REV100086

• Permit No: C1982057
• IBM Index Class Name: Revision
• Doc Date: 8/25/2005
• Doc Name: Adequacy Review Letter
• From: DMG
• To: Seneca Coal Company
• Type & Sequence: PR5
• Media Type: D
• Archive: No

PR-5 PRELIMINARY ADEQUACY REVIEW (2"e Part)
2.05.314) Ponds, impoundments, other treatment facilities and diversions
10. Exhibit 20-2A shows that disturbed area drainage from Past mining Channel PM-4
discharges to a light-use road. Drainage from the disturbed area shall be passed
through a sedimentation pond or other treatment facility. It appears that there should
be a culvert beneath the light-use road to convey the runoff to the drainage above
Pond 015. Please clarify this and provide a culvert design for this location, or
otherwise, provide alternate treatment for the disturbed area runoff from PM-4.
11. The vegetated channel design for PM-1A (page 20-1-148) does not provide sufficient
freeboard for the given capacity analysis design. The construction depth is always
taken from the capacity analysis (1.08 ft for the 7% slope). As proposed the channel
has a 1-foot bottom width and 3.0:1 side slopes giving a total channel depth of 1 foot.
As designed the channel will overtop during the passage of the 10-year, 24-hour
precipitation event. The Division recommends increasing the bottom width or using a
triangular channel with a minimum depth of 1.25 feet. Please revise the vegetated
channel design accordingly.
12. The riprap channel designs for PM 2-2 and PM-2A-1 do not have sufficient bottom
widths for the 2-6% and 8-10°h designs, respectively. For both designs at lesser
slopes, the one-foot bottom width and 3.0:1 side slopes give total channel depths of
one foot. This does not provide sufficient freeboard during the passage of the 10-year,
24-hour event. Please revise the channel designs for PM-2-2 and PM-2A-1
accordingly.
13. There appear to be several minor inconsistencies or typographical errors on the
Hydrologic Input Parameter Tables submitted with the Post mining Channel designs and
stock pond information with regard to runoff curve numbers. Approved curve numbers
and related information are contained on page 3 of Attachment 13-5B Drainage and
Sediment Control Plan II-W South Extension. The cover type Aspen/Mountain Shrub is
described as Aspen Mountain Brush numerous times in most of the Hydrologic Input
Parameters Tables, and for the Reclaimed cover type the Hydrologic Condition should
be listed as Good (>70%) and the Hydrologic Soil Group should C rather than B.
Please revise all Hydrologic Input Parameters Tables accordingly to ensure consistency
with Table 1 of Attachment 13-5B.
14. Please provide a Hydrologic Input Parameters Table for Stock Pond T-25.
15. It does not appear that all of the watershed boundaries are drawn on Exhibit 20-2A.
Please ensure that all of the watershed boundaries are drawn on Exhibk 20-2A and
clearly labeled. Please also include the watershed boundaries designation in the Legend
of Exhibit 20-2A.
16. Please update and revise Section 6.0 of Attachment 13-56 Drainage and Sediment
Control Plan II-W South Extension to include the new and revised information
submitted with Permit Revision No. 5.
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