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SPILLWAYS <br />PECOCO will have separate principal and emergency spillways (see Exhibit 7 -18 for location <br />and configuration of spillways). The principal spillway will consist of a 24" diameter <br />CMP with a trash rack at the inlet. This pipe will maintain the water level to the <br />6774.9' elevation. Peak stage at the principal spillway during the passage of the <br />10-year, 24 -hour event will be elevation 6775.10. This results in 1.9 feet of freeboard <br />to the inlet of the emergency spillway. <br />The emergency spillway will consist of a trapezoidal channel. Assuming the principal <br />spillway was plugged, the trapezoidal channel could safely pass the 100 -year, 24 -hour <br />storm runoff volume. Peak stage elevation during the passage of the 100 -year storm is <br />6777.4, leaving 3.6 feet of freeboard to the top of embankment. With a velocity of 2.90 <br />fps, the grass lined emergency spillway channel and outslope will be stable. Please refer <br />to Addendum 2 for capacities of both spillways during passage of the 10 year, and 100-year <br />storm events. The design details for the emergency spillway are as follows. <br />Type <br />Bottom Width <br />Length <br />Side Slopes <br />Outfall Slope <br />Inlet Elevation <br />100-year, 24 -hour Peak Stage <br />SUMMARY <br />Trapezoidal Channel <br />10 feet <br />40 feet <br />3H:1V <br />2 9 6 <br />6777.0 <br />6777.4 <br />Impoundment capacity below the principal spillway is completely incised. During passage <br />of the 10 -year, 24-hour, storm the emergency spillway does not :iischarne >ediment <br />downstream nor is there any flow through the emergency spillway. the pond will safely <br />pass the flow from the 100-year, 24 -hour precipitation event. <br />7 -6q -17 <br />NOR 006577 <br />