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Elk Creek Mine TR-60 <br />2nd Adequacy Review <br />16 Dec 2008 <br />Page 3 of 9 <br />The Division has no further concerns. The check dams were eliminated from the <br />drainage plan proposed in the November 26, 2008 submittal. <br />6. There are no Sedcad designs for the check dams that show the construction <br />requirements for the check dams. Since.the check dams are part of a designed <br />sediment control structure, please revise the Sedcad designs to show their <br />construction requirements. <br />The Division has no further concerns. The check dams were taken out of the <br />proposed drainage plan in the November 26, 2008 submittal. <br />7. The Sedcad designs for the small area exemptions at the north area, topsoil <br />storage area and the lay down area do not include sediment analyses to show that <br />the 0.5 m111 settable solids limit can be achieved. Please revise the Sedcad designs <br />accordingly. <br />In the submittal dated November 26, 2008, Oxbow presented Sedcad designs to <br />show that silt fence or straw bales can be used to meet the settable solids limit. <br />However, the Division has several questions and comments concerning the <br />Sedcad designs. <br />A. Oxbow used the pond routine in Sedcad to model the straw bale sediment <br />trap, with user-input discharge values based on the permissivity of straw <br />bales. The submittal cover letter states that a permissivity value of 5.6 <br />gpm/ft2 was used for straw bales. The Division attempted to calculate the <br />user-input discharge values for the topsoil SAE using this permissivity rate <br />but was not successful. The Division's back calculation appeared to show <br />that a permissivity rate of about 10 gpm/ftz was used for straw bales. If <br />Oxbow wishes to continue to use the pond routine for straw bale sediment <br />control modeling, please explain the methodology used in calculating the <br />user-input discharge values for the straw bales and show an example <br />calculation that was used to obtain the discharge rate used in the Sedcad <br />design for straw bales. Otherwise, please see comment number 7D below. <br />B. Although user-input values for the discharge rates were used in the Sedcad <br />pond routine for straw bales, it does not appear that any filtering of <br />sediment by the straw bales was modeled in the Sedcad designs. For <br />example, in the Sedcad design for straw bales for the topsoil SAE, the <br />particle size distribution at structure #1 shows that 100 % of the sediment <br />for all particle sizes except the smallest particle size is passing out of the <br />structure. In addition, the trap efficiency that is stated in the pond results <br />for the same Sedcad design is zero. If Oxbow wishes to continue to use the