An Explanation of “TR-20 Computer Program for Project Formulation Hydrology” Soil Conservation Service Feb. 1992 Abstract
Background: The National Resources Conservation Service developed the computer program TR-20 in 1964. The purpose of this computer program was to calculate the peak flow rate (cfs) of defined watersheds. Unfortunately, there is very little documentation available to explain the exact procedure this program uses to calculate the peak flow rate. Findings: This paper outlines a method of calculation that is easy to follow and capable of being input into spreadsheet format. The method is based on the triangular method presented in the National Engineering Handbook, Part 630 Hydrology chapter 16. Like TR-20, the method presented here utilizes the curvilinear dimensionless hydrograph. It addresses the incremental flow time, the time to peak, the NRCS type II storm distribution, the NRCS standard dimensionless unit hydrograph, the design storm, the design hydrograph, the convolusion process and the Gregory-Newton Forward Method for calculating the peak flow rate.
Conclusions: The step-by-step process presented here is an effective method for calculating the peak flow rate from defined watersheds. It also provides clear documentation of the results. Results that can be easily reproduced and verified. After reviewing the material, engineers should have a better understanding of the TR-20 method for calculating the peak flow rate from defined watersheds. This paper is a culmination of six-years of study to document the method of calculation in a clear manner that can be easily reproduced. Ken D. Timpson, PE October 2023
