NRCS Overland Flow Travel Time Calculation

Abstract

Overland flow travel time given by the formula developed by the Natural Resources Conservation Service (NRCS) on the basis of kinematic wave theory can underpredict for short, steep, flow planes having low flow resistance and high degrees of imperviousness. Underestimation is caused by inaccurate representation of average rainfall intensity for durations shorter than approximately 15 min. Although the formula was developed based on only NRCS 24-h Type 2 and Type 3 rainfall distributions, it is used without restriction in regions that experience NRCS Type 1 and Type 1A storms. An alternate overland flow travel time formula based on rainfall intensity-duration relations developed previously by the writer to represent accurately high-intensity portions of each NRCS storm type is presented.