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<br />Mr. David A. Berry <br />Reclamation Specialist -2- <br />CMLRD <br />Trapper has used the CN's of 47 and 49 for undisturbed ground associated with the cover <br />classes of 60% and 54% respectively. The CN's for active mining and reclamation areas <br />range from 67 to 87 and are analogous with similar SCS land use classifications. <br />The SCS combines infiltration losses with surface storage estimating rainfall excess or <br />equivalently the runoff volume by the relationship: <br />Q = P_( 0.2512 when P>0.25 <br />~+0.85 <br />Using this formula, it would take a precipitation event greater than or equal to 2.08" for P <br />to equal 0.25. Since the approved 10-year 24-hour precipitation event is 1.7 inches, <br />Trapper has used the value of zero runoff for all undisturbed areas. <br />Trapper feels that the initial abstraction components (interception, depression storage <br />and infiltration) are sufficient to reduce the accumulated runoff volumes to zero for any <br />event < a 10-yr 24-hr event on undisturbed ground. Therefore Trapper made the <br />statement... "yield no runoff..:' and does not calculate undisturbed acreages into the <br />drainage basin area when calculating excess runoff volumes to size culverts. <br />Sincerely, <br />/~1/,~ta-mot <br />William Agnew <br />Sr. Environmental Engineer <br />pc J M Mattern <br />R W Fish <br />W S Harding <br />R L Fanyo <br />File <br />WA/WSH/rbb/007 <br />