The mitigation bank will provide significant ecological improvement in both watersheds. Proposed hydrologic enhancement would be provided through the removal of culverts, blocking ditches, and the installation of multiple Low Water Crossings. Current conditions have reduced hydrology in the site by allowing surface water and shallow groundwater to be conveyed through a network of ditches and culverts. Installation of Low Water Crossings would eliminate hydrologic reduction, and restore the native/historic hydrologic regimes throughout a majority of the project site.
To model stormwater runoff characteristics within each of the basins, a runoff hydrograph Shape Factor of 100 (UH100) was used, and is appropriate for undeveloped terrains with slopes less than 5% per mile. Curve Number data were assessed using NRCS soils descriptions for St. Johns County, Florida. Time of Concentration data were estimated using site-specific survey, and were supplemented with USGS topographic mapping for each drainage basin.
For the Existing Conditions Model, internal roadways are modeled as extended irregular weirs with multiple openings to simulate the location and physical characteristics of proposed Low Water Crossings. These weirs were implemented for Drainage Basin A, Drainage Basin F, and Drainage Basin G. Figure 3 in Appendix A depicts the Existing Conditions Nodal Diagram.
One of the primary, and important, components of the Proposed Conditions Model would be the re-establishment of a portion of the headwater tributary to Pellicer Creek/Matanzas River. Currently, Drainage Basin A and Drainage Basin E (as a result of the existing ditch network) discharge to the west, and into the Crescent Lake Drainage Basin (Basin 16). Historically, the drainage basins were a primary source of headwater dynamics for the Pellicer Creek/Matanzas River watershed.