Incorporating Non-Equlibrium Ripple Dynamics into Bed Stress Estimates Under Combined Wave and Current Forcing

We present direct measurements of seafloor ripple dimensions, near-bed mean flow Reynolds stresses and near-bed turbulent sediment fluxes on a sandy inner shelf subjected to strong wave and tidal current forcing. The measurements of ripple dimensions (height, wavelength) and Reynolds stresses are used to evaluate the performance of a methodology for the incorporation of non-equilibrium ripple dynamics into the calculations of the drag exerted by the bed on the overlying flow (i.e., the bed stress) using a boundary layer model for wave–current interaction. The methodology is based on the simultaneous use of existing models for the time-dependent evolution of ripple geometry and for the wave–current boundary layer that enable a continuous feedback between bottom drag and small-scale seabed morphology, which determines seabed roughness. The model-data comparison shows good agreement between modeled and measured bed stresses and bedform dimensions. Moreover, the proposed methodology is shown to give better results than combining the wave–current interaction model and standard equilibrium ripple predictors, both in terms of bed stresses and ripple dimensions. The near-bed turbulent vertical sediment fluxes show good correlation with the combined wave–current stresses and are used as a proxy for the resuspension of fine sediments (d < 64 μm) from the sandy seabed matrix. Implications for the modeling of the resuspension processes and erosional fluxes are discussed in light of our findings.