The Hawkesbury-Nepean River Flood Study (May 2024) provides high-resolution spatial and temporal data identifying areas in the Hawkesbury-Nepean Valley affected by flooding, including backwater inundation of tributaries, and assesses the potential impacts of climate change on flood behaviour. The study covers the 21,400km² catchment and the 190km river length from Bents Basin near Wallacia to Brooklyn, encompassing Local Government Areas such as Penrith, Hawkesbury, Blacktown, The Hills, Wollondilly, Liverpool, Hornsby, and Central Coast. This dataset includes outputs from state-of-the-art hydrologic and hydraulic modelling, offering detailed flood information on peak levels, depths, velocities, extents, and hazard classifications for events ranging from frequent floods to the probable maximum flood (PMF).

A 17-page overview of the purpose and methods used for developing the flood study.

This report describes the hydraulic modelling for the first stage of a flood study of the Hawkesbury-Nepean Valley commissioned by the Water Board in 1987. The study is required in order to "determine the effects that various arrangements of major or future works for Warragamba Dam have on flooding in the valley" (Revised Brief for the Preparation of an E.I.S., Water Boards August 1987). Stage 1 required a simplified model to be developed, calibrated and used to provide a measure of the relative effects on flooding of options for such works. The model used is described in Section 2. Section 3 describes the calibration phase, in which three historical floods were used to adjust model parameters in order to match historical and modelled flood heights. The various options for Warragamba Dam flood protection works have an impact on the outflow hydrographs from the dam. Outflow hydrographs were determined for each option for five "design" floods and run through the calibrated hydraulic model. Peak flood heights were then tabulated and compared at three key locations In the valley. Section 4 describes the options while Section 5 discusses the modelling methods used to run these cases. The results are presented as a series of tables and graphs.

The study area of the Hawkesbury Floodplain Risk Management Study & Plan comprises all of the Hawkesbury River and its immediate surrounds that fall within the Hawkesbury Local Government Area. It extends from Agnes Banks/Yarramundi in the south to Wisemans Ferry in the north, representing a river distance of approximately 83 km and an area of some 220 km2 subject to inundation in the Probable Maximum Flood (PMF). The main area of focus is for the area from Agnes Banks/Yarramundi to Wilberforce, including the flood?prone communities of Richmond and Windsor (see Figure 1.2). The Hawkesbury?Nepean catchment covers about 22,500 km2 and is one of the largest of all coastal rivers in New South Wales (see Figure 1.3). It includes extensive grazing areas in the south?west and large National Parks in the Blue Mountains to the north?west. Urban development in the catchment area includes towns such as Goulburn and Lithgow and outer suburbs of western Sydney including Camden and Penrith (ERM Mitchell McCotter, 1995). More than 40% of the total Hawkesbury?Nepean catchment – about 9,000 km2 – is upstream of Warragamba Dam. Half of this area comes from the Wollondilly River. The Warragamba River joins the Nepean River 3.5 km below the dam. The Grose River is a major tributary which joins the Nepean at Yarramundi, after which the Nepean is known as the Hawkesbury. Whilst the Grose has a catchment of only 650 km2, it drains a high rainfall area and can have a significant effect on flooding at Windsor. In particular, it can cause flood levels to rise quickly in the early part of major storms (ERM Mitchell McCotter, 1995). The catchment area at the Windsor gauge is about 12,800 km2. South Creek joins just downstream of the Windsor gauge. Whilst its catchment area of 640 km2 is virtually the same as the Grose, it receives less rainfall and thus has less impact on Hawkesbury River flooding. At Lower Portland the Hawkesbury is joined by the Colo River, which drains an area of 4,640 km2 (ERM Mitchell McCotter, 1995). The Colo can influence flooding in the Hawkesbury River depending on the movement of flood producing rainfall over the Hawkesbury and Colo River catchments. The Colo has a shorter response time to rainfall and as shown in the 1978 flood, it can have a large impact on Hawkesbury River levels, particularly downstream of Sackville. A study of the joint probabilities of floods originating from the Hawkesbury and the Colo has been carried out (AWACS, 1997). AWACS found that the 100 year design flood levels in the Hawkesbury downstream of the Colo confluence were relatively insensitive to the assumed Colo contributions. Nevertheless in some events, flooding in the Hawkesbury River within the lower portions of the study area can be significantly influenced by the Colo subject to the spatial and temporal distribution of the rainfall. When measured in 2000, the Hawkesbury River was subject to tidal influence up to Yarramundi Bridge (MHL, 2005). However, the limit of tidal influence is rarely constant. There are short?term cyclical changes in response to the ever?changing ocean tides, and changes over long time spans according to both natural processes and artificial disturbance. Sand extraction in the vicinity of the limit of tidal influence in the Hawkesbury River is reported to have caused the tidal limit to move a further 10 km upstream over the 20th century (Estuaries Branch, 2010). RECOMMENDATIONS The Floodplain Risk Management Plan (FRMP) showing the preferred floodplain risk management measures for the Hawkesbury study area is presented in this chapter. The recommended measures have been selected from the range of measures discussed in Chapter 6, after an assessment of each measure’s impact on flood risk, as well as consideration of environmental, social, and economic factors. The recommended measures are presented in Table 9.1. The principal components of the Plan are presented below according to priority, which is assessed on the basis

Hydrological model outputs for all design events.

Post-event Flood Behaviour Analysis for the Richmond River - February 2022 Event URBS Hydrology Model

Post-event Flood Behaviour Analysis for the Richmond River - February 2022 Event Hydraulic Model

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

The Post 2022 Event Flood Behaviour Analysis - Brunswick River aims to understand the areas impacted by the recent 2022 flood event and determin the current approved flood model's ability to replicate the 2022 event. The study area includes the townships of Mullumbimby, Brunswick Heads, Ocean Shores, and villages of New Brighton, South Golden Beach and Billinudgel. The study area covers an area of approximately 230 square kilometres, and includes diverse environments, generally comprising of dense vegetation in the hinterlands and coastal villages to the east of the catchment. The Post 2022 Event Flood Behaviour Analysis of the Brunswick River involved a review of the hydrological and hydraulic models, and IFD estimates. Flood Frequency Analysis and Regional Flood Frequency Estimation methods were undertaken to assess the model perfomance. A damage assessment was undertaken to determine the cost of damage and the degree of disruption to the community caused duing flooding event.