Erosive power of debris flows: predictive modelling by a simple empirical approach

Debris flows are massively erosive mass movements that pose an increasing threat to infrastructure and settlements in mountainous areas due to more intense heavy rainfall events in the future. A major contributor to the magnitude for runoff generated debris flow is the parameter of effective erosion. It directly translates to the hazard potential of debris flows, but it is yet to be sufficiently implemented in models to achieve a predictive performance.

We developed a simple predictive erosive debris-flow model calibrated on active channels in the northern Bavarian Alps. The debris-flows at the study sites recently occurred in 2015 and in 2021, and all entrained more than 80% of their final volume from the sediment channel bed. Geomorphic change was calculated from pre- and post-event LiDAR data and the total volume of the flow was then compared to catchment characteristics. For a detailed analysis we divided the channel into equal segments and compared the respective eroded volume to flow parameters of the adjacent cross section which were simulated in a numerical model. The initiation volume was estimated by a runoff calculation from the respective heavy precipitation events recorded with radar data. We were able to obtain a correlation that can be used in a predictive debris-flow model to iteratively calculate the erosion for runoff-generated debris flows that are triggered by intense rainstorms. This model allows improved predictions of the magnitude of debris-flow prone channels through a forward-modelling approach.