Rain Gardens

Design considerations for proposal review | Bioretention on commercial sites | Bioretention projects in Connecticut

Rain gardens are shallow depressions in the landscape that typically include plants and a mulch layer or ground cover. In addition to providing increased groundwater recharge, they are expected to provide pollutant treatment. Pollutant treatment in rain gardens has been attributed to adsorption, decomposition, ion exchange, and volatilization (Prince George's County Bioretention Manual, 2002). Rain gardens can be used in residential settings to accept runoff from a roof or other impervious surface. In a commercial setting, bioretention areas are similar to rain gardens, but are often larger, and have an engineered design.

Design considerations during review

One important note: rain gardens are not water gardens! The gardens are typically designed to infiltrate water very quickly: around 4-6 hours. This rapid infiltration will eliminate the chances of creating mosquito habitat. If an improperly installed rain garden is holding water for excessive periods of time after a storm, the installer should repair the problem. Compaction of the rain garden material and/or premature clogging through siltation of the infiltration surface will lead to excessive ponding.

Residential rain gardens are often sized to hold one inch of runoff in above-ground storage from the contributing impervious surface. This sizing method has two benefits: first, the garden will capture the dirtiest portion of the runoff, or the "first flush". Second, in many parts of the country, the majority (around 90%) of storms are one inch or less in size. What this means is that sizing the garden to contain this volume of water will give the most benefit, without oversizing it to try to contain larger, more infrequent events that are a much smaller portion of the total yearly runoff volume.

Bioretention areas at larger commercial sites should be designed by an engineer. The Bioretention Manual referenced above contains guidance on this. Although bioretention areas at larger commercial sites should have engineered design, you can easily design and create a rain garden for your own home! A guide for residential rain gardens in Connecticut is available through the University of Connecticut extension.A manual from the Wisconsin DNR also provides thorough guidance.

Rain gardens in residential areas may be part of an overall LID approach to a site. There are several things to look for on such a plan:

• Look for the rain garden to be placed so that it will accept runoff from an impervious area such as a roof, driveway, or patio. How will the water enter the garden? Will it be diverted from a gutter and flow over grass, or will there be a pipe from the gutter downspout to the garden? A rain garden with a direct pipe will need to have stone or some other durable material where the pipe enters the garden to slow the water down and reduce erosion. A garden that accepts flow from a grassed area needs to have a broad entrance point for the water so that the flow does not become concentrated and cause erosion. These types of items should be shown on the plan detail.

• The garden should not be placed closer than 10 feet to a house that has a foundation with below-grade space, to avoid water problems in the basement. The garden also should not be placed closer than about 10 feet to a well, or near a septic system.
  

• A rain garden should not be in or very close to wetland soils. If the garden is proposed to be placed near wetland delineation lines, odds are the soils won't have a sufficient infiltration rate, and ponding may occur for long periods of time. This will create a nuisance condition, and will increase the likelihood of mosquito production.
  

• A rain garden should not be placed in an area that has a shallow depth to bedrock, or less than 2 feet from the bottom of the garden.
  

• Check the proposed plant list, and make sure that wetland plants are not proposed. A rain garden is not designed to have wetland plants. Plants listed in Appendix A of the CT Stormwater Quality manual that are in "Zone 4 Riparian Fringe" are most appropriate for rain gardens. Another list of suggested rain garden plants for Connecticut has been compiled by John Alexopoulos in the Plant Science department at the University of Connecticut.
  

• A special soil mix may be recommended, or native soils may be used if the infiltration rate is sufficient. If a special mix is not recommended, look for some justification as to why the native soils are sufficient.
  

• An underdrain may be installed beneath the soil layer, to prevent excessive ponding. This drain may be connected to the stormwater system, but if it is possible, it is better to let it run out onto a lawn or wooded area, as a foundation drain does. This will minimize the amount of water that enters the stormwater system, and help to preserve the predevelopment hydrology of the site.

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Bioretention on commercial sites

Bioretention areas may be part of an LID approach for a commercial site. There are several things to look for on such a plan:

• Look at the plan for the site and find out how stormwater is entering the bioretention area. This may be accomplished through the use of curb cuts (see photo), or by a larger area utilizing sheet flow off a paved surface. Either way, the entrance point into the garden should be stabilized with stone or another durable material to reduce erosion.

• The profile detail of the bioretention area should show the ponding area depth, which is typically 6 inches. This can be greater to increase storage, but aesthetics and safety issues need to be considered also.

• The detail should also show the specifications for the media. Typically, a mix should have a high sand content (at least 50%) to encourage rapid infiltration. The remainder of the mix is typically composed of some sort of compost or peat (20-30%), and topsoil (20-30%) with low clay content.

• Older designs called for geotextile fabric to be placed beneath the soil media, to protect the gravel around an underdrain. However, the 2002 Bioretention Manual recommends a pea gravel blanket in place of the geotextile. The fabrics have been found to clog over time, and have led to decreased infiltration rates.

• An underdrain is typical on commercial bioretention, and is likely tied in to the stormwater system. The drainpipe may also drain to grade if slope permits. These components should be shown in the detail on the plan.

• A structured overflow is also typical of commercial bioretention. This may be accomplished by a raised structure in the garden itself, or a bypass to a catch basin outside of the facility when the maximum ponding depth is reached. Although either design is sufficient, the use of both is redundant and adds unnecessary cost.

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Local Examples

Are you looking for more local examples of bioretention? Visit CT NEMO's LID database for Connecticut. This site contains searchable database for LID practices, engineering firms familiar with LID, and construction companies who have installed LID practices.

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