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

1996; Wiley; Volume: 32; Issue: 9 Linguagem: Inglês

10.1029/96wr01628

1944-7973

Peter Richard Wilcock, A. F. Barta, Conor Shea, G. Mathias Kondolf, W. V. Graham Matthews, John Pitlick,

Landslides and related hazards

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.