Title: An Experimental Study of Sand Transport over an Immobile Gravel Substrate Authors
Submitted to: Federal Interagency Sedimentation Conference Proceedings
Publication Type: Proceedings
Publication Acceptance Date: May 10, 2010
Publication Date: June 28, 2010
Citation: Kuhnle, R.A., Wren, D.G., Langendoen, E.J. 2010. An Experimental Study of Sand Transport over an Immobile Gravel Substrate. 2nd Joint Federal Interagency Sedimentation Conference, Las Vegas, NV, June 27-July 1, 2010. CD (Conference Proceedings) Interpretive Summary: An accurate knowledge of the rate of sediment being moved in streams and rivers by the flowing water is necessary because the 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. A special case of sediment movement is in the streams downstream of dams or other impoundments which block nearly all of the sediment moving on the bottom of the stream. This often causes the bottom of the stream to become depleted in finer sediment sizes and prevents motion of the accumulated coarser sediment except in all but the largest flows. A stream in which the bottom is made up 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 or by sand bypassing the dam. Movement of sand sediment in armored streams is 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 strength of the flow and the height of the sand relative to that of the gravel. The relations developed in this study will be useful for 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 effects of a stepwise addition of sand to an immobile gravel bed on the sand transport rate and configuration of the sand bed was investigated in a laboratory flume channel. Detailed measurements of sand transport rate, bed texture, and bed topography were collected for four different discharges (Fr~0.2, 0.3, 0.5, and 0.6) for each sand and gravel bed mixture. Sand transport was measured using both physical samples and a density cell. The elevation of the sand relative to the gravel and percent coverage of the sand bed were both evaluated in predicting the sand transport rate. For the highest two discharges the sand amalgamated into a small number of large, slow moving bed forms. A mechanism for this phenomenon is proposed. 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 mean size of the bed sediment, with the critical shear stress adjusted by the normalized height of the sand relative to the gravel bed and the flow rate.