Tracer‐pebble movement along a concave river profile: Virtual velocity in relation to grain size and shear stress

Abstract

Over 1400 tracer pebbles 16–256 mm in diameter were tracked for 2 years in six reaches of Allt Dubhaig, Scotland, a small gravel‐bed river along which shear stress and bed surface grain size decrease toward a local base level. Pebble movement was size‐selective both within and between reaches. Within reaches the decrease in mean travel distance with increasing grain size is strongest in the coarse tail of the size distribution. Particle shape has a minor secondary effect. A nondimensional grain velocity, averaged over the duration of competent flow, is used to compare different size classes and reaches. Over 90% of its variance is explained by relative grain size and reach Shields stress. The pattern of size selectivity is consistent with single‐event tracer results elsewhere, bedload trap data from our distal reach, and the concept of partial mobility. It provides a mechanism for strong downstream fining by selective transport and deposition along rivers in which stress declines toward base level. The nondimensional prediction equation for grain velocity may be of use in other rivers but requires testing.