Modeling flow in a coral reef requires a closure model that links the local drag force to the local mean velocity. However, the spatial flow variations make it difficult to predict the distribution of the local drag. Here we report on vertical profiles of measured drag and velocity in a laboratory reef that was made of 81 Pocillopora Meandrina colony skeletons, densely arranged along a tilted flume. Two corals were CT‐scanned, sliced horizontally, and printed using a 3‐D printer. Drag was measured as a function of height above the bottom by connecting the slices to drag sensors. Profiles of velocity were measured in‐between the coral branches and above the reef. Measured drag of whole colonies shows an excellent agreement with previous field and laboratory studies; however, these studies never showed how drag varies vertically. The vertical distribution of drag is reported as a function of flow rate and water level. When the water level is the same as the reef height, Reynolds stresses are negligible and the drag force per unit fluid mass is nearly constant. However, when the water depth is larger, Reynolds stress gradients become significant and drag increases with height. An excellent agreement was found between the drag calculated by a momentum budget and the measured drag of the individual printed slices. Finally, we propose a modified formulation of the drag coefficient that includes the normal dispersive stress term and results in reduced variations of the drag coefficient at the cost of introducing an additional coefficient.

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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