2025-03-03_PERMIT FILE - M2024056 (2)

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1. A detailed analysis of the inflow/outflow structure is required. This requirement is described in the Division's Floodplain Protection Standards for Sand and Gravel Pits Adjacent to Rivers and Perennial Streams (attached). The operator has included a detailed design drawing(F-2 Flow Structures), but additional information is needed. Is the design based on a standard methodology from a drainage criteria manual or other source? This must be discussed in Exhibit G.Also, Exhibit G requires a description of how the velocity on the F-2 figure (7.09 feet/second) was determined. Further discussion on the inflow/outflow structure design basis was added to Exhibit G by Lewicki's hydrologist, Ben Langenfeld. The design is based on the Mile High Flood Control District's Technical Review Guidelines For Gravel Mining & Water Storage Activities Within Or Adjacent To 100-Year Floodplains dated January 2013, specifically Section 2.4.5 Side Channel Spillway. The velocity used in the design for sizing riprap is based on previous designs along the South Platte River with flood water velocity within the river of—7 feet/second. A HEC-RAS model was developed to demonstrate that the assumed velocity is sufficient for the flood conditions at the Evans site. Please see the revised Exhibit G for more details. 2. Additional information is required to explain the inputs to the Hydrology Calculations in Appendix G-1. In particular, how were runoff coefficients determined?Revise Exhibit G to include this information. Exhibit G has been revised to include a cover page for Appendix G-1 that explains the hydrology calculation. Runoff coefficients are sourced from the American Society of Civil Engineers for different land uses unless otherwise required by federal, state, or local regulations. 3. The applicant should explain the difference between the volumes in Table G-3 on page G-4 fifth column from the left) and the volumes in the Hydrograph Summary Report in Appendix G-1 (sixth column from the left). These values are different. Please explain what each represents and why they are different. Revisions should be made to Exhibit G, as appropriate. Thank you for pointing out this discrepancy. The 5-year runoff volume was mistakenly reported as the 100-year runoff on the map and text. These have been updated to reflect the hydrograph summary which contains the correct values. Please see the revised Map G-1 and Exhibit G that fixes this error. Attachments • Exhibit G • Map G-1 Evans Mining Resource 2 Lewicki&Associates February 2025