ABSTRACT

Flume experients with heterogeneous sediment mixtures show that all the grain sizes present in suspension are involved in the process of formation of a bed when the flow velocity is decreased. The size distribution deposited closely reflects the one present in suspension. This finding is contradictory to the common belief that the coarser grains are the first to be discarded at decreasing current velocity. The existing equations after Rouse (1938) and Hunt (1969) also tend to overestimate depletion of the coarser grains.

A mathematical model for deposition, based on the idea that the process of depletion of the coarser grains is actually dampened in nature, has been developed in this work. This model gives results which compare well (particularly for low velocities) with the observed values comprising wide range of grain sizes. Using the model, it should be possible to estimate the grain-size distribution of the deposited material, given the size distribution present in suspension at a higher velocity. Since the size distribution of the deposits closely reflects that present in suspension, it should also be possible to work out paleoflow velocities of suspension currents from grain-size parameters of ancient deposits, following the relationship established earlier between mean grain size in suspension and the shear velocity (Ghosh and Mazumder 1981).