Large map of the Lower Hawkesbury region illustrating inundation with contour and depth markings.

Flood Study Overview, Flood Study Report, 12 technical volumes, 4 map book volumes, Engagement Outcomes Report

Atlas map (42 components) showcasing the Hornsby Local Government Area. Details include: Overland flow path Overland parcels Hawkesbury flood extent Hawkesbury parcels Cadastre Atlas map (42 components) showcasing the Hornsby Local Government Area. Details include: Overland flow path Overland parcels Hawkesbury flood extent Hawkesbury parcels Cadastre

This report was prepared by Webb McKeown and Associates on behalf of Hawkesbury City Council (HCC) and details the engineering investigations carried out to determine the design flood levels, flows …Show full descriptionThis report was prepared by Webb McKeown and Associates on behalf of Hawkesbury City Council (HCC) and details the engineering investigations carried out to determine the design flood levels, flows and velocities for the Lower Macdonald River floodplain. It represents the first step in the process to develop a Floodplain Risk Management Plan for the study area. The Macdonald River catchment is a tributary of the Hawkesbury River and drains an area of approximately 1845 km². Significant flooding has occurred in the past including a severe event in 1949. All available topographic, rainfall and flood data were collected and analysed as part of the study. To extend the existing data sets, Council undertook detailed survey to provide cross-section information and to define the heights of historical flood levels provided by local residents. A WBNM hydrologic model was established to represent the entire catchment draining to the Hawkesbury River. A MIKE-11 hydraulic model was created to represent the Lower Macdonald River downstream of the confluence with Womerah Creek. The lower reaches of Wrights Creek were also included in the hydraulic model. The hydrologic and hydraulic models were calibrated making best use of available historical data to ensure that they reasonably simulated recorded historical floods. The models were calibrated to the March 1978 flood and verified against the August 1990 event. For both models, parameter values found to be applicable in previous studies of similar catchments and those from established texts were used to determine appropriate values for the present study. In addition, a flood frequency analysis was undertaken on the streamflow estimates obtained from the gauge located on the Macdonald River at St Albans. The adopted set of design flows were used to define inflow hydrographs to the hydraulic model. The lower reaches of the Macdonald River are influenced by a combination of flows entering from the upper catchment in addition to elevated water levels associated with concurrent flooding in the Hawkesbury River. Design flood information was therefore obtained using the calibrated hydraulic model in conjunction with a range of tailwater conditions. The main outcomes of this study are documented in this report and include: a full description of the methodology and results, the reporting of design flood information including the preparation of flood information maps for the Lower Macdonald River catchment. The information contained herein is suitable for the preparation of a subsequent Floodplain Risk Management Study and Plan. A glossary of flood related terms is provided in Appendix A.

This is a Floodplain Risk Management Study and Plan for the Hawkesbury River floodplain within the Hawkesbury LGA.\r The study has been commissioned by Hawkesbury City Council with financial assistance from the Office of Environment and Heritage (OEH). The study has been prepared by Council’s Floodplain Risk Management Advisory Committee with the assistance of specialist floodplain risk management consultants.\r \r The study's documentation comprises three volumes:\r \r \x02* #Volume 1 \x03 Main Report\r * #Volume 2 \x03 Town Planning Issues; and\r * #Volume 3 \x03 Flood Maps and Annotated Bibliography of Reviewed Reports\r \r

Reasons for the Study \r

The Hawkesbury \x01Nepean Valley has one of the most significant flood risk exposures within Australia. The risks to both property and people resulting from flooding in the Valley have been recognised for some time. Following the establishment of the Hawkesbury-Nepean Flood Management Advisory Committee in 1997, the Hawkesbury-Nepean Floodplain Management Strategy (HNFMS) was prepared under the guidance of the State Government. The Strategy was developed to enable all levels of government and the wider community to recognise more fully and respond more appropriately to the flood risks in the Valley.\r \r An important outcome of the HNFMS was a Regional Floodplain Management Study which provided tools to assist each council in the Valley develop its own local floodplain risk management plans. This included preparation of a flood hazard definition tool and a set of best practice guidelines covering land use planning, subdivision and building on flood prone land.\r \r The Hawkesbury Floodplain Risk Management Study and Plan builds on the significant work done at the regional level. It advances local floodplain management initiatives including the revision of local planning policies and the provision of advice to Hawkesbury City Council and the State Government concerning the evacuation risk exposure of future development proposals.

The Hawkesbury-Nepean River Flood Study (May 2024) identifies areas in the Valley affected by flooding from the Hawkesbury-Nepean River (including backwater flooding up tributaries) and assesses the potential impacts of climate change on flooding. The study area falls mainly within the Penrith, Hawkesbury, Blacktown and The Hills Local Government Areas but also includes parts of Wollondilly, Liverpool, Hornsby and Central Coast. It accounts for flows from the entire 21,400km2 Hawkesbury-Nepean catchment and provides detailed flood information for the 190km length of the Hawkesbury-Nepean River from Bents Basin near Wallacia through to Brooklyn. Councils and other agencies reviewed the draft study (2023), and 2 independent reviews were also conducted. All comments were carefully considered prior to finalisation. The Study, including all technical data and map books, is considered the most up-to-date and reliable source of flood information for the Hawkesbury-Nepean River and should be used to inform all flood-related decision making from June 2024.

This report is a compendium of data for the August, 1986 storm event as it affected the lower Hawkesbury River. The area considered is shown on the map of Fig.1 in the report, and includes the catchment areas of Nepean, Grose, Colo and McDonald Rivers and Webbs , South and Mangrove Creeks.