Numerical investigation of an estuarine front and its associated eddy

An estuarine frontal system was investigated with a series of numerical experiments using a 3D hydrodynamic model. The model results were confirmed with field observations and compared with theoretical analysis. Various factors, singly or jointly, contributing to the frontogenesis were investigated. The numerical computations demonstrate that the theoretical critical densimetric Froude number is exceeded as flood current increases and the front is pushed forward to the deeper water. The numerical experiments indicate that two factors contribute significantly to the flow convergence by advancing the phase of flood current on the shoals relative to that in the channel. They are differential friction resulting from depth contrast between deep channel and shoals, and enhanced surface ebb current in the channel resulting from stratification and density induced circulation. Numerical experiments also show that the front is stronger and lasts longer during neap tide than during spring tide. Weaker salinity contrast during spring tide reduces the buoyancy force that is required for the diving of the denser water mass. Furthermore, the stronger flood current during spring tide causes the densimetric Froude number to exceed its critical value earlier, as predicted by the theoretical analysis, and pushes the front away from the region of steep bottom slope, causing it to dissipate.