Analysis field measurements Paulina polder : looking for sediment-fluid interaction

The area of intertidal areas in the Netherlands has tremendously decreased over the last decades. For management of the remaining area of inter tidal areas, knowledge of how these basins catch sediment is indispensable. One of the processes which is expected to be responsible for catching fine sediment en intertidal areas is part of this study: the interaction between turbulence and sediment induced buoyancy effects, which might lead to sediment-induced density currents and the possible collapse of the turbulence field and concentration profile when the saturation concentration is exceeded. In this project this feed-back mechanism is investigated by means of a data analysis. The data analysis concerned velocity, turbidity and water level measurements at the Paulina intertidal area in the WesterScheldt. The velocity and turbidity data were collected at two levels. This allows one to gain information on vertical gradients. The generation of density currents was expected to be visible in the vertical concentration gradients. Unfortunately, due to an inadequate calibration of the turbidity sensors, the concentration values near the bed are larger than the ones higher in the water column. This makes it impossible to say anything about vertical gradients in concentration die profile from the data. The conclusions from the data analysis are: 1) The supposed collapse of the entire concentration profile and of the turbulent fluctuations did not occur in the measurements. Possibly the concentrations were too low for this effect to occur. 2) Another phenomenon may indicate the presence of sediment-induced effects: many velocities recordings show a 10 to 30 degree veering with height, possibly indicating the presence of a density current. The hypothesis is that a bottom density current is formed by die interaction between turbulence and sediment concentration. This under-current is then directed by local topography, while the higher layer is governed by larger scale hydrodynamic features. However, the veering may also be the result of wind and/or Coriolis effects. It is recommended to focus further research en this observation. 3) Maximum sediment fluxes en the tidal flats seem to occur in the shallows (< 0.5 deep, at the beginning and end of immersion), when concentrations, velocities and turbulence, are largest. Any residual sediment transport is therefore determined as the difference between the large values of the ebb and flood fluxes.