Going Underground

Across the country, stiff EPA regulations are kicking in for managing stormwater in small cities. As municipalities work to meet those regulations, new types of stormwater management systems are coming to the fore. In Cranston, RI, a large shopping center was recently constructed on the site of an old brewery. The Brewery Parkade sits on about 42 ac. of land, much of it paved or covered by buildings: a large grocery store, a Lowe’s home-improvement center, and a Kmart retailer.

“We were putting this shopping plaza on the site of an old gravel pit, which had virtually no runoff leaving the site,” explains Marco Schiappa, P.E., of Crossman Engineering in Warwick, RI, which designed the Brewery Parkade project. “We are not allowed to have an increase in runoff exiting the site greater than existing conditions. We had to take it all underground because the land was too valuable to use up for a surface detention pond.”Under the National Pollutant Discharge Elimination System Phase II stormwater rule, cities of 10,000 residents (or at least 1,000 people per square mile) that are not part of a larger municipality’s storm sewer system must manage their own stormwater, incorporating best management practices that improve the water quality for the affected watershed. The regulations also affect the amount of runoff from an area into the surrounding waterways; runoff cannot increase after the land is developed. Commercial property owners developing their land are now feeling the effects of the new regulations. Many don’t have the space or the desire to put a retention or detention pond on their property. Such was the case for the Brewery Parkade. The solution in this instance was a custom-engineered stormwater management system. Designed by engineers at the HDPE pipe manufacturer Advanced Drainage Systems Inc. (ADS), the retention system provides 1.2 million ft.³ of runoff storage inside 15,000 ft. of 48-in.-diameter N-12 pipe.“When I first looked at this project, my first impression was that it was huge … and I have done a lot of airports,” says contractor George Mellow, superintendent of Fleet Construction of Smithfield, RI, which installed the system last February. Size was not the only issue: The system also had to be soil-tight and corrosion-resistant and could not allow infiltration or exfiltration at the joints.“What the new regulations meant to a project like this was that a customized system would have to be designed to manage additional stormwater runoff that would be generated,” states Greg Baryluk, ADS regional engineer on the project. The layout of the system has three straight sides, all coming together at 90° angles, and one angled side that makes the whole system look almost like the side view of a box with its lid propped open. The angled side is the top section, an intricate system of steps, making the runs of pipe anywhere from 160 to 540 ft. long.

The odd layout of the system was engineered in part by ADS to accommodate the design of the shopping center. It was imperative to create a design that could be completely underground so as not to take up land space. It was also necessary to capture as much water as possible to limit runoff. That meant placing the majority of the pipe under the parking lot of the shopping center and determining how to split up the functions of retention and detention according to the needs of the shopping center and the regulatory requirements. ADS engineers calculated that it would take 10,000 ft. of perforated 48-in. pipe to make up the retention system and nearly 5,000 ft. of solid 48-in. pipe for the detention system. The perforated pipe is wrapped in geotextile and buried with 20,000 tons of stone.“When you’ve got 42 acres of land and 35 of it is going to be paved, there is going to be a tremendous rush of water coming off of that during and after a storm,” Mellow points out. “This system will take 70% of that runoff and reroute it into the water table. It’s an environmentally friendly approach engineered with outstanding drainage pipe.” At the time this system was designed, it was the largest retention/detention system to date. When ADS first heard of the Brewery Parkade project, it took it as a challenge to figure out how to manufacture, fabricate, and deliver pipe and fittings necessary to manage that much stormwater. Engineers from ADS and Crossman Engineering spent two years working on the design and about nine months to ensure the design would sail through the Rhode Island Department of Environmental Management. The magnitude of the project surprised Crossman’s Schiappa, although he knew it was necessary. “When you have all of that pavement and all of those rooftops on one piece of land, the water has to go somewhere,” he says.After the initial design was completed and the project went out to bid, the developer, New First Hartford Realty, looked into two different corrugated pipe options: ADS N-12 and steel. Although the steel pipe was significantly less expensive–25% cheaper than N-12 pipe–several factors led the developer to choose N-12 as the most financially sound option for the long run. Unlike steel, N-12 pipe does not rust, and it also has proven high joint performance. Speed and ease of installation, aided by technical and engineering support from ADS, was another factor. Besides being enormous, the design is complex. “It was like installing pieces to one giant puzzle,” describes Mellow. “The pipe went in so easily, and it fit together perfectly–even with all of those different angles. It went together exactly as planned.” In part, Mellow attributes the ease of installation to the fact that N-12 pipe is made out of HDPE, which is of lighter weight and is easier to handle than other types of pipe. “Concrete would have been more labor-intensive,” he says. “For this application, this pipe is absolutely perfect. Besides, I like HDPE for its longevity because there’s nothing that is going to bother this system.” Fleet Construction estimated that it would take 18 days to do the installation, and it took only 15.“The job went so well, I would do it again in a minute,” remarks Mellow–and he might get his chance. Although the system, at the time it was designed, was the largest retention/detention system in the country, EPA regulations are here to stay, which means more of its kind–and larger. And at the other end of the spectrum, ADS has also designed a system for the smallest of footprints, an unconventional retention/recharge system that will allow chain stores and restaurants to meet stormwater management needs in severely restricted spaces.Schiappa agrees that there will be an increased number of underground retention/detention systems. “Land all across the country is getting to be so expensive, developers need to maximize its use, and they cannot afford to use land for the construction of detention ponds. The only place to go is underground.”

Going Underground

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