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(TANSER lJTJAN -&-_ ASSO('IATES <br />Hvdrogeological and Environmental Consultants <br />overbank flows. Given the gravelly nature of the overbank material and abundant <br />vegetation along the banks of the river channel that includes large cottonwoods, an "n" <br />value of 0.040 appears more appropriate than 0.035 that was used in the D&A model. <br />Water Level Surface Error <br />Water level elevations should decrease from upstream stations to downstream stations. <br />Between Station 60 and Station 50, the water elevation erroneously increases 0.34 feet. <br />Conclusions <br />Results of the model simulations support the historical and current observations <br />that construction activities at the Everist mine have not only altered the <br />hydrologic regime of the South Platte River but have also changed the <br />morphology and topography of the original floodplain on the west side of the <br />river causing a flow constriction. Storm flows, which in the past could spread and <br />dissipate across the pre -mining flood plain and have no effect on the Bachofer <br />property on the east side, now flood his land. <br />• Results by D&A showed negligible impact of mining operations on the water <br />level surface from the 100 year event when compared to pre -mining conditions. <br />Modeling results from GL&A indicate a significant difference (an increase of <br />approximately 2 feet) in surface water elevations in the vicinity of the Bachofer <br />property between pre -mining and existing mining surface topography. <br />• Model simulations using the 5 -year event (7,900 cfs) indicate flood levels are now <br />close to the 100year event previously determined by the Flood Insurance Study <br />(FIS) for the Town of Fort Lupton (USDH and UD, 1978). <br />The GL&A model was used to conduct a flood plain analysis for the 5-, 10-, and <br />100 -year design events, which corresponds to 7,900, 10,000, and 29,000 cfs. The <br />100 -year flood event (29,000 cfs) was used as the baseline flood event as <br />previously modeled by Deere and Ault in their Flood Hazard Development Permit <br />#355 submitted to Weld County December 9, 2008. Evaluation of only the 100 - <br />year event did not identify adverse impacts for the lessor and much more common <br />flood events, such as the 5- and 10 -year flow events. <br />Pre -mining topographic data for this model were obtained from the 7.5 -minute <br />digital elevation model (DEM) data files from the USGS national elevation <br />dataset (NED). These data are digital representations of cartographic information <br />in a raster form. The 3-D raster data file is from the same period of time that the <br />1950 7.5 minute Fort Lupton quadrangle map was generated and was used to <br />create a 2-D contour map with 2 -foot contours by interpolation algorithms using <br />Autocad® Civil 3-D. <br />- 13 - <br />