2008-12-04_PERMIT FILE - C1994082

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~~/ The 25-year 24-hour storm event SEDCAD demonstration presents a user input storage volume that corresponds to the storage volume calculated in the 10- year, 24-hour demonstration. The demonstration also assumes the principal spillway is clogged and, consequently, the peak stage and discharge are overestimated. • • Spillways. In accordance with Rule 4.05.9(2), Pond 010 was designed and constructed with both a principal and emergency spillway. Exhibit 13-4A shows spillway locations and specifications. The principal spillway consists of a 6-inch 'diameter de-watering orifice cut into a 36-inch CMP riser which drops into a 24-inch barrel pipe. The spillway will keep the pond level at the 6,784.0 elevation. The SEDCAD4 demonstration in Appendix 13-3A.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 13-4A. The invert elevation is at 6,787.6 feet, with a corresponding storage volume of 19.4 ac-ft. The spillway geometry and peak discharge presented on Exhibit 13-4A are based on the 100-year, 24-hour storm event (.see Part II of the As-Built Report for the 100-year, 24-hour demonstration). Consequently, the emergency spillway is over designed and over constructed for the 25-year, 24-hour. storm event discharge expected under reclaimed conditions (see Appendix 13-3A.1-B). Summary The capacity of Pond 010 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 (Rule 4.05.9(2)(c)(iii}). 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 demonstrations. Exhibit 13-4A, As- Built Pond 010, 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 of this As-Built Report). TR-37 13-3A.1-4 Revised 08/08