Role of bioengineering structures made of willow cuttings in marly sediment trapping: assessment of three real-size experiments in the Southern French Alps

Improving knowledge on the ability of bioengineering structures made of willow cuttings to enhance efficient and sustainable sediment trapping in marly gullies in the Southern French Alps under a mountainous Mediterranean climate, to decrease sediment yield at their outlets, is a key issue today for the international scientific community working in geosciences and ecology. This study therefore aims to assess three real-size experiments (A, B and C) carried out between 2003 and 2013 in this environment. A total of 157 bioengineering structures using purple and white willow (Salix purpurea and Salix incana) cuttings–which have been shown to resprout and survive more than 2 years after their installation, corresponding to brush layers with brush mats on wooden sills (BLM), 1.2 m wide and 2 m long, installed on the floors of 33 experimental marly gullies, were monitored. The results showed that sediment trapping occurred upstream of the vegetation barriers from the 1st year onwards. Considering the depth of sediment trapped per experiment, the mean annual values reached 11.2 cm yr-1 after 3 years in experiment A, 7.7 cm yr-1 after 2–4 years in experiment C and 5.1 cm yr-1 after 5 years in experiment B. Occasionally, BLMs showed that they could trap up to 16 and 15 cm.yr-1 in experiments A and C, respectively. Considering the volumes of sediment trapped per experiment, the mean annual values reached 0.25, 0.14 and 0.08 m3 yr-1 in experiments C, A and B, respectively. Maximum values for one structure reached 1.94 m3 per year in experiment C. The significance of the volumes of trapped sediment and the sustainability of sediment trapping are discussed, and rules for bioengineering strategies are proposed.