Does flow variance affect bedload flux when the bed is dominated by grain roughness?

Cooper, James R ORCID: 0000-0003-4957-2774 (2012) Does flow variance affect bedload flux when the bed is dominated by grain roughness? Geomorphology, 141-14. pp. 160-169.

Text Published in Issue.pdf - Author Accepted Manuscript Download (730kB)

Abstract

Previous studies have shown that spatial variance in fluid and critical shear stress, caused by form roughness, can increase bedload flux. Others have revealed that variance in flow velocity increases with relative submergence and that bed mobility is reduced at lower submergences. The paper explores the link between these observations and addresses the following questions: is grain roughness sufficient to cause variance in fluid shear stress and an increase in bedload flux; if this variance changes with submergence, does this mean that the increase is dependent on submergence; and does this explain the change in mobility with submergence? A simple, statistical bedload model, based on spatial distributions of fluid and critical shear stress, has been used to explore these effects over a water-worked gravel deposit. Estimates of spatially distributed fluid shear stress were gained from laboratory flume measurements of near-bed flow velocity, and a distribution of critical shear stress was simulated using a discrete particle model of the sediment distribution used in the flume. The velocity data were used to describe the change in the spatial distribution of near-bed velocity with relative submergence, which allowed the effects of submergence on flux to also be considered. The main conclusions were: (i) spatial variance in fluid shear stress from grain roughness was not sufficient to have an appreciable effect on bedload flux over water-worked gravel beds with a spatial distribution of critical shear stress; (ii) spatial variance in critical shear stress, caused by grain roughness, had a much larger influence and increased bedload flux. This was a similar level of increase observed in studies for conditions where form roughness was high; (iii) spatially averaged estimates of fluid and critical shear stress should not be used to estimate bedload flux even if form roughness is low; and (iv) a rise in relative submergence increased bedload flux. This was due to changes in the spatial distribution of near-bed velocity and not due to a lowering in the local flow velocity as has been suggested by previous studies. © 2011 Elsevier B.V..

Item Type:	Article
Additional Information:	## TULIP Type: Articles/Papers (Journal) ##
Depositing User:	Symplectic Admin
Date Deposited:	12 Dec 2017 08:04
Last Modified:	30 Oct 2023 09:50
DOI:	10.1016/j.geomorph.2011.12.039
Related URLs:	Publisher
URI:	https://livrepository.liverpool.ac.uk/id/eprint/3013948