INSPEC40802

• Permit No: M1977215
• IBM Index Class Name: Inspection
• Doc Date: 3/15/1994
• Doc Name: BULLDOG MINE HYDROLOGIC EVALUATION
• From: CANONIE ENVIRONMENTAL
• To: ENVIRONMENTAL AFFAIRS
• Media Type: D
• Archive: No

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999
CanonleEnvironmental
March 15, 1994
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Canonie Environmental Services Corp.
333 Haggerty Lane
Bozeman, Montana 59715
Phone [phone]
Fax: 406~586~9724
94-012
Mr. Luke Russell
Corporate Manager, Environmental Affairs
650 California Street
San Francisco, CA 94108
Bulldog Mine Hvdrologic Evaluation
Dear Luke:
This letter responds to a February 23, 1994 correspondence from Allen Sorenson,
State of Colorado Division of Minerals and geology, regarding the Hydrologic
Evaluation of Bulldog Mine, File No. M-77-215. In his letter, Mr. Sorenson enclosed
SCS soil survey maps of the mine area.
The maps and accompanying soil unit descriptions indicate that essentially all soils in
the Windy Gulch drainage basin have been classified as being in the Hydrologic Soil
Group C, with minor occurrences in Groups D and B. In his letter, Mr. Sorenson also
identifies the soils as being in the C soil group, with minor occurrences in Groups D
and B. He also states that by duplicating the Curve Number (CN) selection
methodology described in my letter to you dated January 22, 1994, "... the•
placement of the soils in hydrologic group C (instead of B) is, in its self, justification
for using a Curve Number in the range of 75 to 82". The SCS Engineering Field
Manual (SCS, 1984) Table 2-3c, from which a CN is selected, does not support Mr.
Sorenson's assignment of a CN in the range of 75-82 for the Windy Gulch drainage
basin.
Based on Table 2-3c, the CN for a C type soil having aPinyon-juniper and grass
understory cover type, average runoff condition, and good hydrologic condition is 61.
Based on my field review of the drainage basin I believe there are only small areas of
fair and poor hydrologic conditions present. Allowing for tfiese small areas of fair and
poor hydrologic conditions, I believe a CN of 68 is reasonable to use for an evaluation
which is used to develop theoretical hydraulic channel design criteria. Mr. Sorenson's
recommended CN of 75 to 82 would be representative if the hydrologic condition of
the entire drainage basin were fair to poor, as shown in Table 2-3c.
Using the SCS methodology for estimating runoff described in my September 21,
1993 letter and a CN of 68, the computed runoff from a 100-year, 24-hour
precipitation event (2.8 inches) is approximately 140 cfs. Using the same hydraulic

82:W:\94~012\LETTERS\BULLDOG2.LTR IMarch 15, 19931