The overall objective of this study is to develop a Floodplain Risk Management Study where management issues are assessed, management options are investigated and recommendations are made. Thereafter a Floodplain Risk Management Plan detailing how flood prone land within the study area is to be managed can be completed. The objectives of the Flood Risk Management Study are to:,

The objectives of the Flood Study are to: • Identify all the flood-related data by searching all relevant data sources. • Determine the likely extent and nature of flooding and identify potential hydraulic controls by carrying out detailed site visits of the study area. • Define existing catchment condition flood behaviour for mainstream flooding in the catchment with due consideration to the impact of Lake Macquarie levels on flooding characteristics. • Define design flood levels, velocities and flow distributions for the catchment. • Define the extent of flooding for the 200 year, 100 year, 20 year, 10 year and 5 year ARI floods and Probable Maximum Flood (PMF) for the catchment. • Define Provisional Flood Hazard for the flood-affected areas. • Define the Hydraulic Categories for the flood-affected areas. Two numerical modelling tools were developed: • A hydrologic model to convert rainfall on the catchment into runoff. The hydrologic model combines rainfall information with local catchment characteristics to estimate runoff hydrographs. • A hydraulic model to convert runoff hydrographs into water levels and velocities throughout the study area. The model simulates the hydraulic behaviour of the water within the study area by accounting for flow in the major channels as well as all the potential overland flowpaths, which develop when the capacity of the channels is exceeded. It relies on boundary conditions, which include the runoff hydrographs produced by the hydrologic model and the appropriate downstream boundary level from Lake Macquarie.

The objectives of this study are: to define the nature and extent of the flood hazard, to identify, assess and optimise strategies and measures aimed at reducing the impact of flooding on both existing and future development, to determine the effects of development on flooding, to determine flood damages.

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.

In view of the increasing catchment development and the need to accurately define the flood problem, Lake Macquarie City Council engaged Webb, McKeown & Associates, to undertake a Flood Study. The primary objectives of this Flood Study are: • to define the flood behaviour of the North Creek catchment by producing information on flood levels, velocities and flows for a range of design flood events under existing catchment and floodplain conditions, • to assess the hydraulic categories and undertake provisional flood hazard mapping, • to assess the extent of the flood problem by undertaking a damages assessment, • to formulate suitable hydrologic/hydraulic models that can be used in a subsequent Floodplain Risk Management Study to assess various floodplain management measures, including the effects of further development. This report details the results and findings of the Flood Study investigations. The key elements include: • a summary of available data, • reasons for the choice of hydrologic and hydraulic models, • calibration of these models, • establishment of design flood behaviour, • flood damages assessment. The Flood Study does not consider flooding from local drainage which may result from inadequate urban drainage provisions.

This Flood Study comprised the following principal stages: collection and assessment of available data; investigation of coastal processes to determine design ocean levels for tailwater boundary conditions for the hydraulic model and to determine ocean storm inflows into the lagoon; development and calibration of an hydrologic model to convert design storm rainfalls into design flood hydrographs; and development and calibration of an hydraulic model to determine design flood levels in the lagoon and on the floodplain. Available data comprises: Council records of lagoon openings (the most comprehensive available); Department of Land and Water Conservation (DLWC) Rainfall data at Kincumber Sewage Treatment Plant; and Bureau of Meteorology Daily Rainfall Records. The initial flood studies, outlined in this report, indicated significant sections of the Three Points Avenue area of MacMasters Beach would be flood liable. Ground survey of individual properties showed that the established model was not sufficiently refine to accurately represent the separate flow paths. An individual flood study was undertaken of the drain running parallel to Newell Road and Three Points Avenue. The study report appears as Addendum A to this document, while the results have been included on Figure 10.

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.

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.

AIMS OF THE CURRENT STUDY In broad terms, the current Floodplain Risk Management Study has investigated what can be done to minimise the effects of flooding in the Haslams Creek catchment and recommended a strategy in the form of a draft Floodplain Risk Management Plan. Specific objectives of the study include: a review of the results from the 1999 Flood Study; consideration of the potential for culvert blockages and associated flood impacts; the implementation of a community consultation strategy, to ensure community input is obtained at key times throughout the study; a description and quantification of the flood problems in the Haslams Creek catchment including the likely cost of flooding to the local community; the identification and assessment of potential floodplain risk management measures to reduce the risks and hazards of flooding; a detailed review of issues relating to planning and development controls within the floodplain. Don Fox Planning Pty Ltd has prepared a document entitled Report on Planning Issues as part of this study. It is reproduced in Appendix A. Included in this report, is a proposed draft Development Control Plan (DCP) entitled “Managing Our Flood Risks”; the development of a recommended draft Floodplain Risk Management Plan for the Haslams Creek catchment that outlines the best measures to reduce flood damage, based on environmental, social, economic, financial and engineering considerations.

Flood study centres around the opening of Cockrone Lagoon to the ocean when the water level exceeds the let-out-level of RL2.476m AHD. The policy is aimed at minimising the flooding of properties adjacent to the lagoon. 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