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Mr. Joseph Dudash <br />May 16, 2006 <br />Page 2 <br />the analysis and recommended further evaluation of the landslide. Our office has reviewed the letterwith respect <br />to the current application and has found the concerns to still be valid. Displacement of the landslide into the <br />reservoir area could result in overtopping of the dam and/or blockage of the intake structure. From a dam safety <br />viewpoint, this is equally significant to the potential for seismic damage on the embankment dam itself. Our <br />office recommends that concerns raised in the June 11, 2003 setter with respect to the landslide stability analysis <br />be addressed prior to long-wall mining within cone-mile radius of the dam. <br />As stated above, Stipulation Number 9 requires a detailed monitoring plan specific to potential seismicity <br />and any other potential impacts induced by longwall mining. Exhibit 18 is presented in the application as the <br />seismic monitoring program. It is our position that an important part of the seismic monitoring program is to <br />confirm or contradict assumptions made in the geotechnical study. The study and mine application states that <br />longwall mining is proposed within 1,300 feet horizontally to Bruce Park Dam with anticipated mined-induced <br />Richter magnitudes between 2.0 and 4.2. It is unclear from the submitted seismic monitoring plan if seismometers <br />have been or will be placed within 1,300 feet horizontally from the dam. It seems logical that this condition <br />should be met in the seismic monitoring plan for developing both baseline data and monitoring yield thresholds <br />during mining. <br />The geotechnical study concludes that the yield acceleration for the dam is 0.20g for a magnitude 4.2 <br />event located 1,300 feet horizontally from Bruce Park Dam. The seismic monitoring p]an indicates that <br />seismometers will record continuous waveforms of seismic activity. For reporting purposes, the data should be <br />provided in a format that can be directly compared to the geotechnical study to determine if threshold yield <br />accelerations are exceeded. Monitoring thresholds should be established at the minimum yield acceleration found <br />from slope stability analyses of the dam and landslide. Our office and DMG should be notified whenever peak <br />ground accelerations are exceeded during the monitoring period, whether mine-induced or naturally occurring. <br />Additionally, a frequency should be established for submitting historic seismic data to DMG and DWR. <br />The current permit submittal does not appear to address monitoring of existing peizometers on Bruce Park <br />Dam or surveying of movement monuments on the dam crest. Monitoring of these instruments is just as cmcial as <br />the proposed seismic monitoring to evaluate the potential impacts of mining. Again, we refer to our letter ofJune <br />11, 2003 for recommendations on monitoring frequencies of these important performance indicators. <br />We appreciate the opportunity to review and provide comment to the Colorado Division of Minerals & <br />Geology and hope the information provided is helpful in your review ofthe submitted coal mine permit revision. <br />Sincerel <br />Jason P. Ward, Ph.D., P.E. <br />Dam Safety Engineer <br />cc: Frank Kugel, Division Engineer <br />Steve Tuck, Water Commissioner <br />Mark Haynes, Chief of the Safety of Dams Program <br />John Mathewson <br />Terror Creek Ditch & Reservoir Company <br />17445 Garvin Mesa Road <br />Paonia, CO 81428 <br />