Restoration of an alluvial mattress using artificial armour layer Experimental study on the river Ouvèze (France)

摘要

Sogreah has been commissioned by the Syndicat Ouvèze Vive to perform a comprehensive study of solid transport andmorphodynamics for the river Ouvèze (Ardèche, France)The river Ouvèze (catchment area = 123.5 km²) is a right-bank tributary of the river Rhone that is subject to typicalCévennes area rainfall. Most of its bed currently consists of outcropping substratum over almost its entire length. Thisriver was subject to very little natural bed movement, with a relatively high level of solid transport (estimated at4,000 m3/year at the confluence with the river Rhone), but a rather thin alluvial mattress (probably around 1 to 2 m).The volume of alluvial materials extracted over the past forty years is estimated to be comparable to a century ofnatural transit: a generalised incision of the bed has occurred, leading to the disappearance of the alluvial mattress,thereby exposing a fragile substratum.This bed incision raises problems, not so much because of the deepening of the riverbed but rather because of theexposure of the substratum. Indeed, the river currently flows over large marly-chalk slabs, leading to negativeenvironmental impacts and, in particular, the destruction of fish habitats and the loss of attractive river features. Theaim is therefore to restore an alluvial mattress.The traditional method used to restore an alluvial mattress is to build sills across the river bed. This method has anumber of drawbacks: in particular, it blocks alluvium, thus creating a shortage downstream of the sill, so this solutionwas not selected for this project.Sogreah proposes an innovative, experimental technique: the restoration of an alluvial mattress using artificial armourlayer. This technique consists in covering the riverbed with coarse alluvium, which resists the tear-out force of the flow.The dimensions of the materials brought in must therefore be sufficient for this purpose.The plan is to try various solutions and materials on several pilot sites before generalising the protection process to theentire length of the river affected.