Title: Flow and sand transport over an immobile gravel bed. Authors
Submitted to: Congress of International Association for Hydraulic Research Proceedings
Publication Type: Proceedings
Publication Acceptance Date: June 30, 2009
Publication Date: September 4, 2009
Citation: Wren, D.G., Langendoen, E.J., Kuhnle, R.A. 2009. Flow and sand transport over an immobile gravel bed. Congress of International Association for Hydraulic Research Proceedings. 8 pages. August 9-14/2009. Interpretive Summary: Many dams in the USA and elsewhere have exceeded their design life and are being considered for remediation or removal, which will result in increased amounts of sediment downstream of the dam. Very little sediment makes it past dams, so the stream bed downstream is often clean, with no sediment particles on it. When sediments are reintroduced, they change the nature of the water flow. The amount of sediment that can be stored and transported and the effect on water depth are all information that is needed when making planning decisions regarding dam removal. The work is also applicable to the general case of sand transport over gravel beds, which is common in some areas of the United States. In the work described here, incremental amounts of sand were added to a clean gravel bed. The effects on flow and sediment transport were measured after each increment over a range of water velocities. The depth of the sand relative to the gravel was found to be an important control on sediment transport.
Technical Abstract: Many dams in the USA and elsewhere have exceeded their design life and are being considered for remediation or removal, which will result in the reintroduction of fine sediments, often into coarse grained armored substrates, downstream of dams. The deposition of sand in the interstices of the gravel substrate complicates the prediction of sand transport due to difficulties in predicting the effective shear stress, the exposed area of the sand, and the resulting changes in water depth. An adjustable-slope, recirculating laboratory flume was used to evaluate the effects of a stepwise addition of sand to an immobile gravel bed on the properties of flow and sediment transport. Detailed measurements of fluid velocity, sand transport rate, bed texture, and bed topography were collected for four different discharges (Fr=0.1, 0.3, 0.5, and 0.6). A Laser Doppler Anemometer and an Acoustic Doppler Velocimeter were used to measure mean and turbulent flow quantities while a flow-through density cell and physical samples were used to monitor sand transport. Additions of sand had no measurable effect on flow structure until the depth of sand was approximately 5 cm below the top of the gravel layer. It was found that elevation of the sand bed relative to the top of the gravel and grain shear stress were the major factors controlling the rate of sand transport.