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Water 2016, 8(4), 131; doi:10.3390/w8040131

Variability of Bed Drag on Cohesive Beds under Wave Action

United States Geological Survey, 384 Woods Hole Road, Woods Hole, MA 02543, USA
Academic Editor: Clelia Luisa Marti
Received: 1 February 2016 / Revised: 25 March 2016 / Accepted: 29 March 2016 / Published: 1 April 2016
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Abstract

Drag force at the bed acting on water flow is a major control on water circulation and sediment transport. Bed drag has been thoroughly studied in sandy waters, but less so in muddy coastal waters. The variation of bed drag on a muddy shelf is investigated here using field observations of currents, waves, and sediment concentration collected during moderate wind and wave events. To estimate bottom shear stress and the bed drag coefficient, an indirect empirical method of logarithmic fitting to current velocity profiles (log-law), a bottom boundary layer model for combined wave-current flow, and a direct method that uses turbulent fluctuations of velocity are used. The overestimation by the log-law is significantly reduced by taking turbulence suppression due to sediment-induced stratification into account. The best agreement between the model and the direct estimates is obtained by using a hydraulic roughness of 10 - 4 m in the model. Direct estimate of bed drag on the muddy bed is found to have a decreasing trend with increasing current speed, and is estimated to be around 0.0025 in conditions where wave-induced flow is relatively weak. Bed drag shows an increase (up to fourfold) with increasing wave energy. These findings can be used to test the bed drag parameterizations in hydrodynamic and sediment transport models and the skills of these models in predicting flows in muddy environments. View Full-Text
Keywords: water waves; muddy waters; coastal waters; currents; bed drag; drag coefficient; bottom shear stress; bottom friction; mud; cohesive sediment water waves; muddy waters; coastal waters; currents; bed drag; drag coefficient; bottom shear stress; bottom friction; mud; cohesive sediment
This is an open access article distributed under the Creative Commons Attribution License which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. (CC BY 4.0).

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Safak, I. Variability of Bed Drag on Cohesive Beds under Wave Action. Water 2016, 8, 131.

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