2012-04-26_PERMIT FILE - C1981012A (2)

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An erosion estimate was done for the proposed DVWDA and as a result, a small sediment basin of 250 cubic yards is recommended if no cover management is practiced on the DVWDA. The sediment basin should be installed at the inlet to the culvert, the entire volume of which is below the pipe invert. The sediment basin will require regular maintenance. Culverts C -21 through C -25 direct undisturbed runoff south of the fan and substation to the Purgatorie River. Sizing data is provided in Exhibit 19— Sediment Design Specifications. Culvert C -25 was inspected and found to be draining all runoff adequately. The Operator is unsure if it is a drop structure or simple culvert. (It would be necessary to excavate the inlet to determine what type of structure exists. Pond 004 was surveyed in 1995 and found to have 7.19 acre -feet of volume. A 10 -year 24 -hour storm of the contributing watershed is 0.9 acre -feet, a 25 -year 24 -hour storm is 1.13 acre -feet and a 100 -year 24 -hour storm is 1.5 acre -feet. There are 3.78 acre -feet below the lowest dewatering hole, of which SEDCAD+ estimates there are three years of sediment storage between 0.98 and 1.28 acre -feet. The Operator retains the option to use all 4.03 acre -feet as a combination of sediment or permanent pool storage. Sediment would be cleaned out at an elevation of 7,449 feet. The Operator periodically stores drilling fluids in this pond to the elevation of the lowest dewatering orifice and, depending on existing water storage in the pond, there would be a maximum of 2.75 acre -feet (895,950 gallons) of fluids in this pond. The duration of storage will depend upon the quality of the pond's water and the size of subsequent storm events. The Operator constructed the preparation plant in the spring of 1984. After several months of operation, it was determined that a plant process pond was required to accommodate plant discharges. The Operator submitted a permit revision whereby Pond 006 was taken out of the surface drainage plan and modified to a plant process pond. Pond 007 was sufficiently oversized to handle the additional runoff formerly reporting to Pond 006. is Exhibit 19— Sediment Design Specifications and West Portal Drainage Plan, confirm that Pond 007 (as- built) has sufficient storage capacity to contain 10 -year 24 hour storm even runoff from pond 006 and 007 drainage areas a total of 6.6 acre feet. In addition the Universal Soil Loss equation was used to estimate sediment yield from these areas at 0.3 acre -ft /yr (0.9 acre feet for three years) at a conservative 1.0 sediment delivery ratio. The total required capacity for Pond 7 is therefore 7.5 acre -ft. Construction of the Bates Portal will add in incremental amount of water (0.21 A -F) and sediment (0.01 A -F) to pond 7. Therefore the required storage volume for sediment pond 007 is 7.7 acre -ft. As shown by the stage volume curve for Pond 007, 9.2 acre -ft of storage is available in Pond 007. The emergency spillway is adequate for passing peak 25 -year 24 -hour flows from both drainage areas while maintaining 1 foot freeboard. Exhibit 19, Section 1 contains SedCad documentations that indicate Pond 005 and Pond 007 can be operated with the gate valves open and that sediment effluent limitation can be met. However, precipitation events sampled in 2003 indicate that under current conditions, effluent limits at Pond 007 may be exceeded. Thus the operator will operate this pond with the gates normally closed, until such time as it can be demonstrated, either by maintenance or reclamation practices, that effluent limitations will be met. All sampling during discharge events will be conducted in accordance with the approved NPDES permit to determine actual compliance with limitations. The SedCad analysis for Ponds 005 and 007 was an analysis of an installed perforated riser at each pond . under existing subwatershed conditions that had been previously analyzed in the permit. The data used in this analysis, including watershed area calculations, was taken from calculations performed for the design specification of Pond 5 and 7 in Exhibit 19 of the permit. The drainage areas map in Exhibit 19 shows the watershed drainages for ponds 5 and 7. Drainage area 005 -1 is the total drainage area for Pond 005. Drainage areas 006 -1 through 006 -5 and 007 -1 through 007 -6 all constitute the total drainage area for Pond 007. The operator will also open the gates on all other sites. A staff gage will be installed on or adjacent to • the riser pope of Pond 007 identifying the water level (7373.3 ft) adequate to retain a 10 -year 24 -hour storm without discharging over the emergency spillway. This elevation was arrived at through a review of the Pond 007 as -built stage volume curve dated 5/31/85 in Exhibit 19. The crest TR 53 Page 2.05 -10a Revised 1 -3 -11