Mathematical Formulations of NRCS 24-Hour Design Storms

Abstract

Four standard design storms (called Types 1, 1A, 2, and 3) developed by the National Resource Conservation Service (NRCS) are used widely throughout the United States for hydrological design of flood control structures and storm-water best management practices. The 24-h rainfall patterns are intended for use in catchments with areas of 250 km2 or less, and they are considered applicable to storms of any average recurrence interval. Presented as ratios of accumulated rainfall depth to the total 24-h rainfall depth at 6 min intervals, interpolation of the tabulated values usually is needed to obtain rainfall intensities at desired storm times. A mathematical formulation of rainfall intensity consisting of two exponential terms and a constant term is fit to each of the four distributions, and optimal values of the model coefficients are found. The formula reproduces closely all of the four NRCS 24-h design storms, and can be used to extract shorter duration subsets of the 24-h rainfall patterns in an uncompl...