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Two Rivers Gravel Pit Mine <br /> Riverside Berm Failure Analysis <br /> and Flood Control Mitigation Plan <br /> 7ech"olooicess January 22,2020 <br /> 3.2.3 Hydrograph Development <br /> WinDAM C is driven by a user-derived inflow flood hydrograph based on the 100-yr peak <br /> discharge of the South Platte River at the confluence of the Big Thompson River. Being that the <br /> Site is located just upstream from the confluence, that is a representative 100-yr peak flood <br /> because it includes both rivers. <br /> Flood hydrographs on the South Platte River representative of the 100-yr peak could not be <br /> located. Also, the stream gage at Kersey, Colorado was destroyed during the 2013 flood and that <br /> hydrograph data is not available. In lieu of that, a flood hydrograph of the Big Thompson River <br /> (Griffin, 2014; Kimbrough—Homes, 2014) at I-25 from the 2013 flood was modified by <br /> adjusting the ordinates to meet the South Platte River 100-yr peak discharge (FEMA, Flood <br /> Insurance Study, January 20, 2016). This is demonstrated in the Appendix. <br /> An earthen flood control berm—likely built by local industry (farmers and oil and gas <br /> companies)— surrounds the site. Based on engineering judgment, it is estimated the earthen berm <br /> will control a 10-yr flood event and that amount of flow was subtracted from the inflow <br /> hydrograph that drives WinDAM C, and also for pit fill-time calculations. <br /> For analysis, flow was also separated across the site to reflect the influence of each river on it's <br /> adjacent floodplain. Both rivers and the intermediate floodplain would flow as a single river <br /> during a large flood event with the South Platte River carrying the majority of flow, and <br /> floodplain flow gradually decreasing across the floodplain from south to north (South Platte to <br /> Big Thompson Rivers) due to the majority of flood flow in the South Platte River floodplain. <br /> The flood hydrograph at the site was proportioned -based on peak flow of each river(FEMA, <br /> Flood Insurance Study, January 20, 2016) -by having two-thirds flow through the south side of <br /> the Site (Central Field) and the remaining one third flow through the north side of the site, or <br /> Northwest and Northeast Fields. The proportioned inflow hydrographs are included in the <br /> Appendix. <br /> A thunderstorm event(vs., snowmelt)was the flood of choice in the analyses because there <br /> would be little warning time for Varra Company, Inc. to stop dewatering. Thus, it would take <br /> longer for the pit to fill via groundwater ingress and allowing more time for head cutting/erosion <br /> consistent with the conservative approach to this study. <br /> 3.3. Determine Pit Fill Time and Compare to Head cutting Time <br /> Pit fill time is critical because if the pit is full, head cutting/erosion will not occur. And if head <br /> cutting is in progress, then once head cut elevation intersects pit water surface elevation, head <br /> cutting/erosion will cease. Analyses were performed for the case of each pit initially empty for <br /> conservatism. <br /> 3.3.1 Determine Pit Fill Time <br /> pg. 13 <br />