Bioretention Systems Reduce Pollution and Flooding

Wisconsin Expert Advocates for Rain Gardens

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Roger Bannerman is a water resources management specialist with the Wisconsin Department of Natural Resources.

Posted: Wednesday, March 7, 2012 1:47 pm

When a hard rain falls, water splashes through the leaves of towering trees, drips among the limbs, and slowly seeps into the ground, regenerating plant life, renewing habitat, and recharging the aquifer below. Unless of course that forest has been replaced by an urban landscape complete with asphalt roofs and impermeable pavement. In that case, the water rushes along, picking up sediment and junk until, shimmering with petroleum goop, it rips apart the banks of shallow local streams.

Scenario A is a distant memory for much of our natural environment, and unfortunately, Scenario B has been taking its place. Thanks to water resource experts like Roger Bannerman of the Wisconsin Department of Natural Resources, a third scenario featuring the restoration of natural processes is already underway.

Bannerman has been an advocate of bioretention systems, also known as rain gardens, for more than 20 years. A water resources management specialist, Bannerman describes first hearing about bioretention systems at a conference in the early 1990s, in Baltimore, Maryland. That’s where he learned it was possible to “dig a hole, put in the right sort of dirt and plants, and keep water from rushing down the driveway” when it rained.

When Bannerman was experimenting with the right combination of hole depth, growing media, and plant type in his yard, an acquaintance from his garden group wrote about it for their newsletter. Then the local newspaper picked up the story, then the local television station, then some national magazines. Before Bannerman knew it, he said he was giving media interviews or speaking to master gardener gatherings several times a week, as far away as New Zealand. Eventually he wrote a rain garden manual and made it available online, complete with user friendly instructions.

“People are looking for simple alternatives to what would normally be done to control high flows,” Bannerman said. “Traditionally we’ve leaned towards widening channels and lining them with concrete. But bioretention systems or rain gardens can be incorporated into an urban environment as a beautiful landscaping feature. It is a win-win combination of attributes. These systems meet multiple needs, they lower cost, are easy to do, and add value to property.”

Bannerman spoke to a gathering of engineers, contractors, and water experts at the fifth annual Low Impact Development (LID) workshop in Dubuque, Iowa, in February. Event organizer Eric Schmechel is the watershed coordinator with Dubuque Soil and Water Conservation District (SWCD). He said the partnership between the SWCD and the City of Dubuque attempts to bring LID practices, education, and policies to the Dubuque area.

LID, he explained, is an “innovative stormwater management approach with a basic principle that is modeled after nature. LID’s goal is to mimic a site’s pre-development hydrology by using design techniques that infiltrate, filter, store, and detain runoff close to its source. Techniques are based on the premise that stormwater management should not be seen as stormwater disposal.”

Schmechel said the purpose of this LID workshop was to “cover new topics and talk about different projects both locally and throughout the Midwest. We feel like education is something that never really ends. Locally, we have new erosion control and stormwater ordinances, and we are always trying to reach a broad audience, from engineers, contractors, and stormwater professionals to college students, citizens and policy makers. It is important we keep educating and demonstrating ways that green infrastructure is important for both sustainable economic growth and environmental stewardship,” Schmechel said.

At the LID conference, Bannerman described his "quest" to develop the right depth of pond, be it one, two or three feet; the right proportion of sand and compost in the media; and the right combination of plants. In many cases bioretention systems are placed in large expansive parking lots where stormwater has no other choice but to run unabated to the nearest dry creek bed. Increasingly, bioretention systems are placed several to a parking lot. These are not the same as a standard plant island though they may look like it at first glance. For one thing, they are usually deeper, with a bottom design to allow water to seep in and then “infiltrate” the ground far below asphalt level. For another, at the surface they are not convex, but concave. They are designed with the capacity to store water for a time during a heavy rain, allowing water to seep slowly into the ground. Many have grate-like curbs that guide water into the bioretention zone. That design traps eroded materials in the area to be an addition to the media materials of sand and compost.

“We’re still learning about the fine points, but we’re getting better; we still have the basic concept of digging a hole, putting in engineered soil, and planting plants,” he said.

The EPA Stormwater Fact Sheet describes what happens when bodies of water like small streams receive too much rainwater. “Unable to handle the increased water volume and flow, these water bodies often experience eroded banks, incised channels, loss of habitat and aquatic life, and increased flooding and property damage. In addition, stormwater carries a broad mix of toxic chemicals, bacteria, sediments, fertilizers, oil and grease to nearby water bodies.”

For these reasons, Bannerman said many communities have begun regulating how much runoff can occur in new developments. Bannerman said the Wisconsin DNR is doing a great deal of work on the science of the most efficient bioretention systems. For that audience of experts he explained details like the SAR effect, and how to avoid phosphorus discharge, and how to manage a heavy clay soil. But he said Wisconsin DNR is only one of the players in this field: many government and university researchers are also working to perfect bioretention systems and suit them for the area of the country, the type of plant growth, and the existing urban infrastructure. They also take advantage of local partnerships. In fact, Bannerman said, Wisconsin DNR could not have accomplished what they have without them.

For example, a study they conducted at Neenah, Wisconsin, first required financial and manpower assistance from the U.S. Geological Service. Local businesses allowed the project to proceed in their asphalt-covered parking lots. In addition, local consulting firms aided with design work and local services provided material such as compost. “The people partnering with DNR believe in what we are doing and want solutions too, so they are using their funds as an investment in the future,” Bannerman said. “Some people might make money if their product offers a solution – that’s ok. Others help just because they think it is the right thing to do. I’m always impressed with that – it keeps our country going that people want to do the right thing and step up for clean water.”

In addition to doing the right thing, some communities have additional incentives to manage storm runoff, in the form of environmental laws. Further, some major cities such as Philadelphia and Chicago struggle with storm systems that are tied to the sanitary sewer system, Bannerman explained. When it rains very hard, stormwater runoff creates flooding in those underground concrete sewers, mingling water with sewage, and causing effluent to overflow. Cities like these “go under judges orders to do more with clean up,” Bannerman said, “so they’ll sometimes turn toward green infrastructure like bioretention to keep water out of sewers. Bioretention is one part of their solution.”

Beyond the state and legal requirements, there is a realization among people that the groundwater table is dropping, “so you can’t keep pumping water out of the ground and covering it with concrete parking lots without a long-term impact,” Bannerman said.

Some areas of the country are not as concerned about water runoff. Particularly in the arid southwest, there are laws prohibiting the “harvesting” of water, such as in rain barrels. Because water is scarce in these areas, since territorial times people downstream have fought to prevent people upstream from taking what they perceive of as theirs. As an example, a percentage of water formed in the high mountain peaks of Wyoming and Colorado legally belongs to the people of Nebraska and beyond. As a result of this complex legal knot, and the different circumstances of these arid locations, bioretention systems are not as common outside of the Northwest, Northeast, Midwest and Southeast, Bannerman said.

The bottom line is that bioretention is a solution to a local-level problem. It gets water from a parking lot back into the ground below it, or makes water that rushes down a steep street slow down and ooze quietly back into the ground, nurturing the plants cultivated at the bottom of the hill. Bioretention is not a way to move water from stream to larger stream into a larger body of water or a way to fill reservoirs a thousand miles away.

As Bannerman explained, the concept of holding water back in cisterns dates to pre-Roman times; this is another approach of holding it back but instead of storing it for use, we’re infiltrating it back into the ground. “We’re not going backwards anymore,” Bannerman said. “Whether through state regulation, judges’ orders, or so forth, this is becoming more of the norm.”

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