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<br />VIII - EFFECT OF WATER CONTROL PLN,
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<br />J-Ol. General. The Pueblo Dam and Reservoir will regulate
<br />floodflows so far as possible to obtain the maximum practical
<br />reduction in flood damages. The project controls about one-
<br />third of the Arkansas River Basin above Las Animas and thus has
<br />major flood control effects. Control of the design flood witllUut
<br />releases from the reservoir will generally be required because
<br />Fountain Creek and the St. Charles River will normally be in flood
<br />stage when there are large inflows into the reservoir.
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<br />8-02. Spillway Design Flood.
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<br />a. General. A design storm based on the envelopment of
<br />the adjusted values obtained from the transposition of the
<br />2-6 June 1921, above Pueblo, storm: the 17 June 1965, over Kiowa
<br />Creek storm: and the 14-18 June 1965, near Holly, Colorado, storm
<br />WAS developed. This design storm study presented precipitation
<br />values for varioua-sized drainalle areas above Pueblo Oa,. sit"
<br />ranKinr, fron. 500 to 2,500 square miles. It was d"tennineu tllUt
<br />the critical-sized areas for the desilln rainflood contribution
<br />were 500 square miles snd l,500 square miles. A I-hour uniq;raph
<br />fOT the SOO-square-mile area and II 2-hour unitgral'b. for the
<br />l,500-squllre-mile area was L1etermined by the Clark Method. The
<br />maximum snowmelt flood of record at Cauon City occurreLl in
<br />June-JulY 1957, with the maximum l5-day period 26 June - 10 July
<br />1957. This adopted snowmelt hyLlrogral'h was combined with the
<br />rainflood hydro graph to become the uesign flood hydrograph. The
<br />timing is such that the peak of the rain flood hydro graph occurs
<br />6 days sfter the snowmelt peak.
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<br />b. Volume Inflow Design Flood (Spillway Desi~n Flood).
<br />The volume design rain flood was computed using the design rain
<br />occurring over 1,500 square miles, the 2-hour unitgraph, and
<br />a constant retention loss of 0.415 inch per hour. This, when
<br />combined with the snowmelt base flood, resulteLl in a peak flow
<br />of 270,000 c.f.s., a maximum 2-day volume of 399,000 acre-feet,
<br />and a maximum l4-day volume of 582,000 acre-feet. This flood,
<br />being critical, was adopted as the Spillway lJesign Flood. Plate
<br />:\-1 show" the operational and' flood hydrOI\raphs anu re"ervoir
<br />scar.e.
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<br />c. Peak Inflow llesign Flood. Using a constant ret"ution
<br />loss of 0.40 inch per hOUT, the excesses from the 3-hour storm,
<br />given a critical arrangement according to the I-hour unitgraph
<br />for 500 square miles were applied to that unitgraph to compute
<br />the peak rainflood. TIlis, combined with the snowmelt base flood,
<br />re"ulted in a peak flow of 550,000 c.f.s., a maximum 6-day volume
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