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Return Period (Years) Precipitation (Inches) for 29-Hour+ <br />10 1.70 <br />25 2.10 <br />100 2.50 <br />•These amounts also include water emanating from snox cover as snowmelt in a limited <br />period of time. <br />A standard SCS Type II storm was utilized for each analysis. <br />SCS Curve Number Selection. Perhaps no parameter In hydrology Sa 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 questl on, 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 pe rfonna satlsfacto rlly Sn <br />practice. Curve numbers are a function of three principal variables: vegetation type and <br />cover, and the hyd rologlc soil group of [he watershed soils. The curve numbers are <br />weighted based on major soil groups and vegetation types Sn the watershed. Table 1 and <br />Figure 1 provides the basis for all curve numbers utilized. These curve numbers are part <br />of [he revised S.C.S., TR-55 publication and the 1980, "Peak Flows in Colorado" SCS • <br />Manual. <br />Seneca Coal Company will primarily use curve numbers within the range, land use. The <br />reclaimed areas will be evaluated using the herbaceousr 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 [he life of mining and reclamation. <br />These curve numbers will be utilized when reviewing existing structures and designing all <br />new structures. <br />where the cost of a proposed faculty might be extremely large, more extensive anal ysls <br />will be performed. <br />The calculated values for curve numbers reflect an Antecedent Moisture Condition (AMC( II. <br />SCS criteria defines PMC II as between 0.5 and 1.1 inches of rainfall in the flue days <br />prior to the design event for the vegetation "dormant^ season and between 1.4 and 2.1 <br />inches during the growing season. As the most intense precipitation events are su mmer <br />thunderstorms durin rowin <br />the <br />season <br />usin <br />AMC II re <br />uires that a minimum 2-year 29- <br /> g g <br />g <br />, <br />g <br />q <br />, <br />hour event occurs in the flue days preceding the design event in ques tlon. To ens ure <br />13-1-2 Revised 09/28/95 <br />