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4.6.1.2 Peak Runoff Determination <br /> Runoff from the drainage area into the Moffat Area sediment pond was <br /> calculated using the computer program SEDIMOT II. The computer model <br /> simulates the runoff hydrograph of a 10-year, 24-hour storm event. In order <br /> to simulate the storm event, the drainage area was divided into subwatersheds <br /> according to the mining and reclamation sequence and input parameters were <br /> determined for each subwatershed. Input parameters include area, hydraulic <br /> length and slope of the subwatershed, hydraulic length and slope from the <br /> subwatershed to the pond, and curve number. Of the parameters, only curve <br /> number is not measured directly from a map. <br /> Curve numbers were dependent upon the soil type and the vegetation type <br /> and quantity found in the subwatersheds. Each soil type has a hydrologic <br /> classification which is based on the souls runoff characteristics The soil <br /> inventory (Section 2.3) and Exhibit 2.3-1 were used to find the soil types and <br /> quantity existing in the subwatershed. The hydrologic classification for the <br /> soils was then determined from the publication "Procedure for Determining Peak <br />• Flows in Colorado" - Technical Release No. 55, S.C.S., March 1980. The <br /> hydrologic classification of the soil types that occur within the MoffaC <br /> drainage area can be found in Table 4.6-4 along with their corresponding <br /> acreages. As the table shows the majority of the soils are type "B" and "C". <br /> in order to simplify calculations and since topsoil removal will combine <br /> several soil types, all soil types except mine spoil were assigned hydrologic <br /> classification "C^. Mine spoil was assigned a hydrologic classification of <br /> "B". After the soil was classified, the amount and types of vegetation were <br /> estimated for each subwatershed. Then Figures S-3 and S-9 in the above <br /> mentioned S.C.S. publication were used to find the curve numbers for the <br /> subwatersheds. The curve numbers that were assigned to the different <br /> subwatersheds can be found in Tab 1e 4.6-5. <br /> Depending on the mining and reclamation sequence, there is a year of <br /> highest runoff for the drainage area. Also, since the size of the watershed <br /> draining into the pond enlarges over time, evaluation of the sediment pond <br /> design was based on peak flow and peak stage of the runoff at the pond. The <br /> year of highest runoff was determined through a time-step process determining <br />. the runoff from the watershed over the mining years. Although Year 1 had the <br />9.6-15 <br />November 1,1989 <br />