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 concentra- tion 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 expe- riences 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 demon- strated 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 resi- dents, 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 in- novative, regional projects in lieu of traditional ponds or BMPs as a first option.
Figure 4: Initial tracer concentration; all scenarios.
Figure 5: Tracer concentration after 7 days; existing conditions.
Figure 6: Tracer concentration after 7 days; 200 ft opening.
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.
24
csengineermag.com
may 2020
Made with FlippingBook Annual report