Unequal Mobility of Gravel and Sand in Weakly Bimodal River Sediments

Abstract

Bed load was trapped during flood events over a 20‐month period at the lower end of the Allt Dubhaig, a small river in Scotland with rapid downstream fining of gravel bed material on a slowly aggrading concave long profile. The channel bed near the trap is predominantly gravel with a secondary sand mode. Total transport in each event depended mainly on peak shear stress, rather than duration over a threshold. Bed load was mainly sand in smaller events, bimodal in intermediate events, and mainly gravel in the biggest floods. Mean and maximum grain diameter both increased with peak shear stress, but in different ways. Analysis of fractional transport rates and maximum grain size in relation to peak shear stress suggests that gravel transport is slightly size selective but sand transport is close to equal mobility. The slight selectivity in gravel transport is consistent with previous field studies of near‐equilibrium unimodal beds and supports assumptions made in the numerical model of Hoey and Ferguson (1994), which successfully simulates the observed amount of downstream fining over the 2.5‐km upstream of the bed load trap.