2017-08-11_PERMIT FILE - C1981028 ‎(2)‎

• Permit No: C1981028
• IBM Index Class Name: Permit File
• Doc Date: 8/11/2017
• Section_Exhibit Name: Appendix K-6 Hydrologic Analysis Sediment Pond & Ditches
• Media Type: D
• Archive: No

t4wftt fire
11161216+i;i August 31, 2016
Table 1. Sediment Pond Spillway Analysis
Structure Peak
Spillway
Peak Water Surface
Top of Pond
Freeboard
Discharge (cfs)
Elevation
Elevation
Elevation
(ft)
(ft amsl)
(ft amsl)
(ft amsl)
Dugout Pond 3.97
4779.00
4779.06
4780.00
0.94
Sediment 13.39
4773.50
4774.03
4775.00
0.97
Pond 2
Ditch Analysis
A separate SEDCAD analysis of the permanent ditches that convey stormwater to Sediment Pond 2 was
conducted to verify the ditches have sufficient capacity to handle a 10 -year, 24-hour storm. The
majority of the Sediment Pond 2 watershed enters the pond in the northwest corner via the Permanent
West Perimeter Ditch (Exhibit 2). The Permanent East Perimeter Ditch has a relatively small watershed
that flows into the northeast corner of Sediment Pond 2. Both ditches were modeled separately. The
maximum modeled discharge for the Permanent West Perimeter Ditch is 2.36 cfs, while the Permanent
East Perimeter Ditch has a modeled peak discharge of 0.02 cfs. Both ditches were modeled as vegetated
v -shaped channels with 2.5:1 sideslopes. The modeled depth of flow for both channels is less than 1.0 -ft
deep. The ditch design in the permit is 3 -feet deep. Additionally, the ditch was designed to handle 95
cfs. The ditch design geometry and design flow are referenced from the December 1, 1994 letter from
David Berry to Shawn Smith. Since the new hydrologic analysis utilizing current climate information
produces a lower discharge than the original ditch design, and since the modeled depth of flow has well
over 0.3 ft of freeboard compared with the ditch design depth, the previously designed permanent
perimeter ditches have adequate capacity.
Summary
The Dugout Pond and Sediment Pond 2 were analyzed using current climate data from NOAA Atlas 14 to
verify they are sufficiently sized to handle the 25 -year, 24-hour storm. Both ponds have sufficient
capacity to contain the 25 -year, 24-hour storm. Furthermore, if the ponds happen to be full (of water or
sediment) the spillway configuration has sufficient capacity to safely pass the 25 -year, 24-hour storm.
Additionally, the two permanent ditches were analyzed to verify they are sufficiently sized to handle the
10 -year, 24-hour storm based on current climate data. The modeled flow depth and discharge are
significantly lower than the original design values, therefore the existing ditch designs have adequate
capacity. SEDCAD modeling is available in Appendix A of this report. This hydrological analysis assumes
the entire watershed has positive drainage to Sediment Pond 2. However, based on aerial photography
there are several roads and numerous depressions present outside of the CEC site, but within the
watershed boundary that likely interrupt drainage and provide water storage, effectively reducing the
stormwater runoff that actually drains to Sediment Pond 2. Therefore, this hydrological analysis is
conservative in nature, and consistent with anecdotal evidence that Sediment Pond 2 has never
discharged, and that there is rarely evidence of flow in the drainage ditches.
Keenesburg Mine Hydrologic Analysis 2 of 2