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for these spillways have been calculated to be 8.2 cfs and 5.7 cfs respectively. The capacity <br />of the 18-inch diameter culvert is estimated to be 12.7 cfs, and is therefore capable of <br />transporting anticipated flows. <br />If the sediment pond was storing water and sediment generated by the 10-year, 24hour <br />event, the outlet pipe has the capacity to transport calculated flows at approximately 8.2 <br />cubic feet per second (cfs, or 3,680 gpm at average depth), dewatering the pond in about 25 <br />minutes. Maximum flows are not recommended, however, due to high outlet velocities and <br />potential erosion of the hillside below the railroad embankment. If high flow rates are <br />intended, it is suggested that grouted-in-place riprap be placed below the discharge point down <br />to the Highway 133 drainage ditch. <br />The concrete retaining wall will include an emergency spillway. This. spillway is located <br />immediately west of the 8-inch steel pipe outlet, as shown in Drawing 95255-03. This <br />spillway will be constructed within the concrete wall to transport flows generated by a 25- <br />year, 24hour storm event (Rule 4.05.615)(b)). Side slopes for the spillway will be 1:1, with <br />. a 1.5-foot bottom width and 1.5-foot depth. Flows generated by the storm will produce fl- <br />inches of depth on the 1 percent bottom grade (leaving 12-inches of dry freeboard), at a <br />calculated flow rate of 5.76 cfs. Riprap placed below the spillway on the downstream side <br />of the sediment pond wall should be 24-inch maximum rock size, grouted into place. Heavy <br />rip-rap must be in place to ensure the integrity of the track embankment under falling water <br />discharging from the emergency spillway. The protection should extend several feet beyond <br />the spillway on both sides, and within the existing railroad track drainage ditch. Discharge <br />from the spillway will be routed down the railroad track drainage ditch to be transported to the <br />opposite side of the tracks via existing downstream culverts. <br />4.2.6 Maintenance <br />Maintenance of the sediment pond will be concentrated on the removal of accumulated <br />sediment. The frequency of the sediment removal will have to be determined in the field, <br />depending on rainfall conditions. A calibrated pole or other device should be set within the <br />sediment pond to track sediment depth so proper maintenance can be employed when the <br />sediment is too deep to adequately hold the design storm. Vehicle access to the sediment <br />• pond will be via the sediment pond access road as described in Section 4.1.4. <br />96266/1424.RPT Coel Mina Weete Bank Design for the Senborn Creek Mins <br />October 1998 WESTEC, Inc. 19 <br />