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2013-03-19_REVISION - C1981019
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2013-03-19_REVISION - C1981019
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Last modified
8/24/2016 5:18:41 PM
Creation date
3/25/2013 8:38:41 AM
Metadata
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Template:
DRMS Permit Index
Permit No
C1981019
IBM Index Class Name
REVISION
Doc Date
3/19/2013
Doc Name
3rd Adequacy Letter Response (Hand Delivered)
From
Colowyo Coal Company
To
DRMS
Type & Sequence
TR95
Email Name
RDZ
DIH
Media Type
D
Archive
No
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Per Colowyo's December 13, 2012 letter, another justification for using <br />different curve numbers is the impact on the reclamation schedule for the <br />mine site. The Division does not accept this as a valid reason to update the <br />curve numbers in the SEDCAD models. However, we do appreciate that <br />Colowyo has provided this information; it helps us understand Colowyo's <br />underlying concerns related to this technical revision. <br />COMMENT #2 <br />Colowyo is correct in the assumption about the Division's error. In our July <br />2, 2012 letter, we did mean to state "13 soils" rather than "C soils." Thank <br />you for pointing out this error. <br />Colowyo states that in the past " SEDCAD modeling overestimated actual <br />flows witnessed at Colowyo Mine." What is this assertion based on? Does <br />Colowyo have flow data or other information to confirm this? It has been the <br />Division's belief in the past that the models have underestimated flows. This <br />was part of the impetus for TR -73. <br />Colow oy s Response: <br />Colowyo's experience has been that under storm event conditions that are not <br />short duration /high intensity events, the model predicts a much greater volume of <br />runoff generation than is witnessed as inflow into the ponds. This suggests that <br />Colowyo's reclaimed areas are intercepting and holding a much greater volume <br />of precipitation than the modeling parameters assume. This phenomenon held to <br />be true utilizing the curve numbers in place prior to the approval of TR -73. <br />Colowyo is required by regulation to design sediment control /stormwater control <br />structures to treat and process the resulting runoff for various defined storm <br />events for 10 year, 25 year, and 100 year 24 -hour events, depending on their <br />purpose and intended use. There are many instances when a set of storm <br />conditions can generate stormwater volumes that exceed the design capacities of <br />these structures, and the modeling itself bears this out. This was known prior to <br />the incorporation of these regulations, and it was determined that these defined, <br />prescribed storm scenarios were protective. <br />Promulgated regulations concerning the design, structure, and function of <br />sediment control /stormwater control structures do not mandate or expect <br />operators to design and build structures that would be protective under all storm <br />conditions. Colowyo believes that the structures currently in place are protective <br />to the public under the condition assumptions utilized to design them. In all but <br />two instances to date, the structures have been protective to the public regardless <br />of the size or intensity of storm events in the past several decades. Colowyo <br />contends this fact should be a source of confidence in previous actions and <br />A mining property of Western Fuels - Colorado, A Limited Liability Company <br />
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