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).

INTRODUCTION The Tabourie Lake Flood Study has been prepared for Shoalhaven City Council (Council) to define the existing flood behaviour in the Tabourie Lake catchment and establish the basis for subsequent floodplain management activities. This project has been conducted under the State Assisted Floodplain Management Program and received State financial support.

This report documents the Burrill Lake Floodplain Risk Management Study and Plan which investigates and presents a flood risk management strategy for the catchment. The study identifies the existing flooding characteristics and canvasses various measures to mitigate the effects of flooding. The end product is the Floodplain Management Plan, which describes how flood liable lands within the Burrill Lake catchment are to be managed in the future.

Final Report

The Flood Study had three main components:,

The Flood Study comprises two volumes: Volume 1: (i.e., this document) comprises the report text and appendices; Volume 2: contains all accompanying report figures

This report presents the results of a detailed technical investigation of flooding in the Scotts Creek catchment and has been sponsored by Willoughby City Council (WCC) and Department of Environment and Climate Change. Figure 1.1 shows the location of the catchment which drains the suburbs of Chatswood and Castle Cove before discharging to Sugarloaf Bay in Middle Harbour. The investigation defined flooding as far downstream as the Eastern Valley Way. Mathematical models of the catchment and the floodplain were developed using detailed field surveys and interpreted to present a comprehensive picture of flooding under present day conditions. The study objective was to define flood behaviour in the stream in terms of flows, levels and flooding behaviour for floods ranging between 5 and 200 years average recurrence interval (ARI), as well as the PMF. The investigation involved hydrologic modelling to assess flows in Scotts Creek between its headwaters in the Chatswood CBD area and the Eastern Valley Way. These flows were applied to a hydraulic model of the main arm of Scotts Creek to assess peak water levels and flow patterns. The hydraulic modelling extended from Havilah Street on the eastern side of Chatswood Chase to the Eastern Valley Way. The trunk drainage system of Scotts Creek to Penshurst Street is denoted Stormwater Catchment 26 and is owned by Sydney Water. It comprises sections of piped drains, as well as covered and open concrete lined stormwater channels. The piped drainage system extends through the Chatswood CBD to the eastern side of Chatswood Chase in Havilah Street (Figure 1.2). Between Havilah Street and Baldry Street, flows on the main arm of Scotts Creek are conveyed in Sydney Water’s covered stormwater channel which follows the route of the original creek and extends as far as the Temple Emanuel School near Chatswood Avenue. An open concrete lined stormwater channel also owned by Sydney Water continues downstream and conveys flows to Penshurst Street. Willoughby City Council’s channel continues downstream of Penshurst Street to the Eastern Valley Way, and comprises sections of rock lined and “pillow concrete” lined trapezoidal channel. The total catchment area at the Eastern Valley Way is 3 km2. The headwaters of the catchment are drained by separate piped trunk drainage systems which drain the northern and southern portions of the catchment. Overland flows from the southern trunk drain are diverted into Chatswood Oval, which functions as an off-line detention basin during periods of heavy rainfall. Peak flows from the northern portion of the catchment will be influenced by a flood storage/ water harvesting basin which will be excavated adjacent to Ferguson Lane as part of the Chatswood Civic Place project, due for construction in 2009 and situated just upstream of the intersection of that street with Archer Street. The storage characteristics of both Chatswood Oval and the flood storage basin in Ferguson Lane have been included in the hydrologic model of Scotts Creek developed in this study for the estimation of design flows. The trunk drainage systems meet at the intersection of Archer Street and Victoria Avenue and continue eastwards as separate piped systems along both the northern and southern sides of Victoria Avenue to Havilah Street. The drainage system is of limited hydraulic capacity and has been surcharged, most recently in April 1998, when flooding was experienced in several commercial properties bordering Victoria Avenue. It was reported that overland flows heading eastwards along Victoria Avenue in the April 1998 flood were captured by the prevailing natural surface grade and flowed into the basement car park of Chatswood Chase via the driveway entrance near the intersection with Havilah Street. Overland flows heading eastwards down Mills Lane also entered Chatswood Chase. The car park then acted as a detention basin and had a local influence on downstream flows in the creek. To include the

This document constitutes the first and second stages of the management process for the North Creek catchment. It presents a compilation of the data collected and has defined flood behaviour and flood risk for the catchment area. This study was commissioned under the 2005 NSW Floodplain Development Manual (Reference 1), however, it is recognised that the 2023 Flood Risk Management Manual (Reference 2) was gazetted shortly after the project commenced. While the study was undertaken in accordance with the 2005 manual, there are elements that are consistent to both the 2005 and 2023 manuals. Where appropriate, the 2023 manual is referenced where project methodology or outputs are consistent with the new 2023 manual

Report is two parts:,

This report details the results and findings of the Flood Study investigations. The key elements include: • a description of the study area; • a summary of available historical flood related data; • establishment and calibration of the hydrologic and hydraulic models; • the estimation of design flood behaviour for existing catchment conditions; • sensitivity analysis of the model results to variation of input parameters; • potential implications of climate change projections; and • identification of the level of flood risk for individual properties in the catchment.