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

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 т 0 was estimated using local observations of depth‐averaged velocity. Gravel entrainment was measured using large tracer gravel installations. Lateral variability of т 0 is large, even for straight channels with simple, trough‐like geometry. No simple relation exists between local and cross‐section mean values of т 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 т g * ≈ 0.035, for т g * formed using local т 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 т g * . At smaller т g * , entrainment and transport rates decrease rapidly, becoming vanishingly small at т g * ≈ 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.