March-April 2008

A Rain Garden to Stop the Rainfall

Using an arboretum to help control flooding

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By Margaret Buranen

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Shrubs and vines in the Mississippi Embayment include eastern swamp privet, wild honeysuckle, buttonbush, and wild plum. Native trees include persimmon, swamp honey locust, bald cypress, water elm, water hickory, green ash, and several species of oak (basket, swamp white, shingle, cherry bark, and pin).

Describing his work on the Mississippi Embayment, Lempke says, “One of my challenges is not just to curate an exhibit of interesting plants or ecosystems but to have a wetland system that is functioning, that uses water.”

He cites eastern gamma grass as one example of the effective plants in the arboretum’s wetland. “It’s good at breaking through compacted soil because it produces an incredible amount of organic material—leaves, root system. Gamma grass is a very good biofilter.”

Wetlands, he cautions, “are complex and take a long time to develop. It would be arrogant to claim that you could just magically create a functioning wetland.”

Because every wetland contains ranges of moisture level, plants growing in some parts will not grow in other parts of the same wetland. Within the Mississippi Embayment, “We tried to simulate four different zones,” Lempke explains. The four planting zones are wet with occasional standing water expected, wet to mesic (moderately moist) transition, mesic, and upland dry.

Besides selecting plants that would thrive within each specific zone, Lempke also chose plants that would tolerate the impact of people and a variety of different conditions. Plants that are rare or those with a high index of conservation were not included.

When the construction for the stormwater project was finished, the soil where machines and trucks had been was “as hard as a parking lot,” Lempke says. He and Farris were concerned that there would be erosion if a lot of rain fell, because the plants were not yet reestablished. Fortunately, the end of construction coincided with the beginning of a very dry summer. That seasonal aberration, plus the coconut-fiber (coir) mats put down by the contractor, slowed soil erosion until the plants began to grow.

Now that the shrub layer and the tree layer are established, Lempke’s goal is to have no erosion at the site. As sedges, rushes, and grasses begin to cover the bare patches of ground, their roots will stabilize the soil. Eventually they will form a dense ground cover, along with the wildflowers.

He looks forward to the summer of 2008, when, with a full year of growth behind them, “Masses of wildflowers, [producing] a real ‘wow’ factor,” should bloom throughout the Mississippi Embayment.

Even in the first summer after completion of the project, Lempke has noticed that the swamp milkweed has drawn more butterflies, including swallowtails and skippers, to the area. “You can see and hear more bird life over here than anywhere else in the arboretum,” he adds.

The idea that “wetlands are swamps, ugly mosquito-filled undesirable places, is a common misconception,” Lempke says. He hopes that this part of the arboretum “will show that wetlands are essential to cities, farms, nature. They have the ability to provide diverse life forms.”

Bringing people into the environment and demonstrating that it is uniquely functional and beautiful, and that it’s cared for, was planned for as part of the design. A major component of that design is the 280-linear-foot boardwalk, or footbridge, that replaced a section of the arboretum’s regular asphalt path.

Made of recycled plastic that looks like wooden boards, the 6-foot-wide footbridge allows walkers and joggers to travel through the section without getting their shoes damp or trampling on plants or soil. Visitors can pause at any point to look a few feet down to the plants and ecosystem below.

Steps lead down to the area, but they are temporarily blocked, because the newly situated plants “are in intensive care,” Lempke says. Next year he will permit visitors to go down the steps and walk on a path within the wetland area itself.

Lempke, who has more than enough to do at the arboretum, would probably never have wished to become involved with a stormwater project. However, he sees both the need for such a project and the diverse interests of the parties involved. “I don’t know how we bring together the contractor and the biologist. No matter how good their intentions are, [there are conflicts]. Our technology only takes us so far, and there is much demand for speed and efficiency. But Mother Nature has other requirements. We have to find ways for new technology to manipulate the land but not destroy the ability of the land to respond and heal. Given half a chance, Mother Nature will heal.”

Mynear agrees with Lubeck that the most challenging aspect was to devise a concept that was acceptable to all of the parties involved. He thought that the most interesting part was “looking at more environment-friendly alternatives for reducing runoff discharge from the site.”

The final concept includes plantings of native species, a boardwalk, and a series of three water-storage features that are intended to detain stormwater flows up to the 25-year, 24-hour event. (That measurement is approximately 2 acre-feet or 5.1 inches of rainfall within 24 hours.)

Two berms with rock spillway outcroppings were created to slow stormwater and function as shallow detention basins. Water that continues to flow will reach the largest water-storage feature, a permanent pool approximately 2 feet deep located on the south to southwest side—the lowest point—of the area.

Areas of native plants upslope of the three basins intercept sheet runoff and increase infiltration and evapotranspiration rates by virtue of their deep root systems. They reduce the overall amount of runoff reaching the basins.

The native plants chosen by Lempke for the project fit into the arboretum’s master plan. As part of the contract, the city government paid for the purchase, planting, and care of the plants, which was all done under Lempke’s supervision. While the city government has an urban forester, arborists, and landscape employees, everyone involved felt that Lempke was best suited for this responsibility. The cost was approximately $139,000.

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Construction cost was approximately $470,000. This figure included payment for approximately 1,800 cubic yards of excavation, 280 linear feet of recycled plastic boardwalk, 1,500 linear feet of 2-inch waterline, and 88 tons of gray bluegrass region limestone bulk rock.

The West Glendover Stormwater Improvement Project meets the EPA’s stormwater guidelines for public education. As schoolchildren, recreational and fitness walkers, and other people tour the area, they learn about the problem of stormwater and an effective and aesthetic way to manage it. In its cost range division, the project won a Best of the Year 2007 award from the Kentucky chapter of the American Public Works Association. 

Author's Bio: Margaret Buranen of Lexington, KY, writes on environmental and business topics for a number of national publications.

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