New Jersey Department of Transportation (NJDOT) is required to quantify and mitigate the stormwater impacts of certain roadway projects. Acceptable runoff calculation methods include the Natural Resources Conservation Service Method and the Rational Method. Existing coefficients are often insufficient for representing land treatments utilized in roadway design but have never been investigated before. The objective of this project was to develop new curve numbers (CNs) for four land treatments, which included: (1) bare soil, (2) gravel, (3) vegetation, and (4) porous hot mix asphalt (HMA). To achieve this objective, laboratory studies were conducted to measure rainfall, runoff, and infiltration for these four land treatments. Each land treatment was tested as a composite column, where the treatment was installed on top of subsoil. The subsoils utilized in this project had ten different hydraulic conductivities, covering all four Hydrologic Soil Groups, A, B, C and D, that may be encountered at NJDOT roadway projects. The rainfall and runoff data were collected and analyzed to quantify CNs for the four land treatments under laboratory conditions. Laboratory derived CNs were then applied to the field conditions and compared with the established CNs of corresponding land treatments. CNs for bare soil and vegetation agreed well with the existing values, CNs of gravel were significantly smaller than the existing values, and CNs of porous HMA were not established prior to this project and were not available for comparison. The CNs developed from this project can be used to quantify runoff from these four land treatments for any rainfall events.NJDOT Research
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