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re~ch. The physIcal characteristics of these roadways created con- dItions I~ this study which resulted In an increase In water surface elevation Immediately upstream from these roadways ranging from 0 to 4 feet. The upstream water surface elevations would Incr~sedras- tlcally 11 the bridge area were substantially reduced by debrIs, espechlly at those roadways havIng high embankments. These higher water surface elevations would flood more area than shown In this report and Increase velocitIes through the unobstructed portions of the bridges. expocted to peak ~bout 2 1/2 hours after the time of InItial rise. Th9 flood would have 11 durMlon of IIbout 13 hours. Similarly for ~ewl9tt Gulch, ~III Gulch, and Falls Gulch, theS9 values would generaIIVbel-ll/2hoursand5-6hoursrespectlvely. The time to flood peak and flood duration for Watha Gulch and Unnamed Gulch for a 100-year or 500-ye~r flood would be about 1/2 - I hour and 2-3hours respectively. The profiles on plates 6 through 12 show the extent to whiCh flood stages are Increased at tn9 bridges. An appreciable stage Incr9ase occurs on the CaChe la Poudr9 Rlv9r at bridges located at reference point 59A, 67A, and 72A. Appreciable stage Increases also occur on Hil I Gulch at reference point II and 18A. . , I , [. Y!l9cltles of flow - Water velocities during floods depend upon suCh factors as sIze and shape of the c~annel and flood plain cross se<;tlon, condItions 01 the stream, and bad slope; all of t~esfl f3<::tors vary on different streams and at dlfforent loc3tlons on the same stream. On the C"c;~e la Poudre River In the study rMch, t~e ch3nnal velocltv for the 100-ye3r f 100<1, In g"n!,r~l, """'g9S f!'"O"l 9 to 20 feet per second. Theoverbankvelocltyfortne 100-year flood ranges, In general, from .3 to 6 feet per second. These vlllues for the Cache la Poudre Rlvflr tributarIes studied In this report are generally 5 to 9 leet per second for the Channel and 2 to 4 feet p"rsecondfortheoverbank. Rate 01 rIse and duration 01 floodlnq - Rate of 1'159 and duration 01 flooding. for lloodscomp/lrabie to tha IOo--ya/lror500- vearflood,Canvllryoverawld<lranqe. Inth"P(ludrePlIrk3rea,II I OO.year or a 500-year llood on thfl C<lche la Poudro R;~er would be 15 16