2014-02-13_REVISION - M1977493 (2)

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Review Assistance Memorandum — Mayflower Tailings Storage Facility (aka 5 Dam) February 13, 2014 Page 5 of 6 Section 4.5.3.2 — Instrumentation 20. Page 4 -7, last paragraph, "Piezometer levels are automated... Comments and Questions: a. Can the peizometers also be read manually to check and confirm the automated readings? b. See additional comments in Section 6 Section 5.2 — Barges and Mayflower Tunnel Systems 21. This section describes how the decant lines will be replaced with a tunnel system to act as an emergency spillway for the flood routing requirements presented in Section 3.6 and in the 2005 "Climax Mine Tailings Impoundments Hydrology Report" prepared by W.W. Wheeler and Associates. However, timing of the decant line closure with construction of the tunnel is not clearly presented. Additionally, review of the 2005 Hydrology Report provides design information on the capacity of the decant lines to pass the inflow design flood, but no information has been provided for design of the tunnel system. Recommendations: a. Design of the tunnel system and flood hydrology should be provided to DRMS for review prior to construction. b. Timing of the decant line closure and tunnel construction should be clarified. c. Is permitting of Mayflower TSF operations contingent on construction of the tunnel system? 22. Page 5 -2, Section 5.2.1 Decant Pond Operation, Success of the tunnel system to act as an emergency spillway to safely route extreme flood events relies on maintaining the level of the decant pond according to the operating limits outlined in this section; the 800 -foot beach width, 2,000 acre -feet of flood storage, or 6 -feet below the dam crest. However, there is no discussion of how these parameters are physically measured to ensure the "prescribed upper operating level" is maintained at or below this level. Recommendations: a. It may be beneficial to provide a way to gage the performance of the dam and operations against these specific parameters. This information could be a incorporated into the Daily Checklist and Operating Logs, or by separate periodic monitoring and reporting. b. It would be helpful to have clarification on how each of these parameters will be physically measured and monitored. Section 5.3.1.1 — Subdrain System, Operation and Maintenance 23. The discussion indicates that "the drains were inspected in 2012" and that "remote video inspections" may be conducted in the future. Questions and Recommendations: a. Was DRMS provided a copy of the 2012 video inspections? b. Was a report of the existing condition of the drains provided to DRMS? c. It is recommended that design of the drain extension be provided to DRMS for review prior to construction. Section 5.3.1.2 — Subdrain System, Instrumentation 24. The text indicates "there is no instrumentation associated with the subdrain system" Comments and Recommendations a. Monitoring of flows produced by internal drainage systems in dams can be an important indicator of safety and performance.