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Processes in regions of freshwater influence

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Numerical models for Regions of Freshwater Influence (ROFI) have been tested involving physical processes controlling water-property distributions, suspended sediments and microbiology. A fully-coupled 3-dimensional prognostic model was developed and tested. It has high resolution (typically 1hr time-step, a few k horizontally, a few m vertically) and describes: atmospheric forcing (winds, heat), boundary forcing (river inflows, tides and other variables); currents, temperature, salinity, turbulence, diffusion, stratification, tidal and weather effects; suspended particulate matter, including deposition and erosion; microbiology and nutrients; contaminants. A fully-coupled run was carried out for a measurement period in the Rhine outflow, April 1994. Measurements were carried out, and compared with models, in contrasted ROFIs: the Rhine outflow (1990, 1992, 1994), the Clyde (1993-1994) and Thermaikos Bay (1994-1995). In each case there were moorings and regular surveys: time series to determine dynamics and intermittency; long duration to cover variability over a full year (excepting the Rhine, 7 months); continuous salinity and fluorescence records; good resolution of vertical structure suitable for testing the model. Data from German and Italian experiments, in the German Bight and Po outflow, were used for comparison with models. Results show the importance of wind and wave forcing and mixing, quantity of freshwater and ambient stratification on ROFI dynamics. Other factors also apply: Rhine, tidal cycles in stratification and mixing; Clyde, almost permanent stratification with overlying fresher water, cross-sill exchange; Thermaikos Bay, extreme seasonal heating, cooling, fresh water, wind and Aegean influences. Thus new measurements were made of a seasonal cycle in contrasted ROFIs, and a corresponding model developed combining physics, suspended particulate matter and microbiology. A focusing study informs strategy for embedding local detail in regional or global models.

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