Monitoring and modeling of debris-flow surges at the Lattenbach creek, Austria

Abstract

Debris-flow events are often comprised by a sequence of surges, sometimes termed roll waves. The reason for this surging behavior is still a matter of debate. Explanations include the growth of hydraulic instabilities, periodic sediment deposition and release, or grain size sorting. Also, the shape and the velocity of single surges and the implications for hazard mitigation are hard to predict. Here we present results of several years of monitoring debris-flow events at the Lattenbach creek (AUT). The monitoring system includes radar sensors for measuring flow depth at different locations along the channel, as well as a 2-D rotational laser sensor installed over a fixed cross-section that yields a 3-D surface model of the passing debris-flow event. We find that the debris flows at Lattenbach creek exhibited surges for each observed event. The celerity of the surges were up to twice as high as the front velocity. Often, the first surges had highest flow depth and discharge, and showed an irregular geometry. Video recordings reveal that this might be connected to the presence of large boulders and woody debris. On the contrary, the shape of the surges in the second half of the flow, which carried smaller grain sizes and less woody debris, were rather regular and showed a striking geometric similarity, but still high velocities. We tested a recently derived wave equation based on hydraulic theory and found that the shape of these regular roll waves can be reasonably reproduced by that model. The results of our monitoring efforts aim to improve our understanding of the surging behavior of debris flows and provide data for model testing for the scientific community.