NUMERICAL MODELING OF DEBRIS FLOW RUNOUT IN THE SWISS ALPS

Debris flow runout models are in demand for applications including developing hazard intensity maps, evaluating the effectiveness of mitigation measures, and assisting in the interpretation of events for practical and scientific applications. Dynamical simulation models intended for practical use usually include simple relations to describe the bulk frictional behavior of the water-sediment mixture and cannot be used to explain many of the features commonly observed in real debris flows. Experience illustrates that these models may be useful if the limitations are known and caution is exercised in interpreting the results. Herein we describe the application of the RAMMS (Rapid Mass Movements) model which is based on a 2D solution to the shallow water equations for granular flows and describes the frictional behavior of debris flows using the Voellmy relation. This friction relation was chosen because it has been applied in a variety of other models with some success in steep catchments. First we summarize the variability of the friction coefficients and explain several persistent problems in applying the model. The debris flows we studied vary in volume from a few 100 m³ to more than 500'000 m³. Several of the events are from the Illgraben debris flow observation station where both front velocity and flow depth data are available, providing an opportunity to evaluate the model performance in more detail. High-resolution topographic information is essential for accurate modeling, and significant topographic changes within an event due to erosion and deposition have also been observed in some cases and may play a large role in determining the pattern of inundation and deposition on an alluvial fan. When highly detailed topographic information and event data are available for model evaluation, the limits of the simple bulk friction approach become apparent, supporting the need to implement more physically realistic two-phase (sediment and water) descriptions of the frictional behavior of debris flows into debris flow models. However phenomena such as entrainment and deposition present significant challenges that will continue to complicate the application of debris flow runout models for practical problems.