Observation of the hydrologic response in the source area of a debris flow catchment.

During intense rainstorms, runoff plays a major role in triggering debris flows in the Dolomite region of Italy. Here the landscape is dominated by steep dolomite massifs where rainfall rapidly concentrates in steep headwater catchments. Debris flow channels originate at the outlet of headwater catchments and erode thick talus deposits delivering sediments to the fan. Headwater catchments are typically very steep (45°-60° on the average) and mostly consist of exposed bedrock with no vegetation and sparse to none soil cover. Debris flows occur in the summer season during short and intense thunderstorms. In order to gain a better understanding of debris flow initiation by surface water runoff, we monitored rainfalls and pore pressures in the source area of a small catchment. We also obtained video footage of surface runoff and debris flow initiation from two cameras. Observations show that incoming water infiltrates in highly porous channel bed debris flowing downstream as subsurface stormflow. Only when increasing discharge exceeds the capacity of the channel bed, surface flow appears and debris flows are triggered by progressive erosion. Although we don’t have a measure of water discharge, we can distinguish three types of hydrologic response in the initiation area: subsurface runoff; surface runoff and debris flow occurrence. Data are sufficient to empirically define the debris flow rainfall triggering threshold in the duration-intensity space. By analyzing type and timing of the response in relation to rainfall forcing, we can calibrate a simple hydrologic model based on SCS method to reproduce our observations. The model is well suited to describe the empirical rainfall triggering threshold. Even more simple estimates of concentration time and initial loss would be sufficient to explain the fundamental processes ruling the shape and slope of the rainfall threshold. The relative simplicity of the mechanisms at the base of the hydrologic response of debris flow catchments in the Dolomites suggests that a regionalization of debris flow rainfall threshold should be possible.