2013-04-05_PERMIT FILE - C1981019A (9)

Home Browse Search

upstream of the West Taylor Sediment Pond is included in the design to achieve discharge standards. The analysis shows the West Taylor Sedimentation Pond will still meet the applicable standard in the 10 year, 24 hour event, and therefore requires no modifications for this condition. The 25 year, 24 hour rainfall event was also modeled for the truncated watershed, and it does not engage the emergency spillway. The results of the updated modeling are nevertheless now included along with the previous SEDCAD results in this Exhibit 7, Item 20, Part C. Revisions Pursuant to TR -81 The final PMT associated with TR -81 and the final locations of the permanent drainage channels as presented on Map 41A were used to model the watershed for the post- mining condition. Six in- stream stock tanks were included in the permanent drainage channels (West Taylor West Tributary and West Taylor East Tributary) to maximize the sediment control as the reclaimed watershed progresses toward bond release. These stock tanks will be constructed as the final grading of the reclaimed lands and construction of the channels proceeds to the south. Their dimensions will be as described earlier in this section. The SEDCAD modeling was done using the CNs as specified in TR -73 (see Figures 1 C -6 and 1 C -7). The 10 year, 24 hour storm produces 12.35 acre feet of runoff at the West Taylor Sedimentation pond. As summarized in Table 4, peak reservoir stage while passing this storm is at Elevation 7474.89, about 3.89 feet above the overflow of the primary (drop inlet) spillway and 0.9 feet below the invert of the emergency spillway. Settleable solids released form the pond in the downstream "null" modeling node are 0.0 ml /l. The 25 year, 24 hour storm produces flow of 1.1 feet deep in the emergency spillway, leaving 4.1 feet of about 4.1 feet to the dam crest. The normally dry temporary impoundment approximately 0.5 acre feet in size immediately upstream of the West Taylor Sediment Pond, required for the active mining case of TR -88, is no longer required and will likely have been removed as reclamation progresses southward. It was not included in the post- mining hydrologic modeling. Pagination Tracking History, Exhibit 7 -20C Because TR -81 includes revisions to the PMT that make the original modeling associated with PR -02 no longer valid, the original modeling has been excised from this section to prevent confusion. However, the "active mining" condition modeling done pursuant to TR -88 needs to be retained. Rather than re- paginate the entire TR -88, it has been left intact as it was approved, including its page numbers as they appeared in TR -88. The pages associated with the no longer valid PR -02 topography have been removed and in most cases not replaced. Specifically, Fig. 1C -1 which formerly appeared on page Exh. 7 -20C -2 has been deleted, and that page is now occupied by the continuation of the introductory text. Fig 1C -2 which formerly occupied page Exh. 7 -20C -3 has been deleted and replaced with that page intentionally left blank. Original Fig. 1C-3, the "as built" of the sedimentation pond, has been left intact and appears on page Exh. 7- 20C -4. The SEDCAD runs for the no longer valid PR -02 topography have been deleted and not replaced. The TR -88 pages remain as they were submitted and start at page Exh. 7- 20C -83, leaving a gap in the pagination from Exh. 7 -20C -5 through Exh. 7- 20C -82. The TR -88 modeling continues as it was submitted in TR -88 through page Exh. 7 -20C -112. The figures and SEDCAD modeling associated with TR -81 begin immediately after the TR -88 pages, starting on page Exh. 7 -20C -113 through Exh. 7- 20C -194. Finally, with RN -06 this section was repaginated to correct pagination errors and to include a page that delineated the gap in pagination. All modeling associated with TR -88 and TR -81, as specified above, was retained with original footers. Exh. 7 -20C -2 Revision Date: 7/26/12 Revision No.: RN -06