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Abstract Coral reef roughness produces turbulent boundary layers and bottom stresses that are important for reef metabolism monitoring and reef circulation modeling. However, there is some uncertainty as to whether field methods for estimating bottom stress are applicable in shallow canopy environments as found on coral reefs. Friction velocities () and drag coefficients () were estimated using five independent methods and compared across 14 sites on a shallow forereef (2–9 m deep) in Palau with large and spatially variable coral roughness elements (0.4–1 m tall). The methods included the following: (a) momentum balance closure, (b) log‐fitting to velocity profiles, (c) Reynolds stresses, (d) turbulence dissipation, and (e) roughness characterization from digital elevation models (DEMs). Both velocity profiles and point turbulence measurements indicated good agreement with log‐layer scaling, suggesting that measurements were taken within a well‐developed turbulent boundary layer and that canopy effects were minimal. However, estimated from the DEMs, momentum budget and log‐profile fitting were consistently larger than those estimated from direct turbulence measurements. Near‐bed Reynolds stresses only contributed about 1/3 of the total bottom stress and drag produced by the reef. Thus, effects of topographical heterogeneity that induce mean velocity fluxes, dispersive stresses, and form drag are expected to be important. This decoupling of total drag and local turbulence implies that both rates of mass transfer as well as values of fluxes inferred from concentration measurements may be proportional to smaller, turbulence‐derived values of rather than to those based on larger‐scale flow structure.
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Boundary layer dynamics and bottom friction in combined wave–current flows over large roughness elements
In the coastal ocean, interactions of waves and currents with large roughness elements, similar in size to wave orbital excursions, generate drag and dissipate energy. These boundary layer dynamics differ significantly from well-studied small-scale roughness. To address this problem, we derived spatially and phase-averaged momentum equations for combined wave–current flows over rough bottoms, including the canopy layer containing obstacles. These equations were decomposed into steady and oscillatory parts to investigate the effects of waves on currents, and currents on waves. We applied this framework to analyse large-eddy simulations of combined oscillatory and steady flows over hemisphere arrays (diameter $$D$$ ), in which current ( $$U_c$$ ), wave velocity ( $$U_w$$ ) and period ( $$T$$ ) were varied. In the steady momentum budget, waves increase drag on the current, and this is balanced by the total stress at the canopy top. Dispersive stresses from oscillatory flow around obstacles are increasingly important as $$U_w/U_c$$ increases. In the oscillatory momentum budget, acceleration in the canopy is balanced by pressure gradient, added-mass and form drag forces; stress gradients are small compared to other terms. Form drag is increasingly important as the Keulegan–Carpenter number $$KC=U_wT/D$$ and $$U_c/U_w$$ increase. Decomposing the drag term illustrates that a quadratic relationship predicts the observed dependences of steady and oscillatory drag on $$U_c/U_w$$ and $KC$ . For large roughness elements, bottom friction is well represented by a friction factor ( $$f_w$$ ) defined using combined wave and current velocities in the canopy layer, which is proportional to drag coefficient and frontal area per unit plan area, and increases with $KC$ and $$U_c/U_w$$ .
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- PAR ID:
- 10344679
- Date Published:
- Journal Name:
- Journal of Fluid Mechanics
- Volume:
- 931
- ISSN:
- 0022-1120
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1017/jfm.2021.941
