2016-12-22_REVISION - M1980244 (3)

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using a more realistic infiltration assumption, the aforementioned conditions need to be factored into the runoff estimate. b. Please provide the Division with a watershed delineation map demonstrating the 74- acre contributing area. Response 1a). Newmont engaged Ecological Resource Consultants, Inc. (ERC)to design an engineered diversion channel with the purpose of preventing surface runoff from entering the future Globe Hill Pit during a 100-year, 24-hour event. The attached ERC Technical Memo dated December 16, 2016 presents details concerning the design calculations. Response 1 b). Presented in Figure 1 on Page 2 of the attached ERC Technical Memo, the 74 acre Poverty Gulch Drainage Basin is indicated by an orange outline and remains consistent with the figure submitted in the original TR. The drainage basin is bound by Teller County Road 821 to the north (Figure 1) as runoff from areas up gradient of Road 821 is captured in the roadside ditch and conveyed out of the basin area. Additionally, acreage that will be converted to future pit was not included in the drainage basin as flows will still report to the original drainage until the pit is constructed. After pit construction stormwater will report to the diversion channel or be captured inside the pit with additional stormwater controls. Comment 2). Diversion Channel I Hydraulic Design. The first paragraph under "Stormwater Design"on the second page of the TR request indicates the channel is proposed to be five feet wide and two feet deep, lined with clay, and armored with riprap. It is unclear as to whether the five feet is the top or bottom width. The placement of riprap on top of fine clay where high flow velocity is expected requires a filter layer between the clay and the riprap to prevent the clay from migrating out of the base under high flow conditions. This might be a granular (e.g., Terzaghi criteria) or a non-woven geotextile filter. No hydraulic analysis for the proposed channel design was provided. No riprap size, sizing analysis, or gradation was provided. No details were provided as to how the inlet will be designed to ensure capture of the upgradient Poverty Gulch flows. No outlet energy dissipation size analyses, dimensions or details for transitioning back to the natural channel were provided. Please provide the following: a. Hydraulic analysis (e.g., Manning's equation) for the channel using the appropriate peak flow and justification for a selected roughness parameter; b. Riprap sizing (e.g., D50 - the Division can provide some suggestions for appropriate methods upon request); c. Diversion outlet energy dissipation sizing analysis, d. If a granular filter is proposed, please provide the appropriate analysis, e. Engineering drawings showing the channel profile, plan view, typical cross-section detail, inlet, and outlet details; f. References to appropriate engineering specifications as approved in TR-78. If new or additional specifications are required, please provide them for review and approval. Response 2a). Please see Page 3 in the attached Technical Memo for the hydraulic analysis. Response 2b). Please see Page 3 in the attached Technical Memo for the riprap sizing. Response 2c). Please see Page 4 in the attached Technical Memo for channel outlet and inlet design. Response 2d). The geotextile underneath the riprap lined channel will act sufficiently as a granular filter. Please see attached Technical Memo for further design information. Response 2e). Please see Appendix A in the attached Technical Memo for engineering drawings.