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Mr. Rob Zuber <br />August 3, 2015 <br />Page 10 <br />ITEM 35. BMPs (e.g., silt fence) will be needed prior to grass establishment. This should be <br />discussed in text. <br />Response: The area being proposed for a grass filter is existing undisturbed vegetation. Since <br />the vegetation is already mature, there is no need for additional BMPs such as silt fences to serve <br />while the vegetation is growing. However, as discussed in the final paragraph of the narrative as <br />previously submitted, another BMP is proposed if needed. That is the use of straw wattles in the <br />event that concentrated drainage channels might develop. The narrative has therefore not been <br />revised for this issue. <br />Exhibit 7, Item 25 <br />ITEM36. It appears on the maps that large areas that are within the tie-in boundary (presumed <br />to be same as disturbance boundary) do not drain to a pond. This includes the west side of the <br />pit, in Sections 2, 3, and 34. Please explain this in light of 4.05.5(1) and other relevant rules. <br />Response: The response to this comment is still being prepared, and it will be subsequently <br />addressed at a later date under a separate cover. <br />ITEM37. The Division has observed that the modeled flows in the SEDCAD submittal in PR -04 <br />for the reaches in Little Collom are much smaller than the flows in PR -03, despite the fact that <br />watershed areas are approximately the same and slopes are comparable. Please explain what <br />caused these significant decreases inflows. <br />Response: It is agreed that the peak flow rates under PR -03 were higher than those same <br />watersheds subsequently modeled under PR -04, particularly for the larger channels. For <br />example, for the Little Collom Reach 2, which is designed for the 100 year event, the peak flow <br />under PR -03 was 512 cfs while it is predicted at 337 cfs under PR -04. Also note that the total <br />runoff volumes compare much more closely. This change in peak flows is primarily due to the <br />fact that no time of concentrations were entered in the earlier model. Instead, we modeled using <br />Muskingum parameters in recognition of the fact that the main channel of interest receives flows <br />from numerous smaller upstream channels. Under the PR -04, we used the simpler approach of <br />individual time of concentration for each tributary stream, and ignored the attenuating effect that <br />Muskingum routing would have been expected to produce. <br />Upon further review for this submittal, it was decided to rerun the analysis including both normal <br />the time of concentration lag for each subwatershed, as well as Muskingum parameters for four <br />of the subwatersheds that report to the main channel significantly above the point of interest. <br />This is the most accurate model. Its results produced a slight (3%) reduction in the flow from the <br />upper reach (Reach 1), but surprisingly a slight (2.5%) increase in the total flow in the lower <br />reach (Reach 2). While this is technically a more exact model, the differences between this <br />