Effects of hydraulic roughness on surface textures of gravel‐bed rivers

Water Resources Research
By:  and 



Field studies of forest gravel‐bed rivers in northwestern Washington and southeastern Alaska demonstrate that bed‐surface grain size is responsive to hydraulic roughness caused by bank irregularities, bars, and wood debris. We evaluate textural response by comparing reach‐average median grain size (D50) to that predicted from the total bank‐full boundary shear stress (т0bf), representing a hypothetical reference condition of low hydraulic roughness. For a given т0bf, channels with progressively greater hydraulic roughness have systematically finer bed surfaces, presumably due to reduced bed shear stress, resulting in lower channel competence and diminished bed load transport capacity, both of which promote textural fining. In channels with significant hydraulic roughness, observed values D50 can be up to 90% smaller than those predicted from т0bf. We find that wood debris plays an important role at our study sites, not only providing hydraulic roughness but also influencing pool spacing, frequency of textural patches, and the amplitude and wavelength of bank and bar topography and their consequent roughness. Our observations also have biological implications. We find that textural fining due to hydraulic roughness can create usable salmonid spawning gravels in channels that otherwise would be too coarse.

Additional publication details

Publication type Article
Publication Subtype Journal Article
Title Effects of hydraulic roughness on surface textures of gravel‐bed rivers
Series title Water Resources Research
DOI 10.1029/1999WR900138
Volume 35
Issue 11
Year Published 1999
Language English
Publisher American Geophysical Union
Description 15 p.
First page 3507
Last page 3521