PERMFILE125715

• Permit No: C1982056
• IBM Index Class Name: Permit File
• Doc Date: 12/11/2001
• Section_Exhibit Name: Exhibit 07i Review of Subsidence Effects on Streams
• Media Type: D
• Archive: No

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• Consultants in Hydrological
And Geological Engineering
MEMORANDUM
DATE: February 15, 1996
TO: Rick Mills, Cyprus Twentymile Coal Company
FROM: R.M. McDermid
165 S. Union Blvd., Suite 400
Lakewood, Colorado 80228
Phone: [phone]
Facsimile: [phone]
SUBJECT: Review of Cyprus Twentymile Subsidence Effects on Streams
The streambed profiles for Foidel Crcek and Middle Creek as they cross over the proposed Right
I.ongwalls of Foidel Creek Mine have been reviewed. The review is based on the revised Middle
Crcek and Foidel Creek Subsidence Prediction tables and figures received from SubTerra Inc. The
Subsidence Prediction Tables and Figures were prepared in February 1996 based on a revised plan
for the Eastern Mining District. This review replaces a previous review titled "Review Cyprus
Twenrymile Subsidence Effects on Stream" prepared by Ramsay M. McDermid of Hydro-Geo
Consultants, Inc on April 20, 1995.
• A comparison has been made between the present stream profile and that which is expected after
subsidence. The following material provides a discussion of specific areas along the two stream
profiles. At this point it appears that the effects of subsidence will probably not cause disturbance
to the stream profile to the extent that significant damage is caused and mitigation required. The
stream gradient in most cases will quickly adjust to approximately pazallel pre-existing slopes after
several high flow events.
Criteria have been defined for determining at what point stream channel changes might occur that
require mitigation. If headcutting is greater than throe fcet or if incising of two feet or greater
occurs more than 400 feet from the point of origin of the head cut, channel morphology should be
evaluated. It should then be determined if mitigation or repair work is necessary.
Foidel Creek (see Fires Si end Si(I)
The average channel slope, based on the profile presented in Figure Si, is about 0.4796 upstream of
the rib between Panels 7 Right and 8 Right and 0.7396 downstream. Based on the assumption that
the system is in approximate equilibrium, the subsided channel will probably adjust itself W about
0.70% f.
1) As the creek crosses over Panel 6 Right; after subsidence there will be a head cut as much
as throe feet deep tapering off to zero fcet about 1,000 to 2,000 feet upstream, th about the
• same channel cross section as exists now.
This will occur as alluvial silts and clays from the streambed are eroded and redeposited in
the subsided area in the center of Panel 6 Right, and the natural streambed gradient is re-
established. No mitigazion is anticipated to be required.
lu\c: \cyprus\tweety\suefftt.mem
JIS/96