A $10 million development anchored by a Walmart Neighborhood Market Store, their smaller grocery format, in the Village of Menomonee Falls, Wisconsin. Construction design for this 40,000-sf Walmart and separate 5,000-sf building for other retail tenants included a stormwater management plan.
The Wisconsin Department of Natural Resources (WDNR) requires total suspended solids (TSS) reduction in stormwater leaving the project — 80% for new developments and 40% for redevelopments. The 9.22 acre Walmart project was being constructed on the site of a closed supermarket, making this a redevelopment. Additionally, the flow rates for stormwater leaving the site needed to be addressed.
Walmart contacted R.A. Smith National of Brookfield, WI, to design the civil portion of the project, which included the stormwater management plan for the site. Because this project is in an urban setting, R.A Smith National did not have the option of an aboveground stormwater pond for detention and TSS removal. Instead, they looked at economical “mechanical” options to contain stormwater beneath the parking lot to reduce the footprint and maximize land use.
“Due to the dense redevelopment plan and the need to provide water quality, it was apparent that the only way to achieve our goals was to go with an underground facility”, said project engineer Jeremy Jeffery of R.A. Smith National.
R.A. Smith National chose a corrugated metal pipe (CMP) detention and treatment system from Contech Engineered Solutions to satisfy the runoff release rates and TSS removal requirements set forth by the Village of Menomonee Falls, the Milwaukee Metropolitan Sewerage District and the WDNR.
“The Contech design proved to give us the most bang for the buck,” said Jeffery. “We were looking for a product that would provide the smallest footprint, superior durability and service life, and easy installation, all while being cost-effective. But the most important of all — a product that would meet all regulating agencies requirements.”
The underground wet detention facility from Contech consisted of 600-feet of 9-foot diameter Aluminized Type 2 (ALT2) CMP pipe interconnected by a header. The bottom four feet of pipe remains wet, and the top five feet were designed for stormwater storage, allowing a 21,422-cubic-foot volume to reduce the runoff release rate by more than 20% from the original rate for both the 2-year and 100-year events.
Site water quality is attained by directing watershed to the underground wet detention system. The bottom four feet of the underground detention system functions as a large wet sump with a permanent wet pool area. The wet pool area was modeled in WinSLAMM (Source Loading And Management Model for Windows), and designed to settle particles and provide volume for sediment storage. To meet the WDNR’s 40% TSS removal requirement, a total of 1,359 pounds of sediment removal was required. The underground wet detention system removes 1,086 pounds, more than a 60% removal rate. Six Catch basins with two-foot sumps were designed for a second drainage area to remove an additional 275 pounds, allowing a combined total 1,361-pound TSS removal, which exceeded the 40% WDNR requirement.
Contech’s underground wet pond detention and treatment system reduced peak runoff rate and met the sediment collection needs for the project, while keeping the project within budget.