Green Stormwater Infrastructure Tips
Let's say you want to design a rain garden, but the heavy clay soil won’t allow water to infiltrate effectively. Or you want to direct roof runoff into a swale, but you’re dealing with a tiny lot.
“Green infrastructure” refers to the host of techniques and strategies that slow stormwater runoff, clean and filter stormwater and enhance groundwater recharge. These include rain gardens, bioswales, permeable pavement, “green” parking areas, trees and disconnected downspouts. Designers must always choose from this menu of practices, selecting those best suited to the site. However, sometimes restrictive site conditions can be addressed—and the options widened. Here are the most common issues, along with the solutions that will help you design around them.
1. Slow Soils
Soils that are dominated by clay and/or glacial till often have low infiltration rates, and stormwater may run off before it can be captured on the site. But it is still possible to design infiltration-based stormwater controls, such as rain gardens and swales, for sites with these soils. Take these steps to ensure your plan succeeds:
Measure infiltration rates. This should be done before designing stormwater controls. A generally accepted guideline is that the infiltration rate of native soils beneath swales and rain gardens should be greater than 0.25 to 0.5 inches/hour.
Amend soils. Adding compost or other organic matter can increase soil infiltration rates, while improving the soil’s fertility and its ability to remove pollutants
Go deep. Enhance soil infiltration rates by planting deep-rooted vegetation. The roots create small conduits for water to infiltrate and increase biological activity in the soil. The U.S. Geologic Survey found that the median infiltration rate of a clay soil planted with prairie species was more than three times the rate of a clay soil planted in turf (0.88 inches/hour compared to 0.28 inches/hour).
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