Stormwater: Rain Gardens and Drywells
Temporary Water Storage Helps Recharge Aquifers and Reduce Water Pollution
Handling runoff sensibly
Rainwater runoff is often sent to storm drains and, eventually, to overloaded municipal wastewater treatment plants. There are at least two alternatives, both of which capture runoff and hold it temporarily.
Both rain gardens and drywells allow runoff to soak back into the ground, which minimizes the risk of flooding and recharges underground water supplies.
Most rain gardens are built from scratch
Rain gardens are best designed for a specific site. They must be sized to handle the volume of water that the local climate and roof size are likely to produce. Local weather patterns also dictate the choice of plants that will grow best. For these reasons, rain gardens are usually made from local materials and adapted to site requirements.
But manufactured components can give a head start. One manufacturer of plastic drywell components is Flo-Well Water Management. The Massachusetts company makes the O-Well, which can be adapted to various designs, and offers general design advice about rain gardens on its Web site.
Healthy landscapes mimic nature
Gardens are often flat, but what really serves nature is a variety in terrain-–dips and hollows strategically placed to catch and hold rainwater. These undulations allow water to soak into the ground rather than run into the street and overwhelm municipal storm drains.
Size it. A rain garden should be large enough to collect water from all but the biggest storms. An online calculator can help predict the volume of water that rain gutters will produce for every inch of rainfall.
Choose plants. Choices, of course, vary by region. But even within the same garden, moisture levels will vary, so a mix of plants with different moisture tolerances will be most successful. Some types of trees also may create habitats for birds and other animals.
See below for:
HIDDEN UNDERGROUND RESERVOIRS FOR RUNOFF
Three steps for building a rain garden
Choose a spot. Find a low area near hard surfaces such as driveways and sidewalks, or someplace to which runoff from the roof can be directed. Keep the garden at least 10 feet from the foundation to prevent water problems in the basement.
Prepare the bed. A mix of organic material, topsoil, and sandy soil works best. Clay content should be limited. Water that collects in the garden should disappear within a few hours, at most a day or two. Water can be directed to the rain garden by a swale or plastic pipe.
Top it with mulch. A layer of mulch will help keep the weeds down and will trap some contaminants before they percolate into groundwater.
Building code specifies grade but allows for alternatives
Surface drainage should be directed to a storm sewer or other approved collection point so that it doesn’t create a hazard (401.3). Grade around foundations should slope away from the structure in all directions, falling 6 inches within the first 10 feet.
Exceptions are allowed when the structure is too close to the property line or when physical barriers prevent the required 6-inch slope (401.3E). The the minimum slope can be 5%, with the water directed to a swale.
Swales and impervious surfaces like conventional concrete driveways and walkways must slope away from the structure by a minimum of 2% when they’re located within 10 feet of the building’s foundation.
ABOUT RAIN GARDENS
Returning water to the earth gently
A rain garden is a depression in the ground that’s been filled with permeable materials and capped with plants such as wildflowers, shrubs, and small trees. Unlike roadside swales, rain gardens are intended to hold the runoff that enters rather than act as a pass-through.
Water disperses slowly through percolation and the transpiration of water back into the atmosphere through plant leaves. The process is called “biofiltration.”
A less expensive alternative
The rain garden is a relatively new concept in landscape design, but it is growing in popularity.
Rain gardens were originally developed in 1990 by a developer of a residential housing project in Maryland who was looking for an alternative to conventional retention ponds for runoff control. Each house in the subdivision was designed with a rain garden that covered between 300 square feet and 400 square feet. The approach was effective and cheaper.
Since then, rain gardens have been used in projects all over the country. In one Seattle demonstration, rain gardens incorporated into an overhaul of a residential street cut the amount of stormwater runoff by 98%.
Rain gardens mimic natural ecosystems
Residential development creates impervious surfaces that prevent runoff from dispersing slowly and percolating into the soil. Erosion, the pollution of lakes and streams, and more strain on municipal sewage plants and stormwater systems are three unfortunate results.
Rain gardens are a relatively simple but highly effective tool for managing runoff. They work in much the same way as an undisturbed ecosystem.
Drywells are hollow or rock-filled cavities set below ground level that catch and gradually disperse water runoff from sump pumps, gutters, and other drainage systems.
Like rain gardens, drywells (where they are feasible) are preferable to dumping water into municipal sewers or storm drains because they recharge underground water supplies and relieve pressure on overloaded municipal treatment plants.
In situations where runoff may contain sediment or pollutants, drywells can trap contaminants and prevent them from spreading into nearby streams or wet areas.
There are a lot of variations in drywell design and construction. Here are some general considerations:
Your soil has to percolate. Drywells provide temporary water storage. They only work when the water they collect can percolate into the ground, so some soil types are more suitable than others. An impermeable layer of clay or soil with high clay content will trap water. Sand, sandy loam, and gravel will disperse it.
A test hole and “perc” test are a good way of finding out whether underlying soil can absorb water fast enough to do any good. If water in a test hole doesn’t drop appreciably overnight, a drywell won't work in that location.
Your water table must be low. High groundwater levels also will render a drywell ineffective.
Keep drywells away from the house. Drywells should be kept far enough away from the house—a minimum of 10 feet from the foundation wall—to prevent stored water from seeping into the basement.
Allow for overflow and cleanout. When the volume of water draining into the drywell is more than the system can handle—during a pounding rainstorm, for example—an overflow pipe keeps water from backing up. Cleanouts allow accumulated silt and other debris to be cleared away periodically.
Make it big enough. The size of the drywell will depend on a variety of factors, including soil type and the volume of incoming water. (A 1,000-suare-foot roof collects more than 600 gallons of water for every inch of rain that falls.)
Fine Homebuilding magazine explains how to design and build one type of rain garden.
The U.S. Environmental Protection Agency offers a number of links and suggestions for managing stormwater runoff.
The Low Impact Development Center can get you started on selecting plants for a rain garden.
- Michael Chandler
- Lee Ann White/Fine Gardening #74