Since late November, drivers passing Pioneer High School have been puzzled by enormous mounds of dirt at the southwest corner of Stadium and Main. The “Allen Creek Stormwater Project at Pioneer”–a joint venture of the City of Ann Arbor, Ann Arbor Public Schools, and the county Water Resources Commissioner (the former drain commissioner)–is a $3.1 million dollar effort, with 40 percent of the bill paid by a federal stimulus grant.

It aims to improve water quality in the Huron River by intercepting one of its tributaries, Allen Creek–which passes through the corner in an underground pipe. The creek was one of the defining natural features of Ann Arbor. City cofounder John Allen sited the town along its banks, and the configuration of Liberty, Miller, and Huron streets traces the high ground between its main branches. But it gradually devolved into an all-purpose drainage ditch and, beginning in 1925, was redirected into underground pipes.

“When the creek was first buried, the pipes were able to handle the amount of water that came through, most of the time,” explains Harry Sheehan, environmental manager for the Water Resources Commission. “But just look at the west side of Ann Arbor, and see what’s been developed since the 1920s. Behind Westgate and Maple Village shopping center, that’s pretty much the top of the drainage area. Where there was once open land, now there’s just a lot of pavement. The Allen Creek watershed is somewhere around 40 to 45 percent impervious–it’s paved over, or it’s rooftops or parking lots. The pipe that was buried in 1926 didn’t account for all that.”

At Pioneer, two large underground pipes will bring storm water to four buried vertical concrete cylinders. These “swirl concentrators”–each seven feet high and eight feet in diameter–use an ingenious aluminum baffle design to set up a whirlpool as the water spirals downward. Silt, debris, and heavier-than-water pollutants fall to the slower-moving center of the vortex like undissolved sugar coming to rest in a stirred glass of iced tea. Debris that floats is redirected to a separate chamber. If E. coli is present, the majority of that will be removed, since it typically attaches to sediment.

From the swirl concentrators, the cleaned water enters a pair of massive concrete rectangular basins buried twenty-five feet underground, about the level of the old Allen Creek. How big is “massive”? One basin, fed by a fifty-four-inch-diameter pipe, is 160 feet wide, 176 feet long, and six feet high. The other basin, connected to a forty-two-inch pipe, is the same length, half as wide, and two feet higher. The bottoms of the tanks are perforated with scores of holes the diameter of a tennis ball. Most of the newly cleaned water will seep through those holes, filter back through the sandy ground, and become groundwater.

Greg Marker, a 1993 Pioneer grad, is a field engineer on the project for contractor OHM Engineering Advisors. According to Marker, the system will handle the majority of storms–those that produce less than a half inch of rain–before the flow becomes so great that storm water passes directly through the tank outflow pipes rather than seeping into the ground. Either way, the water will head down the pipes at a slower speed. “The exit flow is no different than if everything upstream was all porous or grass,” says Marker. It will be as if the 308 acres of the creekshed upstream from Pioneer–about 9 percent of the total–had never been developed.

Once the construction is done and the holes filled in, the site will be landscaped with a small plaza at the corner and 130 native riverbank trees tracing the course of the buried creek.

Ecological concerns weren’t the only factors considered in the project: it’s also been shaped by U-M football. The site previously belonged to the U-M, which sold it to the Ann Arbor Public Schools in 1950 with the stipulation that it remain open for football parking. Construction couldn’t start until after the Ohio State game in November, and the landscaping is designed so that cars will still be able to park on the site.

There’s a proposal on the table to increase the parking fee from $30 to $35 per car this year. If it takes effect, the public schools will collect nearly $1 million this year from U-M football fans.