Vertical variations of coral reef drag forces

Asher, Shai; Niewerth, Stephan; Koll, Katinka; Shavit, Uri

doi:10.1002/2015jc011428

Cited by 15 publications

(27 citation statements)

References 51 publications

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“…9. It is notable that in this framework the z o values inferred from turbulence estimates (Reidenbach et al 2006) and laboratory studies (McDonald et al 2006;Asher et al 2016) are consistent with most of the field estimates of z o based on momentum balances. Log-profile estimates for 10 sites on FIG.…”

supporting

confidence: 63%

“…9 to the physical roughness at the different coral reef sites without first accounting for the water depth variations. It is encouraging that z o determined from the laboratory studies using real coral heads (McDonald et al 2006;Asher et al 2016) are typical of the field estimates, sandy regions like the Palau back reef have relatively small z o , and the exceptionally rough Moorea back reef has large z o .…”

Section: Discussionmentioning

confidence: 73%

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Coral Reef Drag Coefficients – Water Depth Dependence

Lentz

Davis

Churchill

et al. 2017

Journal of Physical Oceanography

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A major challenge in modeling the circulation over coral reefs is uncertainty in the drag coefficient because existing estimates span two orders of magnitude. Current and pressure measurements from five coral reefs are used to estimate drag coefficients based on depth-average flow, assuming a balance between the cross-reef pressure gradient and the bottom stress. At two sites wind stress is a significant term in the cross-reef momentum balance and is included in estimating the drag coefficient. For the five coral reef sites and a previous laboratory study, estimated drag coefficients increase as the water depth decreases consistent with open channel flow theory. For example, for a typical coral reef hydrodynamic roughness of 5 cm, observational estimates, and the theory indicate that the drag coefficient decreases from 0.4 in 20 cm of water to 0.005 in 10 m of water. Synthesis of results from the new field observations with estimates from previous field and laboratory studies indicate that coral reef drag coefficients range from 0.2 to 0.005 and hydrodynamic roughnesses generally range from 2 to 8 cm. While coral reef drag coefficients depend on factors such as physical roughness and surface waves, a substantial fraction of the scatter in estimates of coral reef drag coefficients is due to variations in water depth.

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supporting

confidence: 63%

Section: Discussionmentioning

confidence: 73%

Coral Reef Drag Coefficients – Water Depth Dependence

Lentz

Davis

Churchill

et al. 2017

Journal of Physical Oceanography

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“…This work complements the earlier study reported by Asher et al (), in which full details of the experimental techniques can be found. Experiments were carried out in a 32‐m‐long, 0.6‐m‐wide, and 0.4‐m‐deep tilting flume in the laboratory of the Leichtweiß‐Institute for Hydraulic Engineering and Water Resources, TU Braunschweig, Germany.…”

Section: Methodssupporting

confidence: 82%

“…where D x (N) is the directly measured mean drag applied on a coral colony in the test section in the direction of the flow (data extracted from Asher et al, ) and A 0 is the total frontal area calculated as

A_{0} = \sum_{1}^{n} \sum_{1}^{m ((), z)} {A_{italicbr}^{x}}_{i} ((), z)

,where n is the number of slices. Rosman and Hench () have used another definition for the reef drag (Rosman & Hench, )

C_{D}^{U} = \frac{1}{U^{2}} \int_{0}^{h_{c}} \frac{1}{2} c_{d} a {⟨\overset{u}{true}⟩}^{2} italicdz,

where c d is the variable sectional drag coefficient (data extracted from Asher et al, ). While c d changes with z and is used to calculate the force per fluid mass at a given height, C D and

C_{D}^{U}

are used to calculate the total force, such as the force applied by a coral colony.…”

Section: Methodsmentioning

confidence: 99%

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The Effect of Water Depth and Internal Geometry on the Turbulent Flow Inside a Coral Reef

Asher

Shavit

2019

JGR Oceans

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The flow of water through coral reefs controls the transport of mass, momentum, and energy and, as a result, affects key processes such as feeding, photosynthesis, and reproduction. While it is often analyzed as a typical canopy flow, the flow through coral reefs is different from both terrestrial and aquatic canopies. A combination of a nonuniform vertical distribution of porosity and resistance and variations in relative submergence generates regions of high‐velocity gradients, increased integral length scales and instabilities which have not previously been quantified in detail. Here we report on velocity measurements inside and above a laboratory reef made of 81 Pocillopora Meandrina skeletons, for a range of relative submergence and flow rates. Under the action of a pressure gradient, the mean velocity shows an increase in the wake zone, generating a potential source for mixing due to a second inflection point. Unlike classical boundary layers and other canopy flows, a quadrant analysis shows that the number of inward/outward interactions events is larger than sweeps/ejections inside the canopy wake zone, due to the sign change in the mean velocity gradient. The vertical distribution of the integral length scales shows a local maximum in the lower part of the wake zone. Finally, under fully submerged conditions, the stream‐wise turbulent energy spectra inside the reef follow an approximate kx−7/3, as opposed to the expected kx−5/3 behavior above the reef. Under near‐emergent conditions, no fit to such a power law could be obtained.

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Hydrodynamic forces on emergent cylinders in non-uniform flow

Uchida

Ato²,

Kobayashi

et al. 2022

Environ Fluid Mech

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The main objective of conducting numerical simulations of flows in rivers with vegetation is to investigate the complex flow dynamics involved in non-equilibrium conditions. In such cases, it is inappropriate to apply the drag coefficient CD, which is typically derived based on uniform flows involving groups of infinitely long cylinders. This paper presents a method for evaluating the drag forces acting on emergent obstacles for non-uniform open-channel flows. This method is devised based on two sets of experiments: on flows with small-diameter cylinders, focusing on the water surface profiles through the group; and on flows with large-diameter cylinders, focusing on the local pressure distribution and local water surface profile around a target cylinder. In addition to the conventional drag force expression that includes CD, two new terms are proposed to account for the effects of water surface variation and pressure gradient in non-uniform open-channel flow on the drag. The first of these terms, which introduces the use of the Froude number to account for the effect of water surface variation, is derived theoretically and evaluated against past and present experimental results under uniform-flow conditions. On the other hand, the second of these terms, which includes the representative length of the separation zone to evaluate the effect of pressure gradient, is confirmed to be a necessity through numerical calculation of the longitudinal water surface profile in emergent cylinders. The incorporation of these two terms using a simple unified expression can help improve the accuracy of numerical simulations for practical problems of flows with emergent obstacles.

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Vertical variations of coral reef drag forces

Cited by 15 publications

References 51 publications

Coral Reef Drag Coefficients – Water Depth Dependence

Coral Reef Drag Coefficients – Water Depth Dependence

The Effect of Water Depth and Internal Geometry on the Turbulent Flow Inside a Coral Reef

Hydrodynamic forces on emergent cylinders in non-uniform flow

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