Effect of Bimodal Gravel Size Distribution on the Clean-Out Depth of Sand

The clogging of the gravel surface layer in gravel-bedded channels with finer sediment (<2 mm) is a common problem that negatively impacts streambed habitat quality. Most existing measurements of surface-layer 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 widely distributed bimodal gravel fractions on the clean-out depth of interstitial sand. Experiments were used to measure the terminal clean-out depth of 0.115-mm sand from bed material substrates composed of unimodal median diameter gravel of 7.16 mm and bimodal bed material with modes of 7.31 and 34.66 mm and a median diameter of 33.46 mm. It was found that the clean-out depth of sand from immobile gravel beds could be predicted by scaling the bed shear stress using the cumulative probability distribution of the elevations of the surface of the gravel bed.