Taking Stock After Hitting Bedrock
Date & Time
Wednesday, September 15, 2021, 9:30 AM - 10:00 AM

Gwinnett County Department of Water Resources (DWR) is working to become a leader in smart stormwater management and watershed protection with their Watershed Improvement Program (WIP). A key element of this program is preparing for future water quality and stormwater management regulations through the implementation and testing of the Gwinnett County Stormwater Management Manual (GCSMM) Best Management Practices (BMPs). Ronald Reagan Park was selected as an ideal location to demonstrate innovation in stormwater BMP design, construction, and function because of its frequency of use by park patrons and natural setting. The park is a popular 25-acre park with amenities including a playground, skatepark, dog park, walking trails, and a stream restoration engagement zone. Stormwater BMPs designed and constructed at the park include two bioretention basins and a Regenerative Stormwater Conveyance (RSC) system. This project is unique because of the adaptation of the RSC design to site constraints discovered during construction when bedrock was encountered at shallower elevations than expected. DWR and Tetra Tech responded quickly with design modifications and innovative additions to the system to maximize water quality treatment. After encountering bedrock, the project was immediately converted to a design-build process to allow a redesign of the conveyance structure. After mapping the bedrock profile, DWR and Tetra Tech discussed design options that would incorporate the bedrock while still maintaining some features, as well as the intent, of the original design. The high elevation of the bedrock would not allow the conveyance structure’s step pools to reach their design depths, nor would it provide the desired infiltration of stormwater. To offset the reduced capacity, DWR and Tetra Tech added a pocket wetland at the base of the conveyance structure for additional storage and water quality benefits. During construction, DWR noticed significant runoff from the nearby skatepark flowing adjacent to the designed conveyance that could potentially lead to future erosion. Since the project was already in a redesign process, Tetra Tech and DWR decided to add a bioslope feature to capture and treat the skatepark’s runoff into the RSC. This project highlights several lessons in the design and construction process including the necessity of conducting a thorough geotechnical analysis prior to designing a stormwater BMP, the effectiveness of the design-build process in adapting a design to a site’s real-world conditions, and the importance of thinking critically when modifying a design to determine which features can be added and which can be removed.