Bottom Stress and Drag on a Shallow Coral Reef

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 ($u_{\ast }$) and drag coefficients ($C_D$) 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, $u_{\ast }$ 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 $u_{\ast }$ rather than to those based on larger-scale flow structure.