FIELD OBSERVATIONS OF DEBRIS FLOW PROPERTIES AT THE ILLGRABEN CATCHMENT, SWITZERLAND

The interaction of soil debris and the interstitial fluid is central to understanding the dynamics of debris flows, yet few observations are available for natural debris flows. The Illgraben debris flow observation station has been in operation since the year 2000, and since 2004 results are available from a large (8 m²) force plate on the channel bed. We report on flow depth, front velocity, and bulk density for 15 natural debris flows which were triggered by summer thunderstorms. The events we describe cover a wide range of debris flows observed at the Illgraben, from slow granular flows to rapid and turbulent muddy flows, with front velocities ranging from 0.7 to 9.0 m/s, maximum flow depths from 1.0 to 2.64 m, and event volumes estimated from 4,000 to more than 80,000 m³.

Using the force plate and flow surface elevation data, we calculate the mass density of the debris flows as they pass over the plate: mean values for the flow head (defined as where the flow depth is more than 90% of the maximum) of each flow range from 1200 to 2200 kg/m³, corresponding to qualitative estimates of sediment concentration from our videos. Assuming that the pores are completely filled with fluid, which is supported by observations of fluid on the surface of the flow after the passage of the granular front, it is also possible to estimate the volumetric water content of the flows, with values from 27 to 88%. While some variability is present in the data set, there is a clear trend for larger flow velocities with increasing water content. Friction coefficients (e.g. a simple application of the Manning-Strickler relation from flood flows) also show clear trends with water content. Although further investigations are necessary to confirm if the variability of flows observed the Illgraben is typical of other field sites, the results indicate that the properties of debris flows are strongly related to the water content of the flow, suggesting that some method of predicting water content will be necessary for accurately modeling the runout of debris flows.