2006-05-31_REVISION - M1978056 (7)

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It. ANdERSON CONSUITlNCt ENCIINEERS, INC. Civil • Water Resources • Environmental May 25, 2006 Mr- Terry McKee Department of the Army CiOvf Corps of Engineers, Omaha ,strict Denver Regulatory Office 9307 South Wadsworth Blvd. 1 CO 80128 6901 tl "4, 4""7' RECEIVED 4AY 2 6 2006 Y wd r P I sp, Lttt eton, - - - . - -_ - . / - Re: Durham Flit Long Term Flood Control Mitigation Plan - Review of Final Construction Drawings. / Permit No. M-1978-0A, Corps File No. 200380651, [ACE Project No. COVC03-01] Dear Terry: On behalf of the Varra Companies, Inc., Anderson Consulting Engineers, Inc. (ACE) is pleased to submit the attached Durham Pit Flood Control Mitigation Plan (FCMP). This Long Term Flood Control Mitigation Plan is required as part of the State of Colorado Division of Minerals and Geology (DMG) and Mined Land Reclamation Board's October 27, 2004 Findings of Fact, Conclusions of Law and Order for the Varra Companies, Inc. Mining Permit Amendment (See Attachment A). The Long Term FCMP is required to minimize any potential adverse offsite impacts due to a 100-year flood event. The FCMP consists of final construction drawings, a copy ofwhich is shown in Attachment B. As requested in the Corps of Engineers' March 30, 2004 letter (See Attachment C) this material is being forwarded to you for your review. This letter follows our letter dated November 18, 2005 in which we requested a preliminary review by your office based on the draft Durham Pit :FCMP. As shown in Attachment B, a spillway will be installed in a low area at the west end of the existing berm on the right bank of the Cache la Poudre River. The spillway will be approximately 1270 feet long and installed at an average elevation of 4921.5 feet. The spillway will allow a portion of the floodwaters in excess of the 10-year flood event to exit the main channel and be conveyed in the right overbank. Conveyance of flows through the pre- and post-reclamation Durham Pit will occur in a manner similar to the pre-mining condition. The spillway will consist of a 1-foot wide concrete control sill and a.20-foot wide aggregate driving surface. Slopes landside of the spillway will be very gradual on the order of l OH: IV and may be protected from the effects of erosion by either a reinforced vegetative cover or a rock armor layer. The concrete sill maybe protected from potential main channel erosion by buried riprap revetment. Any areas disturbed during construction will be revegetated with an appropriate seed mixture. The FCMP shows final reclamation grading around the Durham Pit in order to support the proposed floodplain limits. COVC03-01 USAGE Terry McKee LTl-F1NAL.doc 772 Whalers Way, Suite 200 - Fort Collins, CO. 80525 Phone: (970) 226-0120 - Fax: (970) 226-0121 - www.acewater.com