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The velocity used in the design for sizing riprap is based on previous designs along the South <br /> Platte River with flood water velocity within the river of—7 feet/second. In order to confirm that <br /> this velocity is still sufficient for riprap sizing, a HEC-RAS model was run for the Evans Mining <br /> Resource with the pits mined out and empty. The HEC-RAS model uses the publicly available <br /> data from the Colorado Water Conservancy Board for floodplain and floodway modeling along <br /> the South Platte River from Denver to Nebraska. USGS digital elevation models (DEMs) for this <br /> area were modified to include the presence of the mining pits proposed. <br /> The cross sections in the CWCB flood model were updated based on the modified DEM, and <br /> the HEC-RAS model rerun for the 100-YR (1%) event. A velocity map was generated from the <br /> model outputs to show the velocity of floodwater in the highly unlikely event of a 100-YR flood <br /> entering all three empty pits. It should be noted that the three pits will never be simultaneously <br /> empty, but this modeling scenario is conservative. The resulting model shows the velocity of <br /> floodwater entering empty pits. The velocity in the river and along the berm separating the river <br /> from the pits is around 2.0 feet/second, well below the 7.09 feet/second used in the design. <br /> sei= veer x <br /> i�- <br /> E"va <br /> -"vans Mining <br /> ' F <br /> 1 <br /> I <br /> 6- <br /> a- <br /> 0 <br /> iauni J <br /> Floodwater Velocity — Reclaimed Conditions (Modified DEM) <br /> As an additional check, the HEC-RAS model was run without the pits to see what the velocity of <br /> flood water was in this area under current conditions. This gives insight to the conditions that <br /> might be experienced at the site without large open depressions (pits) to take on flood water, <br /> Evans Mining Resource <br /> February 2025 G-3 Lewicki&Associates <br />