Levees are flood mitigation structures which aim to reduce flood damages and enhance public safety, by creating a barrier between floodwaters and elements at risk. However, unless designed for Probable Maximum Flood (PMF) conditions, all levees will potentially be overtopped. They may also fail through lack of maintenance, inadequate construction or unforeseen circumstances.

The NSW State Emergency Service (SES) is the government agency responsible for controlling the response to floods in NSW. Its combat role is comprehensive, encompassing floodplain risk management, community education for floods, flood planning and flood response. Specifically, the SES has a responsibility to collect, collate, assess and distribute information in regards to floods in NSW. This information is commonly called ‘Flood Intelligence’. The SES maintains a large Flood Intelligence database, detailing flood consequences which occur at key gauge heights for approximately 300 flood gauges state wide. This information is used to assist the SES in conducting flood planning, flood response and community education.

The aim of this project was to undertake a desktop review of existing studies and data to produce a report detailing levee information requested by the SES, for significant urban levees in NSW. These levees are listed in Appendix A. The SES will use this report to audit and update its flood intelligence.

The information required by the SES for each identified levee location is summarised below:

• Description of each levee, detailing the location, construction type and the communities protected.
• The name, identification number and gauge zero in metres (m) to Australian Height Datum (AHD) for the flood warning gauge relevant to each levee.
• The levee design height, overtopping heights of levee low points, levee spillway heights and imminent failure height relative to the relevant flood warning gauge, and the Annual Exceedance Probability (AEP) of the respective heights.
• Likely locations of levee overtopping and the sequence of overtopping and flooding. Size of the population, the number of residential and commercial properties, and critical infrastructure affected by levee overtopping or failure.
• Scope for additional development in areas protected by levees, considering the size of available zoned land.
• The height relative to the relevant flood warning gauge at which backwater flooding commences, impacting upon urban areas behind each levee and the pattern of inundation.
• Once overtopped, the length of time taken to fill the basin area behind each levee and the pattern (evolution) and behaviour of inundation.
• Details of ground profile (topography) inside each levee relative to the relevant flood warning gauge, and the location of high ground suitable for evacuation.
• Location of any parts of each levee which need to be closed (such as gates for roadways and railways), and the height relative to the relevant flood warning gauge that action must be completed by.
• Knowledge of any critical issues including structural integrity affecting each levee.

The project has been undertaken in two stages. Stage 1 consisted primarily of a review of available information sources, whilst Stage 2 consisted of collecting, analysing and compiling actual data for each levee. This report documents the methodology and outcomes from the investigation. Key recommendations to improve and build upon the present study in the future are also provided.