ABSTRACT

The deposits formed in the lee of ripples, dunes, and sand deltas depend for their properties on the behavior of grains when subjected to fluid shearing, turbulent diffusion, and free-gravity slip. Reduced to essentials, the case is that of diffusion in a turbulent reattaching half-jet of low speed with normal gravity. A brief physical exploration is made of the behavior of grains under these conditions. Given knowledge of the sediment discharge and the local properties of the sediment load, the rate of settling of grains at a station downstream from the sand-body crest is regarded as a function of the time-average and fluctuating properties of the flow. The central tendency of the grain distribution is considered to depend on the mean properties of the motion, while the dispersion of the distribution is thought to depend on the fluctuating properties. But the grain distribution is likely to be very strongly skewed toward the larger values of downstream distance, and, under the range of conditions of interest in connection with natural flows, the point of maximum deposition rate lies probably very close to the crest of the sand body.

Experiments with three sands of contrasted properties, at total load transport rates varying over a one-hundred-fold range, yield distributions of grain settling rate which approximate to log-normal distributions. Over the practical range of conditions, however, the distributions can be more easily represented by the general expression

[W_x]/[W_L] = ([X]/[L])^-n

in which W_X is the settling rate at a horizontal distance X from the crest of the body, W_L is the settling rate at a reference distance L, and n is an exponent empirically related to the ratio of the mean flow velocity at the crest of the body to the sediment free falling velocity. However, there is no simple empirical relationship between the reference settling rate and the total load transport rate, nor can the reference rate be predicted.