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n <br />U <br />ALLEN MINE - FILE N0. C-012-81 <br />Technical Revision No. 1 <br />Adequacy Response <br />This response addresses concerns of the Division in the <br />order that the issues were raised in the letter to CF&I Steel <br />Corporation dated April 12, 1982. Therefore, these responses <br />are numbered 1 through 5. <br />1. A CN value ranging from 68 to 82 could be expected <br />with ground covers ranging from 35% to 45X and Juniper- <br />Grass soil groups ranging from B (Table Mountain) to C <br />(Fuera) to D (Varner). Using the SCS method of peak <br />flow estimates from Procedures for Determining Peak <br />Flows in Colorado, peak flows would range from 7. <br />CFS to 124.0 CFS. However, inspection of the site <br />reveals that the predominant grot~ttd cover is Ponderosa <br />Pine, not Juniper-Grass as indicated by the MLR <br />inspector. It should be noted that north facing <br />slopes in the vicinity of Allen Mine are generally <br />covered with Ponderosa Pine and CF14I's permit appli- <br />• cation shows Ponderosa Pine as the predominant cover <br />for the slope in question. From this investigation, <br />curve numbers ranging from 64 to 76 could be expected, <br />resulting in peak flows ranging from 33.2 CFS to P6.3 <br />CFS and averaging 59.8 CFS. With these ranges of <br />runoff estimation and the general range of ground <br />cover types and groups, the original CF&I estimate of <br />59.9 CFS is not unreasonable. This is evidenced by <br />the heavy storm events of July and August of 1981, <br />during which time the ditches did not overflow at any <br />point. Tables and calculations for the figures <br />presented in this response are included at the end of <br />this response letter. <br />2. The Manning's N value chosen by CF&I is a low number, <br />but CF&I believes the use of a low N value is <br />justified in this case. N values for smooth surfaces <br />can be as low as 0.009 (Hydraulics by King, Wisler, <br />Woodburn and Handbook of Engineering Fundamentals by <br />Eschbach). The smooth metal construction of CF&I's <br />proposed design certainly has a lower N value than the <br />Division's recommended value of 0.014 which would be a <br />realistic value for unfinished concrete. The Division <br />is correct in stating that the upper flume does not <br />have the freeboard as required by Rule 4.05.3(6). <br />• <br />