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models. <br />P. In Exhibit 7, Item 20, Part C it appears that some of the SEDCAD models are missing the <br />Subwatershed Sedimentology Detail. Please review the SEDCAD models and provide the <br />sedimentology detail. <br />Response: This comment is dealing with the West Taylor Pond. The sedimentology details <br />are only included on the 10 year 24 hour storm runs for use in assessing compliance with the <br />effluent limitations. These 10 year 24 hour sedimentology runs were performed for two cases, <br />the active mining case where 70% of the watershed does not report but there is recent <br />immature revegetation immediately above the sediment pond, and the PMT case where the <br />entire watershed is reporting. Reviewing pages Exh. 7-20C-20 for the "Active Mining" case , <br />and pages Exh. 7-20C-57 and 58 for the 'Post -Mining" case, all of the subwatersheds have <br />their respective hydrology and sedimentology details. <br />If the review comment deals with the SEDCAD runs made for the 25 year 24 hour storms, <br />again for both cases, please note that those are used to assess the adequacy (safety) of the <br />emergency spillway. There are no relevant effluent limitations on this storm. Accordingly, we <br />do not include the sedimentology input for these spillway design runs. No changes have been <br />made as a result of this comment. <br />q. Please discuss the use of stock ponds in the introductory text of Exhibit 7, Item 20, <br />Part E. <br />Response: We have added a few sentences on page Exh. 7-20E-2 to address the small stock <br />tanks in the upper portion of the watershed. Pages Exh. 7-20E-1 through 5 are resubmitted <br />with this submittal. <br />r. Please re -run the model with actual elevations for the stock ponds (rather than 90, <br />100, etc.) in Exhibit 7, Item 20, Part E. <br />Response: We commonly use this technique to model the situation where a proposed <br />stock tank (typically about 2 ac -ft) is needed to attenuate flows or runoff volumes. The <br />technique has been included in numerous modeling efforts approved under other <br />Technical Revisions. In the future post -mining case that may be 10 to 15 years hence, <br />the exact elevations on the stock ponds inverts and spillways cannot be known nor <br />their exact shape determined until the post -mining topography elevations are known. <br />However the "as -built" tanks at final post -mining time frame need only provide the 2 <br />ac -ft volume and can be constructed as more natural depressions blended into the <br />spoil rather than the prismatic ponds assumed at the time of modeling. <br />The simple generic "relative elevation" stock tanks are needed for modeling because <br />SEDCAD is extremely sensitive to the flow elevations in and out of ditches entering <br />and leaving an on-line reservoir such as these stock tanks, especially if the intent is to <br />store much of the area runoff and sediment higher up in the watershed. That is usually <br />the case since that is the reason they are included in the first place. SEDCAD will <br />Page 9 <br />