A framework to quantify flow through coral reefs of varying coral cover and morphology

Flow velocities within coral reefs are greatly reduced relative to those at the water surface. The in-reef flow controls key processes that flush heat, cycle nutrients and transport sediment from the reef to adjacent beaches, all key considerations in assessments of reef resilience and restoration interventions. An analytical framework is proposed and tested with a suite of high-resolution numerical experiments. We demonstrate a single parameter that describes the total coral frontal area explains variation of horizontally averaged velocity within a reef canopy across morphologies, densities, and flow depths. With the integration of existing data of coral cover and geometry, this framework is a practical step towards the prediction of near-bed flows in diverse reef environments

The global distribution of the three archetypal benthos colony forms analyzed here (“table”, “massive” and “branching”).

Recorded observations [18] of the representative species (A) Acropora hyacinthus [15], (B) Porites lobata [15] and (C) Pocillopora edyouxi [16] are indicated by colored dots in (D) with grey shading indicating ecoregions [19] where these broad morphology types are likely to be found [17]. Panels A-C compare example morphologies of each representative species with the model forms embedded within numerical and experimental simulations (derived from reef colonies digitized in the field, kindly provided by The Hydrous, http://www.thehydro.us). The three coral archetypes are broadly representative of a reef benthos observed globally.</p

The range of parameter values considered in the numerical simulations for each archetype coral colony morphology.

The range of parameter values considered in the numerical simulations for each archetype coral colony morphology.</p

The performance of the predictive model (Eq 8) in collapsing the effects of reef colony morphology, coral cover, water depth, orientation and flow velocity into a single predictor variable for in-reef flow attenuation.

The solid markers indicate the coral cover cases for the three archetypal coral forms. The open markers indicate cases where depth, orientation or velocity (as indicated in brackets in the legend) were varied systematically for one coral morphology. The equation for the line of best fit is indicated on the plot, with the shading representing the 95% prediction interval.</p

Fig 3 -

(A) Example of an experimental setup of the branching morphology used for directly measuring velocities in reefs comprised of 3-D printed coral specimens. (B) Comparison of within- and above-reef velocities predicted by the model with those observed in the laboratory. There is excellent agreement between model and experimental velocity values (RMSE = 0.008).</p