Debris-flow modeling at Meretschibach and Bondasca catchments, Switzerland: sensitivity testing of field-data-based entrainment model

Florian Frank,Marc Christen,Nicole Oggier,Patrick Baer,Brian W Mcardell,Andreas Vieli

doi:10.5194/nhess-17-801-2017

Abstract

Abstract. Debris-flow volumes can increase due to the incorporation of sediment into the flow as a consequence of channel-bed erosion along the flow path. This study describes a sensitivity analysis of the recently introduced RAMMS (Rapid Mass Movements) debris-flow entrainment model, which is intended to help solve problems related to predicting the runout of debris flows. The entrainment algorithm predicts the depth and rate of erosion as a function of basal shear stress based on an analysis of erosion measurements at the Illgraben catchment, Switzerland (Frank et al., 2015). Starting with a landslide-type initiation in the RAMMS model, the volume of entrained sediment was calculated for recent well-documented debris-flow events at the Bondasca and the Meretschibach catchments, Switzerland. The sensitivity to the initial landslide volume was investigated by systematically varying the initial landslide volume and comparing the resulting debris-flow volume with estimates from the field sites. In both cases, the friction coefficients in the RAMMS runout model were calibrated using the model, whereby the entrainment module was (1) inactivated to find plausible values for general flow properties by adjusting both coefficients (ξ and μ) and then (2) activated to further refine coefficient μ, which controls erosion (patterns). The results indicate that the model predicts plausible erosion volumes in comparison with field data. By including bulking due to entrainment in runout models, more realistic runout patterns are predicted in comparison to starting the model with the entire debris-flow volume (initial landslide plus entrained sediment). In particular, lateral bank overflow – not observed during these events – is prevented when using the sediment entrainment model, even in very steep (≈ 60–65 %) and narrow (4–6 m) torrent channels. Predicted sediment entrainment volumes are sensitive to the initial landslide volume, suggesting that the model may be useful for both reconstruction of historical events and the modeling of scenarios as part of a hazard analysis.

Highlights

Sediment erosion caused by debris flows causes flow bulking, which strongly influences the runout behavior of debris flows
The results show that the total volume of eroded sediment, at both field sites, depends strongly on the initial landslide volume
Information on the style of initial landslide failure are is available for either field site; we focus the discussion on other factors related to the runout modeling

Summary

Introduction

Sediment erosion caused by debris flows causes flow bulking (in our case an increase in flow mass; e.g., Iverson, 1997), which strongly influences the runout behavior of debris flows. The entrainment of eroded sediment along the channel has been observed to considerably increase the volume of debris flows (i.e., bulking process) at many different locations (e.g., Hungr et al, 2005; Scheuner et al, 2009; Iverson et al, 2011; Berger et al, 2010a, 2011; Schürch et al, 2011; McCoy et al, 2012; Tobler et al, 2014; Frank et al, 2015). The debris-flow entrainment and bulking process should be included in debris-flow runout models to increase the accuracy of runout predictions including the overall runout distance, location, and amplitude of lateral bank overflow and – importantly for hazard assessment – the flow and depositional pattern on the fan (Gamma, 2000; Scheuner et al, 2009; Hussin et al, 2012; Han et al, 2015; Frank et al, 2015)

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