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 spe- cies and presence that is best explained by the salinity regime that cur- rently 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 abun- dance, 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 ef- fects 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
Figure 1: Tampa Bay (black) and nested (red) model domains.
One such alternative treatment system involves the Courtney Camp- bell 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 east- ern 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
Figure 2: Bathymetric contours for the Tampa Bay model domain.
Figure 3: Bathymetric contours for the nested model domain.
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