Size-selective entrainment of bedload in gravel bed streams

Entrainment of mixed size gravel bed material was studied in nine reaches of three high-power streams in Scotland and Norway. Paired measurements of at-a-point shear stress (estimated from velocity profiles) and bed load transport (by hand-held sampler) were made. They extended to very high values (400 N m −2, 3.5 kg m-1 s−1). Analyses of maximum bed load diameter, mean bed load diameter, transport rates of individual size fractions, and tracer pebble movements all show some dependence of threshold shear stress for entrainment on absolute particle size, despite strong relative size effects. Precise equal mobility of all sizes was approached in the data set with the highest shear stresses and transport rates. Size-selective transport in the streams studied is also indicated by clear downstream and downbar reductions in surface sediment size over distances too short for abrasion to be significant.