Hawkesbury-Nepean Development Dilemma

The recent flooding of Brisbane has raised the question of why so much development has occurred below the 1974 flood level and revived knowledge of the much higher 1893 flood event.

Can NSW see a similar flooding disaster?

The prime responsibility for planning and management of flood prone lands in New South Wales (NSW) rests with local government (councils). The NSW Government’s Flood Prone Land Policy and the 2005 Floodplain Development Manual form the basis of floodplain management in NSW.

The NSW Government assists councils on state-wide policy issues and provides specialist technical support. Financial assistance is also provided to undertake flood and floodplain risk management studies, and for the implementation of works identified in these studies.

As outlined in the Floodplain Management Manual, the designated flood (or flood standard) is the flood level selected for planning purposes and directly determines the area of land subjected to flood-related building and development controls. The level of the designated flood also directly determines by how much the local community will be exposed to the risk of flooding. Over the past 10-20 years, communities have become 'comfortable' with the 100 year average recurrence interval (ARI) flood as their designated flood. However, this has led to a 'false sense of security' that bigger floods are not possible. This is important in the Hawkesbury-Nepean valley, particularly at Windsor, where the probable maximum flood (PMF) would be more than 9m higher than the 100 year flood. The highest known flood, deemed to be about a 250 year ARI flood, occurred in 1867 and is about 2.4m above the 100 year ARI flood level. This will result in flood water up to the ceiling of future houses built to the current minimum required floor level.

The extent of the 1867 flooding in the Hawkesbury-Nepean valley is available at: http://www.ses.nsw.gov.au/multiversions/2533/FileName/Map_of_1867_Flood_for_the_Hawkesbury-Nepean_Region.pdf.

In 1997 the Hawkesbury-Nepean Flood Management Advisory Committee published its Hawkesbury-Nepean floodplain management strategy. The report lists that in a repeat of the 1867 flood, the following would result:

• 200 sq kms of catchment would be inundated by flood waters
• 40,000 people would need to be evacuated
• 6,000 people needing alternative accommodation for up to 12 months with perhaps 4,500 needing medical treatment
• almost 7,000 houses would be inundated
• perhaps 2,000 houses destroyed
• Direct damage costs could be around $1.4 billion with total costs possibly being as high as $2.5 billion.

Since 1997 extensive infill development has occurred and new precincts such as at Riverstone have been developed. The current total liability to damage from an 1867 flood is not publicly known.