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Return Period (years) Precipitation (inches) for 24-Hour • <br />10 1.70 ~ <br />L <br />25 2.10 !~ <br />100 2.50 i <br />These amounts also include water emanating from snow cover as snowmelt in a limited <br />period of time. <br />A standard SCS Type !1 storm was utilized for each analysis. <br />SCS Curve Number Selection. Perhaps no parameter in hydrology is as subjective as the <br />selection of the proper runoff curve number for a given Watershed. Traditional methods of <br />curve number estimation involve the engineer or hydrologist, With some soils and <br />vegetation information, visiting the Watershed in question, observing the vegetation and <br />soils, and then selecting a curve number. Curve number selection relies heavily upon the <br />judgment of the designer, but this selection process usually performs satisfactorily in <br />practice. Curve numbers are a function of three principal variables: vegetation type and <br />cover, and the hydrologic soil group of the watershed soils. The curve numbers are <br />neigh [ed based on major soil groups and vegetation types in the watershed. Table 1 and • <br />Figure 1 provides the basis for all curve numbers utiLiied. These curve numbers are part <br />of the revised SCS, TR-55 publication and the 1980, "Peak Flows in Colorado" SCS Manual. <br />Peabody Coal Company Will primarily use curve numbers Within the range, land use. The <br />reclaimed areas will be evaluated using [he herbaceous, land use. Curve numbers for <br />disturbed areas will mainly be based upon curve numbers for "Street and Roads", and <br />"Disturbed Lands", and a review of the land use during the life of mining and reclamation. <br />These curve numbers will be utilized when revieui ng existing structures and designing all <br />new structures. <br />Where the Lost of a proposed facility might be extremely large, more extensive analysis <br />will be performed. <br />The calculated values for curve numbers reflect an Antecedent Moisture Condition (AMC) II. <br />SCS criteria defines AMC II as between 0.5 and 1.1 inches of rainfall in the five days <br />prior to the design event for the vegetation "dormant" season and bet wean 1.4 and 2.1 <br />inches during the gr owing season. As She most intense precipitation events are summer <br />thunderstorms during the growing season, using AMC II requires that a minimum 2-year, <br />24-hour event occurs in the five days preceding the design event in question. To ensure <br /> 13-1-2 Revised 05/23/95 <br />