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

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Limits of Application

The regional flood-frequency equations in this report have been
derived from data on unregulated rural streams. Therefore, they are not
applicable to regulated streams, such as the Des Moines River downstream
from Saylorville Lake, the Iowa River downstream from Coralville Lake, or
other streams where changes by people have appreciably altered the flow
regime. Furthermore they are not applicable on sections of mainstems where
the magnitudes of floods are attenuated due to drainage basin geometry or
channel and valley storage,

Illustrative Examples

Example 1.-- Estimate the 50 and 100-year floods for Dry Creek at the
road crossing in see, l, T. 84 N., R. 8 W., in Linn County.

Solution:
1. Drainage area (A) is 11.4 square miles (Larimer 1957, p.258).
2. Drainage area is located in hydrologic region 3 (fig. 1).
3. From table 2, the applicable equations are:

695AO.54 and

Q50
QlOO

85lAO.53.
,

substituting 11.4 square miles for A,

Q50
QlOO

2,590 cubic feet per second, and

3,090 cubic feet per second.

Example 2.-- Of interest to many users is the recurrence interval of
previous floods of known magnitude. For instance, determine the
recurrence interval of a peak discharge equal to 10,500 cubic feet per
second on Potato Creek at a road crossing near the north county line,
in Pottawattamie County.

Solution:
l. Drainage area (A) is 23.0 square miles (Larimer, 1957, p.340).

2. Drainage area is located in hydrologic region 1 (fig. 1).

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