The plastic chambers carry excess runoff to the stormwater outlet. Photo: Cultec Inc.
By Gina Carolan
Project: Annex Complex
Owner: Bay St. Louis, Miss., Public Works Department
Consultant: Compton Engineering Inc.
Contractor: Keith Waits Construction
Cost: $5.50/cubic foot installed
Hancock County's annex and human services buildings were two of five county facilities that Hurricane Katrina took out in 2005, and replacing them turned out to be easier said than done.
Though the new buildings qualified as a “critical action” under Federal Emergency Management Agency guidelines, making the project eligible for $10 million in emergency grants, they had to be above the 500-year floodplain at a finished floor elevation of 23 feet above mean sea level.
A local ordinance limiting post-development stormwater runoff to pre-development levels also complicated the project. The buildings' 10.4-acre site was tight, housing the buildings themselves at 50,200 square feet; a 305-space parking lot; and .7 acres of overflow parking.
With no space for aboveground treatment like a pond, concrete reservoir, or a traditional pipe conveyance system, excess rain would have to be collected and stored underground. And because cars and county vehicles would be traveling and parking in a lot above, the solution had to withstand heavy traffic loads.
Ultimately, engineers chose high-density polyethylene Recharger V8 underground retention chambers, made by Cultec Inc., as the site's best management practice. At 32 inches high and 60 inches wide, each chamber holds 100 cubic feet of water. To provide 45,232 cubic feet of storage — enough to withstand a 50-year storm event — 667 units would have to be installed below street level.
“We had to factor the lack of land into our design, and this system afforded us the flexibility of installing the chambers in multiple beds to fit the system onsite,” says Compton Engineering Project Manager John Studstill.
Installation began by excavating a 30,000-square-foot bed, laying non-woven polypropylene filter fabric along the sides and the bottom, and adding a layer of 1- to 2-inch washed crushed angular stone. The units were arranged in the bed and fed using their internal manifold, a feature that eliminates the need for fabricated pipe manifolds and provides extra design flexibility and a smaller footprint. Two side portals located on each chamber allow manifolding to take place at any point within the system.
After the chambers were in place and covered with 6 inches of crushed stone and a layer of filter fabric, the site was ready to be paved with 6 inches of concrete to meet American Association of State Highway and Transportation Officials' H-25 wheel load requirements.
“We were able to save about 15% off the project cost compared to another prospective system,” says Keith Waits of Keith Waits Construction. Although installation costs are comparable to competitive products, Cultec's system required 30% less backfill.
The chambers are stackable, making them easier and lighter to ship and don't require heavy installation equipment.
— Gina Carolan (firstname.lastname@example.org) is chief operating officer and marketing director for Cultec Inc. in Brookfield, Conn.