Section 7: NRCS Runoff Curve Number Methods
Anchor: #i1026683Introduction
The department has adopted the following two specific runoff determination techniques developed by the U.S. Department of Agriculture and Natural Resources Conservation Service (NRCS), formerly known as the Soil Conservation Service (SCS):
- graphical peak discharge (TR 55) procedure
- NRCS dimensionless unit hydrograph.
The procedures presented here are derived from the NRCS National Engineering Handbook, Section 4 and Hydrology for Small Urban Watersheds, TR55.
Anchor: #i1026708NRCS Runoff Curve Aspects
The techniques require basic data similar to that used in the Rational Method. However, the NRCS approach is more sophisticated in that it considers the following:
- time distribution of rainfall
- initial rainfall losses to interception and depression storage
- an infiltration rate that decreases during the course of a storm.
NRCS methods produce the direct runoff for a storm, either real or fabricated, by subtracting infiltration and other losses from the total rainfall using a method sometimes termed the Runoff Curve Number Method.
The primary input variables for the NRCS methods are as follows:
- drainage area size (A) in square miles (square kilometers)
- time of concentration (Tc) in hours
- weighted runoff curve number (RCN)
- rainfall distribution (NRCS Type II or III for Texas)
- total design rainfall (P) in inches (millimeters).
NRCS Rainfall-Runoff Equation. Equation 5-8 represents a relationship between accumulated rainfall and accumulated runoff. This was derived by NRCS from experimental plots for numerous soils and vegetative cover conditions. Data for land treatment measures, such as contouring and terracing, from experimental watersheds were included.
Equation 5-8.
where:
- R = accumulated direct runoff (in. or mm)
- P = accumulated rainfall (potential maximum runoff) (in. or mm)
- Ia = initial abstraction including surface storage, interception, and infiltration prior to runoff (in. or mm)
- S = potential maximum retention (in. or mm).
You may compute the potential maximum retention (S) using Equation 5-9:
Equation 5-9.
where: