The Flood Study had three main components:,

This report presents the results of an investigation of flooding in the Sailors Bay Creek catchment and has been jointly sponsored by Willoughby City Council (WCC) and the NSW Government, via the Office of Environment and Heritage (OEH). Figure 1.1 shows the location of the catchment, which drains residential and commercial areas in the suburbs of Willoughby, Northbridge and Castlecrag before discharging to Sailors Bay in Middle Harbour. The study objective was to define flood behaviour in terms of flows, water levels and flooding patterns for floods ranging between 5 and 100 year ARI, as well as for the PMF. The investigation involved rainfall-runoff hydrologic modelling of the catchment and drainage system to assess flows in the Sailors Bay Creek drainage system, and application of these flows to a hydraulic model of the main arm of Sailors Bay Creek and its overland flow paths to assess peak water levels and flow patterns. The model results were interpreted to present a detailed picture of flooding under present day conditions. The scope of the study included investigation of both main stream flood behaviour along the main arm of Sailors Bay Creek as well as overland flooding throughout the study area which occurs either as a result of surcharges of the piped drainage system or upstream of the commencement of the formal drainage system. Flooding in the lower reaches of the study area as a result of storm-driven elevated harbour water levels was also investigated. The study forms the first step in the floodplain risk management process for the Sailors Bay Creek catchment (refer process diagram presented in the Foreword) , and is a precursor of the future FRMS sponsored by WCC which will consider the impacts of flooding on existing and future urban development, as well as potential flood mitigation and management measures. Note that the results of the present study supersede those presented in the Overland Flooding Investigation undertaken for the whole of the Willoughby City Local Government Area (LGA) (L&A, 2009). The work undertaken in that study is summarised in Chapter 2.

Wingecarribee Shire Council engaged Catchment Simulations Solutions to prepare a Flood Study for the Nattai River catchment. The extent of the catchment is shown in Figure 1 in report (refer Flood Study: Volume 2). The objectives of the Nattai River Catchment Flood Study are:,

WBNM

The study was commissioned by Bayside Council, with the assistance of the NSW Government Office of Environment and Heritage (OEH). Additional information has been provided by Sydney Water Corporation (SWC). The Flood Study comprises the development of computational hydrologic and hydraulic models that define design flood behaviour for the 20% AEP (0.2 EY), 10% AEP, 5% AEP, 1% AEP and 0.5% Annual Exceedance Probability (AEP) design storms and the Probable Maximum Flood (PMF) in the Bardwell Creek and Wolli Creek catchments.

TUFLOW

This study updates the previous Wrights Creek Flood Study (Patterson Britton & Partners, 2007) and provides Council with a suitable platform for undertaking the next steps in the Floodplain Risk Management Process. The XP-RAFTS hydrologic model developed in the 2007 Flood Study has been adopted for this study. A TUFLOW hydraulic model of the catchment has been established using up to date survey of the ground levels and culvert and bridge structures. The TUFLOW hydraulic model has been used to reproduce the historical flood behaviour from events in 1995 and 2002 and to define flood behaviour for a range of design events. Actions undertaken as part of this flood study include: • collection of additional survey, • development of a TUFLOW hydraulic model, • definition of the design flood behaviour, • assessment of provisional hydraulic hazard, • assessment of hydraulic categorization, • assessment of the model sensitivity to parameter changes, • assessment of climate change impacts on model results, • assessment of sensitivity of model results to the new ARR 2016 design inputs, • community consultation.

Complete report

Volume 1 Report This flood study has been undertaken to determine the flood behaviour in the catchment due to local storm runoff for the 0.5% Annual Exceedance Probability (AEP), 1% AEP, 2% AEP, 5% AEP, 10% AEP, 20% AEP, 50% AEP and 100% AEP flood events and the Probable Maximum Flood (PMF). In accordance with its objectives, the study has determined the nature and extent of flooding through the estimation of design flood flows, levels and velocities. Flood impacts due to storm events in the Brisbane Water estuary are detailed in the Brisbane Water Foreshore Flood Study (2009). In undertaking the flood study, a hydrologic-hydraulic computer model of the major channels and floodplain within the catchment was established and verified against historical flood event observations. The hydraulic model was then used with design rainfall conditions to simulate design flood behaviour in the catchment. The study has defined Provisional Flood Hazard and Hydraulic Categories for the flood affected areas. The objective of this Study is to define the nature of the existing flood behaviour due to local runoff only in the Davistown catchment. To achieve the objectives, the following tasks were undertaken:,

Chapter 2 contains background information including a brief description of the study catchment and its drainage system, details of previous flooding investigations, a summary of community consultation undertaken as part of this present study (refer Appendix A for details), and a brief history of flooding within the catchment. Chapter 3 deals with the hydrology of the Lane Cove River and Swaines Creek catchments, and describes the development of the RORB and DRAINS hydrologic models which were used to generate discharge hydrographs for input to the hydraulic model. Chapter 4 deals with the development of the TUFLOW hydraulic model which was used to analyse flood behaviour in the study area. Chapter 5 deals with the derivation of design discharge hydrographs, which involved the determination of design storm rainfall depths over the catchments for a range of storm durations and conversion of the rainfalls to discharge hydrographs. Chapter 6 details the results of the hydraulic modelling of the design floods. Results are presented as water surface profiles and plans showing indicative extents of inundation for a range of design flood events up to and including the PMF. A provisional assessment of flood hazard and hydraulic categorisation is also presented. (The assessment of flood hazard according to velocity and depth of floodwaters is necessarily “provisional”, pending a more detailed assessment which includes other flood related criteria, to be undertaken during the future FRMS.) The results of various sensitivity studies undertaken using the TUFLOW model are also presented, including the effects of changes in hydraulic roughness, partial blockage of the piped stormwater system, and potential increases in rainfall intensities and sea levels due to future climate change. This chapter also deals with the selection of Interim FPL’s for the study area. Chapter 7 contains a list of references. Chapter 8 contains a list of flood-related terminology that is relevant to the scope of the study. Appendix A provides details of the collection of historic flood data and describes the testing of the hydrologic and hydraulic models. Figures referred to in both the main report and the appendices are bound in a separate volume of the report (refer Volume 2).