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portion, we input into a SEDCAD drop down menu the overland flow ground condition (e.g., <br />pasture, gully, etc.) and the horizontal distances and vertical drops determined as described <br />above for channels. SEDCAD then transforms that data into the slope that appears in the <br />Subwatershed Sedimentology Details table that appear on pages Exh. 7 -PP -243 through 345. <br />In each case we measure the slope using the distance and vertical drop, and do not revise <br />them simply to achieve a higher degree of conservativeness, although as described above we <br />usually select the steepest reach of a stream if there are varying slopes within it. <br />We reviewed most of the slopes in the model and were unable to find any that looked unusual. <br />We therefore believe they are correct as submitted. <br />k. The SEDCAD model in Exhibit 7 -PP shows the contour ditches being riprapped. This is not <br />a common practice for contour ditches. Please explain why the SEDCAD model has riprapped <br />contour ditches. <br />Response: The ditches that were called out as "contour ditches" in the SEDCAD model are <br />not contour ditches in the common sense of the term but are collector ditches that happen to <br />run close to the existing contours. <br />The three that were so designated (SEDCAD Strs. 21, 23 and 25) are the three reaches of the <br />South Collector Ditch. It was constructed early in the mine's history as unlined erodible <br />channel in shales and hardpans. We have re -run the model with those ditches designed as <br />"erodible channel, hardpan and shales", and the velocities are within allowable values in <br />SEDCAD for that ditch condition. We have removed the word "contour" from their descriptions <br />in SEDCAD. <br />We also noticed a missing "Med" in the UHS column for Str. 32 in this model (similar to your <br />observation with regard to Structure 3 in Review Comment 18b above). Finally we observed <br />two Curve Numbers that were incorrect in Str 12 and corrected them. Specifically the values <br />of 77 and 85 respectively in Subwatersheds 1 and 2 should have been 74 and 77. All of the <br />above described changes have been incorporated in the revised SEDCAD output. There was <br />no significant change in the runoff delivered to Prospect Pond or the effluent quality leaving the <br />pond, although the 10 year 24 volume dropped slightly from 6.35 ac ft to 4.90 ac ft. <br />A revised SEDCAD model (pages Exh. 7 -PP -299 — 350) and the introductory text provided on <br />page Exh. 7 -PP -107 are included with this submittal. <br />I. Page Exh. 7-20C-1 (in Exhibit 7, Item 20, Part C) contains errors that need to be edited. It <br />mentions the East Taylor Tributary Ditches, which are removed with this submittal. Additionally, <br />there is extraneous text on fourth line ("Exhibit 7, Item 20"). Please review this page and make <br />corrections as necessary. <br />Response: Text in Exhibit 7, Item 20 Part C has been modified as requested. <br />in. There appear to be several errors with Table 4 in Exhibit 7, Item 20, Part C. For the West <br />Taylor Pond, the emergency spillway elevation and the riser height are different than the drawing. <br />Under West Taylor, post -mining, the emergency spillway discharge says "Final." This appears to <br />Page 7 <br />