|Romkens, Mathias - Matt|
Submitted to: Proceedings of the International Yellow River Forum
Publication Type: Proceedings
Publication Acceptance Date: 3/15/2005
Publication Date: 10/16/2005
Citation: Prasad, S.N., Suryadevara, M.R., Romkens, M.J. 2005. Experimental observations of the effect of particle interactions on the transport capacity in shallow overland flow. Proceedings of the 2nd International Yellow River Forum, Zhengzhou, P. R. China, Oct. 16-21, 2005. II:351-361. Interpretive Summary: Sediment transport is a highly complex process that commonly involves hydraulic pressures and shear forces on water and sediment flow in a convoluted system with changing boundaries and boundary conditions. In most overland flow problems and models, calculations of sediment transport are based on the concepts of excess shear or streampower relative to some critical value that is a characteristic of the medium. In these models, the sediment concentrations in the flow is usually assumed to be homogeneous. Not much attention is given to the micro-mechanical nature and the physical processes involved relative to the sediment phase. Our overland flow studies have shawn that a high degree of interaction takes place between sediment particles in transport that substantially affect the flow transport capacity. This article presents the results of an experimental study concerning the effect of sediment concentration on the sediment transport mode shallow overland flow for different size materials. The observations indicate that, even under a steady state flow regime and a constant "feeding" rate of granular material at the upstream end of the stream bed, the mode of transport drastically changes with concentration from a saltation made at very low concentrations of less than 45 grams/min into a 21.6 l/min flow rate to a strip mode of a wave-like nature in a feeding rate from 45-150 g/min, and the formation of a meandering bedform for rates larger than 150 g/min for granular material of about 1.2 mm and bedslope steepness of 1 degree. These findings suggest, once more, that sedimentary fluid mechanics may play a far greater role in sediment transport than thus far has been assumed.
Technical Abstract: 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. At a grain addition rate above the transport capacity the initial saltation mode of sediment movement quickly changed into organized structures such as strips. 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 was considered.