Prediction of sand transport over immobile gravel from supply limited to capacity conditions

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

Submitted to: Journal of Hydraulic Engineering
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/14/2016
Publication Date: 7/1/2017
Publication URL: https://handle.nal.usda.gov/10113/5763087
Citation: Kuhnle, R.A., Langendoen, E.J., Wren, D.G. 2017. Prediction of sand transport over immobile gravel from supply limited to capacity conditions. Journal of Hydraulic Engineering. 143(7):04017010. doi:10.1061/(ASCE)HY.1943-7900.0001292.

Interpretive Summary: Knowledge of the rate of sediment being moved by the flowing water of a stream is important for a number of reasons. As streams flow into reservoirs, deposition of the sediment will reduce the storage capacity of the reservoir. Sediment that is deposited in a reach of channel may decrease capacity of the channel and lead to flooding or destabilization of the channel and accelerate erosion of the channel banks, threaten roads and bridges and valuable agricultural land. The movement of sediment downstream of dams is generally a special case because most of the sediment from upstream is captured in the reservoir and the channel is starved for sediment downstream of the reservoir. The bottoms of channels exiting reservoirs often become depleted in fine sediments and armored with coarse sediments. Sediments finer than gravel, such as sands are introduced to armored streams by tributary streams downstream of the dam or by sand bypassing the dam. The goal of this study was to predict the transport of sand in a gravel armored channel for increasing sand amounts up to and including a gravel bed completely buried with sand. A series of experiments were conducted in a model stream channel in the laboratory to measure and characterize the movement of sand from supply limited to capacity conditions over a gravel bed. Equations were developed that accurately describe sand movement in exposed or buried gravel beds. The sand transport relation developed in this study will enable watershed managers to predict sand movement in streams which have similar characteristics to those used in the experiments. Information of this type 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: The prediction of the transport of sand in armored gravel reaches downstream of dams is complicated by variable bed conditions ranging from sand transported through gravel to sand in transport over buried gravel. Knowledge of the rate of sand transport in these conditions, however, is necessary for the successful management of streams and rivers that flow downstream of dams. It has been shown from previous studies that the elevation of sand relative to gravel along with shear stress at the elevation of the sand are needed to successfully predict sand transport. A series of experiments were conducted in a laboratory flume channel with increasing sand elevations to document the transition from supply limited sand transport over gravel to capacity sand transport. A modified sand transport relation was developed for the prediction of sand transport over an immobile gravel bed from supply limited to capacity transport conditions. It was found that the new relation predicted sand transport over gravel substrates and sand beds well for the data from this and two previous studies.