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.

Calculations have shown that the adoption of a 100 year ARI flood as the designated flood in the Hawkesbury-Nepean valley would cause the average annual flood damage bill to increase by 2½ times within 50 years to $100 million each year. If a PMF is adopted as the designated flood, there would be a minimal increase in future flood risk, but large areas of land would be affected by planning and development controls. However, a flood damage assessment and adoption of a particular designated flood, makes up only one component of a much larger equation of 'non-flooding' issues.

Therefore, the issue of flood risk in the Hawkesbury-Nepean Valley should ideally be tackled with a total catchment management approach, with cooperation among all councils and the community. It is not unreasonable to expect compliance with the Hawkesbury-Nepean Floodplain Management Strategy (1997) prepared by the Hawkesbury-Nepean Floodplain Management Committee and as a minimum similar emergency response outcomes required in the above documentation should be required to be undertaken before occupation of new growth centre release areas.

The recent opening up of the North West and South West sectors of Sydney to greatly increased development will potentially expose possibly thousands of homes and businesses to one of the most devastating floodplains in Australia, the Hawkesbury-Nepean River and its tributaries.

If the NSW State Government and strategic land use planners are to learn a lesson from the horrors, distress and cost of the floods of Brisbane and its surrounding areas, it is that flood risk is related not just to the chance of a severe flood occurring but also to the consequences thereof. The 2011 Brisbane flood was not a “flood of record”, that flood was some 2m higher than the 1974 flood; nor was the 2011 flood as high as the 1974 flood. Indeed, the 2011 flood event at Brisbane is considered to be less than a 50 yr ARI flood event, yet major damage has occurred. The financial and social impacts of a flood disaster must be fully considered and appropriate planning undertaken.

If today’s politicians and planners do not wish to be condemned by tomorrow’s victims of destructive floods, they must learn that floodplains deserve respect and appropriate planning. At the very least, when building in the floodplain, resilience measures should be mandatory; where developments are allowed which require evacuation in flood events then appropriate notation on all planning certificates and individual property floodsafe plans must be compulsory. Resilience measures are well known and documented. Planners need to be decidedly flood aware when zoning land, approving sub-division design and in requiring flood-compatible designs for dwellings and buildings even well above the current planning levels. A set of three guidelines covering these aspects should be available at most council libraries within the Hawkesbury area and can be found on the internet at:

Reducing Vulnerability of Buildings to Flood Damage: Guidance on Building in Flood Prone Areas

Designing Safer Subdivisions: Guidance on Subdivision Design in Flood Prone Areas

Managing FLood Risk through Planning Opportunities: Guidance on Land Use Planning in Flood Prone Areas

These are simple measures which if adopted will add a small cost to development but provide significant reductions in the costs of recovery from flooding and could save lives. Proper consideration of floods can greatly reduce the risk to life, damage and inconvenience to individual owners and occupiers and reduce the large scale liability to governments, insurers, commerce and charities.

There are three questions that I pose:

• What is the duty of care of politicians and planners to the community?
• Can the Community recover from the inevitable flood disaster that will happen in the Hawkesbury-Nepean?
• With the benefit of the Brisbane experience, can our politicians and planners meet the challenge or will they continue to make the same mistakes?