A laterally averaged hydrodynamical model is used to investigate episode intrusions of denser, more oceanic water across the shallow double-silled entrance of Puget Sound and the resulting circulation within the interior deep basin. Tidal, freshwater discharge and wind effects are also encompassed in model results. Model simulations were made of the 4-month period from December 1983 through March 1984. During that period, hourly measurements of currents and salinity were made at the estuary entrance and on four moorings in the interior basin. The first 60 days of salinity data were used with the model to choose rates of vertical turbulent mixing and the lag times between fresh water appearance at river gauges and its arrival in the main basin of the sound. Comparison of model results with the full 120 days of salinity and velocity data shows that the model incorporates the important circulation features of this estuary. The model was then used to study intrusions and mixing in the sill region.

The model identifies these factors as controls on the occurrence and strength of intrusions: the trans-sill salinity gradient, the intensity of tidal stirring and mixing over the sills, the supply of fresher surface water resulting from river runoff, and the winds. Time series of salinities in the interior basin show local maxima following local maxima in the trans-sill salinity difference. Vertical transport in the sill region at both tidal and subtidal time scales is dominated by advection rather than diffusion. Though much of the vertical exchange occurs over the shallower sill, a depth depression between the two entrance sills appears to be the site of significant overturning. Downwelling of surface water occurs on the landward side of both sills. Substantial freshening of the interior basin occurs during the winter months because river discharges are large and are stirred downward in the sill region to be entrained into landward bottom flows. Extraordinarily large river discharges can suppress the effect of large trans-sill salinity differences with the effect of weakening potential intrusions. Winds can indirectly influence intrusions by longitudinally positioning fresher, river-influenced surface water in the sill region. Winds can directly influence intrusions via bottom compensation flows in Admiralty Inlet, with northward winds encouraging intrusions, and by changing salinities at the entrance to the sound, and thus the trans-sill salinity gradient.