Why and how do soil bioengineering structures for river bank stabilization fail? Feedback on a large dataset from the GeniVeg database
Processus et causes de défaillance du génie végétal pour la stabilisation des berges de rivière : retour d’expérience sur un large jeu de données issues de la BD GeniVeg
Résumé
Soil bioengineering structures for riverbank protection take advantage of vegetation to structure the riverbanks by mimicking natural patterns. This approach is mainly based on expert knowledge and is limited by a lack of precise design criteria. This study focuses on damaged structures. Understanding the deterioration processes and the causes of failure through an empirical approach makes it possible to establish technical recommendations. We investigated 223 degraded bioengineering structures involving 269 techniques and included 454 observations of degradations. We observed two main failure types: (i) vegetation mortality, or absence of growth; and (ii) the mechanical failure of the structure and the adjacent bank. The most prevalent causes of vegetation mortality were submersion, lack of water, too much shade, lack of contact with the substrate, and species competition. From most to least frequent, the processes leading to mechanical failure were: scouring at the toe, erosion behind and at the ends of the structures, scouring around hard points in or around the structures, and partial failure of the internal structure. Toe scouring and erosion behind the structure remove substrate and therefore seriously affect vegetation growth. Multiple erosions quickly lead to the total collapse of the structure. Therefore, anticipating these processes from initial project conception and repairing the structure as soon as the first signs of erosion occur can help prevent failure and facilitate vegetation establishment by maintaining plant contact with the substrate. Finally, the causes of deterioration point to a need for special care such as watering, replanting, pruning and maintenance during the first years after installing bioengineering structures.