Erosion of sand from a gravel bed

Authors: Kuhnle, Roger; Wren, Daniel; Langendoen, Eddy

Submitted to: Journal of Hydraulic Engineering
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/16/2015
Publication Date: 2/17/2016
Publication URL: http://handle.nal.usda.gov/10113/61971
Citation: Kuhnle, R.A., Wren, D.G., Langendoen, E.J. 2016. Erosion of sand from a gravel bed. Journal of Hydraulic Engineering. 142(2)04015052-1-8. doi: 10.1061/(ASCE)HY.1943-7900.0001071, 2016.

Interpretive Summary: Effective land management requires an accurate knowledge of the rate of sediment being moved by streams and rivers. A channel in which the amount of sediment it is receiving is out of balance with its capacity to transport sediment will tend to become unstable and cause flooding or erosion of valuable top soil and pose a threat to nearby infrastructure. The bottom of streams downstream of unstable channels or other large sources of fine sediments often become filled with fine sediment which negatively affects the habitat of organisms which live or spawn on the stream bottom. To restore this environment, it is desirable for mangers to assure that flows enter a reach of stream which cause the excess fine sediments to be cleaned out of the stream bottom material yet do not cause the beneficial coarse material to be eroded away. A series of experiments were conducted in a model stream channel in the laboratory to measure and characterize the erosion of sand from an immobile gravel bed. It was found that the depth of clean out of the sand from the gravel substrate was well predicted by using the fall velocity of the mean size of sand and an effective strength of the flow which varied with the depth of penetration into the gravel substrate. Watershed managers will find the relations developed in this study useful to predict the depth which sand may be washed out of gravel beds. This type of information is critical for designing restoration strategies for streams that have been impacted by an excess of sand in a gravel substrate and will allow agricultural and other watersheds to be managed in a more informed and environmentally sensitive manner.

Technical Abstract: 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.