2022-04-04_PERMIT FILE - M2022013 (38)

Home Browse Search

Two Rivers Gravel Pit Mine Riverside Berm Failure Analysis and Flood Control Mitigation Plan 7ech"olooicess January 22,2020 Central Field is located on the South Platte River side of the site and riverside berm failure analyses will focus on that river's berm. And, Northwest and Northeast Fields are located on the Big Thompson River side of the site and berm failure analyses will focus on that river's berm. There will also be a riverside berm developed on the east side of Northeast pit as the result of mining. Head cutting will be evaluated with respect to possibility of a Northeast Pit berm head cut entering the Big Thompson River. Modeling analyses is based on a 100-ft-wide riverside berm throughout the Site. Should it be determined that riverside berm width has possibility of river capture, then wider riverside berm widths will be evaluated via WinDAM C to determine adequacy. Because there is possibility for flooding and head cutting/erosion on any of the three pits, Varra Companies, Inc. plans on mitigation. That mitigation will be accomplished via a six-step approach: (1) Predict likely riverside berm head cut/erosion locations. (2) Perform head cutting and erosion analysis to predict riverside berm head cutting, length, width, and failure time (time from initial head cutting on pit side, to riverbank on riverside). (3) Determine pit fill time based on flood and groundwater inflows and compare to riverside berm head cut time and length. (4) If head cut length is less than riverside berm width at the time that head cut elevation equals pit water surface elevation, then head cutting/erosion ceases and will not capture the river. (5) If head cut length is not sufficient to prevent river capture assuming a 100-ft-wide berm, assumed width will be increased and then repeating steps (2)—(4)until a sufficient riverside berm width is estimated. (6) Provide hard armoring in locations most susceptible to head cutting/erosion during reclamation as an additional measure of safety. The above six items are explained in detail below in sub-sections 3.1 through 3.6. A conservative philosophy was applied throughout the study. Because of the many "unknowns" when evaluating natural occurrences, analytical approaches, scenarios, and modeling input parameters were biased so that results tended toward the most head cutting and erosion. As with any computer models of natural processes—it is not an exact science. There are many uncertainties without the luxury of comparing results to an actual event. However, such modeling can still provide valuable information for planning and decision making. pg. 10