REV04747

• Permit No: C1982055
• IBM Index Class Name: Revision
• Doc Date: 2/14/2003
• Doc Name: Response to Adequacy Review Comments
• From: MWH
• To: DMG
• Type & Sequence: RN4
• Media Type: D
• Archive: No

POST MINING DRAINAGE
Watershed Diversion Ditches 1 and 2, which run along the northern edge of the refuse pile, will remain
with the exception of Section C of Ditch 1. This section will be eliminated allowing the ditch to run in
the natural drainage and discharge directly into the permanent stock pond. Designs for these channels
can be found in Section 2.05.3(4) of the Permit. Post Mining Ditch (PMD) No. 1, Refuse Ditch 1, and
Mine Access Road Diversion Ditches will remain, Watershed Diversion Ditches (WDD) 3, 4, and 5
and Refuse Ditch 2 will be reclaimed.
Following removal of WDD 3 and 4, PMD 2, 3, and 4 will be constructed. These ditches will collect
runoff from the narural undisturbed drainages and divert it to the permanent watershed diversion
ditch. After WDD 5 is removed, PMD 5 will be constructed to collect runoff from the natural
drainage and divert it through the topsoil stockpile/trailer area to the existing Access Road Diversion
Ditch,
The PMD ditches are designed to safely pass the 10-year, 24-hour storm event and are shown on the
Post Mine Topography Map (Map 17). Peak flows were calculated using SEDCAD+ computer model.
All inputs and design details aze presented in Addendum D. Design details and channel characteristics
for each section are summazized as follows:
Channel
Characteristics PM 2 Upper PM 3 PM 2 Middle PM 4 PM 5
Desi Flow cfs 24.7 22.4 50.8 6.1 3.6
Channel Sha e Tra ezoidal Tra ezoidal Tra ezoidal Tra ezoidal Tra ezoidal
Bottom Width ft 5.0 5.0 5.0 5.0 2.0
Sideslo es H:1 3 3 3 3 3
Slo e % 6.0 10.0 1.7 15.0 14.0
Mannin 's n 0.042 0.043 0.037 0.045 0.043
Veloci f s 5.5 6.2 4.7 4.5 4.6
Flow De th ft 0.7 0.6 1.3 0.3 0.3
Freeboard ft 0.3 0.3 0.3 0.3 0.3
Ri ra Dso in 3 3 3 3 3
PERMANENT POND
The existing pond will remain as a permanent structure. Once vegetation is established in the
reclaimed areas, and all regulatory requirements are met, the emergency spillway will be lowered to
6,346 feet. The final pond capacity will total 1.8 acre feet assuming a sediment level of 6,341 feet. To
increase the ponds dead storage, EFMC will add 2.9 feet to the primary spillway (flinch riser) to bring
the invert elevation up to 6345 feet.
The pond will effectively enhance and support the proposed post mine land use (range land). The
water level in the pond should remain constant due to a natural spring in the pond area and the pond
can be safely accessed by livestock from multiple locations.
The discharge from the pond has been monitored for the past 16 years. There has been no indication
of any problems with warer quality and no material has been identified on site which could potentially
cause any future degradation of water quality.
PR-04 427Rc Revised 02/03