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II-18 I I I I I I I I I I I I I I I I I I I years and resulting reservoir volume is slxlwn in Table II-7. This study shows that the reseI"~oir volume would be reduced fran 196,000 to 106,500 ac-ft with- in 100 years. The sediment was distri buted wi thin the reservoir by means of the Empirical Area-Reduction Method (1) for a Type II (floodplain-foothill) reseI'voir. The initial, 25-, 50-, 75- and 100~year elevation-volume CUl"ves based on this distribution are shown in Fig. II-8. The 50-year sediment ele- vation at the dam is estimated to be El 4990 and the 100-year level is El 5010. H. FLOODING AND SEDIMENT TRANSPORT NEAR PARACHUTE 1 . Gen'lral The effects of the proposed reservoir upon existing flood stages and sediment tl'ansport capacities of the Colorado Ri Vel' near the town of Para- chute were studied based upon approximate hydraulic and sediment transport calculations. The study considered a nood with a return period of 100 years and indicated a mode of reservoir operation that would prevent aggravation of nood stages near Parachute while maintaining the El 5070 nonnal maximum pool for the majority of the year and thus maximizing water storage. 2. Wat'lr Surface Profiles Canpl.rati ve flood stages and other hydraulic parameters were cal- culated for the Colorado Ri VeI' reach between the dam site and at a point approximately 3,000 feet upstream of the bridge over the Colorado River at Parachute (Parachute Bridge) which is beyond tbe pI'Oposed reservoir's 1 DO-year (13) flood backwater. The Anny Corps of Engineers computer program HEC-2 was utili zed. Col,~rado Ri Vel' channel cross-sectional data extending fran 10,000 feet downstream of the Parachute Bridge to 3,000 feet upstream of the bridge were estimated fran existing topography and cross-sections produced at a scale of 1 inch equals 500 feet by Frasier and Gingery, Inc. in 1975. Channel infonnation fOJ~ the remaining 44,000-foot long ri VeI' reach downstream to the