Regional Calibration of the NRCS Unit Hydrograph Peak Rate Factor for New Jersey as a Result of Hurricane Irene

Abstract

Synthetic unit hydrographs are used to predict the response of a watershed to a precipitation event. They utilize the physical characteristics of a watershed for their development. Although a variety of synthetic unit hydrograph methods are available in the profession, history shows that the Natural Resources Conservation Service’s (NRCS) Dimensionless Unit Hydrograph is the most commonly used method. Within the NRCS’s peak flow equation, the peak rate factor (PRF) is a required user input parameter; however, little guidance is given throughout the literature for selecting an appropriate value. The goal of this case study was to back-calculate site-specific PRFs for various watersheds throughout New Jersey as a result of rainfall attributed to Hurricane Irene in August 2011. A total of 26 watersheds were investigated with results showing PRFs ranging between 170 and 975. Regional trends were sought but found to be inconclusive. Additionally, statistical analysis showed no correlation between the PRF and various watershed characteristics such as longest hydraulic length, slope, area, percent impervious, among others.