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the recommendations of NEH Chapter 7 and the results of the 1981 Striffler and <br />Rhodes infiltration study. <br />A graph of the `accepted" curve numbers over the years is presented as Figure 1. <br />Figure 1 is for illustration purposes only and is not intended to be part of the TR- <br />95 revision. It is evident that TR -73 resulted in an abrupt rise in curve numbers <br />for reclaimed lands with no technical basis. It is also evident that the proposed <br />curve numbers in TR -95 would return only part of the way back to the historic <br />curve numbers. <br />• CCC references the 1981 report by Striffler and Rhodes, "Hydrologic and Erosional <br />Characteristics of Regraded Surface Coal Mined Land in Colorado," which includes <br />sample sites at eleven surface mines in Colorado. Specific to Colowyo, the sampling was <br />done on 11 acres of lands reclaimed at Colowyo in 1978. Colowyo now has thousands of <br />acres of reclaimed lands. On page 14 of this report, the authors state, "However, any <br />generalizations about the similarities of surface hydrology or regraded coal mined land <br />from state to state, between areas within a mining region, or even between sites within a <br />mine are risky due to critical site specificity." Thus it is not clear that past studies at <br />Colowyo, let alone studies at other mine sites, provide justification for altering CN values. <br />Response: CCC is aware of the generalization described on page 14. We further agree based on <br />annual report mapping there was considerably less area of reclaimed spoils in 1981 <br />than exists today, and that acreage was likely in the near the toe of the Streeter Fill <br />reclamation. However, we believe the regional nature of the report is in fact a <br />strength, not a weakness, as discussed below. <br />The justification for changing curve numbers is nevertheless straight- forward and <br />rests on three premises. <br />First, the NEH, which is the recognized document for hydrologic design, clearly <br />states that it is not appropriate to use native soil mapping to assign Hydrologic Soil <br />Group (HSG) values to disturbed lands. Reference is in part 630.0702, which states: <br />"As a result of construction and other disturbances, the soil profile can be altered <br />from its natural state and the listed group assignments generally no longer apply, <br />nor can any supposition based 011 the natural soil be made that will accurately <br />describe the hydrologic properties of the disturbed soil. In these circumstances, an <br />onsite investigation should be made to determine the hydrologic soil group". <br />Second, the appropriate curve numbers to use for regraded lands are largely <br />determined by the assumptions made regarding infiltration rates. CCC maintains <br />