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

Alteration of bathymetry by the dredging of shipping channels and the construction of bridges and causeways are found to change residence time in a coastal plain estuary. Two identical three-year simulations are performed using realistic numerical circulation models of Tampa Bay that differ only in their bathymetry. The first bathymetry is based on present-day depth measurements and contains the modern infrastructure; the second is based on depth soundings from the pre-construction year 1879. Both models are seeded evenly with over 456,000 passively advected particles at the beginning of three distinct 90-day time periods within the simulations representing low, average, and high fresh water inflow conditions. Two types of Lagrangian residence time are studied: 1) The baywide residence time based on the total number of particles remaining in the bay. 2) The gridscale residence time based on the total number of particles in each model grid cell. The largest change in baywide residence time due to infrastructure is found during a period when the subtidal Eulerian circulation is strongly impacted by the infrastructure. During periods of weak to moderate Eulerian impact the baywide residence time is largely unaffected by infrastructure. At the grid-scale there are significant impacts such as decreased residence time headward of the bridges and decreased residence time in the portion of the bay where relatively deep dredging has occurred. A related experiment considers the effects on circulation of the bay due to the potential loss of Egmont Key, the barrier island that separates Tampa Bay from the Gulf of Mexico. Changes in the amplitude and phase of the dominant astronomical tides, tidal velocities, maximum tidal water levels and current speeds, and storm surge impacts vary spatially and are found to be strongest in the vicinity of present-day Egmont Key and around the estuarine coastline.