Seminole County Cassel Creek Stormwater Master Plan

AMEC-BCI was retained by Seminole County to evaluate the 860±-acre urbanized Cassel Creek basin to determine the priority stormwater and base flow pollutant loads to the creek which discharges to the Lake Howell/ Lake Jessup/Middle St. Johns River system.

AMEC-BCI tasks included the following:

• Water quality monitoring (six automated stations with two rainfall collection sites) to assess loads throughout system (storm event & base flow)
• Sub-basin & in-stream pollutant load estimations
• Prioritization of sub-basins for load reduction
• Assessment of relic wastewater treatment facility for water quality treatment
• Alternative Best Management Practices (BMP) development & cost estimations
• Conceptual BMP load reduction estimates
• Concept BMP plans development
• Level of Service (LOS) evaluation of system conveyance elements

The LOS assessment task included evaluating the system culverts at various roadway crossings under numerous design storm events. Bottlenecks, or systems having LOS lower than the required LOS by County code, were identified and concept alternatives including adding additional crossdrain capacity were developed. Prioritization of the concept improvements were made for both water quantity and water quality enhancements.

AMEC-BCI is currently advancing the highest priority BMP, a three acre in-line extended wet detention system to serve 419 acres of the watershed, through the construction plan preparation and permitting stage. The pollutant load removal analyses was based on monitoring data which differed appreciably from land-use-based load estimates and adjustments for baseflow loads and existing removal from surface water and wetlands within the flow-way had to be made.

A Section 319 grant application is currently being prepared and annual pollutant load removal estimates are approximately 100 pounds Total Phosphorus (TP) and 376 pounds Total Nitrogen (TN) (based on monitoring data which was chosen over land-use-based load estimates). There will be public meetings on this project since it is in a neighborhood location and the public is eager to see the conceptual passive recreation elements that are envisioned for the facility.