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<br />2-04
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<br />of the project. other canmon uses of frequency curves include the
<br />determination of reservoir stage frequency for,.real estat~acqu1s1-,
<br />tion ll.Dd reservoir-use purposes, the selection of ra1l'1f'all frequency
<br />for storm-drain design" and the selection of runoff frequencies ,for
<br />interior drainage, pu:m;ping plant, and local-protectionproj~t. ~0s1gn.
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<br />',', ,fr...
<br />!:,. The basic frequency clln'e used 1nh;ydrologiceug1neering ,
<br />is the frequency Ctlr'le afannual m~...illlnm or annual m1n1mume~nts 'c':' '.
<br />,A second curve, the partial-duration curve,. represents tQ.~ frequency".-
<br />of all events above a given base value, rega.rdJ.ess of whether :two e,.
<br />or more occurred in the same year. Either curvemus'tbe sUPPleD!ented,
<br />by considerations of seasonal effects and other factors :1,0. appli.ca... ,.
<br />tion, as exp1s.ined in reference..5. When both the frequep.cy.,clWVe. of.,
<br />annual floods and the partial-duration curve are prepared, cs.re must
<br />be exercised to assureths.t the two s.re ,consistent.,. Normal re1s.tion-
<br />ships between the two s.re given in pa.rs.graph4-oJi..
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<br />. ,
<br />d. In almost all locations there are seasons during .which ..
<br />stormS or floods do nat occur Or. are ,not severe,. and .otherseasons ,~,...
<br />when they s.re more severe. Also, ds.mages associated withaflood"
<br />often vary with season. of the .year, among other. factors.. In ~ ..,
<br />types of studies, the seasonal '\'IU'1.at1cn factor .1s of primary,impor-.
<br />tance, and it becomes necessary to establish frequency curves,for,
<br />each lDOo.th or ather subdivision of the year..For eJaIII!Ple, one .1;re-:-
<br />quency curve might represent the largest floods.:that occur l'lach. . .<
<br />January, a second one would represent the largest flood,s the<\; occur ,
<br />each February, etc. In another case, one frequency CurTe might repre-
<br />sent floods during the snowmelt season, while. asecondmigpt represen.t
<br />floods during the rain season. Occasionally, when seasons s.re studied
<br />separately, an annual.,.event cury'e covering all seasons 1s 8J.so .pre-
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<br />, . ... ,"
<br />sonal curves are ,consistent with the annual curve {reference 19)". ..
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<br />. , .' .." '.' ,~
<br />~. In connection with power s,tudies forrun.,.of-r1verplants
<br />particularly, and in some phases of ,sediment studies, the flow., , ..
<br />duration curve serves a useful ptU"llOse. .. It s;lll\PlYrepresents.. the .. .~
<br />percent of time during which specified flow .ratess.re .exceededa~~ .
<br />a given location. Ordinarily, variations wi thin; :periods lell,S. ,than . _
<br />one ds;y' s.re o.ot of consequence, and the curves s.re therefore based.
<br />on observed mean-da1ly flows. For the purpose servlro. byflow-d~., .
<br />t1on-curves, the extreme rates of flow are not 1mportant, and cqnse-: .,
<br />quently there is no need for, ref'i.n1ng the curre1n regions of .ll.1sh. "
<br />flow. The procedure. ordinarily used in thep-reparat1on, of a. :Uow.. ;,i ..
<br />duration curve consists of counti.ng the number of mean'"\i&1ly.J',l<;lW8... .
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<br />., 6-
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