Biogeomorphic assessment of natural streambank structures in the Alps: A step to improve soil and water bioengineering techniques

On mountain streambanks, the feedback loops between vegetation and geomorphic processes remain largely unknown, though the mineral and vegetation components of steep mountain streams play a key joint role in bank stability. This lack of knowledge concerning the structure and functioning of high-elevation streambanks is a major obstacle to the development of soil and water bioengineering techniques. Thus, many of these banks have already been protected with civil engineering structures. Our study attempts to overcome this challenge by observing natural streambanks and mimicking their processes through nature-based solutions.

We provide herein a first analysis of intermingled geomorphological and ecological units along natural banks in mountain streams, aiming to identify the bio-geomorphological structures that remain stable under erosion. The study focused on streambank reaches located in the Vanoise mountain range (northern French Alps) that had characteristic natural riparian vegetation and stable bank toes. The sites were representative of the altitudinal range (1.480–2.150 m a.s.l.) and of various bed slopes (0.8–28%). Geomorphic surveys (slope, particle size, shear stress) and ecological surveys (vegetation cover and biological features) were conducted on the bank toes.

Our results highlight two bio-geomorphologic types of streambanks: 1) those along steep-sloped streams, where mixed mineral and plant units, dominated by green alder and a few willow species, jointly contribute to bank stability; and 2) low-gradient streambanks, where bank stability is entirely ensured by the vegetation, composed of willow species, which vary according to elevation. This study provides additional knowledge on bank structure and stability in mature high-elevation streams.