OBSERVATIONS OF FLOW AND SEDIMENT ENTRAINMENT ON A LARGE GRAVEL-BED RIVER

Constant-discharge reservoir releases on the Trinity River, California , provide an unusual opportunity to unambiguously relate flow and grav el entrainment on a large gravel-bed river. Bed shear stress tau(0) wa s estimated using local observations of depth-averaged velocity, Grave l entrainment was measured using large tracer gravel installations. La teral variability of tau(0) is large, even for straight channels with simple, trough-like geometry. No simple relation exists between local and cross-section mean values of tau(0). Fine grains (less than 8 mm; 20-30% of the bed material) are transported at lower discharges than c oarse grains. Scour to the base of the bed surface layer occurs at a d imensionless shear stress tau(g) approximate to 0.035, for tau(g)* fo rmed using local tau(0)and the median grain size of the gravel portion of the bed. The dimensionless reference transport rate W = 0.002, of ten used as a surrogate for the threshold of grain motion, occurs at n early the same tau(g). At smaller tau(g)*, entrainment and transport rates decrease rapidly, becoming vanishingly small at tau(g) approxim ate to 0.031. Even at very small gravel transport rates, all sizes are transported: although the coarsest sizes are in a state of partial tr ansport in which only a portion of the exposed grains are entrained. B oth entrainment and cumulative transport observations suggest that max imum scour depth for plane-bed transport is slightly less than twice t he surface layer thickness.