|
(i:.mod S:~c,es
<br />'', arab!c «:acrs?:eds, frequency- curve
<br />':eta ',ic!d ;ccPptablc insults for csti-
<br />t .:~_:-. :.r ;:cod a.^,d n:a;: be es-
<br />~p',:ai•,. ;0 :..~.i.^.:^ •. r~C lt''J{:Par ~GGd ~t'a}7 a
<br />... .... ... f:C!' 0: 0~::-... ....',h ::CCCp~a-bit CalllES.
<br />'.Ian}' :..-,::^ocs c: :'~vnd frecucnc>• do:ermina-
<br />m1= 1:::-;,1 c. ~.ra.-:"oo: data nave been pub-
<br />c ?. .::~,::;:•.; sur.•.m;,nes o` these meth-
<br />. . :I"::,_ vit}. cc......:na on factor, .:TCctine
<br />:rfr acc,;:::c.: ..::,.1 1:-i;a;ions, are coma:ned
<br />• !hc ;,:.;•P:s en;;Jrr.. "Rcrieu' of rood Fre-
<br />n„';r•: ',!r1:'.~1;..... ..~~.. ".',Ir;!:ntls of 1--10«~ t:rc-
<br />,irC~~•. :\na:;~:.~..."' lC inlc the man.: me;!:ods
<br />1L~..c`7'-;n:c:le:: dcterlninatior.s made from
<br />'.n..o•: da,.. ... - al'. b..^.sPd on acceptable
<br />~.. ::, ..! p:oc~•du:•, ~ di:"Prone^_ .., meth-
<br />'r,~„~;. c::n ~i•:e• a}';~:rr:alu':;: d:'crent results
<br />n c>::"'.sla~.s are r,-.:.dc ue:•'ond the ranee
<br />. ::C t'.a.:IlC Ua:.^.. i O i7f0'::(:c fOr 3 L'.^.::J'm:C';
<br />1'.'.I: ..., .. .. a': :'C•0:., ."CS ;flan:~. i!1 C' \~~-aIC:'
<br />.<mn~r.~ C'o:::xi! h;:> :rcar.:n',ended that all
<br />n•:~~~::.:'^~. ;rc::c`.c~ use the Lug-?car_on
<br />,~pc ]:] c: <; r:hu:ion ,.., _ base method. The
<br />r,•:=:::il •'i r', ;}:c }ra,lic;.tton, ..:\ L ni-
<br />~' ,
<br />,cnniqur ;'ur llctc:'minin Ficud Flotiv
<br />,•
<br />is
<br />c..
<br />is
<br />,;
<br />~,.
<br />n'
<br />r,.
<br />,'
<br />(~
<br />f,
<br />.,
<br />l ~!~~~;t~.~'.c.i,~...'' Il::zcc•s \ic[}7n(: rl': CS 1'CCtI!t5
<br />Il:ai :,!'i C^::1;,.C'::~:`.<' tG ;}7CSC fron] inc Lo^-
<br />?'"•t::c~:: .:';)C' i i ] ^:CL'1J~ aad 15 ca3lrr 10 llSC
<br />," •"":! ('/`^I~,'J;.7;!OIl< :: !'C n:adC by hand R'l:h Or
<br />..~;h~~:a .::!' ai~i o.` .,. chanir.a calcu',at:^•. ^-,a,
<br />.IC„•.. :\ }noccs!'.u'..I aa{:n_• for such IIaz^_n
<br />":::;,;:;::~ inner I~ I,rc>c::;cd i^ sccUOn ~9.
<br />rnr .. .., z:'si: cc;, e: bcrc runcli C':C:.^.atCS
<br />?r,~:n raft:.^,Il :::ul for v: hich s;:'can:.".o~•: data
<br />'"' I:n: ...;;ii;:Llc I:al!: Ill' small tiSalC:'shcds),
<br />.. .n,ilr;;:;p;: n( Ii0(~(i .!'C j^C.^.C'IC3 C:n :C Ob-
<br />' I o.....li runo.. ,ram pm-
<br />'' ..az,n c!:,t.: o` the desired ircqucncy.
<br />~'!'~:,:I,:c raini::!I into^,itics for short dura-
<br />nC~ c;,n Lc aLlainrd :rom Xatio;:al v\'ca:her
<br />'~r:ice pu'uliration. t.o^ bibliography, sec. GO),
<br />..C., •... , ••..,,, F.-~.,.n.. `.. ,~ ..' Fm.: u, n,.., . ~ re. See.
<br />' ' '.. ..~ . ~~~ i,~. r-~ ~ C. r. m,,.. ,~n 1.~. - . Tr.m ASCE.
<br />,, .. ... , , ~ ...., ~. , ,~....,~ .„ s.., ,,m.
<br />,... ... n .,. -, i,.•. •.n.~ ., ~ .., ~,.,~,. , , ".~., v. .~....
<br />~ i, ., r .. ., .,,., e. .... R. ...., •... \.u n.Ji, U..nro•
<br />,,.,, ,.
<br />n ., ,. ,,. ,~. ~„~~. n.
<br />45
<br />er in some instances by direct frequenc.: anal-
<br />yses o: records at nearby- precipi;~tion sta-
<br />tics. These data provide means of ob:ainir.~
<br />probable ":-pear" prccipita'--ion ca!ucs for car-
<br />ious time dura:ior.s. A storm duratio^ L'me is
<br />assumed equal to ;hat of the time of concentra-
<br />tion, i„ estimaud for the v:atershed. Tri-
<br />an~u?ar ;:;:dro~r::p•r.s represenur.~ t},e desired
<br />fee;uency flood are computed by the procP~itn'e
<br />pl'CSC;1tPd in section 53. The unccr;air.ti<~ in-
<br />herrr.t in estimating the ~^atmt of runo5
<br />n~hich ~•:il! resu!t ;turn a ;:ICC.^. amuur.t o.` nin-
<br />:all n;akP the results of ;his procedure less re-
<br />liable than that of usir.~ stream"o:e data :•: here
<br />such data arc availahle.
<br />4°. Special Coses.-The discussions in this
<br />cha!',ter have deal: primarily oath flood 9ov:s
<br />ricta:ir.p :turn runoT irons rain :aL':nP oi, an
<br />c::fro2en .:atcrshcd. T}lcm arc areas <tit'nin
<br />the L`nited State_ v,'r.cre !arse fioods ha.'e orl~-
<br />i::atcd from rapid srlov: melt on a i roz:: ~•::ac•'-
<br />shed or frorn a combir.a;ion of rain :a!li.^.~ on a
<br />snout cover over a i:'ozen v::u~•rshed. The
<br />ea; tern and ~<-estern coastal region, c` the
<br />Lr.aed S;;aes arc subj~'ct to floods caused ~•~•
<br />rain iallin~ on a sno~. cover over ~tatcrs^., •-
<br />hacin~ usually' unfrozen soil. These ty-pPS ci
<br />Cl'f^Lti rCq u;rC SpCClal S;l1dp l0 a9C'C r:a:r: 1 is
<br />amoultt of ~•:atcr that becomes ava:!abic ±or
<br />nlr.u?. Uncc that is dctcrmincd, dis:ribs;:ol:
<br />of the runo.`T b}• a ur,itoraph usua„p is feasible.
<br />Although such special cases ma}• be rare <vith:r.
<br />at: area of interest, they ma)' represe:',; the
<br />maximu:r, probable flood-prod::ci::o ce::daions.
<br />Instances In t•:hich sno•,vmclt runo.`T provides
<br />the m.^.jor portion of : m.^.~imu:n p robab:e flood
<br />usually incclvc major' su•car.:s anc larpc drain-
<br />: Fc areas. Design o: structun•s for such i;
<br />beyond the scope oC this text. Ho<•: over, prcb-
<br />Icros ma}• arise concerning shall, n:oun;ainous
<br />v:atcnc~ds uhcrc sno<•~melt runo i is a dom-
<br />inant characteristic. Ir, these areas. the floe
<br />in the stream due to sno::nnclt runofT TMav be
<br />larpP and ~hotad be acidC•d to the computed r::iu
<br />Oocd. Alpo. ~,hcr. cs;un:::u:q nmufr (rom
<br />r:ur.f;tll i.: such ana>, i; should Inc rcc•o;-nc.cd
<br />that ;ctr; of the ~•:atcshcd r:i!i br emceed by
<br />mclur.): sntrc: v:iueh is r.:U.~fplnr retention los.-
<br />cs under the snot~~-co~~crc•d :trca. The runo'
<br />from r:u:: f:,llinl; ou the sno•s-covu~ed area may'
<br />i
<br />1
<br />!'
<br />l;
<br />l
<br />i
<br />1.
<br />r
<br />t.
<br />
|