Rapho Township implemented a detention basin retrofit project on property they own that will reduce their sediment loading rates by 25,537 pounds per year, phosphorus rates by 10 pounds per year, and nitrogen rates by 336 pounds per year. This project achieves a significant portion of the Township’s required pollutant load reductions for their 2018 to 2023 MS4 Permit.
RETTEW worked with Rapho Township to secure a $161,360 grant from the Pennsylvania Department of Environmental Protection through the Local Stormwater Best Management Practice Implementation Program to retrofit an existing dry detention basin into a dry extended detention basin. The original basin was constructed with a concrete low-flow channel and had no water quality components. During small storm events, polluted stormwater discharge traveled directly through the basin on the concrete low-flow channel and discharged directly into neighboring Chiques Creek.
The retrofit project included removing the concrete low-flow channel, installing a large sediment forebay, and updating the outlet structure. Benefits from the achieved water quality include providing up to 72 hours of water storage that allows sediment to settle into the forebay prior to discharging downstream. The basin is seeded with a native grass mix including switchgrasses, sedges, and wildrye. These native species have deep root systems that promote groundwater infiltration.