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off. The Bench Pond was analyzed using the USDA Soil <br />Conversation Service computer program TR -20 for 30 yr., 25 yr., <br />and 100 yr. 24 hour storm events. Consideration was given for <br />ponding behind the inlet culverts and for additional drainage <br />which might split and contribute to the run off into the pond. <br />The results of this analysis indicate that the pond will function <br />properly even during the 100 year event. <br />Considering the fact that the pond is totally excavated in <br />a erosion resistant formation, that there is no embankment to <br />fail and place persons or property in danger, and that the TR -20 <br />analysis indicated the pond will function with major storm events <br />an emergency spillway does not appear to be necessary or even <br />practical. Ponding on the mine site can be utilized to the point <br />where the water would spill into the mine. The mine entries are <br />bermed to prevent water from entering the mine. Approximately 2 <br />feet of ponding above the 24 inch culvert could be provided with <br />out danger of flooding the mine. <br />Since constructed, the pond has been subjected to several major <br />storm events and performed very well. Three years ago a major <br />storm occurred in the area which raised the flow in the Arkansas <br />River at Portland (about 5 miles down stream from Florence) <br />10,000 cfs. Mast roads in the immediate were washed closed. The <br />pond level was less than 6 inches above the spillway. <br />QDWER BENCH PDND <br />The lower bench pond was analyzed using TR -20 fqr the 10 yr., 25 <br />yr., and 100 yr. 24 hour storm events. The results of the <br />analysis indicate that this pond will function properly for the <br />storm events. Considering the embankment an emergency spillway <br />has been designed for the pond. The contemplated spillway would <br />be 4 feet wide and 2 feet deep. The "GEDWEB" cellular confinement <br />system will be used to protect the embankment. The invert of the <br />spillway will be 1 foot above the top of the existing spillway. <br />The capacity of the spillway is approximately 12.85 cfs, running <br />12 inches deep. The crest of the embankment will have to be <br />raised about 12 inches to provide sufficient free board. The <br />existing crest is wide enough to provide for the additional <br />height. <br />The information required for a Technical Revision for <br />installation of the spillway far the lower sedimentation pond is <br />enclosed. If there is any additional information required or if <br />you have any questions, please let me know. <br />Very truly yoursq <br />Lawrence G. White P.E. <br />2 <br />