A hydrodynamic model of St. Joseph Bay, FL has been developed based on a finite-element circulation model. We have tested the model against predicted astronomical tides and field measurements of water levels at three locations inside and outside the bay. Good agreement has been found, including the correct prediction of the change of the tidal regime from diurnal to mixed tides and the high-frequency oscillations in St. Joseph Bay excited by meteorological forcing. The validated hydrodynamic model was employed to partition the tidal prism of St. Joseph Bay in the event of a breach at Stump Hole north of Cape San Blas and to estimate the flushing through a new inlet at Stump Hole, or the portion of tidal prism controlled by a breach. The existing methodology for inlet stability analysis of a single-inlet system is extended to this two-inlet system if stump Hole is breached by a tropical cyclone. A ten-step procedure has been developed for the breach stability analysis, which has three major components. First, the empirical relationships of inlet geometric parameters were employed to develop a series of possible breach configurations. Second, the validated hydrodynamic model was modified to accommodate the new inlet and numerical simulations were carried out to determine the hydraulic conditions corresponding to each inlet configuration. Third, the computer model results of this two-inlet system were combined with the empirical relationship of tidal prism and inlet cross-sectional area for equilibrium inlets to predict the stable inlet configuration and the fate of a breach using the maximum velocity curve in an Escoffier-type diagram. Similar to the use of an equilibrium beach profile in the design of beach nourishment projects, the concept of an equilibrium inlet and the empirical relationship for stable inlets provide a useful tool to predict the fate of a breach in a multi-inlet, multi-tidal-regime system.