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

Abstract

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.

Mass Transport and Turbulent Statistics within Two Branching Coral Colonies

“…For example, the transport of eggs and sperm from the surface of an Acropora reef to the surrounding water and the transport of nutrients from the water to the interior of the reef depends on the turbulent stress at the top of the reef. Turbulent flow statistics are also important for calculating the Reynolds stress and the bed shear stress, and determining the transport momentum and sedimentation rate in the bottom layer of the reef or over a colony or reef [16,29,30]. Though numerous studies have explored the effects of these quantities on coral reef systems, we reiterate here that computing the detailed, local turbulent flow in the interior of a reef or a single colony is still a challenging task.…”

Hydro-Morphological Characterization of Coral Reefs for Wave Runup Prediction

“…The presented methodology has been demonstrated with many coral reef properties set as constant. It is well-established that changes in the roughness and friction (Lowe et al, 2005;Pomeroy et al, 2012;Monismith et al, 2015;Buckley et al, 2016;Rogers et al, 2017Rogers et al, , 2018Osorio-Cano et al, 2019;Reguero et al, 2019), porosity (Lowe et al, 2008;Asher et al, 2016;Asher and Shavit, 2019;Zhu et al, 2019), and storm surge (Hoeke et al, 2013;Smithers and Hoeke, 2014;Tajima et al, 2016) will affect the hydrodynamics over a coral reef, and in turn the resulting wave runup at the shoreline. Therefore, the RCPs as they are currently do provide a means to estimate wave runup based on reef morphology, although the estimate is limited to coral reefs with similar properties used in this study.…”

Turbulence and Coral Reefs

“…is the spatially averaged total stress, in which an extra term, ū w , called the dispersive stress, appears, accounting for the spatial correlations in the time-averaged velocity field. Inside coral canopies, spatially variable geometry can generate persistent spatial gradients in flow, making the dispersive stress of similar importance to the Reynolds stress (Asher & Shavit 2019). The last term in Equation 7, F x , represents the spatially averaged drag force exerted by the canopy elements onto the surrounding flow.…”

“…Lastly, most of what we know about within-canopy flows is from studies considering idealized geometry or the uniform vertical and horizontal distribution of canopy roughness elements [although idealized studies of nonuniform roughness by Rominger & Nepf (2011) examined flow adjustments within a canopy]. However, the multiscale, multifractal complexity of coral reef structures results in spatially variable resistance, and this nonuniform structure can be important to within-canopy flow structure (Asher & Shavit 2019, Duvall et al 2019.…”