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When rains fall on land that has been largely covered by impervious surfaces, rainwater runs off into the sewer system via storm drains, becoming stormwater. As rain flows off roofs, driveways and streets, it picks up pollutants, such as oil, debris, and bird and animal feces. Most sewer systems can handle the rainfall of a typical storm, but rainfall can be so heavy that the sewer system can't handle the volume of water. When stormwater combines with sewage, as is the case in some areas of San Francisco, both sewage and stormwater overflow untreated into natural waterways. The untreated stormwater is released into surrounding streams, rivers, estuaries and oceans, harming the nearby ecosystem.
The rain garden is a shallow basin with porous soil planted with a variety of trees, shrubs and grasses. Its purpose is to direct and capture runoff from impervious/impermeable urban areas such as roofs, driveways and walkways. As the water percolates through, soil and plant roots provide natural filtration. Rain gardens also biologically filter and sequester pollutants, excess nutrients and toxins through porous media of gravel, sand and soil. According to the Environmental Protection Agency, rain gardens can remove large amounts of pollutants from runoff: approximately 95% of copper, 98% of phosphorus, 20% of nitrate, and 50% of total Kjeldhal nitrogen. Although typically used in new developments to reduce necessary sewer infrastructure, it's also a relatively easy and attractive addition to existing landscaping. Not only that, rain gardens infiltrate 30% more rainwater than turf.
Rain Garden Benefits
In addition to reducing and filtering stormwater runoff and increasing groundwater recharge, rain gardens provide many other benefits:
- Reduce storm drain overload and flooding when adopted on a community or neighborhood scale.
- Provide habitat for wildlife and, with the proper plants, increase the number and diversity of birds and butterflies for those who enjoy watching them.
- Provide an attractive and creative alternative to traditional lawn landscapes.
- Require less maintenance than lawns – they do not need to be mowed, fertilized, or watered once established.
- Increase property values with creative landscaping designs.
- Increase fertility and help provide water to nearby plants longer into the dry season
- Help cities and counties comply with the National Pollution Discharge Elimination System Permits (NPDES) and comply with the Clean Water Act.
- Reduce sediment and pollution into streams and rivers and do not contribute to Total Maximum Daily Load (TMDL) limit.
Rain Garden Design Considerations for Residential Landscapes
Rain gardens should be strategically located to capture as much runoff as possible, often from a nearby paved area or the roof of a building. The best location is between the points where stormwater leaves its runoff area and where gravity would naturally deposit it in the landscape. The goal is maximum water infiltration into the soil (percolation).
For instance, stormwater ponding on a property is a clear signal that the area is over capacity for timely infiltration. Perhaps the water table is close to the surface or a layer of rock or dense clay directly below the topsoil prevents infiltration. The rain garden should be situated at a point before water can pond in this area.
To be clear, water shouldn’t form a pond in the rain garden area either—the basin should drain in 48 hours or less after a large storm so it doesn’t become a breeding ground for mosquitoes. For this reason, it’s critical to consider the subsurface of the garden when choosing its location. At least three feet of permeable soil must be available above impermeable bedrock. Slow draining clay soils can be made more permeable but if impermeable bedrock exists a short distance from the surface you won’t be able to install a rain garden there.
To determine soil permeability, dig a hole six inches deep and fill it with water. If the water is still there in 24 hours, the soil doesn't infiltrate fast enough. First check to see if bedrock is within three feet of the surface. If you don't hit rock and the subsurface is just dense soil, there are a number of ways to remedy the problem.
In addition to drainage flow and subsurface, there are several other points to consider before finalizing the location:- The garden should be located 10 feet from a building so there's no seepage near the building's foundation. Water can be conducted from the downspout or drainage area to the rain garden with contoured ditches (called swales) or via a buried pipe.
- Sunny areas are preferable to shady areas because heat aids evaporation.
- Rain gardens are easier to build on flat ground than sloping ground, but it is possible to build up the lower side of the garden if no flat ground is available. If the slope is greater than 12%, choose a different location.
- Don't locate it over a septic field.
- Call 311 to find out where your underground utilities are before you finalize the location.
- If possible, have water flow through a grassy area before it enters the rain garden so that larger particles are filtered out along the way.
- If the rain garden area is next to a wooded area, its edge should not extend more than 1 foot into the dripline of existing trees. This ensures root structures won't be damaged when you create the garden.
To determine the proper size for the rain garden, estimate how much water will flow into it. Does the rainwater come from all of the building's downspouts or just a portion? Is rainwater flowing into it from any other impervious surfaces? Don't forget to include the amount of water falling directly onto the garden itself. You must also determine the general pace of the water's infiltration into the soil. Is it fast or relatively slow? Ideally the garden should be a minimum of 10 feet wide and its length should be twice as long as its width. The longer side of the garden should face upslope, if one exists. Measure the area of the surface that the water will be draining from and multiply that by the number associated with the type of soil you have.
- For sandy soil multiply by 20%
- For loam use 30-35%
- For clay use 45-60%.
These numbers estimate an area larger than will probably be needed but will ensure that during a heavy storm all of the water will be held by the depression. Even if you've overestimated the typical volume of water that will collect, overflow may happen during a large storm and must be accounted for in the design, especially if the garden is close to roads, buildings etc. Extra water can flow into another rain garden, a surface drain, or an underdrain and then into the sewer system. It can also be directed to an area where flooding is permissible.
Selecting Plants for the Garden
Credit: Good Nature Publishing
A rain garden is designed to dry out between storms, not retain water like a pond or bog. Therefore the plants selected should be able to handle both flooding and dry conditions. For California's summer, dry climate plants should be drought-tolerant for several months. Graywater can be used to water during the summer dry spell. (Hey! Integrated water management!) Native plants are accustomed to California weather and also provide habitat for local wildlife better than non-natives. However, any non-invasive drought-tolerant plant can be used provided it tolerates some flooding.
Generally trees and shrubs can handle flooding and drought conditions better than plants with shallower root systems. Their deep roots also assist with infiltration. Perennials and grasses that thrive at the margins of ponds and bogs can be used because they are accustomed to the occasional flood. Grasses in particular are effective where water enters the garden. Not only will they filter sediment from incoming water but their fibrous root systems filter chemical pollutants. Use only grasses that don't need to be mowed because mowing will be difficult on the slopes of the rain garden.
Rainwater for Edibles
Common sense should dictate whether to use rainwater on vegetables. If the rainwater collection system has been designed to minimize the exposure of rainwater to leach-able synthetic compounds and includes proper filtration, than rainwater is generally considered safe to use on vegetables. If a rainwater system has been improperly designed with non ANSI/NSF materials and does not filter out organic material before storage, there is a greater health risk when using the rainwater on vegetables.
While research has been done on the uptake of synthetic organic compounds by vegetable plants, it is hard to make broad generalizations about relative health concerns in individual backyards. The chance of any one synthetic organic compound being absorbed through the roots of a plant into a fruit depends upon characteristics of the compound, the soil and the plant.
Greater health risks may come from urination and defecation of domestic and non-domestic in proximity to edible plants. Also of concern is lead and other heavy metals from older buildings being present in the soil. Sanitary handling of vegetables and other edibles often is the best strategy for reducing health risks. Careful washing of vegetables and edibles can reduce risks associated with physical contact with rainwater or existing contaminants in the soil.
Installing the Rain Garden
Credit: SUNY, College of Environmental Science and Forestry
Once the location, design and plants are chosen installing the garden can begin:
- Remove existing vegetation. Remove the soil and form the depression that will form the rain garden. Dig the depression with gently sloping sides. It should measure an average of 6 inches below where water will enter. With gently sloping sides, the edges of the garden should be 4 inches below where water will enter and the lowest point should be 8 inches from where water will enter. To help retain water, you can use the soil you remove to create a lip on the side that is furthest from where water enters. Soil that is removed to create the depression shouldn't be piled deeper than 0.5 inch on top of existing tree roots.
- For moderately dense soils you can simply loosen the soil at the base of the garden with a spading fork. For very dense soils you can replace the subsurface soil with a rain garden mix of 50% grit or sharp sand, 20-30% topsoil, 20-30% organic compost. In extreme cases you can even add an underdrain of perforated pipe in a gravel base at the bottom of the basin.
- Spread 2-3 inches of compost and incorporate it into the first several inches of topsoil to increase permeability and soil health.
- You can either plant or seed the rain garden. Seeding is cheaper and denser than planting plants from a nursery, but seeds will take much longer to grow. It’s best if both methods are used—purchase some grown perennials, trees or shrubs and then seed the areas in between so that the vegetation eventually will cover the entire garden. It’s important to plant densely; plants aid in filtration and infiltration and the structure of bare soil can disintegrate during heavy storms. Plants that will best tolerate flooding go in the deepest part of the basin; plants that don't like as much water go on the edges.
- Mulch the area with woodchips to protect soil and decrease evaporation during the dry season. Wood chips should be 1-2 inches across so they don't float or blow away.
Maintenance Considerations
Credit: donsutherland1@flickr
- Weed thoroughly while plants become established so that weeds don't become established or choke out desired plants.
- Cut back dead growth as necessary to allow for new growth.
- Water plants the first summer to help them get established. You can use rainwater or graywater for this.
- Fertilization should not be necessary. You shouldn't need to irrigate after the first year unless there is a particularly hot dry summer.
- You may need to clear leaves from downspouts and water channels annually
- Remulch as needed—probably once or twice annually.
- Avoid walking on or otherwise compacting the soil. Do maintenance only when the ground is dry; walking on wet soil ruins soil structure.
Cost
Credit: bluelivingideas.com
LID strategies use natural drainage features like swales and vegetation to reduce the required amount of materials in paving roads and driveways to install curbs, gutters and piping.
The initial costs can be significantly higher than traditional landscaping techniques but the benefits are worth it. As an example, the use of drought-tolerant plants can help decrease water bills and avoid the cost of expensive irrigation systems.
Major energy expenses in landscape maintenance are water pumping and treatment, the manufacture, purchase and shipping of fertilizers, insecticides, and other gardening chemicals that are not used in LID techniques. Air Quality improvement, decrease in urban heat island effect, reduction of energy costs, noise reduction, roof life extension, aesthetic improvements, and LEED point awards are some of the benefits that come from using these green design techniques. LID strategies also can increase property values due to the desirability and marketability of a green and sustainable habitat, and their proximity to open space.
Although initial costs are high, implementing water-efficient design is important; forests, agriculture, suburban and undeveloped urban lands are replaced with impervious surfaces with urban development. Green development practices are not just beneficial to our health, they also reduce environmental impacts and reduce long-term costs.
Codes and Regulations
No permits are needed to construct a rain garden. Because it's illegal for water to flow onto another property, it must be designed to direct overflow to an appropriate drainage area. Locate the garden at least 10 feet from the foundations of buildings. Make sure you don't create the rain garden on top of a septic field or hit underground utility lines and pipes when digging.
Additional Information
- Rain Gardens
- What is a Rain Garden?
- Rain Gardens - A How-to Manual for Homeowners
- How to Create a Rain Garden
- Bioretention (Rain Gardens), (US EPA, webpage)
- Rain Garden Construction and Sequence of Construction (LID Center, webpage)
- "How to Build a Rain garden" (Rain Garden Network, webpage)
- Fact Sheet: Reducing Stormwater Costs through Low Impact Development (LID) Strategies and Practices
- Rain Gardens: Managing Water Sustainably in the Garden and Designed Landscape, Dunnett, Nigel and Clayden, 2007 (Book)
- Rain Gardens—a How To Manual for Homeowners, University of Wisconsin, 2003 (Book)
