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Silver Lake Plaza <br />• Sand replacement frequency is not well established and will depend on suspended solids <br />levels entering the filter (the effectiveness of the pretreatment BMP can be a significant <br />factor). <br />• Frequent overflow into the spillway or overflow structure or slow drawdown are indicators of <br />plugging problems. A sand filter should empty in 24 hours following a storm event (24 hours <br />for the pre -settling chamber), depending on pond depth. If the hydraulic conductivity drops <br />to one (1) inch per hour corrective action is needed, e.g.: <br />o Scraping the top layer of fine-grain sediment accumulation (mid -winter scraping is <br />suggested) <br />o Removal of thatch <br />o Aerating the filter surface <br />o Tilling the filter surface (late -summer rototilling is suggested) <br />o Replacing the top 4 inches of sand <br />o Inspecting geotextiles for clogging <br />• Rapid drawdown in the sand bed (greater than 12 inches per hour) indicates short-circuiting <br />of the filter. Inspect the cleanouts on the underdrain pipes and along the base of the <br />embankment for leakage. <br />• Drawdown tests for the sand bed could be conducted, as needed, during the wet season. <br />These tests can be conducted by allowing the filter to fill (or partially fill) during a storm <br />event, then measuring the decline in water level over a 4-8 hour period. An inlet and an <br />underdrain outlet valve would be necessary to conduct such a test. <br />• Formation of rills and gullies on the surface of the filter indicates improper function of the <br />inlet flow spreader, or poor sand compaction. Check for accumulation of debris on or in the <br />flow spreader and refill rills and gullies with sand. <br />• Avoid driving heavy equipment on the filter to prevent compaction and rut formation. <br />BMP T5.13 Post -Construction Soil Quality and Depth <br />Maintenance <br />• Establish soil quality and depth toward the end of construction and once established, protect <br />from compaction, such as from large machinery use, and from erosion. <br />• Plant vegetation and mulch the amended soil area after installation. <br />• Leave plant debris or its equivalent on the soil surface to replenish organic matter. <br />• Reduce and adjust, where possible, the use of irrigation, fertilizers, herbicides and pesticides, <br />rather than continuing to implement formerly established practices. <br />WESTERN ENGINEERS, INC. 42 PROJECT: 15-1415-A <br />