MAPPING OVERLAND FLOWPATHS ― GOOD PRACTICE OR POLITICAL SUICIDE?

The advent of two dimensional flood modelling and the availability of air-borne laser scanning survey data have given flood risk practitioners the ability to map inundation over the majority of our urban landscape, albeit at very shallow depths. As the model resolutions become finer, the areas of shallow inundation grow larger (particularly with the inclusion of rainfall on the two dimensional model grid). Difficulties arise when public flood maps are prepared based on this modelling. These difficulties are typified by the following questions that often emerge during floodplain risk management committee discussions: • Should all inundation in a 100 year flood event be mapped as “flooding”? • At what inundation depth should special flood controls be applied and at what depth is it sufficient to fall back on normal building controls, (e.g. BCA)? • What terminology should be used to describe this inundation to the public? • What notations should be provided on Section 149(2) and 149(5) certificates? Communities can become very angry if all inundation is classified as flooding (as has been the practice with riverine flooding derived from more traditional one dimensional models). This is especially so in shallow overland flow areas distant from recognised watercourses. The public outrage in these areas will demand a different approach. This paper presents a pragmatic solution which balances Council’s responsibility to carefully manage the floodplain without unnecessarily classifying shallow inundation areas as ‘flooding’ and incurring the wrath of the community due to the perceived impacts on property values or by triggering planning restrictions that provide no beneficial flood risk management return.