2014-07-08_HYDROLOGY - M2002020 - Laserfiche WebLink

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25`h Ave SWSP July 8, 2014 Dewatering Page 3 of 7 Dewatering at Flatiron Pits 3 and 4 began in June 2012 and is anticipated to continue throughout this plan period. Water pumped from Flatiron Pits 3 and 4 is continuously discharged directly to the Cache to Poudre River with no beneficial use. The initial dewatering rate, required to empty the filled pits, was approximately 6,800 gallons per minute ( "gpm "). After the initial high dewatering rate, a lower pumping rate of 850 gpm was required to maintain a dewatered state. The initial dewatering was assumed to be made up of both lake water and alluvial aquifer (ground) water. For the purposes of this SWSP it was assumed that the initial dewatering depleted the alluvial aquifer at a rate equal to the maintenance pumping rate. Therefore, only 850 gpm of the initial pumping rate of 6,800 gpm was assumed to be a ground water depletion. After the initial dewatering, pumping occurred at the maintenance dewatering rate of 850 gpm (1,700 gpm at 50% daily duty cycle) and was assumed to be made up of only alluvial aquifer water. The maintenance dewatering rate decreased beginning in December 2013 to the current pumping rate of approximately 180 gpm. For the purposes of this SWSP, you have estimated that Flatiron Pits 3 and 4 will be dewatered at a constant rate of 180 gpm during this plan period. Dewatering depletions were lagged to the river using the AWAS program with the parameters identified above. Lagged depletions associated with the dewatering of Flatiron Pits 3 and 4 are estimated to total 374.02 acre -feet during the first year of this plan period, and 330.27 acre -feet during the second year of this plan period, which are partially offset with a credit of 291.13 acre -feet per year for dewatering water discharged to the river. Actual monthly meter readings must be used to determine the actual dewatering rate and the resulting depletions under this SWSP. Flatiron Pit 5 was temporarily dewatered from August through October of 2012 at a rate of 850 gpm. After October 2012, Flatiron Pit 5 began to refill and was estimated to have refilled completely within two months. Lagged depletions associated with the dewatering and refilling of Flatiron Pit 5 are estimated to total 37.92 acre -feet during the first year of this plan period, and 17.98 acre -feet during the second year of this plan period. Replacement The proposed source of replacement water under this SWSP is water rights owned or controlled by the City of Greeley in the Cache la Poudre river basin. In particular, Greeley will use 138 shares in the Boyd and Freeman Ditch (WDID 0300935) that were previously owned by Flatiron Paving Co. and used in previous gravel pit SWSPs for the Greeley West Pit. Greeley now owns these shares and will dedicate these shares as a replacement source in this SWSP. This SWSP relied on a historical use analysis for the 138 Boyd and Freeman Ditch shares included in the SWSP request submitted by W.W. Wheeler and Associates, Inc. on July 13, 1990 for the Greeley West Pit. The 138 shares represent 13.9% of the 993 total shares in the Boyd and Freeman Ditch. The analysis assumed a ditch loss of 30% of the pro -rata headgate diversion and a field irrigation efficiency of 60 %. The 138 shares were historically used for the irrigation of 107.5 acres with a typical crop rotation of corn (33 %), alfalfa (33 %), and pasture grass (33 %). The attached Table 3 summarizes the historical consumptive use and return flow factors for the 138 Boyd and Freeman Ditch shares. Based on this analysis, the historical consumptive use of the subject shares was determined to be 63.4 acre -feet per year. The Wheeler analysis appears to rely on the average headgate diversions and crop consumptive use for the period of 1950 -1980, as opposed to a yearly analysis. In addition, the