Roots mechanical effects on hydraulic riverbanks erosion and on shallow landslides: tools for the protection forest management along channels

Bern University of Applied Sciences, School of Agricultural, Forest and Food Sciences, COSCI, Hydraulic Platform LCH, Institute of Civil Engineering, EPFL-ENAC, Lausanne, CH and University of Lausanne, Institute of Earth Surface Dynamics (marceline.vuaridel@unil.ch)

Floods and intense surface runoff are recurring hazards known for triggering erosion processes at the channel and the catchment slope scales, respectively. Whilst the firsts determine the removal of streambank material, also referred to as hydraulic streambank erosion (e.g., Ruiz-Villanueva et al., 2014), the seconds are typically responsible for destabilizing shallow landslides. Both processes are exacerbated by extreme precipitation events, and can cause important damages to forests, agriculture, civil structures, and settlements through the loss of land masses. Moreover, streambank erosion and shallow landslides can be responsible for the recruitment of large wood (LW), whose transport during floods may strongly impacts on downstream infrastructures of urbanized areas (e.g Ruiz-Villanueva et al., 2014).

Via augmented mechanical stabilization, plant roots may significantly decrease the susceptibility of riverbanks to hydraulic erosion as well as shallow landslides. Under certain conditions, plant roots can be considered an alternative protection against such processes with respect to other civil engineering measures (Stokes et al., 2014). However, root reinforcement effectiveness depends on many factors such as roots density, soil properties, and soil thickness (Cohen and Schwarz, 2017), which implies that some vegetated areas have a more significant effect than others. Most available models ignore the contribution of plant roots with acceptable spatial resolution.

In this work, we present BankforNET and SlideforNET, two physically-based modelling tools, which have been developed to take the different stabilizing effects of soil reinforcement mechanism by plant roots into account. This is important for proper modeling of bank erosion and landslides processes during extreme events, and to optimize forest protection strategies. BankforNET is a one-dimensional, probabilistic model which simulates expected hydraulic streambank erosion by considering channel morphology, bank sediment material, vegetation roots, and a certain discharge scenario. The SlideforNET is a probabilistic model based on the 3D analysis of slope stability and takes the lateral and basal root reinforcement into account. Ultimately, it gives an estimation of the degree of protection of a forest against landslides.

These tools are currently being tested in a catchment of 29 km² in NW Switzerland for the priorisation of protective forests against risks related to LW transport during floods. Based on the model results, the possible silvicultural measures are defined considering quantitative criteria such as the risk mitigation effect of the forest stands, or their risk increment due to LW recruitment and transport. This study is an example of how quantitative tools can be use by decision makers to priories the role of protection forest in a catchment and to support the definition of silvicultural measure to mitigate the risks due to LW transport.