WATER EXCHANGE AND DENSITY STRUCTURE IN A MULTIBASIN ESTUARY

A mathematical model of the Himmerfjard estuary. divided into four basins. has been formulated and validated against measured data for 1986. The structure of each sub-basin is assumed to be horizontally homogeneous with vertical mean velocities based upon in- and outflows from adjacent basins and freshwater supply to each basin. The horizontal water exchange is formulated as a quasi-stationary Bernioulli flow, driven by horizontal pressure gradients over the sounds and instantaneously interleaved at a neutral buoyancy level. Thc salinity and temperature profiles measured outside the mouth of the estuary serve as forcing, as do the water level changes, the freshwater run-off and the local wind. Inherent in the model assumptions of the horizontal exchange over the sounds is that only a fraction, alpha, of the pressure gradient is used to accelerate each stratum. Variation of the alpha-value shows that the best statistical fit is found for alpha = 0.15 when compared with water exchange estimates based on measurements in one of the internal sounds tor almost an entire month. Using this alpha-value, in combination with standard mixing parameters and hypsographical data, the model satisfactorily captures the major features of the salinity and temperature-profiles development for the year 1986. This is substantiated by statistical analysis of the salinity profiles in the sub-basins for which different measures of similarity between simulated and measured data give the best fit for the same alpha-value as above.