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~ERr:~es co-osi-si <br />The pond size required to retain the runoff volume from the <br />10-yr, 24-hr precipitation event and to allow for sediment <br />storage is estimated to be 0.10 AF as follows: <br />Drainage area: 2.1 acres <br />Runoff/acre: 0.03 AF/ac (l0yr 24hr, from MULTSED model) <br />Runoff volume: 0.065 AF <br />Sediment volume: 0.035 AF (from MULTSED model) <br />Total pond volume: 0.10 AF <br />The peak discharge for the 10-yr, 24-hr and 25-yr, 24-hr <br />events are calculated as: <br />Q l0yr 24hr = 0.37 cfs/ac x 2.1 ac 0.78 cfs <br />Q 25yr 24hr = 0.59 cfs/ac x 2.1 ac = 1.24 cfs <br />As can be seen from the attached volume - depth curve, the <br />pond has an adequate volume to meet the above criteria. The <br />current volume will increase by about 0.05 AF when the <br />accumulated sediments, now about 0.5 ft in depth, are cleaned out <br />prior to mine operation. <br />The pond was surveyed to determine dimension and elevations. <br />An arbitrary datum, based on the southwest corner of the <br />maintenance shed as 100.00, was used since no vertical control <br />was readily available in the area. <br />The principal spillway now consists of a 12 inch diameter <br />stand pipe with a cap made up of an open ended 25 gallon drum <br />(See drawing attached). The top of the drum is at elevation <br />97.76 which is above the emergency spillway elevation of 97.67 <br />and only 0.24 feet below the crest of the embankment. Also, the <br />standpipe does not appear to have any means of draining the pond, <br />since there are no openings visible other than the open end of <br />the standpipe. <br />