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• (Page 2) MINE ID f OR PROSPECTING ID ~ M-94-117 INSPE:CTION DATE 2/28/95 INSPECTOR'S INITIALS ACS OBSERVATIONS At a Formal Public Hearing held on January 25, 1995, the applicant (C.O.M., Inc.) indicated that the freeboard in the Gold Hill impoundment stood at 32 inches. It was further indicated that two of the three decant standpipes in the pond are functional. This inspection was conducted to verify these representations. It w:xs verified that two standpipes in the pond are functional. This was done by running water- into the pipes and verifying that the water was reporting to the bulkheaded Hazel-A ad it ae specified in Permit M-83-141. It was recommended to the operator that trash racks be placed over the standpipes. In ox•der to determine freeboard in the pond, a theodolite was set up and levelled on the dam crest:. Differential levelling was utilized to determine relative elevations of a number of point:e on the tailing surface and the dam crest. Plan dimensions were determined by pacing. The e~levatione determined by this survey are relative to an assumed elevation of 8580 for the top of the southernmost rebar driven into the dam crest. A copy of the survey notes is atta<hed to this report. This survey information was utilized to prepare a tailing surface contour map, a copy of which is attached to this report. Freeboard ie a term that is part of the nomenclature for reservoirs that impound water, and is deefined as the vertical distance between the low point on the crest of the embankment and the reservoir water surface. The application of the term freeboard at a tailing impoundment ie complicated by the fact that the elevation of the tailing surface fluctuates. If freeboard at that Gold Hill impoundment were to be defined as the difference in elevation betwExen the low point of the dam crest and the high point of the tailing surface, then the survey completed by the Division indicates that the freeboard at the impoundment is less than zero.. At the present time, the more critical evaluation factor may be consideration of available storage capacity. The storage capacity at the Gold Hill impoundment was determined using the survey data and the volumetrice feature of the engineering software "EarthVieion". Available storage capacity based on this analysis is 10,700 cu. ft. It is estimated that one-third to one-half of this capacity was taken up by ice, slush and water as of the 2/28/95 site inspection, which would leave as little ae 5,350 cu. ft. of available capacity in the pond. An analysis was conducted to determine the volume of water that would report to the pond during a 100 year - 24 hour storm event. This storm would amount to 4.6 inches of rain at the rite according to Ted Zorich and Associates report contained in Permit M-83-141. This evaluation assumed that the upland diversion ditches would be 100 percent efficient. Storm water- reporting to the impoundment would include the rain that falls below the elevation of the lowest ditch in the impoundment area, ae well ae runoff from the mill yard and the mill building roof, which has been routed into the pond ae part of the site storm water management plan. Running the analysis using hydrologic software "Storm•' indicates that 28,750 cu. ft. of storm water would report to the tailing impoundment during the 100 yr. - 24 hr. event, with a peak discharge of 5.18 cfe. A copy of the computer print-out is attached to this report. This analysis indicates that discharge to the pond during the design event would greatly exceeed available storage capacity. If the decant standpipes were to become plugged, or were otherwise unable to drain the pond, the dam would overtop. The Division considers any relizince on the decant standpipes to diminish storm inflows to be an unsound practice; one reaecm being that a pair of 2 in. pvc pipes could only pass a small percentage of the inflows that would be realized during a large rainfall at the site. The 5.18 cfa peak discharge woulcl be highly erosive on the steep outslope of the dam, and stability of the impoundment could be threatened. The Division considers the lack of storage capacity to be the primary immecliate concern at the Gold Hill Mill. A proposal to restore adequate capacity, probably by raising the dam crest, will have to be submitted by the applicant, approved by the Division, and implemented at the site, prior to the June 14, 1995 decision date for the application. The :following guidance should be utilized to determine adequate storage capacity, or freeboard, for the Gold Hill Impoundment: