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 gravel entrainment on a large gravel-bed river. Bed shear stress tau(0) was estimated using local observations of depth-averaged velocity, Gravel entrainment was measured using large tracer gravel installations. Lateral 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 coarse grains. Scour to the base of the bed surface layer occurs at a dimensionless shear stress tau(g)* approximate to 0.035, for tau(g)* formed using local tau(0)and the median grain size of the gravel portion of the bed. The dimensionless reference transport rate W* = 0.002, often used as a surrogate for the threshold of grain motion, occurs at nearly the same tau(g)*. At smaller tau(g)*, entrainment and transport rates decrease rapidly, becoming vanishingly small at tau(g)* approximate to 0.031. Even at very small gravel transport rates, all sizes are transported: although the coarsest sizes are in a state of partial transport in which only a portion of the exposed grains are entrained. Both entrainment and cumulative transport observations suggest that maximum scour depth for plane-bed transport is slightly less than twice the surface layer thickness.