Measurement of friction in debris flows, floods, and intermediate flows

Many different rheological models describing the behavior of debris flows are available, yet there is no general agreement on the appropriate rheology for a given problem. Here we report on full-scale observations of friction in debris flows, which can be used to help constrain the selection of flow rheology. Using measurements from a large force plate (area = 8m2), we investigate the frictional behavior using the ratio of shear (s) to normal force (n) of debris flows recorded at the Illgraben debris flow observation station, in southwest Switzerland. Due to practical constraints the force plate is installed in a horizontal concrete structure in the channel bed, and not tilted to match the slope of the natural channel bed upstream of the force plate (slope S=0.08), which may induce a small deceleration of the flow, which we assume is negligible, especially for flows with large depths. Flow depth is recorded at the center of the force plate using either a laser sensor (point measurement) or radar sensor (average value). Debris flows are characterized with relatively large friction values (s/n ~0.15) at the front of the flows which are about twice as large as the slope of the channel bed. This result is consistent with ideas from the literature describing large friction at the flow front. Flood flows, in contrast, have frontal friction values (s/n ~0.1) approximately equal to the slope of the channel, indicating approximately steady and uniform flow over periods of 10’s of seconds.  Several transitional events have been recorded with properties intermediate between debris flows and flood flows, with corresponding s/n values also intermediate between debris flows and floods. These friction observations were recently incorporated into a debris flow model (Meyrat et al., 2023) which is capable of predicting the transition between debris-flows and glacial-lake outburst floods. While these results are promising, more research is necessary to further explore the controls on debris-flow friction, also for events characterized by multiple roll waves or erosion-depostion waves. Citation:Meyrat, G., Munch, J., Cicoira, A., McArdell, B., Müller, C. R., Frey, H., & Bartelt, P. (2023). Simulating glacier lake outburst floods (GLOFs) with a two-phase/layer debris flow model considering fluid-solid flow transitions. Landslides. https://doi.org/10.1007/s10346-023-02157-w.