ALBERT

Description: ABSTRACT [1]  Saltation of bedload particles on bedrock surfaces is important for landscape evolution and bedrock incision in steep landscapes. However, few studies have investigated saltation in bedrock channels where, unlike alluvial channels, the bed-roughness height and the sediment size may be independent. To address this data gap, we measured the saltation hop height, hop length, and velocity of gravel saltating over a planar bed using 80-160 readings from high-speed photography and direct measurements. Two separate dimensional analyses are used: one leading to a bed-shear-stress scaling and another leading to a Froude-number (Fr) scaling. Our new saltation data coupled with numerous data from previous studies suggest that both shear-stress and Fr-scaling analyses are valid in characterizing bedload saltation dynamics with bed roughness ranging from smooth to alluvial beds. However, the Fr-approach has the advantages that (1) there is no need to estimate a critical Shields stress ( ), which alone can vary up to two orders of magnitude (e.g., 0.001 – 0.1) due to changes in relative bed roughness and slope, and (2) the Fr-based scaling fits the saltation dataset better in a least-squared sense. Results show that the saltation velocity of bedload is independent of grain density and grain size, and linearly proportional to flow velocity. Saltation height has a non-linear dependence on grain size. Saltation length increases primarily with flow velocity and it is inversely proportional to submerged specific density. Our results suggest that either or bed roughness coefficient must be properly estimated to yield accurate results in saltation-abrasion models.