2015-08-31_PERMIT FILE - C1981010 ‎(59)‎

• Permit No: C1981010
• IBM Index Class Name: Permit File
• Doc Date: 8/31/2015
• Doc Name: Reconstructed Drainage Channel Design Model
• Section_Exhibit Name: Appendix Q Section XXXVII
• Media Type: D
• Archive: No

GUIDELINE FOR THE DESIGN AND NUMBER OF CHECK DAMS AND DOZER BASINS
FOR EROSIONAL CONTROL OF RECLAIMED LANDS FOR BOND RELEASE UNDER A
VARIETY OF CHANNEL CONDITIONS USING SEDCAD MODELING
This guideline is specifically applicable to reclaimed lands and associated drainage
channels under 640 acres prior to bond release. The guideline is a result of various
SEDCAD based models. Section 4.05.3 of Colorado DRMS regulations addresses the
design of postmining drainage channels with drainage areas under one square mile. In
this case, the applicable design criteria are based on a precipitation event with a 10 year
recurrent interval except as otherwise specified. While a similar type of analysis might
also be appropriate for larger drainages, this guideline does not apply to watersheds in
excess of one square mile.
The 10 year, 24 hour rain storm event used here is 1.40 inches, as approved by CDRMS
pursuant to RN -03 (July, 1992) wherein Trapper used NOAA Atlas 2, Volume III.
Appendix Q of the current permit document lists representative curve numbers, hydrologic
soil groups, and cover for native and reclaimed land. It should be noted that Part 630
Hydrology, Chapter 7 Hydrologic Soil Groups, subsection 630.0702 of the National
Engineering Handbook published by National Resources Conservation Services, USDA
specifically cautions against use of mapped soils groups when attempting to determine
runoff curve numbers for disturbed lands such as surface mines. Mining disturbance
completely rearranges the shallow soil structure and generally results in a significant
increase in shallow infiltration, a finding that was borne out by the 1981 Striffler and
Rhodes study commissioned by OSM. They performed infiltration tests on numerous
surface mined lands and showed infiltration rates well in excess of those that would
otherwise be selected based on simple mapped soil groups and land use condition.
At Trapper Mine, Appendix Q presents a curve number of 67 for land in a 2+ year
reclamation condition, but does not present a curve number for land at a bond release
condition, which would normally imply at least 4 years of growth and a vegetative cover
value of about 70%. A runoff curve number value of 62 is suggested as a reasonable
value for Trapper for the final reclamation condition.
Typically Trapper Mine uses a trapezoidal channel with a bottom width of 20 feet for
postmining channels. This width is the easiest to shape with a large dozer. These
channels are generally about 2.5 feet deep with 2H:1 V side slopes. Channel slopes are
variable but are typically about 10 percent. Several types of check dams have been used
at the mine. The check dams are built from coarse riprap obtained from competent rock
selected from the mine area. Where possible, the rock should be angular, well graded,
and properly placed to minimize excess voids. One type consists of alternating deflectors
while the second type consists of water bars across the channel. This analysis specifically
addresses this second type.
These are typically three feet in height, with a 1.51-1:11V upstream face and a downstream
face of 3H:1 V. Beneath the crest of the check dam, a rectangular keyway at least 2.5
foot in width and 3 foot in depth should be excavated and subsequently backfilled with
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