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For disturbed lands the use of mapped SCS soil groups is not appropriate. Instead, Section 630.0702 <br />"Disturbed Soils" in Chapter 7 of Part 630 of the NRCS National Engineering Handbook (NEH) <br />recommends that onsite investigations should be made to determine the HSG, including on -site infiltration <br />tests, grain size characteristics and lab permeability tests. A program meeting these requirements was <br />carried out and reported in Striffler and Rhodes (1981), and it included tests performed at the Colowyo <br />mine, as well as ten other Colorado coal mines. <br />All of the infiltration tests performed at the Colowyo Mine reported in Table C -5 of that study indicated <br />infiltration rates well in excess of a "C" soil, and in all cases except one reflected rates that would be <br />associated with a HSG "A" soil. That exceptional case indicated infiltration rate between HSG "A" and <br />HSG B" soils. The selection of HSG "B" for the disturbed lands is therefore a conservative assumption, <br />more likely to overestimate than to underestimate runoff rates. <br />With the HSG determined as described above for undisturbed and disturbed land, the CN is then dermined <br />based on the land cover condition as described in numerous hydrologic references, all ultimately based on <br />NEH cover and condition tables. <br />The resulting hydrologic curve numbers (CN) for use in SEDCAD for the various categories of land at the <br />mine are presented below in Table 1. Table 7 provides a scanned image of published curve numbers <br />typical for disturbed areas; these values will not be used directly but are only provided to support the <br />changes to Table 1. Table 8 provides a similar listing from the National Engineering Handbook (NEH -4) <br />(USDA -SCS, 1985); these values are included within the drop down menus of the SEDCAD model. Of the <br />several tables included here as table 8, the last table presented here in Table 8 is applies to Range lands in <br />and and semi -arid areas, and is the most appropriate set of Curve Numbers for the Colowyo Mine <br />applications. Values in Table 8 will not be used directly but are provided to support the values presented <br />below in Table 1. <br />Exhibit 7 Page 3 Revision Date: 3/18/13 <br />Revision No.: TR -95 <br />