Project Case Study: Private project stores public stormwater

Underground detention system cost-effectively handles parking lot and roadway runoff

Project: Costco retail store stormwater detention
Civil engineer: WRG Engineering, Portland, Ore.
Product application: An underground manifold system of corrugated steel pipe detains stormwater runoff from a retail center parking lot and an adjacent public road.

Costco Wholesale Corporation, a membership warehouse club, is the sixth largest retailer in the United States. When the company built its new store in Woodinville, Wash., part of the construction process included installing underground stormwater detention systems to handle runoff from the store’s 8-acre parking lot (called the on-site system), as well as from an adjacent public roadway (called the offsite system). In an unusual arrangement, stormwater from a public road is stored on private land in a privately built project. The systems, both located beneath Costco’s parking lot on two sides of the new building, capture rain when it falls or runs onto the parking lot, slowly releasing it into the groundwater at the same rate it would be released if there were no pavement.

Detention design

WRG Engineering, Portland, Ore., in partnership with CONTECH Sales Engineer Mike Tolman, determined that CONTECH’s corrugated steel pipe, fabricated from 16-gauge, aluminized Type II steel, with 5-inch by 1-inch corrugations, would fit the plan for a 75-year design life on the stormwater detention system.

Site layout information was sent to CONTECH’s Middletown Engineering Services, which designed the detention system. It includes a manifold system comprising one, 164-foot-long run of 96-inch-diameter pipe with 14 perpendicular runs of 96-inchdiameter pipe of varying lengths. Vertical access risers are fitted with castings and grates to allow stormwater to enter the system.

The manifold system ends were fabricated from the same 16- gauge, aluminized Type II steel, but Middletown elected to manufacture the pipe in the manifold run with heavier 14-gauge steel.

Each run of 96-inch-diameter pipe, including the manifold, has end caps to detain the water and keep it from flowing out the end of the system. Flow control structures fabricated inside two, 96- inch-diameter, subsurface outlet catch basins allow stormwater in the system to soak into the ground slowly.

A high groundwater table required placement of a PVC liner under the pipe bedding to ensure the stormwater in the system was not contaminated by groundwater infiltration. (The site was formerly an auto wrecking yard.) The 96- inch-diameter pipe plus the bedding required an excavation depth of 10 feet.

One design change occurred during the construction process when it was determined that the planter boxes for trees placed around the parking lot required more cover. Consequently, the pipe diameter under the planter boxes was reduced from 96 inches to 72 inches.

Construction challenges

Before starting fabrication of the corrugated pipe, CONTECH worked with the contractor, Nelson and Sons Construction of Woodinville, Wash., to establish a delivery schedule. Once delivery started, the companies mutually decided to have all material on hand within three weeks.

Nelson and Sons leveled the site and installed the 14-run manifold system consisting of 4,768 linear feet of 96-inchdiameter pipe for the on-site system, and 728 linear feet of 72-inch-diameter pipe for the off-site system. They also installed a manifold system for the off-site portion of the project, which consisted of an additional 1,157 linear feet of 96-inch-diameter, aluminized Type II steel pipe.

The detention system’s capacity is 318,630 cubic feet of storage. The pipe meets all American Association of State Highway and Transportation Officials specifications for H-20 loading.

What proved challenging during the project was the size and complexity of these systems. They required CONTECH to develop an elaborate sequential numbering system identifying the exact location and orientation of each of the 199 pieces of pipe used in the project. In addition, most of the pieces of pipe were 30 feet long, making it difficult to find enough trucks to deliver it within the prescribed time frame.

"The alternative to this pipe was to install a cast-in-place concrete vault," said CONTECH’s Gene Zande. "The contractor saved approximately $50,000 using the corrugated steel pipe system.

Additionally, using the steel alternative allowed the contractor to use his own workforce to install the system, whereas, the concrete system would have been installed by a sub-contractor."

Andrew J. Sabadox is regional vice president, Northwest Region for CONTECH Construction Products, Inc., located in Portland, Ore. He can be contacted at

Posted in Uncategorized | January 29th, 2014 by

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