2008-12-04_PERMIT FILE - C1994082 (6)

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Predicted sediment storage capacity = Predicted maximum sediment storage elevation (ft msl) _ Number of storage years (Y) _ 2.2 ac-ft 7,002.0 ft >200 years A user input storage volume that maximizes the available sediment storage in accordance with Warner et al., (1998) guidelines was input into the model for Pond 011. The storage volume for Pond O11A was calculated by increasing the number of storage years until the elevation criterion was met. Spillways. In accordance with Rule 4.05.9(2), Pond 011 was designed and constructed with both a principal and emergency spillway. Exhibit 13-5A shows spillway locations and specifications. The principal spillway consists of a 12-inch CMP barrel pipe. The spillway will keep the pond level at the 7,004.4 elevation. The SEDCAD4 demonstration in Appendix 13-4A.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 rock riprap lined trapezoidal channel, underlain with Mirafi 600X geotextile, as shown on Exhibit 13-5A. The invert elevation is at 7,007.0 feet, with a corresponding storage volume of 2.4 ac-ft. The emergency spillway is constructed to safely pass the runoff volume from the 25-year, 24-hour storm (see Addendum 13-4A- C). The spillway geometry and peak discharge presented on Exhibit 13-5A are based on the 100-year, 24-hour storm event (see 1997 As-built Report, Addendum 13-4A-C). Consequently, the emergency spillway is over designed and over constructed for the estimated 25-year, 24-hour storm event discharge expected under reclaimed conditions (see Appendix 13-4A.1-B). Summary The capacity of Pond 011 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 postmine condition SEDCAD demonstration. Exhibit 13-5A, As- Bui1t Pond o11, 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 13-4A.1-4 Revised 02/08