Natural Stormwater Management: Rain Gardens

September 17, 2009 at 6:23 AM | Posted in Environmentally Friendly Living | 3 Comments

If you’ve never heard of rain gardens, you should read this article. They are very exciting, easy to build and maintain, and very gratifying to operate. Let’s start by talking a little bit about stormwater runoff.

I am a water resources/environmental engineering major, so I get pretty excited about this stuff, but rain gardens are just fun. In nature, when it rains (think tallgrass prairie here), almost all of that water either gets sucked up by the plants or goes into the ground. Once water has entered the ground and escaped the plant roots, it trickles down into the deep soil and to the groundwater table, where it flows to the nearest stream. Streams are supplied by the groundwater, and this is called baseflow.

Now, take that nice prairie and set an Omaha cookie-cutter homebuilder loose on it. This guy plows the native flora under, strips the topsoil, compacts the remaining soil and then sells you the topsoil back after he builds a house on it. To make matters worse, he doesn’t till the soil, and you get the typical Kentucky bluegrass sod that dies within a year (unless you flood the shit). You spend the rest of your life sweating over dandelions, your five applications of chemical fertilizer per year and the occasional power-raking or aeration.

Pretty dim, eh? But what’s all this got to do with stormwater runoff? When you compact soil, it loses a lot of its ability to absorb water. That’s step 1. Step 2 is when you get a crappy layer of Kentucky bluegrass that grows on top of a tiny layer of topsoil atop nutrient-poor soil. Now you’ve go to keep fertilizing it. But the worst news is that when it rains, not only is your Kentucky bluegrass in poor health and not absorbing much water, but the soil is in bad condition as well, so you have lots of water running off the lawn and into the streets. Big deal, right? Your father’s father’s tax dollars paid the city to build a sewer system so your lawn can send its water to the city’s water treatment plant. Right?

Wrong. Sure, you paid tax dollars on a sewer system (or your family did at some point). But many cities, especially older cities, have a decaying, inefficient sewer system that handles both municipal waste (read: toilet flushing) and stormwater together. In order to avoid overload, the system has overflow outlets at various points for emergency situations. The trouble is, during rainstorms, the system nearly ALWAYS overflows, and we dump raw sewage into our lakes and streams. This happens ALL OVER the country.

Anyways, there’s more than one reason to dig a rain garden. Besides doing something really nice for your sewer system and your local lakes and streams, you’ll also be doing the environment another, more direct favor. In cities, streams generally don’t support aquatic life because not only are they horribly polluted, but they also are very low on baseflow (that groundwater stuff). If all of the runoff from a storm goes to a sewer drain, even if it drains into a stream, it goes there right away rather than at a slow, sustained rate. This results in degradation of water quality, degradation of riparian (streambanks) environments and reduced capacity to support wildlife, and it just looks worse. By building a rain garden, you establish for your nearest stream a higher level of baseflow by returning the water to the streams naturally.

So, let’s talk about actual rain gardens. A rain garden is a depression where water runs down to and pools. The basin bottom is usually made of dirt, so the water pools and then infiltrates into the soil at a natural rate. Because you’ll have water pooling, you’ll need hardy plants that can tolerate standing water. This usually amounts to shrubs, woody perennials and tall grasses.

But there’s another thing rain gardens do. Any water that runs into them is naturally filtered and cleansed through a combination of the plants’ root systems and the soil itself. Not only does the water return to the stream at a steady, sustained rate, but it also returns at a much higher quality than that at which the sewers would deliver it. When you consider the variety of urban pollutants in runoff (fertilizer chemicals, nutrient overloads, gravel, sand, oil, rubber and all kinds of nasty things), this is definitely a good thing.

Rain gardens aren’t new, but they haven’t found a lot of use yet. Some innovative designers use rain gardens as parking lot islands. They grade the parking lot so that water that collects on the lot goes to the pools and then infiltrates naturally. This is really awesome for the environment, as one of the chief contributors to huge runoff in cities is concrete. Here’s a great picture of this idea in action:

These rain gardens are full of pretty, native plants with extensive root systems.  They help to stabilize the soil under the parking lot and act as local purifiers of stormwater runoff from the lot.  The water will return to the stream at a natural rate at very high quality.

These rain gardens are full of pretty, native plants with extensive root systems. They help to stabilize the soil under the parking lot and act as local purifiers of stormwater runoff from the lot. The water will return to the stream at a natural rate at very high quality.

You can do this same thing in your yard. Here are some awesome rain gardens that homeowners landscaped themselves.

 

This is a really ornate rain garden that features a detention basin with a pump and waterfalls. Gardens that normally have standing water can be designed as effective rain gardens.

You can get really creative with rain gardens, and they can be quite visually pleasing.  River rocks accent the native plants, and the gardens form is as beautiful as its function.

You can get really creative with rain gardens, and they can be quite visually pleasing. River rocks accent the native plants, and the garden's form is as beautiful as its function.

This type of garden is what you might expect to design in Nebraska.  The look of a rain garden varies by location and they are a unique opportunity to display the natural beauty of your local region.

This type of garden is what you might expect to design in Nebraska. The look of a rain garden varies by location and they are a unique opportunity to display the natural beauty of your local region.

You can see from these shots that rain gardens can look great and still do an awesome job. If you use them to collect runoff from your lawn, you’re really doing everyone a favor: the sewer system, the local natural drainage and the (dwindling number of) plants and animals that call it home. You’ll be removing the chemical fertilizer remnants from the water, removing sediment and any other urban pollutants normally found in runoff. Rain gardens can also be a GREAT way to cure drainage problems you may be having with your lawn. If you have extremely dense soil (like clay or fine silt) that doesn’t drain well during rainstorms, you can dig a rain garden and direct drainage into it.

There are some technicalities that go into rain garden planning. You can get very involved, putting down subgrade-quality aggregate several feet into the ground or placing a pipe below the area. But the beauty of the system is that you can also simply dig a small depression and fill it with nice plants. Just make sure your lawn actually drains into the rain garden, otherwise it won’t do its job as best it could. Here are a few schematics taken from the greater internet.

 

This is an elaborate design. It includes a drainage pipe encased by coarse stone and several layers of well-sorted substrate below the garden.

This rain garden is just a hole dug into the ground and filled with native plants.  This kind of design is good for quickly-draining soils, but will work for slowly-draining soils if made large enough.

This rain garden is just a hole dug into the ground and filled with native plants. This kind of design is good for quickly-draining soils, but will work for slowly-draining soils if made large enough.

The last picture shows an important feature of rain garden design. If you are placing your rain garden in an area that would be sensitive to flood damage, provide the garden with an overflow exit of some sort. This can be a discrete pipe, above or underground, or a channel dug to some other location. The only problem with this is that you may lose sediment through the overflow water, but this shouldn’t be a significant issue because (hopefully) overflow doesn’t happen that often. In most cases, overflow protection is unnecessary as the degree of flooding that may occur is small and infrequent. Overflow considerations are normally for commercial projects.

Rain gardens are a great way to add beauty to a neighborhood or a home, and they serve their purpose very discretely. Most rain gardens do not look any different from a regular garden, except that they may be located in a depression. They are fairly cheap – most of the cost is in plants. If you have more than one year to establish your garden, you can plant many plants by seed and vastly reduce the cost of a rain garden – just be sure not to flood the plants while they are young. Planting a rain garden is also a great community activity and can be a great way to build a sense of pride between neighbors. Here are a few more pictures of rain gardens to leave you with.

This is a picture of a rain garden at its inception.  The plants are not mature yet, so there is an outlet of river rocks to be sure they dont drown.

This is a picture of a rain garden at its inception. The plants are not mature yet, so there is an outlet of river rocks to be sure they don't drown.

This rain garden is another good example of something you might expect to see in Nebraska.

This rain garden is another good example of something you might expect to see in Nebraska.

This rain garden makes use of wetland plants.

This rain garden makes use of wetland plants.

Finally, here’s a list of plants you can use in rain gardens in Nebraska.

Grasses: Big bluestem, Indiangrass, switchgrass, prairie cordgrass, prairie sandreed (I’d be wary of this one), little bluestem, watercress. Avoid shortgrasses like buffalo grass and most gamagrasses (eastern gama grass is tall enough to withstand pooling).

Trees: Silver maple, cottonwood, sycamore, most dogwood (not ornamentals), willow.

Shrubs: Elderberry, button bush, swamp milkweed (also an herb), sedge, marsh skullcap (mint), black cherry, chokecherry, American plum.

Some hardy wildflowers can survive standing water conditions. They are usually things like thistles, vines and relatively tall (1-2ft) perennials.

Of course, this list isn’t exhaustive, and you can tamper with your garden as much as you want to experiment with different looks and see what works well. Some plants are extremely effective at sucking up moisture (especially native tallgrasses), and some are not so great. Either way, planting a rain garden will be one of the most satisfying things you can do to boost your environmental standing in the world, and it’s extremely easy to do.

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3 Comments »

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  1. This is great. I’ve done this informally. Iris Pseudocarus works well!

  2. Raingardens need to be mandatory with new construction and incentives for business and homeowners should be offered by municipalities ($) for existing buildings. All pawned parks and golf courses’ runoff should be directed to rain gardens.

  3. hi nice project


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