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<br />Location <br /> <br />Dr~in~ge <br />Area <br />(sq. ",1.) <br /> <br />10-Year <br />Discharge <br />Palik <br />Ce.f.s.) <br /> <br />50-Year <br />Oischarge <br />Peak <br />(c.t.s.l <br /> <br />100-Year <br />Discharge <br />Peak <br /><C.f.s.l <br /> <br />SOD-Year <br />Discharge <br />Peak <br /><c.f.s.) <br /> <br />situations, water surface elevations rise until relief Is <br />achieved by flows over the flood plain. Historical data, howover, <br />Indicate that obstructions due to Ice are not likely to occur In <br />the study reach. The formation of obstructions at bridges In the <br />study reach Is, however, quite possible - especially due to debris <br />accumulation. <br /> <br />Su..."ary <br />for the <br /> <br />Table 6 <br />of Pertinent Discharges <br />Big Thomo~on River Basin <br /> <br />BIG THOMPSON RIVER <br /> <br />Pry Creek <br /> <br />'" <br /> <br />',000 12,100 19,000 40,000 <br />4,300 8,BOO 11,500 21,000 <br />3,600 7,600 10,000 18,500 <br />LITTLE THOMPSON RIVER <br />3,300 6,600 8,800 17,000 <br />2,850 s,aoo 7,500 13,400 <br />2,800 5,500 7,200 12,800 <br /> <br />HAZARDS OF LARGE FLOODS <br />The extent of damage caused by any flood depends on the <br />topography of the area flooded, depth and duration of flooding, <br />velocity of flow, rate of rise, developments on the flood plain, <br />lIIIlOunt of debris In the f loodw~ter, and Ice conditions, I f Ice <br />Is a problem in that particular area. A IDO-year flood or a 500~ <br />year flood on the Big Thompson River or the Little Thomp~n River <br />In the study reach would result In the Inundation of and subsequent <br />damage to primarily agricultural properties and associated devel- <br />opments as well as public utilities and public roadways. Deep <br />floodwater flowing at high velocity and carrying floating debris <br />would create conditions hazardous to persons and vehicles attempt- <br />[ng to cross flOOded areas. In general, lloo1...ater 3 or more feet <br />deep and flowing at a velocity ot .3 or more teef per second could <br />e~slly sweep adult parsons ott their feet, thus creating definite <br />d~nger of Injury or drowning. Rapidly rising ~nd swiftly flowing <br />floodwater may trap persons tn homes that are ultimately destroyed <br />or In vehicles that are ultlmatol y submerged or tlo~ted. Isolation <br />of areaS by floodwaters could cre~te hazards In tarms of medical, <br />tire, or law enforcoment emergencies. <br /> <br />Upstream limit <br />of study <br /> <br />Interstate <br />Highway 25 <br /> <br />Larimer-Weld <br />GountyLlne <br /> <br />Upstream limit 101 <br />of study <br /> <br />Larimer-Weld <br />County Line <br /> <br />fREQUENCY <br /> <br />The 500-year flood Is not the lar~est flOOd that can <br />o<;:cur, but the probability of largerfloodslsramote. AS can be <br />Seen from the gaging records for the 81g Thompson and Little <br />Thompson Rivers, dl$chllrges smaller than elthar the I ~O-year or <br />500~year floods are much more common. Large floods, however, can <br />happen; thIs was clearly demonstrated by the Denver area floods <br />ot 1965, tha June 1972 flood at Raoid City, South Dakota, and the <br />August 1976 flood In the 81g Thompson canyon. <br /> <br />flOOding can occur with relatively little diSCharge In <br />the event of Channel blockaaecausedbydebrlsorlce. Inthese <br /> <br />Flooded areas and flood damages - Water surface profiles <br />for the tloods studied In this report were developed using the <br />ba<:kwatercomputerprogramHEC-2,"waterSurfaceProllles",deve1- <br />opedbythelJ. S. Army Corps of Engineers. Thecom;:>utatlonswore <br /> <br />,. <br /> <br />" <br />