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

.

The flood-frequency equations presented in this report may be used to
estimate the magnitude and frequency of floods in most unregulated rural
streams in Iowa. These equations have been defined for streams with
drainage areas ranging from 0.7 to 374 square miles in hydrologic region l,
from 0.08 to l,670 square miles in hydrologic region 2, from 0.04 to 5,146
square miles in hydrologic region 3, from 7.9 to 3,440 square miles in
hydrologic region 4, and from 45 to 2,256 square miles in hydrologic region
5. Techniques to estimate flood discharges on the Mississippi and Missouri
rivers are descri.bed in reports by Patterson and Gamble (1968), Patterson
(1966), and Matthai(1968).

To estimate the magnitude and frequency of a flood at an ungaged site
proceed as follows:

1. Determine the size of the drainage area upstream from the site, in
square miles. Drainage areas larger than 5 square miles are listed in
Larimer (l957).

2. Determine the hydrologic region in which the site is located using
figure 1. If lOhe site is on the mains tern of the following rivers,
obtain the flood discharge directly from figures 2 to 8: Turkey River
downstream from Elkader at river mile 38 (site 27); Wapsipinicon River
downstream from Independence at river mile 145 (site 54); Cedar River
downstream from Waterloo at river mile 190 (site 116); Skunk River
downstream from Coppock at river mile 68 (site l40); Little Sioux River
downstream from Gillett Grove at river mile 124 (site 206); West
Nishnabotna River downstream from Hancock at river mile 75 (site 228);
East Nishnabotna River downstream from Atlantic at river mile 85 (site
236).

-I

3. Select the appropriate set of equations from table 2 and compute the
flood discharges.

Commonly the project site will be near a border of a hydrologic
region. Such borders are not defined lines but transition zones where the
physiographic characteristics of one hydrologic region gradually merge into
the next. Within these transition zones, the selection of the appropriate
set of equations from table 2 is a matter of judgement. If part of the
stream begins in or flows across another hydrologic region, there may be
the need to use equations from both regions and estimate a weighted average
based on drainage-area ratios. Many examples of stations located in
interregional transition zones are listed in table 1.

l5

.'
.
.",
:"':-".
v