FLOOD07843

• County: Statewide
• Basin: Statewide
• Title: Nationwide Summary of US Geological Survey Regional Regression Equations for Estimating Magnitude and Frequency of Floods or Ungaged Ssites 1993
• Date: 1/1/1993
• Prepared By: USGS
• Floodplain - Doc Type: Educational/Technical/Reference Information

Nationwide Summary of U.S. Geological Survey
Regional Regression Equations for Estimating
Magnitude and Frequency of Floods for
Ungaged Sites, 1993

Compiled by M.E. Jennings, W.O. Thomas, Jr., and H.C. Riggs

Abstract

For many years, the U.S. Geological Survey
(USGS) has been involved in the development of
regional regression equations for estimating flood
magnitude and frequency at ungaged sites. These
regression equations are used to transfer flood
characteristics from gaged to ungaged sites
through the use of watershed and climatic charac-
teristics as explanatory or predictor variables.
Generally these equations have been developed on
a statewide or metropolitan area basis as part of
cooperative study programs with specific State
Departments of Transportation or specific cities.

The USGS, in cooperation with the Federal
Highway Administration and the Federal Emer-
gency Management Agency, has compiled all the
current (as of September 1993) statewide and met-
ropolitan area regression equations into a micro-
computer program titled the National Flood
Frequency Program. This program includes
regression equations for estimating flood-peak dis-
charges and techniques for estimating a typical
flood hydrograph for a given recurrence interval
peak discharge for unregulated rural and urban
watersheds. These techniques should be useful to
engineers and hydrologists for planning and
design applications. This report summarizes the
statewide regression equations for rural water-
sheds in each State, summarizes the applicable
metropolitan area or statewide regression equa-
tions for urban watersheds, describes the National
Flood Frequency Program for making these com-
putations, and provides much of the reference

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information on the extrapolation variables needed
to run the program.

INTRODUCTION

By W.O, Thomas, Jr., and M.E. Jennings

Estimates of the magnitude and frequency of
flood-peak discharges and flood hydrographs are used
for a variety of purposes, such as the design of bridges
and culverts, flood-control structures, and flood-plain
management. These estimates are often needed at
ungaged sites where no observed flood data are avail-
able for frequency analysis. Basically, two approaches
are used for estimating the frequency of flood-peak dis-
charges and flood hydrographs at ungaged sites-those
methods based on the statistical (regression) analysis of
data collected at gaging stations and those methods
based on rainfall characteristics and a deterministic
watershed model that uses equations and algorithms to
convert rainfall excess to flood runoff. This report
describes a microcomputer program, the National
Flood Frequency (NFF) Program, that provides esti-
mates of flood frequency based on the statistical
approach. A disk of the program is included at the back
of this report.

Support and justification for the applicability of
regression equations developed by the USGS for esti-
mating flood-peak discharges for rural watersheds is
given by the U.S. Water Resources Council (1981) and
by Newton and Herrin (1982). These reports summa-
rize a test of nine different procedures, statistical and
deterministic, for estimating flood-peak discharges for
rural watersheds. The results of this test indicate that
USGS-developed regression equations are unbiased,
reproducible, and easy to apply.

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