Dynamic Diagnosis And Analysis For Residence Time Of Seawater In Tampa Bay

A three-dimensional, numerical circulation model, with resolution as high as20m at important mass conveyances (channels, inlets, bridge causeways, and rivers) isapplied to Tampa Bay by using Finite Volume Coastal Ocean Model (FVCOM). Asatisfactory model evaluation is made against in situ data and a previously published,relatively lower resolution version.Firstly, the tidal and non-tidal circulations of Tampa Bay are addressed. We rantwo parallel model experiments, driven by tides plus rivers, and tides, rivers pluswinds, to separate the gravitational convection and wind-driven circulation. For thecase without winds, Tampa Bay exhibits the classical two-layered estuarinecirculation: relative fresher water moves seaward in the upper layer and relativedenser salty water moves landward in the lower layer. With the winds affecting, thesurface flow is enhanced and converged near Tampa Bay mouth, causing a meanseaward directed sea surface slope, which further invigorates the bottom flow. Windstress could permeate into the bottom layer in shallower water area due to theturbulent mixing, resulting in a flow change nearly in the entire water columnconsistent with the mean wind direction. In deep area such as shipping channels,however, the change of surface flow is consistent while that of bottom flow is reversewith the mean wind direction.Secondly, a systematically diagnosis of salt balance is investigated. Both the saltbalances and the salt fluxes are fully three dimensional. On experimental (threemonth) average, the total (horizontal plus vertical) advective salt flux divergence ismainly balanced by the vertical diffusive salt flux divergence, except near the bottomof deep shipping channel where horizontal diffusive salt flux divergence is alsoimportant. Instantaneously, the local rate of change of salinity is primarily controlledby the advective salt flux divergence, with a secondary contribution by the verticaldiffusive salt flux divergence everywhere and the horizontal diffusive salt fluxdivergence only near the channel bottom. To examine the role of tidal pumping, theadvective salt fluxes and divergences are further decomposed into the products of the mean salinity and velocity, and the correlation between the salinity and velocityfluctuations. The horizontal and vertical advective salt flux divergences by the meanquantities are equally large and counterbalancing, with their sum being a small, butsignificant residual. The horizontal and vertical advective salt flux divergences due totidal pumping are relatively small and counterbalancing, but, when summed theirresidual is comparable in magnitude to that by the mean quantities. The salt fluxesvary along the estuary axis with varying morphology and channel complexity; but,regardless of these variations, the salt fluxes by the mean quantities are much largerthan the salt fluxes by tidal pumping.Thirdly, the residence time distributions for the entirety of Tampa Bay and fiveof its sub-regions are estimated by using an Eulerian passive tracer technique todistinguish the effects of tides, rivers, winds and multiple inlets. Tides alone have aminor effect. An exception pertains to sub-regions with multiple inlets, wheredifferential flows related to tidal phase differences aid in flushing. Rivers and windsare important contributors for three-dimensional circulation, thus for flushing. Aspatial variation of residence time is found for Tampa Bay, with a relative lowerresidence time in inlets, channels and river ways, and the highest residence time in thewestern and northern portions of Old Tampa Bay. A double exponential method isalso employed to extend the residence time estimates beyond the simulation intervalby empirical curve fitting.Finally, the high resolution Tampa Bay model is used to address how the tidaland non-tidal circulation and flushing may be affected by potential channel deepeningand widening. Results show that such channel modification would increase tidal rangeand reduce tidal phase, affecting both riparian rights and navigation, and invigoratethe non-tidal circulation, resulting in a significant increase in salinity and attendantconsequences to the estuary’s ecology. Along with these tidal and non-tidal effects,the residence time is found to decreases due to an increase in gravitational convection.