Future extreme flood generation processes in the Alps

Extreme floods are intensifying in magnitude and frequency in a warming climate. While the current drivers of flood changes are well understood, large uncertainties remain regarding the most extreme floods and their drivers due to internal climate variability. This makes it difficult to disentangle changes due to interannual variability from changes due to climate change. In mountain regions such as the Alps, increasing sub-daily rainfall extremes, together with declining snowmelt contributions and evolving antecedent soil moisture conditions, are expected to substantially alter the generation processes of extreme floods. However, how these changing flood drivers jointly affect future extreme flood events in mountain regions remains poorly understood.

We therefore investigate how the generation processes of extreme floods will change in the Alps. Specifically, we examine 1) how the drivers of moderate and extreme flood events differ in a warming climate, 2) the extent to which increasing sub-daily rainfall extremes can compensate for declining snowmelt, 3) potential changes in the timing and volume of flood events, and 4) whether projected changes in flood generation processes are significant relative to internal climate variability.

To address these questions, we analyse hourly simulations from a hydrological model driven by climate projections from a single-model initial-condition large ensemble (SMILE) for 384 catchments in Switzerland and Austria. The SMILE consists of 50 ensemble members and enables a robust quantification of internal climate variability. To analyse future changes in flood generation processes, we classify the projected floods based on their drivers, including precipitation, snowmelt, soil moisture and their interplay using a flood classification framework. We further analyse flood characteristics using indicators such as time to peak, volume, peak magnitude and seasonality. Preliminary results indicate that (1) snowmelt extremes continue to play a dominant role in driving the most extreme floods at high elevations, but are less important for moderate floods; (2) floods occur more frequently under dry antecedent moisture conditions than before, while requiring higher rainfall intensities to be generated; and (3) flashiness increases more strongly for extreme floods than for moderate floods.

Improving our understanding of future changes in the generation processes of extreme floods is essential for supporting local authorities, who are deciding on how to adapt to the effects of climate change on hydrological extremes.