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

摘要

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 storm water infiltration in sandy clay substrate is currently limiting its use.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 unit area of pavement is approximately ten to twenty-five times greater than the runoff from a unit area of grass. In urban areas, thirty to forty percent of the rainfall runs directly into the nearest stream, hi 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. Lost ecological functions due to impervious pavements include retention and infiltration. Storm water infiltration is a critical ecological function within the hydrologic cycle intrinsic to the health, welfare and safety of our potable water supply. Infiltration reduces flooding, recharges our ground water and generates stream base flows during periods of limited precipitation. Infiltration is a quantifiable process that follows the basic laws of physics. Infiltration is not a new concept. In fact the basic principle governing flow in homogeneous porous media, Darcy's Law, was formulated in1856. The model typically used today in quantifying infiltration, the Green-Ampt equation, was formulated in 1911.Two case studies will be presented, showcasing design and implementation of two projects in Mecklenburg County. Mecklenburg County is located in the Piedmont Geographic Region and the Charlotte Belt Geologic Region of North Carolina. According to the Natural Resource Conservation Service (formally the Soil Conservation Service), 64% of Mecklenburg County's land area is composed of sandy clay soils in hydrologic group B. These soils vary in composition of clay, silt and sand that typically fall in the range of 30% clay. They have infiltration rates that vary from 1.5 cm/hr to 5 cm/hr. The soils observed on the sites of the two case studies presented here are in the Cecil and Cecil-Urban series, which compose approximately 59% of the Mecklenburg County land area.