Hawkesbury Floodplain Risk Management Study & Plan

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

Medium Priority Measures
<ul>
<li>Item 2(b): Enhance Emergency Management Assessment Tools</li>
<li>Item 5: Study of Voluntary House Raising and Redevelopment Options</li>
<li>Item 6: Assessment of a Levee and Refuge Mound for McGraths Hill</li>
</ul>

Low Priority Measures
<ul>
<li>Item 2(g): Provide additional crossing of South Creek</li>
<li>Item 7: Update Flood Behaviour Data for Valley</li>
</ul>

Tags: Hawkesbury, Hawkesbury River
Organization: Hawkesbury City Council