The impact of climate change on dam overtopping flood risk

Abstract. There is unequivocal evidence that climate change will change the risk profile of dams, which are critical pieces of infrastructure that safeguard water supply and provide flood mitigation for populated areas. However, the challenges involved in estimating the probability of extreme floods under climate change have meant that few studies have estimated the plausible changes in the risk of extreme floods that have the potential to overtop dams. A recent examination of contemporary scientific findings pertinent to climate change impacts on flood risk has informed the projection of extreme flood risk and dam overtopping risk estimates made here. We project changes in the exceedance probabilities of overtopping risk for 18 large dams in Australia under a range of global warming assumptions, where consideration is given to the impacts of climate change on rainfall depth, rainfall temporal pattern, and rainfall losses resulting from changes in antecedent catchment wetness. We used event-based flood modelling and Monte Carlo sampling to appropriately represent the range of uncertainties associated with projecting estimates of extreme flood risk. Our results are presented in terms of changes per degree of global warming, which facilitates their interpretation in terms of different greenhouse gas emission scenarios and future time horizons. We found that increases in rainfall depth had the largest impact on increasing dam overtopping flood risk for all 18 dams under climate change. Under 4 °C of global warming, which approximates conditions towards the end of this century under a high emissions scenario, the risk of overtopping floods was between 2.4–17 times that of historical conditions for the 18 dams investigated. We also found that the risk of overtopping has more than doubled compared to the historical baseline for four of the dams investigated here as a result of global warming that has already occurred.