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<br />.' <br /> <br />. <br /> <br />"", <br /> <br />, <br />~,. <br /> <br />... () r~ - <br />UU";:J <br /> <br />The Blench equation was applied to the long-time 1915 to 1975 dOQ- <br />1Ilant discharge of 6,930 ft3/s equivalent to a 2-year flood peak <br />as dete~ed from a flood flow frequency analysis for the Overton <br />gage. The regime width for the 6,930 ft3s was 932 feet. This is <br />considered a good check on the channel widths varying from 1,000 <br />to 1.200 feet from the 1971 photographs. In this dominant discharge <br />analysis, the question is whether the dominant discharge downstream <br />from Overton, Nebraska, will change significantly with construction <br />of Narrows Dam. <br /> <br />. <br />Since the dominant discharge is based on the 2-year flood peak dis- <br />charge, a dam located about 300 miles upstream "on the South Platte <br />River should not appreciably affect the lower frequency flood events <br />as far downstream as Overton. This is also supported by the follow- <br />ing comparisons of drainage area: <br /> <br />Location <br /> <br />D i (mi2) <br />ra na~e area _ _ <br /> <br />Proiected Flows. Overton to Grand Island <br /> <br />As indicated under "Basic Data," the mean annual discharge for his- <br />toric flows from Overton to Grand Island for the 1947 to 1970 period <br />shows a decrease of flow in this reach from 1,207 ft3/s to 1,173 ft3/s. <br />Figure 1 shows this annual decrease but when monthly values are plot- <br />ted. there are increases in mean discharges for the 6 months from <br />February through July. Even with the decrease in mean discharge at' <br />Overton without Narrows Dam and with Narrows Dam of 1,150 ft3/s to <br />1.029 ft3/s. respectively, this decrease may have little effect on <br />the open channel widths. The open channel widths would be controlled <br />by the dominant discharge which is determined from the flood peaks. <br /> <br />LV <br />() <br />. " <br />~ <br />