Rain Gardens

One of the more popular tools in the Low Impact Development toolbox is the rain garden. You've probably seen these around town; little strips or bowls of grass and plants, designed to catch runoff from a storm. When designed properly and constructed correctly, they are very useful and beautiful. Not only do they divert rain water, they also help filter pollutants in the runoff, recharge underground aquifers and provide habitat for plants and animals.

As I'm currently a student in Low Impact Development, I had the opportunity to collaborate on a rain garden on campus. We were given a small, unused corner of the campus to work with, a little bit of grant money and a couple of months. Performing some pre-work soil tests, we quickly determined that our little plot was a former parking lot, with about eighteen inches of compressed gravel to show for it. Four hours of bulldozing later, and we had a formidable hole to work with.
After a few weeks of shoveling, amending, planting and mulching, we had a garden that we were proud off. Our rain garden is really more of a wetland, since we were able to catch and sequester such a large amount of rain.

 

We have a nice variety of plants and animals that call this area home now. On my visit today, I saw several dragonflies and butterflies, frogs and tadpoles, and a few different species of birds that had stopped by for a drink. This is certainly an improvement over the cluttered, unattractive and disused space that was here before. Beyond aesthetic issues, our little corner of campus manages to sequester and filter hundreds of gallons of rain water from every storm, providing long term benefits to the school and the environment.

You may notice more pipe planters in the background; these will be planted in the fall. As the site matures, plants will start to fill in the holes in the design, providing more shade and helping to cool the water in the pool. This is another big benefit of the rain garden design; lowering water temperatures before the water reaches streams and rivers helps maintain overall waterway health.

This view shows the overall design of the area. The curve in the channel helps to slow the storm water entering the garden, preventing erosion from cutting away at the bank. The stone in bottom serves a similar purpose, and helps to trap sediment in the water. The adjacent road is visible at the bottom edge of the image; a small speed bump will be added to the road to funnel even more water into the system. At the top right is an existing storm drain. This drain will catch any overflowing water in the event of an unusually large storm.

We have a good mix of (mostly) native plants. Rhododendron and its cousin, (both Rhododendron spp.) are establishing themselves along the back fence and the side road. We put in lots of doghobble (Leucothoe fontanesiana), a nice native shrub with great foliage and flowers. Bee balm (Monarda spp.) and a few varieties of echinacea (Echinacea spp.) provide lots of color this time of year.
It will be interesting to watch the garden mature over the next few years, and heartening to realize just how much storm water it will process.

Edit to add: A fellow student was serving as a guide and technical adviser on this project, and she was kind enough to forward her calculations on the actual impact that this little rain garden is having. Our garden takes up about 840 square feet, and is capturing the rain water from almost 16,000 feet of campus. We can capture and hold almost 1,100 cubic feet of water in our little garden, which keeps it out of the storm drain system and helps prevent downstream erosion. Not bad, eh?
(Thanks to Vicki Eastland for the guidance and the data!)