Effects of sediment load on the velocity field and friction factor of turbulent flow in an open channel

Abstract

An experimental investigation was made of the friction characteristics of streams with sediment load. Measurements of velocity and sediment profiles, and calculations of friction factor, f, and von Karman's constant, k, were made in a 40-foot tilting flume. Several runs were made with uniform flow and various bed configurations using sands of two sizes (.10 mm and .16 mm). For better understanding of the effect of sediment on von Karman's constant k and the friction factor, uniform clear water flows were established on stabilized natural sand beds. The depth and the mean velocity were kept the same as those of the movable bed stream for which the sand bed was stabilized, and a direct comparison was made. Then, by adding loose sand in steps and establishing uniform flow, the change in von Karman's constant and the friction factor with sediment load was studied. It was found that both the friction coefficient f, and von Karman's k, decreased as the sediment load was increased, although the coefficient f decreased by a much smaller percentage than the constant k. It is hypothesized that the sediment load appreciably reduces the rate of turbulent energy diffusion, thus reducing the turbulence level of the fully established uniform flow and changing the balance of turbulence energy. In a very small region near the bed the turbulent energy production, diffusion, viscous action and dissipation of energy due to sediment in suspension are all of about equal importance. A theoretical study was made of the distribution of both the production of turbulent energy and the dissipation of energy by the sediment along a vertical profile for hydrodynamically smooth bed,and it was made possible to integrate them to the bottom of the stream.