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Obstructions to floodflows - Several roadways cross the flood plains of the Big Thompson River and the Little Thompson Riverinthest-udyreach. The physical characteristics of these r~dways created conditIons In the study whIch resulted In an Increase In water surface elevations Immediately upstream from these roadways ranging fromOto 5 feet. Theupstreilmwatersur. face elevations would Increase drastically If the bridge area Is substantially reduced by debrIs especIally at those roadways h aving high embankments. These higher water surface elevat-ions would flood more area t-han shown in thIs report and Increase velocities through the unobstructed portions of the bridges. Velocities of flOw - Water velocities durIng floods depend upon such factors as size and shape of the cross ~ections, conditionS of the stream, and bed slope; all of these factors vary on dIfferent streams and at dIfferent locations along the same stream. On the 81gThompson River In the study reach. the ChannelvelocltyfortheIOO-yearfIOO<.J,lngeneral,rangesfroo 5 to 10 feet per second. The overbank velocIty for the 100-year floodranges,lngeneral,from2t04feetpersecond. Thechan- nel velocity for the 100-year flood on the Little Thompson River In the study reach ranges, In general, from 6 to 12 feet per second. The overbank velocity for the IOO-year flood ranges, in general, from 2 to 4 feet per second. The profiles on plates 9 throu9h 23 show the e~tent to which flood stages are increased at the bridges. An appreciable stage increase occurs on the BIg Thompson River upstream frotn Interstate Highway 25 and a county road at reference coint 49. S"nllarly, an appreclilble stage increase occc,rs on the Little Thompson ~Iver upstream from county roads at reference points 27, 47, 59, 74, 132, 91, 99, and 103 as ...ell as upstream from lJ. S. Highway 287 and tlYe Colorado...n<:l Southern Railroad. All road...ays in til.,BigThomp"on Rivar study reach will beovertoppo<lCythc 100.year flood at some polnt along tho roadway e><Cflpt Interstate HlghW1lY 25 and the Frontage Road at reference point 22. Similarly, all roadways in the Little Thompson Rlvor study reach wi II be ovortopcQd by the IOO-year flood e><cept fhe county road at reter. once points 47, 59, 74, and 82, U. S. Highway 287, and the Colorado and Southern Rilll...i1Y. Rate of rise a~d duration of floodinq - Rate of rise and duration of flooding, for floods comparable to a 100.year or 500_yearfIOOd,Canvaryoverawiderange.llfloodofralnfall origIn on the Big Thompson River at loveland would peak approxi- mately 8 hours from the time it begins to rise. ThIs flood would have an approximate duration of 20 hours. The sa~ type of flood On the little Thompson River at the upstream limit would peak approximately j nours tram the time it begins to r;s... The 11004 would have an approximate duration of 10 hours. Obstructions such as trees and buIldings dO e~lst on the flood plain in the study reach. The obstructive effect Is, ho.... aver, relatively in~lqnificant ~ecause of the sparsity in number. " "