VARIATIONS OF BED SURFACE SEDIMENT SIZE IN A CHANNEL BEND

Variations of sediment size and its gradation of the bed surface layer in a channel bend with nonuniform sediment are investigated experimen tally. Four groups of sediment with the same initial median diameter ( D-o) but different initial size gradation (sigma(o)) have been used fo r experiments which were run until the equilibrium bed topography was achieved. Analyses of experimental data have yielded the following res ults: (1) The time of equilibrium for bed evolution decreases as sigma (o) increases; (2) the median size of sediment (D) for a given section in the bend increases with increasing distance from the inner bank to wards the outer bank, and it also increases with increasing sigma(o); (3) the value of D/D-o along the inner bank decreases with increasing sigma(o), and it also shows a gradual decrease in the upper half of th e bend and a slight recovery in the lower half; and (5) the transverse variation of sigma value exhibits a general trend increasing from the inner bank towards the outer bank.