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Erosion of sand from a gravel bed

Roger A. Kuhnle, Daniel G. Wren, Eddy J. Langendoen
Journal of hydraulic engineering 2016 v.142 no.2 pp. 04015052
cleaning, cumulative distribution, engineering, gravel, habitats, sand, shear stress, stream channels, suspended sediment, water erosion
Cleaning of fine sediment out of gravel stream beds has become an important method to restore impacted 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. Towards this end, a series of 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 were imposed on three gravel beds filled with fine sediments of 0.2, 0.3, and 0.9 mm, respectively. The shear stress of the series of flows ranged from just above the critical shears stress for initiation of motion of the sand sediment up to 0.9 times the critical stress of the 36.1 mm gravel. Experiments were continued until the rate of fine sediment exiting the channel was very small. It was found that the clean-out depth of the sand was reasonably predicted by estimating the shear stress in the gravel as the product of bed shear stress and the cumulative distribution function of the gravel surface evaluated at the level of the sand surface and using a suspended sediment initiation criterion that was a function of the shear velocity of the flow as related to the fall velocity of the fine grains.