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2008-03-21_PERMIT FILE - C1980007A (5)
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2008-03-21_PERMIT FILE - C1980007A (5)
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Last modified
8/24/2016 3:25:41 PM
Creation date
6/20/2008 11:15:04 AM
Metadata
Fields
Template:
DRMS Permit Index
Permit No
C1980007A
IBM Index Class Name
Permit File
Doc Date
3/21/2008
Doc Name
pg 2.05-101 to 2.05-199
Section_Exhibit Name
2.05.5 & 2.05.6 Post-Mining Land Uses and Mitigation of Surface Coal Mining Operation Impacts
Media Type
D
Archive
Yes
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West Elk Mine <br />MCC will work with the Minnesota Ditch and Reservoir Company to obtain the appropriate approvals, <br />permits, and implement a preventative measures construction project to modify and improve the <br />Monument Dam. MCC recognizes that any dam modifications proposed to and carried out by the <br />Minnesota Reservoir and Ditch Company will require the activities be consistent with the reservoir <br />company's existing Forest-issued Special-Use Permit or will require a modification to the permit. <br />Implementation of this project will prevent damage to the dam/reservoir from the potential of mining- <br />induced microseismicity, strengthen the dam against damage due to naturally occurring seismicity, and <br />control damage due to past and future periodic movement of the landslide located on the left (south) <br />abutment of the dam. Exhibit 72, Prevention Measures, contains the general design of the measures to be <br />implemented that will allow the darn to reach a static safety factor of 1.5 and will safely withstand any <br />mine-induced microseismic event. The plans include the construction on the downstream face of the dam <br />of a sand chimney drain covered by stability berm. Additionally, a buttress constructed of erosion <br />resistant fill will be placed at the toe of the dam. The purpose of the bean and buttress is to increase the <br />stability of the dam fill itself and to impede further movement of the landslide located in the left <br />abutment. Other preventative measures to increase dam and landslide stability may include the slip-lining <br />of the existing outlet conduit with HDPE pipe to eliminate the possibility of further damage to the <br />existing cast iron pipe, replacement of the inlet structure to outlet conduit to eliminate leakage around the <br />existing structure, and construction of dewatering trench to further stabilize a portion of the landslide. <br />MCC, in addressing 2.05.6(6)(f)(iv)(A-D), will complete preventative measures on Monument Dam <br />(Exhibit 76) and may do one or more of the following: replace, repair, and otherwise restore structures <br />downstream or will purchase insurance policies addressing downstream damage and will be in effect at <br />the time of longwall mining, should catastrophic failure of the dam occur as a result of mine-induced <br />impacts. The dam and dam failure hazards are addressed in Exhibit 74, and are briefly mentioned here <br />from the Keith Bakeman, Dam Breach Analysis of 2/8/84, contained in Jim Norfleet's letter to Grant <br />Farnsworth, dated May 21, 1990: <br />"The clear weather breach of Minnesota Dam (sic) produced a calculated peak <br />flow at the dam of 10,280 cfs. The peak flow was reduced to 8,230 cfs by the time it <br />reached the mouth of Minnesota Creek at the North Fork of the Gunnison River in <br />Paonia, 9.8 miles downstream. The computed overbank flooding depths vary from 0.3 <br />feet to 3.5 feet in the lower 6 miles of Minnesota Creek, that contain road crossings and <br />buildings. Five gravel road crossings will sustain damage, up to complete failure. <br />Perhaps four houses will sustain shallow flooding but loss of life is not expected. These <br />houses will experience water damage up to structural collapse, depending on the distance <br />from the stream and stream movement due to erosion. Computed flood velocities within <br />the stream channel ranged from 42 feet per second down to 13 feet per second. The <br />quantity and velocity of flood waters are expected to cause extensive erosion and <br />resource damage along Minnesota Creek and probably deposit a sediment bar in the <br />Gunnison River (sic). <br />This evaluation of the clear weather breach indicates that the failure of Minnesota <br />Reservoir would probably not cause loss of life or excessive private or public property <br />damage. If a field check of the flood plain supports the findings of this report, the hazard <br />classification of Minnesota Reservoir will be reduced to moderate. <br />The clear weather breach of Minnesota Reservoir will: <br />1. Cause resource damage along 9.8 miles of Minnesota Creek. <br />-? 2. Damage five gravel road crossings, probably completely breaching the <br />roads. <br />2.05-172 Revised June 2005 PR10, Rev. March 2006; Afay 2006 PRIO. Nov. 2006TR107,Ap7•i12007TR108; Sep. 2007 PR12; Feb. 2008 PR-12
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