The importance of entrainment and bulking on debris flow runout modeling: examples from the Swiss Alps

F Frank,B W Mcardell,A Vieli,C Huggel

doi:10.5194/nhess-15-2569-2015

Abstract

Abstract. This study describes an investigation of channel-bed entrainment of sediment by debris flows. An entrainment model, developed using field data from debris flows at the Illgraben catchment, Switzerland, was incorporated into the existing RAMMS debris-flow model, which solves the 2-D shallow-water equations for granular flows. In the entrainment model, an empirical relationship between maximum shear stress and measured erosion is used to determine the maximum potential erosion depth. Additionally, the average rate of erosion, measured at the same field site, is used to constrain the erosion rate. The model predicts plausible erosion values in comparison with field data from highly erosive debris flow events at the Spreitgraben torrent channel, Switzerland in 2010, without any adjustment to the coefficients in the entrainment model. We find that by including bulking due to entrainment (e.g., by channel erosion) in runout models a more realistic flow pattern is produced than in simulations where entrainment is not included. In detail, simulations without entrainment show more lateral outflow from the channel where it has not been observed in the field. Therefore the entrainment model may be especially useful for practical applications such as hazard analysis and mapping, as well as scientific case studies of erosive debris flows.

Highlights

Debris flows in mountain areas are one of the most important landscape forming processes in high alpine catchments
Using the RAMMS debris flow runout model without entrainment modeling, the most plausible modeled flow properties can be achieved using Voellmy friction coefficients of ξ = 200 m2 s−1 and μ = 0.20. This calibration was done applying the standard RAMMS debris flow runout model without entrainment modeling for the largest debris flow event at Spreitgraben (12 August 2010; Geotest, 2010) which had an estimated volume of 130 000 m3 (Table 1) with the total volume entering the computational domain at the lower firn boundary (Fig. 2b and c)
The entrainment model is based on the relation between calculated basal shear stress and the net erosion (Schürch et al, 2011b) as well as an erosion rate (Berger et al, 2011) which were both measured at the Illgraben channel

Summary

Introduction

Debris flows in mountain areas are one of the most important landscape forming processes in high alpine catchments. Previously stable sediment deposits or rock walls became destabilized in the Swiss Alps (Huggel et al, 2011; Tobler et al, 2014). The problem has become worse due to increased sediment input connected to intense rainfall activity (e.g., Tobler et al, 2014) or in some cases snowmelt activity in early summer (e.g., Graf et al, 2013) which can readily mobilize the debris. The recent large debris flows are unusual in that they have caused unprecedented amounts of erosion on the debris fans or alluvial fans, thereby increasing awareness of the importance of debris-flow entrainment, especially for practical applications such as runout analysis and hazard mapping (e.g., Rickenmann and Zimmermann, 1993; Kienholz et al, 2010)

Objectives

Results

Discussion

Conclusion