2009-10-06_PERMIT FILE - C1980005 ‎(2)‎

• Permit No: C1980005
• IBM Index Class Name: Permit File
• Doc Date: 10/6/2009
• Doc Name: Pond 008 Principal Spillway Replacement As-Built Report
• Section_Exhibit Name: Tab 07 Appendix 7-6f.3
• Media Type: D
• Archive: No

the pond as constructed and operated meets the intentions of Rule
• 4.05.9(2)(b).
The modeled 25-year, 24-hour storm event peak inflow is 35.1 cfs, while the
calculated peak discharge is 11.7 cfs, with a corresponding peak stage of
7,017.3 feet (see Addendum 7-6f.3B). The emergency spillway configuration
was checked using the SEDCAD4 Channel Utility (see Addendum7-6f.3C) to show
that the as-built configuration can safely pass the 25-year, 24-hour
discharge. The channel utility modeling shows a peak flow depth of
approximately 1.1 feet, which would be approximately 3.5 feet below the top
of the embankment (7,020.0 feet). If the principal spillway were to clog, or
become inoperable for some other reason, the modeling shows that the
emergency spillway is of sufficient size to safely pass the peak discharge
from the 25-year, 24-hour storm event. Pond 008 is constructed and operated
in compliance with Rule 4.05.9(2)(c)(ii).
Pond 008 Sedimentoloqy Demonstration
SCC revised the SEDCAD4 demonstration as part of TR-42 to update the volume of
allowable sediment storage in Pond 008. With the cessation of mining operations
is and subsequent revegetation of disturbed areas, the annual sediment yield to the
pond has decreased significantly and, consequently, it is not necessary to
maintain the excessive sediment storage capacity needed to capture the large
volumes of sediment generated from the disturbed and barren areas during coal
resource removal operations.
The watershed area for Pond 008 is presented on Exhibit 7-22, Pond 004 & 008
Watershed Boundary. The area is based on the resurveyed drainage divide
separating Pond 004 and Pond 008 completed in August 2009. The structure
elevations input into the model are based on a physical survey conducted in
September 2005. The stage/storage inputs utilized data presented by SCC in the
as-built report previously approved as part of TR-37.
SEDCAD4 uses an Annual R factor for a set number of storage years (Y) to determine
if the pond capacity is adequate to store the calculated sediment yield from the
design storm. SCC uses an Annual R factor of 10 for reclaimed and revegetated
landscapes. In previous SEDCAD4 modeling exercises (i.e., TR-37 and during coal
resource removal operations) the storage year value (Y) was artificially set at
two (2) years. Use of this value resulted in low sediment storage capacity
• because the model suggested that the pond needed a large amount of capacity to
TR-51 4 Revised 09/09