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Memo - C, Mount - C-81-022 - 2 - February 15, 1991 Section 1 - Evaluation of Responses to Division Review of November 28, 1990 Operations PLEASE MAP THE 100-YEAR FLOOD LEVELS OF THE NORTH FORK OF THE GUNNISON RIVER ON DRAWING 5-002. OPERATIONALLY, IT IS IMPORTANT THAT THE STRUCTURES PROPOSED FOR THE SANBORN CREEK FACILITIES BE ABOVE THE 100-YEAR FLOODPLAIN. The revised Drawing 5-002, has been reviewed. The map is certified as being accurate, and we can assume that the flood boundary has been properly drawn. This issue is resolved. 4. PLEASE PROVIDE DESIGNS FOR THE MINE WATER SETTLING POND PER RULE 2.05.3(4) (a ). This request should have cited Rules 2.05.3(4)(a), (b) and (c ). The plans submitted in response to this request are not adequate. First, the mine water settling pond is considered a sediment pond, and as such, it must be designed in accordance with the performance standards of Rules 4.05.6 and 4.05.9. The applicant must submit detailed design diagrams which portray both plan and cross-section views of the impoundment. Embankment dimensions, volume/stage information, spillway configurations, etc. must be shown. The plans must also portray the anticipated stage/discharge information in sufficient detail to allow an accurate assessment of pond storage capacity and water detention time relative to anticipated inflow and outflow. Second, the operator projects a maximum anticipated pond inflow based upon assumptions which may not be appropriate. Randy will provide an assessment of the applicant's projection for mine water inflows. Pursuant to discussions with Randy, it seems that the operator's mine water inflow projection of 237 g.p.m. may not be sufficient. Additional amounts may be encountered due to the position of the mine workings relative to the ground water table, and the presence of fractures throughout the overlying strata. Also, the estimated dewatering rate of 80 g.p.m. is not substantiated. The mine plan scheme indicates that the one-third estimate may be flawed. The operator should submit additional information to substantiate the figures used. Such information should include an estimate of fracture flow to the mine workings, more detailed information to substantiate the one-third fraction, and detailed information regarding sump construction phasing, sump volume, and sump locations. We will assess the adequacy of the mine water settling pond design after the above-requested details are submitted.