Characteristics and selective mobility of bedload sheets

Abstract

Bedform-driven flux is a fundamental component of bedload transport along riverbeds, resulting in unsteady transport rates. For bedload sheets, these fluctuations emerge through the preferential sorting of coarser grains in the leading front of the bedform. This study establishes key characteristics of bedload sheets formed in a sand-dominated unimodal sediment and, using fractional transport rates, determines the state of grain mobility to evaluate a proposed bedform regime (Venditti et al., 2017). A series of 10 flume experiments were executed using a water-worked sediment bed, with Shields numbers ranging from 0.037 to 0.15. Sediment supply was set by operating the flume in sediment recirculation mode. For each run bedform dimensions and migration rate were measured as well as hydraulic conditions. Sampling and analysis of bedload and bed sediment were used to derive fractional transport rates and grain size distributions to determine scaled transport rates and transport-supply ratios. Bedload sampling partitioned transport into bedload sheets and the load between sheets. Bedload sheets initially formed at a Shields number equal to 0.056 and persisted to a value of 0.13. Over this range, sheets grew in size and frequency, migrated faster, and produced elevated bedload transport rates that were consistently coarser than the flux between sheets. Scaled transport rates indicate selective mobility when sheets existed but this state of grain mobility does not uniquely define bedload sheets. Nonetheless, the mobility of larger grains relative to those in the surface sediment and the sediment supply conditions during runs fall within the bounds that define bedload sheets in the bedform regime diagram evaluated. Incorporating these conditions for bedload sheets into fractional bedload transport rates could improve predictions of bedload fluxes.