Analysis of debris-flow velocities due to superelevation

Gravitational mass-movements like debris flows endanger human settlements all over the world. Especially in mountainous regions of dense population, people meet a challenge to find an accurate balance between spatial development (economy, ecology) and potential hazards (concept of safety). This approach, also known as integral risk management, needs fundamental knowledge of the expected natural hazardous processes. For debris flows the estimation of flow parameters like flow-height, flow-velocity or maximum discharge, are considered to be essential. Often the runout or the degree of exposure of a debris-flow event can only be predicted, based on these parameters. The research of this project is on the estimation of dynamic parameters of debris flows and will be carried out at the Swiss Federal Institute for Forest, Snow and Landscape Research (WSL) in Switzerland. Since geomorphologic traces like flood marks on banks provide important information about the flowing process of a debris flow, the emphasis of the project will be on the determination of the maximum flow velocity due to superelevation of the flowing mass within a distinct channel. Superelevation can be observed in bending channels, where the flow-height of the inner-curvature is lower than the flow-height of the outer-curvature, caused by the centrifugal acceleration of the flow. For the research project experimental flume investigations will be done at the laboratory of the host institution (WSL). Material mixtures as well as water/sediment concentrations will be varied to apply for the complexity of a debris-flow process. Further the geometric boundaries of the experimental setup (slope, curvature radii, curvature length) will be changed to cover a wider range of topographical conditions in nature. Flow parameters of debris flows from natural events in the field and/or from large scale experiments will be compiled and compared to the experimental observations in the flume (physical model). In detail, geomorphologic investigations along channel bends with superelevation deposits will be performed to test existing velocity equations using superelevation data. The main objective of this project is a detailed analysis of observed debris-flow events (in the field and laboratory) and the related superelevation data. The results of the planned project will improve existing estimation methods of flow parameters for the event documentation. The results will further provide better input parameters for debris- flow simulation models to perform more accurate back-calculations of observed debris-flow events. Further the experiences made by the proposed project should support the physical modeling of debris flows at the Institute of Mountain Risk Engineering at the University of Natural Resources and Applied Life Sciences. The necessary time scheduled for the proposed project is 18 month. The project will be supervised by Dr. Dieter Rickenmann and Dr. Brian McArdell.

Gravitational mass-movements like debris flows endanger human settlements all over the world. Especially in mountainous regions of dense population, people meet a challenge to find an accurate balance between spatial development (economy, ecology) and potential hazards (concept of safety). This approach, also known as integral risk management, needs fundamental knowledge of the expected natural hazardous processes. The estimation of flow parameters like discharge, flow-velocity or flow-height, are considered to be essential. A possible approach to estimate maximum flow velocities of debris flows is based on the vortex equation by using superelevation marks. Superelevation can be observed in bending channels, where the flow-height of the inner-curve is lower than the flow-height of the outer-curve. Several studies propose an adaption of the vortex equation by introducing a correction factor to account for different rheological characteristics of debris-flows. However, those studies also suggest a wide range of the correction factor when compared to observed debris-flow velocities. In this project we analysed debris-flow velocities in a curved flume construction and back-calculated correction factors for all our experimental debris-flows. We measured superelevation and investigated the influence of different material mixtures as well as bend geometries. Experimental flume investigations were conducted within a flexible plastic half-pipe, mounted on a wooden plane construction. Two different bend radii (1.0 m and 1.5 m) with a bend angle of 60 were implemented. The total length of the flume, of about 8 m, was covered with 40 grit silicon carbide sandpaper, reflecting a constant basal friction layer. To apply for the complexity of a debris-flow process, four different material mixtures based on four different grain size distributions, were defined. 69 experiments have been conducted, based on the maximum channel slope of 20. Another 16 experiments were performed with a channel slope of 15. We found that the channel slope as well as the centreline radius has a significant influence on the correction factor. A difference between finer mixture setups and coarse setups were found when comparing the measured superelevation angles, showing steeper angles for coarser debris-flow setups. Further, we could observe a significant relation between the correction factor and the Froude number. Our experiments indicate that correction factors of 1 < k < 5 might be considered for supercritical flow regimes. However, for subcritical flow conditions the correction factor shows a higher deviation in the relation to the Froude number, which leads to an adaption of the forced vortex formula by considering active and passive earth pressure. Finally, we present a forced vortex equation for debris flow velocity estimation without correction factor, based on our empirical findings.