2011-06-06_PERMIT FILE - M2011001 ‎(8)‎

• Permit No: M2011001
• IBM Index Class Name: PERMIT FILE
• Doc Date: 6/6/2011
• Doc Name: Response to 2nd Adequacy Review
• From: Tetra Tech
• To: DRMS
• Email Name: PSH
• Media Type: D
• Archive: No

TETRA TECH
June 6, 2011
Mr. Peter S. Hays
Environmental Protection Specialist
Division of Reclamation, Mining and Safety
1313 Sherman Street, Room 215
Denver, CO 80203
RE: Response to Iverson Mine (M-2011-001) Second Adequacy Review Comments
Dear Mr. Hays:
We received your Second Adequacy Review comments regarding the 112 Reclamation Permit
Application for the Iverson Mine dated May 17, 2011. Below is a listing of each of your comments
followed by our responses in italics.
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 discussed this comment with Tony Waldron to understand how the Urban Drainage and Flood
Control District (UDFCD) Publication "Technical Review Guidelines for Gravel Mining Activities within
or Adjacent to 100-year Floodplains" would apply to the Iverson Mine project which is outside of the
UDFCD. Mr. Waldron explained that the DRMS would like us to analyze the Iverson Mine project in
relation to the Cache la Poudre River and use the UDFCD Guidelines to propose mitigation measures
that make sense for the Iverson Mine project.
Therefore, Tetra Tech conducted a site visit to the Iverson property to reevaluate the impact flow of
the Cache la Poudre River may have on the Iverson Mine. The meandering nature of the Cache La
1900 S. Sunset Street, Suite I-F, Longmont, CO 80501
Tel [phone] Fax [phone] www.tetratech.com