Development and Characteristics of Bedload Sheets in a Gravelly Sand Mixture
The development of bedforms increases sediment transport rates in fluvial channels, which directly impacts channel morphology and sediment transfers. Bedforms known as bedload sheets have been documented as increasing sediment transport rates but the range of flow conditions for their development and the resulting properties of geometric shape, abundance, and migration rates have not been fully elucidated. This study investigated the control of flow magnitude on bedload sheet properties formed in a sand-gravel mixture by using flume experimentation. The sediment mixture containing 56% sand with a 50th percentile (D50) of 1.8 mm and D90 of 5.2 mm was subjected to a series of 10 different steady flows covering critical dimensionless shear stress values (τ *) from 0.043 to 0.17. Bedload sheets developed and persisted when τ * exceeded 0.05 but was less than 0.17. Sheet geometric dimensions, frequency, and migration and transport rates increase with τ *. Additionally, bedload sheets exhibit larger concentrations of coarser grains compared to intervening non-sheet bedload as flow increases. Bedload sheets where the dominate bedform when τ * was equal to 0.05 to 0.10 with higher flows transitioning fully to low relief bed waves around τ * of 0.17. Based on these findings it is clear that bedload sheets, like many better known bedforms, are governed by specific flow conditions. Results from this study improve the understanding of bedload sheet developmental conditions and characteristics, which can more generally be applied in river management by further quantifying large scale sediment fluxes, paleo flow conditions.