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,~,5 Flooc Sr~oies 5;.
<br />•
<br />u
<br />O-•
<br />storm that produced the flood h}'drobraph un-
<br />dcr ~wd_ should be detcrmir.ed from the mass
<br />eurces for each station in and near t}le drair,-
<br />a:z area. "r or a ;food }:>'dm~raph consis:ink
<br />of :... .'c ecen;, t is ~•:ill be the total depth of
<br />prrcipi;;~;ion occu!:ir.F durir.e the storm pe-
<br />r:or.. For a compound h;:drnrraph, in «:Fich
<br />r.n!.tid::::'. ;•ortlon~ of the h::drograph arc stud-
<br />i_d : _. ...-;cl>', temporan' cessations of r.:in-
<br />;:a: ,':i:: r.: ua!i}' be ir,riicated in the mass curses,
<br />;u:ri frnnl inspection it usuall}~ ,'.'ill bz apparent
<br />c:h..:r. nl tee iacron•.zn;s e; rainfall caused ;he
<br />r:::lo.". step: a^dcr stuc.:. The approprizte
<br />depths of rainfal! arc then used to dra`.c an
<br />i_~;; ~t::' R;ap, usin;; standard procedures. A
<br />;epic:a [sehy'etal map is o-hoc:n in figure °_i(E).
<br />lip;:'c:ne c:ru;ion should be used in dras:lr.g
<br />,.~. i<oh;:e;al pa;te:'r. in inOllnta:110;15 areas
<br />a.i:~'rc :::c erograpr.lc e.`Tec: is an important
<br />`..'.tor ir. the :-rr.:! d;s;rlbuaon of rair.`a!;.
<br />i'cr csan:plc, if there is :: prec;pity;ion station
<br />.. •: aGc on ol:c side c,` a nl nll nl9ln 1'fl n;IC :L':C
<br />.~:. :l:ter .;:rtion in ~ •ra!!cy c•n ;he opposite side
<br />;tiro ra:;~c .;'r. r.o in;en'enine station, it
<br />,., .co; be ... _rncd teat :h^ rainfall durir~ a
<br />~'ui'.T. «: mild •; ar•: linca:•'. •.' uet,cccr. the t'.ao sta-
<br />_... 1: Ix ~::;cl•.' that t^e r.:infa!1 v:owd in-
<br />crz:rye s':ith increases in eleea:ioa on ;he v:iRd-
<br />lc;,rd .ids cif ;hc di•.Ids. ,chess: s on the lecv:ard
<br />•.~'.. :,",: r'[~:[;a;I01: ,1'oUld dl`C I'CaSC rapldl~ «'Ith
<br />.. ..:::cc item ;hc dnide. This trpc of dls-
<br />•.I;I~•,I:!nn can usual)>' be ccrifmd in moon:ain-
<br />'.I< :,'~ra< «~hrrc thcrr arc sufilcient prceipita-
<br />:~•~:: s;a:iur.:: to duflnc the isohyrtal patters.
<br />:•.~ c'.:r:uc~s.
<br />:1l:rr t!te pr~dimir.ar}• )1}'dropra, hs and the
<br />:•aFy~;;,! snap: i:ace beer. dra«: n, the a;, pica!
<br />I~~ ~d csrnts for u!lit hpdro~.rap'r.s dctermir.a-
<br />'"~:: :-::}' readily' is e!innr.ated. Those /?ooas
<br />,, •:r~~,~; c rornLiur.:inr; ul (r.~'nc rolumc, xuilurn:
<br />"f, :,>if~rs, isolr.(cc prrtoc's of rcirlJc'l, cr.d
<br />,•,;~,~u+ areal tilshlLu;:oe of rail+jcl! should
<br />r" ,'i,,,.•eu for frn'(lrcr.ch,cu.
<br />1:;1 :it•rrr.,rc rair,lali Ly Thiessn: pu!ir~ars.
<br />--•I''.,• ::sera; r r:,:l:iaa u:: n dralllag2 arc:: can
<br />~~ ..,;rrn::n:•r! :roc; ivccl.nu:;ion station rcc-
<br />'"''• I,;.' the T!liea<rn po!1;•on method. .y ,:rm.
<br />,.:, can:p;;;;:;ins u' ..crra~e huurlp rai::[a!:
<br />''"R~ th~~ r•:::ss c::nr~ I:: f:;;tn'c ::, (;1 ), usin^
<br />4
<br />Thiessen pol>•gons indicated in fi);uI'e 25(B),
<br />is picen in. table 3.
<br />The firs: step is to cor,s;ruct the Thiessen
<br />pol.:gons, v:hick are rite areas bounded b;: the
<br />perpznd:c:::ar bisectors of lines jcinia~ adjn-
<br />cent precipitation stations. ThC pcrcen:ape
<br />of the drainage area cor:ro::cd b;: each sta-
<br />tion's pol;;on is planirnc:ercd ar.d en:zmd In
<br />table v. .text, the acer•~gz depth of ra..:fail
<br />o,er each station'; pe!}'von is determined L_:
<br />p1ar.:mctering areas bc;o:een isoh~'e;s rn 6ccrz
<br />:~(B). A factor to be used i^. v:e:~'n,r.~ =ra-
<br />tion rainfail values is Jo;ained bs m::!;: p'. }': r.;
<br />the percentage of the dra:r,a~e area controlled
<br />b_: e:,ch s;atior.'s pel}'~o ; L>' the ratio a1 the
<br />average depth of rainfall user each station's
<br />poir;:on to the oLserccd rainfall a; the s::,tion
<br />and dicidil:g b;' 100.
<br />}ioarl}' incremental rair..`a11 value= arc deter.
<br />mined for each precipitation 5:;:;10L iri`^-, l:.c
<br />mass cones of prose ^~(:',) ; nd arc -... ;:lied
<br />bF ;hc a;,propria:c tcc';;ht f:.c:e:>.: < s;~c,:. ^ ;..
<br />U,blz 3, to oLtair. the total for the dry:n:::
<br />area.
<br />.-1CdI;lGnal 1nfO rRla t;G1: O:1 dC:C.'.T.]':::'~ a\'-
<br />CR-:,^,e :alllfa!! IS ~:Ce:1 ;1: "CGOpe1'a;l':C ~-., ..._
<br />Technlca! Yapcr \o. 1," pd:,ashed br .he \;•\
<br />liof~ai 11'eathzr Sen'ice.
<br />(.1 Drtrr;,riur.;inu u/ rr.nrlcl! c~ress.-Tutu
<br />method; r.:ac be used to dcterm:^r ralnfaa ~•~-
<br />(•ef.,': b': a?S^min}; .^- COn stool a': Cr:,;:C I'~'lCa-
<br />tion raL t!`ruu~hou: the >:orm pcrio;i. ::n;l Ls
<br />assum:nyr .- retcnlior: r:uc ca r~'ICy; ,~`.:i; tirr.r.
<br />The capacity' rate of retention decrease> pro•
<br />f;ressicclc ,?lrouphout the storm period u::;:i ,r
<br />C0:!s::.at m11L'mU^; talc 1F rC.^-C fiCC! if (1'.C salt) 15
<br />su;):crcn;!y' prolon;;ed. \t ::h dr}' ar.tccedcnt
<br />conditions, the Initial capacity' sae ,via be
<br />greater ...Rd t•:ill dca;ne is>tc:•. ECCallx :hc
<br />use of a cary'ing retention rate requlre~ a com-
<br />plicated method of compL':a::0.^., :; is e+;c:;
<br />prccrablc to assume an a•:erage rrtcntiou rate
<br />(sometime referred to as "•
<br />in.:.;rar.ol: lodes)
<br />~':it}: an e,Lr.::rtc of initial loss bony; mails if
<br />:uncccdcnt condaior.s ; rc rc!a:isr;s dec.
<br />T!;c ru;hod of do;rrn'r•nin~ the period ci
<br />rain;: it excess. «: hen an asst::^.' retention rate
<br />iv u,.rd. is :: u'cr!-:rod-error prnrcas in u'hirh a
<br />rctcnuml rate is ::>sumcd and :;:Lu-:eted iruln
<br />
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