1622 E MARINE VIEW DR WHOLE SITE 2022-02-01

separation for deeper ponds. Other designs that satisfy the intent of this provision are allowed as long as the divider is permeable, structurally sound, and designed to prevent erosion under or around the barrier. To aid in determining sediment depth, one -foot intervals shall be prominently marked on the riser. If an embankment of more than 6 feet is proposed, the pond must comply with the criteria contained in Volume III regarding dam safety for detention BMPs. • The most common structural failure of sedimentation basins is caused by piping. Piping refers to two phenomena: (1) water seeping through fine-grained soil, eroding the soil grain by grain and forming pipes or tunnels; and, (2) water under pressure flowing upward through a granular soil with a head of sufficient magnitude to cause soil grains to lose contact and capability for support. The most critical construction sequences to prevent piping will be: 1. Tight connections between riser and barrel and other pipe connections. 2. Adequate anchoring of riser. 3. Proper soil compaction of the embankment and riser footing. • 4. Proper construction of anti -seep devices. Maintenance • Sediment shall be removed from the pond when it reaches 1—foot in Standards depth. • Any damage to the pond embankments or slopes shall be repaired. BMP C250: Construction Stormwater Chemical Treatment Purpose This BMP applies when using stormwater chemicals in batch treatment or flow -through treatment. Turbidity is difficult to control once fine particles are suspended in stormwater runoff from a construction site. Sedimentation ponds are effective at removing larger particulate matter by gravity settling, but are ineffective at removing smaller particulates such as clay and fine silt. Traditional erosion and sediment control BMPs may not be adequate to. ensure compliance with the water quality standards for turbidity in receiving water. Chemical treatment can reliably provide exceptional reductions of turbidity and associated pollutants. Chemical treatment may be required to meet turbidity stormwater discharge requirements, especially when construction is to proceed through the wet season. Conditions of Use Formal written approval from Ecology is required for the use of chemical treatment regardless of site size. The Local Permitting Authority may also Volume II — Construction Stormwater Pollution Prevention - August 2012 4-110