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, CD, referenced to measured depth‐averaged velocities, V, were inferred. Reflecting the fact that seagrass blades deflect in the presence of flow, we find that CD 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. Key Points Measurements of drag in a flow over a shallow seagrass bed were made using simultaneous observations of pressure gradients and velocities Reflecting deflection of the seagrass shoots by flow, drag coefficients were O(1) at low flows and tended to 0.03 at the highest velocities Canopy flow theory provides a good description of the observed drag coefficient variability with flow strength