Background/Objectives
McKay Bay is a heavily altered shallow sub-estuary at the northeastern end of Tampa Bay, adjacent to the Port of Tampa. This 3.5 km2 Bay is an integral part of the Palmetto Beach community by providing recreational and subsistence-based fishing, as well as an opportunity for education and waterfront views for this low-income underserved community. The circulation of McKay Bay is driven by tides from the Gulf of Mexico, and discharge from the Tampa Bypass Canal. This Bypass Canal was engineered from the original Sixmile River, a tidal creek, to route urban stormwater to Tampa Bay. Due to its proximity to the Port of Tampa, McKay Bay has been heavily engineered since the 1920s. A bridge and causeway were built crossing the mouth of the bay in the late 1920s. From the 1950s through the 1960s this area was heavily altered through numerous dredge and fill projects near the mouth of the Sixmile river, and within McKay Bay. These significant fill projects resulted in a 10% reduction of the bay area and greatly altered the flow pattern, along with negative environmental impacts on the Palmetto Beach community. The heavily engineered system provides an opportunity to understand how historical management decisions and practices can influence the current and future health of the bay and to explore nature-based solutions to improve the health of the system.
Approach/Activities
A detailed hydrographic survey was conducted. The majority of the bay’s bathymetry is shallow ranging from about 0.5 to 2.0 m, relative to mean sea level, except a 3.0 to 5.0 m dredged channel from the Tampa Bypass Canal to the mouth of the bay. The tides in the study area are mixed with semi-diurnal neap tides of ~0.7 m and diurnal spring tides of up to 1.2 m. A numerical depth-averaged flow model was built using the U.S. Army Corps of Engineer’s Coastal Modeling Systems (CMS). The numerical model was calibrated and verified using measured current velocities at various locations within the bay. The natural flow pattern was modeled based on the 1885 bathymetry before any human alterations. The results were compared with the existing bathymetry and various anthropogenic modifications including filling the channel to varying degrees, and the construction of shoals of various sizes and orientations, in an attempt to improve the circulation of the bay.
Results/Lessons Learned
Under present day conditions both peak ebb and flood velocities show similar values at about 0.2-0.3 m/s through the bridge at the entrance of the bay. The discharge from the Tampa Bypass Canal suppresses flood flow and enhances ebb flow, as expected. These simulations also show low flow “dead zones” along the northern portion of the bay, i.e., the Palmetto Beach Community area, and southern portion. These two low flow areas were not computed over the 1885 bathymetry before anthropogenic modifications. Various production runs were tested with different bathymetry alterations with the goal of improving circulation in these low-flow areas. The best results were achieved by filling the channel coupled with an elongated shoal and spur constructed as well. The results of this analysis show that it is important to quantify potential influences on bay-wide flow patterns when making engineering and management decisions. The modeling results were used to assist in evaluating various nature-based solutions to mitigate negative impacts from past engineering activities.