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
Forests can decrease the risk of shallow landslides by mechanically reinforcing the soil and positively influencing its water balance. However, little is known about the effect of different forest structures on slope stability. In the study area in St Antönien, Switzerland, we applied statistical prediction models and a physically‐based model for spatial distribution of root reinforcement in order to quantify the influence of forest structure on slope stability. We designed a generalized linear regression model and a random forest model including variables describing forest structure along with terrain parameters for a set of landslide and control points facing similar slope angle and tree coverage. The root distribution measured at regular distances from seven trees in the same study area was used to calibrate a root distribution model. The root reinforcement was calculated as a function of tree dimension and distance from tree with the root bundle model (RBMw). Based on the modelled values of root reinforcement, we introduced a proxy‐variable for root reinforcement of the nearest tree using a gamma distribution. The results of the statistical analysis show that variables related to forest structure significantly influence landslide susceptibility along with terrain parameters. Significant effects were found for gap length, the distance to the nearest trees and the proxy‐variable for root reinforcement of the nearest tree. Gaps longer than 20 m critically increased the susceptibility to landslides. Root reinforcement decreased with increasing distance from trees and is smaller in landslide plots compared to control plots. Furthermore, the influence of forest structure strongly depends on geomorphological and hydrological conditions. Our results enhance the quantitative knowledge about the influence of forest structure on root reinforcement and landslide susceptibility and support existing management recommendations for protection against gravitational natural hazards. Copyright © 2015 John Wiley & Sons, Ltd.
Abstract. Coarse particulate organic matter (CPOM) particles span sizes from 1 mm, with a dry mass less than 1 mg, to large logs and entire trees, which can have a dry mass of several hundred kilograms. Pieces of different size and mass play different roles in stream environments, from being the prime source of energy in stream ecosystems to macroscopically determining channel morphology and local hydraulics. We show that a single scaling exponent can describe the mass distribution of CPOM heavier than 0.1 g transported in the Erlenbach, a steep mountain stream in the Swiss pre-Alps. This exponent takes an average value of −1.8, is independent of discharge and valid for particle masses spanning almost seven orders of magnitude. Similarly, the mass distribution of in-stream large woody debris (LWD) in several Swiss streams can be described by power law scaling distributions, with exponents varying between −1.8 and −2.0, if all in-stream LWD is considered, and between −1.3 and −1.8 for material locked in log jams. We found similar values for in-stream and transported material in the literature. We had expected that scaling exponents are determined by stream type, vegetation, climate, substrate properties, and the connectivity between channels and hillslopes. However, none of the descriptor variables tested here, including drainage area, channel bed slope and the percentage of forested area, show a strong control on exponent value. Together with a rating curve of CPOM transport rates with discharge, the scaling exponents can be used in the design of measuring strategies and in natural hazard mitigation.
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