2019-01-18_PERMIT FILE - C2010089

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POND NHN-001 AS -BUILT Pond Construction and Compliance Demonstration The as -built drawing for Pond NHN-001 is provided on Map 2.05.3(4)-6 and includes a plan view, cross-sections and stage -storage curve. The principal spillway consists of a riser at elevation 5675.50 feet, with a 24 -inch diameter pipe, an 18 -inch diameter restrictor plate, and a pair of valved 6 -inch diameter weepholes at elevation 5671.50 feet. The emergency spillway is a combination rip -rapped trapezoidal channel at elevation 5676.50 feet (minimum 15 feet wide with 2h:1 v or flatter side slopes) which flows into a 72" concrete drop inlet. The drop inlet feeds a 24 -inch diameter CMP which then flows to outlet (see Map 2.05.3(4)-6). SEDCAD was used to analyze hydrology and sedimentology and the demonstration indicates Pond NHN-001 is adequate to store the 10 -YR, 24 -HR storm event and five (5) years of accumulated sediment. With the weepholes open, the peak water level is 5674.35 feet, which is 1.15 feet below the principal spillway riser and 2.15 feet below the emergency spillway channel. The SEDCAD demonstration for the 25 -YR, 24 -HR storm event indicates that with weepholes closed, Pond NHN-001 will safely convey the peak discharge with 1.01 feet of freeboard from the peak level (5676.99 feet) to the top of the structure (5678.00 feet). Embankment Foundation preparation for the embankment of Pond NHN-001 included removal of vegetative and organic materials and scarification of the ground. The embankment fill material was constructed 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. The faces and slopes of the embankment are being planted during the first growing season to establish vegetation. Additionally, Lambert and Associates performed a slope stability analysis of the embankment using "Slope Stability Analysis" by Geosoft computer software. Their stability analysis considered a crest width of 10 feet, embankment height of 9.5 feet and embankment inclination of 3h:1 v (upstream and downstream). This configuration applies only to the theoretical stability assessment, for actual side slopes measurements at specific locations, refer to Map 2.05.3(4)-6. The analysis indicated a theoretical factor of safety of 1.53 for the pond embankment. Appendix 2.05.3(4)-6 Page 1 December 2018 (TR -19)