FLOOD06431

• County: Statewide
• Basin: Statewide
• Title: Estimating Design-Flood Discharches for Streams in Iowa Using Drainage-Basin and Channel-Geometry Characteristics
• Date: 1/1/1993
• Prepared For: Iowa City
• Prepared By: USGS
• Floodplain - Doc Type: Educational/Technical/Reference Information

ungaged streams in Iowa because most such
stream sites in the State have no flood data
available, particularly sites on smaller streams.

Flood runoff is a function of many
interrelated factors that include, but are not
limited to climate, soils, land use, and the
physiography of drainage basins. Previous
investigations for Iowa (Schwob, 1953, 1966;
Lara, 1973, 1987) have been limited to the types
of basin characteristics that can be investigated
as potential explanatory val"iables for the
development of multiple-regression flood-
estimation equations because many of the
flood-runoff factors are difficult to measure.
Previous investigations defined hydrologic
regions to account for factors affecting flood
runoff that were difficult to measure directly.
The hydrologic regions were delineated on the
basis of physiographic differences of broad
geographic landform regions. However, two
major limitations are encountered when using
the hydrologic-region method to estimate flood
discharges for ungaged sites. First, it is difficult
to weight flood estimates for drainage basins
located in more than one hydrologic region or
located near the boundaries of hydrologic
regions because the boundaries are not well
defined. Regional boundaries are tran6itional
zones where the physiographic characteristics of
one hydrologic region gradua.lly merge into
another. Second, because large hydrologic
regions may contain drainage basins with
physiographies that are anomalous to the region
in which they are located, it is difficult to
correlate their physiographic differences to
another hydrologic region, or to weight their
flood estimates. Quantitative measurements of
basin morphology to determine appropriate
regional equations for drainage basins are not
applicable for resolving the6e regional
limitations. As a result, flood estimates for some
ungaged sites become very subj.ective.

To address the need to minimize the
subjectivity encountered in applying regional
flood-estimation methods, a study using two
different flood-estimation methods was made by
the U.S. Geological Survey in cooperation with
the Iowa Highway Research Board and the
Highway Division of the Iowa Department of
Transportation. Two new flood-estimation
methods for Iowa, which are presumed to be
independent from each other, were used in this

study. An advantage in developing flood-
frequency equations using two independent
flood-estimation methods is that each method
can be used to verify the results of the other, and
the estimates obtained from each method can be
used to calculate a weighted average.

Methods are now available to more easily
quantify 6elected morphologic and climatic
characteristics for a large number of drainage
basins. A geographic-information-6ystem (GIS)
procedure developed by the U.S. Geological
Survey uses topographic maps and digital
cartographic data to quantify several basin
characteristics that typically were not
quantified previously. This GIS procedure
expand6 the capability to relate drainage-basin
characteristics to the magnitude and frequency
of floods for stream sites in Iowa and provides a
flood-estimation method that is independent of
hydrologic regionalization.

Measurements of channel-geometry
characteristics have been used to estimate the
magnitude and frequency of floods in
investigations conducted by Fields (1975),
Webber and Roberts (1981), Parrett and others
(1987), Hedman and Kastner (1977), and
Osterkamp and Hedman (1982). These
investigations have shown that measurements
of specific channel-geometry characteristics
provide a reliable method (or estimating flood
discharges because channel cross-sectional
characteristics are assumed to be a function of
flow volume and sediment-load transport
(Pickup and Rieger, 1979, p. 41; Osterkamp,
1979, p. 2).

Purpose and Scope

The purpose of this report is to: (1) define
equations for Iowa that relate measurable
drainage-basin characteristics to design-flood
discharges having recurrence intervals of 2, 5,
10, 25, 50, and 100 years that are independent
of hydrologic regionalization; (2) define
corroborative equations for Iowa that relate
channel-geometry ch8racteristics to the same
design-flood recurrence intervals; and (3) define
application and reliability of drainage-basin and
channel-geometry flood-estimation methods.

Both the drainage-basin and channel-
geometry flood-estimation methods described in

2 ESTIMATING DESIGN-FLOOD DISCHo\RGES FOR STREAMS IN IOWA