Willows root distribution on riverbanks through a descriptive approach

The use of Nature-based solutions for riverbank stabilization purposes in Western Europe is mainly based on willows implemented on engineered structures stabilizing riverbanks as they grow. Willows are ubiquitous and pioneer species capable of survival in submerged conditions when drowned a part of the year, therefore particularly well suited for soil and water bioengineering techniques. In the context of bend scour or of bed incision, bank toe bellow the line of brush growth is a weak point to control. This weaker zone presenting a low vegetation surface cover is in fact strongly subject to erosion leading to undermining and bank failure. This situation raised up the question: what is the effect on the soil stability of willows root reinforcement in this weak zone, knowing that their root system is allegdly able to develop in this part time drowned soil? In order to understand the stabilizing function of these species bellow the brush line, willows root system implemented on riverbanks from two different alpine watersheds in France have been surveyed in autumn 2023. The root development is studied in regard of the ground water level and soil texture. Nine soil profiles with willows root systems are described in the study and the respective root area ratios have been estimated manually. Soil profiles were dug up to the last visible root and never exceeded 80 cm, corresponding to the ground water level of the low flows that were running during the measurements. The study describe three different species root system aged from 2 to 20 years: Salix purpurea, S. daphnoides and S. eleagnos.