|
<br />FREQUENCY
<br />
<br />at a ve 10<:1 ty 01 3 or "",re le",t per ~econd cou I d O~5 i Iy "'mop a~
<br />ad,,1 t p~r5on 011 ~ i s I"et, Ihus croati n<] del i ni tf) ~,'nqcr 0'
<br />i~j'.J~y or d~owninq. Rllpidlv ~isin~ dn~ swiftly flC)win~ flood.
<br />w~te~ ro2JY trap p"~sons i" 11()"n()5 thar ara ul+lm~t"ly do~troyod
<br />o~ in ~ehiclo~ that are ulti"",tely suomcnled o~ floateo, I~(')I~-
<br />tio" of a~e,,~ by Iloodwat",~~ c(')"ld cr,,~te hazards in terms of
<br />mn~ical, llrn, o~ I~w ~~Iarc"me~t eme~qe~cies.
<br />
<br />The SOO-year flood is not the largest flood that c~n
<br />OCClIr ~lIt the pra~a~illty 01 larger floods Is r"rT'IOte. As C(ln be
<br />seen Irom the ga(ling reCO~dS lor Ch"rry Cr'lek, d l~charqe'S <;I"Ia Iler
<br />t"an oither the lOO-ye~~ o~ S00-yea~ flood~ ~re much ""'~~ common,
<br />LiI~ga floods, howe~er. CM h~ppen: IhiS was cl~"rly derT'lOnslrated
<br />by the Oon~",~ i1~eil floods 01 1%5, th<< June 1972 llood at Raold
<br />CI ty, South Dakota, ~nd fhe Ju Iy 1976 flood 10 the Biq Thompson
<br />Canyon near Lov61~nd. Colo~~do.
<br />
<br />FIOO~6n areas and llood dama~os - ~ater su~f(lce
<br />
<br />Flooding can occur with relatIvely little discharqe io
<br />the event of channel blockagecausodbydobrisor Ice. Infhase
<br />situ~tions, water surface 0 lev~tions rise unti I rei lei 15 achle~ed
<br />by flo...s o~o~ the flood plain. Histodcal dah. however, innl-
<br />cate that obstructions due to ice are not likely to occur in tho
<br />study ~oach, The formation of obstructions at bridges in the
<br />study roach is, ho"evor, quite possible, ospociallydue todobr ls
<br />accumuliltion,
<br />
<br />p~ol i los lo~ the flonds stud led in fh i s ~eport w,,~e deve loped
<br />usina fhe backwilto~ como;Jtor proq~am HEC-2, .'Wafe~ Su~lace Pro-
<br />Illes", developed by the U. S. Army Coro~ of Enqinee~s. The CO"-
<br />pufilflons were bilsod on ch~nnel and flood plain conditions as
<br />r"p~esented by 5urv8Y dafa ~atht>red in Apr i I 1975 and as ~upple-
<br />."enf"d by lafe~ I ield i n~esti'l"tlnns.
<br />
<br />n"PrOfileon olates 29 through 53 shows tl,<<wafer
<br />~ur face e I o~a t Ions and Wd t,,, dODtl' of the I O-YfJdr, ~O-year, 100-
<br />
<br />y".,r, imG 5C8-yeM I loods r~lafive to fhe strOil,"bcd,
<br />
<br />!nte.ls
<br />
<br />HAZAllOS DF LARGE F'LQOOS
<br />
<br />p;,n i cu lar c,'~" and Ihrouql'OLJt thi s studv, f~e streambed i 5
<br />c>etu,JI I Y tho ch<lnnel low flow waler surlact> c lo~atlon since tho
<br />,lJrvrws '~o~e cond'lct".j hv ilnrinl phofan~"",,",,f~lc methDds. Tho
<br />wate~ su~lace elevations al t~c 108-yo~~ "nd ~OO-y~~r floods we~~
<br />;JS"O to determine flood liroitsonfl1ecrosss"ctiansilndtopo-
<br />n~,'ohlc "'''rs. Tn'" ,esul I~ ~~e s~awn an fhe Flooded Areil~, illd~"S
<br />
<br />The extenf 01 d~mage cau~d by ~ny II00d depend~ on the
<br />topography of the ~r6a flooded, depth ~nd duraTion of flood in",
<br />velocity of 110", rate of rise. developments on tho Ilood plain,
<br />amounT of debris inthellOOdwater,aod Icecooditlons, if!ce
<br />1$ a problem in that pa~ticular "rea. ^ IOD-yoar flood o~ ~
<br />
<br />~OO-yo~r flood o~
<br />
<br />"'~n~~..
<br />-"-",
<br />
<br />C~oo~ i:': t~"
<br />
<br />~~~dy ,co<;h
<br />
<br />~'ou ~ d
<br />
<br />rc :; u ~ t
<br />
<br />~ i i.. uuqi. 20.
<br />
<br />;", inJ"x iu
<br />
<br />ii,,,
<br />
<br />riuuJ"d !" "d~ "i..,i,,~
<br />
<br />;;,u"pi"i"
<br />
<br />in the inund"tlan of "nd suhse'1ucnt <:1""'''90 to prlma~! Iy agr!cul-
<br />tural prope~tles OInd i1ssocl"tod Mwelopments as w,,'1 015 ~ubl ic
<br />IIli Iltles ilnd public r""dways. OOOD 1 loodwate~ f lowinq "f hiqh
<br />v"locity and carrying floating d"brls wnuld c~eate conditions
<br />~aza~dous to pe~sons ilnd vehIcles ~ttompting to c~oss floodod
<br />areas. In gonerill, I )oodwilte~ 3 or more leet deep dnd f lowl"9
<br />
<br />.
<br />~.
<br />
<br />f(eP~(JSi.,n~" t i ~e cross snct i on S ,l~e II I us~rate.j on pi afe S~.
<br />
<br />Tn" cro5S ,-"ctians show o~olJnd "'''Viltio~s ,1C~a~;~ tho: viii lev on
<br />hot" Sid"., "I h" chen""I e."1 tho cj"nth~ o' ov,,~bank floodin'l.
<br />I{"f,,~enca points, coincidi~q with ~ho locafions 01 ~he c~oss Sec-
<br />I it>ns. "~,, SCK)wn on tl,,, Flo<H.J",j Ar""s ",nd Prof i Ie lor co~re laflan
<br />l",f~~'.'n ~he dr;'w;nns, T,,~I,,!i Dr<wid"e, :t.'la ...t r"I,,~,,~c" noint
<br />
<br />"
<br />
<br />"
<br />
|