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<br />2.2 Hvdroloqy. <br />l.ere those used in the <br />'rhe discharges are shown <br /> <br />The discharqes used for this study <br />1978 Flood Plain Information S1:udy. <br />in Table One. <br /> <br />TABLE ONE: <br />S011th Platte River :Discharges <br />at sterling, Cc,l.<::.J:'ado <br /> <br />P:eak Disc:harge <br /> <br />Return Interval <br /> <br />10-Year <br />50-Year <br />100-Year <br />SOO-Year <br /> <br />18,000 c:.f,s. <br />55,000 c:.f.s. <br />B4,000 c:.f.s" <br />207,000 c.f,s" <br /> <br />~ HYDRAULIC ANALYSIS. <br /> <br />;3.1 Existinq Conditions. Wa'ter surface profiles <br />were computed using the U. S, Army Corps of Engineers HEC-2 <br />backwater program. The September 1.989 rl~lease with error <br />correction three for an IBM compa1::ible microcomputers was <br />used. The program uses the standard ;step backwater method and <br />is applicable to steady gradually varied flo.r in na1:ural <br />channels. <br /> <br />The first three cross sections of the HEC-2 model were <br />taken from the Section 22 Study performed at Sterling in 1989. <br />'rhe three sections provide a suitable reach length for the <br />model to approach normal depth.. The model used 1:he starting <br />lOO-year water surface elevati(m calculated for t:he previous <br />Section 22 study. These cross sections were supplemented with <br />cross sections taken from the 10 foot. contour Quadrangle maps. <br />The channel for these cross sections ,..as takem from a surveyed <br />cross section used in the Flood Plain Information Study and <br />then adjusted in elevation and stat:ion for e;ach cross section <br />taken from the Quadrangle map. This allowed for a more <br />accurate channel shape to be deterndned for each of the cross <br />sections. This channel shape does not reflect: the actual <br />qeometry of the channel, but does provide a bet:ter defined <br />,;hannel than can be obtained using 10 foot contour mapping. <br /> <br />The "n" values were assigned based on t,he field <br />inspection of the area and engineering judgement.. The <br />selected values are shown in Table Two. All existing <br />agricul tural levees in the overbanks were! determined to be <br />:Eailed for the existing condit:ions water sur:Eace, elevations <br />calculations. Existing conditions water surfa.::e profiles are <br />shown on Plate 1. <br /> <br />5 <br />