Experiments on Downstream Fining of Gravel: I. Narrow-Channel Runs

Abstract

We present the results of three laboratory experiments in which longitudinally sorted deposits were formed by feeding poorly sorted sediment at the upstream end of a narrow, 45-m-long channel. The input sediment had a median size of 6 mm and included significant amounts of material up to 64 mm and down to 0.2 mm. Water discharge was constant at 49 L/s and sediment discharge varied from 0.048 to 0.19 kg/s. Downstream fining was produced in all three runs; the variation in sediment-feed rate had relatively little effect on the fining profiles. In all three runs, the formation of a longitudinally sorted deposit was mediated by the formation of a coarse surface layer. The surface layer remained at the top of the deposit during aggradation by continually reforming itself at the deposit surface. The coarse surface layer fined by approximately a factor of 2 in D90 and D50 consistently in all three experiments. The deposit (subsurface) fined less, with D90 fining more strongly than D50. The short channel length a...