Tidal Circulation and Old Tampa Bay
By Shayne Paynter, PhD, PE, PG, Ed Cronyn, PWS, Daniel Lauricello, PE, Virginia Creighton, PWS, David Tomasko, PhD, and Mike Salisbury, PE
In highly urban areas, large transportation projects often spend tens of millions of dollars purchasing right-of-way for stormwater management, including the purchase of developed property to be demolished to construct ponds. In lieu of traditional stormwater best management practices, Atkins, a member of the SNC-Lavalin Group and the Florida Department of Transportation (FDOT) looked at an area of Old Tampa Bay (OTB) that historically has had poor water quality and evaluated improving this offsite area to compensate for other projects that drain to Tampa Bay.
The Florida Department of Environmental Protection (FDEP) placed OTB on its list of impaired water bodies in 1998. For waterbodies categorized as having impaired water quality, the typical management approach has been to transition towards the development of a total maximum daily load (TMDL). Rather than waiting for FDEP to produce a TMDL for Tampa Bay, local governments, state agencies (including FDOT, FDEP and the Southwest Florida Water Management District (SWFWMD)) and various other stakeholders joined forces and took a proactive approach to produce a Reasonable Assurance Plan (RAP) to guide the management of water quality in Tampa Bay. The RAP requires an estimated 85 tons of additional nitrogen load reduction projects for each five-year planning period, equal to a 17-ton-per-year reduction.
The most widely adopted stormwater treatment system in Florida, wet detention ponds, remove only about 30 to 40 percent of incoming nitrogen loads from stormwater. Dry retention ponds have nitrogen removal efficiencies in excess of 90 percent, but they often require much larger construction costs or areas of land to meet design standards and are often impossible in areas with poor soils or high-water tables.
However, the Florida legislature passed House Bill 559 in 2012 which included direction to the water management districts and the FDEP to “…allow alternatives to onsite treatment, including, but not limited to (emphasis added) regional stormwater treatment systems.” Upon the governor’s signature, this provision was enacted into law as Section 373.413(6), Florida Statutes (F.S.). Additionally, Section 4.0 of the SWFWMD Applicant’s Handbook Volume II (AH Vol II) states that “The applicant may also provide reasonable assurance of compliance with state water quality standards by the use of alternative methods that will provide treatment equivalent to systems designed using the criteria specified in this section.”
One such alternative treatment system involves the Courtney Campbell Causeway (CCC), which was constructed in the early 1930s when OTB was considered to have good water quality. Aerial photographs from 1948 showed evidence of extensive seagrass meadows in most of OTB. However, the shallow waters of OTB north of the CCC at its eastern terminus appeared to be devoid of seagrass in 1948. These findings indicated that the construction of the CCC changed the environment to the extent that seagrass could not grow in that area, even while adjacent waters supported extensive meadows of these underwater plants.
In January 2015, Atkins engineers completed a feasibility study characterizing existing seagrass, water quality, sediments, and other parameters in OTB. The feasibility study was initiated to evaluate if the replacement of a portion of the CCC with a conveyance structure such as a bridge would likely bring about an ecological response in OTB similar to or greater than that which would be expected to occur by treating stormwater runoff alone.
The results of the preliminary study revealed a pattern of seagrass species and presence that is best explained by the salinity regime that currently occurs in the area north of the CCC. The CCC has also altered hydrologic flow and residence times, which has placed stressors on seagrass meadows within the assessment area. The current state of the seagrass resources in the assessment area can be most likely attributed to the lack of tidal flushing causing the differences in seagrass species and abundance. The current pattern of seagrasses is such that shoal grass (Halodule wrightii) and widgeon grass (Ruppia maritima) are the dominant species north of the CCC, while mixtures of turtle grass (Thalassia testudinum), shoal grass, and manatee grass (Syringodium filiforme) dominate the seagrass areas south.
Based on the results of the feasibility study, including significant, demonstrated differences in nitrogen, salinity, seagrass type and abundance, as well as a link between lack of seagrass and distance to open water, FDOT, Atkins and permitting agencies agreed that placing a cut within the CCC would likely result in significant ecological uplift. As such, a hydrodynamic model and further data collection was provided to help size and locate a bridge cut through the causeway and evaluate anticipated changes in salinity, nitrogen, and residence time.
The hydrodynamic model applied in this effort was the Delft3D model, a widely used and validated numerical model that incorporates the effects of astronomic tides, wind, waves and meteorological forces to simulate time-varying hydrodynamics in two or three dimensions. The grid associated with the model is depicted in Figure 1. Figure 2 and Figure 3, respectively, illustrate the bathymetric contours for the Tampa Bay and nested domains.
Models were driven by tides, wind, and precipitation, and the nested domain included a conservative tracer to simulate residence time in the area of interest. Model predictions were validated against actual data prior to running bridge simulations.
The team evaluated modelling scenarios of various bridge opening lengths and locations along CCC. The intent was to balance costs versus having an opening long enough to exchange enough water to significantly reduce residence time north of the causeway and improve water quality.
Figure 4 illustrates the initial distribution of a 1 kg/m3 concentration of the conservative tracer. This distribution is identical for all model simulations. Figure 5 and Figure 6 represent the final tracer concentration for the existing and proposed conditions at the end of the seven-day model run. After seven days, the highest concentration in the area of concern is 0.25 kg/m3 under existing conditions and 0.15 kg/m3 with a modeled 200-foot opening.
The addition of the modeled 200-foot opening generally decreases peak velocities to the north of CCC compared to existing conditions except in the immediate vicinity of the opening, while the opening itself experiences a peak depth-averaged velocity of 2.5 ft/s. A sediment analysis was also performed to ensure no significant impacts on erosion or siltation would be caused by the bridge or changes in velocities. After seven days, the peak concentrations in the area of concern are about 50 percent lower with the 200-foot opening versus without. Within the area of concern, the modelled 200-foot opening reduces residence time (defined at time to reach 50 percent of initial concentration) from three days to approximately one day, depending on the location.
Because both the field study and hydrodynamic modeling demonstrated that adding a bridge cut under CCC was extremely likely to significantly improve water conditions, the team began the design and permitting phase. A final bridge length of 229 feet was designed and permits with the SWFWMD, FDEP, Tampa Port Authority, United States Army Corps of Engineers, Florida Fish, and Wildlife and the United States Coast Guard were obtained. As part of the permitting conditions, a two-year water quality monitoring program and success criteria for releasing both water quality and seagrass credits were developed. Credits could be applied, on a case-by-case basis to any FDOT projects within the Tampa Bay Coastal Floodplain.
The OTB project was completed in the summer of 2019. Water quality monitoring as part of the permit success criteria has been ongoing since the bridge opened in December 2018. As of spring 2020, tidal flux, salinity, Chlorophyll-a, total nitrogen, seagrass coverage and seagrass species variation have all met their targets, and 80 percent of water quality (10,161 Kg TN) and seagrass credits have been requested or released. In fact, while a residence time reduction of 50 percent was modeled and was generally the basis for parameter improvement, all parameters increased more than 50 percent.
The completed project is a win-win for the environment and for FDOT. The project has already saved $100 million by internal FDOT cost analysis, it improves water quality and ecological habitat far more than more expensive ponds could possibly have, and the local public residents, boaters and water enthusiasts receive direct benefit. This benefit will exist in perpetuity and will not require the ongoing maintenance costs that traditional ponds would have. Also, if seagrass growth and abundance increase over time as anticipated, asignificantlong-term ecological benefit for benthic organisms, seagrass and marine life is anticipated. Due in part to the success of this project, FDOT and Atkins are developing state-wide guidance, promoting the use of similar innovative, regional projects in lieu of traditional ponds or BMPs as a first option.