| Autor: |
Wilcock, Peter R., Barta, Alan F., Shea, Conor C., Kondolf, G. Mathias, Matthews, W. V. Graham, Pitlick, John |
| Zdroj: |
Water Resources Research; 1996, Vol. 32 Issue 9, p2897-2909, 13p |
| Abstrakt: |
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. [ABSTRACT FROM AUTHOR] |
| Databáze: |
Complementary Index |
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