Analysis of Dimensionless Variables to Study the Problem of Sediment Threshhold

Abstract

Sediment transport constitutes a vital part of the field of fluvial hydraulics with its newlineapplications going as far as coastal engineering. The initial motion of the sediment which is newlinecommonly called the incipient motion marks the beginning of the scouring process in natural newlinestreams. Sediment threshold is a condition where the destabilizing forces equal the stabilizing newlineforces and the sediment particle is about to experience the incipient motion. Two commonly newlineused parameters for sediment threshold, the Shields parameter and the Movability Number, newlinehave been studied by various researchers. The primary focus in the present study is on the newlineMovability Number with Shields parameter frequently used for the comparison purposes. newlineThe geometry of a sediment bed in natural channels is complicated. Its configuration newlineis defined by various characteristics. Here, two such features, sediment grain protrusion and newlinesediment non-uniformity, are considered which are important vis-à-vis sediment threshold. newlineGrain protrusion, an important characteristic of non-flat sediment beds, is a measure of the newlineexposure of a sediment particle to the streamflow. It is an important determinant of the newlinethreshold of sediment motion and its transport. Its effect on the Shields parameter has already newlinebeen examined while as this research brings attention to the Movability Number. The results newlineshow that grain protrusion has profound impact on the Movability Number. Sediment on a newlinechannel bed is generally not composed of particles of the same size. This non-uniformity newlineaffects its incipient motion condition. As a model of an otherwise very complicated situation, newlinetwo sediment sizes are considered here, the overlying sediment size d and the underlying newlinesediment size D, while as the ratio d/D is considered the parameter of non-uniformity. It is newlinefound that the critical Movability Number decreases when the d/D ratio increases, that is, as newlinethe overlying sediment size increases and there is less likelihood of grains being hidden in newlineinterstices of the underlying layer. newlineVario