FLOOD10216

• County: Statewide
• Basin: Statewide
• Title: Method for Estimating the Magnitude and Frequency of Floods at Ungaged Sites on Unregulated Rural Streams in Iowa
• Date: 1/1/1987
• Prepared For: State of Iowa
• Prepared By: USGS
• Floodplain - Doc Type: Educational/Technical/Reference Information

METHOD FOR ESTIMATING THE MAGNITUDE AND

FREQUENCY OF FLOODS AT UNGAGED SITES ON

UNREGULATED RURAL STREAMS IN IOWA

By Oscar G. Lara

ABSTRACT

This report provides techniques and procedures for estimating the
probable magnitude and frequency of floods at ungaged sites on Iowa
streams. Physiographic characteristics were used to define the boundaries
of five hydrologic regions. Regional regression equations that relate the
size of the drainage area to flood magnitude are defined for estimating
peak discharges having specified recurrence intervals of 2, 5, 10, 25, 50,
and 100 years. Regional regression equations are applicable to sites on
streams that have drainage areas ranging from 0.04 to 5,150 square miles
provided that the streams are not affected significantly by regulation
upstream from the sites and that the drainage areas upstream from the sites
are not mostly urban areas. Flood-frequency characteristics for the
mainstems of selected rivers are presented in graphs as a function of
drainage area.

INTRODUCTION

This report represents the fourth update in the last 33 years of the
magnitude and frequency of floods in Iowa. Flood reports were updated to
provide more dependable flood-data and more accurate and reliable methods
for estimating the magnitude and frequency of floods. These estimates are
needed to implement efficient flood-plain management strategies and for
economical design of highway structures, levees, and buildings in the flood
plain. Economic design criteria require the availability of adequate data
with long periods of record, improved analytical methods, and better
understanding of the flood hydrology of Iowa.

The first of three previous reports (Schwob, 1953) was limited to an
analysis of the magnitude and frequency of floods that was based on data
collected in Iowa before 1950. A second report (Schwob, 1963) included
updated data and a method for estimating magnitude and frequency of floods
at sites on ungaged rural streams that had drainage areas of 10 square
miles or more. The third report (Lara, 1973) was prepared using data
updated through 1972. This report also included a method for estimating
the magnitude and frequency of floods at sites on ungaged rural streams
that had drainage areas of two square miles or more.

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