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~~ SPILLWAY DESIGN <br />Each pond in East Pyeatt and Middle Pyeatt Gulches, along with the existing <br />Johnson ponds, has a combination of principal and emergency spillways that <br />will safely discharge the runoff from a 25-year, 24-hour precipitation event. <br />The principal spillway in the last pond in each series shall safely discharge <br />the runoff from a 10-year, 24-hour precipitation event. <br />The principal and emergency spillways for the Pyeatt dams have been designed <br />as a combination structure at one side of the sedimentation pond embankment. <br />The locations of the spillways are shown in Figure 4, Plan View of Typical <br />Sedimentation Pond Embankment and Spillways. The principal spillway design <br />calls for a corrugated semi-circular metal flume which is anchored in a con- <br />crete base. The concrete base shall be placed in excavated material on one <br />bank of the pond. The principal spillway will discharge into a diversion <br />channel to route outflow back to the natural drainage channel. This outflow <br />channel will be lined with suitable riprap or other appropriate material to <br />prevent excessive erosion. The principal spillway design is shown on Figure <br />3, Principal and Emergency Spillway Front Detail, and Figure 4, Plan View of <br />Typical Sedimentation Pond Embankment and Spillways. Discharge rates for each <br />principal spillway are shown in Table 3, Open Channel Spillway Discharge <br />Rates. Designs for each principal spillway outflow ditch are shown in Figure <br />5, Principal Spillway Outflow Ditch Cross Section, and Table 4, Principal <br />Spillway Outflow Ditch Designs. <br />The emergency spillway is designed as an excavated depression at one side of <br />the sedimetation pond embankment. Each emergency spillway is sloped down- <br />,, stream at lY, and has side slopes of 3H:1V. The top of the embankment shall be <br />constructed such that at least one foot of Freeboard is present when the emer- <br />gency spillway is flowing at design depth. The emergency spillway design is <br />shown in Figure 3, Typical Sedimentation Pond Embankment Design, and Figure 4, <br />Plan View of Typical Sedimentation Pond Embankment and Spillways. Discharge <br />rates Eor the emergency spillways are shown in Table 3, Open Channel Spillway <br />Discharge Rates. <br />9 Revised 8/86 <br />