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Unraveling bed slope from relative roughness in initial sediment motion

Prancevic, Jeff P. and Lamb, Michael P. (2015) Unraveling bed slope from relative roughness in initial sediment motion. Journal of Geophysical Research. Earth Surface, 120 (3). pp. 474-489. ISSN 2169-9003. doi:10.1002/2014JF003323.

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Understanding incipient sediment transport is crucial for predicting landscape evolution, mitigating flood hazards, and restoring riverine habitats. Observations show that the critical Shields stress increases with increasing channel bed slope, and proposed explanations for this counterintuitive finding include enhanced form drag from bed forms, particle interlocking across the channel width, and large bed sediment relative to flow depth (relative roughness). Here we use scaled flume experiments with variable channel widths, bed slopes, and particle densities to separate these effects which otherwise covary in natural streams. The critical Shields stress increased with bed slope for both natural gravel (ρ_s = 2.65 g/cm^3) and acrylic particles (ρ_s = 1.15 g/cm^3), and adjusting channel width had no significant effect. However, the lighter acrylic particles required a threefold higher critical Shields stress for mobilization relative to the natural gravel at a fixed slope, which is unexpected because particle density is accounted for directly in the definition of Shields stress. A comparison with model predictions indicates that changes in local velocity and turbulence associated with increasing relative roughness for lighter materials are responsible for increasing the critical Shields stress in our experiments. These changes lead to concurrent changes in the hydraulic resistance and a nearly constant critical stream power value at initial motion. Increased relative roughness can explain much of the observed heightened critical Shields stresses and reduced sediment transport rates in steep channels and also may bias paleohydraulic reconstructions in environments with exotic submerged densities such as iron ore, pumice, or ice clasts on Titan.

Item Type:Article
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URLURL TypeDescription DOIArticle
Prancevic, Jeff P.0000-0003-1890-7551
Lamb, Michael P.0000-0002-5701-0504
Additional Information:© 2015 American Geophysical Union. Received 25 AUG 2014; Accepted 21 JAN 2015; Accepted article online 26 JAN 2015; Published online 14 MAR 2015. Funding was provided by NSF grants EAR-0922199 and EAR-1349115, the Terrestrial Hazards Observations and Reporting center (THOR) at Caltech, and the Keck Institute for Space Studies. The experimental data presented in Figures 5 to 9 are either present in Table 1 or may be calculated from data in Table 1 using equations included in the manuscript. We thank Brian Fuller for help with experiments. This paper was improved by the helpful comments of two anonymous reviewers.
Group:Keck Institute for Space Studies
Funding AgencyGrant Number
Caltech Terrestrial Hazards Observations and Reporting center (THOR)UNSPECIFIED
Keck Institute for Space Studies (KISS)UNSPECIFIED
Subject Keywords:sediment transport; steep channels; shallow flows; Shields stress; stream power; flow resistance
Issue or Number:3
Record Number:CaltechAUTHORS:20150522-094313365
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Official Citation:Prancevic, J. P., and M. P. Lamb (2015), Unraveling bed slope from relative roughness in initial sediment motion, J. Geophys. Res. Earth Surf., 120, 474–489, doi:10.1002/2014JF003323
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:57768
Deposited By: Tony Diaz
Deposited On:22 May 2015 20:25
Last Modified:10 Nov 2021 21:55

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