Recent developments on the mechanics of sediment suspension

Abstract

Some developments in our knowledge of the properties of the sediment suspension in turbulent boundary-layer flow are reviewed, the emphasis being placed on the processes close to the bed of a turbulent open channel. The suspension process considered in this paper is the one which is due only to the action of turbulence. This is normally what occurs in water flows with silt and sand size sediment particles. The processes which involve gravel size sediment suspension in water flow and also sand suspension in air flow are excluded in the present review. The work which relates the sediment suspension to the near-bed turbulence structures (namely the bursting process) is reviewed with special reference to the author’s own work. Lift forces on a moving particle near the bed are discussed. Available information is given, covering both the smooth and rough bed cases. Finally, the paper addresses to one other important aspect of the suspension process: the limiting stages of suspension. Information on our knowledge of 1) the termination of suspension (of the particles which are put into the flow above the constant-stress layer) and 2) the initiation of suspension (of bed-material particles) is reviewed. In this context the paper discusses some new data obtained using 1) a conventional flume facility with a loose particle which is a member of a bed of like particles and 2) a turbulent-spot facility with a sediment bed (of size small compared with spot dimensions) placed on an otherwise rigid wall.