0.10
Average Error (m) 0.22 0.13
The model slightly overestimates the recorded depth at the observed locations, with a reasonable average error below 0.15 m using the Malabar pluviometer pattern. The same sources of uncertainty as for the 2015 storm also apply here.
6.4.3. Validation to Qualitative Flood Behaviour
While residents provided some descriptions of flooding that occurred in the late 1990s, there was generally little confidence regarding the year it occurred, or the exact depth. There were several residents who indicated that flooding above floor level occurred in this period, suggesting at least one storm caused relatively severe flooding.
A qualitative assessment was undertaken at each location that reported flood behaviour for the following the events listed below utilising both the Randwick Racecourse and the Malabar STP pluviometer data: • December 2015 • March 2014
The assessment analysed whether the model replicated the observed flood behaviour, flood inundation or produced a flood extent similar to the reported behaviour. The results are presented as a simple Yes or No and are displayed in Table 26. The flood depth grids for each event in conjunction with the observed flood behaviour locations are shown in Figure B2 to
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Figure B16. The modelling showed a reasonable match to reported behaviour across the majority of events and temporal pattern combinations that were considered.
Table 26: Validation to Observed Flood Behaviour Reproduce Reported Flood Behaviour ID Resident Description Approx. Depth (m) Dec-15 Mar-14 RRC M STP 566099 566088 BG011 Drains back up into our property 1 m Y Y BG012 2 motor of the front gate, flooded above motors 1 m Y N BG039 Water comes into property and garage and home 0.65 m Y Y the water just pours down to the flat point of the street BG040 - Y Y and up to the front doors of a few of our Cannot walk onto footpath. Totally flooded and comes BG045 high N N into our property at the front. BG049 Up to air vents in 1st row of bricks above ground - Y Y BG051 Water came up to the step at the front door - Y Y BG052 No comment 0.12 m Y N Most of the water comes from Gale Rd and the great BG054 0 Y Y volume that comes to the gully that’s in front The front yard at number 52 floods. The backyard at BG063 0.1 m to 0.15 m Y Y number 56 BG064 Plants and bags for garage 0.6 m Y Y BG066 Front and the sided of the house 0.5 m Y Y BG136 Front yard entry from Botany St, Kingsford 0.3 m - 0.6 m Y Y BG144 Water floods footpath on south side of road 0.02 m to 0.05 m Y Y BG148 Basement level 2 carpark 0.1 m N N BG150 Floods garage near the water drain 0.01 m N N BG151 Driveway and garage (sun-room) 0.07 to m 0.1 m N N The water gushes down 29's driveway and enters my BG173 0.25 m N N property via driveway BG174 Belongings in garage wet garden messed up 0.5 m Y Y BG016 2 motorbikes complete flooded 0.20 m Y Y Garage completely flooded with major damage to BG031 0.40 m Y Y property. BG033 Basement and backyard only 0.40 m Y Y BG043 flooded 0.05 m N N flooding from Rainbow to Paton St, then through our BG044 0.55 m Y N backyards BG037 Building 2 lifts, resident car damage 0.37 m Y Y BG139 in front of 3 Apsley Ave, Kingsford 0.20 m Y Y BG145 Water on floorboards in entry to the house. 0.30 m Y Y BG010 Flooding at rear granny flat 0.20 m Y Y BG010 Pic of front gate 0.10 m Y Y Water runs down from street into backyard and BG065 0.30 m Y Y backyard
There are some points where the model does not replicate the reported flood behaviour. These
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points are listed below with a brief explanation: • BG148 – This point is located on the model boundary and it is not justified to delineate the subcatchments small enough to allocated flows in this area. The issues appear to be related to intra-lot drainage rather than flooding. • BG173 – The reported issues here appear to relate to intra-lot drainage rather than catchment overland flow.
Note: For the calibration events the following details were not been included in the model: • Sydney Water works at Astrolabe Park • Stormwater upgrade Beauchamp Road • Development of Heffron Park
The above works were included as part of the design event modelling.
6.4.4. Validation Conclusions
The adopted modelling parameters utilised in the validation process produce a good match to the observed flood behaviour. Where observed flood depths were available the model typically matched those depths to within 0.2 m which is considered to be within a reasonable range when considering the reliability of the available data. For locations where the community reported flood behaviour the model has replicated that behaviour in the majority of cases.
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7. DESIGN EVENT MODELLING
7.1. Overview
Design flood levels in the catchment are a combination of flooding from rainfall over the local catchment (overland flooding), as well as elevated water level in open channels (mainstream flooding) and tail water levels in Botany Bay for the southern part of the catchment.
7.2. Critical Duration
To determine the critical storm duration for various parts of the catchment (i.e. produce the highest flood level), modelling of the 1% AEP, 5% AEP and 10% AEP events from separate temporal pattern bins was undertaken for a range of design storm durations from 20 minutes to 6 hours. Each duration utilised ten temporal patterns from AR&R 2016 (Reference 3). The result analysed to represent both the mainstream and overland flooding. The following process was undertaken in order to determine the critical duration for each temporal pattern bin: 1. Run 10 temporal patterns for each duration for the 1% AEP, 5% AEP and 10% AEP events. 2. Determine the mean enveloped level across the catchment from each duration modelled. 3. In order to determine which temporal pattern to use for each duration analyse each of the 10 flood level grids by producing difference mapping of each flood level grid against the mean enveloped level grid. 4. Statistically analyse the afflux grids utilising the mean, min, max and sum of difference. 5. The grid that produces statistics that is the closest to just above the mean level grid across the catchment was chosen, and then checked that it produced results that were a reasonable match to the mean across the catchment. 6. Two durations were chosen to reflect differences in behaviour between smaller subcatchments with faster response times, and the broader catchment behaviour.
It was found that for the 1 EY, 0.5 EY and 20% AEP events the 30 min duration event was critical for upper areas of the catchment affected by overland flow and the 2 hour event critical for lower areas of the catchment affected by mainstream flooding.
Modelling of the 10% and 5% AEP events determined that the 1 hour duration event was critical for upper areas of the catchment affected by overland flow and the 3 hour event critical for lower areas of the catchment affected by mainstream flooding.
Modelling of the 2%, 1% and 0.5% AEP events determined that the 1 hour duration event was critical for upper areas of the catchment affected by overland flow and the 3 hour event critical for lower areas of the catchment affected by mainstream flooding.
Modelling of the PMF determined that the 1 hour event was critical across the entire
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catchment.
The critical durations that were used for each duration are shown in Table 27.
Table 27: Design event critical duration Mainstream Critical Design Event Overland Critical Duration Duration 1 EY 30 min 120 min 0.5 EY 30 min 120 min 20% AEP 30 min 120 min 10% AEP 60 min 180 min 5% AEP 60 min 180 min 2% AEP 60 min 90 min 1% AEP 60 min 90 min 0.5% EP 60 min 90 min PMF 60 min
The temporal pattern selected for each design event and duration is shown in Table 28.
Table 28: Temporal Pattern Selected
Overland Temporal Mainstream Temporal Design Event Pattern Pattern 1 EY 4523 4641 0.5 EY 4523 4641 20% AEP 4523 4641 10% AEP 4568 4639 5% AEP 4568 4639 2% AEP 4557 4588 1% AEP 4557 4588 0.5% EP 4557 4588 PMF GDSM Method
7.3. Design Results
The results from this study are presented as: • Peak flood depths in Figure C1 to Figure C9; • Peak flood velocities in Figure C10 to Figure C18; • Provisional hydraulic hazard in Figure C19 to Figure C22; and • Provisional hydraulic categorisation in Figure C23.
The results were provided in digital format compatible Council’s Geographic Information Systems. The digital data should be used in preference to the figures in this report as they provide more detail.
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7.3.1. Summary of Results
Peak flood levels, depths and flows at key locations within the catchment are summarised below. These key locations coincide with the key locations used for the sensitivity analysis discussed in Section 8. A tabulated summary of peak flood depth and level results at key locations as shown in Figure 21 are detailed in Table 29.
Table 29: Peak Flood Levels (m AHD) and Depths (m) at Key Locations 1 0.5 20% 10% 5% 2% 1% 0.5% ID Location Type PMF EY EY AEP AEP AEP AEP AEP AEP Level 1.9 2.1 2.5 2.7 3.0 3.3 3.6 3.9 5.6 H_01 Upstream Botany Road Depth 0.3 0.5 0.8 1.0 1.3 1.5 1.8 2.1 3.8 Denison Street and Perry Level - - - - - 7.4 7.6 7.7 9.1 H_02 Street crossing Depth - - - - - 0.1 0.3 0.4 1.8 Level 7.6 7.7 7.8 7.8 7.8 7.9 7.9 8.0 8.3 H_03 Australia Avenue Depth 0.3 0.3 0.4 0.5 0.5 0.5 0.6 0.6 0.9 Baird Avenue and Perry Level 14.7 14.8 14.9 14.9 15.0 15.1 15.1 15.2 15.5 H_04 Street crossing Depth 0.0 0.1 0.2 0.3 0.3 0.4 0.4 0.5 0.8 Level - - - 10.9 11.0 11.1 11.2 11.2 11.7 H_05 Beauchamp Road Depth - - - 0.0 0.1 0.2 0.3 0.4 0.8 Grace Campbell Crescent Level - 11.1 11.1 11.3 11.3 11.4 11.4 11.4 11.8 H_06 and Nilsson Avenue - 0.1 0.1 0.2 0.2 0.3 0.3 0.4 0.7 Depth crossing Level 12.3 12.4 12.5 12.7 12.7 12.7 12.7 12.8 13.2 H_07 Beauchamp Road Depth 0.0 0.1 0.2 0.4 0.4 0.4 0.4 0.4 0.8 Bunnerong Open channel Level 13.6 14.2 14.6 14.8 14.8 14.8 14.8 14.9 15.6 H_08 at Matraville Public School Depth 1.6 2.1 2.5 2.7 2.7 2.7 2.7 2.8 3.5 Level - - 12.4 12.9 13.2 13.5 13.7 14.2 16.2 H_09 Rhodes Street Reserve Depth - - 0.1 0.6 1.0 1.2 1.5 1.9 3.9 Level 21.7 21.9 22.0 22.1 22.1 22.2 22.2 22.2 22.4 H_10 Jersey Road - West Depth 0.2 0.3 0.5 0.5 0.6 0.6 0.6 0.7 0.8 Level - 16.3 16.4 16.4 16.5 16.5 16.6 16.7 17.4 H_11 Jauncey Place Depth - 0.1 0.2 0.2 0.3 0.4 0.4 0.5 1.2 Level 16.4 16.4 16.6 16.6 16.6 16.8 16.9 17.1 18.0 H_12 Boonah Avenue Depth 0.1 0.2 0.3 0.4 0.4 0.6 0.7 0.9 1.7 Bunnerong Open Channel Level 17.9 17.9 18.1 18.2 18.6 18.9 19.0 19.2 21.2 H_13 at Fitzgerald Avenue Depth 0.9 1.0 1.1 1.3 1.7 1.9 2.0 2.2 4.2 Parer Street and Ulm Level 19.4 19.5 19.6 19.7 19.7 19.8 19.9 20.0 21.2 H_14 Street crossing Depth 0.3 0.4 0.5 0.5 0.6 0.7 0.8 0.9 2.1 Paine Street and Level 20.8 20.9 21.0 21.2 21.4 21.5 21.6 21.6 21.9 H_15 Fitzgerald Avenue 0.1 0.1 0.2 0.5 0.7 0.8 0.8 0.9 1.2 Depth crossing Level 21.6 21.7 21.8 21.9 22.0 22.1 22.1 22.2 22.4 H_16 Jersey Road - East Depth 0.0 0.1 0.1 0.2 0.4 0.4 0.5 0.5 0.7 Level - 22.0 22.0 22.1 22.1 22.2 22.2 22.3 23.2 H_17 Maroubra Road Depth - 0.0 0.1 0.1 0.2 0.2 0.3 0.3 1.2 Piccadilly Place and Bruce Level - - 24.4 24.5 24.6 24.6 24.7 24.7 25.7 H_18 Bennetts Place crossing Depth - - 0.0 0.1 0.1 0.2 0.2 0.3 1.3 H_19 Upstream Bunnerong Level 19.8 19.8 19.8 19.8 19.8 19.8 19.9 19.9 21.7
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1 0.5 20% 10% 5% 2% 1% 0.5% ID Location Type PMF EY EY AEP AEP AEP AEP AEP AEP Open Channel at Nagle 0.5 0.5 0.6 0.6 0.6 0.6 0.6 0.6 2.5 Depth Park Level 25.3 25.4 25.5 25.6 25.6 25.7 25.7 25.7 28.3 H_20 Gale Road Low Point Depth 0.1 0.2 0.3 0.4 0.4 0.5 0.5 0.5 3.1 Level 23.5 23.5 23.6 23.7 23.8 23.9 24.0 24.0 25.2 H_21 Snape Park Basin Depth 0.3 0.4 0.5 0.6 0.6 0.7 0.8 0.9 2.0 Level - 24.1 24.2 24.3 24.4 24.5 24.6 24.7 25.4 H_22 Percival Street Depth - 0.1 0.2 0.3 0.4 0.4 0.5 0.7 1.4 Prince Edward Circuit and Level 22.5 22.6 22.7 22.8 22.9 23.0 23.0 23.1 24.0 H_23 Towner gardens crossing Depth 0.2 0.2 0.3 0.5 0.5 0.6 0.7 0.8 1.6 Level 23.2 23.2 23.3 23.3 23.3 23.3 23.3 23.3 23.4 H_24 Prince Edward Circuit Depth 0.3 0.3 0.3 0.4 0.4 0.4 0.4 0.4 0.5 Level 25.7 25.8 26.0 26.2 26.3 26.4 26.5 26.6 28.5 H_25 Gale Road Depth 0.1 0.2 0.3 0.5 0.6 0.8 0.9 1.0 2.8 Holmes Street and Avoca Level - 27.5 27.6 27.7 27.8 27.9 27.9 28.1 28.8 H_26 Street Crossing Depth - 0.1 0.2 0.4 0.4 0.5 0.6 0.7 1.4 Level 60.3 60.4 60.5 60.5 60.6 60.6 60.6 60.6 60.8 H_27 Tucabia Street Depth 0.1 0.2 0.3 0.3 0.4 0.4 0.4 0.4 0.6 Level 25.8 25.8 25.9 26.0 26.0 26.1 26.1 26.2 27.6 H_28 Irvine Street Depth 0.3 0.4 0.5 0.5 0.5 0.6 0.7 0.7 2.1 Botany Street and Marville Level 24.2 24.3 24.3 24.4 24.4 24.5 24.5 24.5 25.1 H_29 Avenue crossing Depth 0.0 0.1 0.1 0.2 0.2 0.2 0.3 0.3 0.9 Level - 19.2 19.4 19.5 19.9 20.1 20.2 20.3 21.7 H_30 Astrolabe Park Depth - 0.1 0.3 0.4 0.7 1.0 1.0 1.1 2.5 Anzac Parade near Level 22.8 23.1 23.3 23.3 23.4 23.4 23.4 23.5 23.8 H_31 Rainbow Street Depth 0.3 0.6 0.8 0.8 0.9 0.9 0.9 1.0 1.3 Byrd Avenue near Anzac Level 29.3 29.5 29.5 29.5 29.6 29.6 29.6 29.6 29.8 H_32 Parade Depth 0.5 0.7 0.7 0.7 0.7 0.8 0.8 0.8 1.0 Level 25.1 25.3 25.4 25.5 25.6 25.7 25.8 26.0 26.6 H_33 Harbourne Road Depth 0.2 0.4 0.5 0.5 0.7 0.8 0.9 1.1 1.7 Level - - 31.4 31.5 31.6 31.7 31.7 31.8 32.3 H_34 Araluen Street - East Depth - - 0.0 0.1 0.2 0.3 0.3 0.4 0.9 Rainbow Street at Level - 37.1 37.2 37.3 37.4 37.5 37.5 37.6 38.1 H_35 Randwick High School Depth - 0.0 0.1 0.3 0.3 0.4 0.4 0.5 1.0 Level - - 54.4 54.4 54.5 54.5 54.5 54.6 54.9 H_36 Blenheim Street Depth - - 0.0 0.1 0.1 0.1 0.2 0.2 0.5 Level - 49.4 49.5 49.6 49.6 49.6 49.7 49.7 49.9 H_37 Elphinstone Road Depth - 0.2 0.3 0.4 0.4 0.4 0.5 0.5 0.7 Level 33.4 33.5 33.7 34.2 34.5 34.7 34.8 34.9 35.7 H_38 Byrd Avenue Low Point Depth 0.0 0.1 0.4 0.9 1.1 1.3 1.4 1.6 2.4 Level 33.9 33.9 34.0 34.3 34.5 34.7 34.8 34.9 35.6 H_39 Paton Street Depth 0.1 0.1 0.2 0.5 0.6 0.8 0.9 1.0 1.7 Bunnerong Road near Level 23.4 23.5 23.5 23.6 23.6 23.8 23.8 23.8 24.3 H_40 Rowland Park Depth 0.2 0.3 0.3 0.3 0.4 0.6 0.6 0.6 1.1 Level 26.4 26.4 26.6 26.9 27.1 27.2 27.2 27.2 27.6 H_41 Isis Lane Depth 0.3 0.4 0.5 0.9 1.0 1.1 1.1 1.1 1.5 Glanfield Street near Level 21.5 21.6 21.7 21.9 22.0 22.1 22.1 22.2 23.2 H_42 Bunnerong Road Depth 0.1 0.2 0.3 0.5 0.6 0.7 0.8 0.9 1.8
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1 0.5 20% 10% 5% 2% 1% 0.5% ID Location Type PMF EY EY AEP AEP AEP AEP AEP AEP Level 22.5 22.6 22.6 22.7 22.7 22.8 22.8 22.9 23.4 H_44 Mason Street - West Depth 0.3 0.3 0.4 0.4 0.5 0.6 0.6 0.7 1.1 Level 23.9 24.0 24.1 24.3 24.4 24.6 24.6 24.7 25.3 H_45 Glanfield Street - East Depth 0.0 0.1 0.3 0.4 0.6 0.7 0.8 0.8 1.5 Level 25.6 25.7 25.9 26.1 26.3 26.4 26.5 26.6 28.5 H_46 Alma Road Depth 0.1 0.3 0.4 0.7 0.8 0.9 1.0 1.2 3.0 Level - - 19.0 19.3 19.5 19.8 20.0 20.4 21.8 H_47 Jersey Road Depth - - 0.0 0.3 0.5 0.8 1.0 1.4 2.8 Randwick Environmental Level 31.2 31.2 31.2 31.2 31.2 31.3 31.4 31.4 32.8 H_48 Park Depth 1.3 1.3 1.3 1.3 1.4 1.4 1.5 1.6 2.9
The tabulated summary of peak flows at the key locations sown in Figure 22 are detailed in Table 30.
Table 30: Peak Flows (m3/s) at Key Locations 20 10 0.5 5% 2% 1% 0.5 % % % ID Location Type 1 EY AE AE AE PMF EY AE AE AE P P P P P P Overland 0.1 0.2 0.4 0.6 0.9 1.7 2.5 4.5 57.8 Q_01 Flint Street Pipe/ 12.3 14.1 16.6 17.9 19.0 19.0 19.0 19.1 21.9 Channel Overland - - - - 0.2 0.9 1.6 2.6 57.5 Upstream Tierney Q_02 Avenue Pipe/ 11.8 13.3 15.2 17.0 18.0 18.2 18.5 18.5 23.1 Channel Overland 0.1 0.1 0.1 0.1 0.1 1.1 2.7 5.3 78.7 Q_03 Donovan Avenue Pipe/ 7.6 8.1 9.3 10.5 11.1 12.0 12.6 12.7 17.1 Channel Overland 0.4 0.6 0.8 0.9 1.0 1.1 1.2 1.3 6.8 Q_04 Boyce Road Pipe/ 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.1 Channel Overland ------0.3 7.6 Downstream Parer Q_05 Street Pipe/ 0.5 0.5 0.5 0.6 0.6 0.6 0.6 0.6 0.5 Channel Overland - 0.1 0.2 0.2 0.2 0.2 0.3 0.5 11.4 Maroubra Road - Q_06 West Pipe/ 0.2 0.3 0.3 0.4 0.4 0.4 0.4 0.4 0.5 Channel Overland - - - - 0.0 0.1 0.1 0.3 10.7 Maroubra Road - Q_07 West Downstream Pipe/ 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.4 Channel Overland - - 0.2 0.2 0.2 0.2 0.2 0.2 1.2 Glanfield Street - Q_08 West Pipe/ 0.2 0.2 0.3 0.3 0.4 0.4 0.4 0.4 0.3 Channel Overland 0.0 0.1 0.1 0.2 0.2 1.7 5.4 9.3 95.9 Q_09 Perry Street - West Pipe/ 18.5 20.4 21.3 22.8 24.1 25.1 25.0 25.1 24.5 Channel Overland 0.0 0.1 0.2 0.5 0.9 1.6 2.2 3.3 17.2 Q_10 Australia Avenue Pipe/ 1.3 1.2 1.3 1.2 1.2 1.3 1.3 1.3 1.4 Channel
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20 10 0.5 5% 2% 1% 0.5 % % % ID Location Type 1 EY AE AE AE PMF EY AE AE AE P P P P P P Overland 0.1 0.2 0.2 0.5 1.0 1.8 2.4 3.6 16.8 Q_11 Harold Street Pipe/ 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.5 Channel Nilson Ave and Overland - - 0.0 0.3 0.7 1.5 2.0 2.7 12.0 Q_12 Grace Campbell Pipe/ 5.0 5.1 5.2 5.1 5.1 5.2 5.2 5.2 6.3 Cres Channel Overland 0.1 0.2 0.4 0.4 0.5 0.8 1.0 1.2 4.3 Grace Campbell Q_13 Cres - North Pipe/ 4.8 5.4 5.3 5.3 5.3 5.4 5.4 5.3 6.0 Channel Overland - - 0.0 0.3 0.6 0.7 1.2 2.3 17.7 Fitzgerald Avenue - Q_14 East Pipe/ 1.6 1.7 1.9 1.9 1.9 1.9 1.9 1.9 1.9 Channel Overland 0.0 0.2 0.5 0.5 0.4 0.8 2.2 4.2 65.6 Q_15 Alma Road Pipe/ 0.5 0.6 0.8 1.1 1.3 1.4 1.5 1.5 0.3 Channel Overland 0.2 0.2 0.2 0.4 0.6 1.2 1.5 1.9 41.1 Upstream Walengre Q_16 Avenue Pipe/ 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4 Channel Overland 0.1 0.1 0.2 0.2 0.3 0.3 0.4 0.6 33.1 Irvine Street at Q_17 Fisher Street Pipe/ 0.4 0.4 0.4 0.4 0.4 0.5 0.5 0.5 0.6 Channel Overland 0.2 0.4 0.8 1.3 2.6 5.0 9.7 17.4 119.5 Upstream Ainslie Q_18 Street Pipe/ 0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8 1.0 Channel Overland 0.0 0.1 0.2 1.1 2.1 5.2 9.4 16.3 105.5 Q_19 Sturt Street Pipe/ 0.4 0.5 0.7 0.7 0.7 0.7 0.7 0.7 0.9 Channel Overland 0.5 1.2 2.1 3.0 3.7 5.1 5.9 6.9 22.1 Downstream Q_20 Hayward Street Pipe/ 9.3 9.2 9.2 8.7 8.7 9.3 9.6 9.3 10.0 Channel Overland - - 0.8 2.6 3.9 6.0 7.1 8.8 43.7 Bunnerong Road - Q_21 North Pipe/ 9.7 10.6 11.2 11.0 10.1 10.0 10.1 10.2 11.5 Channel Overland - - 0.1 0.3 0.6 0.7 0.9 1.4 9.0 Q_22 High Street Pipe/ 1.6 1.9 2.2 2.3 2.5 2.5 2.6 2.7 3.3 Channel Overland 0.4 0.5 0.7 1.0 2.1 2.7 3.9 5.7 31.5 Q_23 Barker Street Pipe/ 2.6 3.2 3.6 3.7 3.8 3.7 3.8 3.8 4.8 Channel Overland 0.7 1.6 2.7 3.6 4.8 6.1 7.8 10.5 49.7 Q_24 Middle Street Pipe/ 2.3 2.4 2.4 2.4 2.4 2.4 2.4 2.4 2.5 Channel Overland 0.1 0.3 1.8 4.5 7.4 11.6 15.1 21.2 111.4 Q_25 Rainbow Street Pipe/ 9.2 10.0 10.9 11.1 11.3 11.3 11.4 11.4 11.9 Channel Overland 0.0 0.0 0.0 0.0 0.1 0.1 0.1 0.2 5.3 Upstream Snape Q_26 Street Pipe/ 0.0 0.0 0.1 0.1 0.0 0.1 0.1 0.1 0.1 Channel Q_27 Overland 0.3 0.5 0.8 1.1 1.5 2.0 2.6 3.6 38.2
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20 10 0.5 5% 2% 1% 0.5 % % % ID Location Type 1 EY AE AE AE PMF EY AE AE AE P P P P P P Downstream Isaac Pipe/ 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 Smith Street Channel Overland 0.0 0.0 0.1 0.2 1.0 2.5 4.2 6.3 37.0 Downstream Cook Q_28 Avenue Pipe/ 12.4 13.2 14.0 13.9 13.3 13.5 13.5 13.7 14.8 Channel Overland 0.1 0.1 1.1 1.5 2.3 2.8 3.2 3.5 9.9 Q_29 Boonah Avenue Pipe/ 3.7 4.2 4.2 4.0 4.1 4.1 4.2 4.2 5.4 Channel Overland 0.0 0.0 0.1 0.4 0.6 0.8 1.1 1.4 25.3 Brittain Crescent - Q_30 South Pipe/ 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.5 Channel Overland ------0.0 Brittain Crescent - Q_31 South East Pipe/ ------Channel Overland - - - 0.0 0.5 1.1 1.8 3.0 27.9 Q_32 Richardson Walk Pipe/ 12.7 14.8 17.2 18.6 18.5 18.7 18.6 18.7 20.4 Channel Overland 0.2 0.3 0.5 0.8 1.3 2.0 2.7 4.8 57.7 Downstream Flint Q_33 Street Pipe/ 12.3 14.1 16.2 18.0 18.5 18.7 18.6 18.8 20.8 Channel Overland 0.6 1.0 1.5 2.0 2.5 4.6 8.8 15.8 135.8 Avoca Street and N Q_34 Benvenue Street Pipe/ 1.1 1.1 1.1 1.1 1.1 1.1 1.2 1.2 1.3 Channel Parer Street at Overland 0.4 0.7 1.0 1.2 1.4 1.5 1.7 2.0 9.1 Q_35 Hinkler street Pipe/ 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 intersection Channel Overland 0.3 0.5 0.8 1.0 1.3 2.4 3.0 3.9 16.2 Grace Campbell Q_36 Crescent - South Pipe/ 4.7 4.7 4.7 4.7 4.7 4.7 4.7 4.7 5.0 Channel Overland 0.0 0.0 0.0 0.0 0.1 0.7 1.3 2.3 23.1 Beauchamp’s Road Q_37 - West Pipe/ ------Channel Overland 0.0 0.1 0.2 2.7 3.3 4.6 5.5 6.8 51.5 Downstream Q_38 Beauchamp’s Road Pipe/ 14.5 16.1 16.5 17.0 17.1 17.2 17.3 17.4 18.1 Channel Downstream Overland - - 0.2 4.0 4.0 5.2 6.3 8.0 60.5 Q_39 Matraville Public Pipe/ 13.7 15.6 16.2 16.4 16.3 16.3 16.3 16.3 16.0 School Channel Overland 0.0 0.0 0.1 0.1 0.1 0.2 0.7 1.4 37.4 Bunnerong Road at Q_40 Rowland Park Pipe/ 0.9 0.9 0.9 0.8 0.8 0.9 0.9 0.9 1.0 Channel Overland - - 0.0 0.1 0.3 0.7 1.1 1.5 36.0 Botany Street - Q_41 West Pipe/ 0.7 0.7 0.8 0.8 0.8 0.8 0.8 0.8 0.9 Channel Overland 0.3 0.8 1.5 2.1 2.5 3.1 3.4 3.9 17.7 Anzac Parade at Q_42 Byrd Avenue Pipe/ 8.7 8.8 8.8 8.7 8.7 8.8 8.8 8.9 9.3 Channel Raymond Avenue Overland 0.1 0.1 0.1 0.1 0.1 0.3 0.4 0.7 11.8 Q_43 and McCauley 0.9 0.9 1.0 1.0 1.0 1.0 1.0 1.0 1.1 Street Pipe/
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20 10 0.5 5% 2% 1% 0.5 % % % ID Location Type 1 EY AE AE AE PMF EY AE AE AE P P P P P P Channel
Overland 0.2 0.4 0.5 0.8 1.1 1.7 2.3 3.4 15.0 Perry Street and Q_44 Harold Street Pipe/ 0.4 0.4 0.4 0.4 0.5 0.5 0.5 0.5 0.5 Channel Perry Street and Overland 0.4 0.8 1.2 1.4 1.6 1.7 2.0 2.8 13.7 Q_45 Bunnerong Road Pipe/ 1.1 1.1 1.1 1.1 1.1 1.0 1.0 1.0 1.1 Intersection Channel Overland 0.0 0.1 0.2 0.4 0.6 0.8 1.0 1.2 93.6 Q_46 Wild Street Pipe/ 7.2 7.6 8.2 8.6 8.8 9.3 9.9 10.8 22.5 Channel Overland 0.4 0.6 0.9 1.1 1.4 1.6 1.7 2.2 90.9 Upstream Paine Q_47 Street Pipe/ 2.6 2.6 2.7 2.7 2.7 2.7 2.7 2.7 2.8 Channel Overland - - - 1.1 2.2 3.7 4.5 5.7 26.9 Q_48 Cook Avenue Pipe/ 11.2 12.3 12.5 12.3 11.8 11.7 11.7 12.0 13.8 Channel Overland 0.1 0.2 0.3 0.5 0.7 1.4 2.6 6.5 106.7 Q_49 Storey Street - West Pipe/ 0.2 0.2 0.2 0.2 0.3 0.2 0.2 0.3 0.5 Channel Overland 0.0 0.1 0.3 1.4 1.2 2.3 3.8 8.8 153.6 Upstream Moverly Q_50 Road - West Pipe/ 5.4 5.9 6.1 6.4 6.4 6.5 6.5 6.5 6.8 Channel Downstream Overland 0.0 0.1 0.6 1.3 1.7 2.6 4.5 10.4 127.7 Q_51 Holmes Street - Pipe/ ------West Channel Overland 0.3 0.4 0.5 0.9 2.2 4.4 5.8 7.9 50.0 Cook Avenue and Q_52 Banks Avenue Pipe/ 11.4 12.1 12.6 12.8 12.4 12.2 12.2 12.5 14.8 Channel
7.3.2. Provisional Flood Hazard Categorisation
Hazard classification plays an important role in informing floodplain risk management in an area. In the Floodplain Development Manual (Reference 25) hazard classifications are essentially binary – either Low or High Hazard as described in Figure L2 of that document. However, in recent years there has been a number of developments in the classification of hazard especially in Managing the floodplain: a guide to best practice in flood risk management in Australia (Reference 17)
For this study Provisional Flood Hazard Categorisation mapping has been provided utilising techniques form both of the above mentioned references. The techniques are outlined in the following reference material:
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Managing the Floodplain: A Guide to the Best Practice in Flood Risk Management in Australia
Managing the floodplain: a guide to best practice in flood risk management in Australia (Reference 17) provides revised hazard classifications which add clarity to the hazard categories and what they mean in practice. The classification is divided into 6 categories which indicate the restrictions on people, buildings and vehicles: • H1 - No constraints; • H2 – Unsafe for small vehicles; • H3 - Unsafe for all vehicles, children and the elderly; • H4 - Unsafe for all people and all vehicles; • H5 - Unsafe for all people and all vehicles. Buildings require special engineering design and construction; and • H6 – Unsafe for people or vehicles. All buildings types considered vulnerable to failure.
Diagram 3: Hazard Classifications
Figure C19a to Figure C22a provide the hazard classification for all the design events, according to the above classification. Under this classification, the most hazardous areas of the floodplain are generally constrained to the non-habitable areas, the parks, reserves, golf courses etc., lying adjacent to the waterways.
NSW Floodplain Development Manual
Provisional hazard categories were determined in accordance with Appendix L of the NSW Floodplain Development Manual (Reference 25), the relevant section of which is shown in Diagram 4. For the purposes of this report, the transition zone presented in Diagram 4 (L2) was considered to be high hazard.
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Diagram 4: Provisional Hydraulic Hazard Categories
Figure C19b to Figure C22b provide the hazard classification for all the design events, according to the above classification.
7.3.3. Provisional Hydraulic Categorisation
The 2005 NSW Government’s Floodplain Development Manual (Reference 25) defines three hydraulic categories which can be applied to define different areas of the floodplain, namely; • Floodways; • Flood Storage; and • Flood Fringe.
Floodways are areas of the floodplain where a significant discharge of water occurs during flood events and by definition, if blocked would have a significant effect on flood flows, velocities and/or depths. Flood storage are areas of importance for the temporary storage of floodwaters and if filled would significantly increase flood levels due to the loss of flood attenuation. The remainder of the floodplain is usually defined as flood fringe.
There is no quantitative definition of these three categories or accepted approach to differentiate between the various classifications. The delineation of these areas is somewhat subjective based on knowledge of an area, hydraulic modelling and previous experiences. A number of approaches, such as that of Howells et al (Reference 16), suggest the use of the product of velocity and depth as well as velocity itself to establish hydraulic categories.
Hydraulic categorisation has been mapped and is shown on Figure C23. The hydraulic categories were defies according to the following parameters:
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• Flood Fringe (base layer): PMF extent for peak depth greater than 0.15 m. • Flood Storage (takes precedence over Flood Fringe when overlapping): 1% AEP extent for peak depth greater than 0.15 m. • Floodway (takes precedence over Flood Fringe and Flood Storage when overlapping): Extent of 1% AEP peak velocity depth product when greater than 0.3 m2/s; or Extent of 1% AEP peak velocity when greater than 0.5 m/s.
Hydraulic categories based on the above criteria are considered provisional and may be revisited as part of a subsequent FRMS/P.
7.3.4. Preliminary Flood Emergency Response Classification
The design flood modelling was assessed in accordance with guidelines for Emergency Response Planning (ERP) outlined in Reference 22. These guidelines are generally more applicable to riverine flooding where significant flood warning time is available and emergency response action can be taken prior to the flood, or where long-term isolation may occur requiring possible resupply or medical evacuation. It is unclear how to apply the classifications in flash flood areas where there is little or no warning, and isolation times will be relatively short.
In urban areas like the Birds Gully and Bunnerong Road catchment, flash flooding from local catchment and overland flow will generally occur as a direct response to intense rainfall without significant warning. For most flood affected properties in the catchment, remaining inside the home or building is likely to present less risk to life than attempting to drive or wade through floodwaters, as flow velocities and depths are likely to be greater in the roadway.
The design modelling indicates that in the PMF event some properties will be subject to high hazard flooding with depths greater than 1.0 m covering access routes, prior to potential flooding of buildings. As floor level survey is unavailable for these properties, it is unclear what the depth of above-floor inundation of buildings on these properties may be. If estimated depths were life-threatening, these properties would need to be classified as “Low Flood Island” according to Reference 22, as by the time above-floor inundation occurs the roadways at the property frontages would already be inundated with high hazard flooding.
If a property is unaffected by above floor flooding but nearby streets are flooded, vehicular access from the area may be blocked, causing inconvenience or potentially threatening life if emergency medical care is required during a flood. This issue of flood isolation is less critical for urban flash flooding than for rural flooding as it is unlikely that access will be cut for more than a few hours. For example it is unlikely that provision of food or other supplies to isolated areas will be required in the Birds Gully and Bunnerong Road catchment. The following four emergency response classifications were identified as the most appropriate for the Birds Gully and Bunnerong Road catchment:
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Low Flood Island Low Flood Island was assessed as any property that was totally inundated in the PMF and with all potential evacuation routes unavailable at the peak of the flood, due to flood waters, topography or impassable structures. This corresponds approximately with the PMF High Hazard classification utilising the technique outlined in the Floodplain Development Manual (Reference 25).
High Flood Island Any area totally surrounded by Low Flood Island that is not inundated with all access roads closed and no overland or alternate road access possible. There is enough land higher that the flood level to cope with the number of people in the area.
Overland Escape Route Any park, open space or recreation area that is not heavily inundated that can be used as an overland escape route.
Rising Road Access The remaining area of the catchment where inhabited properties are above the predicted flood level. It is safe to assume that during the PMF that all areas of the catchment will be affected in some manner by a storm event of this magnitude, but these areas will still have all-weather uninterrupted rising road access.
For this preliminary assessment, some areas have been classified as “Low Flood Island” where it was assessed that there is a real risk of injury or death if residents become trapped in their homes during a flood. The SES does not provide definitive guidance on flood depth or velocity threshold before a road is “cut,” or on “acceptable” isolation times. For this study, roads have been assessed as potentially cut based on consideration of flood hazard in the PMF.
In light of these considerations, preliminary classification for the majority of the study area catchment is as “Rising Road Access” with some areas marked as “Low/High Flood Island” or as “Overland Escape” or “Overland Refuge” areas. Mapping of the classifications is shown on Figure D1.
7.4. Road Inundation
Flood level hydrographs showing flooding of key road reserves and creek crossings are provided in Figure D2 to Figure D14. These figures are included to provide the SES with an understanding of the period of time that the roads may be subject to hazard, and the range of inundation depths for the design events considered.
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7.4.1. Preliminary “True” Flood Hazard Categorisation
The provisional hazards were reviewed in this study to consider other factors such as rate of rise of floodwaters, duration, threat to life, danger and difficulty in evacuating people and possessions and the potential for damage, social disruption and loss of production. These factors and related comments are given in Table 31.
Table 31: Weightings for Assessment of True Hazard Criteria Weight Comment (1) Rate of Rise of High The rate of rise in the creek channels and onset of overland flow along roads Floodwaters would be very rapid, which would not allow time for residents to prepare for the onset of flooding. Duration of Flooding Low The duration for local catchment flooding will generally be less than around 6 hours, resulting in inconvenience to affected residents but not necessarily a significant increase in hazard. Effective Flood High Roads within the catchment will generally be inundated prior to property Access inundation, which may restrict vehicular access during a flood. Size of the Flood Moderate The hazard can change significantly at some locations with the magnitude of the flood, particularly between Canberra Street and Hobart Street. However, these changes in hazard are generally captured by mapping a range of events using the provisional hazard criteria. Effective Warning and High There is very little, if any, warning time. During the day residents will be Evacuation Times aware of the heavy rain but at night (if asleep) residential and non-residential building floors may be inundated with no prior warning. Additional Concerns Low These issues are a relatively minor consideration in urban environments like such as Bank the Little Creek catchment. Erosion, Debris, Wind Wave Action Evacuation Low Given the quick response of the catchment pre-flood evacuation is unlikely Difficulties to occur. There may be significant difficulties evacuating people who become trapped in their houses, but only if the depth is sufficient to present a risk to life. This factor is already captured by the provisional hydraulic hazard classification, and therefore was not given significant weight for assessing true hazard. Flood Awareness of Moderate Urban communities in general have relatively low flood awareness and a the Community short “community memory” for historical flood events. Depth and Velocity of High In areas of overland flow roads are subject to fast flowing water. In the main Floodwaters creek channels velocities and depth would be high. There is always a risk of a car or pedestrian being swept into the open channel while attempting to cross swiftly flowing waters at major creek crossings. However this factor is largely included in the provisional hydraulic hazard calculation metrics. Note: (1) Relative weighting in assessing the preliminary true hazard.
For the Birds Gully & Bunnerong Road catchment, the factors with high weighting in relation to assessment of true hazard are generally related to the lack of flood warning, the dangers of driving on flooded roads, and the potential for flooding of access to residential properties prior to above-floor flooding of buildings occurring. In many cases, it is likely that remaining inside the property will present less risk to life than attempting evacuation via flooded routes, as refuge can generally be taken on furniture etc. There may be some properties where remaining inside would present a high risk to life due to very high flood depths, but these properties will generally already be classified as high hazard using provisional hazard criteria.
In general it was found that areas where a high flood hazard would be justified based on consideration of the high-weight criteria in Table 31, the area was already designated high hazard as a result of the depth/velocity criteria used to develop the provisional hazard.
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Therefore the preliminary “true” hazard categories were assessed to be the same as the provisional hydraulic hazard (see Section 7.3.2).
7.4.2. Pipe Capacity Assessment
The design flood modelling was analysed to determine how frequently the stormwater pipe system capacity is likely to be exceeded throughout the catchment. Defining the maximum capacity of a pipe is not straightforward, as it depends on multiple factors including the shape of the pipe, the flow regime (e.g. upstream or downstream controlled), the inlet and outlet connections, the pipe grade, and other factors. For example, the nominal flow capacity of a pipe may increase with significant head to drive flow at the upstream end, but this “maximum” flow may be only slightly larger than when the soffit of the pipe is first exceeded, and the upstream afflux is an undesirable outcome in terms of reducing surface flooding.
TUFLOW provides output indicating the proportion of the cross-section area of the pipe that has flow in it. For the purposes of the pipe capacity assessment, pipes were assumed to be “full” when the flow area equalled or exceeded 85% of the pipe cross-sectional area. This is the point at which circular pipes tend to be close to their most efficient, since at 100% of cross- sectional area the additional friction from the top of the pipe reduces the pipe conveyance more than the slight increase in flow area. Similarly, box culverts designed for a supercritical flow regime will typically be designed for free surface flow approximately 80% of the depth of the culvert, as when flow touches the pipe soffit it will typically “trip” the flow regime to become sub-critical, resulting in lower capacity, depending on the pipe grade. Furthermore, due to energy losses associated with adjoining pits, inlets, culvert bends etc., some culverts may never become “100% full,” although they may be 90% full for a range of design flood events (e.g. from the 5% AEP through to the PMF). In such circumstances, it is informative to know the design storm for which the pipe is almost at its maximum capacity.
Figure 23 shows the outcomes of the pipe capacity assessment. In the catchment, the majority of pipes have less than 50% AEP capacity (that is, they are effectively “full” in the 50% AEP event), except for isolated pipes throughout the catchment.
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8. SENSITIVITY ANALYSIS
8.1. Overview
A number of sensitivity analyses were undertaken to establish the variation in design flood levels and flow that may occur if different parameter assumptions were made. These sensitivity scenarios are summarise in Table 32.
Table 32: Overview of Sensitivity Analyses
Scenario Description
Manning’s “n” The hydraulic roughness values were increased and decreased by 20%
Infiltration The Infiltration values were increased and decreased by 20%
Culvert and Bridge Sensitivity to blockage of culverts and bridges on open channel sections was assessed Blockage for 25%, 50% and 75% blockage
Pit, inlet Blockage Sensitivity to blockage of all culverts was assessed for a combination of: • 50% grade blockage, 100% sag blockage; • 100% blockage of all pits; and • 0% blockage of all pits.
Climate Change Sensitivity to rainfall and runoff estimates were assessed by increasing the rainfall intensities by 10%, 20% and 30%.
Sea level rise scenarios of 0.4 m and 0.9 m were assessed.
8.2. Climate Change Background
Intensive scientific investigation is ongoing to estimate the effects that increasing amounts of greenhouse gases (water vapour, carbon dioxide, methane, nitrous oxide, ozone) are having on the average earth surface temperature. Changes to surface and atmospheric temperatures are likely to change the future climate and sea levels. The extent of any permanent climatic or sea level change can only be established with certainty through scientific observations over several decades. Nevertheless, it is prudent to consider the possible range of impacts with regard to flooding and the level of flood protection provided by any mitigation works.
Based on the latest research by the United Nations Intergovernmental Panel on Climate Change (Reference 26), evidence is emerging on the likelihood of climate change and sea level rise as a result of increasing greenhouse gasses. In this regard, the following points can be made: • greenhouse gas concentrations continue to increase; • global sea levels have risen about 0.1 m to 0.25 m in the past century; • many uncertainties limit the accuracy to which future rainfall intensity changes and sea level rises can be projected and predicted.
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8.2.1. Rainfall Increase
The Bureau of Meteorology has indicated that there is no intention at present to revise design rainfalls to take account of the impact of climate change, as the implications of temperature changes on extreme rainfall intensities are presently unclear, and there is uncertainty about whether the changes would in fact increase design rainfalls for major flood producing storms.
Any increase in design flood rainfall intensities will increase the frequency, depth and extent of inundation across the catchment. It has also been suggested that the cyclone belt may move further southwards. The possible impacts of this on design rainfalls cannot be ascertained at this time as little is known about the mechanisms that determine the movement of cyclones under existing conditions.
Projected increases to evaporation are also an important consideration because increased evaporation would lead to generally dryer catchment conditions, resulting in lower runoff from rainfall. Mean annual rainfall is projected to decrease, which will also result in generally dryer catchment conditions.
The combination of uncertainty about projected changes in rainfall and evaporation makes it extremely difficult to predict with confidence the likely changes to peak flows for large flood events within the catchment under warmer climate scenarios.
In light of this uncertainty, the NSW State Government’s (Reference 18) advice recommends sensitivity analysis on flood modelling should be undertaken to develop an understanding of the effect of various levels of change in the hydrologic regime on the project at hand. Specifically, it is suggested that increases of 10%, 20% and 30% to rainfall intensity be considered.
There is no IFD data for events greater than the 1% AEP event for durations less than 24 hours, therefore only a comparison of the suggested climate change increases can only be made with the 24 hour event. The 24 hr IFDs and the climate change increases are shown in Table 33.
Table 33: 24hr Duration and Climate Change comparison 1% AEP 1% AEP 1% AEP 1% AEP 0.5 % 0.2 % 0.1% 0.05% + 10% + 20% + 30% AEP AEP AEP AEP 301 mm 331 mm 361 mm 391 mm 327 mm 370 mm 404 mm 437 mm
Table 33 indicates that for daily rainfalls: • A 10% increase in rainfall for the 1% AEP event is approximately equivalent to a 0.5% AEP event • A 20% increase in rainfall for the 1% AEP event is approximately equivalent to a 0.2% AEP event
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• A 30% increase in rainfall for the 1% AEP event is approximately equivalent to a 0.1 % AEP event
8.2.2. Sea Level Rise
The NSW Sea Level Rise Policy Statement (Reference 19) was released by the NSW Government in October 2009. This Policy Statement was accompanied by the Derivation of the NSW Government’s sea level rise planning benchmarks (Reference 20) which provided technical details on how the sea level rise assessment was undertaken. Additional guidelines were issued by OEH, including the Flood Risk Management Guide: Incorporating sea level rise benchmarks in flood risk assessments 2010 (Reference 21).
The Policy Statement says: “Over the period 1870-2001, global sea levels rose by 20 cm, with a current global average rate of increase approximately twice the historical average. Sea levels are expected to continue rising throughout the twenty-first century and there is no scientific evidence to suggest that sea levels will stop rising beyond 2100 or that current trends will be reversed… However, the 4th Intergovernmental Panel on Climate Change in 2007 also acknowledged that higher rates of sea level rise are possible” (Reference 19).
In light of this uncertainty, the NSW State Government’s advice is subject to periodical review. As of 2012 the NSW State Government withdrew endorsement of sea level rise predictions but still requires sea level rise to be considered. In the absence of any other advice the previous NSW State Government benchmarks of sea level rise of 0.4 m by the year 2050 and 0.9 m by the year 2100, relative to 1990 levels have been adopted in this study.
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8.3. Sensitivity Analysis Results
The sensitivity scenario results were compared to the 1% AEP event. A summary of peak flood level and peak flow differences at various locations are provided in: • Table 34 for variations in infiltration and roughness; • Table 35 for variations in pit/inlet and structure blockage; • Table 36 for variations in climate conditions.
8.3.1. Roughness and Infiltration Variations
Overall peak flood level results were shown to be relatively insensitive to 20% variations in the roughness and infiltration parameter. These results were found to be within ± 0.1 m. The results for the roughness sensitivity analysis are shown in Table 34.
Table 34: Results of Roughness and Infiltration Sensitivity Analysis – 1% AEP Levels (m) Peak Difference with 1% AEP (m) Flood Roughness Roughness Infiltration Infiltration ID Location Level Decreased Increased Decreased Increased 1% by 20% by 20% by 20% by 20% AEP H01 Upstream Botany Road 3.59 0.06 -0.07 -0.11 0.03 Denison Street and Perry Street 7.56 0.01 -0.01 -0.02 0.01 H02 crossing H03 Australia Avenue 7.90 0.00 0.00 0.00 -0.01 Baird Avenue and Perry Street 15.12 0.00 0.00 0.01 -0.01 H04 crossing H05 Beauchamp Road 11.15 0.01 -0.01 0.00 0.00 Grace Campbell Crescent and 11.41 0.00 0.00 0.01 -0.01 H06 Nilsson Avenue crossing H07 Beauchamp Road 12.73 0.01 0.00 0.01 -0.01 Bunnerong Open channel at 14.85 0.01 0.00 0.01 -0.02 H08 Matraville Public School H09 Rhodes Street Reserve 13.75 0.01 -0.01 -0.01 -0.04 H10 Jersey Road - West 22.18 0.00 0.00 0.01 -0.03 H11 Jauncey Place 16.60 0.00 0.00 0.00 -0.01 H12 Boonah Avenue 16.93 0.01 -0.01 0.00 0.00 Bunnerong Open Channel at 19.03 0.00 -0.01 -0.01 -0.02 H13 Fitzgerald Avenue Parer Street and Ulm Street 19.93 0.01 -0.01 0.00 0.00 H14 crossing Paine Street and Fitzgerald Avenue 21.58 0.00 0.00 0.00 -0.01 H15 crossing H16 Jersey Road - East 22.13 0.00 0.00 0.00 -0.03 H17 Maroubra Road 22.22 0.00 0.00 0.01 -0.01 Piccadilly Place and Bruce Bennetts 24.67 0.00 0.00 0.00 0.00 H18 Place crossing Upstream Bunnerong Open 19.85 0.00 0.00 0.00 -0.01 H19 Channel at Nagle Park
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Peak Difference with 1% AEP (m) Flood Roughness Roughness Infiltration Infiltration ID Location Level Decreased Increased Decreased Increased 1% by 20% by 20% by 20% by 20% AEP H20 Gale Road Low Point 25.69 0.00 0.00 0.00 0.00 H21 Snape Park Basin 23.96 0.01 -0.01 0.01 -0.08 H22 Percival Street 24.56 0.01 -0.01 0.00 0.01 Prince Edward Circuit and Towner 23.02 0.01 -0.01 0.00 0.00 H23 gardens crossing H24 Prince Edward Circuit 23.32 0.00 0.00 0.01 -0.02 H25 Gale Road 26.52 0.01 -0.01 -0.01 0.01 Holmes Street and Avoca Street 27.93 0.01 -0.01 -0.02 0.00 H26 Crossing H27 Tucabia Street 60.61 0.00 0.00 0.01 -0.01 H28 Irvine Street 26.14 0.01 -0.01 0.00 0.00 Botany Street and Marville Avenue 24.48 0.00 0.00 0.00 0.00 H29 crossing H30 Astrolabe Park 20.20 0.01 -0.01 0.00 -0.03 H31 Anzac Parade near Rainbow Street 23.45 0.00 0.00 0.01 -0.01 H32 Byrd Avenue near Anzac Parade 29.58 0.00 0.00 0.01 -0.04 H33 Harbourne Road 25.85 0.01 -0.01 0.01 -0.03 H34 Araluen Street - East 31.73 0.01 -0.01 -0.01 0.00 Rainbow Street at Randwick High 37.51 0.00 -0.01 0.00 0.00 H35 School H36 Blenheim Street 54.53 0.00 0.00 0.01 -0.01 H37 Elphinstone Road 49.66 0.00 0.00 0.01 -0.01 H_38 Byrd Avenue Low Point 34.78 0.01 -0.01 0.00 -0.02 H_39 Paton Street 34.77 0.01 -0.01 0.00 -0.01 Bunnerong Road near Rowland 23.82 0.01 -0.01 -0.01 0.00 H_40 Park H_41 Isis Lane 27.19 0.00 0.00 0.00 -0.01 Glanfield Street near Bunnerong 22.14 0.01 -0.01 -0.01 0.01 H_42 Road H_44 Mason Street - West 22.84 0.00 -0.01 0.00 0.00 H_45 Glanfield Street - East 24.64 0.01 -0.01 0.00 0.00 H_46 Alma Road 26.50 0.01 -0.01 -0.01 0.01 H_47 Jersey Road 20.02 0.02 -0.02 -0.03 -0.01 H_48 Randwick Environmental Park 31.36 0.01 -0.01 0.00 -0.09
8.3.2. Blockage Variations
The culverts and bridges in the Bunnerong open channel alignment were tested for blockage sensitivity. The design scenarios assumed 10% blockage at these same culverts. Overall peak flood levels were only affected at areas adjacent to Bunnerong open channel, particularly upstream of Botany Road at the downstream end of the catchment, where there is a relatively large culvert crossing. Modelling showed variation in peak flood levels at that location of between 0.8 m and 1.4 m for the scenarios tested that increase blockage and a decrease in peak flood level of 0.55 m in the scenario where the culverts are unblocked. The culvert
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blockage sensitivity results are shown in Table 35.
Table 35: Bunnerong Open Channel Blockage Sensitivity Analysis – 1% AEP Depths (m) ID Location Peak Difference with 1% AEP (m) Flood Culverts Culverts Culverts Culverts Level unblocked blocked blocked blocked 1% 25% 50% 75% AEP H_01 Upstream Botany Road 3.59 -0.55 0.81 1.35 1.42 H_02 Denison Street and Perry Street 7.56 -0.03 0.05 0.61 0.17 crossing H_03 Australia Avenue 7.90 0.00 0.00 0.00 0.00 H_04 Baird Avenue and Perry Street 15.12 0.00 0.00 0.00 0.00 crossing H_05 Beauchamp Road 11.15 0.00 0.00 0.02 0.03 H_06 Grace Campbell Crescent and 11.41 0.00 0.00 0.00 0.01 Nilsson Avenue crossing H_07 Beauchamp Road 12.73 0.00 0.01 0.06 0.07 H_08 Bunnerong Open channel at 14.85 -0.02 0.03 0.14 0.15 Matraville Public School H_09 Rhodes Street Reserve 13.75 -0.01 0.01 0.06 0.09 H_10 Jersey Road - West 22.18 0.00 0.00 0.00 0.00 H_11 Jauncey Place 16.60 0.00 -0.01 -0.05 -0.08 H_12 Boonah Avenue 16.93 0.00 0.00 0.00 0.00 H_13 Bunnerong Open Channel at 19.03 -0.01 0.03 0.21 0.78 Fitzgerald Avenue H_14 Parer Street and Ulm Street 19.93 0.00 0.00 0.02 0.06 crossing H_15 Paine Street and Fitzgerald 21.58 0.00 0.00 -0.01 -0.01 Avenue crossing H_16 Jersey Road - East 22.13 0.00 0.00 -0.01 -0.01 H_17 Maroubra Road 22.22 0.00 0.00 0.00 0.01 H_18 Piccadilly Place and Bruce 24.67 0.00 0.00 0.00 0.00 Bennetts Place crossing H_19 Upstream Bunnerong Open 19.85 0.00 0.00 0.09 0.39 Channel at Nagle Park H_20 Gale Road Low Point 25.69 0.00 0.00 0.00 0.00 H_21 Snape Park Basin 23.96 0.00 0.00 0.00 0.00 H_22 Percival Street 24.56 0.00 0.00 0.00 0.00 H_23 Prince Edward Circuit and Towner 23.02 0.00 0.00 0.00 0.00 gardens crossing H_24 Prince Edward Circuit 23.32 0.00 0.00 0.00 0.00 H_25 Gale Road 26.52 0.00 0.00 0.00 0.00 H_26 Holmes Street and Avoca Street 27.93 0.00 0.00 0.00 0.00 Crossing H_27 Tucabia Street 60.61 0.00 0.00 0.00 0.00 H_28 Irvine Street 26.14 0.00 0.00 0.00 0.00 H_29 Botany Street and Marville Avenue 24.48 0.00 0.00 0.00 0.00 crossing H_30 Astrolabe Park 20.20 0.00 0.00 0.00 0.00
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ID Location Peak Difference with 1% AEP (m) Flood Culverts Culverts Culverts Culverts Level unblocked blocked blocked blocked 1% 25% 50% 75% AEP H_31 Anzac Parade near Rainbow 23.45 0.00 0.00 0.00 0.00 Street H_32 Byrd Avenue near Anzac Parade 29.58 0.00 0.00 0.00 0.00 H_33 Harbourne Road 25.85 0.00 0.00 0.00 0.00 H_34 Araluen Street - East 31.73 0.00 0.00 0.00 0.00 H_35 Rainbow Street at Randwick High 37.51 0.00 0.00 0.00 0.00 School H_36 Blenheim Street 54.53 0.00 0.00 0.00 0.00 H_37 Elphinstone Road 49.66 0.00 0.00 0.00 0.00 H_38 Byrd Avenue Low Point 34.78 0.00 0.00 0.00 0.00 H_39 Paton Street 34.77 0.00 0.00 0.00 0.00 H_40 Bunnerong Road near Rowland 23.82 0.00 0.00 0.00 0.00 Park H_41 Isis Lane 27.19 0.00 0.00 0.00 0.00 H_42 Glanfield Street near Bunnerong 22.14 0.00 0.00 0.00 0.03 Road H_44 Mason Street - West 22.84 0.00 0.00 0.00 0.01 H_45 Glanfield Street - East 24.64 0.00 0.00 0.00 0.00 H_46 Alma Road 26.50 0.00 0.00 0.00 0.00 H_47 Jersey Road 20.02 0.00 0.00 0.00 0.00 H_48 Randwick Environmental Park 31.36 0.00 -0.01 0.00 0.00
The following three pit blockage scenarios were tested: 1. Inlets totally unblocked 2. Graded inlets 50% blocked and sag inlets 100% blocked 3. All inlets totally blocked
Modelling indicates that the 50% blocking of graded inlet pits and the 100% blocking of sag inlet pits has minimal effect on peak flood levels except upstream of Botany Road where a decrease in flood level of 0.30 m was modelled, Rhodes Street Reserve where an increase in flood level of 0.2 m was modelled, and Bunnerong Open Channel at Fitzgerald Avenue where a decrease of 0.28 m was modelled. The total blocking of all inlet pits has a significant effect on flood levels across the catchment with the following locations all recording a difference in peak flood level in excess than +/- 0.5 m: • Upstream Botany Road • Bunnerong Open channel at Matraville Public School • Rhodes Street Reserve • Bunnerong Open Channel at Fitzgerald Avenue • Alma Road • Jersey Road
The results for the pit inlet blockage analysis is shown in Table 36.
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Table 36: Results of Pit Inlet Blockage Sensitivity Analysis – 1% AEP Depths (m) ID Location Peak Difference with 1% AEP (m) Flood Inlets Graded Inlets Blocked Inlets Level Unblocked 50% Completely 1% AEP Sag Inlet Blocked 100% Blocked H_01 Upstream Botany Road 3.59 0.03 -0.30 -1.64 H_02 Denison Street and Perry 7.56 0.00 -0.16 -0.19 Street crossing H_03 Australia Avenue 7.90 0.00 0.00 0.04 H_04 Baird Avenue and Perry 15.12 0.00 0.00 0.10 Street crossing H_05 Beauchamp Road 11.15 -0.04 0.02 0.08 H_06 Grace Campbell Crescent and 11.41 0.00 -0.04 -0.02 Nilsson Avenue crossing H_07 Beauchamp Road 12.73 0.00 -0.01 -0.11 H_08 Bunnerong Open channel at 14.85 0.00 -0.01 -1.57 Matraville Public School H_09 Rhodes Street Reserve 13.75 0.00 0.20 0.84 H_10 Jersey Road - West 22.18 0.00 -0.02 0.03 H_11 Jauncey Place 16.60 0.00 0.04 0.09 H_12 Boonah Avenue 16.93 0.04 0.18 0.22 H_13 Bunnerong Open Channel at 19.03 -0.03 -0.28 -1.20 Fitzgerald Avenue H_14 Parer Street and Ulm Street 19.93 0.00 0.00 0.20 crossing H_15 Paine Street and Fitzgerald 21.58 0.01 0.01 0.09 Avenue crossing H_16 Jersey Road - East 22.13 0.00 0.01 0.05 H_17 Maroubra Road 22.22 0.00 0.03 0.14 H_18 Piccadilly Place and Bruce 24.67 0.00 -0.01 0.08 Bennetts Place crossing H_19 Upstream Bunnerong Open 19.85 0.00 -0.02 -0.31 Channel at Nagle Park H_20 Gale Road Low Point 25.69 0.00 0.01 0.18 H_21 Snape Park Basin 23.96 0.00 -0.10 -0.48 H_22 Percival Street 24.56 0.00 -0.05 0.27 H_23 Prince Edward Circuit and 23.02 0.00 0.02 0.17 Towner gardens crossing H_24 Prince Edward Circuit 23.32 0.00 0.01 0.01 H_25 Gale Road 26.52 0.00 0.05 0.89 H_26 Holmes Street and Avoca 27.93 0.00 0.02 0.23 Street Crossing H_27 Tucabia Street 60.61 0.00 0.03 0.05 H_28 Irvine Street 26.14 0.01 0.00 0.03 H_29 Botany Street and Marville 24.48 0.01 0.00 0.04 Avenue crossing H_30 Astrolabe Park 20.20 0.00 -0.02 0.27 H_31 Anzac Parade near Rainbow 23.45 0.00 -0.02 -0.02 Street H_32 Byrd Avenue near Anzac 29.58 0.01 -0.02 -0.04
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ID Location Peak Difference with 1% AEP (m) Flood Inlets Graded Inlets Blocked Inlets Level Unblocked 50% Completely 1% AEP Sag Inlet Blocked 100% Blocked Parade H_33 Harbourne Road 25.85 0.01 0.08 0.41 H_34 Araluen Street - East 31.73 0.00 0.01 0.12 H_35 Rainbow Street at Randwick 37.51 0.00 -0.01 0.09 High School H_36 Blenheim Street 54.53 0.00 0.06 0.11 H_37 Elphinstone Road 49.66 0.00 0.00 0.05 H_38 Byrd Avenue Low Point 34.78 -0.01 0.04 0.23 H_39 Paton Street 34.77 -0.01 0.04 0.20 H_40 Bunnerong Road near 23.82 0.01 -0.01 0.05 Rowland Park H_41 Isis Lane 27.19 0.01 0.00 -0.01 H_42 Glanfield Street near 22.14 0.00 0.05 0.22 Bunnerong Road H_44 Mason Street - West 22.84 0.00 0.04 0.10 H_45 Glanfield Street - East 24.64 0.00 -0.10 0.15 H_46 Alma Road 26.50 0.00 0.06 0.91 H_47 Jersey Road 20.02 -0.01 0.09 0.89 H_48 Randwick Environmental 31.36 -0.01 -0.11 0.01 Park
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8.3.3. Climate Variations
The effect of increasing the design rainfalls by 10%, 20% and 30% was evaluated for the 1% AEP rainfall event with impacts on peak flood levels observed throughout the study area. Generally speaking, each incremental 10% increase in rainfall results in an increase in peak flood levels at most of the locations analysed. The largest variation in flood level occurred upstream of Botany Road within the open channel. Sea level rise scenarios have the greatest effect on the downstream reaches of the catchment, near Botany Bay. The climate change sensitivity results are shown in Table 37.
Table 37: Results of Climate Change Analysis – 1% AEP Depths (m) Peak Difference with 1% AEP (m) Flood 2050 Sea 2100 Sea ID Location Depth Rain Rain Rain Level Level 1% +10% +20% +30% Rise Rise AEP + 0.4 m + 0.9 m H01 Upstream Botany Road 3.59 0.26 0.44 0.60 0.40 0.86 Denison Street and Perry Street 7.56 0.09 0.16 0.25 0.00 0.00 H02 crossing H03 Australia Avenue 7.90 0.03 0.06 0.08 0.00 0.00 Baird Avenue and Perry Street 15.12 0.04 0.07 0.09 0.00 0.00 H04 crossing H05 Beauchamp Road 11.15 0.04 0.07 0.11 0.00 0.00 Grace Campbell Crescent and 11.41 0.02 0.05 0.07 0.00 0.00 H06 Nilsson Avenue crossing H07 Beauchamp Road 12.73 0.02 0.03 0.04 0.00 0.00 Bunnerong Open channel at 14.85 0.03 0.06 0.07 0.00 0.00 H08 Matraville Public School H09 Rhodes Street Reserve 13.75 0.26 0.55 1.00 0.00 0.00 H10 Jersey Road - West 22.18 0.02 0.04 0.06 0.00 0.00 H11 Jauncey Place 16.60 0.05 0.09 0.13 0.00 0.00 H12 Boonah Avenue 16.93 0.11 0.21 0.28 0.00 0.00 Bunnerong Open Channel at 19.03 0.13 0.24 0.33 0.00 0.00 H13 Fitzgerald Avenue Parer Street and Ulm Street 19.93 0.07 0.13 0.17 0.00 0.00 H14 crossing Paine Street and Fitzgerald Avenue 21.58 0.04 0.07 0.09 0.00 0.00 H15 crossing H16 Jersey Road - East 22.13 0.02 0.04 0.05 0.00 0.00 H17 Maroubra Road 22.22 0.02 0.05 0.07 0.00 0.00 Piccadilly Place and Bruce Bennetts 24.67 0.03 0.05 0.07 0.00 0.00 H18 Place crossing Upstream Bunnerong Open 19.85 0.01 0.03 0.13 0.00 0.00 H19 Channel at Nagle Park H20 Gale Road Low Point 25.69 0.03 0.07 0.12 0.00 0.00 H21 Snape Park Basin 23.96 0.06 0.11 0.15 0.00 0.00 H22 Percival Street 24.56 0.09 0.17 0.22 0.00 0.00 Prince Edward Circuit and Towner 23.02 0.05 0.11 0.16 0.00 0.00 H23 gardens crossing
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Peak Difference with 1% AEP (m) Flood 2050 Sea 2100 Sea ID Location Depth Rain Rain Rain Level Level 1% +10% +20% +30% Rise Rise AEP + 0.4 m + 0.9 m H24 Prince Edward Circuit 23.32 0.01 0.01 0.02 0.00 0.00 H25 Gale Road 26.52 0.08 0.17 0.34 0.00 0.00 Holmes Street and Avoca Street 27.93 0.09 0.16 0.23 0.00 0.00 H26 Crossing H27 Tucabia Street 60.61 0.02 0.03 0.04 0.00 0.00 H28 Irvine Street 26.14 0.04 0.08 0.12 0.00 0.00 Botany Street and Marville Avenue 24.48 0.02 0.04 0.05 0.00 0.00 H29 crossing H30 Astrolabe Park 20.20 0.06 0.16 0.23 0.00 0.00 H31 Anzac Parade near Rainbow Street 23.45 0.02 0.04 0.05 0.00 0.00 H32 Byrd Avenue near Anzac Parade 29.58 0.01 0.02 0.02 0.00 0.00 H33 Harbourne Road 25.85 0.09 0.18 0.25 0.00 0.00 H34 Araluen Street - East 31.73 0.05 0.09 0.12 0.00 0.00 Rainbow Street at Randwick High 37.51 0.04 0.08 0.12 0.00 0.00 H35 School H36 Blenheim Street 54.53 0.03 0.06 0.08 0.00 0.00 H37 Elphinstone Road 49.66 0.01 0.03 0.04 0.00 0.00 H38 Byrd Avenue Low Point 34.78 0.08 0.15 0.21 0.00 0.00 H39 Paton Street 34.77 0.07 0.14 0.19 0.00 0.00 Bunnerong Road near Rowland 23.82 0.03 0.05 0.06 0.00 0.00 H40 Park H41 Isis Lane 27.19 0.01 0.02 0.04 0.00 0.00 Glanfield Street near Bunnerong 22.14 0.06 0.11 0.16 0.00 0.00 H42 Road H44 Mason Street - West 22.84 0.04 0.07 0.10 0.00 0.00 H45 Glanfield Street - East 24.64 0.07 0.12 0.15 0.00 0.00 H46 Alma Road 26.50 0.09 0.18 0.35 0.00 0.00 H47 Jersey Road 20.02 0.24 0.45 0.61 0.00 0.00 H48 Randwick Environmental Park 31.36 0.06 0.13 0.30 -0.02 -0.01
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9. FLOODING HOT SPOTS
Some of the areas where flooding is problematic, sometimes referred to as “hotspots,” are discussed below in further detail. Figure C24 provides an overview of the locations discussed.
9.1. Harold Street and Perry Street Low Point – Matraville
The Harold Street and Perry Street low point occurs in a natural depression that operates as and overland flow path between Harold Street and Australia Avenue. Overland flow originates from the following locations: • Local catchment runoff; • Overland flow originating in Solander Street and Baird Avenue is conveyed down Perry Street in a westerly direction until it reaches the low point; • Overland flow originating in Daunt Avenue is conveyed down Bunnerong Road and into Perry Street where it travels in a westerly direction until it reaches the low point.
Flooding within the low point is controlled by high ground levels on each side of the depression which channels flow through the residential properties between Harold Street and Australia Avenue. The low point in Harold Street is shown in Photo 9. Modelled flood depths of up to 0.6 m inundate the residential properties in the 1% AEP event. Figure C25 shows the location of the low point, topography and flood depths.
Photo 9: Harold Street Low Point
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9.2. Paton Street and Byrd Avenue Low Point – Kingsford
The Paton Street low point occurs in a natural depression of medium density residential land- use, and affects Paton Street, Byrd Avenue, and McNair Avenue. Overland flow originating in the upstream catchment is conveyed in a north to south direction through Randwick Girls High and Randwick Boys High, Inglis Newmarket Stables, Rainbow Street Public School and Paine Reserve, then enters the Paton Street Low point. Flooding within the low point is controlled by high ground levels along Sturt Street and once the drainage network reaches capacity, flood waters can reach depths of up to 2 m in the 1% AEP event.
The sag point is on the Birds Gully drainage line, but the relief flow path from the low point is across Sturt Street, east of Paton Street, rather than at Byrd Avenue where the drainage line runs. The overtopping flows therefore result in a diversion of flow out of the Birds Gully catchment towards Avoca Street, via low points in Jellicoe Avenue and Ainslie Street.
Modelled flood depths of up to 0.9 m inundate McNair Avenue and Paton Street and modelled flood depths of up to 1.4 m inundate Byrd Avenue in the 1% AEP event. Figure C26 shows the location of the low point, topography and flood depths.
9.3. Holmes Street and Benvenue Street Low Point – Maroubra
The Holmes Street and Benvenue Street low point occurs in a natural depression of medium density residential land-use, which affects Holmes Street and Benvenue Street. Overland flow is conveyed down Avoca Street at a maximum rate of 6 m3/s in the 1% AEP event, and is redirected into the Homes Street and Benvenue Street low point due to the raised elevation of Anzac Parade, which is approximately 1 m above the ground level in Holmes Street. Modelling indicates there may be minor surcharging of the drainage network into the Holmes Street low point.
Design flood depths of up to 0.6 m on Holmes Street and 0.5 m on Benvenue Street were modelled in the 1% AEP event with Figure C27 showing the location of the low point, topography and flood depths.
9.4. Alma Road Low Point – Maroubra
The low points on Alma Road and Gale Road between Anzac Parade and Garden St occur in a depression in the topography exacerbated by the clustered nature of the residential development. Overland flow travelling down Garret Street and Garden St is conveyed into the low points on Alma Road and Gale Road, and the raised profile of Anzac Parade prevents outflow occurring along the natural drainage path to the south-west. The raised residential properties and their close proximity to each other also prevent draining of the low points as shown in Photo 10.
Modelled flood depths of up to 0.9 m inundate Gale Street and Alma Road with surrounding
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residential properties inundated by up to 0.5 m in the 1% AEP event. Figure C28 shows the location of the low point, topography and flood depths.
Photo 10: Alma Road Residential Dwellings
9.5. Glanfield Street Low Point – Maroubra
Photo 11: Glanfield Road Low Point
The low point on Glanfield Street and Boyce Road is a natural feature in the topography that is compounded by the elevated road crest of Maroubra Road. The low point in Glanfield Street is shown in Photo 11.
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Modelling indicates that Glanfield Street is inundated by depths of up to 0.8 m in the 1% AEP event with Figure C29 showing the location of the low point, topography and flood depths.
9.6. Jersey Road Low Point – Matraville
Flooding on Jersey Road is due to the topographic depression between Bunnerong Road and Dive Street. Overland flow enters the low point form four directions: • From the east along Jersey Road • From the north across Heffron Park • From the north-west from Bunnerong Road • From the south-west through residential properties on Bunnerong Road
The crest of Bunnerong Road is 1.5 m above the lowest section of Jersey Road creating a barrier that prevents stormwaters from flowing towards the Bunnerong drain. Although there is a reasonably large 1.05 m pipe conveying a peak flow of 2 m3/s in the 1% AEP event, peak flood depths of 1.0 m are estimated to occur in the 1% AEP event.
Modelled indicates that the Jersey Road low point is inundated by flood depths of up to 1.1 m in the 1% AEP event with Figure C30 showing the location of the low point, topography and flood depths.
9.7. Flack Avenue Low Point – Matraville
The low point on Flack Avenue north of the intersection with Beauchamp Road is inundated primarily by overtopping of the Bunnerong open channel near Matraville Public School, where the open channel enters a 3.5 m x 1.68 m box culvert. There is a peak flow of 16 m3/s in the 1% AEP event through the culvert, but this is less than the total channel flow. The channel overtopping inundates the Flack street low point with a peak flow of 5.5 m3/s through private property in the 1% AEP event.
Photo 12: Flack Avenue Low Point
Flooding in Flack Avenue is exacerbated by the crest of Beauchamp Road being 0.5 m above the low point in Flack Street which prevents overland flow being conveyed downstream. An
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example of historical flooding in Flack Avenue is shown in Photo 12. Modelled flood depths of up to 0.7 m inundate Flack Street in the 1% AEP event with Figure C31 showing the location of the low point, topography and flood depths.
9.8. Denison Street and Nilson Avenue Low Point – Hillsdale
The Denison Street and Nilson Avenue low point encompasses the southern ends of Denison Street and Nilson Avenue as well as Beauchamp Street, with the residential properties bounded by the three streets heavily affected by ponded floodwaters. The flood affectation is caused by the following flood mechanisms: • Local catchment runoff • Overland flow conveyed down Nilson Avenue and Grace Campbell Crescent from the north • Overland flow along Beauchamp Avenue, including flow that originates from the overtopped Bunnerong open channel near Flack Avenue.
Beauchamp Avenue is raised by between 0.4 m and 1.25 m above the low point which acts as a barrier to overland flow. Modelled indicates that the residential properties within the low point are inundated with flood depths of between 0.4 m to 1.2 m with Figure C32 showing the location of the low point, topography and flood depths.
9.9. Boonah Avenue Low Point – Eastgardens
Photo 13: Boonah Avenue Low Point
The Boonah Avenue low point is located between Fraser Avenue and Smith Street with Smith Street acting as a barrier preventing overland flow from escaping the low point. The most severely affected properties are just north of Smith Street. Ponding in this area is caused by two mechanisms:
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• Localised runoff draining to the low point • The drainage pits on Boonah Avenue surcharging
An image of the low point in Boonah Avenue is shown in Photo 13. Boonah Street has modelled flood depths of up to 0.7 m with the residential properties between Boonah Avenue and Smith Street inundated up to a depth of up to 1.2 m in the 1% AEP event. Figure C33 shows the location of the low point, topography and flood depths.
9.10. Parer Street Low Point – Maroubra
The Parer Street sag point is located in between Donavan Avenue and Hinkler Street. Water ponds at the location during a storm once the capacity of the trunk drainage system is exceeded. The source of the flood mechanism is primarily local subcatchment runoff. Modelling indicates that Parer Street is inundated by depths up to 0.9 m in the 1% AEP event with Figure C34 showing the location of the low point, topography and flood depths.
9.11. Glanfield Street / Maroubra Road Low Point – Maroubra
The low points in Glanfield Street and Maroubra Road are located between Bunnerong Road and Royal Street. Water ponds at the location during a storm once the capacity of the trunk drainage system is exceeded. Modelling indicates that Glanfield Street is inundated by up to 0.6 m and Maroubra Street by up to 0.3 m in the 1% AEP event with Figure C35 showing the location of the low point, topography and flood depths.
9.12. Edward Circuit Low Point – Pagewood
The Edward Circuit low point is located between Monash Gardens and Birdwood Avenue. Overland flow is conveyed down Wark Avenue and Towner Gardens towards the Edward Circuit low point. Once the capacity of the drainage system is reached the water begins to pond, to depths up to 0.7 m in the 1% AEP event.
Design flood depths of up to 0.5 m inundate the Edward Street low point in the 1% AEP event, with Figure C36 showing the location of the low point, topography and flood depths
9.13. Bunnerong Road Low Point – Hillsdale
The residential properties between located between Bunnerong Road and Botany Street directly across from Rowland Park are situated in a topographic depression. The low point is affected by overland flow from the following sources: • Localised runoff between Bunnerong Road and Botany Street. • Surcharging pits along Bunnerong Road. • Backwater flow from the Marville Avenue drainage line, across Botany Road and into the low point. This overland flow path originates from as far north as Hincks Street and as far east as Anzac Parade.
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Once the capacity of the drainage network is met water begins to pond in the residential properties on Bunnerong Road Due to the flat topography floodwaters disperse slowly across Bunnerong Road and into Rowland Park.
An image of historical flooding is shown in Photo 14. Design flood depths of up to 0.6 m inundate the residential properties on Bunnerong Road in the 1% AEP event. Figure C37 showing the location of the low point, topography and flood depths.
Photo 14: Reported Flooding Bunnerong Road Low Point
9.14. Irvine Street Low Point – Kingsford
The low point on Irvine Street is located between Beulah Street and Walenore Avenue, directly above the trunk drainage line that continues towards Marville Avenue. Overland flow enters the low point down Irvine Street from a north to south direction and down Fischer Street from an east to west direction where it is trapped by the surrounding topography once the capacity of the drainage system is reached. The overland flow relief path is towards Marville Avenue, but the outflows are limited by an adverse grade of Marville Avenue near Walenore Avenue.
An image of the Irvine Street low point is shown in Photo 15. Modelled flood depths of up to 0.65 m inundate Irvine Street in the 1% AEP event with Figure C38 showing the location of the low point, topography and flood depths.
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Photo 15: Irvine Street Low Point
9.15. Hincks Street Low Point – Kingsford
The Hincks Street low point is located on the residential block between Hincks Street, Irvine Street, Isis Street and Botany Street where residential properties are situated below the adjacent road levels. Overland flow enters the low point from two locations: • Localised runoff from the residential block; and • The overland flow path down Botany Road that originates north of Anzac Parade.
There is a relatively small drainage system inside the residential properties to drain the low point towards the Birds Gully trunk line to the north. Modelled flood depths of up to 1.1 m inundate the residential properties in the Hincks Street low point in the 1% AEP event with Figure C39 showing the location of the low point, topography and flood depths.
9.16. Harbourne Road Low Point – Kingsford
The Harbourne Road low point is located in Kingsford at the intersection of Anzac Parade and Rainbow Street, with the crest of Rainbow Street approximately 0.8 m above the lowest point of Harbourne Road. Overland flow enters the low point from three locations: • Local catchment runoff; • Overland flow conveyed down Rainbow Street from the east inundates Harbourne Street; and • Overland flow from as far east as Kennedy Street is conveyed to Forsyth Street and through the residential properties towards the Harbourne Street low point.
Once the capacity of the drainage system is exceeded, water begins to pond in the low point with depths up to 1.0m in the 1% AEP event. Modelled flood depths of up to 1.1 m inundate the Harbourne Street low point in the 1% AEP event. Figure C40 with showing the location of the low point, topography and flood depths.
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9.17. Hayward Street / Jacques Street Low Point – Kingsford
The low point on Jacques Street near the intersection with Hayward Street (Photo 16) is directly above the Birds Gully trunk drainage line. Overland flow enters the low point primarily down Jacques Street from the south east (following the trunk drainage line), but also from Hayward Street from the south and overland flow coming from Anzac Parade through residential development.
Photo 16: Hayward / Jacques Street Low Point
This water is trapped by the surrounding topography once the capacity of the drainage system is reached. Modelled flood depths reach just over 0.5 m in the street and can inundate nearby houses. The overland flow path relief is along Jacques Street to the north-west, following the Birds Gully trunk drainage line. This is activated once the Hayward Street road crest is overtopped, although flooding remains an issue down Jacques Street to Bunnerong Road. Figure C41 shows the location of the low point, topography and 1% AEP flood depths.
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10. PRELIMINARY FLOOD PLANNING AREA
10.1. Background
Land use planning is one of the most effective means of minimising flood risk and damages from flooding. The Flood Planning Area (FPA) identifies land that is subject to flood related development controls and the Flood Planning Level (FPL) is the minimum floor level applied to development proposals within the FPA.
The process of defining FPAs and FPLs is somewhat complicated by the variability of flow conditions between mainstream and local overland flow, particularly in urban areas. Traditional approaches that were developed for riverine environments and “mainstream” flow areas often cannot be applied in steeper urban overland flow areas.
Defining the area of flood affectation due to overland flow (which by its nature includes shallow flow) often involves determining at which point it becomes significant enough to classify as “flooding” rather than just drainage of local runoff. The difference in peak flood level between events of varying magnitude may be minor in areas of overland flow, such that applying the typical freeboard can result in a FPL much greater than the Probable Maximum Flood (PMF) level.
The FPA should include properties where future development would result in impacts on flood behaviour in the surrounding area and areas of high hazard that pose a risk to safety or life. Further to this, the FPL is determined with the purpose to decrease the likelihood of over-floor flooding of buildings and the associated damages.
The Floodplain Development Manual suggests that the FPL generally be based on the 1% AEP event plus an appropriate freeboard. The typical freeboard cited in the manual is that of 0.5 m; however it also recognises that different freeboards may be deemed more appropriate due to local conditions. In these circumstances, some justification is called for where a lower value is adopted.
Further consideration of flood planning areas and levels are typically undertaken as part of the Floodplain Management Study where council decides which approach to adopt for inclusion in their Floodplain Management Plan.
10.2. Identification of Flood Control Lots
Flood Tagging is the process where lots are identified as flood liable. The “tagged” lots will be subject to Section 149(2) notification (under NSW Local Government Act) indicating that their properties are subject to flood related development controls. This simply means that should development of the lots occur, flooding will need to be considered and Council’s LEP, DCP and any other relevant flood related policies will apply.
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Flood tagging was undertaken using a three step process, shown below.
Diagram 5: Stages for Identification of Flood Control Lots
The methodology used in this report is consistent with that adopted in a number of similar studies throughout the Sydney metropolitan area. Identification of properties subject to flood- related development controls is undertaken by using the 1% AEP model results, with filtering to remove nuisance or non-damaging levels of flow, then applying subsequent ground truthing to determine whether individual properties are tagged or not. For this study, there were no areas where typical mainstream flood techniques (adding freeboard and stretching the results) produced reasonable outcomes. Each of the properties identified were based on overland flow criteria as identified below. • Overland flooding: Lots were originally classified as “flood control lots” and therefore within the FPA, if they were affected by the modelled 1% AEP flood extent (after applying filtering). The flood depth map was filtered to remove areas less than 0.15 m deep. Properties were then identified as preliminary “flood control lots” where 10% or more of the property was affected by this filtered flood extent.
The Desktop Analysis (Step 2) and Ground Truthing (Step 3) processes were then undertaken. Some potentially flooded lots are not identified from the automated GIS Analysis process (Step 1), due to the approximations required to construct the computational model of the catchment, and due to the sensitivities of GIS processing. Furthermore, some lots may be initially identified as flood control lots, which in reality are unlikely to be subject to significant flooding. Ground truthing was undertaken first through desktop analysis, and then a site visit for properties requiring detailed investigation. The results of this process were provided in GIS format to Council. The considerations applied during this process, and categories assigned to various properties as part of this process, are summarised in Table 38.
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Table 38: Ground truthing classifications for flood control lot identifications process Classification Description Tag Removed A1 Initially tagged due to localised ponding within property. Ground truthing indicated that the depression was an artefact of the LIDAR and not a genuine trapped drainage sag point. Tag removed.
A2 Initially tagged because of model grid cells in the gutter overlapping with the property boundary. Ground truthing confirmed that water would be confined to gutter and there is a significant gradient past the property. Tag removed. Tag Added B1 Surrounding lots tagged. Ground truthing confirmed that the topography and flow behaviour for the lot was similar to adjacent tagged lots. Tag added. B2 Lot identified to have a drainage easement. Ground truthing identified that the easement was associated with a potential overland flow path in the case of blockage of stormwater inlets or gutters. Tag added. B3 Property downstream of or adjacent to a sag point. Ground truthing identified that there would be a potential overland flow path resulting from blockage of kerb inlets, pipes or gutters. Tag Retained (confirmed by ground truthing) C1 Confirmed to be inundated or potentially inundated from nearby flow path or sag point. C2 Overland flow area, initial tagging confirmed by ground truthing. Not Tagged (confirmed by ground truthing) D1 Not tagged initially and still not tagged after investigation
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11. PUBLIC EXHIBITION
A draft version of this study was placed on Public Exhibition for a period of four weeks, from 30 April to 27 May 2017. Local residents were informed of the public exhibition period and were invited to provide comments on the draft report. Letters were sent to affected residents, landowners and precinct committees (approximately 5,200 in total), and notifications of the public exhibition period were included in the Southern Courier, Randwick eNews and on the Randwick Council website.
A website was set up (https://www.yoursayrandwick.com.au/BirdsFloodStudy) that included the Draft Flood Study document, an online submission option and a question and answer forum. There were 321 visits to the site during the public exhibition period, with a total of 400 documents downloaded (Draft Flood Study, catchment maps, flood maps, etc). Five submissions were received though the website (including one in response to Council’s 2018- 19 draft budget regarding flood mitigation) and one question was asked. The community consultation report prepared by Council summarising this information is included in Appendix F.
A community information session was also held on 24 May 2018 at the Lionel Bowen Library, Maroubra. It was estimated that approximately 20 people attended the session. Council and WMAwater project staff were available to explain the study, present results and answer questions from the community. Four written submissions were received directly at the drop- in session. In addition to the submissions received online and at the community information session, an additional 5 submissions were received via email directly to Council.
A compilation of the submissions and responses can be found in Appendix F. Generally, the community had concerns regarding the flooding issues within the catchment and how these are to be managed. These issues are mainly concerned with drainage, including the blockage, maintenance and upgrade of the stormwater system. Consideration of potential mitigation of flood issues is part of the next phase – the Floodplain Risk Management Study and Plan.
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12. REFERENCES
1. Bureau of Meteorology Intensity Frequency Duration (http://www.bom.gov.au/water/designRainfalls/ifd/) 24 January 2016.
2. Ball J, Babister M, Nathan R, Weeks W, Weinmann E, Retallick M, Testoni I (Editors) Australian Rainfall and Runoff – A Guide to Flood Estimation Commonwealth of Australia (Geoscience Australia) 2016.
3. Pilgrim DH (Editor in Chief) Australian Rainfall and Runoff – A Guide to Flood Estimation Institution of Engineers, Australia, 1987.
4. Sydney Water Birds Gully SWC 10, Banks to Cook Avenue SWC 12 Capacity Assessment December 1997.
5. Sydney Water Bunnerong to Tasman Sea SWC 11AMP Capacity Assessment June 2002.
6. Sydney Water Bunnerong to Botany Bay (SWC 11) Capacity Assessment July 2002.
7. DRAINS User Manual, Version November 2011 Watercom, July 2011
8. TUFLOW User Manual, 2016-03-AA BMT WBM, March 2016
9. Chow, V.T.
Open Channel Hydraulics McGraw Hill, 1959
10. Henderson, F.M.
Open Channel Flow MacMillian, 1966
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11. Rawls, W., Brakesiek, J. and Miller, N. Green-Ampt Infiltration Parameters from Soils Data Journal of Hydraulic Engineering, 1983
12. Barton, C. L.
Flow through an Abrupt Constriction – 2D Hydrodynamic Model Performance and Influence of Spatial Resolution Griffith University, July 2001
13. Project 15: Two Dimensional Modelling in Urban and Rural Floodplains
Engineers Australia, November 2012
14. Weeks, W.
Australian Rainfall and Runoff: Revision Projects Project 11: Blockage of Hydraulic Structures - Guidelines Engineers Australia, February 2014
15. Floodplain Development Manual NSW Government, April 2005
16. Howells, L., McLuckie, D., Collings, G. and Lawson, N.
Defining the Floodway – Can One Size Fit All? Floodplain Management Authorities of NSW 43rd Annual Conference, Forbes February 2003
17. Australian Institute for Disaster Resilience Managing the Floodplain A Guide to Best Practice in Floodplain Management in Australia Australia Government – Attorney General’s Department, 2017
18. NSW Department of Environment and Climate Change Practical Consideration of Climate Change Coast and Floodplain Management, October 2007
19. NSW Sea Level Rise Policy NSW State Government, October 2009
20. NSW Department of Environment and Climate Change Derivation of the NSW Government’s Sea Level Rise Planning Benchmarks NSW State Government, October 2009
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21. NSW Department of Environment and Climate Change Flood Risk Management Guide Incorporating sea level rise benchmarks in flood risk assessments NSW State Government, August 2010
22. NSW Department of Environment and Climate Change Flood Emergency Response Classification of Communities NSW State Government, October 2007
23. Sydney Water Corporation Cooks River Flood Study Engineering and Planning Services PB MWH Joint Venture February 2009
24. City of Botany Bay Mascot, Roseberry and Eastlakes Flood Study WMAwater, May 2015
25. NSW State Government Floodplain Development Manual April 2005
26. IPCC Climate Change 2014: Synthesis Report, Contribution of Working Groups I, II, III to the Fifth Assessment Report of the Intergovernmental Panel of Climate Change 2014
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FIGURE 1 CATCHMENTS AND PREVIOUS STUDIES
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S A L O B
B N
N T F A A N Y
E O I
BIRDS GULLYBIRDS ROAD BUNNERONG AND 1 I
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($1 J:\Jobs\116083\Admin\CommunityConsultation\Responses\BirdsGully_Questionnaire_Responses.xlsx FIGURE 5A COMMUNITY CONSULTATION RESPONSES Residents Contactable
No response, 59, 40% Yes, 84, 57%
No, 5, 3%
Period Periodof Living/Owning/Working Living at Current Address on Property 60
50
40
30
49 No. of RespondentsNo. 20 No. of RespondentsNo. 30 27 10 13 14 14 0 0 Question 3 Less than 5 5 to 10 10 to 15 15 to 30 Greater than 30 No response YearsYears
Property Type
Other, 1, 1% No Response, 3, 2% Business, 6, 4%
Residential, 140, 93% FIGUREFigure XX5B COMMUNITY CONSULTATION RESPONSES
Properties Affected by Flooding 120
100
80
60 103
No. ResponsesNo. 40
20 45
2 0 Yes No No Response Properties Flooded Above Floor Level
No Response, 1, 2%
No, 20, 46% Yes, 23, 52%
Properties that Experienced Flood Damage No Response, 8, 4%
No, 5, 11%
Yes, 35, 78% FIGURE 5C COMMUNITY CONSULTATION RESPONSES Residents who Observed Water Flowing Overland during Storm Events in their Neighbourhood 100 90
80 70 60 50 86
No. of RespondentsNo. 40 30 52 20 10 11 0 Yes No No Response
Where neighbourhood flooding observed, water was above floor Level
No Response, 4, 8%
Yes, 25, 50% No, 21, 42%
Where neighbourhood flooding observed, damaged was caused as a result
Yes, 18, 37% No Response, 23, 47%
No, 8, 16% FIGURE 6 BIRDS GULLY AND BUNNERONG ROAD
Moverly Road LIDAR DATA DIGITAL ELEVATION MODEL
t e e tr S y e r o t S
H o w a rd S t r e e MAROUBRA t
Avoca Street RANDWICK
Rainbow Street e Parad c F Anza i t t z Maroubra Road
e g
e e
r r t a
S l d
h A g i v
H e
n
u
e
MATRAVILLE
HILLSDALE
KINGSFORD Bu nnero ng Road B anks A venue
d EASTGARDENS a Ro y n B ta anks o PAGEWOOD Ave B nue DACEYVILLE e u n ve A B th e r a o u tw c d n ha p Roa e m W ´
Catchment Boundary Elevation (mAHD) High : 70 Low : 0
0 0.25 0.5 1 1.5 2 Km J:\Jobs\116083\Arc\Maps\ReportFigures\Stage2\Figure06_DEM.mxd FIGURE 7 Dover Heights BIRDS GULLY AND BUNNERONG ROAD (Portland St) RAINFALL GAUGES
Rose Bay (Royal Sydney Golf Club)
COOGEE Little Bay Malabar (The Coast STP Golf Club) Mosmon Council Randwick (Randwick MAROUBRA St) RANDWICK Centennial Park Randwick MATRAVILLE Paddington Racecourse Randwick HILLSDALE (composite Racecourse KINGSFORD site) Eastlakes EASTGARDENS Sydney SW DepotDACEYVILLE PAGEWOOD Botanic Gardens
Sydney (Observatory Hill)
Erskineville Bowling Club
Sydney Airport AMO
Lilyfield Marrickville Kyeemagh Bowling Bowling Club Bowling Club Club
Five Dock SPS065 Marrickville Ashfield Golf Club Ashfield (Ashfield Park Sans Souci Bowling Bowling Club) (Public Club School) ´
Canterbury Study Area Racecourse Bexley Pluviometer Gauges Canterbury Bowling Daily Gauges Concord Greenlees Racecourse Club BC (formerly AWS Wests Rugby Club) 0 1.5 3 6 Km © Department of Finance, Services & Innovation 2017 J:\Jobs\116083\Arc\Maps\ReportFigures\Stage2\Figure07_Rainfall_Gauges.mxd 1000 Eastlakes SW Depot Paddington (composite site) Malabar STP Randwick Racecourse Erskineville Bowling
1% AEP 2% AEP 5% AEP 10% AEP
20% AEP
50% AEP 100 1EY
Intensity (mm/h) Intensity
10 APRIL1998
BURSTINTENSITIES ANDFREQUENCIES
FLOOD EVENT
1
30m 1hr 2hr 3hr 4.5hr 6hr 9hr 12hr 18hr 24hr 48hr 72hr 8 FIGURE
Burst duration
1000 Randwick Racecourse
Malabar STP
Erskineville Bowling Club
1% AEP 2% AEP 5% AEP 10% AEP
20% AEP
50% AEP 100 1EY
Intensity (mm/h) Intensity
10 JANUARY
BURSTINTENSITIES ANDFREQUENCIES 1999 FLOOD EVENT
1
30m 1hr 2hr 3hr 4.5hr 6hr 9hr 12hr 18hr 24hr 48hr 72hr 9 FIGURE
Burst duration
1000 Eastlakes SW Depot Paddington (composite site) Malabar STP Randwick Racecourse Erskineville Bowling
1% AEP 2% AEP 5% AEP 10% AEP
20% AEP
50% AEP 100 1EY
Intensity (mm/h) Intensity
10 MARCH
BURSTINTENSITIES ANDFREQUENCIES 2014 FLOOD EVENT
1 FIGURE 10 FIGURE 30m 1hr 2hr 3hr 4.5hr 6hr 9hr 12hr 18hr 24hr 48hr 72hr
Burst duration
1000 Eastlakes SW Depot Paddington (composite site) Malabar STP Randwick Racecourse Erskineville Bowling
1% AEP 2% AEP 5% AEP 10% AEP
20% AEP
50% AEP 100 1EY
Intensity (mm/h) Intensity
10 DECEMBER 2015 FLOOD EVENT
BURSTINTENSITIES ANDFREQUENCIES
1 FIGURE 11 FIGURE 30m 1hr 2hr 3hr 4.5hr 6hr 9hr 12hr 18hr 24hr 48hr 72hr
Burst duration
D E
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L A N S I T O G L E T R D P A I AU V YN V PD T E LS L B Y E N HUME ST
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O AV T High Street H W B AN R N E D BASS ST T AN
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G RD ENDEAVOUR RD E EASTGARDENS
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D E O S N Y C DENISON ST y CAST RD O B A A ER A LE O P Y n V V DONCASTER BARKER L SO K E M D R D S N ST a
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AV A AY L s C K Av B S e H M ST n e R
D u B A DON R e AN O M A CAS PAGEWOOD K u R D TER A DORAN E S P R B V AV n R MOORAMIE AV A COURT AV N ASTROLABE RD DACEYVILLE E R e Y E D W v CO N T D TE D C NHAM ISAA A A V R
AV A E ST h T A A LEONARD AV H
Y MIT rt O D S B G o B e MINYAAV tw a D n u R e c EAS h p Road TER S a m N AV COTT W ENHA R V M AV E V A N A
E H
Y EASTERN AV D C
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R TUN D R A
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S MAITLAND AV E SWC Trink Drainage R
T Birds Gully Bunnerong to Tasman Sea Bunnerong to Botany Bay Birds Gully Catchment (1.7km²) Bunnerong Road Catchment (8.2km³) SO UTHER N CROS S DR 0 0.25 0.5 1 1.5 2 Km J:\Jobs\116083\Arc\Maps\ReportFigures\Stage2\Figure12_Trunk_Drain_Assets.mxd FIGURE 13 BIRDS GULLY AND BUNNERONG ROAD Moverly Road HYDROLOGIC MODEL SUBCATCHMENTS
t e e tr
y S e r o t S
H o w a rd S t r e e MAROUBRA t
Avoca Street RANDWICK
Rainbow Street e Parad Anzac F i t t z Maroubra Road
e g
e e
r r t a
S l d
h
A g i v
H e
n
u
e
MATRAVILLE
HILLSDALE
KINGSFORD Bu nnero ng Road B anks A venue
d EASTGARDENS a o R y n B ta anks o PAGEWOOD Ave B nue DACEYVILLE e u n ve A B th e r a o u tw c d n ha p Roa e m W ´
Study Area Subcatchments
0 0.25 0.5 1 1.5 2 Km J:\Jobs\116083\Arc\Maps\ReportFigures\Stage2\Figure14_Hydrologic_Model_Subcatchments.mxd FIGURE 14 BIRDS GULLY AND BUNNERONG ROAD Moverly Road HYDRAULIC MODEL BOUNDARY AND DRAINAGE
t e e tr S y e r o t S
H o w a rd S t r e e MAROUBRA t
Avoca Street RANDWICK
Rainbow Street e Parad c F Anza i t t z Maroubra Road
e g
e e
r r t a
S l d
h A g i v
H e
n
u
e
MATRAVILLE
HILLSDALE
KINGSFORD Bu nnero ng Road B anks A venue
d EASTGARDENS a Ro y n B ta anks o PAGEWOOD Ave B nue DACEYVILLE e u n ve A B th e r a o u tw c d n ha p Roa e m W ´
1D boundary Condition 2D HQ Boundary 2D HT Boundary Pipe Open Channel Hydraulic Model Boudary
0 0.25 0.5 1 1.5 2 Km J:\Jobs\116083\Arc\Maps\ReportFigures\Stage2\Figure14_HydraulicModelBoundary.mxd FIGURE 15 BIRDS GULLY AND BUNNERONG ROAD HYDRAULIC MODEL Move rly Road MANNINGS 'N' ROUGHNESS
t e e tr S
y
re o t S
Avoca Street RANDWICK
Rainbow Street e Parad c F Anza i t z Maroubra Road g
e
r
a
l d
High Street A
v
e
n
u
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MATRAVILLE HILLSDALE
Bu nnero
ng Road e
Bank u
s Aven n u e
e Beauchamp Road v
A
h
t
r
o w
t d
n a o B e anks A R ven W y ue n ta o DACEYVILLE PAGEWOOD B ´
Study Area Mannings 'n' Rougness Roads Urban Residential Open Space Industrial Infrastructure Barracks
0 0.25 0.5 1 1.5 2 Km J:\Jobs\116083\Arc\Maps\ReportFigures\Stage2\Figure16_Mannings_n_Roughness.mxd FIGURE 16 BIRDS GULLY AND BUNNERONG ROAD HYDRAULIC MODEL Move rly Road NULL BUILDINGS AND GUTTERS
t e e tr S
y
re o t S
Avoca Street RANDWICK
Rainbow Street e Parad c F Anza i t z Maroubra Road g
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r
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l d
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v
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h
A
v
e
n
u e
Ban ks d Ave a DACEYVILLE nu o PAGEWOOD e R y n ta o B ´
Study Area Buildings Gutters_polyline
0 0.25 0.5 1 1.5 2 Km J:\Jobs\116083\Arc\Maps\ReportFigures\Stage2\Figure17_HydraulicModel_NullBuidlings_Gutters.mxd FIGURE 17 BIRDS GULLY AND BUNNERONG ROAD TUCABIA ST
A HYDRAUIC MODEL
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ST T O T E
S N S M D T H T V P Y ELLEN A A S M S E A U ND O D A T S COO GALE RD H M I PE R J E AR O ST H T ST R C F A S W M ST A O L A E T U G O T GALVIN ST ETHEL A T ST I R S S M S T S T P R C DIJARMA E G O U L AH O D C P S E S R E O K ST BUDJAN ST T L O G E S R S D C N E N T E R O L P O U M IVY D O U CREER ST D G RAINBOW ST BANBURAANG O EDGAR ST A N A L J D P
U N CLA CA ST R L T R N KE BE B NA RRA M GA A S U ST R RD STOREY ST A D AA E T G N L S ALMA RD R ST T R I GA L L SE S RD AEO U EN B O T ST BOYCE RD U PER D D S GREEN ST KA R R G O AR MAROUBRA RA A DE ST N R Y S ES T G M W L ARRE A M T EAST L HENNEGAR ST T U R S DINE ST O T M LION L ST D R H S AMIENS O T R R L VE BY GL T H LOCH L N SOUDAN S A WY DINE L O E GREEN ST D BUNDOCK L Y R MAREE ST Y L M E HELENA L R A MO RA E GARDEN L I WEST L D N G B R BE OW L IAN ST ANZAC ST GALUAV R N NU O O RSES TITANIA L GUS L T R ROBEY ST WY R AVO NORTH A FE
PDE PDE PERONNE U C D CA W LA R ST S TH RA IVE R ON R ST BENVENUE ST ST ANZAC S S I A E B U
BEN ENU S
V T G U Y S
S STRE ER L E
E R
K O E T A
R
RANDWICK A ANZAC C E TB G AVOCA ST PDE R F H RIGNEY AV N A I AV C GAINFORD O HO T M SP HOLMES ST D IT PL B H AL RD S AV HOSPITAL YO R LE EURIMBLA AV UNG S R EVERETT E NAG T E S
ROAD U B L T PICCADILLY
G PAT E T O T B N ST PA ST A INES
O A TO P R PA T N A B S RUNIC L AN ST ST T G O'SULLIVAN AV Y S T S T T JAN ON R T S E Y HAY L S S H T P MAGILL E T H AN R T S S T N A O MI A T N HANNAN H T ST E S BY S 1 S A T R R S
1 D D T
S ST AV T O H W A GATE 10 L A B N R N BASS ST T AN E T V R ST N O U E O S L RT S O C N L BOTANY ST B T N
D B S T N S Y FISCHER ST E D I R D I I AV K ST A D T B R W LE O C RVINE M TA A I ST OU NY R A ON P L O N N V T
T L P I
V S Z SNAPE ST
A D S WALLACE L S DIVE ST H H MASON ST
E OV
A E KEN E A
D H N NED C HO V Y ISIS L L S R L D T A L A E E E K NN R N O I N EDY S L O PER YAL T WALSH AV STEWART T W C ST STURT L U N M IVA F S KENNEDY L ST T S A O AVE
E E A N JERSEY L
R HI D V
V N
L A KL L B A B E
A R I MATRAVILLE P V S
S I T A E E L L L W
L EY R L L L W R
E B G O E W D T L T T TA O A RD S I D C S H S N L A I AY CA Y M K B D AV R P RD STURT ST W E RD S T T R WALLACE ST A N GALE RD BUNNERONG I M R T T BA T D S A C S T DEVITT PL E N L V I W T M S JERSEY S ILLI A BOYCE RD T A S L D L E D HILLSDALE A EY T R G PARE N F F N R R L O O R RSY R ER ER D JA
TH NN S TI UN V
ST C C U O P E E U B R W T G Y ER R C K ST UNS JAMES PL A ST T O N T U B N JENNINGS ST MEEKS L WALLACE B J A S I H C I R O B L AM SO C ST O DL RO R JENNINGS R L A F R ST ULM ST EW DE L D RK E A F N U W R A A TH ST LA O H N A R I E W O R
N T E R M LEMAN R FLACK S
N V
E N T D D C
KINGSFORD A P N
E E B U I
HAROLD ST
U D A S C B
M R
N C T
C AVE K
L N B IT L E R D B R E U
´ R S
MIDDLE R O N E E P A W R G R B M E IN D I A L C T O A
A E F V I RHOD R ES R T ST LL D U Y L S FRASER E O CIR N T H BRITTAINCR D D C S N R B G A AVE NK S H S R E A No Data Available A T C VE E R N H A DE W W GRACE AN S M OL H TO P MCC S L A MPBELL CR AUL RD IL IG CA R E SWC Trink Drainage W AV A D T Y E E S S W E NILSON AVE T G Y AV D Birds Gully ENDEAVOUR RD EASTGARDENS DENISON R R R Y D R V R C E N E O DENISON ST ST A A A OL P Y USS OK M D T BO AV BA DENISON ST N O Bunnerong to Tasman Sea E NKS O B AVE
Bunnerong to Botany Bay D DACEYVILLE R PAGEWOOD Y AC N Study Area ISA A H ST T SMIT O Circular Pipes - Diameter (m) B 0.3 - 0.6 0.6 - 1.2 1.2 - 1.8 Rectangular Pipes - Width (m) 0.3 - 0.6 0.6 - 1.2 1.2 - 1.8 1.8 - 3.0 3.0 - 5.45
0 0.25 0.5 1 1.5 2 Km J:\Jobs\116083\Arc\Maps\ReportFigures\Stage2\Figure17_TUFLOW_Pit_Pipe_Data.mxd FIGURE 18A HISTORICAL RAINFALL EVENTS RAINFALL HYETOGRAPHS
120 Malabar STP - January 1999 Event
100
80
60 Rainfall Intensity Rainfall Intensity (mm/hr)
40
20
0
100 Erskineville Bowling Club - January 1999 Event
80
60 Rainfall Intensity Rainfall Intensity (mm/hr)
40
20
0 FIGURE 18B HISTORICAL RAINFALL EVENTS RAINFALL HYETOGRAPHS
180 Malabar STP - 25 March 2014 Event 160
140
120
100
80
60 Rainfall Intensity Rainfall Intensity (mm/hr)
40
20
0
120
Eastlakes SW Depot - 25 March 2014 Event
100
80
60
40 Rainfall Intensity Rainfall Intensity (mm/hr)
20
0 FIGURE 18C HISTORICAL RAINFALL EVENTS RAINFALL HYETOGRAPHS
140 Malabar STP - 17 December 2015 Event
120
100
80
60
40 Rainfall Intensity Rainfall Intensity (mm/hr)
20
0
140
Randwick Racecourse - 17 December 2015 Event 120
100
80
60
Rainfall Intensity Rainfall Intensity (mm/hr) 40
20
0
J:\Jobs\116083\Arc\Maps\ReportFigures\Stage2\Figure20_17_December_2015_Storm_Rainfall.mxd 20
(! St) (Randwick Randwick (! Racecourse Randwick
Racecourse Randwick 30 (! RANDWICK COOGEE KINGSFORD
(! Depot SW Lakes East 40
DACEYVILLE
70
50 60
PAGEWOOD 80
EASTGARDENS 90
MAROUBRA 100 0 HILLSDALE 0.25 0.5
(! STP Malabar MATRAVILLE BIRDS GULLYBIRDS ROAD BUNNERONG AND 17 DECEMBER 2015 STORM EVENT EVENT STORM 2015 DECEMBER 17 1 Rainfall Distribution (mm) Distribution Rainfall ( ! Low : 2 : Low 104 : High Study Area Gauges Rainfall RAINFALL ISOHEYTS 1.5 FIGURE 19 19 FIGURE 2 Km ´ J:\Jobs\116083\Arc\Maps\ReportFigures\Stage2\Figure21_25_March_2014_Storm_Rainfall.mxd
(! St) (Randwick Randwick (! Racecourse Randwick
(! Racecourse Randwick
30 RANDWICK
COOGEE 60 KINGSFORD
(! Depot SW Lakes East 40 DACEYVILLE
50 PAGEWOOD EASTGARDENS MAROUBRA 0 HILLSDALE 0.25 0.5
(! STP Malabar MATRAVILLE BIRDS GULLYBIRDS ROAD BUNNERONG AND 1 25 MARCH 2014 STORM EVENT EVENT STORM 2014 MARCH 25 Rainfall Distribution (mm) Distribution Rainfall ( ! Low : 6 : Low 64 : High Study Area Gauges Rainfall RAINFALL ISOHEYTS 1.5 FIGURE 20 20 FIGURE 2 Km ´ FIGURE 21 BIRDS GULLY AND BUNNERONG ROAD FLOOD LEVEL REPORT LOCATION
H_27
H_37
H_48
H_25 H_46
H_34 H_26 H_35 H_20 H_36 H_18 H_16 H_39 H_15 H_45
H_38 H_21 H_19 H_32 H_28 H_10 H_41 H_22
H_44 H_13 H 47 H_4 H_29 H_40 H_17 H_42 H_14 H_11 H_8 H_31 H_33 H_24 H_9 H_7 ´ H_23 H_12 H_3 H_6 H_5 H_2
H_30
H_1 Study Area Level/depth Location 100Y Depth (m) 0 - 0.15 0.15 - 0.25 0.25 - 0.5 0.5 - 1.0 > 1.0
0 0.25 0.5 1 1.5 2 Km J:\Jobs\116083\Arc\Maps\ReportFigures\Stage2\Figure21_Report_Location.mxd FIGURE 22 BIRDS GULLY AND BUNNERONG ROAD PEAK FLOW REPORT LOCATION
Q_15
Q_50 Q_49
Q_34 Q_51 Q_18
Q_25 Q_14 Q_22 Q_23 Q_19 Q_24 Q_4 Q_47
Q_42 Q_17
Q_16 Q_46 Q_3 Q_6 Q_26 Q_5 Q_45 Q_20 Q_41 Q_7 Q_2 Q_1 Q_8 Q_5 Q_40 Q_35 Q_33 Q_32 Q_31 Q_21 Q_30 Q_39 Q_44
Q_38 ´ Q_11 Q_48 Q_29 Q_36 Q_10 Q_52 Q_12 Q_13 Q_36 Q_28 Q_37 Q_9 Q_43
Study Area Q_27 Comment Flow Location 100Y Depth (m) 0 - 0.15 0.15 - 0.25 0.25 - 0.5 0.5 - 1.0 > 1.0
0 0.25 0.5 1 1.5 2 Km J:\Jobs\116083\Arc\Maps\ReportFigures\Stage2\Figure22_Report_Location.mxd FIGURE 23 BIRDS GULLY AND BUNNERONG ROAD PIPE CAPACITY ASSESSMENT ´
Study Area Full in the 1 EY Full in the 0.5 EY Full in the 20% AEP Full in the 10% AEP Full in the 5% AEP Full in the 2% AEP Full in the 1% AEP Not Full up to the 1% AEP
0 0.25 0.5 1 1.5 2 Km J:\Jobs\116083\Arc\Maps\ReportFigures\Stage2\Figure23_Stormwater_Capacity.mxd
Birds Gully & Bunnerong Road Flood Study
APPENDIX A. Glossary
Taken from the Floodplain Development Manual (April 2005 edition)
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 sea (AHD) level. Average Annual Damage Depending on its size (or severity), each flood will cause a different amount of flood (AAD) damage to a flood prone area. AAD is the average damage per year that would occur in a nominated development situation from flooding over a very long period of time. Average Recurrence Interval The long term average number of years between the occurrence of a flood as big (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. catchment The land area draining through the main stream, as well as tributary streams, to a particular site. It always relates to an area above a specific location. consent authority The Council, Government agency or person having the function to determine a development application for land use under the EP&A Act. The consent authority is most often the Council, however legislation or an EPI may specify a Minister or public authority (other than a Council), or the Director General of DIPNR, as having the function to determine an application. development Is defined in Part 4 of the Environmental Planning and Assessment Act (EP&A Act). infill development: refers to the development of vacant blocks of land that are generally surrounded by developed properties and is permissible under the current zoning of the land. Conditions such as minimum floor levels may be imposed on infill development. new development: refers to development of a completely different nature to that associated with the former land use. For example, the urban subdivision of an area previously used for rural purposes. New developments involve rezoning and typically require major extensions of existing urban services, such as roads, water supply, sewerage and electric power. redevelopment: refers to rebuilding in an area. For example, as urban areas age, it may become necessary to demolish and reconstruct buildings on a relatively large scale. Redevelopment generally does not require either rezoning or major extensions to urban services. 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). 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.
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flash flooding Flooding which is sudden and unexpected. It is often caused by sudden local or nearby heavy rainfall. Often defined as flooding which peaks within six hours of the causative rain. flood Relatively high stream flow which overtops the natural or artificial banks in any part of a stream, river, estuary, lake or dam, and/or local overland flooding associated with major drainage before entering a watercourse, and/or coastal inundation resulting from super-elevated sea levels and/or waves overtopping coastline defences excluding tsunami. flood awareness Flood awareness is an appreciation of the likely effects of flooding and a knowledge of the relevant flood warning, response and evacuation procedures. flood education Flood education seeks to provide information to raise awareness of the flood problem so as to enable individuals to understand how to manage themselves an their property in response to flood warnings and in a flood event. It invokes a state of flood readiness. flood fringe areas The remaining area of flood prone land after floodway and flood storage areas have been defined. flood liable land Is synonymous with flood prone land (i.e. land susceptible to flooding by the probable maximum flood (PMF) event). Note that the term flood liable land covers the whole of the floodplain, not just that part below the flood planning level (see flood planning area). flood mitigation standard The average recurrence interval of the flood, selected as part of the floodplain risk management process that forms the basis for physical works to modify the impacts of flooding. floodplain Area of land which is subject to inundation by floods up to and including the probable maximum flood event, that is, flood prone land. floodplain risk management The measures that might be feasible for the management of a particular area of the options floodplain. Preparation of a floodplain risk management plan requires a detailed evaluation of floodplain risk management options. floodplain risk management A management plan developed in accordance with the principles and guidelines in plan this manual. Usually includes both written and diagrammatic information describing how particular areas of flood prone land are to be used and managed to achieve defined objectives. 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 “flood liable land” concept in the 1986 Manual. Flood Planning Levels (FPLs) FPL’s are the combinations of flood levels (derived from significant historical flood events or floods of specific AEPs) and freeboards selected for floodplain risk management purposes, as determined in management studies and incorporated in management plans. FPLs supersede the “standard 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
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floods. Flood risk in this manual is divided into 3 types, existing, future and continuing risks. They are described below. existing flood risk: the risk a community is exposed to as a result of its location on the floodplain. future flood risk: the risk a community may be exposed to as a result of new development on the floodplain. continuing flood risk: the risk a community is exposed to after floodplain risk management measures have been implemented. For a town protected by levees, the continuing flood risk is the consequences of the levees being overtopped. For an area without any floodplain risk management measures, the continuing flood risk is simply the existence of its flood exposure. flood storage areas Those parts of the floodplain that are important for the temporary storage of floodwaters during the passage of a flood. The extent and behaviour of flood storage areas may change with flood severity, and loss of flood storage can increase the severity of flood impacts by reducing natural flood attenuation. Hence, it is necessary to investigate a range of flood sizes before defining flood storage areas. floodway areas Those areas of the floodplain where a significant discharge of water occurs during floods. They are often aligned with naturally defined channels. Floodways are areas that, even if only partially blocked, would cause a significant redistribution of flood flows, or a significant increase in flood levels. freeboard Freeboard provides reasonable certainty that the risk exposure selected in deciding on a particular flood chosen as the basis for the FPL is actually provided. It is a factor of safety typically used in relation to the setting of floor levels, levee crest levels, etc. Freeboard is included in the flood planning level. 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. minor, moderate and major Both the State Emergency Service and the Bureau of Meteorology use the following flooding 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. Probable Maximum The PMP is the greatest depth of precipitation for a given duration meteorologically Precipitation (PMP) 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 “water 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.
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