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Direct measurements of lift and drag on shallowly submerged cobbles in steep streams: Implications for flow resistance and sediment transport

Lamb, Michael P. and Brun, Fanny and Fuller, Brian M. (2017) Direct measurements of lift and drag on shallowly submerged cobbles in steep streams: Implications for flow resistance and sediment transport. Water Resources Research . ISSN 0043-1397 . (In Press) http://resolver.caltech.edu/CaltechAUTHORS:20170818-074626750

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Abstract

Steep mountain streams have higher resistance to flow and lower sediment transport rates than expected by comparison with low gradient rivers, and often these differences are attributed to reduced near-bed flow velocities and stresses associated with form drag on channel forms and immobile boulders. However, few studies have directly measured drag and lift forces acting on bed sediment for shallow flows over coarse sediment, which ultimately control sediment transport rates and grain-scale flow resistance. Here we report on particle lift and drag force measurements in flume experiments using a planar, fixed cobble bed over a wide range of channel slopes (0.004 < S < 0.3) and water discharges. Drag coefficients are similar to previous findings for submerged particles (C_D ∼ 0.7) but increase significantly for partially submerged particles. In contrast, lift coefficients decrease from near unity to zero as the flow shallows and are strongly negative for partially submerged particles, indicating a downward force that pulls particles toward the bed. Fluctuating forces in lift and drag decrease with increasing relative roughness, and they scale with the depth-averaged velocity squared rather than the bed shear stress. We find that, even in the absence of complex bed topography, shallow flows over coarse sediment are characterized by high flow resistance because of grain drag within a roughness layer that occupies a significant fraction of the total flow depth, and by heightened critical Shields numbers and reduced sediment fluxes because of reduced lift forces and reduced turbulent fluctuations.


Item Type:Article
Related URLs:
URLURL TypeDescription
https://doi.org/10.1002/2017WR020883DOIArticle
http://onlinelibrary.wiley.com/doi/10.1002/2017WR020883/fullPublisherArticle
ORCID:
AuthorORCID
Lamb, Michael P.0000-0002-5701-0504
Brun, Fanny0000-0001-6607-0667
Additional Information:© 2017 American Geophysical Union. Received 4 APR 2017; Accepted 6 AUG 2017; Accepted article online 18 AUG 2017. Emilio Graff built the force balance and helped with instrument calibration. We thank Florent Gimbert for help interpreting Figure 2b. Funding was provided by the National Science Foundation grants EAR-0922199 and EAR-1349115, and the Caltech Terrestrial Hazard Observation and Reporting program. Thanks to Paul Carling, Mário Franca, and two anonymous reviewers for their useful critiques. Experimental data can be found in the supporting information.
Funders:
Funding AgencyGrant Number
NSFEAR-0922199
NSFEAR-1349115
Caltech Terrestrial Hazard Observation and Reporting (THOR) CenterUNSPECIFIED
Subject Keywords:Drag; Lift; Flow resistance; Friction coefficient; Sediment transport
Record Number:CaltechAUTHORS:20170818-074626750
Persistent URL:http://resolver.caltech.edu/CaltechAUTHORS:20170818-074626750
Official Citation:Lamb, M. P., and F. Brun (2017), Direct measurements of lift and drag on shallowly submerged cobbles in steep streams: Implications for flow resistance and sediment transport, Water Resour. Res., 53, doi:10.1002/2017WR020883
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:80594
Collection:CaltechAUTHORS
Deposited By: Tony Diaz
Deposited On:18 Aug 2017 17:46
Last Modified:01 Sep 2017 16:51

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