Experimental Observations of the Effect of Particle Interactions on the Transport Capacity in Shallow Overland Flow

摘要

In upland areas, the surface flow regime carries sediment in several states varying from very small concentration to relatively high concentrations in hydraulic conditions with supercritical Froude numbers. In our laboratory studies of shallow overland flow, sediments were introduced in a controlled manner by a hopper arrangement and measurements were made to determine the relationship between particle velocity and sediment concentration. The paper will discuss the results of these studies. The data clearly indicated the existence of maxima of sediment transport rates under various hydraulic flow regimes. Observations on moving grains revealed that the mode of transport is by saltation. Under these conditions water flow was dominated by the appearance of roll waves which contain a significant portion of the total kinetic energy of the flow. Hence, roll waves act as primary energy source in transporting eroded sediment in shallow flows. At a grain addition rate above the transport capacity the initial saltation mode of sediment movement quickly changed into organized structures such as stripes. This critical condition is associated with the modulation of roll waves and happens when most of the energy in the roll waves is consumed by the transporting sediment. This study describes the inception, development, and maturity of grain patterns that are formed when the sediment supply is beyond the critical value. It was observed that particle interaction mechanism plays a very significant role and although the energy is supplied by the hydraulic condition of water flow, the bed evolution is embedded in the DNA equivalent of grain movement process. In order to explain the interrelationships between hydraulic condition and sediment characteristics, a two phase flow model based on St. Venant equations of shallow water flow and dry granular flow (Prasad et al. , 2000) was considered. The proposed mathematical model substantiates the critical condition for phase transformation from steady saltation into an organized strip mode.