2008-11-13_ENFORCEMENT - C1981019

• Permit No: C1981019
• IBM Index Class Name: ENFORCEMENT
• Doc Date: 11/13/2008
• Doc Name: SEDCAD Review ‎(Memo)‎
• From: Kent Gorham
• To: Jim Stark, Sandy Brown
• Violation No.: CV2008004
• Email Name: KAG
• Media Type: D
• Archive: No

STATE OF COLORADO
DIVISION OF RECLAMATION, MINING AND SAFETY
Department of Natural Resources
1313 Sherman St., Room 215
Denver, Colorado 80203
Phone: [phone]

FAX: [phone] Interoffice Memo
To: Jim Stark; Sandy Br
From: Kent Gorham ,
Date: 11/13/2008
Re: SEDCAD Review, NOV CV-2008-004, Colowyo, #C-81-019
COLORADO
D IV IS I ON OF
RECLAMATION
MINING
SAFETY
Bill Ritter, )r.
Governor
Harris D. Sherman
Executive Director
Ronald W. Cattany
Division Director
Natural Resource Trustee
I have completed review of the technical materials submitted during the initial assessment
conference for NOV CV-2008-004 issued to Colowyo Coal Company.
One error was noted on page 2 of the accompanying text narrative where it is stated "The 10
year, 24 hour design storm with this distribution produces about .46 inches during the most
intense hour of the storm". This statement should read "The 10 year, 24 hour design storm
with this distribution produces about .92 inches during the most intense hour of the storm".
Colowyo submitted three SEDCAD storm runoff models along with supporting maps and
figures. The three SEDCAD storm models estimated runoff under three different scenarios:
10-year 24-hour storm event occurring on the current Prospect watershed
The actual storm event occurring on the current Prospect watershed
Scenario #2 above with the Prospect pond primary spillway blocked
The Division would concur that based on the rain data provided, the actual storm event
delivery varied from the Type II storm event that was used in the Prospect pond SEDCAD
model for pond design and construction. The rain gauge data shows that the storm appeared
to be more intense, in terms of inches of rain per hour, potentially greater than the design
storm by as much as .6 inches of rain per hour in terms of the rate of delivery. This increase
in the rate of rainfall could have the effect of increasing the actual peak flow and result in
excessive erosion and sediment generation. The effect on Prospect pond could
understandably be blockage of the primary spillway and overtopping of the pond. The
Division believes that the three modeling runs are reasonable and based on the best
available information. We have no reason to refute or question the results of the three
modeling efforts as submitted.
The Division compared the first SEDCAD model (10/24 model on current Prospect
watershed) with the approved model from the permit application used to construct the
Prospect pond (revised 2006). Peak discharge and total storm runoff volumes were
compared.
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