Water exchange of Oeregrundsgrepen. A baroclinic 3D-model study

Hypothetically transport of radionuclides from the SFR repository for low and medium active wastes could be mediated by natural water circulation within receiving coastal basins. In this context a basic equation, free surface 3D-model has been used to compute the water exchange of the Oeregrundsgrepen bay-like area for a representative full year cycle. This has been achieved in two steps in order to provide coherent densimetric and sea level elevation boundary data relative to the adjacent Baltic coastal water. Weather data from 1992 were chosen. The focus is placed entirely on water exchange aspects with no consideration of what the water parcels may contain. Earlier model and measurement programs have also been reviewed. The first phase consisted of running a 3D-model encompassing the entire Baltic Sea. This model resolves the Baltic horizontally in five by five nautical miles (5x5). This model was driven by gridded (approx. 20x20) synoptic weather data with geostrophic wind and the varying density and sea level elevation on the Kattegat border. Freshwater discharge from the major rivers along the Baltic coastline was also taken into account. Initial data prior to December 1991 have been assessed from the available, but relatively scarce, salinity and temperature profile measurements in the Baltic. The time step was 2 hours. The relevant boundary data in the vicinity of the Oeregrundsgrepen area were saved after one full-year cycle of simulation. The second phase consisted of running a local model over the Oeregrundsgrepen with a higher horizontal grid resolution consisting of a 0.1x0.1 grid. This model was driven by the same weather data, combined with the saved densimetric and sea level elevation boundary data that were produced by the Baltic model with the coarser grid. This procedure applies both for the wide northern and the narrow southern interface. The transference of boundary data necessitated development of an appropriate interpolation scheme. This model has also been run for a full-year cycle allowing one month (December 1991) of spin-up time. The time step has been varied between 3 and 6 minutes. The retention time of the Oeregrundsgrepen was found to vary between 12.1 days (surface) and 25.8 days (bottom) as a yearly average. Special regard has been placed on estimating the ventilation of a particular subarea where a biological model study is currently being performed. This subarea is located in the waters above the SFR-depository embedded within the Oeregrundsgrepen model area. The exchange intensity, expressed as a yearly average transit retention time, spanned from 0.5 days (surface) to 1.2 days (bottom) with regard to the depth strata that the model resolves. The bulk volume average for all strata was 0.77 days with a standard deviation of 0.22 days equally for both intra-monthly and inter-monthly variations. The corresponding average total volume flow across the boundary was 2.1x103 m3/s