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i FREOUEr-cy rising ~nd swiftly flowing floodwater may trap persons in homes that are ultimately destroyed or in vehicles that are ultimately submergedorfloatad.lsolatior>ofareilsbyfloodwaterscould creato hazards in te~5 of medical, fire, or law enforcement emer- gencieS. The 50Q-year flood Jsoottheldrgestlloodthlltcan occur, but the probabll ity of largar floods is remote. As eM be soon trom the 939109 records for the Big T~p5on RiVer, discharges smaller than olther the IOQ-yearor50G-year floods are much more common. Large 11=<15, hOWever, can happen; this "oS clearly demon- strated by the Denver area floods 01 1965, the June 1972 flood at Rapid City, South Dakota, and the July 1976 flood in the Big Thompson canyon. , I . Floodod areaS and flood d<lmaqes - W<lter surf<lce profiles for the floods studied In this report were developed using the back- w~ter computer program HEC.2, "W~ter Surface Prof lies", developed by the U.S. Army Corps of Engineers. The computations were based on channel and flood plain conditions ~s represented by survey data g<lthered In June 1975 and as supplemented by later field Invostl- g~tlons. FloodIng can occur with relatively little dtsch~rge In the event of channel blockage caused by debris or Ice. Inthese situations, water surface elevations rise untIl relief Is achIeved by flo"sover the flood plain. Hlstorlcaldata, r.owever, IndlC<lte that obstructions due to Ice are not likely to occur In the study reach. Tho for~tlon of obstructIons ~t bridges In the study reach Is, however, quIte possible-especially duo to debris accumulation. The protlles on plates 12 through 23 sho" the "<Iter sur- faceelevatlonsandwaterdepthoftheIO-year, SO-ye<lr, IOO-year, and 50C-year floods relative to the strea~bod. In this p<lrtlcular case and throughou.t the study, the strea~bed Is actually the chan- nellowflowwdterslJrfaceelev<ltionslncothesurveysworeccnducted by aeri<ll" photo<)ramtnetrlc rnethod~. The water .,,,dace elevations of the IOO-year and 50C-year floodS were used to detormlne flood limits ontheeross sections and topographic maps, Theresultsarllshown on the Flooded Arcas, plates 3 thrOU'Ih II, An lndex TO the Flooded Areas plates Is on plate 2. Representative cross sections are Illustrated on plates 24 through 28. The cross sections show ground elevations across the valley on both stdesof the Channel an<lthe depths of overbank flooding. Roferencepclnts,colncldlngwltntne locations of the cross sections and pertinent pl~nlmetrlc features are shown on the Flooded Aroas and profile for correlation between the drawings. Table 6 provides d<lta at reference point locatIons, including distencoalOllg themdin channel centerline, elevation sof thestrca"'bOd,anoolevatlonsofthe IO-yellr, 50-year,IOO-Yf.lar, and 500-ye"r floods. HAZARDS OF LN1GE FLOODS The extent of damano caused by <lny /Iood depends on the topogrllrhyof the arOa flooded, depth and duratIon of flooding, velocity of flow, rate of riSfl, dovelopments on the flood plaIn, andarnountofdobrls IntheflOOd"ater, A IOO-year flood or YO<lr flood On the 81g Thompson River In the study reach "ould result In the Inundation of and subsequent damage to primarily agricultural propertIes and associated developments as woll as publlc utilities and public roadways. Deepfloodwatorflowlngathlghvelocltyand carryln,! floetlng dobrls would cre<lto conditions h<lzardous to persons and vehicles att'll'lpting to cross flooded areas. In general, floodwator3ormorefeetdeepllnd flowlnq ata velocity of 30r more feeT per second could easIly swoop adult persons off their fBet, thuscreatlngde/initoJnngerof Injury Or drowning. Rapidly 19 20