APPCOR10134

• Permit No: C1994082
• IBM Index Class Name: Application Correspondence
• Doc Date: 1/20/1995
• Doc Name: YOAST MINE C-94-082 PERMIT APPLICATION
• From: DMG
• To: SENECA COAL CO PEABODY WESTERN COAL CO
• Media Type: D
• Archive: No

a. The DMG calculations were based on dividing Watershed 2 into disturbed and
undisturbed subwatersheds. DMG's design assJgned a curve number of 85 for the
pit azea and 52 for the undisturbed area, with the outlets of both watersheds at
culvert YB6. This results in much higher runoff and sediment delivery, which
DMG believes to be accurate. SCC used a weighted average curve number (65)
for the watershed, with one outlet at YB6 resulting in lower runoff and sediment
delivery.
b. DMG's model broke Watershed 9 into four sepazate watersheds, for routing
purposes. This had no impact on the runoff or sediment delivery.
c. Particle size distributions used by DMG and SCC were:
iz mm DMG % SCC %
1z.7 loo
11.2 97
2.0 100 91
1.0 96 87
.5 93 84
.25 91 82
.106 88 79
.045 67 56
.02 49 40
.O1 40 31
.005 33 24
.002 26 17
.001 23 14
.0001 0 0
The reason for the DMG adjustment to the distribution is that particles lazger
than 4 mm would not be mobilized. The SEDCAD+ manual describes the
required input for this pazameter as the "erodible particle size " Including values
higher than 4 mm will distort the sediment delivery value, resulting in less
accurate predictions.
d. DMG included a baseflow of ,017 cfs from pit dewatering, resulting in a slight
increase in peak dischazge.
All other inputs were very similar to those of SCC. Please revise SCCs model to
incorporate the premise of DMG calculations, or provide additional information to
support SCCs model.
39. DMG also modelled the Pond 11 watershed. Again, the resultant runoff and sediment
delivery were much higher than that estimated by SCC, for similar reasons. Assumptions
leading to the disparity were:
a. DMG split the Pond 11 watershed into two watersheds, disturbed and
undisturbed. SCC again used a weighted average of all factors.
b. DMG adjusted the particle size distribution, as noted above.
c. DMG included a baseflow of .017 cfs for pit dewatering:
Please review DMG's calculation and revise SCC's according]y, or provide additional
information to better support SCCs model.
M. Altavilla and'G. Wendt tl January 23, 1995