Coherent vortex structures at the sediment-water-interface

In river, estuarine and coastal environments, the diffusive boundary layer (DBL) model is typically employed to describe solute fluxes between the sediments and the overlying water column. This model assumes that vertical diffusion at the sediment-water interface occurs at molecular rates. Recent studies, however, show that measured fluxes can be several orders of magnitude larger than can be explained by molecular diffusion alone. Here, we use a laboratory experiment to investigate one mechanism for enhanced interfacial fluxes, namely the generation of coherent structures at the sediment-water interface. The experiments use a combination of Particle Tracking Velocimetry (PTV) and Refractive Index Matching (RIM), thus allowing simultaneous acquisition of instantaneous flow fields both above and below the sediment-water interface. The experiments demonstrate the existence of coherent structures at the interface and their influence on interfacial exchange. The results show a strong dependence of the flow characteristics on the permeability Reynolds number. For low ReK the flow resembles an impermeable boundary layer, while for high ReK the flow resembles a mixing-layer-type flow where the DBL model is no longer applicable. In the high-ReK case, swirl of the instantaneous velocity field shows the existence of coherent structures at the interface and these coherent structures extend well into the interstitial fluid in the sediment bed. These coherent structures become more pronounced with increasing ReK.