Erosion of Sand from a Gravel Bed

Abstract

Cleaning of fine sediment out of gravel streambeds has become an important method to restore affected stream habitats. Introducing the increased flows needed to entrain fine sediments without eroding the coarser fractions of the bed and potentially destroying its usefulness as a habitat requires careful selection of flow strength. Toward this end, a series of flume experiments was conducted on the entrainment of sand from immobile gravel beds to determine the depth to which the sand could be eroded without the gravel being mobilized. A series of steady flows was imposed on three gravel beds filled with sand with median sizes of 0.2, 0.3, and 0.9 mm. The shear stress of the series of flows ranged from just above the critical shear stress for initiation of motion of the sand up to 0.9 times the critical stress of the 36.1-mm gravel. The experiments continued until the rate of fine sediment exiting the channel was very small. It was found that the cleanout depth of the sand was reasonably predicted by estimating the shear stress in the gravel as the product of bed shear stress and the value of the cumulative probability distribution function of the elevations of the gravel surface evaluated at the surface of the sand layer. The appropriate initiation of sediment motion criterion was found to be a function of the shear velocity of the flow and the fall velocity of the fine grains.