EXPERIMENTS ON DOWNSTREAM FINING OF GRAVEL .1. NARROW-CHANNEL RUNS

We present the results of three laboratory experiments in which longit udinally sorted deposits were formed by feeding poorly sorted sediment at the upstream end of a narrow, 45-m-long channel. The input sedimen t had a median size of 6 mm and included significant amounts of materi al 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 b y 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 appro ximately a factor of 2 in D-50 and D-50 consistently in all three expe riments. The deposit (subsurface) fined less, with D-50 fining more st rongly than D-50. The short channel length and the relatively high rat e of deposition rule out clast abrasion as the source of fining. The e xperiments suggest that selective deposition of the coarsest clasts du e to unequal mobility is capable of producing fining rates comparable with the highest rates observed in nature.