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6. Attachment 2.05.3(3) -24 — Pond 015 As -Built <br />a) Under the "Design" section on page 1, the three -year projected sediment <br />accumulation is given as 0.002 acre -feet. The volume anticipated in the Pond 015 <br />design (Attachment 2.05.3(3) -22) was 0.04 acre -feet. Please provide an <br />explanation for the 95% reduction in predicted three -year sediment accumulation <br />for Pond 015. <br />Response: The Revised Universal Soil Loss Equation (RUSLE) for sediment <br />runoff from a 10OYr -24hr event was used for both the Design calculations <br />(Attachment 2.05.3(3) -22) and the As -Built calculations (Attachment <br />2.05.3(3) -24. RUSLE has five components (R, K, LS, C & P) and their <br />definition can be seen on page Attachment 2.05.3(3) -22 -2, -3 & -4. <br />A constant value of 0.39, (based upon a 2 -3% slope and 1000ft of horizontal <br />travel), for the LS factor was used in the Design. Since the as -built drainage <br />area for Pond 015 was a different shape than the design, the LS factor was <br />recalculated. <br />An absolute worst case "C" factor with "No appreciable canopy" and 0% <br />ground cover of 1.0 was used for the Design. See pages Attachment 2.05.3(3)- <br />22-6 for more detail. The As -Built "C" factor was derived from the "C" <br />Factors for Permanent Pasture and Rangeland table on page Attachment <br />2.05.3(3) -22 -6. The "No appreciable canopy and 40% Ground Cover" value <br />of 0.10 was used for the rocky Stockpile. The "Canopy of tall forbs or short <br />brush and 60% Ground Cover" value of 0.038, rounded to 0.04 was used for <br />the other three areas of runoff. In conclusion, the design was conservative in <br />different factors to ensure that Pond 015 was sufficient; the as -built <br />calculations show that the actual conditions will generate less sediment than <br />the design assumed. <br />5 <br />b) As -Built parameters for Pond 015 are provided on page 4. The Curve Number <br />(CN) has been reduced to 75.5 from 83.5 presented in the original design for Pond <br />015. USLE inputs are tabulated on page 6, and an " Erodibility" value of 0.37 is <br />used for each of the different land use categories in this table. Please provide an <br />explanation as to how this number was derived. <br />Response: The Erodibility "K" factor for NRCS Map Unit 77- Pinion, zone <br />0 " -5" is 0.28, & 5 " -16" is 0.37. The "K" factor for NRCS Map Unit 77- <br />Progresso, zone 0 " -7" is 0.37. So a value of 0.37 for the Erodibility K factor <br />was selected, as it is the highest K factor for the soil types and would keep the <br />calculation conservative. The design K of 0.39 was based on a soil type 14 <br />that is not present in the as -built area. See NRCS, Soil Survey of San Miguel <br />Area, Colorado, and page 462. <br />The CN of 75.5 was calculated from a composite of the as -built runoff areas <br />that drain to Pond 015. Pond O15's storage volume (0.935 ac -ft) is <br />X: \Western Fuels Nucla \TR- 60 \TR -60 adequacy question #1 \TR -60 PRELIMNARY ADEQUACY RESPONSE <br />25AUG11.doc <br />