Roots mechanical effects on hydraulic riverbanks erosion and on shallow landslides: modelling tools for forest management along alpine streams

<p>Floods, shallow landslides and surface runoff are recurring processes known for mobilizing sediments and large wood in alpine catchments leading to on-site and off-site hazards such as hydraulic bank erosion, debris flows, increased suspended sediment concentration and instream large wood (LW) transport. All these hazards are exacerbated by extreme precipitation events, and can cause important damages to forests, agriculture, civil structures and settlements. Because plant roots may significantly increase the stability of riverbanks as well as of vegetated slopes, forests can be considered an alternative protection against such processes with respect to other civil engineering measures. However, root reinforcement effectiveness depends on many factors such as roots density, soil properties, and soil thickness, which implies that some vegetated areas have a more significant effect than others.</p><p>In this work, we present the application of BankforMAP and SlideforMAP, two physically-based probabilistic models, for the prioritization of silviculture measures along channels aiming to mitigate risks. The first model simulates expected hydraulic bank erosion by considering channel morphology, bank sediment material, tree roots distribution, and a certain discharge scenario. The second model simulates expected precipitation-triggered shallow landslides by analyzing slope stability and takes the spatial distribution of lateral and basal root reinforcement into account. In this study, these models are further used to quantify the protection effect of the forest against these two processes. In addition, we propose a methodology for using these models to quantify the volume of recruited and transported LW considering different triggering event and vegetation scenarios. Furthermore, we propose an approach for assessing the risk related to LW and its spatial distribution in a catchment area.</p><p>These tools and methods have been tested in a catchment of 29 km² in NW Switzerland, allowing the identification of forested areas with high protection effect, the estimation of the volume of LW expected to be supplied by both hydraulic bank erosion and shallow landslides, as well as the volume of wood transported by the stream considering different triggering event and vegetation scenarios. These results were then used to assign spatially-distributed values of risk to LW.</p><p>Altogether, these results highlight areas where the forest has an important protection effect and where it might pose a risk. Ultimately, the results of this study can be used by decision makers to prioritize the role of protection forest in a catchment and to support the definition of silvicultural measures to mitigate the risks due to LW transport.</p>