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January 15, 2009

BAE Stormwater Engineering Group makes it happen for Wal-Mart

Bill Lord points out the profile of the permeable concrete used in the Wal-Mart parking lot. (Photo by Art Latham)

At first, it seemed that Wal-Mart might be running into substantial regulatory roadblocks to its plans for a new “Super Center” on U.S. Highway 64 in Nashville, N.C.

Due to anticipated adverse wetland and stream impacts, in 2007 the U.S. Army Corps of Engineers and the state Division of Water Quality had rejected Wal-Mart’s original site plan, which didn’t include room to meet stormwater-control regulations and destroyed almost an acre of wetlands.

Stormwater runoff from the site eventually drains to the Tar River’s nutrient-sensitive waters, then into Pamlico Sound, part of the U.S. Environmental Protection Agency’s National Estuary Program.

“Flooding and nutrient pollution associated with development and reduced pervious surfaces are major concerns nationwide,” says Bill Lord, environmental educator with North Carolina Cooperative Extension. “Pollution sources in urban stormwater include automobiles, associated roads and parking lots and contributions from atmospheric deposition.”

At about the same time Wal-Mart was facing its regulatory challenges, Dr. Bill Hunt, assistant professor of Biological and Agricultural Engineering (BAE) at N.C. State University’s College of Agriculture and Life Sciences and N.C. State’s Stormwater Engineering Group leader, was looking for a shopping center site on which to conduct low impact development (LID) stormwater practices research.

This was part of an EPA 319(h)-funded project that required installing at least two research-validated stormwater best management practices (BMPs). LID strategies help develop a site so it mimics on-site water’s pre-development properties, distribution and effects on the earth's surface.

The Wal-Mart project seemed a likely prospect, so the group, located in the College of Agriculture and Life Sciences, proposed a win-win solution.

Hunt, a professional engineer and N.C. Cooperative Extension urban stormwater management specialist; Dan Line, a BAE extension associate; and Rob Brown, BAE graduate research assistant, partnered with site designer Paul Smith of Stocks Engineering of Raleigh to create a LID design to lessen the project’s water-quality impact and gain regulatory agency approvals.

Wal-Mart chose bioretention and permeable pavement as its stormwater BMPs, and Smith included stormwater wetlands to further meet water quality and quantity requirements. Such BMPs reduce polluting nitrogen, phosphorus and water input from new developments such as the Wal-Mart site.

Because of NCSU’s involvement and the fact that Smith had reduced the negatively impacted, naturally occurring wetlands area to .1 acre, DWQ accepted the compromise design with undersized stormwater wetlands and larger bioretention beds. The agency also accepted a combined permeable pavement/bioretention system that can treat approximately two inches of rainfall, rather than the one inch usually required.

The Wal-Mart now has 47,600 square feet of permeable concrete parking areas, eight bioretention beds and 20,000- and 10,000-square-foot stormwater wetlands that accept runoff from the store roof, parking lot and out parcels. That includes runoff from 1.47 million square feet of permeable concrete parking and associated underground water storage areas.

And as of 2008, Nashville has a more environmentally friendly Super Wal-Mart.

However, the story wasn’t over yet.

“Based on prior experience,” says Lord, “Stormwater Group members advised the engineer and contractors to protect the bioretention beds from sedimentation during construction, since sediment from unstable parking lot base layers and soil clog and contaminate them if it rains in the interlude between bed-filling and parking lot paving. And it rained.”

In this case, the contractor had filled the beds with a custom nitrogen- and phosphorus-removing soil mix after the parking lot was covered in a gravel-and-sand mix and the beds covered with geotextile, a woven polyester fabric, to protect them from sedimentation before paving. After the parking lot was paved and the bioretention beds planted and mulched, several drained very slowly and remained excessively wet for several days after a rain. Apparently, a fine granite-based sediment layer accumulation in the fill media’s top layer was inhibiting drainage.

Indeed, sediment layer analysis showed high silt and clay levels several inches under the bioretention beds’ surfaces.

“Although attempts were made to protect bioretention beds from sedimentation,” says Lord, “subsequent poor drainage and investigation shows that geotextile provides insufficient protection from sediment. Based on our experience with this project, we need to do more to protect bioretention beds during construction.”

Rob Brown will observe and monitor the poorly draining beds for a year before removing the sediment layer from each to contrast performance before and after sediment removal.

“We are not only collecting water-quality data on the stormwater BMPs as part of a bioretention research project,” says Lord, “but we are also documenting the BMP system’s planning, construction and performance to help us learn how to do a better job on future projects. We present this information as a case study, and it’s all part of the learning process.

“Every site is different, and we appreciate the opportunity to work with Wal-Mart and to monitor the BMPs’ performance over time,” he says. “The design engineers, contractor and Wal-Mart were extremely cooperative during construction, but stormwater BMPs are relatively new technologies and we are learning as we go, even on this project.

“As with all of our field work,” Lord says, “the Stormwater Group, contractors and owners continue to learn from the collective experience and we continue to refine our recommendations on BMP construction and maintenance.”

The Cooperative Institute for Coastal and Estuarine Environmental Technology and the N.C. Department of Environment and Natural Resources helped fund this research project.

-A. Latham

Posted by Art at January 15, 2009 08:46 AM