Lamb, Michael P. and Brun, Fanny and Fuller, Brian M. (2017) Hydrodynamics of steep streams with planar coarse-grained beds: Turbulence, flow resistance, and implications for sediment transport. Water Resources Research . ISSN 0043-1397 . (In Press) http://resolver.caltech.edu/CaltechAUTHORS:20170303-091848044
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The hydraulics of steep mountain streams differ from lower gradient rivers due to shallow and rough flows, energetic subsurface flow, and macro-scale form drag from immobile boulders and channel- and bed-forms. Heightened flow resistance and reduced sediment transport rates in steep streams are commonly attributed to macro-scale form drag; however, little work has explored steep river hydrodynamics in the absence of complex bed geometries. Here we present theory for the vertical structure of flow velocity in steep streams with planar, rough beds that couples surface and subsurface flow. We test it against flume experiments using a bed of fixed cobbles over a wide range of bed slopes (0.4 – 30%). Experimental flows have a nearly logarithmic velocity profile far above the bed; flow velocity decreases less than logarithmically towards the bed and is non-zero at the bed surface. Velocity profiles match theory derived using a hybrid eddy-viscosity model, in which the mixing length is a function of height above the bed and bed roughness. Subsurface flow velocities are large (> 1 m/s) and follow a modified Darcy-Brinkman-Forchheimer relation that accounts for channel slope and shear from overlying surface flow. Near-bed turbulent fluctuations decrease for shallow, rough flows and scale with the depth-averaged flow velocity rather than bed shear velocity. Flow resistance for rough, planar beds closely matches observations in natural steep streams despite the lack of bed- or channel-forms in the experiments, suggesting that macro-scale form drag is smaller than commonly assumed in stress partitioning models for sediment transport.
|Additional Information:||© 2017 American Geophysical Union. Accepted manuscript online: 3 March 2017; Manuscript Revised: 22 February 2017; Manuscript Accepted: 22 February 2017; Manuscript Received: 28 July 2016. Funding for this work was provided by the National Science Foundation EAR-0922199 and EAR-1349115, and the Caltech Terrestrial Hazard Observation and Reporting program. This work benefited from helpful discussions with Rob Ferguson, Jeff Prancevic and Heidi Nepf, and formal reviews by Dieter Rickenmann and other reviewers. Data from the experiments are provided in the Supporting Information Table S1 or by email request to M. Lamb.|
|Subject Keywords:||flow resistance, roughness layer, turbulence, form drag, sediment transport, mountain streams|
|Official Citation:||Lamb, M. P., Brun, F. and Fuller, B. M. (2017), Hydrodynamics of steep streams with planar coarse-grained beds: Turbulence, flow resistance, and implications for sediment transport. Water Resour. Res.. Accepted Author Manuscript. doi:10.1002/2016WR019579|
|Usage Policy:||No commercial reproduction, distribution, display or performance rights in this work are provided.|
|Deposited By:||Tony Diaz|
|Deposited On:||10 Mar 2017 15:51|
|Last Modified:||10 Mar 2017 15:51|
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