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

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 k x − 7 / 3 , as opposed to the expected k x − 5 / 3behavior above the reef. Under near‐emergent conditions, no fit to such a power law could be obtained.