2012-07-03_REVISION - M1984014

• Permit No: M1984014
• IBM Index Class Name: REVISION
• Doc Date: 7/3/2012
• Doc Name: RESPONSE TO ADEQUACY REVIEW #2
• From: COTTER
• To: DRMS
• Type & Sequence: AM1
• Email Name: GRM
• Media Type: D
• Archive: No

Parameter
Spillway
Pond 1
Spillway
Pond 2
Optional
Culvert
Pond 2
Design Flow (cfs)
1.5
3.7
3.7
Bottom Width (ft)
2.0
5.0
Spillway Depth or Culvert Diameter (ft)
1.0
1.0
1.5
Headwater Depth (ft)
0.4
0.5
1.1
Flow depth (ft)
0.2*
0.25*
0.6
Velocity (ft/s)
2.3*
2.6*
5.2 (exit)
Slope (ft/ft)
0.06*
0.05*
0.03
Length (ft)
10
10
30 (est.)
Invert Entry (relative to 10 -year storage elev) (ft)
0.0
0.0
-1.5 **
JD -8 Mine — Drainage Design Plan 11
Excess flows from Retention Pond 2 will be conveyed across a roadway spillway or through a spillway and
roadway culvert (see Figure 1) per the operating requirements of the mine. The culvert invert has been
selected to avoid interference between culvert headwater and correct spillway operation. A culvert design
profile is provided in Attachment lb.
Spillway crest elevations will be identical to the 10 -year 24 -hour storage elevation for all retention ponds.
The spillways will be constructed to the design specifications in Table 10 and will generally follow the
schematic provided in Attachment 3.
Table 10. Mine Site Planned Drainage Structure Parameters
*Down gradient from spillway crest
* * Culvert approach channel to be 1.75 ft deep
In consideration of the exit flow conditions, a gravel mulch lining with a D50 of 2" (see Section 2) has been
selected for both spillways and exit chutes based on Table 2.3 of HEC -15 (FWHA 2005).
4. OPERATIONAL AND POST - MINING DRAINAGE CONDITIONS
When mining resumes, the existing waste rock pile will be built up to house additional mine facilities
including a new maintenance shop and office. During the build -out period, drainage will be modified to
prevent water from accumulating in the facilities area and to prevent excessive erosion and sedimentation
during operations. For the post- mining reclamation period, drainage will be engineered and constructed to
reduce erosion and protect reclaimed surfaces.
4.1 Runoff Areas and Routing
Flow directions and runoff areas during mine operations will resemble the current condition and are shown
in Figure 3 and Table 11. However, the waste rock pile will be expanded to encompass the area currently
occupied by the existing constructed catchment (Pond 2) and planned expansion/construction of the
western catchment (Pond 3) near the topsoil stockpile. When these storm water retention facilities are
removed and replaced by waste rock, the pre- existing road will be realigned between the property boundary
and the toe of the waste rock pile. Water will be routed westward across the site and southward off the face
of the waste rock pile. Two interceptor ditches will be constructed to convey water to the proposed new
storm water Retention Pond 3 located west of the planned final waste rock pile. The upper interceptor ditch
(Figure 3) will divert upland runoff from Sub -Basin N -E to the natural drainage west of the waste rock pile
and Pond 3, thus preventing non - contact flows from entering the retention structure. This upper interceptor
ditch will be constructed on the upslope (south) side of an existing drill road. The lower interceptor ditch
will route water from the waste rock pile and eastern sub -basin along the toe of the pile to Pond 3 and will
be constructed on the upslope (south) side of the realigned road along the property boundary and toe of the
waste rock pile.
4122A.120702 Whetstone Associates •