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<br />Mr. Rick Mills <br />Twentymile Coal Company <br />April I1, 1997 <br />Page 2 <br />The attached Map 25 Revision plan sheets show cross-sections for the newly proposed <br />reclamation grading information. The cross-sections capably demonstrate the reduced, limited <br />disturbance associated with this new reclamation grading. <br />Regazding the second main issue of drainage conveyance, it is our finding that permanent <br />diversion of drainage from the areas "above" (south of) the disturbance area by means of grassed <br />channels is the most effective approach. The channels have been designed to convey peak flows <br />from a 100-year, 24-hour stomp event as developed using the SedCAD+ computer program. <br />The permanent diversion channels are designed to keep ] 00-yeaz peak flow velocities under five <br />feet per second (5 fps) by means of mild slopes and wide channels with shallow depths of flow. <br />The slopes used are at or under 2%, and the depths of flow are typically between two to 2.5 feet. <br />An updated basin map with summary table and SedCAD+ output data aze included for review <br />and records. <br />The permanent diversion channel locations and alignments are indicated on the revised Maps 25 <br />and 31 plan and cross-section views. In addition, dimensioned cross-sections for the channels <br />have been added to the revised Map 25 sheet 4 of 4. <br />Rick, this concludes our comments regazding the attached map revisions and support <br />documentation. Please let me know if you have comments or questions. Thank you. <br />Respectfully, <br />TUTTLE A PLEGA"CB, INC. <br />`' ~;..' <br />. o` <br />-_' ' °~ ~~72 v'a's <br />Daniel P. Giroux, P.E. ,~ <br />a <br />cc: File 95-115dmg •.:~~` °' <br />.1:1951151mil6.Itr <br />