Small Rain Gardens Do a Big Job

Late summer hibiscus and black-eyed Susan with switchgrass fill in the rain garden, with the tall sweetbay magnolia in back. Photo: George Brown

The rear yard of the C Street SE home designed by Capitol Hill landscape architect Ryan Moody is beautifully detailed with a custom rear gate, masonry, a fountain, and bright red accents in furniture, birdfeeder, and flowers. With long, narrow proportions typical of the neighborhood, the space is subdivided into a patio with seating, a walkway to the alley, and a garden area. But not just any garden area. About a third of this yard features a rain garden designed to absorb water from the home’s roof runoff, letting it percolate into the ground instead of being piped into the Anacostia River through a storm drain. 

CSS and CSO’s with Green Infrastructure to the Rescue

Why is this important? Because by disconnecting even one downspout from the city storm drain Moody is helping reduce the load on the Hill’s 19th-century drains, which mix water from rainfall with water used inside the home that must be treated before being released into the Anacostia. This is called CSS, a combined sewer system. One-third of all DC’s water is in combined sewers, including most of Capitol Hill. In the rest of the city only water used inside the home (called “sanitary waste water”) is treated before being released, while the storm water that collects in street sewers and storm drains is diverted directly into the Potomac River. That is the modern method. 

The problem comes during large storms, when the CSS cannot process all the water fast enough, causing a CSO, or combined sewer overflow. This means raw sewage flows directly into the Anacostia, which is not good for water or wildlife. The Anacostia is slower moving than the Potomac, making CSOs particularly harmful. And these CSOs are not only from the District. CSOs from far upstream also eventually flow into the Anacostia River, making matters worse. 

The rain garden Moody designed is part of the solution, and in the lingo is called GI, or green infrastructure. A federally mandated consent decree has led to DC’s Clean Rivers Project, which is expected to spend $2.6 billion implementing a wide range of GI and engineered solutions in its Long Term Control Plan. Green infrastructure will be part of the plan, but not officially on Capitol Hill. Instead the main strategy to clean up the Anacostia River is a system of deep, massive tunnels intended to hold the CSOs until they can be processed by the existing Blue Plains water treatment plant. The Anacostia tunnel system should be complete by 2022 and is expected to reduce CSOs by 98 percent.

This year the DC Clean Rivers Project was revised to include a host of GI practices as part of the clean-up calculations for CSSs in other parts of the city. This means that rain gardens on a larger scale qualify as environmental mitigation, making them more than just a pretty face. Although GI is not an integrated strategy for the Anacostia clean-up, voluntary practices like residential rain gardens will lighten the load on the infrastructure plus appeal to humans and critters. You can look up your home address online and see its location within the local CSS as well as the Anacostia River watershed at “Locate Your Watershed,” http://geospatial.dcgis.dc.gov/watershedfinder/

Experiencing Local Rain Gardens in Action

You may not be able to visit private back-yard rain gardens on the Hill, but fortunately there are several local public spaces where you can see them in action. One notable project, at the Brent Elementary School at 301 North Carolina Ave. SE, was designed by Sustainable Life Designs, a firm founded by local designer and urban planner Michael Lucy with collaboration from a host of agencies and community groups. Built in 2009, the project has just become the first K-5 school in the nation to be certified by the prestigious national Sustainable Sites Initiative. The rain garden at Brent Elementary manages storm water for one-quarter of the site. The project involved removing 2,000 square feet of asphalt, drastically increasing the permeable surface on the site. For more see Lucy’s website: www.sustainablelifedesigns.com/photo_galleries.

Sustainable Life Designs has also created rain gardens at St. Peter's and at Watkins Elementary School in partnership with their PTAs and DC Greenworks (see http://dcgreenworks.org/st-peter-school-3/ and www.caryeuwer.com/watkins-elementary/), so there are several nearby public examples to see. 

One great time to experience a rain garden is during a big storm. Most of our rainfall is half an inch or less, but when an especially big storm passes through, you can observe the water flowing into the rain garden and ponding. Come back the next day and you will not see any standing water. It has all gone down into the ground and up through the plants roots, stems, and leaves. This means that a well designed and built rain garden will not attract mosquitoes. 

How Rain Gardens Work

The basic idea of a rain garden is to disconnect the water running off your roof from the city storm drain. Some residents install large rain barrels under their downspouts, with overflow valves that feed rain gardens. Others, like the Moody garden pictured here, replace the building’s downspout with an attractive rain chain, which directs water into a gently sloping channel at least 10 feet from the house. The outfall leads into a gently convex shaped garden with about 18” of a sandy soil mix (which drains well), into which is planted a host of native plants that can withstand various levels of water, but also drought. There should always be a planned overflow at the other end of a rain garden. In the Moody project the overflow is directed into the alley (never toward a neighbor’s property). As the plants grow and their roots create deeper channels in the soil, water is absorbed into the garden more quickly over time. In winter 70 percent of perennial plant roots die back, leaving tiny channels deep into the soil to absorb water as if it had been aerated. Water is cleaned in this process too. 

In the Moody project deep-rooted native flowering plants help soak up the water, creating bird habitat and filling the space with color during most of the growing season. Rain garden plants include switchgrass (Panicum virgatum), beautyberry (Callicarpa Americana), butterfly weed (Aesclepias tuberosa), hardy hibiscus (Hibiscus moscheutos), cardinal flower (Lobelia cardinalis), and the tree sweetbay magnolia (Magnolia virginiana). See www.moodyla.com/capitol-hill-rain-garden/8g4j3srzc5sxfcrouk4mg1n6sa62no

Rain Garden Resources

For residential projects there are many local talented designers and landscape architects with rain garden experience. DC’s Riversmart Homes is another place to go for a modest rebate for your project (www.capitalcommunitynews.com/content/let-it-rain-and-drain-your-rain-garden). Montgomery County’s Rainscapes Program offers good design templates and technical manuals (www.montgomerycountymd.gov/DEP/water/rain-garden.html). A recent book, “Creating Rain Gardens,” by C. Woelfle-Erskine and A. Uncapher, is a great resource for experienced designers and lay people alike. Note that if your project is over 5,000 square feet you will become well acquainted with the 2013 “Stormwater Management Guidebook” published by the DC Department of the Environment. 

By undertaking a modest rain garden project you will cultivate more than a beautiful space. You’ll cultivate awareness of how natural systems are linked, and you’ll never again take a rain storm for granted.

Water is channeled through the brick patio to the rain garden, upper left. Photo: Ryan Moody, RLA
An attractive rain chain replaces the downspout, guiding water into a small trough below. Ryan Moody, RLA

Cheryl Corson, RLA, ASLA, is a local licensed landscape architect who works on the Hill and beyond. She has designed rain gardens for various sites, recently completing one for the Smithsonian’s National Zoo funded by the DC Department of the Environment. www.cherylcorson.com


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