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In all of these snowflood and rainflood probable maximum events the tailings impoundment completely contains the projected runoff water volumes witPiout overtopping in these "worse case" flood scenarios. In the event of large flood inflows into the tailing pool, emergency spillway would begin discharge upon the pool level reaching 800 feet of beach, the discharge would maximize at a rate of 135 CFS or 270 acre-feet per day. In extreme conditions the decant pipeline would be utilized to discharge an additional 135 CFS or 270 acre-feet per day. Together these spillways would discharge 270 CFS or 540 acre-leet per day. 8y utilizing this emergency discharge the tailing pool would be rapidly drawn down approximately 1 foot per day, thereby reestablishing beach at the rate of 150 feet per day, achieving the desired 1000 foot of beach in approximately 7 days. These discharges would be covered under Henderson's NPDES permit for a discharge in the documented 10 year 24 hour precipitation event. DAM STABILITY Woodward Clyde Consultants conducted a static stability analysis on #1 tailings dam in July 1992 utilizing UTEXAS3 software and the Spencer Method in a report: "Slope Stability Study Henderson Mill Tailings Dam #1". The results of this study (summary attached) indicate a factor of safety of at least 2.39 under existing dam configuration. To evaluate stability during a potential flood event, the study was repeated utilizing a raised phreatic surtace. In this secnario the phreatic surtace was assumed to be located at a level of 10 feet from the dam face in all piezometers, then projected thru the dam interior to the surface at a point 400 feet from the crest, which corresponds to the sand/slime interface. The sand above the sand/slime interface is believed to be free draining and would not impeed water movement even if the pool were to cover this point in a flood condition. The calculated dam safety factor in the raisE~d phreatic condition is at least 1.78. The free draining sand assumption is illustrated during summer spigot deposition when tailing slurry is deposited at the dam crest. As the tailing is deposited at the crest, the water contained drains quickly and does not cause an increase in the phreatic level as measured monthly at the piezometers. Therefore the dam is expected to remain stable during a PM flood situation where the tailing water pool could approach to within 250 feet of the crest line for a short fe~a days. CLOSE The #1 tailing dam impoundment will with stand probable maximum flood events, even should they occur when the process water system is at maximum operation storage limits. All referenced documentation is maintained in the mill library should specific questions arise.