The impact of climate change on dam overtopping floods in Australia

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. A key input to assessing risks to dam safety is a probabilistic estimate of extreme flood magnitudes with the potential to overtop dams. However, few studies have attempted to consider climate change in such estimates due to the challenges involved. A recent examination of contemporary scientific findings pertinent to climate change impacts on the probability of dam overtopping floods has informed the projection of estimates made here. We project changes in the exceedance probabilities of overtopping floods, namely floods that exceed the dam crest flood, for 18 large dams in Australia under a range of global warming assumptions. Explicit 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 quantiles. The analysis is dependent on the degree of global warming, which allows results to be interpreted in terms of different greenhouse gas emission scenarios and future time horizons. Our results are consistent with general expectations that the probability of dam overtopping floods will increase with global warming. Specifically, we found that increases in rainfall depth had the largest impact 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 probability of overtopping floods was between 2.4–17 times that of historical conditions for the dams investigated. We also found that the overtopping probability 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.