REV91895

• Permit No: M1974015
• IBM Index Class Name: Revision
• Doc Name: ATTACHMENT 4 ALLEN SORENSON MEMORANDUM
• Type & Sequence: AM6
• Media Type: D
• Archive: No

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STATE OF COLORADO
DIVISION OF MINERALS AND GEOLOGY
Department of Natural Resources
1313 Sherman 51.. Room 215
Denver. Colorado 60203
Phone [phone]
FAt. 13011 8 72-8106
DATE: January Z8, 2000
TO: Erica Crosby
FROM: Allen Sorenson ~~
RE: Interior Berms, Amendment AM-06, Western Mobile Boulder, Inc.,
Lyons Pit, Permit No. M-74-015
DIVISION O F
MINERALS
GEOLOGY
0.EC LAM AiION
MIN IN G•SAFEiv
Bill Ovens
Governor
Greg E. Walther
&etNive Dnettor
Michael B. Long
Drvrsion Dnector
I discussed the proposed interior berms at the Lyons Pit with Curt Pazker, a consulting engineer
engaged in the permitting of the operation. At issue was the behavior of the berms during a 100-yeaz
flood. The applicant has proposed, and the Division has agreed, that 188-foot crest widths for interior
berms will be adequate to prevent failure of the berms during a flood. Mr. Pazker inquired if the
Division would approve 150-foot crest width berms for two of the interior berms at the Lyons Pit. He
stated that the two berms at issue are both upstream of proposed reservoirs; thus, if the berms were to
overtop during the flood only slight differential hydraulic head will develop assuming the reservoirs aze
full or neaz full at the time the flood occurs. This rational is discussed on page 7 of the "Floodplain
Impact Analysis for the Lyons Pit" dated December 10, 1999 and provided to the Division in support of
the application. It was discussed with Mr. Pazker that the Division's concern is with the differentia]
head that would develop if a flood were_to occur when the reservotrs are drawn down. To address the
Division's concern, a number of additional arguments in favor of [he 150-foot crest width.berms were
discussed.
1. The interior berms where a 150-foot crest width is proposed aze the interior berm located upstream
(west) of the lined reservoir proposed for the south east corner of the permit area and the
embankment of the proposed pond to be located north and west of the cement plant. In both cases,
if the interior berms were to fail the downstream reservoirs would be flooded but there would be no
propagation of successive failures in a downstream direction because there are no pits or ponds
located immediately downstream (See the discussion of propagating failures on page 7 of the
December 10, 1999 "Floodplain Impact Analysis").
2. A probabilistic azgument can be made that although a flood certainly could occur when the
reservoirs aze drawn down, this is not the likely case.
3. The specified 188-foot wide interior berm crest width is considered to be a conservative protective
measure given the magnitude and duration of a flood on the St. Vrain (see discussion on page 5 of
the December 10, 1999 "Floodplain Impact Analysis"). A 150-foot crest width, although less
conservative, is still unlikely to fail during a flood.
Mr. Pazker agreed to compile a proposal for I50-foot minimum crest width berms in the two locations
discussed. I told him that the Division would likely approve the 150-foot crest width berms, but that
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