Effect of a Porous Sand Bed on Incipient Sediment Motion

Incipient motion tests were conducted for flow into and out of a porous bed. Seepage out of a bed does not affect incipient motion measurably because the seepage force is lost once a sediment particle rocks. Seepage into a bed may either enhance or hinder incipient motion, depending upon the relative effect of the boundary shear stress and the seepage force, both of which depend on the seepage flow. For a given hydraulic gradient the size of the sand grains is critical regarding incipient motion. For turbid water flow over a bed, fine particles of the mixture may become deposited in the bed, resulting in a concrete-like, more resistant bed. Typical values of the hydraulic gradient are given for flow out of a canal and in an ocean bed under oscillatory waves.