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I 1 The Flooded A~ea5 show the area that would be flooded by the IOo-year and SOC-year flOOds. The flood limits were located at each cross sectIon and the IntervenIng flood outlines were drawn based upon detaIled topographIc mapolng, engln&erlng Jud9ment. and IIeld observations. It Is, however, possible that mere or less flooding should be shown on the Flooded Areas. For 11 specific 51t- uatlon, wnere..,re dlltalled accuracy of a floodedll..e.. IS""',!ulred, the flood 1 fmlts clln be more accurately est/lbllshed by determining the water surface elevation from the profIle or reference tables and then locating that elevatIon by survey on the flood plain. Flood pllllnwldfhsv".-yfrotnone area Toanofherfhr-ough- out the study reach. The IOO-year flood plain width of t~ Big ThOmpson River v~rles from a minimum near 350 feet to a maximum near 3,500 feet with an averege of 1,750 feet. The meJor portion of the ICO-yeer flood pl~ln of the 81g ThOmpson RIver In the study reach Is In egrlcultural use. Few build Ings of ~ny type exist on this flood plaIn. Thase thet do exl st ere prlmerlly lIgrlcultur~ Ily oriented. The det~lled topographic mapping utIlized to assTst In <klllneatlng the outline of the lOO-yeer and 500-year floods cOn- sists of ortha-photo bese maps having a sellle of I Inch eqUlIls 200 feet with a 2 foot contour Intllrval. Thesll mllps were preperlld In June 1975 by fbgan ~ 01 hausen, Inc. of lovelllnd, Colorado under contract with the Colorado Water Conservlltlon aollrd In cooperation wIth Wold County. TheSllmaps,nowusedforadmlnlstrlltlvllPurposes, are on file and available for lwblle Inspection at the PllInnlng Deoartment, Weld County Courthouse, Greeley, Colorado. Obstructions to floodflows - Several roadways cross the flood plain of the Big ThOmpson RIver In thll study reach. The physlc~1 characteristics of these roadways crellted conditions In thIs study Which resulted In lIn Increase In water surface eleva- tion Immediately upstream from theSe roadways ranging from 0 to 3.5 feet. The upstream water surface elevations would Increase drastically If the bridge area were substllntl~!ly reduced by debris, especIally at those roadwllYs hllvlng highembllnkments. Thesehlgher water surfllce elllvlltlons would flood more lIrell th~n shawn In thIs report lInd Increase velocities through the unobstructed portions of the bridges. A reference ITne I~ ~hown on the Floooe~ Ar~~s for t~~ 81g Thompson River. This line Is used for referenca only and doas not necessarily colnclda with the existing stream chllnnal eVen though It may be referred to as such In thIs report. The profiles On pl~tes 12 through 23 show the extent to whiCh flood st~ges lire Increased at tile bridges. "'n ~ppraclllble stage Increllse occurs On the 81g Thompson River upstre~m from the Grellt Western RaIlroad lInd ~he cOMblnlltlon Union PlIclflc Railroad Bridge lInd the Colorado State HT'lhwlIY 257 Bridge. "'II roadways 1n the study reach, except County RblId 54, Colorado Stllte Highway 257, and the Union PacifIc Rallrolld, would be overtopped by the IOO-year floodlltsomepolntalongtherolldway. CountyRolld54,Colorlldo St~te HlghwllY 257, and the UnIon P~cl fIe Ra Ilroad crossing would, however, be overtopped by the SOO-yellr flood. Thll flood elevlItlons shown on the profIle ~nd In table 6 ~pply I~terally from the chllnnel over most of the flood plaIn width. RO~d crossings IInd other topograpnlc teatures can lIlter the later~1 flood elevations. Depending on whether these flllltures divert or block flows, the flood elevations lit the edge of the flood pl~ In may be higher Or lower tnlln lit the Chllnnel. " 22