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<br />I <br />I- <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br /> <br />5260 under the 100-year flood inflaw conditions. An embankment to elevation 5260 <br />may be classified as a dam under the State Engineer's criteria. If the embankment is <br />considered a dam, it must be designed to pass the PMP flood resulting in considerable <br />additional cost. It was decided to complete the preliminary design on the basis that <br />the embankment will be considered a dam. The cost estimate computed using this <br />approach will be the upper limit of possible construction cost. Two methods of pro- <br />viding for the PMP flood were studied: (1) armor the entire dawnstreom slope of the <br />embankment; and (2) provide an armored overflow emergency spillway. The second <br />solution proved to be much more economical and was adopted. For inflows less than <br />a loo-year flood, discharge will OCCur only through the discharge conduit and will <br />not exceed 450 cfs. For inflows greater than a loo-year flood, discharge will occur <br />through the discharge conduit and over the emergency spi Ilway. The spi IIway dis- <br />charge under these unusual storm conditions will flow onto 1-25 and continue south <br />to a low orea where 1-25 crosses Tributary L, the flaw will then return to Niver <br />Creek via Tributary L. The problem of Niver Creek water flawing on 1-25 will <br />actually be improved by constructing the detention facilities since flood flows of <br />less than 100-year flood frequency dump onto 1-25 under present conditions. <br /> <br />Woter will rise to elevation 5254 under 100-year flood inflows without 'v <br /> <br /> <br />detention. This represents a flood ploin of approximately 16 acres. With deten- <br /> <br />tion, 32 acres must be purchased; therefore, the net amount of land acquisition <br /> <br />due to detention is 16 acres. <br /> <br />Channel Upstream of East 88th Avenue - Plates III-la, 11 and 12 <br /> <br />c/ <br /> <br />Channel Design <br /> <br />This upstream reach of Niver Creek generally flows in a well defined chan- <br />nel and the 100-year flood plain of the existing channel is not excessively wide. The <br />channel is quite steep in places; however, it appears stable in thase areas. It is <br />recommended that this entire reach be flood plain zoned. Any sCOur caused by flood <br />flows will be deposited in the 88th Avenue detention pond and will not create main- <br />tenance problems in the rehabilitated portions of the lower reaches. The detention <br /> <br />111-10 <br />