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

~- -----.

URBAN FLOOD-FREQUENCY ESTIMATING
TECHNIQUES

By V.B. Sauer

INTRODUCTION

The National Flood Frequency (NFF) Program
provides equations for estimating the magnitude and
recurrence intervals for floods in urbanized areas
throughout the conterminous United States and
Hawaii. The seven-parameter nationwide equations
described in USGS Water-Supply Paper (WSP) 2207,
by Sauer and others (1983), are based on urban runoff
data from 199 basins in 56 cities and 31 States. These
equations have been thoroughly tested and proven to
give reasonable estimates for floods having recurrence
intervals between 2 and 500 years. A later study by
Sauer (1985) of urban data at 78 additional sites in the
southeastern United States verified the seven-
parameter equations as unbiased and having standard
errors equal to or better than those reported in WSP
2207.

Additional equations for some urban areas in a
few States have been included in the NFF program as
optional methods to estimate and compare urban flood
frequency. These equations were developed for local
use within their designated urban area and should not
be used for other urban areas.

Nationwide Urban Equations

The following seven-parameter equations and
definitions are excerpted from Sauer and others (1983).
The equations are based on multiple regression analy-
sis of urban flood frequency data from 199 urbanized
basins,

UQ2

= 2.35 A41 SL,I7 (RI2+3P,Q4 (ST+8r,6S
(I3_BDF).,32 IAI5 RQ2.47

standard error of estimate is 38 percent
= 2.70 A35 SVl6 (RI2+3)1.86 (ST+8r.59
(I3.BDF)',31 IAII RQ554

standard error of estimate is 37 percent

UQ5

UQIO = 2.99 A32 SVl5 (RI2+3)1.75 (ST +8r.57
(I3-BDF)-.30 JA,09 RQIO.58
standard error of estimate is 38 percent
UQ25 = 2.78 A31 SV15 (RI2+3)1.76 (ST+8r.55
(l3-BDF).,29JA.0'7 RQ25,60
standard error of estimate is 40 percent
UQ50 = 2.67 A29 SVI5 (RI2+3)1.74 (ST +8r.53
(I3.BDF).,28JA,06 RQ50,62
standard error of estimate is 42 percent
UQloo = 2.50 A29 SV15 (RI2+3)1.76 (ST+8r.52
(l3_BDF).,28JA,06 RQloo,63
standard error of estimate is 44 percent
UQ500 = 2.27 A29 SVI6 (RI2+3)1.86 (ST +8r,54
(I3_BDF).,27JA,05 RQ5oo.63
standard error of estimate is 49 percent

,

I

I

where

Q2, UQS,... UQSOO are the urban peak discharges, in
cubic feet per second (ft3ts), for the 2-, 5-, .., 500-year
recurrence intervals;
A is the contributing drainage area, in square
miles, as determined from the best available topo-
graphic maps; in urban areas, drainage systems some-
times cross topographic divides. Such drainage
changes should be accounted for when computing A;
SL is the main channel slope, in feet per
mile (ft/mi), measured between points which are 10
percent and 85 percent of the main channel length
upstream from the study site (for sites where SL is
greater than 70 ft/mi, 70 ftImi is used in the equations);
RI2 is the rainfall. in inches (in) for the 2-hour.
2-year recurrence interval. determined from U.S.
Weather Bureau (USWB) Technical Paper 40 (1961)
(eastern USA), or from NOAA Atlas 2 (Miller and
others, 1973) (western USA);
ST is basin storage, the percentage of the drain-
age basin occupied by lakes, reservoirs, swamps, and
wetlands; in-channel storage of a temporary nature,
resulting from detention ponds or roadway embank-
ments. should not be included in the computation of
ST;

8 Nationwide Summary of U.S. Geological Survey Regional Regression Equations for Estimating Magnitude end Frequency of
Floods lor Ungsgsd Sitas, 1993