REV96144

• Permit No: C1981041
• IBM Index Class Name: Revision
• Doc Date: 12/29/1997
• Doc Name: ROADSIDE PORTALS MINES C-81-041 TR-23 CRDA-2 DITCH DESIGNS
• From: DMG
• To: DAN MATHEWS AND LARRY ROUTTEN
• Type & Sequence: TR24
• Media Type: D
• Archive: No

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STATE OF COLORADO
DIVISION OF MINERALS AND GEOLOGY
Deparlmenl of Natural Resources
131 3 Sherman 51 ,Roam 215
Denver, Colorado 80203
Phone: [phone]
FAX: [phone]-A 106
DATE: December 29, 1997
TO: Dan Mathews
Larry Rout[en(~~~
FROM: Erica Crosby j,0"
RE: Ide Portal Mines (C-81-041)
~ a~ R- 3• CRDA-2 Ditch Designs
Per your request, I reviewed the technical revision to the Roadside Portal Mine (C-81-041)
revising the ditch design for the pernanent drainage above CRDA-2. I have the following
comments that should be addressed by the operator;
~I~
DEPARTMENT OF
NATURAL
RESOURCES
Roy Romer
Governor
lames 5 Lochheazl
E¢ec W rve Direclrn
Michael B Long
Dmivon Direcbr
[n the previous channel designs a curve number of 89 was used representing a "D" soil
group and soil type of Persayo-Badland-Chipeta. The operator revised the curve number
in the current SedCad model to a factor of 83. Please inform the Division as to why a
different curve number was used in the runoff calculation for the permanent diversion
ditch above CRDA-2, or revise the model using a curve number factor of 89.
2. Powderhom designed the channel using a 5.5 fps maximum velocity, which is based ott
material classified as graded silts to cobbles when colloidal. According [o the text titled,
Design Hydrology and Sedimentology for Small Catchments, by C.T. Hann, B.J.
Barfield, and J.C. Hayes, a limiting velocity for this material is 4.0 fps for clear wrater and
5.5 fps for sediment laden water.
The channel above CRDA-2 collects runoff from an undisturbed watershed which is
predominately a sandstone cliff. The runoff generated from a stone may be more clear in
nature, requiring a limiting velocity of 4.0 fps. Powderhorn has a number of designed
channel segments that range from 4.91 fps to 5.49 fps, and could potentially be erosive.
This applies to the ditch segments with a slope greater than 1.4%. The channels will
carry the designed flow, but will be prone [o erosion. In the steeper segments of the
channel where velocities are greater than 4.91 fps, Powderhorn should revise the designs
for either vegetation or rip-rap armoring. Please revise the designs accordingly.