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Obstructions to floodflows - Several roadwavs varyinn fro~ county roads to hiqhways to railroads cross the Cache la Poudre River floorl olain within the confines of the sturly reach. The physical characteri5tic5 of these rt'Jadways, coupled with thn assumption in the hydraulic cOMoutations that I ittle debris was lodqed in the hridqe openings, created conditions in the study which resulted in an increase in water surfacn elevations iMMediately uostream from these brirlqes ranoinn from n to'4 feet for al I flood5 studied. This chanoe in water surface elevation could increase drastically esrecial Iy at those roadways havino hioh emban~~ents if sub- stantial amounts of debris were to lodoe in the bridoe opening, thus reducing effective floodflow conveyinq area. If this were to occur durinq a large flood such as the IRF or SPF, the increased bridqe velocities would cause added erosion around hridge Diers and abutments with possible damage. Upstream water surface elevations would also increase because of thn debris possibly floodinn more area than shown in this rerort. t'I't . 'ns the main floodflow by alonq this southeasterly path un I I reJol . t q' h ay 25 in the vicinity of reference oOlnts overtopping Intersta e r 19 W ')9-102. Floodflow obstructions are also present in the for~ of irriqa- st'ructures. These structures, by reMoving floodflow tion diversion f the Cache 18 Poudre River channel, not only create conveying area rOM ' localized increases in water surface elevation upstrea~ from the ~truc- ture, but force more floodwater to flow on the adjacent flood plain, thereby creatlnq higher flood plain velocities. were not taken into account in the hydraulic analysis Ice effects . . d to the fact that the occurrence and maanitude of this rerort. ThiS IS ue - indetermlnant factors and to the fact that no records of of ice jams are nroblems with ice were found. AI I roadways within the study reach are overtooped by the IRF and SPF. '10st of these roadWAYS are county roads havlno relatively low roadway embAnkments. This exrlains the relativelv low increase in water surface upstream frOM these roadways as evidnnced by the Profile. of 2 feet the 5tudy reach for the IRF and second, resrectively. Althouqh hazardous, localized velocities the averaqe velocities. Travel Velocities of flow - Veloc.lties of 3 feet per second \>lith deoths or more are considered danoerous. Overbank flow velocities in SPF averaoe about 1.5 and 2.1 feet ner these average velocities do not appear are often two or three tiMes oreater than through the floodflows should be avoided The ro I orado and Southern Ra i I road (reference po i nt fl7), I nter- state Hi qhway 25 (reference po i nt 92), Ced orado State Hi ohwav 392 (rAferencfl roint 115), a road at the Larimer-Weld rounty I ine (reference point 119), and the r,reat Wflstern Railroad (reference noint 135) are such that the co~- bination of roadway eMbankment heinht and hridoe ooeninq size create staqe increases in excess of one foot for the IRF. One of these roadwaY5, Inter- state Hlqhway 25, creates a siqnificant floodwater diversion due to the skewed angle at which it crosses the flood nlain. Floodwater on the right bank which does not nass through the Interstate Hlqhway 25 bridqe at reference point 92 continues to flow downstream in a southeasterly direc- tion. This floodwater flows at an elevation different from that flood- water followlnq the adjacent Cache la Poudre River channel. It continues nxcept for extreMe emeroencies. Pate of rise and duration of floodinn - Flood oeaks in the. study intense rainfall or a combination of rainfall ,each can result from either Storm COMbinations would nroduce two distinct flood and snowmelt. Certain Standard Project Flood under tho double neak conditions, the for tho first peak would be about 9 hours SMa I ler reak resultlnq from ~ountain runoff. could I~st about 7 days. peaks. For a rate of rise or peakin9 time fol lowed 2 1/2 days later by a The total durntion of floodino 32 31