| Autor: |
Boles, Elisabeth1 (AUTHOR) eboles@stanford.edu, Khrizman, Alexandra2 (AUTHOR), Hamilton, Jenny1 (AUTHOR), Mucciarone, David2 (AUTHOR), Dunbar, Robert2 (AUTHOR), Koseff, Jeffrey1 (AUTHOR), Monismith, Stephen1 (AUTHOR) |
| Předmět: |
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| Zdroj: |
Journal of Geophysical Research. Oceans. Nov2024, Vol. 129 Issue 11, p1-24. 24p. |
| Abstrakt: |
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∗ ${u}_{\ast }$) and drag coefficients (CD ${C}_{D}$) 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∗ ${u}_{\ast }$ 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∗ ${u}_{\ast }$ rather than to those based on larger‐scale flow structure. Plain Language Summary: Bottom stress plays a primary role in the dynamics of coral reef flows and is therefore critical to parameterize correctly in reef circulation models. These models are often used to examine transport of the larvae of reef biota, for example. Bottom stress is also important for coral metabolism, as turbulent mixing enhances nutrient uptake and potentially alleviates coral bleaching severity. However, it is difficult to measure directly in the field and there is some uncertainty as to whether commonly used methods are applicable in shallow, canopy environments as might be found on coral reefs. We estimated bottom stress on a shallow forereef with high coral cover in Palau using five common methods at 14 different locations. The methods included point‐measurements of near‐bed turbulence, velocity profile fitting to expected boundary layer scaling, large‐scale momentum budgets, and roughness characterization from digital elevation models of the reef. Estimates of bottom stress derived from "local" measurements of turbulence were consistently smaller than those from more "spatially distributed" methods such as momentum balance closure. Care should be taken in choosing the appropriate measurement of bottom stress to use in circulation modeling or reef health studies. Key Points: Bottom stress was estimated using 5 methods and compared across 14 sites on a shallow forereef in Palau with large coral roughness elementsMeasurements indicate that Reynolds stresses contributed only about 1/3 of the total bottom stress and drag produced by the reefCirculation models should use drag coefficients from momentum budgets; reef metabolism studies should use direct turbulence measurements [ABSTRACT FROM AUTHOR] |
| Databáze: |
GreenFILE |
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