FLOOD08020

• County: Statewide
• Basin: Statewide
• Title: Techniques for Estimating the Magnitude and Frequency of Floods in Minnesota
• Date: 1/1/1988
• Prepared For: USDOT
• Prepared By: USGS
• Floodplain - Doc Type: Educational/Technical/Reference Information

manmade regulation also were excluded_ Application of these criteria resulted
in the selection of 246 gaging stations in Minnesota (57 continuous record,
140 crest stage, and 49 combination sites).

Purpose and Scope

The purposes of this report are to (1) describe the'analytical techniques
used for annual series flood-frequency computations, regionalization, and
development of estimating equations for small watersheds, (2) present flood-
frequency data at gaged sites, and (3) develop equations for es~imating flood-
flows and present examples of flood-flow estimations at gaged and ungaged
sites on unregulated streams_

This report supersedes previous reports by Prior (1949), Prior and Hess
(1961), Wiitala (1965), Patterson and Gamble (1968), and Guetzkow (1977); all
of which dealt with techniques for estimating flood magnitudes in Minnesota.

ANALYTICAL TECHNIQUES

Flood-FreQuencv Ana1vsis of Ga2in2 Stations

An annual series peak-flood-frequency analysis at each gaging station was
prepared according to the procedures outlined in Bulletin 17B [U. S. Water
Resources Council, 1981b.] Federal agencies are requested to use these
guidelines for all analysis of flood frequencies of unregulated streams. The
equation for fitting the log-Pearson Type III frequency-distribution function
to the T-year recurrence interval is defined below:

Log QT - M + KS

where:

QT is the peak discharge for T-year recurrence interval,

M is the mean of the logarithms of annual peaks,

K is a factor dependent on T and the coefficient of skewness

available from Bulletin 17B, appendix 3, and

S is the standard deviation of the logarithms of annual peaks.

An example plot from the Log-Pearson analysis of the Redwood River near
Marshall, Minnesota, (station number 05315000) is shown on figure 1.

Recorded flood peaks at some stations with less than 25 years of record
exceed the high-outlier criterion defined in Bulletin 17B. Stations in an
area in which a well-documented historic flood occurred were considered for
adjustment in the peak-flow analysis. A regional analysis of the occurrence
of the historic floods was used to evaluate whether a peak of record also
should be included in the historic period_ Criteria in Bulletin 17B for
identifying low and high outliers were used in all cases.

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