Comparing a newly developed DEM-based runout model for hillslope debris flows with full-scale experiments and historical events

<p>The increasing urbanization of mountainous areas increased the risk imposed on residential buildings and infrastructure. In Switzerland, shallow landslides and hillslope debris flows are responsible every year for high infrastructure damage, blocking of important highways, evacuations and deaths. Up till now, the assessment of these processes has been mainly based on the experience of experts, especially in the assessment of their run-out extent and expected damage. In this research we present a new computationally efficient Discrete Element Model (DEM) which has been developed for the aim of simulating the run-out of hillslope debris flows.</p><p>YADE-DEM open source code has been extended and an elasto-plastic adhesive contact law have been implemented, which partially account for the presence of the fluid composed of water and find material. This is achieved through the adhesive aspect of the contact law, which would indirectly take the presence of such fluid into account, as this fluid would increase the cohesion of the flowing mass. A parametric study has been carried out to define the most sensitive model parameters, which were found to be the microscopic basal friction angle (Φ<sub>b</sub>) and the ratio between stiffness parameters (loading and unloading) of the flowing particles <img src="data:image/png;base64,%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" alt="">. Data of full-scale experiments of hillslope debris flows were used to compare the flow kinematics with the model’s prediction. A good agreement between the model and experiments was observed concerning the mean front velocity (average margin of error of 8%) and the maximum applied pressure (average margin of error of 5%), with less agreement of the flow height (average margin of error of 13%). Detailed comparisons of pressure evolution between different selected experiments and simulations revealed the model’s capability of reproducing observed pressure curves, especially during the primary loading phase, leading to maximum pressure.</p><p>In order to test the model’s prediction of run-out distance of hillslope debris flow, hundreds of past hillslope debris flow events in the Swiss Alps were analyzed and 30 cases were selected representing different situations (i.e. different release volumes, slopes and forest cover). Due to the discrete nature of results in YADE, a GIS algorithm was developed in order to create envelopes representing the temporal evolution of the simulated propagating processes, which were compared to the those of the historical events. Results of the comparison revealed that, with the calibration of the two sensitive parameters in YADE, a fair to very good agreement was observed between the envelopes of the model and those of historical events for 87% of the tested cases. Difficulties in reproducing the envelopes of the rest of the cases are linked to the uncertainties in the mapping of the envelopes of past events, the role of the forest which is not taken into account in the model, and the lack of direct representation of fluid in the model.</p>

2D Runout Modelling of Hillslope Debris Flows, Based on Well-Documented Events in Switzerland

In mountain areas, mass movements, such as hillslope debris flows, pose a serious threat to people and infrastructure, although size and runout distances are often smaller than those of debris avalanches or in-channel-based processes like debris floods or debris flows. Hillslope debris-flow events can be regarded as a unique process that generally can be observed at steep slopes. The delimitation of endangered areas and the implementation of protective measures are therefore an important instrument within the framework of a risk analysis, especially in the densely populated area of the alpine region. Here, two-dimensional runout prediction methods are helpful tools in estimating possible travel lengths and affected areas. However, not many studies focus on 2D runout estimations specifically for hillslope debris-flow processes. Based on data from 19 well-documented hillslope debris-flow events in Switzerland, we performed a systematic evaluation of runout simulations conducted with the software Rapid Mass Movement Simulation: Debris Flow (RAMMS DF)—a program originally developed for runout estimation of debris flows and snow avalanches. RAMMS offers the possibility to use a conventional Voellmy-type shear stress approach to describe the flow resistance as well as to consider cohesive interaction as it occurs in the core of dense flows with low shear rates, like we also expect for hillslope debris-flow processes. The results of our study show a correlation between the back-calculated dry Coulomb friction parameters and the percentage of clay content of the mobilised soils. Considering cohesive interaction, the performance of all simulations was improved in terms of reducing the overestimation of the observed deposition areas. However, the results also indicate that the parameter which accounts for cohesive interaction can neither be related to soil physical properties nor to different saturation conditions.