Red Horse Hill Road Drainage Improvements

sharon, CT

AI provided civil engineering design services for drainage conveyance and treatment improvements on Red Horse Hill Road, in Sharon, CT. Red Horse Hill Road is a one-mile private gravel road serving several residential properties. In 2011, Eversource (formerly CL&P) replaced the damaged direct buried power supply line serving the properties with a new line buried in conduit, along the entire length of the roadway. The installation of the new power line required the blasting of shallow ledge rock to accommodate the required bury depth of the line. After the installation of the line, there were severe erosion problems with the roadway since it lacked a formal roadside drainage system. Stormwater runoff previously flowed along and on the existing roadway, which had varying terrain with profile grades exceeding 17 percent in areas. The steep grades and lack of drainage system exacerbated the erosion issues by allowing the accumulation of flows over long distances, which gained velocity through the steep grade areas.

AI designed a stormwater conveyance and treatment system to better manage runoff flows to acceptable velocities, and designed permanent treatments to ditches, swales and outfall areas to help prevent future erosion from occurring. The roadside drainage design included a series of riprap lined ditches through steep areas, with a series of intermittent plunge pools, with sumps to provide water quality treatment. Rather than allow flows to accumulate along the road’s length, as it had prior to the new design, each plunge pool conveyed its flow beneath the road via a culvert, with an armored splash pad at the outfall. Each plunge pool was also designed to overflow to the adjacent riprap ditch downgrade, in the event of clogging or overtopping of the pool.  The series of ditches and plunge pools terminate at a large pool used to detain flows prior to entry into a wetland complex at the base of the roadway corridor. The system was designed and modeled using HydroCAD software, and the system was sized to safely convey the 100-year 24-hour storm. Additional stormwater elements were designed to convey flow and protect the area from erosion in challenging areas, where the use of ditches was not possible due to geometric constraints. There were several areas where old stone walls bordered the road, and excavation of ditches would have destroyed the walls. AI designed underdrained scour pads, where runoff would enter the riprap scour pads, and drain through to the underdrain for discharge further downslope. The riprapped and underdrained system provided further water quality treatment, as the underdrains were wrapped in crushed stone and filter fabric. In addition to the stormwater and erosion design, AI also provided wetlands permitting services. The lower end of the road corridor had a stream and associated wetlands that eventually discharged to a wetland pond area. AI obtained wetland impact permitting for construction of the project, and designed restoration of impacted and denuded wetland areas, using soil reinforcement matting and a native wetland plant seed mix.


  • Duration: 2013
  • Project Scope:
    • Drainage improvements
    • Erosion and sedimentation issues
    • Roadway designWetlands Permitting
    • Construction inspection