Flow and Turbulence over an Oyster Reef

Styles, R., 2015. Flow and turbulence over an oyster reef.

Simultaneous measurements of near-bed flow and turbulence were collected on opposite banks of an intertidal channel in the North Inlet/Winyah Bay National Estuary Research Reserve. One bank supported an extensive cover of oysters and the other a mixture of sand and mud. The measurements allow comparisons of flow and turbulence characteristics in a similar flow regime but widely varying roughness conditions. Near-bed velocities are higher over the sandbank and occur during the maximum flood portion of the tidal cycle. In contrast, turbulence parameters are higher over the oyster reef, resulting from the presence of the larger roughness elements. Turbulent kinetic energy and Reynolds stress components increase as a function of flow speed, consistent with equilibrium boundary layer shear flows. For some bursts, the energy spectrum exhibits a −5/3 slope, indicating a defined inertial subrange. Dissipation over the oyster bank is on the order of 10 cm² s⁻³ during maximum flood, when the near-bed current speeds are greatest. Drag coefficient and hydraulic roughness are likewise greater over the oyster bank, with average values of C_D = 0.025 and z₀ = 0.78 cm compared with C_D = 0.004 and z₀ = 0.02 cm for the sandbank. The analysis reveals a simple roughness formula for oysters in which the physical bottom roughness is equal to 5 times the average height of the oysters.