Juvenile and mature alpine sediment fans respond differently to rainfall intensification

Alpine alluvial fans and debris flow cones are central components of the sediment cascade. The projected increase in heavy precipitation due to ongoing climate warming is thought to intensify sediment redistribution dynamics under transport-limited conditions. However, alluvial fan response to increasing heavy precipitation has been shown to strongly differ between individual catchments. In this study, we compare decadal-scale planimetric dynamics of a mature alpine alluvial fan (“Friedergries”, 5 km² catchment area) to juvenile debris flow cones (Lake Plansee, catchment areas mostly < 0.5 km²) in the Main Dolomite region of the Northern Calcareous Alps. We show that the juvenile cones corresponding to small and steep catchments are susceptible to moderate precipitation while floodplain dynamics on the mature fan are only susceptible to extreme precipitation events with supra-regional extent. Our observation indicates that sediment redistribution on juvenile cones with small and steep catchments will shift towards spring and autumn, corresponding to the seasonal shift of moderate precipitation extremes (1-year return level). In contrast, sediment redistribution on mature fans with larger, gentler catchments will continue to occur mainly in summer, as supra-regional extreme events with return levels > 1 year will occur during the hottest months also in a changing climate (Brönnimann et al., 2018). Here we show that catchment morphology and fan maturity control future susceptibility to rainstorms and thus sediment fan evolution over the coming decades.

Brönnimann, S., Rajczak, J., Fischer, E.M., Raible, C.C., Rohrer, M. & Schär, C. (2018): Changing seasonality of moderate and extreme precipitation events in the Alps. – Natural Hazards and Earth System Sciences, 18: 2047 – 2056. DOI: 10.5194/nhess-18-2047-2018.