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culverts was calculated using a computer model. The model simulates the storm <br />• routing of a 10-year, 24-hour storm event from the drainage area to the <br />culvert. In order to simulate the storm event the discharge area of each <br />culvert was divided into subwatersheds according the mining and reclamation <br />sequence. Input parameters for each subwatershed were determined. The <br />included area, hydraulic length and slope of the subwatershed, hydraulic <br />length and slope from the subwatershed to the culvert and curve number. <br />Curve numbers were dependent upon the soil type, and the vegetation type <br />and quantity found within the subwatershed. Each soil type has a hydrologic <br />classification which is based on the soils' runoff characteristics. The <br />hydrologic classification for the soil types vas then determined from the <br />publication "Procedures for Determining Peak Flows in Colorado" - Technical <br />Release No. 55, S.C.S March 1990. The majority of soil types are hydrologic <br />classification "B" and "C". In order to simplify calculations and since <br />topsoil will combine several soil types, all soil types except mine spoil were <br />assigned hydrologic classification "C". Mine spoil vas assigned a hydrologic <br />classification of "B". After the soil was classified, the amount and types of <br />vegetation were estimated for each subwatershed. Figures S-3 and S-4 in the <br />above-mentioned S.C.S publication were used to find the curve numbers for the <br />• subwatersheds. Resultant subwatershed curve numbers can be found in Table <br />3.5-1. <br />3.5.3.3 Peak Flow Determination of Culverts in the Moffat Area <br />Depending on the mining and reclamation sequence, there is a year of <br />highest runoff for each culvert drainage area along the upper haulzoad in the <br />Moffat Area. The year of highest runoff will be determined by weight <br />averaging corresponding curve numbers and acres for the subwatersheds to find <br />the highest curve number that would occur in each of the culvert drainage <br />areas. This year would have the highest volume of runoff and highest peak <br />flow rate for each drainage area. Appendix 3.5-D contains culvert sizing <br />information for the culverts designed to date with additional culvert designs <br />to be added to this appendix as necessary in the future. Mai 3.SD-1 shows the <br />location of the designed culverts (those having an identification number <br />associated with [he culvert), as well as, the location of proposed culverts <br />• 3.5-5 May 14,1993 <br />