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III IIIIIIIIIIIII III • • 9s9 DATE: September 17, 2001 TO: Gregg Squire FROM: Allen Sorenson RE: Floodplain Issues, Mamm Creek Grave/ Pit, Roaring Fork Resources, Inc, File Nn. M-1000-113 The Division of Minerals and Geology has completed a review of the Mamm Creek Pi[ mined land reclamation permit application as revised by submittals received at the Division's office on August 17. 2001 and August 22, 2001. This memorandum specifically addresses the Mamm Creek Gravel Pi[, "Colorado River Floodplain Study" prepared by High Country Engineering and dated August 3.2001 and the revisions [o the study included in a memo from Leslie Hope, High Country Engineering, Inc. and dated August 20, 2001. I was assigned to review [he reclamation permit as modified to determine if the proposed mining and reclamation plans are sufficiently protective to the prevailing hydrologic balance during flooding, and [o determine if any of the floodplain mitigation measures proposed would have a material impact on the amount of reclamation bond [o be required for permit issuance. The following discussion details the clarifications required, either prior to application approval, or prior to the commencement of mining if the applicant agrees to a conditional approval with stipulations. The current flood control plan for the Mamm Creek Gravel Pi[ calls for [he installation of a low berm along the north perimeter of the site to exclude the 50-year Flood from the operations area both during mining and following reclamation. The primary purpose of this berm is to prevent frequent comingling of the Colorado River native fish species with non-native species that may populate the gravel pits following reclamation. Since the low berms will be permanent features in the post-mining landscape under the current plan, there will be no reclamation costs associated with removal or regrading the berms, hence no impact on [he amount of bond to be required. On occasion, mining plans specify [he installation of flood control or drainage control structures at some intermediate point in the life of the operation. In these cases [he Division may require that bond be posted to cover the cost of installation of the necessary structures up until the time that the structures have been installed, inspected, and accepted. This type of bonding does not appear to be necessary for the 50-year flood control berm, as it is stated on page 7 of the Floodplain Study, "The berm will need to be constructed and armoring put is place at the beginning of the gravel extraction." The Operator should be notified that the Division considers installation of the 6emt for each phase concurrent with the earliest stages of mining in each phase to be a binding permit condition. The current flood control plan also calls for the installation of a control sill at the downstream end of the active pit to provide a controlled overflow location. The Operator must specify the timing for the installation of the control sill or sills. If the sills are to be installed at some time after the earliest stages of gravel extraction, the Division will require that the bond amount include a cost for installation. If this is the case, the Division will further require the Operator to supplement Exhibit L to the permit application by providing an estimated cost to complete installation of required control sills.