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separation for deeper ponds. Other designs that satisfy the intent of this <br /> provision are allowed as long as the divider is permeable, structurally <br /> sound, and designed to prevent erosion under or around the barrier. <br /> To aid in determining sediment depth,one-foot intervals shall be <br /> prominently marked on the riser. <br /> i <br /> i If an embankment of more than 5 feet is proposed,the pond must <br /> comply with the criteria contained in Volume III regarding dam safety <br /> for detention BMPs. <br /> E <br /> E The most common structural failure of sedimentation basins is caused <br /> by piping. Piping refers to two phenomena: (1)water seeping through <br /> fine-grained soil, eroding the soil grain by grain and forming pipes or <br /> tunnels; and, (2)water under pressure flowing upward through a <br /> r: granular soil with a head of sufficient magnitude to cause soil grains to <br /> lose contact and capability for support. <br /> The most critical construction sequences to prevent piping will be: <br /> E . <br /> 1. Tight connections between riser and barrel and other pipe <br /> i connections. <br /> i = <br /> 2. Adequate anchoring of riser. <br /> 3. Proper soil compaction of the embankment and riser footing. <br /> 4. Proper construction of anti-seep devices. <br /> k: <br /> Sediment shall be removed from the pond when it reaches 1—foot in <br /> Maintenance ' <br /> Standards depth. <br /> i <br /> i Any damage to the pond embankments or slopes shall be repaired. <br /> ' BMP C260: Construction Stormwater Chemical Treatment <br /> Puipose This BMP applies when using stormwater chemicals in batch treatment or <br /> E <br /> flow-through treatment. <br /> E . <br /> Turbidity is difficult to control once fine particles are suspended in <br /> E stormwater runoff from a construction site. Sedimentation ponds are <br />! _ effective at removing larger particulate matter by gravity settling, but are <br /> ineffective at removing smaller particulates such as clay and fine silt. <br /> Traditional erosion and sediment control BMPs may not be adequate to <br /> ensure compliance with the water quality standards for turbidity in <br /> receiving water. <br /> Chemical treatment can reliably provide exceptional reductions of <br /> turbidity and associated pollutants. Chemical treatment may be required to <br /> meet turbidity stormwater discharge requirements, especially when <br /> construction is to proceed through the wet season. <br /> Con(litions of Use Formal written approval from Ecology is required for the use of chemical <br /> treatment regardless of site size. The Local Permitting Authority may also <br /> 11ohnne H—Conslruelion Stor rnwaler Polhrtion Prevention-August 2012 <br /> 4-110 <br />