2018-09-11_PERMIT FILE - C1981008A (12)

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Pond 016 As -Built Pond Construction and Compliance Demonstrations The as -built drawing for NH2 Pond 016 is provided on Map 2.05.3(3)-36 and includes a plan view, cross-sections, stage -storage curve, and spillway details. Pond 016 was approved and constructed under the requirements of Colorado DRMS Rule 4.05.9 (2)(e)(ii) and will be operated as a zero discharge facility for stormwater runoff up through the 100 -YR, 24 -HR storm. The emergency spillway consists of a minimum 2 -foot deep vegetated channel with a 17 -foot minimum bottom with and side slopes no steeper than 2.9H:1 V. SEDCAD was used to analyze sedimentology and hydrology for Pond 016. First, a sediment volume estimate was conducted using the 10 -YR, 24 -HR storm. This demonstration indicates the capacity of Pond 016 below the emergency spillway is adequate to store the total runoff volume into the pond (2.03 ac -ft) as well as seven years of sediment (0.23 ac -ft). Next, a containment analysis was performed using the 100 -YR, 24 -HR storm. This demonstration shows the capacity of Pond 016 below the emergency spillway is adequate to store the total runoff volume into the pond (5.79 ac -ft), assuming a permanent pool elevation no higher than 5,614.0'. Finally, the emergency spillway was evaluated using the 100 -YR, 24 -HR storm event, assuming the pond is full to the emergency spillway at the time of the storm event. The evaluation shows the peak discharge of 22.83 cfs is carried by the emergency spillway at a depth of 0.6 feet, providing 1.4 feet of freeboard. Therefore, the emergency spillway is adequately sized. Embankment Construction and Stability Analysis Foundation preparation for Pond 016 embankment included removal of vegetation, a key cut and scarification of the ground. The embankment fill material is free of sod, roots, vegetative matter and frozen soil. Lambert and Associates on-site compaction testing services were utilized as - needed 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 95% standard or greater relative compaction. Lambert and Associates also performed a stability analysis of the embankment soil material using "XSTABL" software. Their slope stability analysis was conducted on Section A -A' from the Pond 016 As -Built drawing. The analysis provides a theoretical factor of safety for various conditions as indicated below. Theoretical Condition Factor of Safety Rapid Draw Down: Interior 3.380 Reservoir Full: Exterior 3.504 Reservoir Full: Interior 4.907 Information provided from Lambert and Associates has been provided on the following pages. Attachment 2.05.3(3)-36 Page 1 July 2018 (TR -84)