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 of how easily (quickly) each measure can be implemented and on value for money. The timing of the proposed works will depend on Council’s overall budgetary commitments, and the availability of funds from other sources.

PRIORITISED MEASURES

High Priority Measures

• Item 1: Community Flood Education
• Item 2(a): Dual Outbound Lanes on Jim Anderson Bridge
• Item 2(c): Construction of Community Refuges
• Item 2(d): Flood Emergency Plans for Special Uses and Utilities
• Item 2(e): Flood Emergency Plan Templates for Caravan Parks
• Item 2(f): Review and Update Hawkesbury?Nepean Flood Emergency Sub Plan and NSW State Flood Sub Plan (Annex C)
• Item 2(i): Investigation of Road Duplication Options at the Eastern End of the Jim Anderson Bridge
• Item 3(a): Provision of Evacuation Risk Advice for Existing Development Proposals
• Item 4: A range of Town Planning Measures including revisions to the Flood Risk chapter of Council’s DCP

Medium Priority Measures

• Item 2(b): Enhance Emergency Management Assessment Tools
• Item 5: Study of Voluntary House Raising and Redevelopment Options
• Item 6: Assessment of a Levee and Refuge Mound for McGraths Hill

Low Priority Measures

• Item 2(g): Provide additional crossing of South Creek
• Item 7: Update Flood Behaviour Data for Valley