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<br />In the submittal dated November 26, 2008, Oxbow presented Sedcad designs to show that <br />silt fence or straw bales can be used to meet the settable solids limit. However, the <br />Division has several questions and comments concerning the Sedcad designs. <br />A. Oxbow used the pond routine in Sedcad to model the straw bale sediment trap, <br />with user-input discharge values based on the permissivity of straw bales. The <br />submittal cover letter states that a permissivity value of 5.6 gpm/ft2 was used for <br />straw bales. The Division attempted to calculate the user-input discharge values <br />for the topsoil SAE using this permissivity rate but was not successful. The <br />Division's back calculation appeared to show that a permissivity rate of about 10 <br />gpm/ft2 was used for straw bales. If Oxbow wishes to continue to use the pond <br />routine for straw bale sediment control modeling, please explain the methodology <br />used in calculating the user-input discharge values for the straw bales and show <br />an example calculation that was used to obtain the discharge rate used in the <br />Sedcad 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 pond <br />routine for straw bales, it does not appear that any filtering of sediment by the <br />straw bales was modeled in the Sedcad designs. For example, in the Sedcad <br />design for straw bales for the topsoil SAE, the particle size distribution at <br />structure #I shows that 100 % of the sediment for all particle sizes except the <br />smallest particle size is passing out of the structure. In addition, the trap efficiency <br />that is stated in the pond results for the same Sedcad design is zero. If Oxbow <br />wishes to continue to use the pond routine for straw bale sediment control <br />modeling, please explain. Otherwise, please see comment number 7D below. <br />C. In the Sedcad designs, Oxbow used the pond routine to model the silt fence <br />sediment trap. Please explain why Oxbow used the Sedcad pond routine for the <br />silt fence when a silt fence routine is available in Sedcad. <br />D. It does not appear that there is a satisfactory way of directly modeling straw bales <br />for sediment control in Sedcad. Previous attempts by other operators involved <br />either modeling the straw bales as silt fence or as grass filters, with the <br />assumption that the straw bales act similarly. If Oxbow would like to use another <br />approach to Sedcad modeling for straw bales besides the pond routine, the <br />Division suggests that the silt fence routine be used in Sedcad with a permissivity <br />rate of 5.6 gpm/ft2, similar to that of straw bales. A note should be added that the <br />designs assume that the straw bales will filter sediment similar to silt fence. <br />Oxbow response: In the last srabrnission, S`? r? bale boundaries and Wilt Fences were <br />17?ode ed Ilse g .`he pond rowing %rI SedCad 21ere ?>ere riumeerovsprogra;'n errors <br />encounter"ec' /Zth the gilt Fr-,,n e however-, after some changes in Configurations <br />and col'abor"at on r?i±+? he erea2o7s ?f SedCad, we were able to ran the Silt Fence routine <br />f©r" the SA -Es; zhere;?;fe, S!ro-v,) bale grid Sit Fence sediment traps were modeled using the <br />silt fence rouIire it SedCAD. err2issiv;I- races of 5. b gpm/ft and 10 gpm/ft were used <br />for straw vales and silt fence, "espeetively. <br />• Page 3