Assessment of canal bank stability with vegetation root reinforcement

There is an increasing need for using nature based solutions in protecting canal and stream embankments in the Netherlands and delta areas in general. Vegetation provides additional reinforcement and forms an integral part of many nature-based solutions. However, quantifying this reinforcement in-situ is challenging. This study aims to quantify the root reinforcement of three species prevalent along canal embankments – Salix fragilis L. (SF), Salix purpurea L. (SP), and Crataegus laevigata DC. (CL) – using the corkscrew extraction technique. Furthermore, canal bank stability was analyzed under different bank conditions regarding protection (unprotected, protected by vegetation), bank geometry, and hydraulic conditions.

Quantity of roots and Root Area Ratio (RAR) generally decreased with depth for all species. While root breakage was observed in most samples, all species exhibited increased ductility with higher root densities, except for CL at two depths. SF showed higher root reinforcement at shallower depths (≤ 250 mm), while SP demonstrated greater reinforcement at deeper depths. Results demonstrate that the corkscrew extraction technique is a quick and minimally invasive method for measuring root reinforcement in riparian environments.

Bank stability simulations revealed that vegetation significantly increases the stability of canal banks. Notably, when considering measured root reinforcement, the factor of safety improved dramatically from 1.08 to 2.46, even under analyzed worst case conditions. However, the analysis suggests a limiting root reinforcement beyond which further increases in root reinforcement have minimal impact on stability. Monitoring using the corkscrew apparatus and future design approaches could aim to achieve this minimum reinforcement.