ARS | Publication request: Sand transport over an immobile gravel substrate

Submitted to: Journal of Hydraulic Engineering
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: July 10, 2012
Publication Date: February 1, 2013
Citation: Kuhnle, R.A., Wren, D.G., Langendoen, E.J., Rigby Jr, J.R. 2013. Sand transport over an immobile gravel substrate. Journal of Hydraulic Engineering. 139(2):167-176.

Interpretive Summary: Excess sediment may fill reservoirs and reduce their capacity, may fill channels and cause flooding, may degrade water quality, and may cause instability of the channel banks which can cause the destruction of valuable agricultural and other lands. Thus, accurate predictions of the rate of sediment being moved by streams and rivers are necessary for effective land management. Streams downstream of dams or other impoundments which block nearly all of the sediment moving on the bottom of the stream from upstream make a very challenging environment in which to accurately predict sediment movement rates. The bottom of the stream usually becomes depleted in finer sediment sizes downstream of a dam and prevents motion of the accumulated coarser sediment except in all but the largest flows. A stream in which the bottom consists of coarse gravel sediment which seldom moves is said to be armored. Finer sediments, such as sands are introduced to armored streams by tributary streams downstream of the dam, by sand bypassing the dam, or in some cases after the dam is removed. The characteristics of the flow and the movement of sand sediment in armored streams are very difficult to predict accurately. A series of experiments were conducted in a model stream channel in the laboratory to measure and characterize the movement of sand with an immobile gravel bed. It was found that the movement of the sand over and through the gravel was well predicted by using the mean size of the sediment bed and an effective strength of the flow which varied with the height of the sand relative to that of the gravel. Watershed managers will find the relations developed in this study useful to predict sand movement in streams which have characteristics resembling those used in the experiments. This type of information is critical for improving sediment prediction and sampling techniques and will lead to advances which will allow agricultural and other watersheds to be managed in a more informed and environmentally sensitive manner.

Technical Abstract: Experiments were conducted in a laboratory flume channel to evaluate the effects of increasing amounts of sand with an immobile gravel fraction on the sand transport rate and configuration of the sand bed. Knowledge of the movement of sand in gravel beds is important for the management of streams and rivers with armored beds such as those downstream of dams. Detailed measurements of sand transport rate, bed texture, and bed topography were collected for four different discharges (~20, 30, 50, and 65 l/s) at approximately the same flow depth of 0.2 m for each sand and gravel bed mixture. Sand transport was measured using both physical samples and a density cell. For the highest two discharges, the sand amalgamated into a small number of long and low bed forms that translated through and over the gravel bed. A collapse of the transport data was accomplished by relating the sand transport rate to a power function of the bed shear stress scaled by the critical shear stress of the mean bed material grain size, with the shear stress adjusted by a factor related to the normalized height of the sand relative to the gravel bed and the mean velocity of the flow.