Keyword Site Search

Rain Gardens and Rain Groves

By Buck Abbey, ASLA

A rain garden in Minnesota. Maplewood, Minn., in conjunction with the University of Minnesota's dept. of landscape architecture and the Ramsey Washington Metro Watershed District, has a program to encourage residents to install rain gardens. Many neighborhoods have swales on their properties. Installing a garden at the swale is voluntary. A focus group was held with residents and those particulars were published so that other communities could use it as a resource when planning their own rain garden projects.

"Rain gardens 'manage storm water through a combination of physical, biological, and chemical processes'"--Kalamazoo County, Michigan Drain Commission

Municipal ordinances create landscape codes. These public policies toward construction and landscape design are enacted to protect, preserve or rebuild nature within urbanizing areas. Each community benefits by keeping natural systems a functioning part of city infrastructure. To this end, new practices of storm water management using the calculator and creativity are evolving that are sure to influence the codes.

Onsite Storm Water Management

Micro-detentions are shallow depressions within the graded earthworks of a project site designed and constructed to recharge water that falls on developed property. A portion of this water is infiltrated directly into the ground, rather than flowing into underground pipe or into public storm water conveyance, which has been the conventional practice.

These simple site structures, known as rain gardens, rain groves or circular grass depressions, can be functional and creative additions to any landscape architect's quiver of storm water design ideas.

Rain gardens are landscape features adapted to provide onsite treatment of storm water runoff in highly urbanized areas. They are commonly located in parking lot islands, adjacent to roadsides, near yard buffers or within small pockets of open space in highly congested use areas. They can even be included within formal gardens if designed well.

Surface runoff is directed into these shallow, landscaped depressions by sheet flow or pipe outfall where it may infiltrate, evaporate or be transpired by native wetland plants. These shallow depressions may be round, irregular, geometric or linear in shape and are designed to incorporate many of the pollutant removal mechanisms that operate in forested or wetland ecosystems.

Rain gardens and rain groves can be highly effective in reducing storm water flows and cleaning pollutants from run off while acting as a special site feature.

Rain gardens are usually planted with perennials, grasses, shrubs and small trees. These shallow depressions can be planted with a variety of fresh water herbaceous native plants adapted to wet sites and heavy soils. Native plants are generally preferred. This one is in central Iowa.

Rain Gardens

Rain gardens have been defined in several ways. But perhaps the most direct definition comes from the state of Virginia where rain gardens are known as "a landscape tool to improve water quality." Rain gardens are being used by cities, schools, museums, libraries and back yard gardeners to promote water quality. Indianapolis, Indiana is one community that has committed to building thous ands of rain gardens in their community though the use of an aggressive grant program.

Rain gardens are generally small collections of water-loving plants placed within a low site area or within a concave planting bed that naturally collects rainfall. In some cases the gardens are designed expressly to collect rainfall from roofs, downspouts and parking areas. Water plants are used because they grow well in saturated ground where most bedding plants will not grow.

The Brooklyn Botanical Garden recommends using a strategically located low area. Most rain gardens should be located near the site's natural outfall or on a terrace no more than midway up the site's natural topography. Editor's note: According to the Charles River Watershed Association's "Low Impact Best Management Practices," gardens designed to infiltrate groundwater should "only be placed above uncompacted soils with a minimum infiltration rate of two inches per hour, otherwise, an under drain system is required to carry treated water to a traditional stormwater sewer system."

In commercial construction, rain gardens are always designed with the use of a grading plan and calculations for sizing and filter bed. Grading shapes the site and provide elevations for the use of various drainage features such as curbs, inlets, catch basins, man holes, headwalls and underground pipe. Calculations determine water volume and infiltration rates according to formulas published by various organizations, such as the Michigan's Kalamazoo County Drain Commissioner.

Smaller micro-detentions are often small circular depression less than 20 feet in diameter and not more than 12 inches deep. In many rain gardens, they are only three to six inches deep due to the percolation rate of subsoils. The best micro-detentions should be in full sun and never over one-acre.

Rain gardens consist of several components: a grass buffer strip, a ponding area, a rich thick organic mulch layer and an improved soil mix. Finally they include plants selected for their ability to withstand alternating flooding and drought.

Generally, interesting native wetland plants are used for their flowers, foliage and decorative seeds. These shallow depressions can be planted with variety of fresh water herbaceous native plants adapted to wet sites and heavy soils. The plants, the mulch and the soil are all used to soak up rainwater and to absorb certain chemicals or other pollutants. Water that ponds up in the garden following a rainstorm may also be lost to evaporation and infiltration. The grass buffer is used to filter water as it enters the garden. The garden can be supplemented with natural rock and garden ornamentation.

Rain Groves

Rain Groves are an enlarged version of a rain garden. They are very practical storm water management devices that provide multiple services but on a larger scale. Rain groves add beauty and shade to any building site while modulating its environment. They provide habitat for birds and other urban creatures that live in close proximity to mankind. But more importantly, rain groves can be used to absorb runoff. Since trees need water, building a rain grove to intercept run off will provide suitable habitat to grow a variety of wetland trees and shrubs. These plants will soak up hundreds of gallons of water a day while also removing waterborne pollutants.

Very Little Maintenance

The New Jersey Native Plant Society suggests these special gardens require very little maintenance. At most, they are periodically remulched and perhaps some plants replaced for aesthetic reasons. Plant colonies that are too aggressive might be thinned near slow growing and smaller plants. Unwanted weeds can be removed and all plants should be checked for stress during dry period.

Infiltration Capabilities

These micro-methods of storm water management rely upon ground shaping, surface material selection, recharge capability improvement and vegetation plantings to modify the infiltration capabilities of grassed landforms, beds and planted buffers.

These earth-friendly methods that replicate nature's methods of storm water management make it possible for landscape architects and engineers to design site facilities to better detain, retain and infiltrate storm water.

Parking lots are being targeted more and more as functional locales for simple rain gardens: mulch, shrubs and trees.
Photo: N.C. Cooperative Extension

Calculations Required

But designers must remember that even small storm water facilities must be calculated for water flow as well as water quality impact. Drainage basins must be determined and they must be designed for a design storm that will vary from region to region. Inflow, outflow and flooding must all be calculated using a modified version of the Rational Method formula. Performance criteria for bioretention design must be carefully considered in the design of rain gardens and rain groves.

Properly designed rain gardens can effectively eliminate many pollutants.

  • Total Suspended Solids: 23-81%
  • Total Phosphorus: 38-72%
  • Nitrate (as N): 8-80%
  • Lead: 62-91%
  • Zinc: 63-76%
  • Copper: 53-65%

Source: Charles River Watershed Association's "Low Impact Best Management Practices."

D.G. "Buck" Abbey, Associate Professor of Landscape Architecture at Louisiana State University, is LASN's Associate Editor for Ordinances.

Search Site by Story Keywords

Related Stories

June 27, 2019, 2:00 am PDT

Website problems, report a bug.
Copyright © 2019 Landscape Communications Inc.
Privacy Policy