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! ~ ~ Dlr. Dean Massey ,~ D1arch 15, 1978 Page Four have to be destroyed. It is the opinion of our hydrologist that putting in a sedimentation pond will cause more damage than it will prevent. 1'or a Eull discussion of these alternatives, please see Appendix II (attached). Mr. Charles F. Leaf, Y.C., of Fort Collins, Colorado has done an extensive catalysis of the drainage program to be implemented at the Orchard Valley Minc. Appendix (attached) contains detailed information on storm runoff predictions for all drainage areas involved. In addition, it contains a more detailed discussion of drainage plans and sediment control. Appendices III and IV (also attached) contain further analyses on rile sedimentation pond capacities as required by the OSD1 regulations. Analyses contained in these ap- pendices are based upon the assumption that sr_orm runoff from all disturbed areas would be routed through the settling ponds. It should be noted that ehe series of sedimentation ponds proposed has been recently broken down into a two phase program. CWI intends Co build four sedimentation ponds inunediately and add the remaining four ponds as needed. Section 717.17 (h) Surface Water Monitoring - CWI has had an extensive surface water monitoring program in effect for the past year. The monitoring program will be carried on in accordance with the OSM regulations. The sediment control measures which will be imple- mented by CIdI include designing the sediment retention ponds to retain far in excess of the ten year, 24-hour storm as required by the OSM regulations. Because of the design criteria used, the only time a discharge could he expected would be in a storm event larger than a ten year, 24-hour frequency and according to 717.]7 (a) (1), CWI would not then be subject to the effluent limitations set forth in the OS14 regulations. Section 717.17 (c) Diversion and Conveyance of O~erl.and Flow Away from Distrubed Areas - In order to minimize erosion and prevent water from upstream undisturbed areas heing routed through the mine, the diversion channel as described on page D-17 of the permit application will be installed. The channel has been designed to intercept the runoff from a probable maximum thunderstorm and routed away from the mine area. If this upstream water were allowed to run through the mine area, it could pose a potential hazard to life and property. (See Appendix IV for a detailed discussion of the necessity for the channel. ~ 1~1 i~~l~' ~1 'I ~1 ~. ~ 17~~ ~ . , ~