WATER RESOURCES MONOGRAPH, VOL. 12, PP. 1-50, 1989
Boundary shear stress and sediment transport in river meanders of sand and gravel
Field measurements in a sand-bedded river and in two gravel-bedded ones are compared to examine controls on boundary shear
stress fields, sediment transport processes, and sorting in meanders. Analysis of detailed flow field measurements in the
sand-bedded river meander and over a gravel-bedded alternate bar reveals a well-defined spatial structure to the magnitude
and sign of forces controlling boundary shear stress that arise from topographically-induced spatial accelerations. The relationship
between bedload transport and boundary shear stress fields in river meanders varies with size and heterogeneity of bed material.
In bends of moderately to well sorted sand in flows generating boundary shear stresses well above critical (such as in large
sandy rivers), downstream varying boundary shear stress is matched by topographically-induced cross-stream transport of sediment.
In meanders with high excess shear stress but poorly sorted coarse sand and fine gravel, boundary shear stress variation downstream
is partially matched by surface grain size adjustments and by net cross-stream sediment flux. Maxima of bedload transport
rate and boundary shear stress do not correspond in some areas. In gravel-bedded meanders with low excess boundary shear stress
and low sediment supply, bedload may be much finer than the bed surface, and significant areas of bar surface are covered
with grain sizes that constitute a very small portion of the bedload. Substantial bedload transport may only occur over a
narrow portion of the bed width where boundary shear stress relative to critical stress of the surface is highest and where
the sediment flux from upstream is locally concentrated. In this case, grain size adjustments dominate over topographically-induced
cross-stream sediment transport in controlling the relationship between boundary shear stress and bedload transport fields.
Citation: Dietrich, W. E., and