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in September 2007. The TR-42 SEDCAD4 demonstration is the model design for <br />. the August 2009 SEDCAD 4 demonstration. The only changes between the TR-42 <br />SEDCAD demonstration and the August 2009 SEDCAD demonstration are changes <br />to the watershed areas to reflect the corrected watershed boundary. <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 6,948.2 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 7-12A, 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 6,948.2 ft, with a storage capacity of 7.6 acre-feet (ac-ft). The <br />• SEDCAD4 modeling indicates that runoff volume and peak inflow for the 10- <br />year, 24-hour storm event (see Addendum 7-6c.IA) are 2.10 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 0.7 cfs. The SEDCAD4 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 SEDCAD4 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 004 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 6,949.0 <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, 24-hour storm event. Therefore, <br />the pond as constructed and operated meets the intentions of Rule <br />4.05.9(2)(b). <br />. The modeled 25-year, 24-hour storm event peak inflow is 16.5 cfs, while the <br />calculated peak discharge is 7.2 cfs with a corresponding peak stage <br />TR-51 3 Revised 09/09