Laserfiche WebLink
area and completely incised impoundment, there are no embankments to fail and so only <br />run -off volume calculations have been performed. <br />b) The sedimentation ponds are designed to provide adequate sediment storage volume in <br />compliance with the CDMG regulations. The actual designs are based on 3 year sediment <br />volumes based on USLE calculations. Actual construction may cut more volume below the <br />dam level (incised storage) to lessen expensive cleanout during the operation. <br />c) The principal spillway design is an 18 -24" diameter CMP or other pipe which will have a <br />valve on the entrance. This pipe will be stopped down in the inlet side to a 6" valve which will <br />be controlled by a wheel on the pond embankment. During normal conditions, the valve will <br />be left slightly open to provide 24 hour detention time for each pond during the large events. <br />This is accounted for as weep holes in the SEDCAD design runs. When a large event <br />occurs, the operator will wait until the Imhoff cone test shows that the water meets effluent <br />standards prior to discharge. If an Imhoff cone shows that the water meets the effluent <br />standard of 0.5ml settleable solids per liter, the valve will be opened until the water has <br />dissipated to the valve level. It will then be left slightly open to allow detention time for <br />subsequent storms. For Ponds 008, 011, 012 and 013, SEDCAD analysis has shown that <br />the effluent from a slightly cracked open valve (equivalent of a garden hose) still meets <br />effluent requirements. For these ponds, the operator can decide to leave the valve cracked <br />or shut as described above. <br />d) Appropriate combinations of principal and emergency spillways to safely contain or <br />discharge the runoff from a 25 -year, 24 -hour precipitation or larger event, as required by the <br />CDMG's regulations, will be installed. If a pond is designed to contain the runoff from a 25 <br />year 24 event, an emergency spillway cut into the dam will still be installed. <br />e) Pond freeboard will be a minimum of 1.0 feet from the highest level of water allowed in <br />the pond (or emergency spillway) to the top of dam. <br />0 All pond dams will be designed to have a height of less than 10.0 feet measured from <br />the original ground in the middle of the dam to the top of the dam. The top of dam width will <br />be at least 10.0 feet in all areas. <br />g) All ponds will have a keyway cut along the entire width of the dam as shown on Map <br />2.05.3(3) -6 -1. This keyway will be filled with competent material and compacted to the same <br />standards as the dam foundation. <br />(Revised June 2010) 2.05.3(3) -13 <br />