REV07330 - Laserfiche WebLink

Home Browse Search

r J Memo - C. Mount - C-81-022 - 3 - February 15, 1991 5. PROVIDE CULVERT DESIGNS FOR THE TWO UNNAMED DRAINAGES WHERE THE CREEKS PASS BELOW THE ACCESS ROAD, PER RULE 4.03.2(4)(e). The operator indicates that only one culvert will be installed to pass upland drainage under the haul road/conveyor corridor. Two dugout ponds are proposed to contain upland and disturbed area runoff from the area east of Sanborn Creek. This proposal is cannot be approved. It is allowable to route undisturbed and disturbed area runoff through ponds; however, these ponds must be designed, constructed and maintained in accordance with Rules 2.05.3(4), 4,05.6 and 4.05.9. The applicant must submit the following: a. More detailed pond designs must be submitted, for the drainage area east of Sanborn Creek, in accordance with the requirements of Rules 2.05.3(4), 4.05.6 and 4.05.9. These designs must include both plan view and cross-section designs which include anticipated water and sediment volumes, detention times, stage/discharge/volume details, outlet and spillway specifications, etc. b. Revised peak flow and volume calculations must be submitted, using a revised curve number for the upland area east of Sanborn Creek. According to Map 5-005, the upland area is a juniper woodland community with a cover value of 70%, The soils are Torriorthents - Rock Outcrop, Sandstone complex. According to the "Soil Survey of the Paonia Area, Colorado", (U.S.D.A.), this unit consists of about 30% outcrop and about 60% soil. The soils are composed of loam and/or sandy loam, with stones and rock fragments. According to "Soils of Colorado, Loss Factors and Erodibility, Hydrologic Groupings", S.C.S., 1983, Torriorthents are classified in the D hydrologic group. Based on these variables, and with reference to Table 2-2d (Urban Hydrology for Small Watersheds - TR-55), the appropriate curve number for this area should be 80. This curve number will result in significant runoff generation. The final drainage control plan for the area east of Sanborn Creek must be designed, based on these revised figures. c. Given the apparent lack of space at the mine area, the applicant is strongly encouraged to divert upland flow by way of ditches, and culverts. The small remaining disturbed area would be more easily handled, if all upland drainage is diverted. If diversions and culverts are used, then plans must be submitted. d. The operator should also submit a plan to accommodate pond dewatering, and any associated need to obtain NPDES compliance for the area east of Sanborn Creek. A more detailed review of the drainage control features east of Sanborn Creek will be done when the responses to this issue are submitted.