GENERAL53197

• Permit No: C1981037
• IBM Index Class Name: General Documents
• Doc Date: 11/3/1993
• Doc Name: MEMO GEC WEST PIT DIVERSION DESIGN
• From: DMG
• To: LARRY ROUTTEN ERICA CROSBY
• Permit Index Doc Type: GENERAL CORRESPONDENCE
• Media Type: D
• Archive: No

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STATE OF COLORADO
DIVISION OF MINERALS AND GEOLOGY
Depa«ment of Natural Resources
131 3 Sherman $L, Room 215
Denver, CO 80203
Phone: U031 866-3567
FA%:1303)872-8106
Date: November 3, 1993
To: Larry Routten
Erica Crosby
From: David Berry~~
Re: GEC West Pit Diversion Design
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Roy Romer
Governor
Michael 8. Long
Division Director
As requested, I have reevaluated the GEC West Pit drainage control design submitted to you
previously, and some changes are recommended. My recommendations, and the substantiating
methods and references are provided below. It is my understanding that the landowner and the
Division of Minerals and Geology (DMG) have decided to join the existing channel situated west
of the West Pit, with the Magpie Channel at the location of an existing pond. This pond would
be removed. 1 have based the following designs on these instructions. I have not attempted to
evaluate any alternative design options.
In general, a theoretical 100-year, 24-hour peak flow was derived for the West Pit channel
work, using available site-specific and published reference material, and the SEDCAD software.
Channel configuration and riprap specifications were derived also using the SEDCAD software,
the Surface Mining Water Diversion Design Manual (Simons et al., 19821, and the Urban Storm
Drainage Criteria Manua/ (Urban Drainage and Flood Control District, 1969). Each input variable
is discussed in detail.
Drainage Basin Area
The drainage basin was outlined on the Rockvale, Colorado, Quadrangle (USGS, 1959), then
digitized using the SEDCAD software. The basin is outlined as shown in Figure 1 .
The drainage basin area was determined to be approximately 445 acres.
Precipitation Event
The design precipitation event was derived by plotting the mine site location on the appropriate
map shown within the Precipitation -Frequency Ai/as of the Western United States (Miller
et al., 1973), then interpolating between the isopluvial lines. The 10-year, 24-hour; 25-year,
24-hour; and 100-year, 24-hour maps are attached (Figures 2 through 4), and the mine site is
plotted on each map.
The annual precipitation value was used, as is normally the case with all of our permitting and
design work. I have noted that some previous permitting work has included values mapped