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<br /> PUK FLOWS Of HISTORICAL flOODS
<br /> Gunnison Ri.....
<br />~ Ne.r Grand Junction G.~e D.le Nu,Laze"C.l\e Date
<br />"" 27,400 )un9
<br />1920 35700 May 23
<br />1921 30,100 /unlS
<br />1941 27,500 Mily14
<br />1957 27,800 lun6
<br />1969 12,000 Apr24 10,100 Apr24
<br />1970 11,500 )un29 11,000 Junl9
<br />1973 12,000 May19 9,600 JunlS
<br /> Uncompahgre River
<br />~ Colan. GaRe D.t.. Delta GaRe Date
<br />1921 5,140' lun'13-H ,
<br />1927 3,400 lun26 '^"" Jun29
<br />1938 3,390 Jun22 ,
<br />1941 3,730 MayS
<br />1944 3,340 May16
<br />"" , 3,500 OCl15
<br />1957 '.300 Juo29 ,
<br />1975 3,3&0 Jul4 ,
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<br />'Caa~ no,", ope..OIoo.
<br />'l'''im"od_MNnd.;lyfl'''''.~.aaod"
<br />'Flown010"'mnd"'B_
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<br />irrigationdivc"ionstruClure,andotherriver.
<br />ine improvements. Inundation of agricultural
<br />area, has eroded the land; (lam.ged or
<br />destroyed ClOpS; damaged Irrigation sY'1ems
<br />ilnd other farm Improvements; reduced crop
<br />yieldl due to increased saTinltyof 'oil; and
<br />J",posiledsand,sill,anddebrisoncropland.
<br />livestock have had to be protecled by moving
<br />lhem 10 high ground, Major flood, have
<br />requi,ed e.'acualion of home, and re.ulted ill
<br />damaged Qr Je'troyed public Ulility facililies,
<br />road,.ndroadbridge"andr..ilroadlrackand
<br />railroad bridge" The cmu of recovelY Irom
<br />flood,havebeensubllanlia!.
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<br />Streamflow in lhe Gunnison River ba,in i,
<br />regulaledbyTaylorPark,BlueMt-sa,Morrow
<br />Point, Cry'tal, and Paonia Re,..rvoirs, which
<br />aggregate 1,206,000 .cre-feel of multiple_
<br />purpose 'torage. Ridgway Reservoir, aW..ter
<br />and Power Resources Servke improvement 0"
<br />the U"compahsre River just upwe.m from
<br />Cow Creek, is scheduled for completio" in
<br />1984. Its operation on lhe basis of 'nowmelt
<br />forec..,t. would afford minimal flood control
<br />benefll.Stre..mflow in both the Gunni;<)Il and
<br />Uncompahgre bas'n> i. al,o influenccd by
<br />irtigationdlversions.
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<br />FUTURE FLOODS
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<br />Although flood, with magnitude, and fre-
<br />quenclessimil.r to lhme 01 pasl floods could
<br />recllr, discussion of future flood,in lhlsreporl
<br />i,limited primarily 10 one design.ted.s lhe 100-
<br />year flood. A l00-year flood has a peak flow
<br />magnilude with a 1 percent chane<:' of being
<br />equalled or exceeded i"anygivenyear,ora
<br />frequencyofoccllrrenceolaboutoncei,,'OO
<br />years on the lo"g-lerm aver.ge. II mu,t be
<br />understood that lhe lerm "100-year flood"
<br />rel.leslo flood magnitude and ooes nolmean
<br />lhal lhe flood will occur one time in a 100-year
<br />pedod. {ssenliall~, "prob.bilit~ 01 occurrence"
<br />is implied, Thus, during the term of an average
<br />mortgage (30 years), the chance of a 100-year
<br />Ilood occurring is approximately 1 in 4; during
<br />an average lifetime (70 years), the chance is
<br />approximately 1 in 2. In..l00-ycarperiod,lhe
<br />chanceofa100_yearlloodisapproxlmately3in
<br />S.
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<br />To delermine f1ow5of the100-yearand other
<br />frequenc~ floods for this report, a flow-
<br />frequency analysis was made for five index
<br />pOinlS. (See Drain.gc Ba,in Map.) Threc of lhe
<br />index points (Nos. 1, 2, and 3) were on the
<br />Gu"n;,o" River and two (Nos,4and 5) were on
<br />lhe Uncompahgre River. Tho'e on the Gunni_
<br />son were at Incaliom reflecting signllicant
<br />tribulary inflow, and those ontheUncompah-
<br />g'e were at the localiom of lh" Delta ."d
<br />Colona gagingstatiom.
<br />In general, the flow-frequenCy analysis lor
<br />the Gu"nl,on River involved developing peak
<br />flow curVel reflecting nJtural (unregulated)
<br />flow prior to construction of Blue Mes.
<br />Rescrvoirandadjuslinglhe,ecurvesloaCCount
<br />forlhe regulatory effecl of the reservoir. The
<br />unregulated peak flow CUrves were e,t.blished
<br />from an anal~,ls of unregulated flow al the
<br />"below Gun"ison Tunnel" "nd "near (",rand
<br />Junclion"weam gages (periods of record 1906.
<br />1965and 1897-196S, respectively). Regulated
<br />peak flow-Irequency curves for pre\entcondi_
<br />tlonl were developed from analysis of Ilows
<br />recorded ahe'CO"I1ruction 01 Biue Mes. Dam
<br />(period of record 1966-1976), from reservoir
<br />routingsludiespreviouslymadeb~theCorpsof
<br />E"gineers artd the Water and Powe' Resources
<br />Service, and consideriition of the W.ter and
<br />Power Resources Service objNllve flow of
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<br />1S,OOOcubic feet per second for IheGunnison
<br />River upstream from Delta.
<br />Peak flow-frequency curyesfor the Uncom.
<br />pahgreRiver were based on records of flow al
<br />the Della andColorta gages.' As rtoted ea;lier,
<br />floodfloWI on the Uncompahgre Riverllsually
<br />r"wlt from snowmelt, but m.~ also relull from
<br />gene..1 rain, particularl~ at Delt.. Therefore,
<br />annualpe.k flow. for r..in and ,nowmellwere
<br />.nalyzedindependently,andfrequencycurve,
<br />developed for each. AII-evenl peak flow-
<br />frequency Cl.lrves w"re developed bycombin-
<br />ing the rainfall and snowmelt peak flow_
<br />fre<juen<:ycurvesforeachstatio".Sincethe
<br />all-evenl curves are es'e"tially identicai for the
<br />lWo stations, it was concluded lhat lhe Com-
<br />binedpeak f1ow.freql.lenciesbelweenthemdo
<br />not change.
<br />Peak flow-frequency value, found for lhe
<br />10-, 50-,100-, and 5OO-yearfloodsal ,elecled
<br />location, on the Gun",.on and Uncompahgre
<br />Rivers are ~hown in (he tabulation on page 6.
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<br />Peak flow y.luesfor 10-, So., 100-, and 500-
<br />year floods (logetherwith correlpondl"gwater
<br />surface elevations) at each cro"sectionu.edin
<br />the hydr.utic analysis made for lhis reporlare
<br />shown in Appe"dix A.
<br />Plat'" 2-59 show the areallh.l would be
<br />inundaledby the l00-ye.lfflooo.Ingerleral,the
<br />10_andSO_yearfloodswouldcloselyp.rallellhe
<br />pallem of the 1oo-ye.' 1I""d. Bol.Io would 1.."
<br />largely confined to the channel and low-I~ing
<br />overb.nk area~. Delinealing the 500-ye.r flood
<br />was beyond lhe scope of the study aUlhoriu-
<br />lion. As shown on PI.le522. 23,and 41, some
<br />areasar" subject to sheet flow. That is,broad
<br />overland floodinggene'ally lessth.n 2 feel
<br />deep o"theaverageand characlNized by
<br />unpredictable flow p.th,. Water surface eleva-
<br />lions in sheel flow ar...s are essentially ind,,-
<br />pendent Qf thOle along adjacenl .tre.mways
<br />and are affecwd principally byobllructions and
<br />local lopography i"lhe area flooded.
<br />Waler.urface elevations of the 10-, 50_,100_,
<br />.nd 500-year flood. ~Iong the Ilream reaches
<br />sludi~d are shown on PI.t"s GO-72. flood
<br />"I"yations.hown were compUled lhroughuse
<br />of the Corps of Engineers Slep-backwater
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<br />'p.,;ad, '" ,eco.d: 0<>1" R'te. 1901-1~l' .nd Wle.p,.""nr; Colon. ~'R'. 19OJ.1\1OS .nd 19!'.p,.,eot,
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