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1 <br />staked straw bales wrapped with filter fabric (geotextile) may be used. If the pond is <br />more than 6 feet deep, a different mechanism must be proposed. A riprap embank- <br />ment is one acceptable method of separation for deeper ponds. Other designs that <br />satisfy the intent of this provision are allowed as long as the divider is permeable, <br />structurally sound, and designed to prevent erosion under or around the barrier. <br />To aid in determining sediment depth, one -foot intervals shall be prominently <br />marked on the riser. <br />If an embankment of more than 6 feet is proposed, the pond must comply with the <br />criteria contained in Volume III (p.423) regarding dam safety for detention BMPs. <br />The most common structural failure of sedimentation basins is caused by piping. <br />Piping refers to two phenomena: (1) water seeping through fine-grained soil, erod- <br />ing the soil grain by grain and forming pipes or tunnels; and, (2) water under pres- <br />sure flowing upward through a granular soil with a head of sufficient magnitude to <br />cause soil grains to lose contact and capability for support. <br />The most critical construction sequences to prevent piping will be: <br />1. Tight connections between riser and barrel and other pipe connections. <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 />Maintenance Standards <br />• Sediment shall be removed from the pond when it reaches 1—foot in depth. <br />. Any damage to the pond embankments or slopes shall be repaired. <br />BMP C250: Construction Stormwater Chemical Treatment <br />Purpose <br />This BMP applies when using stormwater chemicals in batch treatment or flow -through <br />treatment. <br />Turbidity is difficult to control once fine particles are suspended in stormwater runoff from <br />a construction site. Sedimentation ponds are effective at removing larger particulate mat- <br />ter by gravity settling, but are ineffective at removing smaller particulates such as clay <br />and fine silt. Traditional erosion and sediment control BMPs may not be adequate to <br />ensure compliance with the water quality standards for turbidity in receiving water. <br />Chemical treatment can reliably provide exceptional reductions of turbidity and asso- <br />ciated pollutants. Chemical treatment may be required to meet turbidity stormwater dis- <br />2014 Stormwater Management Manual for Westem Washington <br />1 <br />1 <br />1 <br />L <br />J <br />1 <br />11 <br />11 <br />1 <br />1 <br />1 <br />1 <br />1 <br />1 <br />1 <br />1 <br />Volume 11- Chapter 4 - Page 396 <br />C� <br />