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Option 6: J -2 Forebay Reservoir. This site is located southeast of Lexington in Gosper <br />County in the Plum Creek basin, south of the J -2 Forebay on the south side of the Canal. <br />This reservoir would be gravity fed from the Canal. The reservoir capacity is estimated to <br />be 3,436 ac -ft. <br />0 Basic Description: <br />Re- regulating reservoirs capture Platte River water beyond that required for irrigation <br />deliveries and mainstem instream flows during periods of excess flow at the critical <br />habitat. In general, water would be diverted from the Central District Supply Canal <br />during periods of excess and released during periods of shortage at the critical habitat. In <br />the case of the Jeffrey Canyon and the J -2 Forebay Reservoirs, water would be supplied <br />from Jeffrey Reservoir and the J -2 Forebay, respectively, as opposed to the Canal. <br />CNPPID is proposing to re- regulate flows in their system, in which case diversions will <br />not be increased or decreased, only return flows will change. <br />0 On -Site Hydrologic Effects: <br />The HDR Report was relied on for yield estimates. The on -site yields presented have not <br />been discounted, therefore, the EIS team will need to consider the reservation of water for <br />Nebraska's future depletions in determining the scores associated with these reservoirs. <br />HDR developed a spreadsheet to analyze the flow regime of each potential reservoir. <br />Reservoir operations were modeled on a daily basis. Daily operation is possible due to <br />the close proximity of the reservoirs to the habitat. Days of excess can occur in months <br />that the monthly flow does not exceed monthly target flows, in which case, the reservoirs <br />could be operated to store on days of excess and release on days of shortage. These <br />reservoirs can take advantage of short-term excesses and shortages in a more efficient <br />manner than other alternatives that are further upstream. <br />The following assumptions and operating rules were used by HDR to determine the yield <br />and timing associated with these reservoirs. <br />• No dead pool was accounted for. All reservoirs were allowed to drop until they <br />were dry. <br />• Type of year for purposes of defining target flows (wet, average, or dry) is known. <br />• Travel time from Overton to Grand Island is two days. Historic flows at Overton <br />were used to determine the amount of water that should be stored or released from <br />the reservoirs to meet the target flows at Grand Island. <br />• Buffers were used to incorporate a factor of safety in the decision to store or <br />release. If the flow at Overton was more than 200 cfs above the target flow, then <br />water was diverted to storage. If the flow at Overton was more than 500 cfs below <br />the target flow, then water was released from storage. Changes to these buffers will <br />affect yield results. <br />\ \DN00\E- DRIVE \PROJECTS\Platte \Work Products\Task 9 \wapc report (Version 7).doc 10 <br />