Sediment Transportation Mechanics: Initiation of Motion

Water flowing over a bed of sediment exerts forces on the grains that tend to move or entrain them. The forces that resist the entraining action of the flowing water differ according to the grain size distribution of the sediment. For coarse sediments, the forces resisting motion are caused mainly by the weight of the particles. Finer sediments tend to be cohesive and resist entrainment mainly by cohesion rather than by the weight of the individual grains. Also, in fine sediments, groups of grains are entrained as units whereas coarse noncohesive sediments are moved as individual grains. Cohesion of the fine sediments is a complicated phenomenon that appears to vary with mineral composition and environment. Although the problem of entrainment resistance of cohesive sediments is an important one it has received less attention than the similar problem of entrainment of noncohesive sediments. This situation is reflected in the fact that most of this report concerns noncohesive sediments. From a scientific viewpoint, the problem of impending motion is one of the simpler of the many complicated sedimentation problems, and therefore one that might be attacked with hope of success.