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<br />500-Ye~r Flood Event
<br />
<br />r~te deri ved in this ~n~lys is. Tnis difference is ~ttri buted to the
<br />different methods used to derive hydrologic inform~t ion. The method
<br />utililed in this report 15 more representative for large drainage
<br />bas i ns than is the Rat i ona 1 Formul a whi,h was ut j 1 i zed in the pre~iOJs
<br />report.
<br />
<br />The SOD-year fl ood event is useful in mak i ng the pub 1 i c aw~re that
<br />floods larger than the IOO-year flood can and do occur. Just because a
<br />person is living outside the lOO-year flood boundary does not n~an that
<br />he is cornpletely safe froln flooding. The 500-year flood event can also
<br />be used for regulating high risk developo:rents within the floodpl ain
<br />such as nuclear ~ower ~l ants, or the storage or 1,ldnufacture of tox ic or
<br />explGsi~e r.uteri~ls,
<br />
<br />Another watershed ln~est 1 gat ion report wa, prepared by the Soil
<br />Conservat ion Service for ~ontrose (Reference 6). This report presented
<br />a IOO-year runoff rate approxi'n~tely four tilnes 9re~ter th~n that indi-
<br />cated by this study. Howe~er, no backup data is avallable to subst~n-
<br />tlate the large value presented, nor to e~~luate the difference between
<br />th~t study ~nd this present report.
<br />Neither of the ~bove studies were ~repared for the purl!ose of
<br />defi ning a floodp1 ain, ~nd for this re~son, a, well as those enumerated
<br />~bove, this present study supersedes the above,
<br />The Corps of Engineers i, pre~aring a floodplain study for the
<br />Uncomp~hgre Ri ~er, It wi 11 be entltled: "Flood Hazard Information
<br />Uncompahgre Ri ~er Montrose County, Colorado", This report presents the
<br />floodplai n data for two tributaries to the Uncompahgre Ri~er: Cedar
<br />Creek (and it, tri buta ry Montrose Arroyo), ~nd Ory Ced~r Creek. The
<br />Corps of Engineers' report will present the floodplain for the
<br />U nCOlnpahgre Ri ~er itse 1f. The two studi es wi 11 be compat in 1 e,
<br />Otller re 1 ated flood studies avai 1 ab 1 e i ncl ude the ~ari ous region~ 1
<br />reports prcpared for the Uncompahgre Ri~er, part icularly regarding the
<br />proposed Ridgeway Dam and Oall as Creek ~rojcct. Other reI ated studies
<br />WhlCh were eXdmined are t~bul~ted in tile bib1 iography.
<br />
<br />Table VI-2 1 ists the disch~rges and Ilater surface ele~ations for
<br />the 10-, 50-, 100-, and SOD-year flood events at various reference
<br />poi nts along the three st udy streams. The water surf dce el e~~t 1 ons
<br />shown on this Tdble represent the peak attained \/atersurface el eva-
<br />t ion. Thls pe~k elevdt10n is telllporary, and does not simultaneously
<br />occur at ~11 points upstream ~nd dO'instream.
<br />
<br />D. M~pping and Surveys
<br />Acri a 1 photogr~phy and orthophoto contour ITlilpping were pro~ i ded by
<br />the Colorado Water Con,ervdtion Board, in cooperation with the ~,S.
<br />Bure~u of Reclam~tion. and prepdre<.l by Aerial Photo Service, Inc.,
<br />Tul $~, Ok1ahor::~. The d~te of the ~erial photography is June, 1977; the
<br />mappl ny 1 s dated Dece~lber, 1 977. The contour i nter~al is 2 feet and
<br />origin~l Inapping scale was 1" ~ 200'. Addition~lly, approxim~tely 270
<br />di~it ized stream cross sections were provided by ttle Inapping contrac.
<br />tor. Mapping ~ccuracy was to the third order as defined by the U,S.
<br />OepartmentofCOfllTl('rce{Reference44).
<br />The City of 110ntrose ~nd CWCB furnished addit ianal technical su;;-
<br />port d~t~ which included contour mapping of the City prepared for the
<br />C ity b,y r~e rr i c k & Compa ny, da. ted November, 1965; z on i n 9 and 1 and use
<br />data; prior drainage reports prepared for or by the City, etc.
<br />
<br />The City of Montrose has ~ reference IIlQl\Unlellt system which is use-
<br />ful for Inaintaining consistency in survey operdtions. Monuments are
<br />~ro"ided at nUlllerous points throughout the city, the locations ~nd ele-
<br />vat ions of whiCh IndY be obtained frOln the City of ~lGntrose.
<br />Sup~lelllental field surv~ information ..as gathered by H.ydro.Triad,
<br />Ltd. st~ff on cuhert5, overflow sections, and general urban basin con~
<br />ditions (including street crowns and intersection draina~e d~ta),
<br />
<br />[, ~elJtcd Flood St~cic.
<br />
<br />T..o rel ~ted studies Ii~ve been iire;iared for the ~ontrose are~,
<br />a 1 though nei ther ;iresented the 100-ye~r floodpl ~i n, One, prepared by
<br />r~erri c~ &. CO"l'any, in 1965, and ent i t1 ed "e i ty of )~Ol\t rose, Co 1 orado
<br />Orai n~!le Stlld,y" (Reference 19), b~sed its hydro10s.y on the Rat ional
<br />fOr"lUla (speSect1on I....B fOI'definition). The 5G-yeM runoff rate
<br />presellted in that report is somel,hat greater than the lOO-yeor runoff
<br />
<br />-8-
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