2018-07-02_PERMIT FILE - C2010089A (15)

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Pond NHN-002 As -Built Pond Construction and Compliance Demonstrations The as -built drawing for Pond NHN-002 is provided on Map 2.05.3(4)-10 and includes a plan view, cross-sections, stage -storage curve, and spillway details. The principal spillway consists of an 18 -inch diameter pipe with a pair of valved 6 -inch diameter weepholes. The emergency spillway was constructed with native fill material and has a minimum bottom width of 8.1 feet. The principal and emergency spillway discharge pipes are buried approximately 350 feet north from Pond NHN-002 as shown on Map 2.05.3(4)-10. At the outlets, an erosion control structure with plastic liner and 2-12 inch rock was constructed. In accordance with Rule 4.05.18, the outlet structure for Pond NHN-002 was not constructed within the buffer zone for Meehan Draw. SEDCAD was used to analyze hydrology and sedimentology and the demonstration indicates Pond NHN-002 is adequate to store the 10 -YR, 24 -HR storm event, pit pumping and three years of sediment. With the weepholes open, the peak water level is 2.3 feet below the principal spillway riser and 3.4 feet below the emergency spillway. The SEDCAD demonstration for the 25 -YR, 24 -HR storm event indicates that with weepholes closed, Pond NHN-002 will safely convey the peak discharge with 2.0 feet of freeboard from the peak level to the top of the structure. As indicated on the Structure Summary, the peak discharge out of Pond NHN-002 is 9.51 cfs, which is the flow that must be discharged via the 18 -inch diameter principal spillway discharge pipe. SEDCAD culvert calculations show that the maximum capacity for the principal spillway discharge pipes is 13.94 cfs, which is adequate to convey the 9.51 cfs peak pond discharge. Embankment Construction and Stability Analysis Foundation preparation for Pond NHN-002 embankment included removal of vegetation and scarification of the ground. The embankment fill material was free of sod, roots, vegetative matter and frozen soil. Lambert and Associates on-site compaction testing services were utilized on an as -needed basis and their testing methods were in accordance with ASTM Test Method, D2922 and D3017, for Nuclear Moisture Content and Dry Density for accepted material testing practices in the area. All final field density tests indicated 90% or greater relative compaction. Additionally, Lambert and Associates performed a stability analysis of the embankment soil material using "XSTABL" software. Their slope stability analysis considered a cross-section 10 feet high with a 3:1 (horizontal to vertical) slope inclination. This embankment configuration applies only to the theoretical stability assessment, for actual side slopes measurements at specific locations, refer to Map 2.05.3(4)-10. The analysis indicated a theoretical factor of safety for stability of 2.4 for the pond embankment slope and 1.7 for rapid drawdown condition. Appendix 2.05.3(4)-10 Page 1 March 2018 (TR -16)