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n <br />u <br />LJ <br />I• <br />Principal Spillway Replacement <br />The 36-inch CMP pipe was removed and replaced with an 8-inch plastic <br />corrugated pipe set at an invert elevation of 6,998.2 feet. Bedding <br />material was backfilled in the trench to the appropriate elevation, and <br />after the pipe was installed, the trench material was compacted back around <br />the pipe. Exhibit 7-12 A, Pond 004 Principal Spillway Replacement As-Built, <br />presents the as-built configuration following the replacement of the <br />principal spillway. It should be noted that the replacement of the <br />principal spillway did not affect or include the existing emergency <br />spillway. <br />An as-built survey was completed in September 2005. Using the as-built <br />survey data, SEDCAD4 was used to check the as-built configuration with the <br />new principal spillway and shows that the pond will meet or exceed the <br />performance requirements. The survey indicates a permanent pool elevation <br />of 6,996.2 ft, with a storage capacity of 7.6 acre-feet (ac-f t). The <br />SEDCAD9 modeling indicates that runoff volume and peak inflow for the 10- <br />year, 29-hour storm event (see Addendum 7-6c.lA) are 2.56 ac-ft and 4.25 <br />cubic feet per second (cfs), respectively, with a calculated peak stage of <br />6,949.0 feet. The corresponding peak discharge from the 10-year, 24-hour <br />storm event was estimated as 0.8 cfs. The SEDCAD9 results also indicate <br />the average discharge settleable solids concentration during the peak 24- <br />hour period will be 0.0 milliliters per liter (ml/1) . The Water Quality <br />Control Commission (WQCC) settleable solids effluent concentration limit is <br />0.5 ml/1. The SEDCAD9 modeling demonstrates there is sufficient storage <br />capacity available, and that the settleable solids effluent concentration <br />is less than the applicable water quality standard. Therefore, Pond 009 as <br />constructed and operated meets the intentions of Rule 4.05.6(3)(a). <br />The calculated peak stage from the 10-year, 24-hour storm event is 6,999.0 <br />feet. This elevation is 1.2 feet below the emergency spillway invert <br />elevation (6,950.2 feet). This demonstrates that no flow will pass through <br />the emergency spillway during the 10-year, 29-hour storm event. Therefore, <br />the pond as constructed and operated meets the intentions of Rule <br />9.05.9(2)(b). <br />The modeled 25-year, 24-hour storm event peak inflow is 17.56 cfs, while <br />the calculated peak discharge is 7.57 cfs with a corresponding peak stage <br />TR-42 3 Revised 09/07 <br />