2008-12-04_PERMIT FILE - C1994082 ‎(19)‎

• Permit No: C1994082
• IBM Index Class Name: Permit File
• Doc Date: 12/4/2008
• Doc Name: Part I Pond 013 Postmine Storage Capacity Addenum ‎(TR37)‎
• Section_Exhibit Name: Tab 13 Attachment 13-20.1
• Media Type: D
• Archive: No

• Provide storage and treatment for the runoff from the 10-year, 24-hour
storm event without experiencing any discharge through the emergency
spillway (Rule 4.05.9(2)(b));
• Provide an emergency spillway capable of passing runoff from the 25-
year, 24-hour storm event in accordance with Rule 4.05.9(2)(c)(ii).
The contributing watershed is approximately 34 acres - mining activities
disturbed 100 percent of the total area (see Exhibit 12-1, Operations Plan
and Exhibit 20-2, Postmining Topography Map). Seneca Coal Company (SCC) re-
evaluated the sediment storage requirement as part of TR-37 because several
of the original design assumptions and permit conditions are no longer
appropriate or applicable (e.g., land condition, sediment delivery).
The TR-37 hydrology and sedimentology demonstration utilizes a watershed
condition based on the September 2007 landscape. This is an appropriate
approach because all coal resource removal activities have ceased and
disturbed areas within the watershed are reclaimed and successfully
revegetated (see Yoast Mine, 2007 Annual Reclamation Report, Chronological
Seeding Map). Consequently, the current sediment delivery is greatly reduced
from the worst-case conditions evaluated in the 2004 as-built evaluation.
The hydrology and sedimentology in the TR-37 demonstrations were analyzed
using the SEDCAD4 computer program (Schwab, 1998). Model methodology and
assumptions are presented in Attachment 13-12, Yoast Mine Drainage and
Sediment Control Plan. Appendix 13-20.1-A and 13-20.1-B present the SEDCAD4
demonstrations for the 10-year, 24-hour and 25-year, 24-hour design storms.
Input consists of precipitation (inches), watershed area (acres), time of
concentration (hours), runoff curve number (CN), representative particle-size
distributions of soils in the area, sedimentology information, spillway
dimensions, etc. The drainage area(s) for the watershed, as well as, slope
lengths and distances for time of concentration estimates were measured on an
approved permit topographic map (see Exhibit 13-12.1A, Permanent Channel and
Pond Watersheds).
This evaluation conservatively assumed the pond(s) are 100 percent full to
the emergency spillway invert elevation (e.g., principal spillway is
inoperable) during the 25-year, 24-hour precipitation event and provides
adequate freeboard to prevent overtopping of the embankment.
SCS Curve Number and "k" Factor. The curve numbers used for the TR-37 SEDCAD
demonstration are presented in Attachment 13-12. The "k" factors were
determined based on soil survey information and National Resource
Conservation Service soil interpretations. The curve numbers and "k" factors
were then "weighted" for each sub-watershed as presented in the Hydrology
Input Parameter tables found at the back of Appendix 13-20.1-A.
TR-37 13-20.1-2 Revised 02/08