La conception de l'aménagement des cours d'eau dans le cadre de la sauvegarde de l'environnement : L'exemple de la Fecht (Haut-Rhin)

Abstract

Author describes how development work was put into effect on the river Fecht, a tributary of the river III in the Haut-Rhin Departement of France. To counteract erosion caused by the river's substantial bed load in the uppermost reaches and gravel accumulation further downstream, the following river engineering works were put into effect :- 1. The longitudinal river profile was stabilised by subdivision into a number of sections at the equilibrium slope for the river bed material, each section being separated from the next by a permanent sill. 2 The river banks, consisting of erodible alluvium, were stabilised by maintaining the river bend radii in either direction at least five times the river width, which almost invariably reduced river bank protection requirements to their simplest expression, and frequently eliminated them altogether on the convex side of the bends. Except for sills excecding 1.5 m in height, which are based on Creager's design, i.e. usually with a paved energy-dissipation basin, most of the river control structures are built of rubble. This method was originally developed in the Austrian Alps about twenty years ago, with artificial ramps usual1y with a slope of 20 % and a width limited to 1/20th of that of t.he river. In plan, the control structures form a curve with a radius equivalent to 1 1/4 times that of the river width, which is designed to keep the flood discharge in the cent.re of the river downstream of the control structure. The ramps are fashioned from large lumps of granite of at least 1 ton weight, which are placed side-by-side in the form of a particularly rough hedgehog structure capable of efficiently absorbing the full kinetic energy of the water. The upstream row of boulders is encased in concrete, whilst the downstream row is maintained in position by one or two rows of rails rammed to a depth of 4 m, into the river bed, spaced between 60 and 80 cm apart. This system offers the following advantages: - It does not mar the landscape ; -It is easily installed, as there are no foundation s; -No dewatering is required for work under water ; - It can undergo deformation without endangering the overall structure ; - Efficient aeration of the water assists self-purification ; - No interference with fish proceeding up-river. The same material is used in smaller sizes (smaller by 100-300 kg) for river bank protection. The material was formerly placed by hand, proceeding fish-scalewise from downstream to upstream, but power shovels are now used for the purpose, except for placing the small stones inserted between the large blocks. The foundatiom of the structure consist of rubble tipped into a pit with a trapezoidal cross-section. To improve the appearance of the site, willow cuttings have been implanted (salix purpurea-usually in autumn) underneath the rubble bed, or more frequently standing more or less upright between the lumps of rock. Thus, after only a few years, the rubble is now hidden behind a curtain of vegetation, which simply requires cutting back every five years.