Full-scale scalar fluxes near the sediment water interface: spatial, temporal, and source location dependencies

Several biological, chemical, and ecological processes occur at the sediment-water interface of the coastal ocean. Near-boundary hydrodynamics are dominated by oscillating flows, while the porous boundary is characterized by different roughness scales including sediments (O[mm]) and bedforms (O[cm-m]). To better understand the exchange dynamics in this region, full-scale laboratory experiments were performed with an oscillating sediment tray to acquire high-resolution velocity and concentration fields using PIV and PLIF. Rhodamine 6G dye was used as the scalar and injected at three different locations along the sediment ripple: crest (CrC), flank (FlC), and trough (TrC). Phase-averaged plume concentrations remained inside the boundary layer, with no accumulation over time. The spread of dye reached 0.5, 1.3, and 0.25 of the boundary layer thickness for CrC, FlC, and TrC, respectively. Phase-averaged concentration fluxes were higher for FlC and much lower for TrC. Fluxes for TrC remain very close to the bed in a layer of roughly 10mm, following the ripple shape. The same was true for CrC during phases with u