REV91714

• Permit No: C1981014
• IBM Index Class Name: Revision
• Doc Date: 3/9/1992
• Doc Name: TR 11 SOUTHFIELD MINE C-81-014
• From: MLRD
• To: EFCI
• Type & Sequence: TR11
• Media Type: D
• Archive: No

STATE OF COLC ~~~ ~~~~~~~~~~~~~ ~~~
MINED LAND RECLAMATION DIVISION
Department of Natural Resources
1373 Sherman SI., Room 215
Denver. CO 80203
303 866-3567
Rax 303 832-8706
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Roy Romer.
Governor
Michael B. Long,
Dwision D~rec{or
March 9, 1992
Mr. Allen Weaver
EFCI
P.0. Box 449
Florence, CO 81226
Re: Technical Revision No. 11, Southfield Mine, C-81-014
Dear Mr. Weaver:
The staff has reviewed the January 9, 1992, submittal for TR-11. The designs
for the emergency spillways are acceptable, but the re-designs on Ponds 1 and
5 were incomplete. The modified watershed for Pond 5 of 32 acres was not
incorporated into the sediment caicuiations on Page 80R. Substitution of this
acreage into the USLE yields an annual sediment load of 1.23 acre-feet.
Consequently, Pond 5 is undersized to handle and treat the 10 year - 24 hour
event. In addition, the Stage 1 Storage curve and the Storage Capacity Table
for Pond 1 (Map 21) were not modified to reflect the smaller watershed and
reduced storm volume.
The technique used to evaluate the sediment volume for Pond 1 is unorthodox
and probably underestimates the sediment generated from the coal stockpile
area. Given the excess volume available below the primary spillway invert,
EFCI would reduce their vulnerability to enforcement actions by increasing the
permissible sediment volume.
To address these concerns, the Division is proposing a conditional approval of
the revision. The following stipulation will accompany the revision decision:
Stipulation No. i9
WITHIN 30 DAYS, EFCI WILL RE-EVALUATE THE ENGINEERED DESIGNS FOR POND 5
TO ACCOMMODATE SEDIMENT GENERATION FROM THE ENTIRE WATERSHED. IN
ADDITION, MAP 21 WILL BE MODIFIED TO REFLECT HYDRAULICS ASSOCIATED WITH
THE REDUCED WATERSHED.
On another matter, riprap installation was not described in detail in the
submittal. Prudent installation requires either a geosynthetic or natural
material filter below the riprap. The stone filter blanket should be graded