Conclusions Provided below is a summary of the key findings of the Flood Study, in particular some of the important considerations for future floodplain risk management in the catchment:,

Warringah Council has identified a need to prepare a drainage and flood mitigation plan for the Dee Why Town Centre. The plan is needed to accompany the Dee Why Town Centre Masterplan and Public Domain Improvements Plan that has been prepared to guide future redevelopment of the town centre. Council recognised that its existing 1D hydraulic model of the Dee Why CBD has limitations and instead decided to upgrade the floodplain model to a1D/2D model to provide a clearer understanding of the flood characteristics in and around Dee Why CBD. In view of Council’s aim to better understand overland flooding of the Dee Why CDB, it was decided to assemble both a SOBEK model and an xpswmm2D model of the study area to allow the estimated overland flood depths, velocities of flow and hazards to be compared and to identify any significant differences in overland flooding within the study area. The SOBEK model was assembled based on a 1m x 1m grid of surface elevations that was created using 12D and the ALS data supplied by Council. Buildings were represented by raising the surface elevations of all buildings within the building outlines around 10 m higher than the surrounding terrain. The xpswmm2D model of the Dee Why CBD was also based on the ALS data supplied by Council. The MapInfo building outline was imported into xpswmm2D and was used to “null” the grid points within the building outlines. The “nulling” of these grid points eliminated them from the computation scheme on the basis that flow was excluded from buildings. Both models were run for both the 100 yr ARI and PMF events using the local inflow hydrographs obtained from Council’s MIKE-11 model of the Dee Why CBD. Overland flood depths, velocities, velocity x depth and provisional flood hazards in the 100 yr ARI and PMF events were estimated in the Dee Why CBD. It was concluded that the SOBEK model and xpswmm2D model estimate similar 100 yr ARI and PMF overland flood levels and patterns of flow. It was concluded that around 60% of the estimated 100 yr ARI flood levels adjacent to properties are equal to or lower than the 100 yr ARI flood levels currently adopted by Council. Conversely around 40% of the estimated 100 yr ARI flood levels are higher than the 100 yr ARI flood levels currently adopted by Council. 10% of the estimated 100 yr ARI flood levels adjacent to properties are more than 0.25 m lower than currently adopted by Council. A further 10% of the estimated 100 yr ARI flood levels adjacent to properties are more than 0.33 m higher than currently adopted by Council. It was noted that overland flows in sections of Oaks Avenue, Howard Avenue, Dee Why Parade and Avon Road exceed the velocity x depth criterion for vehicular stability. Similarly the sections of the overland flowpath located above the trunk drainage line between Oaks Avenue and Howard Avenue and between Dee Why Parade and Richmond Avenue also exceed the velocity x depth criterion for pedestrian and vehicular stability. Similarly these areas were also assessed to have a provisional high hazard rating. It was concluded that around 80% of the estimated PMF flood levels adjacent to properties are equal to or lower than the PMF flood levels currently adopted by Council. Conversely around 20% of the estimated PMF flood levels are higher than the PMF flood levels currently adopted by Council. 10% of the estimated PMF flood levels adjacent to properties are more than around 0.6 m lower than currently adopted by Council. A further 10% of the estimated PMF flood levels adjacent to properties are more than 0.16 m higher than currently adopted by Council. It was noted that the sections of Oaks Avenue, Howard Avenue, Dee Why Parade and Avon Road that exceed the velocity x depth criterion for vehicular stability significantly increase in a PMF. Similarly almost all the overland flowpath located above the trunk drainage line between Oaks Avenue and Richmond Avenue also exceed the velocity x depth criterion for pedestrian and

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:,

The Raymond Terrace Flood Study has been prepared for Port Stephens Council (Council) to define the existing flood behaviour in the western Raymond Terrace catchments and establish the basis for subsequent floodplain management activities. The study will also assist Council in considering future upgrades to the drainage network and flood relief pump capacity to the Hunter River. This project has been conducted under the State Assisted Floodplain Management Program and received State financial support.

The Flood Study had three main components:,

South Creek is a 7.3 km2 catchment which drains to the southern shore of Narrabeen Lagoon through Cromer Golf Course (Figures 1, 2 and 3). The catchment area is predominantly occupied by urban development (59%) including both residential and commercial/light industrial development, with the remainder comprised of open space and forested areas (41%). The catchment has two main tributary branches. South Creek is the major watercourse and Wheeler Creek is the main tributary draining a significant proportion of the western catchment. In view of the increasing urban development and the need to accurately define the nature and extent of the flood problem, Warringah Council engaged Webb, McKeown & Associates to undertake a Flood Study. The primary objectives of this Flood Study are:,

Study Context The preparation of this plan follows on from the Flood Study (WMA, 2006) and Floodplain Risk Management Study (Cardno Lawson Treloar, 2007a) and forms the fifth stage of the Floodplain Risk Management Process which includes:,

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.

The key objective of this Flood Study is to develop a suitable hydrologic/hydraulic model that can project flood and permanent inundation water levels in Lake Macquarie from rainfall, sea level rise and storm surge. These results will be used by Lake Macquarie City Council, in consultation with the community of Lake Macquarie City, to manage flood and permanent inundation risks to low lying land around the Lake Macquarie waterway. The key stages in the process are: Undertaken a comprehensive review of the 1998 Lake Macquarie Flood Study (Part 1 – Reference 1) and develop suitable hydrologic/hydraulic models to define flood behaviour over the full range of design events for existing catchment conditions, Use the hydrologic/hydraulic models to assess various climate change scenarios, including application of the NSW Government’s sea level rise benchmarks, Assess the potential increase in storm surge as a result of climate change and its impact on elevated ocean levels, Review the potential impact of climate change on the local wind/wave climate as this affects the extent of wave runup on the foreshore, Assess the hydraulic and hazard categories for existing and climate change conditions. This report details the results and findings of the above investigations. The key elements include: a summary of available historical flood related data, establishment of the hydrologic and hydraulic models, calibration of the hydrologic and hydraulic models, definition of the design flood behaviour for existing catchment conditions, sensitivity analysis of the design flood behaviour, assessment of the impacts of climate change on the still water and wave runup water levels re-definition of the flood extent and hydraulic and hazard categories mapping for existing and climate change conditions. A Flood Study is a technical document and not easily understood by the general public. A glossary of flood related terms is provided in Appendix A to assist. If more explanation of terms or a better understanding of the approach is required, type “NSW Government Floodplain Development Manual” into an internet search engine and you will be directed to the NSW Government web site which provides a copy of this manual and further explanation. Flood levels given in this report relate only to the water level with the lake itself. Design water levels in the creek systems entering the lake (Cockle Creek, Dora Creek etc.) will be higher than those shown for the lake.

The Narrabeen Lagoon Flood Study has been prepared for Warringah and Pittwater Councils (The Councils) to define the existing flood behaviour in the Narrabeen Lagoon catchment and establish the basis for subsequent floodplain management activities. This study updates previous studies on the Lagoon including the Narrabeen Lagoon Flood Study (PWD, 1990) and studies of the individual tributary streams, providing a holistic assessment of flooding within the catchment. The current Flood Study considers land use changes subsequent to previous modelling investigations, the influence of the Narrabeen Lagoon entrance on flood behaviour and the influence of potential climate change. The primary objective of this Flood Study is to define the flood behaviour under historical, existing and future conditions (incorporating potential impacts of climate change) in the Narrabeen Lagoon catchment for a full range of design flood events. The study provides information on flood levels and depths, velocities, flows, hydraulic categories and provisional hazard categories. The Flood Study has also identified the impact on flood behaviour as a result of future climate change and potential changes in the catchment and lagoon entrance. Specifically, the study incorporates:,