Turbulence-Resolving Numerical Investigation of Fine Sediment Transport over Ripples in the Suborbital Regime

Sediment transport in the oceanic bottom boundary layer (BBL) plays an important role in the cross-shelf exchange of near-bottom particles and their associated geochemical constituents, as well as the cross-shelf gradient of overlying flow energy. Interestingly, during the transport process, bedforms are commonly observed, which retard the overlying flow and enhance the energy dissipation. Our work uses turbulence-resolving numerical simulations to investigate fine sediment (silt and very fine sand) transport over suborbital ripples, a condition found commonly on continental shelves. For a preliminary study, the numerical model is simplified to have a BBL driven by surface gravity waves only. Two classes of sediment are considered, and the model accounts for the important effects of winnowing and armoring (Wu et al. 2018, J. Geophys. Res. Earth Surf.). The ripple bed is fixed and made up of very fine sand, replicating the bedforms observed in Hooshmand et al. (2015, J. Geophys. Res. Ocean.), and with dimensions calculated using a ripple predictor (Wiberg and Harris 1994, J. Geophys. Res.). Finer silt is eroded from the sandy ripple bed into the computational domain, limited by a specified critical shear stress of erosion. Thus, the suspended load consists of the finer silt only, resulting in a near-bed fluid mud layer that could flow across isobaths due to the sediment-induced density stratification. Through the high accuracy numerical simulations, the mechanism governing the transition of different ripple regimes is studied. Moreover, we investigate the role of the ripple bed in enhancing the sediment-carrying capacity and the energy dissipation of flow, which further controls the intensity of wave-supported gravity flows.