dissolved and suspended material in the bay. The content of this paper 
is largely taken from the results of Goodwin (1987) and Goodwin (in 
press). 
As previously mentioned, horizontal circulation is a tidally 
averaged water motion that is caused by interactions between incoming 
(flood) and outgoing (ebb) tidal flows and bay geometry. Figure 3, 
depicting tidal flood (3a), tidal ebb (3b), and tidal 1y averaged (3c) 
water motion in a region at the mouth of Hillsborough Bay (see Figures 1 
and 7), illustrates this phenomenon. A primary feature of the bay’s 
geometry in this region is an east-west oriented ship channel with two 
dredge-material islands on the south side of the channel. The channel is 
about 25 feet deep, and the surrounding bay depths vary from 5 feet or 
less near the eastern shore to about 15 feet on the western edge of the 
illustrated area. 
Visual comparison of the flood and ebb patterns of flow shows a 
large westward component in and along the ship channel during ebb that is 
not balanced by an equivalent eastward flow during flood. The fact that 
the channel lies to the north of the islands provides a path of little 
resistance to help convey ebb flows westward. No similar pattern forms 
during flood flow because the channel is then in the lee of the islands. 
Through an entire tidal cycle, the overall effect of the channel and 
islands is to produce a tidally averaged net or residual motion that is 
westward along the channel (Figure 3c). Because (in a net sense) the 
westward moving water must be replaced, circulation cells are set up to 
accomplish this and maintain continuity of mass throughout the affected 
region. This rather extreme example of geometry-controlled flow and 
circulation patterns is, nonetheless, a valid description of how 
horizontal circulation features are generated throughout Tampa Bay. This 
type of circulation has been called "tidal pumping" by Fischer, List, 
Imberger, and Brooks (1979). 
Circulation features computed in Tampa Bay, using a simulation 
model having a grid size of 1,500 feet, are shown in Figure 4 for 
conditions as they existed in 1985. The 20 or so annotated circulation 
features indicate the complexity of the overall pattern of bay 
circulation that is believed to play a large role in the distribution and 
flushing of dissolved and suspended material. For comparison and a 
visual indication of the cumulative effects of dredge and fill 
activities, Figure 5 shows computed circulation patterns for 1880 
conditions. Impacts of construction of ship channels, causeways, 
islands, and shoreline fills have been to both intensify and distort 
circulation features that existed prior to construction as well as to add 
new circulation features. 
To compare circulation changes between 1880 and 1985, Goodwin 
(1987) plotted a measure of circulation intensity as a function of 
distance (Figure 6) and identified six zones from the Gulf of Mexico to 
the head of Hillsborough Bay having different circulation 
characteristics. Circulation zones are shown in Figures 4 and 5, and 
Table 2 gives the average circulation computed for each zone for 1880 and 
53 
