1991-05-16_REVISION - C1981035 (2)

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Emergency Spillway Type: Rectangular Sharp Crested Weir C = 3.27 L = 8ft Crest Elev. = 97.7 Max. Head = 0. 3 ft where C is the orifice discharge coefficient, A the area of the opening in ftZ, and Ho is the head on the orifice in feet, and Q is the discharge in cfs. The invert of the orifice is at elevation 87 . 17 and the maximum water surface during the 25yr event is 98. A standard discharge coefficient of 0.7 was used. The orifice discharges water into an 12 inch diameter pipe which terminates in the field west of the pond. In order to maintain inlet control at maximum discharge (= 1 cfs) , the outlet pipe must be at a minimum slope 0. 007 (0.7%) . Under these conditions, the pipe will be flowing nearly full. Any slope greater than this will shorten the length of pipe required and increase the discharge capacity (Simon, 1981) . The outlet works characteristics are summarized in the tables and shown in figures 2 and 3 . The discharge rating curve for the orifice can be seen in the HEC-1 run entitled 'Low-Level Outlet Performance' in the appendix 4 . The combined rating curve for the orifice and the weir is seen in the HEC-1 output entitled 'Design Storm, Full Pond and 25yr-24hr Event' , appendix 3 . 4 . West Sediment Pond Hydrology The 10yr-24hr and 25yr-24hr design events were computed and routed through the West Pond using the HEC-1 model (HEC, 1981) . The procedure is identical to that described in section 3 for the East Pond. The following table lists the watershed parameters for the 10yr and 25yr events Watershed Parameters for Westside Drainage Drainage Area = 43 .5 acres Length to Divide A� 2500 ft Curve Number = 72 Average Slope = 12.8% Lag Time = 12 . 6 minutes 7