Council project demonstrates low-impact development

Features allow stormwater to soak in where it falls

Treating wastewater to stringent environmental standards isn’t the only goal of the Metropolitan Council’s recently expanded Empire Wastewater Treatment Plant in Dakota County. Another is reducing the amount of stormwater runoff flowing from the plant to the nearby Vermillion River.

With the encouragement of Dakota County, the Council incorporated a number of innovative stormwater management features into the plant’s design. They include:

Several biofiltration basins, also known as rain gardens, where stormwater can slowly soak into the soil and pollutants are filtered out. The basins are planted with native species of grasses and flowering plants.
A series of swales, or grassy ditches, to convey stormwater to the basins – rather than through pipes – to allow some stormwater to soak into the ground before it reaches the basins.
Pervious pavers. Rain soaks into the ground in the gravel-filled spaces between the thick paving bricks, which are an alternative to asphalt. Several parking areas on the plant property use the pavers. They are expected to have a longer lifespan than typical pavement.
A green roof. Sedums, native grasses and other plants are growing on a specially engineered roof on one of the buildings at the plant. The roof captures stormwater that would otherwise drain away. It aids in cooling the building during the summer, and should last approximately three times as long as a regular roof.

One of the buildings at the Empire Plant has a green roof, which soaks up stormwater and aids in cooling the building during warm weather.

"We want to be able to demonstrate the effectiveness of low-impact development,” explained Karen Jensen, an environmental engineer with the Council who worked on the plant’s design team. “So far, it’s working great – in 2008 there hasn’t been any observable stormwater runoff from the property.”

Low-impact development pays off

Conventional stormwater management involves piping stormwater from places like streets and parking lots to nearby retention ponds or often, directly to nearby streams and rivers. The runoff can carry pollutants like worn tire particles, car oil, fertilizers, grass clippings, eroded soil from construction sites, sand and salt, and animal wastes, which all degrade water quality. The added flow can also erode the banks of streams and rivers.

Low-impact development (LID) aims to allow as much stormwater as possible to soak into the ground at or near where it falls. In 2006, the nonprofit Friends of the Mississippi River (FMR) – in partnership with the Minnesota Department of Natural Resources (DNR), the Minnesota Pollution Control Agency and others – commissioned a study of LID. The study showed that LID is not only better for the environment, but results in a higher profit for the developer, higher property values and a higher tax base for local government.

Stormwater runoff from buildings, sidewalks and streets at the treatment plant are piped just a short distance into grassy ditches, or swales, where the water can soak into the ground rather than being conveyed to a detention pond or flowing directly into a river.

Preserving an outstanding water resource

By keeping stormwater on its property, the Council avoids having an adverse impact on the Vermillion River. The Council owns about 460 acres in Empire Township on both sides of the Vermillion, which is the only world-class trophy trout stream in an urban area in the lower 48 U.S. states, according to Tom Lewanski, Conservation Director for FMR.

While there is no tax benefit from the LID features at the Empire Plant, they serve as a demonstration of the effectiveness such features.

“Some parts of the Vermillion River watershed are developing rapidly, and we wanted to be able to provide a demonstration project where people could come and tour the treatment plant to see how these features look and how they’re working,” Jensen said. “It is our hope that local governments will encourage LID and that developers will implement these features in their projects.”

Restoring the surrounding habitat

In addition to incorporating low-impact stormwater management into the treatment plant expansion, the Council embraced an invitation from FMR in 2002 to develop a natural resources management plan for land surrounding the treatment plant. The management plan identified several restoration projects that were subsequently undertaken, including:

Stabilizing eroded streambanks and restoring trout habitat along the river.
Restoration of 50 acres of a historically wet meadow that had been marginally farmed for several decades.
Removing buckthorn from the Vermillion floodplain forest.
Enhancing species diversity in a 34-acre seeded grassland.

The projects were overseen by FMR and funded, in large part, with grants from the Council’s former MetroEnvironment Partnership program.

The gravel gaps between these pervious pavers, used in several parking areas, allow rainwater to soak into the ground rather than run off into a stormwater pipe..

On a recent tour of the restored wet meadow, which is thick with native wildflower and grass species, FMR Ecologist Karen Schik noted that the number of bird species that have been observed on the site has jumped dramatically. A pre-restoration survey found fewer than 10 species of birds. A few years later, in 2007, observers identified nearly three dozen.

Protecting the river as development continues

The Vermillion River Watershed drains 372 square miles of land, rising from eastern Scott County and passing through portions of Scott, Dakota and Goodhue counties before it empties into the Mississippi River in Red Wing. FMR has chosen to focus much of its conservation efforts in the watershed because of the potential for maintaining or even improving the high quality of the Vermillion River as the area around it develops rapidly, Lewanski said.

Gray-headed coneflower and swamp milkweed grow abundantly on a restored wet meadow on the Empire Plant property.

The Council’s property is part of what is becoming an important wildlife corridor from Empire Township northeast to the Mississippi River, Lewanski explained. The property is adjacent to large areas of open space recently acquired by Dakota County and the DNR. These properties connect with the University of Minnesota’s UMore Park, which in turn abuts open space surrounding the Flint Hills Resources refinery in Rosemount. “It’s wonderful to see these pieces of the puzzle come together,” he said.