Interaction of combined waves and current with high-relief bottom roughness

Johanna H Rosman, University of North Carolina at Chapel Hill, Institute of Marine Sciences, Chapel Hill, NC, United States, Xiao Yu, University of Florida, Department of Civil and Coastal Engineering, Ft Walton Beach, FL, United States and James L Hench, Duke University, Nicholas School of the Environment, Durham, United States
Abstract:
In the coastal ocean, bottom topography varies continuously across a wide range of length scales. While bottom friction associated with small (e.g., sand grain) roughness has been studied extensively, less is understood about friction on waves and currents when roughness element sizes are similar to wave orbital excursions. In this study we investigated bottom friction in combined wave-current flows over large roughness elements using LES. Flow parameters (wave orbital velocity Uw, period T, current Uc) were varied across ranges typical in reef systems to examine dependence of flow dynamics on important dimensionless parameters. Terms of the spatially- and phase- (and time-) averaged momentum budget were computed directly from simulation results to quantify the effects on waves (and currents).

Flow dynamics depended strongly on Keulegan-Carpenter number (KC) and Uw/Uc. The KC dependence was weak when Uw/Uc was small (current dominated) and strong when Uw/Uc was large (wave dominated). When there was no current, flow separation strength and roughness element cd increased with KC. For current-dominated cases (Uw/Uc<1), flow separation was controlled primarily by the current and cd decreased with Uw/Uc. Dispersive stresses arising from persistent spatial flow variations were comparable to Reynolds stresses for wave-dominated cases (Uw/Uc>1) and were particularly important in the momentum budget for the current. The friction coefficient for the current, computed from the time-averaged drag force, increased with Uw/Uc. In the momentum budget for waves, the drag force increased with Uc/Uw for small KC but varied little with Uc/Uw for large KC. These patterns could be reasonably well explained by a quadratic dependence of drag on instantaneous velocity together with observed variations in cd. This study provides new insights into dynamical interactions of waves and currents with topography at low KC, with important consequences for parameterizing bottom friction on reefs.

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