Field Measurements of Forces in Debris Flows at the Illgraben: Implications for Channel-Bed Erosion

Abstract

Using a large force plate as installed in the channel bed at the Illgraben debris-flow observation station, we illustrate flow properties of three typical types of debris flow which occur there, single-surge debris flows, multiple-surge flows, and more complicated surges with many individual roll waves. In all three types of flow, the median value of the normal and shear stresses are largest at the front or fronts of each surge. Additionally, the fluctuations in normal and shear stress are also largest at the flow front, reaching values up to 1.5 times larger than the median value. The presence of large force fluctuations at the front of the surges is in agreement with previously-published results which show that the rate of erosion is largest within the first minute of the arrival of a debris-flow surge.