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Carol Pahlke <br />December 23, 1980 <br />Page 12 <br />One additional comment not <br />not addressed revegetation <br />species diversity (Section <br />Kerr Coal must address this <br />Division. <br />mentioned to date: Kerr Coal has <br />success criteria with respect to <br />4.15.8(b)) at the Kerr mine area. <br />issue to the satisfaction of the <br />Response: Kerr Coal proposes, in view of the <br />present paucity of scientific information or agree- <br />ment on species diversity, that the MLRD, OSM and <br />Kerr Coal stipulate that mutually acceptable suc- <br />cess criteria for species diversity be formulated <br />and agreed to on or before September 1, 1982. <br />Section 780.29(c) - Diversions <br />Kerr Coal has responded to OSM's concern regarding flow veloc- <br />ities of diversions. The seed mixture used appears to be <br />acceptable based on design velocities of ditches. However, <br />LRCWE did not consider the stability of the channels in the <br />unvegetated condition. It may take an entire growing season <br />or longer, to establish enough vegetation to justify the <br />Mannings n used to calculate channel flow velocities. During <br />this semi, or unvegetated condition the Mannings n will be <br />substantially lower, resulting in a much higher flow velocity. <br />This increased velocity may corrode the channels and prevent <br />any plant establishment. <br />The standard method of designing vegetated channels is to <br />design for velocity using unvegetated conditions, then re- <br />design the ditches for capacity using the vegetated condition. <br />The additional flow capacity needed for vegetated channels <br />should then be added on top of the unvegetated ditch design. <br />Kerr Coal should submit an analysis of the ditch stability in <br />the unvegetated condition using either the limiting velocity <br />approach or the limiting tractive force approach. <br />Response: LRCWE did not consider the stability of <br />the channels in an unvegetated condition because the <br />mine site is located in an acid climate where annual <br />rainfall is approximately 11 inches. It was the <br />professional judgment of LRCWE that designing the <br />ditches based upon velocities in an unvegetated <br />condition was not justified. To quote LRCWE: <br />"If the Kerr Coal Company site was back east, <br />or some other location where rainfall was sig- <br />nificantly greater, LRCWE would agree that the <br />ditches should be designed for the worst case <br />