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<br />Fiocd Ludes
<br />:nc pro;cdurc applicable to prcbablc maximum
<br />t'cncral storm iollo~cs:
<br />1) :, i,~hocr po!nt rain`all •:a!ue is obtained
<br />. i ire y:cn, r.t; ! ical locatia: iron an appro-
<br />pr::nc ch:u't.
<br />~ '1 li:• mc;,::- of grap):_, Ibis point rainfa!1
<br />~: ia" i ad}c>tr_' to represent Ghot:r accra~e
<br />1'rec;;,:::,tion oc: r the su!lject drainage area
<br />..::c :::<o ar'.uered to give the cumulative rair.-
<br />',::i; i.a~ lon;;cr durations up [o ;S hours.
<br />t3) Ci:ars ar.d tables arc used to determine
<br />t::-e ::r.•.c•-;.'is;ribction o: the raii;ial!.
<br />(,) Soil and voce: data are trarslated by
<br />m~; ns of taa.E; Into an appro;~riatc runoff
<br />c::r•:~ numi;er i~\) from v:l!ich the runoff
<br />c~,::;n-,E .~ determined for ir.crtments of time
<br />IBS ,,,:=cd or. the cor.ccpt of rcpresentin~
<br />'r.; drop:'a;,hs as triangles, a triangular hydre-
<br />:,;~h`cr 1 inc:, o` n,r.cff (unitgraph) from
<br />,:^.r sui;!cc: v: atersbed is computed from the
<br />„•~~sir.grelauorships:
<br />
<br />
<br />-1~{.~Q
<br />Qo= 7.- 13)
<br />~•: itcrc
<br />T -time to peak, hours,
<br />Dcrair.*::!: Exce~'s period, hours,
<br />T~~time o` concentra:ion in hours, de`ned
<br />as t: ace'. time of water from h.;-
<br />draclica!!.: most list: n: , Dint in the
<br />'rater>hec. to the poir.: of irteres:.
<br />T;r,?me length of base of h}•d:•egraph,
<br />h•ot:-:,
<br />--^~•:::; r.is^_harge ir. second-feet,
<br />Q=`:o!::mc o` nlno`~, i^ inches, and
<br />:1- arcs of •.:atcrs}.c•d in square miles.
<br />Lr~c!o;,^•cn: of :he a`•o~c equations is pre-
<br />(Gt T'::e -rian;.::!ar h;drograp:^ for each in-
<br />.r.... .7, is ccmpu:cd b~ direct ratio of ;he
<br />rcno:` [,~r that in:er•:al to 1 :rich o: runo:" end
<br />.:o:-'d.
<br />(~ 1 ; hE total rur.o~ !':;: dreg rapb is obtained
<br />b.: ~•r:.ph:cal addition of rite incmmcr.:a! h~dro-
<br />4J
<br />The above procedure is also used for cu:n;wt-
<br />ing inilutc desi;;n floods from thwldcrstorm
<br />rainf::'.I ura o.' the ]0;,' r.:cridi:u:, a, discusrod
<br />;a!cr in part P. I3 u~•: ever, a >h,u ;rr time in-
<br />tcrral is cscd fur the tituudersturm unitt•raph
<br />as deli,; n thur.dcrsto: m; preser.usl in tins text
<br />arc o: ]- or 3-};our duration.
<br />T'r.is procedure might be termer: a "; en•
<br />cra!izcd" unitgraph approach. ~.n en;:ince:
<br />using the a'uove equations should ,:.kc rorni-
<br />~ance th:a the relationships of time-to-pc:.~: ar.:!
<br />base-time expressed b the cqua:ion< rcpre-
<br />Sen: at'c.'agC 'Ca!11C5 fGr these 1'e!ni10.^.ala ~, ..
<br />adopted irom nurlcrous anal;:.cc of r._tura!
<br />hood cents made b•: Soi; Consercauon Scr~icc
<br />h_: drolo~ists.
<br />]f rollabic• s:reamfloc: hpdro;:raphs arc
<br />acailai,!c, anal.:ses can be made to d~tc:r.inc•
<br />relationshi;s expressed b_; the abo•:c c:;c:.:inns
<br />that are at,;tiic~b!e tG tnC' $ll UJ I'C: : ate.'S!: 'd.
<br />anG t!iCSe 5..^GUIc be Used. FOC l;n_:: ,:'e(! t•:a:C.'-
<br />shcds, the abo•:e equations shcuid gi':e ;.sai,ie
<br />values fcr rain floo:is i.` the r:a:er;`.ed h::< no
<br />unus;lal runo!f ch :r::CIB.'1`UCS. l.i a:c="r.;: n'.
<br />i '•^ n}'d l'og la~ln ac tr:a^.g lllar ICi;(:::! Of a ;:::-
<br />L•Iai Cllr`: 1!!:1G:r shape h•d^CCS CGm 7U!:a 10.^.$,
<br />The error in shape t:^•ue in:roducc•d r~sula in a
<br />sLgh;lp more severe hyd ro~raph it :!:at a :ri-
<br />angle distributes a given amoan: 01 :uae:? ir. a
<br />s};o::er tme in;en'a! than does a ccrtil::;rsr
<br />h}'dre•graph. (See P.g. ~^(.~).)
<br /><5. InOow Design Flood.-As stated in Section
<br />35, the inPo~c desigr. 'and is usual;p egnn'a!ent
<br />to the maxi.r.um probable Hood, 1::: in cer::,i::
<br />instances, as disrassed in c!:.^.pter il. a ".Dori
<br />ci sm:aler magnitude tha.^.:`:e ^--~'~m;:m p:-ob-
<br />able ma}' be sc'.cctcd for cse as ar. in:;o:c de-
<br />sign !loud. Fioed; smal!cr ;?tan the max:n-._m
<br />probable can be computrd h}' using drs:gn pre-
<br />cipitation c.^.lues Ics; t`.::n the protiaL!~ ^t;lv-
<br />imum ~cithou: changing :cno:f iactcrs, or 'u•:
<br />using lcsrr design prc::pi:at:o:: ca:;:es ;:l :,~:',-
<br />iunc::on •.ci:h •r::rylr.g dcenE< of ra!tof^,' po;":'~-
<br />..a! as dctcrmirc•' `rem ...:teccde^ mois::;rc
<br />cund::ions l-~.:,'C). .... t'r.r.,c ~.::cccd~•r.t
<br />n:o.,:;:r~ conc..iorc, -z':C-;. .1".C-I1. _::c
<br />:1.`•fC-III used to est:mat_ r::r.o.'.' are ce,c*ic_d
<br />in ::,t:endix ~. r~'rC,d.::::CS for ccml,:::icg
<br />^oeds less than the ...ax:mum rro~•able arc
<br />presented in section SS.
<br />
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