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<br />with the Color~do Water Conservation Board. U.S. Geological Survey
<br />7.S-mlnute Quadrangle maps with IO-foot contour TnterVills at a scale
<br />of I inch equals 2,OOQ leet were also used In the study. Brldgednd
<br />ro~dway survey data '~em provided by the Colorado '~ater Cansorvation
<br />8<xlrd.
<br />
<br />FLOOD SEASON AND FLOOD CHARACTERISTICS
<br />The Gla Thompson River basTn Is located wTthTn an area
<br />whiCh can receTve very intense rainlall at cloudburst magnitude.
<br />Floods have resulted Irom storms covering largearedsrecelvlng
<br />heavygoneral rainfall aswell as from storms covering s"",11 areas
<br />but producing extremely Intense rainfall. floodsgenerallyoccur
<br />In the period from May through July. IInnu", I ,Je"k dlschargos in tho
<br />study reach, however, have been exporlenced from March through
<br />September.
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<br />Manmade objects, such as buildings, cars, and fences, as
<br />well as trees, brush, and other vegetatlO,n located On the flood
<br />plaln,canalsocauselloodflowobstructlons. Theseitems,whlla
<br />remaining In place, not only tend to create higher stages on the
<br />flood plain because of reduced flow area and flow blockage but may
<br />also create higher locallled velocities as the fioodwaters flow
<br />around them. flood stagesrnay be lowered Inalocallledarea, If
<br />the floodflows are of sulflclent magnitude to dislodge these flow
<br />obstructions. The debris may then lodge at locations farther down-
<br />stream, thus compounding problems. This floatIng debris commonly
<br />lodges a~alnst bridge piers and bridge decks, the flow area Is then
<br />reducedorthechannellsC01T1pletelyblocked. Theblockagecan
<br />produce severe stage and velocity Tncreases, often accompanied by
<br />bridge damage or complete failure.
<br />
<br />Floods in thu 8ig Thornps<.m Rivur l>asin c<Jn have consid-
<br />erably different Characteristics depending UPo^ the location within
<br />the basin. Floods In the rnounta I oous re<]lonof the basin typically
<br />havera;>ldrlses, highmaxi"'umdischargas, and relatlvel',/short
<br />~U~i1'tr"",s. Floods i~ the ~!aT~s res lon, senerall,', art' of lower
<br />
<br />IceeffectshllVe, historically, not been a problemorl the
<br />8lgThompsonRlver. Ice jams, however, could occur 10 the future,
<br />althou~h the chance of occurrenco of such a phenomonoo Is quite
<br />remote.
<br />
<br />pea~ discharge and have 10nfJer dur,ltiorl. Floods wi thl n the stu<ly
<br />reach generally have a duration 01 "':lout 1 day. FLood peaks Irom
<br />t~,und..rsturms covcrl"9 SO... I I ~r-'M~ ",ro g,,,,,,,rillly r~llidly "ttenu~'t(ld
<br />in th., study reach by va I i€y stora'le 01 I loodw~ters.
<br />
<br />FlOOd carnage redUCTlon meaSures - Olvmpus Dam, constructed
<br />In 19~9 by the Bureau of R{lclamatlQIl, Is located ne",r Estes Park In
<br />th9 upper Big Thompson Rlvcr t>asln. This structure, however, has
<br />neglTglble effect In reducing flOOd damages on the re~ch 01 the 8ig
<br />Thompson River studl€d in this report because 01 the relatively long
<br />distance between th6 two loci!ltlons, Flood damages are redUCe<l to
<br />variou~ extents, d~pending upon the degree of prO~Actlon provldod,
<br />Ly lovees constructed by private Interests along the 81g Thompson
<br />River In the study reach.
<br />
<br />FACTORS AfFECTING FLOOD I NG AND I T5 1~.1f'!lCT
<br />
<br />ODstructlons to floodflows - Thoro ~re I~ cro~~ln~" ovor
<br />the Oiil Th<.>I1lpson Rlv..r I load pia In a~ de~crl~."j in lhi s r'eport.
<br />CountyF<oadS4, theGr"dtW€st€r^f1ailroad,dndthocornLinatlonol
<br />the Union PaclllcRallroad and C<lloradoHighway 2S7 presont tho
<br />r ar!JO~t potontia I for r-,bstructi"9 f lood IO'~5.
<br />
<br />Flood plaln lonlng can be an eltectlve method lor prevent-
<br />InQfuturedamages. To date, flood plain lonlng Tn weld CoUrlty has
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