Soil and water bioengineering on moutain streambanks: Lessons learnt from extensive feedback on projects in the Alps

Channel mobility is essential to maintaining a river's healthy biogeomorphic and ecological functions. Although, local erosion processes can sometimes require stabilization to protect infrastructure. Soil and Water Bioengineering (SWB) techniques use vegetation as living construction materials to provide effective erosion control while preserving riparian ecological functions. Designing these structures depends significantly on expert knowledge and technical documentation that shares best practices and experiences, and provides specific hydraulic stress resistance thresholds. There is currently only limited documentation regarding the use and mechanical resistance of SWB on mountain streambanks where the harsh climate and steep slopes put a strain on vegetation growth and recovering, which in turn makes it challenging to design effective SWB structures. This study aims to fill this gap by providing an exhaustive record of SWB works throughout the French Alps, with additional sites in Italy and Switzerland. Hydraulic parameters, vegetation dynamics, and failure mechanisms were assessed across 48 SWB works. Drawing on our expertise, we assessed the key factors governing the implementation and success of SWB works in mountain streambanks. Our survey revealed that the works could be classified along a “naturalness” gradient, with vegetation playing different structural roles. Riprap was commonly used at the bank toe and technical choices depended mostly on the cultural context of the watershed and the local knowledge of SWB rather than on the hydromorphological context or the assets to be protected. Ultimately, this study enhanced our understanding of the hydraulic stresses experienced by these structures and the most prevalent causes of failure.