Tidal-flow, circulation, and flushing changes caused by dredge and fill in Tampa Bay, Florida

Tampa Bay, Florida, underwent extensive physical changes between 1880 and 1972 because of construction of causeways, islands, channels, and shoreline fills. These changes resulted in a progressive reduction in the quantity of tidal water that enters and leaves the bay. Dredging and filling also changed the magnitude and direction of tidal flows in large parts of the bay.

A two-dimensional, finite-difference, hydrodynamic model was used to simulate flood, ebb, and residual transport of both water and a dissolved constituent for the physical conditions that existed in Tampa Bay during 1880 and 1972 and for the conditions that are likely to exist in 1985. The calibrated and verified model was used to hindcast water- and constituent-transport changes caused by construction in the bay between 1880 and 1972. The model was used also to forecast changes that can be expected to occur as a result of a major Federal dredging project scheduled for completion in 1985.

The model forecasted transport changes caused by the Federal dredging project to be much less areally extensive than the corresponding transport changes caused by construction in Tampa Bay between 1880 and 1972. Dredging-caused changes of more than 50 percent in flood and ebb transport were computed to occur over only 8 or 9 square miles of the bay's 390-square-mile surface area between 1972 and 1985. The model computed that construction between 1880 and 1972 caused changes of similar magnitude over 58 square miles of the bay. Dredging-caused changes of more than 50 percent in residual transport were computed to occur over 58 square miles of the bay between 1972 and 1985. According to the model simulation, construction between 1880 and 1972 caused changes of similar magnitude over 167 square miles.

Computations reveal historical tide-induced circulation patterns. The patterns consist of a series of about 20 interconnected circulatory features that range in diameter from 1 to 6 miles. Dredging- and construction-caused changes in size, position, shape, and intensity of the circulatory features increase tide-induced circulation and flushing throughout most of the bay. As a result of past and projected physical changes, the bay can and will more rapidly transfer waterborne constituents that have landward sources to the Gulf of Mexico. Conversely, the bay can and will more rapidly transfer constituents that have their source in the Gulf into the upper parts of the bay.

Model results show that the bay can be functionally subdivided into eight circulation zones. The zones near the entrances to Tampa Bay and the major bay subunits, Hillsborough Bay and Old Tampa Bay, have several times greater average circulation than do adjacent and more landward zones. Circulation generally decreases from the Gulf of Mexico to the head of Hillsborough and Old Tampa Bays, with a striking exception in zone 3 in mid-Tampa Bay. This 10-mile section of the bay has significantly lower average circulation than that of adjacent zones, particularly for conditions in 1880. The section is thought to be a circulation constriction that reduces the potential transport of dissolved and suspended constituents.

Circulation in the constricted section was computed as having increased 6 percent because of dredge and fill construction between 1880 and 1972. An additional increase of 21 percent, due to the Federal dredging project, is calculated. With these increases, this zone acts as less of a constriction than it did in 1880. This and other circulation increases may have contributed to increased bay salinity and to more rapid flushing of constituents from the bay to the Gulf of Mexico.