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

3. Appropriate equations from table 2 are:

Q2 211AO.62 - l,470 cubic feet per second,
Q5 _ 502AO.60 _ 3,290 cubic feet per second, .
010 - 757AO.60 _ 4,970 cubic feet per second,

Q25 _ 1140AO.57 - 6,810 cubic feet per second,
Q50 - l500A 0.60 - 9,840 cubic feet per second, and
Q100 - l880AO.60 _ 12,300 cubic feet per second.

4. On log-probability paper, plot 0 versus its recurrenee
interval, t, and fit a smooth cu?ve through the points, as shown
in figure 9. From this graph, the recurrence interval of
10,500 cubic feet per second is 65 years. In terms of
probability, the chance of exceedence of this peak in a given
year is 1 in 65 or 4.5 percent.

Example 3. What is the probability that the 10,500 cubic feet per
second of E!xample 2 will be exceeded in a 10-year period?

The probability of a flood being exceeded in n years is given by:

P ..l-(l-l/t)n
n

where :~ probability;
t - recurrence interval of flood, in years; and
n - time, in years.

.1

Substituting for t and n, from example 2, in equation 1:

P10 - l_(1_1/65)10 - 1_(0.985)10 or
P10 - 1-0.856 - 0.144.

Therefore, the probability of the 10,500 cubic feet per second being
exceeded one or more times in the lO-year period is .14. ThE!
probabilit:y of not having a flood of that magnitude in the lO-year
period is .86.

ExamplE! 4. Estimate the 50- and lOa-year floods for the Cedar River
near the eenter of sec. 10, T. 85 N., R. 9 W., at the crossing of
county road M bridge in Benton County.

Solution:

l. The drainage area of the Cedar River at this point is 6,135
square miles (Larimer, 1957).

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