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 ReK = √KU*/ν. 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flow where the DBL model is no longer applicable.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.