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East and West Topsoil Stockpiles Gob Pile #3 Alternate Sediment Control Straw Log substitute for Silt Fences �yQp LIC EN�F/� 2 � c� •1 a The following table summarizes the peak water stage, trap efficiency and si ESSIO %V' fence length for the topsoil stockpiles at gob pile #3. Description Peak Water Stage Trap Efficiency Length Feet % Feet Topsoil Stockpiles- 0.24 50.95 65 Gob Pile #3 The design flow rate for the silt fence is 10 gallons per minute per square foot. The peak water stage is 0.24 feet or 2.88 ". The key factor in sediment control is the trap efficiency. According to data published by American Excelsior Company, its 12" Curlex sediment log has a trap efficiency (% Soil Retained) of 70.3% and a flow rate of 37.5 gallons per minute per square foot. Its 6 and 9 inch sediment logs have a trap efficiency of 53.9% at 42.5 gallons per minute per square foot. The high flow rates of the sediment logs in relation to the silt fence design would indicate run -off will not overtop the straw logs. The straw logs will filter the run -off faster that the sediment fences. The favorable trap efficiency of the straws logs indicate they are as good as or better than the designed silt fence The 6 and 9 inch straw logs have a slightly lower trap efficiency that is not considered significant. Straw logs will be keyed into the natural soil at the low point of the topsoil stockpiles as shown on Map 21 -3. The total length of the straw logs will be comparable to the length indicated in the approved silt fence designs including tie back distances. The straw logs will be overlapped to prevent flow bypass when pieces are joined to achieve the appropriate total linear footage. Flow from the straw logs will then be directed to the Clean Water Diversion ditch. TR -88 Volume XI, App. B -75 05114