Author(s): Natacha Fructus; Zoe Deltombe; Andre Evette; Julie Delaye; Alain Recking

Linked Author(s): Alain Recking

Keywords: Scour; Erosion; Braided River; Fascines; Rootwads

Abstract: Scouring is a fluvial process which contributes to bank erosion by removing weakly cohesive materials from the bank toe. It is one of the leading causes of mechanical failure on riverbanks soil bioengineering structures, as the roots can't reach the bottom of the erosion pit due to anoxic conditions. Moreover, scour is not the result of a single event but a dynamic process involving successive phases of erosion and deposition, making it difficult to assess bed depth during flood events. In channels with significant bed level vertical variability during floods, the use of large wood structures as anti-scour devices can be used as a complementary strategy to the more classical water and soil bioengineering using live vegetation and generally implemented at a higher level where vegetation can develop. A physical model is developed to understand the functioning of large wood riverbank protection structures in a context of intense scouring. The prototype of this study is a reach of the Séveraisse River, a snowmelt -fed alpine stream in south-east France, with a slope of 1.6% and a coarse gravel bed, we scale down in the flume to 1:38. In order to correctly reproduce flow dynamic characteristics, Froude similarity was respected. The scouring patterns are quantified along stabilisation structures during a series of flood event s with sediment input followed by clear water floods (aggradation and degradation of the riverbed). Three techniques are compared: a double-toe fascine; a series of rootwads embedded along the bank; and a combination of the previous two techniques. Photogrammetry is employed to generate digital elevation models at peak flow and at the end of the flood. In addition, videos are taken at peak flood to analyse the velocity field using LSPIV. Preliminary results suggest that rootwads move the main fluid vein away from the bank, allowing lower velocities near the banks . It also appears that scouring is slightly reduced in the presence of rootwads compared to double -toe fascines.

DOI: https://doi.org/10.64697/iahr.proc.ise2026.abs.141

Year: 2026