Flow and Drag in a Seagrass Bed

We report direct measurement of drag forces due to tidal flow over a submerged seagrass bed in Ngeseksau Reef, Koror State, Republic of Palau. In our study, drag is computed using an array of high-resolution pressure measurements, from which values of the drag coefficients, C_D, referenced to measured depth-averaged velocities, V, were inferred. Reflecting the fact that seagrass blades deflect in the presence of flow, we find that C_D is O(1) when flows are weak and tends toward a value of 0.03 at the highest velocities, behavior that is consistent with existing theory for canopy flows with flexible canopy elements. A limited subset of velocity profiles obey the law of the wall, producing values of shear velocity that, while noisy, broadly agree with values inferred from the pressure measurements.