FLOOD00403

• County: Statewide
• Title: Hydrology of a Highway Stream Crossing
• Date: 10/1/1979
• Prepared By: USDOT
• Floodplain - Doc Type: Educational/Technical/Reference Information

The average reCl&1~nce interval for the looo-year record vas
determined for each ot:! four selected lIdnimum peak rates and the results
were tabulated in table 1. Line (1) ot: the table lists the tour selected.
minimum peaks. Line (2) shows the IlIIIIlber ot .....,<1...... ........, peaks tb&t
were equal to or greater than each ot th.e minimum peaks. Line (3) lists
the averq;e recurrence interval which was obtained. by dividins 1000
years by the number ot peaks on line (2). The minimum peak tor &IIY
average recurrence interval is uauaJ.l,y designated. by the capital letter,
Q, with a subscript corresponding to the averq;e recurrence interval.
Thus, 'UO (13,000 c.t.s. tor the lOOO-year record) would be the miniJDwa
peak rate tor an averq;e recurrence interval ot 10 years. Minimum peak
rates are also reterred to by their correspondiD(l average recurrence interval
such as the 10 year Q and the 50 year Q or as the lO-year peak and the
50-year peak.

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Averq;e recurrence intervals ~ be expressed. as averq;e trequencies
iii th llhich max1mwn annual peak rates equal to or greater than some desig-
nated. minimum may be expected to occur in &IIY one year. AI. show on line
(4) of table 1, they may be obtained by divid1l1g one by the average
recurrence interval or, tor the lOOO-year record, they are the lUlJIlber
of peaks on line (2) divid.ea. by 1000.

Probabili ty

The foregoing discussion has demonstrated that if we had a 1000
:,ear streamflow record, we could determine averq;e recurrence intervals
or ave~'lI5e frequencies for various selected minimum peak rates of 1'\.UI01'f'.
~t us asSUIIIII that we have made such a stud;y Md lul.ve determined
values for QlO' Q:;o, Q"o, and. Q:l.25' Just hOll web 1nf'omation do
these values givellS aAd how can lie use them in the design ot our stream
cross1ng1 It we s~ that ~O is equal to 13,000 c.t.s., we meen tb&t
over a long period ot t1me, s~ 1000 years, there will occur a group ot
100 max1llIUm """".1 peaks whose min1l!lulll vahle is 13,000 c.t.s. A glance
at the a:rr&3 ot plotted. points in f'i8Ure 2 shows us wbat such a gl'OUjl
might be like. we note that there is no un1for.m:I:ty in the time ot their

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Y The tour selected peak rates (desigllll.ted. on the graph as Q10' .
Qro, Q.;o, end QJ25) were determined f'raII a frequency stud;y based on actual
streamtiow recorai. The magnitude !II.lld time ot occurrence ot the synthetic
Iila.ximwri annual peaks were then taken 110 that the observed averq;e recurrence
intervals tor the lOOO years wuld be the same as those determined from the
1'requency stud;y.

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