2009-10-06_PERMIT FILE - C1980005 (2)

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demonstration and the August 2009 SEDCAD demonstration are changes to the • watershed areas (and corresponding time of concentration calculations) to reflect the corrected watershed boundary. Principal Spillway Replacement The new 8-inch plastic corrugated pipe was installed in a trench excavated across the width of the embankment. The invert was set at 7,015.3 feet, with a barrel slope of one percent. Because of the embankment height (toe elevation of approximately 7,000 feet) the trench was extended down the face of the embankment, and an 8-inch diameter plastic corrugated pipe extension was fitted to the end of the straight barrel pipe. Trench material also was compacted around this extension pipe. The extension pipe has an outlet elevation of 7,001.0 feet. Exhibit 7-17E, Pond 008 Principal Spillway Replacement As-Built, presents the as-built configuration following the principal spillway replacement. It should be noted that the replacement of the principal spillway did not affect or include the existing emergency spillway. An as-built survey was completed in September 2005. Using the as-built survey data, SEDCAD4 was used to check the as-built configuration with the • new principal spillway and show that the pond will meet or exceed the performance requirements. The survey indicates a permanent pool elevation is 7,015.3 ft, with a storage capacity of 7.5 acre-feet (ac-ft). The SEDCAD4 modeling indicates that runoff volume and peak inflow for the 10- year, 24-hour storm event (see Addendum 7-6f.3A) are 2.6 acre-feet and 11.3 cubic feet per second (cfs), respectively, with a calculated peak stage of 7,016.1 feet. The corresponding peak discharge from the 10-year, 24-hour storm event was calculated as 0.8 cfs. The SEDCAD4 results also indicate the average settleable solids concentration in the pond discharge during the peak 24-hour period will be 0.0 milliliters per liter (ml/1). The Water Quality Control Commission (WQCC) settleable solids effluent concentration limit is 0.5 ml/l. The SEDCAD4 modeling demonstrates there is sufficient storage capacity available, and that the settleable solids effluent concentration is less than the applicable water quality standard. Therefore, Pond 008 as constructed and operated meets the intentions of Rule 4.05.6(3)(a). The calculated peak stage from the 10-year, 24-hour storm event is 7,016.1 feet. This elevation is 0.8 feet below the emergency spillway invert • elevation (7,016.9 feet). This demonstrates that no flow will pass through the emergency spillway during the 10-year, 24-hour storm event. Therefore, TR-51 3 Revised 09/09