2011-05-04_REVISION - C1982057

• Permit No: C1982057
• IBM Index Class Name: REVISION
• Doc Date: 5/4/2011
• Doc Name: Round 1 Adequacy Review
• From: Seneca Coal Company
• To: DRMS
• Type & Sequence: TR73
• Email Name: SLB
• Email Name: SB1
• Media Type: D
• Archive: No

Pond 016 Slide Remediation Plan
Page 8
reports submitted by SCCfor the Pond 016 watershed indicate an average cover of 47% in
2009. TR-55 indicates that for a C type soil, 47% herbaceous vegetation cover (fair),
would result in a curve number of 81. In order to ensure that the ditches are properly
armored and the pond adequately sized, the model should consider the maximum runoff
and sediment loading that would occur during the life of the structures. Please either
revise the curve numbers in the watershed to 81 or provide information supporting SCC's
curve number of 71.
Response: The vegetation monitoring reports submitted in 2009 for the Pond 016 watershed
area does in fact indicate 47% for "Allowable All Hit Herbaceous Cover". This value is used for
determining vegetation success criteria for bond release and not to represent the on-the-ground
conditions that affect runoff. In order to portray the "current ground surface conditions", values
for residual ground cover should be used; Total Foliar Cover (59.9%) plus Litter (25.5%) as
reported in the 2009 Vegetation Monitoring Report. These are the conditions and factors that
affect and control runoff. Therefore the 71 curve number appears to be appropriate.
29. For disturbed areas (the slide repair area), the curve number should be 94; SCC used 86.
Please either revise the SEDCAD run using a curve number of 94, or provide additional
information justifying the use of 86.
Response: Review of Appendix 13-513, Table 1, Seneca II-W Curve Numbers, indicates that the
use of an 86 curve number is appropriate. The majority of disturbed area associated with the
Pond 016A slide remediation area will consist of backfilled and rough-graded spoil material
(bare/rough) rather than Pit and Pre-stripped areas where water will flow unimpeded off of native
in-place consolidated (bare) material.
30. In the "Subwatershed Sedimentology Detail" of the SEDCAD run, SCC uses a C factor of
.035 for reclaimed areas. It looks like that value is for derived from the tables in SEDCAD
for "C Factors for Permanent Pasture, Rangeland, Idle Land, and Grazed Woodland",
"Canopy of tall weeds or short brush (0.5m fall height), 50% canopy cover, 60% ground
cover". Again, looking at recent data, cover values for the reclaimed area are around
47%, which would put the C Factor at approximately .053 (40% cover is.07; 60% is.035).
Please revise the design using a C Factor more representative of documented conditions,
or provide additional information supporting SCC's C Factor of. 035.
Response: Again, the 47% cover value is for "Allowable All Hit Herbaceous Cover". This
value is used for determining vegetation success criteria for bond release and not to represent the
"current conditions" that affect runoff. In order to portray the "current conditions", values for
residual ground cover should be used; Total Foliar Cover (59.9%) plus Litter (25.5%) as reported
in the 2009 Vegetation Monitoring Report. These are the conditions and factors that affect and
control runoff. Therefore the C Factor of 0.035 appears to be appropriate.
31. The inputs for hydrograph response time used by SCC are inconsistent with the currently
approved design and not representative of current ground conditions. The approved
designs for all of the post-mine channels indicate a "medium " response time and are based
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