Flume study of the deposition of fine sediment into river gravel

Increased land erosion and drainage, combined with larger impoundments for water consumption needs, result in increased levels of sediments infiltrating into gravel river beds. This can cause a threat to the ecology of rivers and to fish populations. However, the mechanisms by which transported fine sediments deposit, i.e. pass through the surface layer of gravel before infiltrating into the bed pores, are poorly researched. Several investigators have highlighted the needs for further explorations in that field, as it also has direct implications in phenomena such as flood hydraulics sediment transport, armouring and downstream fining. Previous studies have indicated that deposition rates are proportional to sediment concentrations and fall velocities in still water. A preliminary series of experiments was conducted in an 8m-long flume to compare deposition rates of sand through single layers of gravel to transport rates measured 25mm above the bed surface. It was found that the deposition rates Δ are proportional to the near-bed concentration of fine particles Cb. The constant of proportionality has the dimension of a velocity, and represents an average fall velocity through the bed surface layer, or deposition velocity wd. The ratio between the deposition velocity wd and the fall velocity in still water wd*, referred to as the dimensionless deposition velocity Wd*, gives an indication of the effects of the gravel bed surface on the settling behaviour of the sediment particles. Following the preliminary series of experiments, the main series of experiments was aimed at measuring deposition velocities in different hydraulic and sediment conditions to study the physical mechanisms controlling the deposition process. It was found that the deposition velocity generally increases with grain size, but tends to stabilise in the upper size range (i.e. particles transported by saltation). Deposition velocities tend to decrease as bed shear stress and turbulence level increase, particularly in the case of medium-size sand (~300-350µm). Gravel size does not appear to have a significant influence on the deposition velocity of particles coarser than ~200µm. The deposition velocity results of experiments using medium-size sand were, in general, larger than the fall velocity Ws for particles finer than ~200µm in diameter, indicating a phenomenon of enhanced deposition. This phenomenon has already been observed in previous studies (e.g. Jobson and Sayre, 1970). It was not detected in experiments using very fine sand, but the deposition velocity results in this case followed similar variations with grain size to that observed with medium sand. The experimental results suggested a distinction between three ranges of fine sediment: (1) the Stokes' range includes very fine particles, the diffusion coefficient of which is nearly equal to that of the fluid. The depositional behaviour of these particles is directly influenced by the structure of near-bed turbulence. A bursting-based analysis showed that the so-called deposition parameter wd+ = wd u* / (g d) is in this case related to near-bed turbulence and bed roughness parameters, but not to the grain size; (2) the intermediate range, which is influenced by both turbulence and gravity. A dimensional analysis indicated that, under these circumstances, Wd+ increases with grain size; and (3) the upper range, which corresponds to the range of particles transported by saltation. Deposition is, in this case, mainly influenced by the landing angle of the fine particles and the bed surface topography. The experiments also indicated that the deposition process is influenced by turbulence damping, a phenomenon which consists in the reduction of the eddy diffusivity of the fluid by increasing sediment transport concentration and grain size. This phenomenon can be described using van Rijn's φ coefficient. It was observed that the deposition velocity wd tends to increase with increasing turbulence damping. The experimental results were applied to model downstream sorting of fine particles in gravel-bed rivers in 2-D uniform flow conditions. Methods of computation and examples of applications are described