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GANSEM 1A TJAN h ASSOCIATES <br />llydrogeological and Environmental Consultants <br />the morphology and geometry of the South Platte river and flood plain has changed due to <br />the construction of lined pits on the west side of the river. The statement by Deere & Ault <br />that the mine operator, L.G. Everist has not altered the South Platte River floodplain is <br />unsubstantiated and negated by the very application of the LOMR letter to establish a new <br />100 -year flood level. <br />Based on the Deere & Ault letter of notification dated August 19, 2011, the LOMR <br />application is for a considerable change in flood levels. The original 100 -year flood plain <br />elevation on the finished floor elevation at the Bachofer house was 4874 feet amsl. The new <br />proposed elevation according to the Deere & Ault map attached with the LOMR letter is at <br />4876.5 feet or 2.5 feet above the original flood level. This level was estimated using HEC- <br />RAS surface water modeling and was based on a steady -state discharge of 29,000 cfs <br />representative of a 100 -year event. The results of this modeling appear to be unreliable and <br />unrealistic when compared to actual stream flow data measured at the nearby Fort Lupton <br />gauging station. Recent peak flows measured 9,200 cfs at this station on 9113/2413, while <br />the water level was at 4877.6 feet, 3 feet above the floor elevation of 4874.6 at the Bachofer <br />house. The disparity between the model and measured results suggest that the model is <br />poorly calibrated and does not represent actual hydrologic conditions. As such, the model <br />results should not be used as a basis to re- establish a new 100 -year flood plain map. The <br />model significantly underestimates the stage level in the entire reach of the model area and <br />specifically in the vicinity of the Bachofer property. <br />Response to Deere and Ault Comment #2 <br />The comment states that the South Platte River lies between the L.G. Everist property and <br />the Bachofer property and that hydrologically forms a no -flow boundary and would be <br />modeled as such. This statement is categorically and unequivocally incorrect and indicates <br />that the author is inexperienced in the construction of numerical groundwater models and <br />does not understand boundary conditions in the hydrological system. The river is modeled <br />as a sink or a source depending on the stage in the river and head in the adjoining model <br />cells. A river is usually modeled with the river module or package which allows water to <br />either flow into or out of the river cells through a conductive layer. It is not a no -flow <br />boundary. <br />With respect to hydrogeologic conditions in the area near the Bachofer property, the <br />groundwater gradient is toward the river from both the west and east sides. Hence, <br />groundwater is flowing toward the northeast on the west side and toward the northwest on <br />the east side. While this condition appears to preclude an apparent hydrologic effect to the <br />Bachofer property, the opposite is actually true. Groundwater mounding or rise that occurs <br />on the west side due to the impermeable slurry wall around L.G. Everist pits has created <br />higher heads in this area and as a consequence, the alluvial aquifer is discharging and <br />contributing more water to surface flows in the river than what was occurring prior to the <br />slurry wall construction. <br />8 <br />