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~ o <br />Page 4 <br />Colorado Westmoreland, Inc. <br />MLR D/OSM Joint Review <br />December 2, 1980 <br />Ditch cross-sections and profiles should be cross-referenced to the drainage <br />control map in ot'der to clearly show the Location of eoch type of r:itch and <br />ditch slope. The assumptions and calculations for each ditch should also be <br />cross-referenced so that Cbe ditch or lengths of ditch to which they refer car. <br />be clearly seen. <br />Please provide plan views and cross-sections of each sedirent pond. Pond <br />cross-sections should show maximum sediment storage elevation and runoff storage <br />for the 10 year/24 hour precipitation event above the dewatering device. <br />The storage capacities of the ponds below the mine area must be calcu_'ated <br />using worst case distrrrhance estimates for both the mine site and the refuse <br />disposal area. Estimated detention times for each runoff/sediment control <br />system should be provided for the 20 year/24 hour precipitation event. <br />Provide plan views and cross-sections of each Fond spillway showing deviations, <br />depth of design flow, spillway length, width and slope, length and width of <br />apron, riprapping lunyth, width and size, energy dissipation structures, etc. <br />All assumptions and calculations used to develop design flows, apron length, <br />riprap sizing, Length of riprapping, energy dissipation structures, e[c., <br />need to be provided. <br />AlI assumptions and calculations used to determine peak flows runoff volumes, <br />and sediment volwncs Cram t1~~• area(s) draining to a pond nen_d to be provided <br />for all ponds. Also area(s) draining to each pond should be clearly outlined <br />on a topographic map (Plate 2 - drainage plan and waste disposal area pre- <br />liminary flow lines shoul ri be more clearly illustrated.) <br />Two alternatives for sediment control in the storage and Ioadout area are <br />given in Appendix O, but ncit:hcr appears to be the method shown on map No. 14. <br />A single plan should he presented, ambiguous reference to alternatives deleted. <br />Pumping is not an accepted dewatering method for the parking area sediment <br />pond. A dewatering device should be added at a level such that there is <br />storage space for the .10 year/24 hour event between the dewatering device and <br />the emergency spillway. Also, contrac'y to the statement in Appendix P, the <br />lower most mine area sediment pond must be dewatered after sediment has settled out. <br />Please indicate on map No. 11 the route that water discharge From the sewage <br />treatment facility and node No. 20 will take in order to pass through the <br />sedimen[ation ponds. It appears that unless ^ collector ditch is constructed <br />from beneath thu two culverts near node No. 20 over the large CMP draining the <br />office and shop area, the runoff from this node will nut pass through a <br />sediment pond. Also, if the waste pile is to be permanent, the final <br />diversion ditches draining node No. 20 and the office and shop area will have <br />to be designed accordingly. <br />Water quality date on the nfflucnt di:;charging at node No. 22 should be provided <br />when availnbL:. <br />