Sediment dynamics related to the triggering of debris flows in different alpine watersheds

Debris flows initiate by a critical combination of abundant sediment, steep inclination, and water. The latter is mostly provided by rainfall that can lead to landslides at the hillslope or along the channel and/or erosion and bulking of sediment due to increased runoff. Location of sediment sources and channel recharge are related to short- and long-term geomorphological processes within the watershed. Up to now, there are only a few studies investigating sediment dynamics in high alpine watersheds that are regularly affected by debris flows. In this contribution we report of our ongoing efforts to monitor sediment dynamics and debris-flow activity in three very different watersheds in the Austrian Alps. We use a combination of remote sensing and in-channel monitoring techniques including UAV, air-borne and terrestrial laser scanning before and after debris-flow events. We find that debris-flows frequency and volumes are strongly related to movement rates of landslides present in the watershed. At high movement rates, most of the channel refill occurs within the time scale of hours. In the absence of active landslides, debris-flow activity is limited by rainfall-triggered embankment failures along the channel and continuous transfer of hillslope sediment into the channel. In the steepest and smallest monitored watershed, active landslides and continuous surface erosion from landslide scars leads to a high frequency of debris flows of all magnitudes, even in the absence of rainfall. Our study shall provide the basis for a more complete modeling framework for a better prediction of debris flows now and in a future climate.