How to Manage Rainwater Runoff with a Rain Garden

water management Oct 11, 2021

 

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Michael Judd, Founder + Principal Designer, Ecologica DesignFrederick, Maryland
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Rain gardens basically are upside down baseball mitts in the ground, water-sinking depressions that come in many shapes and forms. They are simple imitations of how a forest floor absorbs and sinks rain water through porous soil.

Valuable water runoff is lost due to poor landscape design contributing to storm water pollution.

Valuable water runoff is lost due to poor landscape design contributing to storm water pollution.

During a good rain, our homescapes are hit with thousands of gallons of water that rush downstream lickity split. Maybe you have a rain barrel or two that catch a few drops, but most of it is going down the gutter across the landscape and into the watershed, picking up all the funky funk on the way. Hopefully, you are catching some of it with swales on contour, which are a form of rain gardens, but adding rain gardens to your landscape increases your site’s multifunctional design while stewarding our watersheds.

Usually rain gardens are sized for the amount of run off from a site, but you can start small, sink what you can, and build in an overflow. Perhaps you’ll decide to create a series of small rain gardens over time and assure every drop of water is staying on your site. The only rule I will give is to start at least 10 feet from your house foundation, unless you are wanting to grow mushrooms in your basement!

Small rain garden dug by hand can ‘sink’ a lot of runoff, just plan for the overflow.

Small rain garden dug by hand can ‘sink’ a lot of runoff, just plan for the overflow.

 

Smaller rain gardens can be dug by hand, such as the picture here, which is approximately 8 by 6 feet by 24 inches deep. This small rain garden sits at the entrance to a restaurant’s edible court yard and harvests the parking lot and walk way runoff, sinking and cleaning the water for the edible landscape beyond.

 

 

 

 

 

 

Rain gardens can add year round architecture to the landscape. Here a small tear drop shaped rain garden captures water for the raised beds beyond.

Rain gardens can add year round architecture to the landscape. Here a small tear drop shaped rain garden captures water for the raised beds beyond.

Regardless of width, deeper is usually better with 2 feet deep an average for home scale rain gardens. Leave 6 to 12 inches from the ground or berm down to the soil mix; this is called the pooling area, and it allows water to accumulate and sink.

 

 

 

Typical rain garden side view that shows a 6″ ‘pooling’ area, water inlet and outlet buffered with small stone, and a stone border for looks.

Typical rain garden side view that shows a 6″ ‘pooling’ area, water inlet and outlet buffered with small stone, and a stone border for looks.

There are equations for factoring rain water run off—volume relative to rain garden size—but more often than not, residential rain gardens are factored by the available area, budget, and interest of the owner. This is totally fine. A rain garden does not need to capture the full amount of water running off your property; any water capture is a benefit. Just plan where the overflow will go. The following is a good example:

 

 

The client on the left, ninja bog gardener Sandi Gill-Smith, had a home built next to her that thoughtfully drained all their water to her side yard. She also had considerable runoff coming from her roof drains and backyard and was dealing with wate…

The client on the left, ninja bog gardener Sandi Gill-Smith, had a home built next to her that thoughtfully drained all their water to her side yard. She also had considerable runoff coming from her roof drains and backyard and was dealing with water sitting alongside the house.

To capture the large volume of runoff we designed a three tiered rain garden that perfectly fit the slope, allowing the water to overflow from one garden to the next, assuring not one drop left the site. Note the French drain leading into the top ba…

To capture the large volume of runoff we designed a three tiered rain garden that perfectly fit the slope, allowing the water to overflow from one garden to the next, assuring not one drop left the site. Note the French drain leading into the top bay that drains water sitting along the house foundation.

Voila! Filled with a 50:25:25 ratio of sand, soil, compost and finished with a permeable hardscape. This rain garden now sinks every drop from both house sites. For further details on soil mixing options and design approaches see the Rain Water Harv…

Voila! Filled with a 50:25:25 ratio of sand, soil, compost and finished with a permeable hardscape. This rain garden now sinks every drop from both house sites. For further details on soil mixing options and design approaches see the Rain Water Harvesting chapter in Edible Landscapes with a Permaculture Twist.

 

Planting

The picture at the beginning of the rain garden section is the same design in only its second year of planting! The irony of a rain garden is that it is more often than not very dry thanks to its fast draining character. Unfortunately most rain gardens are only planted up with a handful of drought tolerant natives when there are loads of function and harvest to be had from all the water capture and loose soil. Some of my rain garden favorites, such as Beach Plum, Juneberry, Elderberry, Aronia, and low bush Blueberry, thrive in the loose and occasionally wet/dry soil. Other beautiful and functional plants to try are Swamp Milk Weed (more attractive than it sounds), Echinacea, Hibuscus moscheutos (edible petals), Winterberry, Spicebush, and Bee Balm to name just a few. For a low care rain garden, native clump grasses will fill the niche. To balance the periods between rains in the first year, I mulch well and set up a simple drip irrigation or sprinkler as back up.

Green Streets & Green Parking Lots

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We can create mini watershed along our streets and within our parking lots by harvesting rainwater runoff into planted basins. Think of a street or parking lot as a watershed and collecting water runoff at multiple points dividing the ‘watershed’ into subwatersheds. Each subwatershed or basin yields an easily managed volume of harvested rainwater that quickly infiltrates to passively irrigate plantings.

Planted with trees these rain garden basins act as living air conditioners, living carports, and living water and air filters. By harvesting storm water onsite we reduce street flooding, decrease the need for flood control and Bay pollution, we beautifies neighborhoods and creates wildlife habitat. .

The use of engineered swales in place of curbs and gutters should be encouraged in low- and medium-density residential zones where soils, slope and housing density permit…This practice promotes grass swales as an alternative to curbs and gutters along residential streets. Curbs and gutters are designed to quickly convey runoff from the street to the storm drain and, ultimately, to a local receiving water. Consequently, they provide little or no removal of stormwater pollutants. Indeed, curbs often act as traps where deposited pollutants remain until the next storm washes them away.
— Virginia Stormwater Design Specifications

*Also see page on fungi used in conjunction with swales to clean up toxic runoff and specific swale design for holistic equine farms.

French Drains

Flagstone pathway sits atop a French drain that pulls water away from the house foundation

Flagstone pathway sits atop a French drain that pulls water away from the house foundation

French drains are great designs to harvest and move rain water where you want in the landscape. They can take on many different functions and looks. French drains in urban/suburban landscapes are usually designed along hardscapes, drives, patios, building foundations etc. to either capture and infiltrate run off water or move standing water further out in the landscape.

The basic design element of a French drain is a stone filled trench, either on contour or slightly off, that allows water to infiltrate quickly or be moved to where it can best be utilized. As with rain gardens French drains are used on flat or gently sloping land, suitable adjacent to patios, paved parking areas, buildings, driveways, walkways and down spouts. Designed with rain gardens or raised bed swales or just on their own French drains fit nicely into the existing landscape, adding a new dynamic that seamlessly combine beauty and function.

Resources

Brad Lancaster’s Rainwater Harvesting for Dry Lands and Beyond volumes 1 & 2 are fantastically written and illustrated. Volume 2 is the real nitty gritty on swale and rain garden design and building.

The Permaculture Manual by Bill Mollison & David Holmgren is the bible on overall permaculture design but also detailed in earth works, swales, rain gardens etc.

Most states have rain garden publications and non-profits dedicated to helping you get started; many even offer financial support toward your project. See what is going on locally, you’ll hopefully be surprised.

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