Observations of Flow and Sediment Entrainment on a Large Gravel‐Bed River

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Abstract
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 т0 was estimated using local observations of depth‐averaged velocity. Gravel entrainment was measured using large tracer gravel installations. Lateral variability of т0 is large, even for straight channels with simple, trough‐like geometry. No simple relation exists between local and cross‐section mean values of т0 . 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 тg* ≈ 0.035, for тg* formed using local т0 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 тg*. At smaller тg*, entrainment and transport rates decrease rapidly, becoming vanishingly small at тg* ≈ 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.