FLOOD08849

• County: Statewide
• Basin: Statewide
• Title: Techniques of Water-Resources Investigations of the US Geological Survey Frequency Curves
• Date: 1/3/1997
• Prepared By: USGS
• Floodplain - Doc Type: Educational/Technical/Reference Information

14

TECHNIQUES OF WATER-RESOURCES INVESTIGATIONS

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RECURRENCE INTERVAL, IN YEARS

50

1.1 1.2

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30

Figure 9.-Fre:que:ncy curve: of annual minimum flows and plot showing serial correlation, South Fork Obion River near
Greenfield, Tenn.

Tenn., in figure 9. Also in figure 9 is shown
the first-order serial correlation between the
annual minimums. The existence of this serial
correlation should warn the analyst that inter-
pretations of the frequency curve in the usual
way will be subject to more than the usual
uncertainty.
The frequency distribution of daily mean dis-
charges of a stream, plotted on a log normal
graph, is called a duration curve (Searcy 1959).
Daily mean discharges are not only serially
correlated, they are nonhomogeneous because
of the yearly cycle in streamflow which pro-
duces different means and ranges of discharge
at different times of the year. Thus, the dura-
tion curve cannot be interpreted as a proba-
bility curve, but it is useful as a description
of the distribution of daily means that has
occurred and may be expected to recur over
a period of several years. The duration curve
also has other uses because its shape and posi-
tion are defined by basin characteristics.
Although the annual flood series is commonly
used for frequency analysis, the partial-duration,

series is also used. This series is made up of all
floods above an arbitrary base regardless of
their time sequence. Such a series is not a true
statistical series and cannot be treated rigor-
ously. Use of the term "recurrence interval"
with the partial-duration series introduces dif-
ficulties if the series includes more floods than
years. The partial-duration flood-frequency
curve is useful in studying the frequency
of inundation. Dalrymple (1960) describes cur-
rent practice. Riggs and Thomas (1965) discuss
the interpretation and use of the method.
Frequency curves of mean discharge for
periods longer than 1 year have been used
in storage analyses (Stall and Neill, 1961;
Stall, 1964). Such curves must be interpreted
carefully; the recurrence interval in years of
an event that extends over a period of more
than 1 year is hard to visualize except for the
No.1 item in the array.
The foregoing special frequency curves pro-
vide some information on probability of ex-
ceedence of an individual event, although this
probability may not be estimated directly or

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100

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