Effects of the depth to coral height ratio on drag coefficients for unidirectional flow over coral

We investigated the effects of the ratio of water depth (H) to coral height (h) on the drag coefficient (Cd) for unidirectional flow over the coral species Porites compressa. Cd, measured in a laboratory flume at several different flow rates and depths over a coral canopy, varied from 0 to 1.68 with a clear dependence on H: h. At higher Reynolds numbers Cd was inversely proportional to H: h and showed signs of becoming Re-independent. Cd was dependent on both Re and H: h at lower Reynolds numbers, indicating laminar or transitional flow within the coral canopy. The relationship between Cd and H: h at the highest available Re for each depth ratio is described by the power law: Cd = 1.01(H: h)^-2.77 + 0.01. This result should be applied with caution, however, as different coral species and bottom roughness may cause a deviation from this relationship.