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What Is A Rain Garden, and Why Should I Care?

You would almost have to be living under a rock to not have heard someone throw the words 'rain garden' around recently. You may even have a pretty good idea of what that is. Simply put, a rain garden is a sunken garden area that captures rain water and allows it to seep slowly into the ground. It's become a rather popular new trend in home landscaping, although, depending on the size of the garden and the amount of runoff it collects, it may be bigger than a typical DIY project.

Now there are new catch phrases to learn. Bio-swale, bio-retention basin, and bio-infiltration systems are all similar in concept to the rain garden, but are semantically a little more accurate (which makes the engineers happy) and they tend to be much larger in scale than the 200 square foot rain garden in your front yard. In fact, a rain garden would be more correctly described as a small bio-retention basin. If you've driven on I71/75 lately in northern Kentucky, you may have noticed a large sunken area next to the new St. Elizabeth building that has some plants dotting the bottom of it. That is actually the largest of 4 bio-infiltration systems that St. E had installed to try and reduce the amount of runoff their new facility (and all its impervious surfaces) would add to the sewers when it rains. And if you've noticed that basin after a major rainfall, you've probably seen just the tips of some of the branches poking out of the top of the water. I believe the landscape architect's exact description of the amount of water that comes off of the highway and into that basin was “scary.” So, the garden is doing its job in keeping some of that water from going directly into the storm drains and allowing it time to infiltrate down into the soil.

A bio-swale is a slightly different concept. Rather than retain water, the bio-swale acts more like a ditch. But this ditch is designed to slow down the flow of water, allowing extra time for the plants and the soil to reclaim some of it before it can get to the sewer drain. The Amberley Village Municipal Center has an excellent example of a bio-swale. A very long section of an existing drainage ditch was filled in with large stones at points to cause some ponding, and then planted with a mix of grasses and perennials. There is still an overflow drain at the far end of the swale, but the speed with which water reaches the drain will be significantly slower thanks to the bio-swale's intentional obstacle course.

Why is all this important for you to know? Most of Cincinnati has what is called a combined sewer system. This means that both storm water (from rainfall events) and waste water (from your drains and toilets) go into the same sewer drains and are routed through treatment facilities before being put back into the river. (Newer construction uses separate sewers for waster water and storm water.) The combined sewers also utilize a combined sewer overflow, commonly referred to as a CSO. Not to be confused with the Cincinnati Symphony Orchestra. During dry weather and even during light rainfall, the combined sewer system works just fine. But when a light rainfall turns into a gulley-washer, the sewers get too full, too fast for all the water to be routed through a treatment facility. This kind of surplus is what causes raw sewage to backup into your basement during periods of heavy rain. Gross, right? Just wait.

Rather than allow raw sewage to backup into people's homes, the excess simply bypasses the treatment facilities and goes directly to the rivers and streams. Let that sink in for a moment. Raw sewage is sent directly to our waterways because the current sewer system just can't handle the enormous amount of water that runs off of all the rooftops, parking lots and roadways in the city. What is the city doing about this? They're working on it. But, as you can imagine, this is a huge project that already has and will continue to take decades to rectify. In the meantime, there might be something you can do to help. This is where rain gardens, bio-swales, and bio-infiltration basins come into play.

By diverting some of the storm water runoff from going directly into the sewer system, individual home owners can have a profound impact on the CSO as a whole. This doesn't mean just diverting water so it can pond in your yard, killing the grass and creating a new mosquito breeding facility. It means making an attractive garden, that doesn't have to look much different than any other garden on your property, but that serves a unique purpose. Scooping out a basin and replacing the clay soil with a mixture that allows better water infiltration and then planting with appropriate trees, shrubs, and perennials, can make a useful storm water diversion disguised as a beautiful garden. Funneling just a little of the water that sheets off of your roof, your driveway, or the street in front of your home, can have a big impact.

If you would like to see the effect that a few garden plants can have on slowing down storm water runoff, the Sanitation District No. 1 in northern Kentucky has a superb educational display built in front of their facility. They also have a bio-infiltration system designed in the center of their parking lot that is the gold standard in what is possible if everyone put in the effort. The address is 1045 Eaton Dr., Ft. Wright, KY 41017. I highly recommend it for an outing with the kids. If you'd like to visit other rain gardens or bio-swales in the region, there are dozens listed on (including the ones mentioned above) along with a map and photographs. And if you would like more information about creating a rain garden in your own yard, start with the Rain Garden Guidelines handbook published by the Hamilton County Extension Office.

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