Changes in Residence Time due to Large-Scale Infrastructure in a Coastal Plain Estuary

The alteration of residence time in a coastal plain estuary due to dredging and building of barrier-type structures is found to vary with freshwater conditions. Two identical 3-year simulations with realistic boundary conditions were performed using numerical circulation models of Tampa Bay, Florida, that differ only in their bathymetry. The first simulation used present-day bathymetry. The second used bathymetry based on depth soundings from the preconstruction year 1879. Both models were seeded evenly with over 456,000 passive tracers at the beginning of three separate 90-day time periods. These times were initially chosen according to the relative change in subtidal circulation but found to correspond to different vertical mixing conditions. Two types of Lagrangian residence time were studied: The first, baywide residence time (TR), is based on the total number of particles in the bay. The largest change in TR was found during a weakly mixed time period with strong baroclinic circulation. In other time periods best described as well- or partially mixed, TR was relatively unaffected. The second, grid scale residence time (), is based on the total number of particles in each model grid cell. Increased was found near bridges and causeways, and decreased was found where relatively deep dredging has occurred. Some regions within the estuary experienced increased in well-mixed conditions but decreased in weakly mixed conditions. The Lagrangian method reveals a rapid, largely transverse redistribution of particles that indicates the results are not sensitive to the initial particle distribution