PERMFILE65956

• Permit No: C1981010
• IBM Index Class Name: Permit File
• Doc Date: 12/11/2001
• Doc Name: STOCK POND CERTIFICATIONS
• Section_Exhibit Name: APPENDIX Q SECTION XXIX
• Media Type: D
• Archive: No

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TerraMatrix
Engineering a Environrnetatal Settviees
165 S. Union Boulevard • Suite 400
Lakewood, Colorado 80226
303-763-5140 • fax [phone]
February 2, 1995
Mr. Forrest Luke
Trapper Mining Company
P.O. Box 187
Craig, Colorado 81626
Re: Surface Water Hydrology Calculations for Existing Stock Ponds
Dear Mr Luke:
(via: U.S. Mail)
In January 1994, TerraMatrix conduced the necessary hydrologic calculations to evaluate designs
and sizing of existing stock ponds at the Trapper Mine located just south of Craig, Colorado.
Since that time, several ponds were eliminated and several new ponds have been added. In
addition, several contour ditches were removed. Given these changes, TerraMatrix has revised
the hydraulic calculations to evaluate designs and sizing of the existing ponds. The following
describes the assumptions, methodology, calculations, and results of TerraMatrix's study.
1.0 INTRODUCTION
Trapper Mine is a surface strip mine located just south of Craig, Colorado. As mining progresses,
reclamation of disturbed areas is completed behind the active mining areas. As part of
reclamation, Trapper Mining Company has constructed a system of contour furrows, stock ponds,
and designed drainage to effectively convey run-0ff and retain water oo the reclaimed slopes which
provides a reliable water source for wildlife use. The ponds are permanent, and under current
regulatory interpretations, the spillways need to be able to safely pass the peak discharge from a
24-hour, 25-year event.
To determine the peak flow for each pond, TerraMatrix utilized the SEDCAD+ computer model
assuming aworst-case scenario where each pond would be full. Spillway flows and erosion
protection requirements were determined utilizing Mannings Equation for open channel flow. A
summary of all model inputs and results are shown on the attached table. Following is a brief
discussion of the modeling methodology and assumptions.
2.0 METHODOLOGIES AND ASSUMPTIONS
The required hydrology calculations for each stock pond were performed utilizing the SEDCAD+
Computer Modeling Program developed by Civil Software Design.
SEDCAD+ is a hydrologic and hydraulic calculation model designed for use on computer systems.
The SEDCAD+ model calculates run-off and peak flow via a numerical modeling technique based
• on user inputs of a design storm event, (i.e., rainfall amount and duration, selection of rainfall
distribution, and convolution increment). Hydrograph's are developed on a subwatershed basis
with the input of area, time of concentration, SCS Curve Number, and the selection of 1 of 3
dimensionless double triangle unit hydrograph shapes. Routing of hydrographs is accomplished
by Muskingum's Method.
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