Revised conceptual model of gravitational circulation in Suisun Bay

Velocity data collected during the 1990s and a complete review of all historical current-meter data collected in Suisun Bay and the Sacramento River have been used to develop a revised conceptual model of gravitational circulation in Suisun Bay. The data suggest that gravitational circulation has the following characteristics:

• it dominates residual transport in Carquinez Strait unless freshwater inflows are so high that the waters in the strait are completely fresh (Burau and others, 1993; Monismith and others, 1996);
• it is rare in the southern ship channel during the spring, but consistently has been measured during the autumn in the northern part of Suisun Bay (Burau and others, in press);
• it has been measured in the lower Sacramento River when local, near-bottom salinities have exceeded about 2 psu (Nichol, 1996);
• it is weakest during spring tides and strongest during neap tides (Burau and others, 1993); and
• it can occur as landward pulses of water that develop along the bottom at the beginning of flood tides during weak neap tides, when the water column is stratified (Monismith and others, 1996).

These observations differ from the existing conceptual model of gravitational circulation. For example, gravitational circulation is not dependent on a particular salinity, it varies in strength in Suisun Bay, and it is altered by the spring-neap tidal cycle. A revised conceptual model (table 1) was developed using results from other studies and quantitative scaling of stratification and mixing to explain these observations (Burau and others, in press).

Table 1: Revisions to the conceptual model of gravitational circulation and the estuarine turbidity maximum in Suisun Bay, California [psu, practical salinity units]. Click on table for a larger version (30 KB)

The revised conceptual model includes the following changes:

• gravitational circulation increases with water depth (Walters and others, 1985);
• a semipermanent null zone is located near the Benicia Bridge during spring (Burau and others, in press), and other geographically fixed null zones may be located elsewhere in Suisun Bay, where deep channels become shallower in the landward direction (see bathymetry in fig. 4);
• gravitational circulation is suppressed by increased vertical mixing during spring tides (Walters and others, 1985); and
• the horizontal salinity gradient (rate of change of salinity along the estuary), not salinity, drives gravitational circulation (Hansen and Rattray, 1965).

Figure 4: Suisun Bay bathymetry. Click for a larger version (55 KB)

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