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.

Separate Flood studies have been undertaken for four coastal lagoons in the Gosford area, that is, Avoca, Cockrone, Terrigal and Wamberal Lagoons. Different analytical approaches have been used for Avoca-Cockrone and Terrigal-Wamberal Lagoons. The physical processes involved in the break-out of the lagoons through the beach are complex. Thus, there is a potential for the different analytical processes to produce significantly different results. For this study, separate MIKE-11 models were established for Terrigal-Wamberal Lagoons. These models were calibrated against the documented openings of August and September 1993. MIKE-11 models have been previously established for Avoca and Cockrone Lagoons as part of the Flood Studies. The Avoca Lagoon hydraulic model established for the flood study was modified to remove the source of numerical noise which resulted in the over-estimation of the volume of water discharged to the ocean during the opening calibration event. The re-calibration did not alter the parameters for initial breakout of the lagoon. The amendment of channel lengths in the Avoca model was also undertaken to remove a minor over-estimation of storage. The revised hydraulic model produced an estimated one percent AEP design flood level 20mm lower than that obtained in the flood study. This variation in estimated flood levels is not considered significant. It is noted that the critical duration storm (that is, the design storm that produces the greatest water level in the lagoon) was the 9 hour storm for each lagoon. The design flood level estimates for the one percent AEP flood in Terrigal Lagoon and Wamberal Lagoon determined by the review hydraulic model were within 30mm of the flood study estimates. This can be considered to provide independent verification of the flood study estimates.

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

The main objective of this study (in two parts) has been to determine water levels in Brisbane Water for the full range of flood and ocean events that can occur due to the various natural physical processes, taking joint occurrence issues into account to some extent. The flooding behaviour can be influenced, either separately, or by a combination of, catchment rainfall runoff flooding, rainfall directly onto Brisbane Water, elevated ocean levels, local winds and, to a minor extent, the condition of the ocean entrance channel near Ettalong. The outcomes from this study include: -,

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 Gloucester and Avon Rivers Flood Study has been prepared for Gloucester Shire Council (Council) to define the existing flood behaviour in the catchment and establish the basis for subsequent floodplain management activities. This project has been implemented through the Gloucester Water Study Project. The primary objective of the Flood Study is to define the flood behaviour within the Gloucester and Avon Rivers catchment through the establishment of appropriate numerical models. The study has produced information on flood flows, velocities, levels and extents for a range of flood event magnitudes under existing catchment and floodplain conditions. Specifically, the study incorporates: Compilation and review of existing information pertinent to the study and acquisition of additional data including survey as required; Development and calibration of appropriate hydrologic and hydraulic models; Determination of design flood conditions for a range of design event including the 50% AEP, 20% AEP, 10% AEP, 5% AEP, 2% AEP, 1% AEP, 0.5% AEP, 0.2% AEP and PMF event; and Presentation of study methodology, results and findings in a comprehensive report incorporating appropriate flood mapping.

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.

Storage Volumes: The modelling showed that the approach developed for Terrigal and Wamberal Lagoons could be satisfactorily applied to Avoca Lagoon for the opening of 14 September 1993. In discussions with Paterson Consultants it became apparent that the storage volume of the Lagoon was critical for establishing the peak outflow from the Lagoon. A considerable effort was undertaken in this regard which revealed that a table of height/area of Avoca Lagoon was in error at a level of I.0 m. In addition it was determined that the Mike-II and RUBICON models determine different storage capacities of the lagoon. There was insufficient time to completely resolve this issue, however preliminary work suggested that the Mike- 11 model was over-calculating storage. The differences in storage make no significant difference to the peak flood level. Comparison of Results: For Wamberal, Avoca and Cockrone Lagoons the peak flood levels from both approaches should be very similar. This is because the berm height is the major control and the peak flood level is only just (0.3m) above the berm level. For Terrigal Lagoon the differences in the two approaches are likely to be more significant as the beach berm is much lower (2.5m AHD) and the peak flood level approximately 0.5m higher than the berm. Additional Work: It is recommended that data be collected for future openings. If the data are suitable they should be used in further verification of the hydraulic models.

This study investigates the hydraulic effectiveness of the current levee, assess the need for future improvements and also attempt to establish the flood overtopping levels of the levee, for the benefit of emergency services.

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.