Exchange flows in estuaries and archipelagos

Abstract: Different aspects of the exchange of water and tracersbetween adjacent coastal basins, and between the coast and thesea, are addressed. Deviations from the inviscid hydraulicprediction of flow rates and interface heights in maximaltwo-layer exchange flows due to viscous effects are quantifiedby means of non-hydrostatic, three-dimensional numericalmodeling. Simulations are made for different strait geometriesand with a time-dependent barotropic flow to determine when andwhere vertical mixing and the resulting recirculation oftracers preferentially take place (Paper I). Non-hydrostaticthree-dimensional numerical modeling is also performed to studythe variability in circulation and stratification in a tidallyinfluenced estuary. In particular, two processes that act todecrease the rate of flushing of tracers are addressed?turbulent mixing and secondary circulation, i.e. circulationperpendicular to the longitudinal estuary axis (Paper II).Schemes for modeling water exchange in an archipelago, i.e. ina complex system of basins and interconnecting straits, arediscussed (Paper III). Problems relating to Bernoulliaspiration of dense water from below the sill crest,multi-layer stratification, and to some extent rotationalcontrol in parallel straits are addressed.Methodological aspects of both three dimensional numericalmodeling and of solution of the inviscid two-layer straitexchange problem are discussed. In the three-dimensionalnumerical modeling two issues are central: (i) theimplementation of open boundary conditions that allow a meanflow to be forced from one of the boundaries, while at the sametime let barotropic and baroclinic waves be radiated out of thedomain with minimum reflection, and (ii) the implementation ofa subgrid closure scheme to calculate the vertical eddyviscosity and diffusivity coefficients locally fromresolved-scale shear and stratification.