Estimates of Precipitation IDF Curves and Design Discharges for Road-Crossing Drainage Structures: Case Study in Four Small Forested Watersheds in the Southeastern US

Abstract

We compared precipitation intensity-duration-frequency (PIDF) curves developed for four small forested watersheds to spatially interpolated estimates from the National Oceanic and Atmospheric Administration’s (NOAA) Atlas-14. We also evaluated the Rational Method (RM) using on-site PIDFs and USGS Regional Regression Equations by comparing their estimated design discharges with a given exceedance probability p (Qp) to values computed from on-site data fitted to the Log-Pearson (LPIII) distribution. Overall, NOAA’s PIDF estimates were not substantially different from the on-site PIDFs. The 25-year and larger Qp by the RM were in closer alignment with LPIII estimates in the smaller watersheds, whereas Qp by the USGS were a better fit for the larger ones in most cases. Adapting return period-dependent runoff coefficient improved estimates by the RM in the large lowland watershed, but not in the other smaller high-relief watersheds. We recommend RM with 1-h duration NOAA-PIDF for designing road drainage structures in small and possibly the USGS method for large forested watersheds. However, future studies should focus on validation in watersheds of different sizes and topography.