There's growing recognition that the best way to cleaner lakes is to improve the watershed around them--including more infiltration of rainwater, less erosion, and more biodiversity.
The Friends of Lake Wingra undertook a survey of the watershed. One finding was that swales--shallow grassy depressions used to manage stormwater--were present everywhere and could be modified to improve infiltration. Every school we surveyed had swales, often in areas that have no other use.
Swale at Midvale School during a severe storm.
Swales are linear depressions in turf, usually in sunny locations. With little effort, they can be turned into rain gardens by the creation of shallow dams, then planted with native prairie species.
What's needed is a test of this concept, to see if we can find a method that requires minimal labor and expense. We hope to demonstrate that benefits will outweigh any increased maintenance or loss of open lawn.
- Improved infiltration. Turf is only a little better than than pavement. Infiltration of rain improves springflow--the lakes receive pure groundwater instead of dirty stormwater. Raising the water table means lower costs in providing tapwater to Madisonians.
- Reduction in flooding in the neighborhood. When stormwater arrives at a rain garden, first the plants drink deeply like camels. Then their long roots provide channels for water to sink into the ground. Then the garden's basin starts to fill like a reservoir. All this helps to reduce flooding in the neighborhood during a big storm.
- Health from Increased biodiversity. Studies show that biodiversity improves the health of human communities. Taller native plants help to sweep harmful dust from the air. A variety of species provides "exercise" for the immune system of children, leading to fewer children with asthma. There is less West Nile virus in communities with more biodiversity.
- Monarch rescue. Populations of monarch butterflies have been declining rapidly, due to a shortage of their preferred plants--members of the milkweed family. Swales can be planted with common milkweed, butterfly weed, and marsh (red) milkweed. The last two are considered beautiful plants for any garden.
- Classroom opportunities. The Friends of Lake Wingra are prepared to show teachers how rain gardens can be used for education. Children can sweep a garden with a butterfly net, then examine the hundreds of different kinds of insects attracted. Students can participate in "adaptive management" of the garden. This is a new concept that involves careful observation, then change in how the garden is managed, to adjust to changing conditions. Each year, the swale garden can be extended by one or two links, so teachers can continue to develop and use their biodiversity lesson.
- Increased beauty of school grounds. With a variety of species planted in swales, they will be a riot of color from spring through fall. Initially, we will plant a limited number of species, to see which grow best.
- School pride. This project will bring committed volunteers to the school. It's likely that their care may spread to other parts of the grounds nearby. For example, this project may produce surplus squares of turf, which can be used to patch an eroding area.
We propose to build small rain gardens in swales--which are shallow channels in grassy areas of all school grounds. We chose Hamilton school for a test, in part because Hamilton has a large puddle next to the building, which this project may help to drain.
Swales are an ideal location for a rain garden, because they receive ample water, are already a depression, and are usually in sunny locations away from trees. All that's needed is to create a shallow dam across the swale. If the dam is not too high, it won't impair the purpose of the swale, which is to carry stormwater away. In a big storm, water will overflow the dams and continue down the swale.
Rectangular blocks of turf will be removed from the center of the swale. If removed with a flat shovel, the blocks will measure approximately 8 x 12 x 3 inches. Removal of the turf creates a basin 3" deep. Stacking three layers of turf downsream from the basin will produce a dam about a foot high (diagram below). The exact dimensions will depend on the width of the swale and the tools used.
Besides finding an easy way to create the basin, we hope to find attractive plants that thrive under these conditions. If we find plants that spread on their own, then the "volunteer" seedlings can be used the following years to plant a second, then a third basin along the swale.
Maintenance and other issues
The plants are hardy perennials adapted to local conditions. Watering and weeding are necessary while the plants are becoming established the first summer. Afterward, weeding is required 1-2 times a year, and watering only during severe drought. The garden will look better if dried, old growth is removed by hand or by burning in the fall (or spring), but this is not essential.
The gardens should be easy to mow around, because of their linear shape. If the gardens are later abandoned, the dams will be shallow enough so the mower can go over them.
Snow plows may be the biggest issue, if the gardens are located near pavement. But if plows do push snow over the garden, probably there will be no damage, because there will be no berm around the garden, and the dam won't project above the surface of the swale.
What about trampling of the plants by students, or mud? We can place a rope barrier around the garden if necessary. If the garden is used in classroom studies, students will learn to respect it. So far, rain gardens at other schools haven't been trampled. We plan to avoid high-traffic areas.
Can a rain garden help to drain a pesky puddle?
Rain gardens can probably reduce puddles, but not eliminate them. If the garden near a puddle has a deep basin, that basin will fill first, possibly preventing the puddle after moderate rain. The roots of the plants (providing a channel into the soil) will help to drain the puddle faster after a storm. But a rain garden won't help much when the soil is frozen, or when plants are dormant.
Another way rain gardens can help is by starving the puddle for water. If the gardens are placed upstream, then less water will flow to the puddle. Likewise, once people start to think about how water flows on school grounds, they may find another solution to the problem.
For example, for the big puddle at Hamilton School, we think there may be a clogged and buried stormwater inlet near the puddle. We will try to find it and make it functional again. With the extra sod the project will produce, we think we can create a berm to direct stormwater way from the puddle.
Location of two test gardens at Hamilton School
Location of Garden #1 in front of Hamilton. It receives some runoff from parking in the rear. Brown areas indicate limits of snowplow damage. This area feeds runoff down the asphalt flume (below) towards the big puddle.
Same view, during spring thaw, showing location of snow piles. Photo shows how turf is pushed around, so the swale doesn't drain as well.
The man is standing at the location of Garden #2. This location won't affect runoff towards the big puddle.
The location of the big puddle.
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* "Swales changed..." At Toki School, the swale near the parking area no longer drains effectively. Possibly, the ground settled after construction. But also it's clear that snow plowing has changed the area, because turf is pushed around by the blades. It's also possible a stromwater inlet has become clogged and buried.