2008-05-05_PERMIT FILE - C1980001A

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• (Topographic Factor) was determined for our situation from Table I for an average slope length of 200 feet and a maximum percent slope of 12.0%. The "C" factor (Cover or Crop Management Factor) changes from year to year during our mining and reclamation sequence. The factor that we used for the Aspen Woodland was conservative for the area, since we have close to 100°h tree canopy along with almost 100% ground cover by forest litter. The factor that was used for our mine spoiis was based on the high infiltration rate of the old spoils that virtually eliminates rainfall runoff. Also, the area between spoil ridges acts as a closed collection system for what little runoff does occur. The factors that were used for vegetation or ground cover were also conservative since w e have observed more ground cover during each of the years of growth than the percentages stated. The factors were taken from Tables 3, 4 and 5 of the publication, as noted on The tables. The "P" factor (Erosion Control Practice Factor) is said to be I.0 for all areas except cropland. For the area that we are mining and reclaiming, contour furrows or small pits will be used. The factor 0.30 was selected for this condition (8 to 12%o slope) and was found in Table 6 of the publication. • The sediment yield was calculated to be 12.5917 acre-feet for the area during the mining and reclamation sequence. The summary sheet for the total sediment yield, an example calculation, and individual calculations for each area of clas- sification can be found in Tables 4.6-18, 19 and 20 respectively. Refer to Exhibit 4.6- 17 for information regarding the areas of sediment yield. 4.6.4.2 Peok Runofi Determination for West Ridge Area Curve numbers, selected for peak runoff determination, were dependent upon the soil type and the vegetation type and quantity found in the subwatersheds. Each soil type has a hydrologic classification which is based on the soils runoff characteristics. The soil types and hydrologic classifications are listed in Table 4.6- 21. As the table shows, the soils are type "C" with the exception of mine spoil which was assigned o hydrologic classification of "B". After the soil was classified, the amount and types of vegetation were estimated for each ores. Then Figures S-3 and S-4 in the above mentioned S.C.S. publication were used to find the curve numbers for the areas. The curve numbers that were assigned to the different areas during the mining and reclamation sequence can be found in Table 4.5-22. Revised 7-81 4.6-64