The effect of a bimodal gravel size distribution on clean-out depth of sand

Authors: KUHNLE, ROGER - Retired ARS Employee; SMITH, JAMES - Us Geological Survey (USGS); Wren, Daniel; Langendoen, Eddy

Submitted to: Journal of Hydraulic Engineering
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/9/2024
Publication Date: N/A
Citation: N/A

Interpretive Summary: Channels in which the boundary is unstable and eroding destroy valuable topsoil and are detrimental to organisms which live in the water column and on the bottom of the channel. The bottom of unstable gravel channels often become filled with fine sediment which negatively affects the habitat of organisms which live or spawn on the stream bottom. To restore these impacted stream environments, where possible, watershed managers often need to make sure that flows enter a stream channel that promotes the removal of excess fine sediments from the stream bottom material, but yet does not cause the beneficial coarse material to be eroded away. A series of experiments were conducted in a model stream channel to characterize the erosion of sand from an immobile gravel bed. In the first series of experiments the sediment on the bottom of the channel was composed of gravel with a narrow distribution of sediment sizes. In the second series the bottom sediment was composed of a mixture of sizes more representative of natural streams. Two published methods that predict the depth of fine sediment clean-out did not match measured data for the second experimental series while a third method yielded predictions much closer to the measured data. Accurate predictions of the clean-out depth were found to require a technique that accounts for the entire distribution of sizes of the sediment on the surface of the channel bottom. Watershed managers will find the analyses conducted in this study useful when choosing a technique that will produce an accurate prediction of clean-out depths in a given gravel stream bed. This type of information is critical for designing restoration strategies for streams that have been impacted by an excess of fine sediment in a gravel substrate and will allow agricultural and other watersheds to be managed in a more informed and environmentally sensitive manner.

Technical Abstract: The clogging of the surface layer of gravel bed streams with finer sediment (< 2 mm) is a common problem with negative impacts on the suitability of bed sediment as a habitat. Most existing measurements of sediment clean-out depth are from experiments with narrow gravel size distributions; therefore, a new set of experiments was conducted to explore the effect of more widely distributed bimodal gravel fractions on the clean-out of interstitial sand. Experiments were used to measure the terminal clean-out depth of 0.14 mm sand for bed material substrates composed of unimodal median diameter gravel of 7 mm and bimodal bed material with modes of 7 and 44 mm and a median diameter of 33 mm. It was determined that the clean-out depth of the sand was predicted well by characterizing the gravel bed using the cumulative probability distribution of the elevations of the surface of the gravel bed (CPDG) to scale the shear stress of the flow acting within the surface layer of the gravel.