Bottom Stress and Drag on a Shallow Coral Reef

IF 3.3 2区 地球科学 Q1 OCEANOGRAPHY
Elisabeth Boles, Alexandra Khrizman, Jenny Hamilton, David Mucciarone, Robert Dunbar, Jeffrey Koseff, Stephen Monismith
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引用次数: 0

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 ( u ${u}_{\ast }$ ) and drag coefficients ( C D ${C}_{D}$ ) 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.

浅海珊瑚礁的海底应力和阻力
珊瑚礁粗糙度会产生湍流边界层和海底应力,这对珊瑚礁新陈代谢监测和珊瑚礁环流建模非常重要。然而,对于估计海底应力的实地方法是否适用于珊瑚礁上的浅冠环境,还存在一些不确定性。采用五种独立的方法估算了摩擦速度(u ∗ ${u}_{ast }$)和阻力系数(C D ${C}_{D}$ ),并对帕劳浅海前礁(2-9 米深)上的 14 个地点进行了比较,这些地点有大型且空间可变的珊瑚粗糙度要素(0.4-1 米高)。这些方法包括(a) 动量平衡封闭,(b) 速度剖面对数拟合,(c) 雷诺应力,(d) 湍流耗散,(e) 根据数字高程模型(DEM)确定粗糙度特征。速度剖面和点湍流测量结果都与对数层缩放比例十分吻合,表明测量是在发达的湍流边界层内进行的,冠层影响很小。然而,根据 DEM、动量预算和对数剖面拟合估算出的 u ∗ ${u}_{\ast }$ 始终大于直接湍流测量估算出的值。近海底雷诺应力只占暗礁产生的总海底应力和阻力的 1/3。因此,引起平均速度通量、分散应力和形态阻力的地形异质性效应预计会很重要。总阻力与局部湍流的脱钩意味着,无论是质量转移率还是通过浓度测量推断出的通量值,都可能与较小的、由湍流推导出的 u ∗ ${u}_{\ast }$ 值成正比,而不是与基于较大尺度流动结构的值成正比。
本文章由计算机程序翻译,如有差异,请以英文原文为准。
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来源期刊
Journal of Geophysical Research-Oceans
Journal of Geophysical Research-Oceans Earth and Planetary Sciences-Oceanography
CiteScore
7.00
自引率
13.90%
发文量
429
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