<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 />
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