Impacts of flow path water-saturation for debris-flow erosion modelling at Illgraben (Switzerland)

Debris flows can significantly grow along their flow path by entraining sediments stored in the channel bed and banks. This entrainment process is influenced by various factors such as flow properties (e.g., flow momentum, basal shear stress) and environmental conditions (e.g., soil water saturation, sediment availability). In recent years, different attempts to include the entrainment process in runout models have improved modelled flow properties and runout behavior by empirically linking entrainment volumes to individual modelled flow properties. Linking entrainment to environmental factors, however, has remained challenging.

Here, we aim at implementing and testing the influence of flow path water-saturated conditions in debris-flow runout modelling in a Swiss debris-flow basin (Illgraben). To this end, the modified RAMMS runout model, which includes an empirical algorithm to describe entrainment as a function of basal shear stress (Frank et al., 2015), is coupled with a simple hydrological model to predict soil water saturation. In a first step, the RAMMS model was calibrated for the Illgraben site for seven events with detailed data on erosion/deposition along the fan as well as flow properties at the outflow of the simulation domain (de Haas et al., 2022). In the calibration procedure, the focus was placed on the erosion proportionality factor dz/dtau [m/kPa] (which links the maximum potential erosion depth to the basal shear stress) as it is assumed to be the driving saturation-induced increase of entrained volume. Preliminary results show that in most cases, including the entrainment process improves the reproduction of the flow properties, especially the ‘hydrograph’ front, and that the erosion proportionality factor dz/dt shows a significant degree of variation for different events. In a second step, the relationship between soil moisture conditions and maximum erosion depth expected along the flow path was investigated. The hydrologic conditions are simulated with a conceptual model solving the water balance for the basin’s headwaters. The headwater discharge serves as the water input for the channel on the fan, where an infiltration model is applied, and entrainment is investigated. The presented framework, which could be incorporated into other runout models, is expected to be useful for debris-flow entrainment modelling, as well as for assessing climate change impacts on debris-flow runout.

References

de Haas, T., McArdell, B.W., Nijland, W., Åberg, A.S., Hirschberg, J., Huguenin, P., 2022. Flow and Bed Conditions Jointly Control Debris‐Flow Erosion and Bulking. Geophysical Research Letters 49. https://doi.org/10.1029/2021GL097611

Frank, F., McArdell, B.W., Huggel, C., Vieli, A., 2015. The importance of entrainment and bulking on debris flow runout modeling: examples from the Swiss Alps. Nat. Hazards Earth Syst. Sci. 15, 2569–2583. https://doi.org/10.5194/nhess-15-2569-2015