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Technical Revision 90-06 • The primary outlet structure selected for the ponds is an outlet culvert with a flap valve on the outlet to prevent backflow during spring runoff. The outlet culvert extends through the pond embankment within the impoundment area as shown by the sedimentation pond design maps. The outlet culverts, which are located at the design storage level, are provided to allow gradual discharge of impounded storm runoff. The size of the outlet culverts have been determined so as to allow a minimum detention time of 24 hours. By providing for a detention time of 24 hours or more, any suspended sediment contained in the runoff is allowed to settle out before the water is discharged to the receiving drainage. The emergency spillways for the pond structures are riprap lined broad-crested weirs and outlet channels. The emergency spillway invert elevation is a minimum of 1.0 foot above the primary outlet structure. The primary outlet structures are designed so that any runoff • increment greater than the pond storage volume will overflow through the outlet culvert. In this way, the primary outlet structure supplements the emergency spillway to discharge excess storm runoff. The emergency spillways for Pond M3-S and M3-N have been sized for the 100 year-24 hour and 25 year-24 hour storm events, respectively. The emergency spillways have been sized to provide a minimum of 1.0 foot of freeboard between the maximum anticipated water level for the design storm event and the top of the embankment structure. In addition to the design freeboard, the construction height specifications for the embankment structure have been increased by 5°~ to allow for any possible settlement. The design equations utilized to size the primary outlet structure and emergency spillway are standard hydraulic design equations obtained from the Civil Engineering Handbook, TR 90-06 Exhibit 3-10 Revised 11!16!90