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.