Experimental Study on Bed Load Control in Torrents by Open Slit Dams

Deposit basins closed by an open slit dam are often implemented for bed load control in torrents. Retaining bed load is of course the first function of these dams. However, these dams usually retain too much material causing erosion in the downstream channel. An experiment with scaled models was set up to represent the downstream part of a deposit basin. Natural material with a large range of grain sizes was used. For given widths of the slit, outgoing discharges, deposition volumes, slopes and backwater elevations were measured under controlled liquid and solid discharges. As long as there is no blocking of the slit, the outgoing solid discharge does not depend on the width of the slit. Solid discharging through the slit appears to be an all or nothing process since firstly only clear water is discharging at the outlet during deposition followed very rapidly by the same liquid and solid discharge as upstream of the basin. Evolution of the bed profile is controlled by the backwater elevation and the equilibrium topset slope. Backwater elevations depend on the liquid discharge, on the width of the slit and increases when bed load occurs through the slit. The topset slope increases with the solid concentration and slightly decreases with the liquid discharge for a given concentration. The topset slope during deposition is the same as the final equilibrium slope. A dam meeting the requirement of both retaining and discharging bed load has been proposed for further testing.