Shallow coral reef ecosystems are particularly sensitive to anthropogenic impacts, notably climate change. Temperature increases associated with global warming contribute to coral bleaching. Ocean acidification associated with increasing levels of atmospheric CO₂ will slow calcification on coral reefs and may lead to erosion and dissolution of reefs. Sea level rise and changes in the frequency and strength of storms will impact the flow and wave characteristics over coral reefs and hence exchange with the surrounding water. Predicting the impact of climate change on coral reef ecosystems requires accurate physical models that are presently limited by our poor understanding of drag. 

A defining physical characteristic of coral reefs is that they are rough. As a result, coral reefs exert an enormous drag on the flow above the reef, 100 to 1000 times larger than over sandy beaches or continental shelves. The drag exerted by the reef on the flow is represented by a drag coefficient in physical models of flow over coral reefs. Accurate drag coefficients are an essential element of physical and bio-chemical models of coral reefs because of the dominance of drag. However, drag coefficients over reefs are estimated using a wide variety of approaches and reported drag coefficients span almost two orders of magnitude without a clear framework for interpreting the consistency of the various estimates. In a recent review of this issue, Rosman and Hench (2011) noted, “the broad range of reported drag coefficient values thus presents a significant challenge to predictive modeling.” They conclude their review paper “… we urge caution when using published (drag coefficient) or (hydrodynamic roughness) values for coral reefs.”

This study advanced our understanding of drag over coral reefs by showing that drag coefficients over coral reefs depend on water depth and on the surface gravity wave orbital velocities over the reef. Using observations from five different reefs I showed that the drag coefficient increases as the water depth decreases in a manner consistent with existing theory from engineering studies of flow in channels. Consequently drag coefficients vary on tidal and longer time scales due to substantial variations in water depth over shallow coral reefs. Drag coefficients also vary over different reefs because of differences in the water depth. Compiling results from previous laboratory studies and previous studies of other coral reefs I showed that a substantial fraction of the variation in drag coefficients noted by Rosman and Hench (2011) is due to variations in water depth.

Surface gravity waves propagating from the open ocean onto a shallow coral reef break (just like waves on a beach) but then smaller waves continue to propagate across the reef. Previous studies have not addressed whether the orbital velocities associated with the waves propagating across the reef enhance the drag on the mean current over the reef. Analysis of observations from four coral reefs in the Red Sea indicates that even small surface gravity waves propagating across reefs can substantially enhance the drag—by a factor of 3–5 when wave orbital velocities are twice as large as the mean cross-reef current. I also show that a simple model can be used to estimate this drag enhancement. Previous work has shown that surface gravity waves evolve across coral reefs due to the large drag on the waves themselves. This implies that the surface gravity wave enhancement of drag will vary across reefs and consequently accurate models of currents over coral reefs require also accurately modeling the evolution of the surface gravity waves across reefs.

Last Modified: 05/10/2018
Modified by: Steven J Lentz