2008-12-04_PERMIT FILE - C1994082 (19)

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Principal spillway invert elevation (ft msl) = 7,367.2 ft Total pond storage capacity (at principal spillway elev.) = 5.0 ac-ft Storm predicted effluent peak settleable concentration = 0.0 ml/1 Predicted annual sediment loading = <0.01 ac-ft Predicted sediment storage capacity = 3.7 ac-ft Predicted maximum sediment storage elevation (ft msl) = 7,305.3 ft Number of storage years (Y) _ >100 years The 25-year, 24-hour event SEDCAD demonstration presents a user input storage volume that corresponds to the storage volume calculated in the 10-year, 24- hour demonstration. This demonstration assumes the principal spillway is clogged and, consequently, the peak stage and discharge are over estimated. Spillways. In accordance with Rule 4.05.9(2), Pond 013 was designed and constructed with both a principal and emergency spillway. Exhibit 13-7 shows spillway locations and specifications. The principal spillway consists of a 12-inch CMP riser that drops into a 12-inch CMP barrel pipe. The spillway will keep the pond level at the 7,367.2 elevation. The SEDCAD4 demonstration in Appendix 13-20.1-A shows, in accordance with Rule 4.05.9(2)(b), there is no discharge through the emergency spillway during the 10-year, 24-hour event. The embankment portion of the emergency spillway consists of a 10-foot wide trapezoidal channel, as shown on Exhibit 13-7. The invert elevation is at 7,368.3 feet, with a corresponding storage volume of 5.9 ac-ft. The emergency spillway is constructed to safely pass the runoff volume from the 25-year, 24-hour storm (see Appendix 13-20.1-B). Summary The capacity of Pond 013 is attributed partially to the embankment and partially to the pond excavation. During passage of the 10-year, 24-hour storm the discharge through the principal spillway meets effluent limitations. In addition, there is no flow through the emergency spillway. The emergency spillway was designed and constructed to safely handle a storm event equal too, or larger than the 25-year, 24-hour event. Revised Storage Capacity Certification I am a registered professional engineer in the State of Colorado experienced in the design, construction and operation of earthen structures. I have reviewed the storage capacity SEDCAD demonstrations. Exhibit 13-7, As-Built Pond 013, presents the existing configuration of the impoundment. The earthen impoundment and spillway configuration are pre-existing and have been certified by others (see Part II). TR-37 13-20.1-4 Revised 02/08