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Sediment Ponds <br />The general plan calls for two sediment ponds controlling surface drainage from the sources <br />described above. <br />Surface runoff from the gravel operations is directed to and retained in a separate sediment pond, <br />which was designed and constructed as part of the gravel operations. Sediment pond sizing, design, <br />and operation is explained in Appendices 5-2 and 5-9. <br />Based upon the latest two foot topography, the current volume of Sediment Pond #2 is 4.4 acre- <br />feet. <br />Design Volume Calculations Sediment Pond # 2 <br />Area Drainage Area (acres) 11.19 <br />Design Event 10 yr. storm <br />Design Precipitation 2.4 <br />Average CN 86 <br />Calculated S 1.62790697674 <br />Description Exposed loose soil some vegetation <br />Calculated Runoff Volume (~ 1.16 <br />Calculated Runoff Volume (acre-ft.) 1.08 <br />Ponding Percentage 0.00 <br />Actual Runoff Volume (acre/ft.) 1.08 <br />Adding 0.4 ac-ft for sediment storage yields a total volume of 1.48 acre-feet needed in the pond <br />above the spillway. The pond has 4.4 acre feet, therefore, it is more than adequate. The pond will <br />actually contain the runoff for the 11.19 acre drainage area for the 100 year 24 hour event, therefore, <br />no spillway is needed. A principal and emergency spillway are installed in the pond and past <br />calculations show that the pond will route large storms through the spillway structures while <br />providing detention time for sediment and greatly lowering the peak flows. <br />Once Phase II bond release is granted, this pond will be removed. <br />Collection Ditches and Channels <br />Two collection ditches and one collection channel route surface runoff to the sediment ponds. The <br />East Collection Ditch collects runoff from an area south and west of the gravel operations and <br />conveys flow to Sediment Pond No. 1. A compacted berm was constructed below the east <br />TR09, TRl1, TR12 (v1.0) 5-5 Revised 3/98, 8/02, 9/03 <br />