With assets in poor condition and the uncertain effects of climate change, New Zealand is facing an impending need for investing further and wider to deliver effective flood infrastructure. Yet today’s method of adopting a fixed line flood map, coupled with a precautionary approach to stormwater infrastructure, can lead to significant over investment up front and across the life of the assets. This paper discusses planning for exceedance as an approach to flood risk management, against the backdrop of a fiscally-constrained economy and pressure on land use in flood risk areas.
Land is effectively classified in one of two ways - ‘no flooding’ or ‘flooding’ - and it usually adopts a precautionary approach representing the worst case situation. But in reality the flooding that actually occurs rarely matches what is shown on maps, nor provides planners with an understanding of how the flood map can vary.
As pressure on land availability increases, it is becoming increasingly important for planners to understand the ‘grey zone’ between the best and worst case flooding scenario when making land use and infrastructure decisions. In this paper we review methods for identifying exceedance areas and introduce the concept of probabilistic mapping - an alternative approach to floodplain mapping that can help planners understand uncertainty and probability.
The predicted high cost of future infrastructure investment as a result of climate change means protecting overland flowpaths is becoming an increasingly important feature of delivering effective flood management practices. Councils can apply policy to restrict new development in floodplains or on overland flowpaths, however managing exceedance of the stormwater system in existing urban areas represents a more significant challenge both in practicality and funding. This paper discusses approaches to designing for the exceedance of existing stormwater infrastructure. With the objective of mitigating potentially high costs, key success factors and design approaches are identified that recognise the uncertainty in magnitude, the necessary resilience and adaptability in design, and importance of community awareness.