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Low Impact Development Reduces Storm Water Runoff and Development Costs

Among the many techniques to limit runoff when building on finished lots, mitigating for soil compaction is one of the most important things builders can do, according to Marolyn Parson, who manages NAHB’s Water and Wetlands Policy Department.

“The lot is as hard as cement,” said Parson, “and if it slopes back rain will run off it just like from concrete.”

Once the original topsoil has been disturbed, builders need to restore it to its original state as much as possible so that rain water can soak into it, according to Scott Brown, a site design specialist for the Pennsylvania Housing Research/Resource Center. While hydrology varies widely by location, in Pennsylvania’s Piedmont region, 82% of the rain that falls infiltrates the soil, he said, with about a third of that amount percolating deeply into the ground water and the balance being used by plants or evaporating.

Builders “have to break up the compacted layer” of soil, he said. Otherwise, in addition to excessive runoff every time it rains, they also risk moisture problems on the site because water that can filter down into the earth can start flowing laterally. A chisel plow can be used to loosen things up, he said.

“Conservation of natural areas and maintaining topsoil are two of the best things you can do,” Brown said. “Topsoil is one of our friends” and acts as “a huge sponge” that can soak up about four inches of rain because 35%-45% of it is void space from plant root systems.

Brown suggested four approaches to replacing the natural infiltration function removed by the construction process:

• Minimize the amount of the natural site that is disturbed by the project and preserve as much natural land area as possible.
• Minimize impervious areas and use pervious materials where possible. This includes reducing road and sidewalk widths and lengths and considering multi-story buildings.
• Direct runoff over grass or other permeable surface areas, or collect water in rain barrels or cisterns that can later be used for the landscape.
• Provide an engineered infiltration facility to receive the increased surface runoff from impervious surfaces. These can include bio-retention areas, infiltration beds and infiltration trenches.

In urban areas, which represent “the systematic destruction of hydrological function,” Larry Coffman, associate director of the Prince George’s County, MD, Department of Environmental Resources, said there are hundreds of techniques that can be integrated into site design to encourage water to drain into the soil.

These low impact designs can reduce development costs by 30%-40%, he said, by eliminating curb and gutter, storm water ponds and other infrastructure.

Coffman cited case studies and resources from the NAHB Research Center showing significant benefits from low impact development.

For further information on low impact development, e-mail Marolyn Parson or call her at 800-368-5242 x8157.

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