As more lanes are added to SR 400 via the SR 400 Express Lanes project, additional stormwater runoff will occur along the corridor. Detention ponds are essential to reducing the risk of flooding on neighboring properties. The roadway’s drainage design will be brought to current standards, where possible, as part of the overall design process and conform to best management practices. The project corridor falls within a Municipal Separate Storm Sewer System (MS4) Permitted area which consists of a system of conveyances owned by the state or local municipalities that discharges to waters of the U.S. Therefore, this project is subject to the requirements of the MS4 Permit. The stipulations to comply with this permit will require treatment of the runoff and protection of area surface waters and water quality. To address drainage concerns, design features in this project include, but are not limited to: detention basins, curb inlets and pipes (e.g., a closed drainage system), and ditches along the corridor where feasible to convey stormwater away from the roadway.

The detention ponds are stormwater management systems that will be used to both remove some of the pollutants in the roadway runoff (improve water quality) during a storm as well as to prevent downstream flooding. They are not intended to be permanently wet but rather to detain the water and allow it to trickle out at a slow rate at or near the pre-widened roadway rate so as not to flood downstream. It is intended for these ponds to release all the water within 48 hours of most rain events and not to serve as permanently wet ponds. Stormwater retention ponds will include fencing to discourage unauthorized access to GDOT right-of-way.

The detention ponds are being designed by registered design professionals in accordance with GDOT policies and standards. All stream alignments will be designed to safely convey water back to the existing riverbed. As part of the installation of these basins, routine maintenance will be conducted. Being dry between rain events greatly reduces the attractiveness to nuisance species.