What Makes a Garden, a Rain Garden?
By Paul Woodward, PE, CFM
What is something that almost all projects -- roads, buildings, parks, dams, etc. -- have in common?
Give up? It's stormwater.
No matter what the project is or where it is located, it is affected by rainfall and its by-product, stormwater runoff. If you haven't been following all of the latest and greatest information lately, the perception of how we deal with rainfall and stormwater is rapidly changing. No longer considered a nuisance that we need to get rid of as fast as possible, rain water today has the potential to be conserved and used as part of our landscape.
One of the many methods employed to conserve and use stormwater is a rain garden. A rain garden can simply be thought of as a bed for flowers, shrubs, or trees that is designed as a depression to hold rainfall from its surroundings until it can be used by the plants or can infiltrate back into the ground. This seemingly simple concept can achieve numerous environmental benefits while still being aesthetically pleasing. Such benefits include less runoff, reduced pollutants, improved stream function, and reduced erosion, just to name a few.
Although many suggestions are available for building a rain garden, don't be frightened; it's just as easy as a regular garden and probably less work than picking peas or tomatoes. With some helpful information from the University of Nebraska Extension, I was able to gain firsthand knowledge by building a rain garden in my own backyard. Many of your local universities, cities, and municipalites have guides available online that can help you through designing, selecting plants for, and installing your own rain garden with plants and techniques appropriate for your region.
The first step is to determine if a rain garden will fit your particular project. One easy way to find out is to simply go outside when it's raining and observe where stormwater travels. I used maps with two foot contours available on the Internet and recorded the location of roof downspouts and existing depression areas. You can capture as little or as much runoff as you want; however, a general rule of thumb is to size your rain garden for the volume of water that would run off during a one-inch storm. Simply identify what area (in square feet) will drain to your rain garden, and divide it by 12; this is the area (in square feet) recommended for your rain garden. Bigger rain events will require one side of the rain garden to be built slightly lower than the others to provide an outlet for excess stormwater.
Once you've located a potential site that will receive stormwater (either naturally or by some minimal grading), you will need to determine how much water your rain garden can handle. One easy method that I found helpful was to dig a small uniform hole with a post-hole digger or auger about eight inches deep. Fill the hole with water and let it soak in for awhile. Then come back and fill the hole with water again. This time, mark the top of the water; come back about every half hour to measure how much the water level has decreased. This will tell you how much water will soak into the soil in 24 hours. For example, if the water level drops about 0.5 inches each hour, you would be able to create a rain garden that is a foot deep, and all the water would disappear (infiltrate) in one day.
Best of all, most rain gardens can be designed to minimize the amount of time you have to spend with your shovel. In many instances, the soil that is excavated can be used to build a small uniform berm around the outlet of the garden. In my case, I was able to use the excavated soil to fill in behind a retaining wall.
Once you have a plan for where, what size, and how to grade your rain garden, you need to select some plants. Not being a master gardener myself, I took my guide of suggested plants to a local nursery and asked them for advice. They were very helpful, and I was able to get a variety of shrubs, grasses, and flowers that seem to be doing very well after just one year of growth. Once you have finished planting, simply bring in mulch or other ground covering to fill in around the plants to help prevent weeds.
Lastly, rain gardens vary in size -- from small gardens that handle runoff from a residential roof, like the one in my backyard, to larger, more complex facilities that are incorporated into the overall stormwater management plan for a project, such as the rain gardens at Omaha's Saddlebrook Elementary School, which receive runoff from the school parking lots and roof tops. Larger or more complex stormwater drainage conditions require special considerations and additional design. Practitioners at Olsson Associates have handled a variety of stormwater management practices, including large rain gardens, and know it's the little details that make stormwater features successful and attractive. We can put our experience to work for you by designing and implementing cost-effective green stormwater management solutions.
The only feature that really makes a garden, a rain garden is that it can temporarily hold and infiltrate water. If you are excited about rain gardens and want to learn more, give us a call.
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You can reach Paul Woodward with Olsson's Water Resources team at pwoodward@oaconsulting.com.