Auto-suspension of transported sediment; turbidity currents

Abstract

It has been apparent for some time past that fine sediment material carried in suspension by a turbulent water stream flowing by gravity is apt to behave inconsistently with conventional theory. This demands that the concentration of suspended solids, which being heavier than the fluid tend to fall through it, must always increase downwards towards the bed. In fact, however, the concentrations of fine sediment grades present in rivers are often found to increase upwards instead of downwards. Further, while the discharge or transport rate of the coarser grades is found to be a function of the stream flow, that of the finer grades appears to be unlimited. The critical grain size below which these anomalies occur is usually put at about 50 µ for sediments of natural mineral density. But the factors on which the critical size depends have not previously been looked into. The reason for these anomalies has remained mysterious. Equally mysterious have been the conditions which enable a submarine turbidity current to maintain itself and to transport sand and silt in turbulent suspension for distances of many hundreds of miles over a very gently sloping ocean bed.