Main Report; Appendix A - Flood Mapping; Appendix B - Community Consultation; Appendix C - Design Hydrographs

This report forms the Flood Study for Newport. It documents flood behaviour across the catchment for a range of historic and design floods. This includes information on flood discharges, levels, depths and flow velocities. It also provides estimates of the variation in flood hazard and hydraulic categories across the catchment and provides an assessment of the potential impacts of climate change on existing flood behaviour. The flood study comprises two volumes: Volume 1 contains the report text and appendices, Volume 2: contains all figures/maps.

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:,

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:,

This Floodplain Risk Management Study and Plan for Mona Vale/Bayview has been undertaken by Cardno Lawson Treloar for Pittwater Council to identify and examine options for the management of flooding within the Mona Vale Main Drain and Cahill Creek floodplain. The study has been undertaken in accordance with the NSW Government Floodplain Development Manual (2005). A locality plan can be found in Figure 1.1. The outline of the study area can be found in Figure 1.2. Study Context This study forms two of the multiple stages of the Floodplain Management process which includes:,

This report comprises an overview of the work that has been undertaken by WMAwater on the Manly Lagoon Floodplain Risk Management Study and Plan. It builds on prior stage reports and, in accordance with the Brief, it additionally includes a discussion of: • hydraulic and hazard categorisation; • future development scenarios; • review of climate change; • flood damages assessment; and • emergency management.

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

Objective This Floodplain Management Study defines the nature and extent of the flood hazard in the three catchments. It also identifies and assesses strategies and measures aimed at reducing the impact of flooding on both existing and future development, and measures to prevent future development from making flooding worse. The results of this Floodplain Management Study are being used to prepare a Floodplain Management Plan (FPMP). The objective of the FPMP is to establish a cost-effective floodplain management strategy for the area and to recommend a programme for implementation of the Plan. The Study Brief is attached as Appendix A. What is in the Study The Floodplain Management Study and Plan will not totally eliminate flooding. Flooding is a natural phenomenon which cannot be fully controlled by human works. In fact, excessive artificial controls are likely to be very costly and to have adverse environmental effects. Therefore the Floodplain Management Study and Plan aim to provide a package of best management practices which give a balance between reducing flood hazard and flood damages, allowing appropriate development, and protecting and enhancing the environment of the floodplain. This Report has been divided into the following sections: Section 1: Introduction (page 1) Establishes the context and basis for the plan. Section 2: Background (page 5) Describes the study process, including previous studies, and the background technical information which is available. It includes a review of the previous Muddy Creek Flood Studies. Section 3: Existing Flood Behaviour (page 15) Summarises the available data and modelling results concerning flood behaviour. Section 4: Social and Economic Impacts of Flooding (page 23) Examines the potential flood damages and other social and economic effects of flooding for a range of flood events, including an extreme flood. Section 5: Choosing the Designated Flood (page 28) Discusses the issued to be considered by Council in choosing a designated flood for Rockdale, and recommends that the 1% AEP flood be adopted. Section 6: Available Floodplain Management Options (page 35) Describes in broad terms the range of flood mitigation and other management works and other measures available to address the existing, future and residual flooding problems in the study area. Sections 7, 8 and 9: Options for Management Areas (page 48) These three sections describe in detail, for the Muddy Creek, Spring St Drain and Scarborough Ponds catchments respectively, the feasible options and their likely benefits and costs. Section 10: Assessment of Management Options (page 86) Reviews the financial aspects and the justification for the proposed management measures, identifies priorities and possible sources of funding. These issues will be further addressed in the future Floodplain Management Plan. Section 11: References (page 90) Technical detail which supports the main report is provided in the Appendices, in Volume 2.

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:,

The investigation process used in this flood study was: - collection of available data; - assessment of available data; - overview of lagoon behaviour; - assessment of coastal processes; - hydrologic modelling of the catchment to determine flood inflows; and - hydraulic modelling of the lagoon to assess flood levels from flood inflows.