2011-04-29_PERMIT FILE - M2011001 (21)

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Page 3 of 8 TETRA TECH Mr. Peter Hays April 29, 2011 14. The mining plan proposes a 100-foot setback from the Cache La Poudre River. In order to mine within 400 feet of the river, plans for engineered inflow and outflow structures must be submitted to the Division for review. This comment relates to comment 16 which we are still working through. Therefore, we will respond to this comment and provide the required information at a later date. 15. Due to the prevailing wind direction and fetch distance of the final lake configuration, the Division will require the Applicant to complete a wave analysis and wind erosion study for the site. A wave analysis and wind erosion study for this site is not necessary. As a below grade reservoir, any wave induced erosion significant enough to cause embankment failure would not result in a significant public safety hazard for the following reasons: • The embankment width, as currently designed, is no less than 100 feet. Full erosion of an embankment of this width is very unlikely. If a wave of significant height were to overtop the embankment, the width would allow the water to spread out and dissipate. • The maximum water surface in the reclaimed mine would only be slightly higher at select times of the year. In the unlikely event of an embankment failure that connects the river and the reclaimed mine, any release of water would be contained within the normal banks of the river. • The reclamation slopes are proposed to be 3HAV. The gentleness of the slopes significantly reduces the risk of failure. 16. Portions of the proposed mine site lie within the 100-year flood plain of the Cache la Poudre River. In the event of a significant flood event (100-year flood or greater) it is likely the proposed pit will be captured by the Cache La Poudre River. Wherever mining will occur within 400 feet of the river channel, a flood analysis and flood control plan must be evaluated and submitted. This would include pit side armoring, river side armoring, inflow and outflow channels, or other appropriate measures. Accordingly, the Division requests the Applicant provide the flood elevations(s) to be expected under a "worst case" flooding scenario and specific mitigation measures that will be implemented to minimize the potential for any offsite impacts. The potential for "berms" around gravel pits to be damaged during flooding is discussed in detail in the 1987 Urban Drainage and Flood Control District (UDFCD) Publication "Technical Review Guidelines for Gravel Mining Activities within or Adjacent to 100-year Floodplains." Strips of native ground or constructed fill between the stream and the gravel pit, and generally aligned with the flow direction of the stream are referred to in the UDFCD document as riverside berms. Riverside berms are proposed in the Iverson Mine application, which may be prone to erosion during a flood event. The required inflow/outflow structures are intended to mitigate possible slope failure during flood events for these berms. The Iverson Mine is outside of the political boundaries of the UDFCD, but the technical floodplain factors that led to the formation of the UDFCD guidance still apply and will be used to review the adequacy of the flood mitigation structures proposed for the site. Additionally, the site and surrounding area have a history of flooding as demonstrated by flooding event last summer which caused extensive damage to the 83rd Joint Venture mine site located east of the Iverson Mine and involved the Boyd Freeman Ditch located adjacent to the Iverson site. We will provide a response to this comment at a later date. 17. The Applicant intends to install a conveyor structure to transport material to the 35th Avenue site, File No. M-1977-036, for processing. The conceptual drawings included in Exhibit C are not adequate. Please provide additional information regarding the conveyor structure (i.e. composition, length, width, height, foundations, etc.) and a specific plan for reclamation of the structures. Additional details of the conveyor structure have been added to Exhibit C (Attachment 1). The conveyor structure will be anchored to the ground with rebar; no permanent foundations will be needed. Therefore, there will be little ground disturbance under the conveyor, once the conveyor is removed, the land will just need to be reseeded.