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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 1ntervals 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 bas1n Is located w1th1n an area
<br />whiCh can rece1ve very intense rainfall ot cloudburst magnitude.
<br />Floods have resulted Irom storms covering largearedsreceivlng
<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. Annu", 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 floodw~ters flow
<br />around them. floodstagesrnaybelowerad 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 flo~tlng debris commonly
<br />lodges a~elnst bridge piers and bridge decks, the flow area Is then
<br />reducedorthechannellsC01T1pletelyblocked. Theblockagecen
<br />produce severe stege and velocity 1ncraases, often accompanied by
<br />bridge damage or complete failure.
<br />
<br />Floods in thu8igThornpS<.lnRivurt>asinc<Jnhaveconsid-
<br />erably different Characteristics depending upon the location within
<br />the b~sln. Floods In the mountainous re<]lon of the basin typically
<br />havera;>ldrlses, highmaxi"'umdischargas, and relatlvel-y short
<br />~u~dtr"",S. Floods i~ the ~!a1~s res lon, Senerall,', art' of 10'll<.::r
<br />
<br />Iceeffectshllve, historical lv, not been a probremorl the
<br />8lgThompsonRlver. Ice jams, hO'llever, could occur in the future,
<br />althou~h the chance of occurrenco of such a phenomenon is qu1te
<br />remote.
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<br />pea~ discharge and have 10nfJer dur,ltion. floods wi thl n the stu<ly
<br />reach generally have a duration 01 a':>out 1 day. FLood pBaks Irom
<br />tr,understurms covcr,ro9 SO... I I ~r-'M~ ",1'0 g,,,,,,rally r~llidly "ttenu~-t(ld
<br />in th., study reach by va I i"y stora'le 01 I loodw~ters.
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<br />FlOOd damage redUCTlon measures - Olympus Dam, constructed
<br />In 19~9 by the Bureau of R{lclamatIQIl, Is located ne",r Estes Park In
<br />th9 upper Big Thompson Rlvor t>asin. This structure, however, has
<br />nogl1glble etfect In reducing flOOd damages on the re~ch 01 the 8ig
<br />Thompson River studl"d 1n th1s report because 01 the relatively long
<br />distance between the two loci!ltlons, FlOOd damages are redUCed to
<br />varlou~ extents, d~pending upon the degree of prorActlon providod,
<br />Ly lovees constructed by private Interests along the 8ig Thompson
<br />River in the study reach.
<br />
<br />FACTORS AFFECTING FLOOD I NG AND I TS 1~.1f'ACT
<br />
<br />ODstructlons to floodflows - Thoro ~re I~ cro~sln~" oYor
<br />the Oiil Th<.>I1lpson Riv..r I load pia In a~ de~crl~..ll in Ih! s r-eport.
<br />County "<oild 54, the Great West~rn Rai Iroad, dnd tho combinatlon 01
<br />th9UnlonPaclllcRallroaJandC<lloradoHighwily2S7presonttho
<br />I ar!je~t p<ltontia I for r-,bstructi "9 f lood IO-~5.
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<br />Flood plaln zoning can be an eftectlve method lor prevent-
<br />InQfuturedarnages. To date, tlood plain zoning 1n Weld CoUrlty has
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