Sediment transfer patterns at the Illgraben catchment, Switzerland: Implications for the time scales of debris flow activities

Debris flows strongly control sediment transfer patterns in mountainous catchments. We quantify sediment transfer at the Illgraben, Switzerland, where debris flows are frequent and geomorphic change is rapid. Four sequential aerial image series (from fall 2007 to fall 2009) were used to measure landscape change in relation to debris flows. The debris, often originating from bedrock landslides, was transported in patterns of erosion, storage, and remobilization. The landslides typically stopped on the downslope hillslope or in the channel, and they did not transform directly into debris flows. The magnitude and nature of sediment transfer patterns show large spatial and temporal variability, and the storage time of the deposits was shorter than one year. Landslide volumes were an order of magnitude smaller than the debris flows at the catchment outlet. While the mechanism of debris flow initiation could not be determined unambiguously, clearly debris flows must entrain substantial amounts of sediment along the flow path to attain the volumes estimated at the distal end of the fan.