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Ditch designs are summazized in Figure i, Permanent Ditch Design Summary. Ditch locations are shown on the Postmining Drainage and Sediment Control Map, • • (Map R-1). Pond 4 The proposed slope reduction activities east of Pond 4 will reduce pond capacity. The new pond configuration will meet all requirements for sediment ponds in accordance with Rules 4.05.6 of the CMLRD Rules and Regulations. The outlet structure has been designed to safely pass the peak dischazge from the 25-year, 24hour event while providing at least 1 foot of freeboard. Pond 4 will employ a 30•inch diameter CMP barrel extending through the pond embankment with a 42" riser as the primary outlet structure. A 4" orifice at the design sediment level will serve to dewater accumulated storm runoff until effective runoff control is reestablished for the ~, contributing drainage area. At that point in time, the lower orifice will be plugged to establish permanent storage. An upper orifice will then function to dewater any accumulated storage in excess of the design maximum storage capacity of 2.0 acre-feet. • Pond 4 will utilize a riprapped trapezoidal channel with a 5 foot bottom and 3H:1V sideslopes • as an emergency spillway. The pond design and stage/storage curve aze shown on the Pond 4 Design Map (Map R-3). The stage/storage curve illustrates sediment and runoff storage capacity. The following summarizes the revised Pond 4 design: Three-Year Sediment Yield (ac-ft) 0.35 Elevation Three-Yeaz Sediment (fc) 7539.6 Elevation Discharge Orifice 7539.6 Volume 10-year, 24Hour Event (ac-ft) 2.57 Volume Runoff and Sediment (ac-ft} 2.92 Elevation Discharge Orifice (Permanent Storage, ft) 7545.5 Elevation 10-year, 24hour Event (ft} 7547.5 Elevation Crest of Riser Structure (ft) 7547.6 Elevation Emergency Spillway Invert (ft) 7547.6 Elevation Maximum Water Level 25-year, 24hour Event (ft} 7548.6 • Elevation Top of Structure (ft) 7550.00 • Freeboazd (ft) 1.4 ..voa,.,., 17R Revised -May 1992 ov~vn s~