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Daniel Hernandez April 21, 2006 Page 2 to all items related to Monument Dam. Due to the large size of the response document, we have elected to correspond our review comments with the alpha-numeric sections and sub-sections. Please advise if we have missed any sections of importance that pertain to Monument Dam. Section 2.05.6(61(a-fl -Subsidence Monitoring and Subsidence Control Plan Adequacy Item: MCC indicates that if damage to the dam and related structures occurs, the notification plan outlined in Section 2.0506(6)(e)(i)(D)tyill be activated. Additionally, if significant damage is imminent or human life is in danger, all necessary precautions and actions will be immediately implemented by MCC to lessen or eliminate the threat. DWR Response: In addition to the notification procedure, MCC should immediately contact the Minnesota Ditch and Reservoir Company and/or the Division of Water Resources for activation of the Emergency Prepazedness Plan (EPP) for Monument Dam. The EPP is on file with our office, the Ditch Company, and with Local Emergency Managers and provides immediate defensive action to prevent failure of the dam and minimize property damage and loss of life. Section 2.05.6(6)(e)(i)(D) -Detailed Description of Predicted Subsidence Phenomena Adequacy Item: MCC proposes several monitoring procedures to confirm that Monument Dam is outside the angle-of--draw ofmining-induced subsidence. Monitoring is to occur prior to, during, and after mining in the area at specified locations and frequencies. DWR Response: Overall, the proposed monitoring systems and the proposed ongoing and conditional monitoring frequencies are acceptable to our office. We request to be informed ofplacement of all new survey monuments and to be copied on all survey, piezometer, and accelerometer/seismometer monitoring data in and around the dam. Our office will use the subject document to ensure that monitoring frequencies and submittals are followed as indicated. Reporting ofpeak ground accelerations and subsidence monitoring data will also help validate the slope stability model assumptions for the dam and landslide. Adequacy Item: MCC summarizes the results of the slope stability analysis and implementation of proposed preventive measures contained in the report by GEI Consultants, Inc. entitled Geotechnical Evaluation of Mine-Induced Seismicity on Monument Dam. The text states, "...The construction and implementation of these structures and activities will allow the dam to withstand a seismic event of at least magnitude 2.3 (Richter scale) generating a peakground acceleration (pga) of 0.16 g. As stated in the GEI report, a maximum seismic event and pga anticipated to be generated by mining in the SOD will be ML 2.3 and 0.06 g, respectively. " The report goes on to identify the monitoring "threshold event" pga of 0.16 g. DWR Response: The statement is correct that the GEI report establishes a maximum potential Mc of 2.3 from long-wall coal mining. However, MCC incorrectly identifies a yield acceleration of 0.16 g