2017-01-09_PERMIT FILE - C1981008

Home Browse Search

Pond 018 As -Built Pond Construction and Compliance Demonstrations The as -built drawing for NH2 Pond 018 is provided on Map 2.05.3(3)-35 and includes a plan view, cross-sections, stage -storage curve, and spillway details. Pond 018 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 4 -foot deep vegetated channel with a 18 -foot minimum bottom with and side slopes no steeper than 3H:1 V. SEDCAD was used to analyze hydrology and sedimentology and the demonstration indicates the 4.03 ac -ft capacity of Pond 018 below the emergency spillway is adequate to store the total runoff volume into the pond of 1.03 ac -ft as well as five years of sediment generated by the 10 - YR, 24 -HR storm event. The SEDCAD demonstration for the 100 -YR, 24 -HR storm event indicates the 4.03 ac -ft capacity of Pond 018 below the emergency spillway is adequate to store the total runoff volume into the pond of 2.25 ac -ft generated by the 100 -YR, 24 -HR storm event. The 100 -YR, 24 -HR peak storm flow of 15.58 cfs was used to check the emergency spillway capacity. The emergency spillway can discharge 15.58 cfs at a depth of 0.46 feet, which provides 3.54 feet of freeboard. Therefore, emergency spillway is adequately sized. Embankment Construction and Stability Analysis Foundation preparation for Pond 018 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 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 95% 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 018 As -Built drawing. The analysis provides a theoretical factor of safety for various conditions as indicated below. Theoretical Condition Factor of Safety Reservoir Empty: Reservoir Side 1.806 Rapid Draw Down: Reservoir Side 1.405 Reservoir Full: Reservoir Side 2.001 Reservoir Full: Exterior 2.030 Information from Lambert and Associates including test results and analysis has been provided on the following pages. Attachment 2.05.3(3)-35 Page 1 October 2016 (TR -76)