INTEGRATED ASSESSMENT AND ANALYSIS OF PHYSICAL LANDSCAPE PROCESSES THAT IMPACT THE QUALITY AND MANAGEMENT OF AGRICULTURAL WATERSHEDS
Location: Watershed Physical Processes Research Unit
Title: Sand Transport, Flow Turbulence, and Bed Forms over an Immobile Gravel Bed
Submitted to: International Symposium on River Sedimentation
Publication Type: Proceedings
Publication Acceptance Date: July 7, 2010
Publication Date: September 6, 2010
Citation: Kuhnle, R.A., Wren, D.G., Langendoen, E.J. 2010. Sand Transport, Flow Turbulence, and Bed Forms over an Immobile Gravel Bed. 11th International Symposium on River Sedimentation, Stellenbosch, South Africa, 6-9 September, 2010. CD (Conference Proceedings)
Interpretive Summary: Accurate predictions of the rate of sediment being moved in streams and rivers by 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 streams downstream of dams or other impoundments which block nearly all of the sediment moving on the bottom of the stream from upstream. 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. 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 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.
Channels downstream of dams often become armored because the sediment supply from upstream is cut off. Sand is generally supplied to these armored reaches intermittently from tributaries downstream of the dam or from sand bypassing. Accurate predictions of the rate of transport of sand over and through a gravel substrate is complicated and difficult To simulate these conditions, the effects of a stepwise addition of sand to an immobile gravel bed on the flow turbulence, sand transport rate, and configuration of the sand bed were investigated in a laboratory flume channel. Detailed measurements of flow, sand transport rate, bed texture, and bed topography were collected for five different discharges (10, 20, 30, 50, and 65 l/s) for each sand and gravel bed mixture. Flow velocity, measured with an ADV, was found to remain nearly constant, while turbulence intensity and bed shear stress were found to decrease with increasing sand elevation. Sand transport was measured using both physical samples and a density cell. The elevation of the sand relative to the gravel and percent exposure 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 that translated through and over the immobile gravel substrate. 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 shear stress adjusted by the normalized height of the sand relative to the gravel bed and the mean flow rate.