Effects of vegetation on debris flow mobility and erosion

Abstract

Debris flows are dangerous soil mass movements which can pose a great risk for people in the downstream area of the flow. To lower the risk, hazard mitigation measures are implemented which can stabilize slopes and hinder the debris flow on its way down. Mitigation measures for slope stabilization already use vegetation, for example., by using the roots to stabilize the slope and thus preventing the debris flow from happening. At the moment however, mitigation measures against moving debris flows often consist of structures like concrete walls. The use of vegetation (either planted or naturally present in the debris flow channel) as a mitigation measures to hinder debris flow movement in channels not done. This research looks into how vegetation in a debris flow channel influences the debris flows development and the depositional character of the flow. The results are used to understand how vegetation can alter the debris flow risk. To investigate this, debris flow experiments were executed in the small-scale debris flow-flume at Utrecht University using varying forest densities (low to high forest density) and forest clustering (random distribution to few local forests; with the same amount of trees) set ups. Also, the effect of entrained vegetation in the debris flow was investigated to see how this influences the debris flow development and its depositional character. It became apparent that the implementation of a forest on an erodible bed lowers the debris flows velocity and increases the deposition of material transported in the flow. Obstructions (e.g., dams), formed on the erodible bed, due to the capturing of gravel and entrained vegetation (in the debris flow) in between narrow passages (e.g., closely spaced trees). These obstructions lead to localised spots of relatively high deposition and the formation of preferential flow paths. Unlike previously thought, the addition of (more) entrained vegetation to the debris flow does not necessarily result in a lower debris flow velocity. These results demonstrate that vegetation in the debris flow channel can be used as hazard mitigation measures by lowering flow velocity and promoting deposition. This will lower the debris flow volume and runout distance which will in turn decreases the risk posed to humans. This is, for example, important for areas at risk from debris flows, which also suffer from deforestation or forest fires, because a removal of the forest could lead to a dangerous increase in the risk posted by the hazard.