In Ludlow, Mass., a new development under construction provided a challenge for engineers at Sherman & Frydryk. Before the development began, the discharge rate ranged from 0.11 cu ft per second (cfs) for a one-year storm to 0.28 cfs for a 100-year storm. According to the Massachusetts Storm Water Management Policy, the post-development rate of runoff had to be equal or less than the pre-development rate.
Engineers considered an open infiltration basin but the space onsite was limited. In addition to the buildings, the development featured 125 parking spaces and 24-ft-wide paved access aisles throughout the site. Space was also required to accommodate improvements such as town water and sewer services, underground power and communication utilities, and natural gas service.
CULTEC’s subsurface chamber system, constructed of 210 Recharger 330XLHD units, was selected to infiltrate runoff and detain it to balance peak flows from the site.
“The requirements for infiltration differ from site to site and depend on the type of soil,” said Donald Frydryk, project engineer with Sherman & Frydryk. “In this case, the volume of water to be infiltrated was based on 1 in. of runoff from all impervious surfaces for about 3,700 cu ft.”
Another onsite challenge was a high level of groundwater, which varied from 5.5 to 7 ft throughout the location. Town regulations required a 3-ft separation between the storm water management practice and the groundwater. The Recharger unit is able to hold a minimum of 11.32 cu ft of water per linear ft when installed.
The system was installed in a 7,650-sq-ft bed, above which a parking lot was built. The system provides 17,000 cu ft of storage. Additionally, the plaza incorporated lawn areas for infiltration of the roof runoff, deep sump hooded catch basins, and a quality unit to deliver runoff flows to the subsurface infiltration system.
