2016-11-28_REVISION - M1980244 - Laserfiche WebLink

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"CC&V is evaluating an alternative design for a stream bypass system that will intercept the flow in Poverty Gulch above that location, convey it around the mine to the Poverty Gulch streambed and discharge the flow into Poverty Gulch using a suitable energy dissipation structure". CC&V must commit in writing to the Division that no surface mining activity (including topsoil stripping and overburden removal) will take place within 400 feet of Poverty Gulch until a technical revision is submitted and approved by the Division for an acceptable reclamation/closure design alternative to the pipeline. Newmont's response to Adequacy Review Comment number 56 on September 28, 2016 is as follows: Response: The design of the Globe Hill mine area was approved by the Division under Amendment 10. As pointed out by the Division above, Newmont is evaluating alternative designs for managing stormwater in Poverty Gulch once the North Cresson mine area is developed. We agree that a pipeline is not an acceptable permanent alternative for reclamation or closure design. This alternative was envisioned as a temporary measure for routing stormwater during the development of the Glove Hill mine area. Newmont will continue to evaluate alternative designs for stormwater management in Poverty Gulch and will commit to discussing these alternatives with DRMS in advance of submitting a technical revision for this activity. During a meeting with DRMS on November 9, 2016 a verbal agreement was made that Newmont would submit a TR to Amendment 10 outlining a stormwater diversion structure to allow Globe Hill Pit preparation and mining activities to occur within the 400 foot buffer described above. The proposed stormwater diversion channel is further described below. Stormwater Design The Globe Hill Pit design takes the northwest corner of the pit across a small section of Poverty Gulch. It is important to note, that Poverty Gulch has been identified in Amendment 10 to have historically little to no flow, however, in order to ensure mining activity does not impede any stormwater flow, Newmont proposes the installation of an inlet collection structure, a diversion channel, and an outlet structure (Figure 1). The inlet collection structure will be placed in the drainage above where the pit intersects the gulch and will include wing walls to funnel flow into the channel (Figure 2). The 5 foot wide and 2 foot deep clay lined diversion channel with rip-rap will carry collected stormwater around the edge of the future pit to an outlet energy dissipation structure below where the pit intersects the gulch returning stormwater flow into the gulch (Figure 2). The design of the proposed stormwater diversion channel has taken into consideration existing and future topography of the area under conditions of a 100-year 24-hour stormwater event. The 100-year 24-hour event as defined in Amendment 10 is 3.5 inches and the total watershed contribution is 74 acres. A conservative approach was taken for the runoff calculation assuming 0% infiltration. This calculation estimates the 100 year 24-hour storm event will result in a maximum calculated flow of 11 cubic feet per second reporting to the channel. Due to site equipment capability and construction methods the channel design has a calculated capacity of 74 cubic feet per second providing an additional factor of safety. The channel will be 525 feet in length with a 65 foot elevation change from diversion inlet to diversion outlet resulting in a total gradient of 12.5%.