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<br />ilcfnrc a decision is made as to the exact
<br />m:~gn~tudc of the in'taw flood to Lc adopted for
<br />design, a comparison should be made beuceen
<br />computed synthetic values and records of foods
<br />that have occurred in the general area of in-
<br />terest A method o: making such compari-
<br />wr.s is discussed in the following section.
<br />{6. Envelope Curves.-Peal: discharge enve-
<br />lope cones and flood volume envelope cun•cs
<br />r: r.• be prc;.arcd by drav:ing cur:cs envclopir.g
<br />plotted points representing r..aximum recorded
<br />;truce for Various drainage areas. The calucs
<br />yloucd should represent similar t} pc floor's
<br />(rain Coods or snow:melt floods) that have oc-
<br />curred ~cithin the broad geographical subdivi-
<br />sion tc:;Fin ~•:hich the subjec: watershed lies,
<br />ar.d should no: b: li.^.,ited to erects o[ a single
<br />;r,:rll ricer scstem. Preparation of er.celcpe
<br />cu n'es for a general area provides an engineer
<br />v:ith valuable information or. pas: flood histor•;
<br />and an indication o` the food of record com-
<br />paraLie to the subject area. 1ia'.cever, they'
<br />s:nould no: Le retied upon as .. means of es-
<br />timatmg max,mum p:ebab!e flood calucs. De-
<br />si°a ood values p~rpor:i~b to be the n:ax-
<br />imam probable should be highs than those ob-
<br />:ained front em'clopE cur: es. On1}' in .=pecific
<br />,r.su:noes tehcre a v:atersied has defm:c!}• low-
<br />er flood potential than neighboring v:atcrsheds
<br />due to soil type, surface storage, etc., tivould it
<br />he good judgment to adopt zn inflow design
<br />Pood of smaller magnitude than that of a fiood
<br />~ehich has occurred r.carbc.
<br />.~ sir-iple me:hod of prepara;,on of envelope
<br />curvas is to tabulate maximum peas, discharges
<br />(or volumes of a selected dur:ion) and respec-
<br />tive drainage arcs prior to plotting poins.
<br />]n most instances, the drainage area above a
<br />stream gsFirg s:at,on or the poi r.; of a large
<br />flood d,scha:ge measurement is given in the
<br />C.S. Geological Sunep n•ater supple paper
<br />listing the flood. ',']ter. it is known that only'
<br />a portion of the drainage zrca abo•:e a point of
<br />measurement conuib:aed to z fiood, the size o.*
<br />that contributing portion should be used in the
<br />enceiope r.:rve anahs~s. Doc^a:ges err cn!-
<br />umes are plowed versus respec:n•e drainage
<br />areas usir.~ log-loo paper. Data the; plotted
<br />usual!r indicate a cured lire envelopment on
<br />tog-Iog p~^,.cr which map be approximzted by
<br />V :Div,. ... ..
<br />a straight line for small rangers in areas, li,gh
<br />discharges from local thur.dc:'<torms may sug-
<br />best consideration of taco curves---erne [or
<br />smaller areas subject to such occurrences and
<br />another for larger areas o:herr ma>:imum dis-
<br />chargcs originate from ~;one:al storr^s.
<br />{7, Fslimoles of Frrq uenry o! Ouurrence o/
<br />Flood:.-~sti,-ores of the r.:a~aitude o` floods
<br />v:hich ha•:e freq::encies of 1 is ~, 1 in 1G, or 1 in
<br />°_J }'Cars a1'C hC!pful in cs;imaung rcq,:immcnts
<br />for stream diversion during construc:ior..
<br />These floods arc normally' termed tnc "5-, 10-,
<br />or 2v-year good." The r.:agr.itudc of more
<br />rare events such as the 5U- or 100-}'car P,uod
<br />ma}• be required for reasons such as to estab-
<br />lish sill location of emergency spill•.v~;s, to de-
<br />sign diversion dams, etc. The usual term of
<br />expression, =-}'car flood," s:nould no: lead ;o
<br />the wronn conclusion that t:^.e e':en! i..~:catc•d
<br />can happen onlc once in = ;:ears, a-.d :.::vin;
<br />occurred. •: ill r,o: F.cpyen pair. for ..nc;her
<br />penod of z years. 12 does mean .hat Deer a
<br />long span Of \'Ea:S vie C3C ES'~CCt c -"^~S
<br />z-year ;'Dods (or )anger) ac :^c:e are .r-pear-
<br />Ior.g periods v:i:hin gnat s-an. Fiocds occur
<br />random!}' ar.d ma_: be be.r_hec er :ire::d our
<br />uneve^.l}' v::;h resrec: to ;ir.,e. \e ~rrdic:ion=
<br />are possibic• for de:crmining their der:r:x:ion:
<br />the maximum probable flood cc,l occur :hc firs:
<br />}•ear after the project is burl:, though o: co;:rse,
<br />the odds are hcavil.: aga;ns: i:.
<br />The fregacnc}' of z food should he consid-
<br />ered as the chances o: occurrEnce of a flood of
<br />that size (or one ]arrcr) in any ore year.
<br />Stated another c:ay, the chances of :he good in
<br />any one year being equ::?ed o: exceeded by
<br />floods of t'r.e magm:udes i^.cicated as the 5-,
<br />10-, ~5-, or 100-}'ear foods have ratios of
<br />2G :100, 10:100, 4:100, 2nd 1:100, res~ee,ivelc.
<br />The hydregraph of a particular frrquenc)'
<br />flood is usuaP.y sL:e:chcd *-o co.^.ccaaoral s^aPr'
<br />using the peak disc'r,arge valve ar.d correspond-
<br />ing volume value ob~ined from computed fre-
<br />quency cun'es. ]n some instances, a peak dis-
<br />ci:zrge and associated velame o! a recorded
<br />food :oil! eorre_pond close!} w•lth a p.^.rtieu'.ar
<br />ircqucncp value, ir. tchich case the recorded
<br />fiood h;•cro,•--.^. p!: is Lsed.
<br />Zf streac,:iow data for a period of 20 )'cars
<br />or more are ac•ailable for the subjec: watershed
<br />
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