Location: Watershed Physical Processes ResearchTitle: Erosion depth of sand from an immobile gravel bed Author
Submitted to: Meeting Proceedings
Publication Type: Proceedings
Publication Acceptance Date: 7/1/2014
Publication Date: 12/11/2014
Citation: Kuhnle, R.A., Wren, D.G., Langendoen, E.J. 2014. Erosion depth of sand from an immobile gravel bed. In: Sediment Dynamics from the Summit to the Sea, Y.J Xu (ed) IAHS, Wallingford, UK. pp. 117-121. Interpretive Summary: Accurate predictions of the rate of sediment being moved by streams and rivers are necessary for effective land management. A channel which is receiving too much or too little sediment compared to its capacity will tend to become unstable and cause flooding or erosion of valuable top soil and pose a threat to nearby infrastructure. The bottom of streams downstream of unstable channels or other large sources of fine sediments often become filled with fine sediment which negatively affects the habitat of organisms which live or spawn on the stream bottom. To restore this environment, it is desirable for mangers to assure that flows enter a reach of stream which cause the excess fine sediments to be cleaned out of the stream bottom material yet do not cause the beneficial coarse material to be eroded away. A series of experiments were conducted in a model stream channel in the laboratory to measure and characterize the erosion of sand from an immobile gravel bed. It was found that the depth of clean out of the sand from the gravel substrate was well predicted by using the fall velocity of the mean size of sand and an effective strength of the flow which varied with the depth of penetration into the gravel substrate. Watershed managers will find the relations developed in this study useful to predict the depth which sand may be washed out of gravel beds. This type of information is critical for designing restoration strategies for streams that have been impacted by an excess of sand in a gravel substrate and will allow agricultural and other watersheds to be managed in a more informed and environmentally sensitive manner.
Technical Abstract: Abstract This study was conducted to provide information on the depth of erosion of sand (D50 = 0.3, 0.9 mm) from immobile gravel (D50 = 36.1 mm) under steady uniform flows with bed shear stresses from 0.1 to 0.9 of that required to entrain the gravel. This situation, often encountered downstream of dams, has important implications for habitat restoration. Steady uniform flows were imposed on a channel containing a mixture of sand and gravel until sediment concentrations in the flow exiting the channel became small. The elevation of sand relative to gravel was measured after each experiment and compared poorly to calculated depths from published relations whose predictions were based in part on the equivalent grain roughness of the bed. An improved predictive relation was developed by using the cumulative distribution function of the surface gravel elevations to scale the shear velocity available for transporting sand from the gravel substrate.