Field and laboratory analysis of the runout characteristics of hillslope debris flows in Switzerland

Abstract

Hillslope debris flows are unconfined flows that originate by shallow failures in unconsolidated material at steep slopes. In spite of their significant hazard for persons and infrastructure in mountainous regions, research on hillslope debris flows is rather scarce in comparison to other landslide types. This study focusses on the runout characteristics of hillslope debris flows applying two different approaches. First, detailed landslide inventories, which include field measurements of 548 slope failures that occurred during the last two decades in seven parts of Switzerland, were analysed. Second, laboratory tests were carried out to study the effect of the soil water content, grain-size distribution and mobilized volume on the runout behaviour of hillslope debris flows. Most of the failures in the field started as shallow translational slides at terrain slopes between 25° and 45° and involved volumes of some tens to a few hundred cubic meters. An analysis of the runout distance of 117 hillslope debris flows showed that they normally travelled some tens of meters, but sometimes the runout exceeded 300 m. A positive relation between volume and runout distance and between volume and affected area was observed, although there is considerable scatter in the data. The affected area of 63 hillslope debris flows ranged from ~ 100 to ~ 1500 m2. Based on the field data, a 7.5 m long laboratory hillslope was designed with a geometrical scale factor of 20. A total of 75 runs with volumes from 4 to 20 dm3, water contents from 18% to 38%, and four grain-size distributions were carried out. The laboratory tests revealed that water content is the dominant control, but also the clay content strongly influences the runout distance and the affected area. Even a small increase in water or clay content produces a considerably larger or smaller runout distance, respectively. In contrast, the influence of the volume on the runout was smaller, and a positive relation was observed between these two parameters. The field and laboratory results are in general agreement and consistent with the results of other studies. The results of this work improve the understanding of hillslope debris flows and may aid in the hazard assessments of these processes.Peer ReviewedPostprint (author’s final draft

The ability of tree stems to intercept debris flows in forested fan areas: A laboratory modelling study

“…In agreement with de Haas et al (2015), Hürlimann et al (2015), and Scheidl et al (2013), the results of this study highlight that small variations in water content are associated with large o n l y changes in maximum runout distances. This effect was found especially by Hürlimann et al (2015) and D' Agostino et al (2013) using grain-size distributions containing relatively small amounts of clay and silt, similar to the mixture material used during our tests.…”

“…No bifurcation of the front was observed during the tests. Segregation and accumulation of coarse particles were observed in the distal part of the deposit, as also observed by Hürlimann et al (2015) in their laboratory experiments. An overview of the morphological features measured after each run are listed in Table 3 and plotted in Figure 3, together with the mean deposit width W, the debris-flow velocities at the flume outlet (Uout), and the resistance coefficient H/L (Iverson, 1997).…”

“…Empirical-statistical methods are based on regression analysis of data from past debris-flow events to define relationships between factors such as runout length and sediment volume. Due the difficulties involved in studying debris flows at the field scale, physical scale-model experiments are considered to be very helpful for understanding the runout of hillslope debris flows (Scheidl et al, 2013), and several different scale laboratory experiments have been carried out to simulate debris flows (e.g., Major and Iverson, 1999;D'Agostino et al, 2010;Hürlimann et al, 2015;de Haas, 2015). The majority of these studies have focused on the rheology and flow behaviour of debris flows and the formation of frontal accumulations of coarse debris.…”

DebrisInterMixing-2.3: a finite volume solver for three-dimensional debris-flow simulations with two calibration parameters – Part 2: Model validation with experiments

“…The first experimental case used for validation is based on flume experiments from Hürlimann et al (2015), simulating hill-slope debris flows on a wide laboratory slope to exclude side-wall effects as suggested by Jop et al (2008). We use this case study to illustrate to what extent the calibrated model can predict flow behavior with different water contents without recalibration.…”

“…In our modeling approach, the rheology of the slurry phase (the fine material suspension) depends on its yield stress, which is known to be exponentially dependent on water content (e.g., Hampton, 1975;O'Brian and Julien, 1988;Yu et al, 2013;Hürlimann et al, 2015), with increasing exponents for higher clay fractions. Therefore, a minor variation in water content may cause a strong change in flow depths and run-out distance.…”