Storm Water Infiltration in Clay Soils: A Case Study of Storm Water Retention and Infiltration Techniques in the North Carolina Piedmont

Abstract

As developers and regulators alike struggle with increasing pressure to protect our streams and water quality, the perceived lack of sufficient data supporting the merits of the concept of storm water infiltration in sandy clay substrate is currently limiting its use. According to the nonprofit Center for Watershed Protection, as much as sixty-five percent of the total impervious cover over America’s landscape consists of streets, parking lots, and driveways. The estimated growth of paved areas nationally is 250 square miles per year. Runoff from an acre of pavement is approximately ten to twenty-five times greater than the runoff from an acre of grass. In urban areas, thirty to forty percent of the rainfall runs directly into the nearest stream. In heavily urbanized areas, such as central business districts, precipitation run-off can be more than fifty percent. Compare this to the amount of runoff from woodlands, which is often less than five percent. Mitigating environmental damage caused by impervious pavements can be successful and economical. The function that is lost when impervious pavement is installed is the natural process of retention and infiltration. This loss causes devastating effects on the landscape. Targeting the composition of the pavements themselves to mitigate this function makes the most environmental sense, and in many cases also the most economic sense. The degree of retention and infiltration that naturally existed prior to the installation of pavements can be matched, and in many cases easily exceeded, utilizing the readily-available tested technology of porous pavements. When, for various reasons, impervious pavements must be utilized, careful design of other infiltration techniques can mitigate the lost retention and infiltration function. Each infiltration technique, including bio-retention, infiltration basins/swales and porous pavements, has its own unique advantages and caveats.