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21 <br />pond 007. The engineering evaluation of the DWDA #2 is included as Exhibit 39 <br />Sediment Design Specification- Development Waste Disposal Area. As part of TR- <br />60 the DWDA #2 was expanded to the west in order to gain more storage area for <br />development waste. The drainage area evaluated for TR -60 is 5.1 acres and includes <br />the DWDA #2, and a small portion of the haul road north of the pile that will flow <br />into Ditch D -12. The 100 years, 24 -hour storm precipitation is 4.2 inches. SedCad <br />calculates the peak flow to be 15.58 cfs with 1.47 acre -feet of total runoff. The <br />required ditch size to convey that flow is a trapezoidal channel of bottom width 3.0 <br />ft, height of 1.5 ft, side slopes of 1:1 and a minimum slope of 1 %. Existing ditches <br />are of this recommended size and shape. The minimum pipe diameter needed to <br />convey the 100 -year, 24 -hour peak flow to pond 007 is 24 inches. The existing <br />culvert is 24 inches. The pipe installation should have a minimum of 1.2 feet of <br />material from the top of the pipe to the top of the bank (i.e. the lowest point on any <br />top of berm /top of bank around the DWDA #2 shall be no less than 1.2 feet above the <br />top of the 24 inch outlet pipe. <br />A small area exemption has been established for a material storage area and <br />associated activities located south of sediment pond 7 due to its location outside of an <br />area where runoff will be routed to a sediment pond. Berms and a silt fence provide <br />alternative sedimentation control for this area. The design of the silt fence is included <br />Exhibit 19(17). This SAE area is shown on Map 11 Sheets 1, 2 and 3. <br />The cut slopes above the upper diversion ditch at the Bates Portal fall under Rule <br />4.05.2(4), which states in part; for the purposes of 4.05.2, "disturbed area" shall not <br />include those areas in which only diversion ditches sedimentation ponds, other <br />treatment facilities, or roads are installed in accordance with the performance <br />standard requirements of this Rule. The cut slopes will be covered with a straw <br />blanket and seeded since there is concern the slopes may contribute suspended solids <br />to stream flow or runoff outside the permit area. The straw blanket will be <br />maintained until vegetation is established. <br />An erosion estimate was done for the proposed DWDA #2 and as a result, a small <br />sediment basin of 250 cubic yards is recommended if no cover management is <br />practiced on the DWDA #2. The sediment basin should be installed at the inlet to the <br />culvert, the entire volume of which is below the pipe invert. The sediment basin will <br />require regular maintenance. <br />Culverts C -21 through C -25 direct undisturbed runoff south of the fan and substation <br />to the Purgatoire River. Sizing data is provided in Exhibit 19- Sediment Design <br />Specifications. Culvert C -25 was inspected and found to be draining all runoff <br />adequately. The Operator is unsure if it is a drop structure or simple culvert. (It <br />would be necessary to excavate the inlet to determine what type of structure exists. <br />Pond 004 was surveyed in 1995 and found to have 7.19 acre -feet of volume. A 10- <br />year 24 -hour storm of the contributing watershed is 0.9 acre -feet, a 25 -year 24 -hour <br />storm is 1.13 acre -feet and a 100 -year 24 -hour storm is 1.5 acre -feet. There are 3.78 <br />acre -feet below the lowest dewatering hole, of which SEDCAD+ estimates there are <br />Section 2.05 TR -68 12/10/2014 <br />