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construction tends to dog the soils and reduce their capacity to infiltrate. If permanent infilt- ration BMP footprints are used, the sides and bottom of the temporary sediment pond must only be rough excavated to a minimum of 2 feet above final grade of the permanent infiltration BMP. Final grading of the permanent infiltration BMP shall occur only when all contributing drainage areas are fully stabilized. Any proposed permanent pretreatment BMP prior to the infiltration BMP should be fully constructed and used with the temporary sediment pond to help prevent dogging of the soils. See Element 13: Protect Low Impact Development BMPs for more information about protecting permanent infiltration BMPs. The pond shall be divided into two roughly equal volume cells by a permeable divider that will reduce turbulence while allowing movement of water between the cells. The divider shall be at least one-half the height of the riser, and at least one foot below the top of the riser. Wire - backed, 2- to 3-foot high, high strength geotextile fabric supported by treated 4"x4"s can be used as a divider. Alternatively, staked straw bales wrapped with geotextile fabric may be used. If the pond is more than 6 feet deep, a different divider design must be proposed. A riprap embankment is one acceptable method of separation for deeper ponds. Other designs that satisfy the intent of this provision are allowed as long as the divider is permeable, struc- turally sound, and designed to prevent erosion under and around the divider. The most common structural failure of sediment ponds 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 gran- ular soil with a head of sufficient magnitude to cause soil grains to lose contact and capability for support. The most critical construction practices to prevent piping are: o Tight connections between the riser and outlet pipe, and other pipe connections. • Adequate anchoring of the riser. • Proper soil compaction of the embankment and riser footing. • Proper construction of anti -seep devices. Sediment Pond Geometry To determine the sediment pond geometry, first calculate the design surface area (SA) of the pond, measured at the top of the riser pipe. Use the following equation: SA = 2 x Q2/0.00096 or 2080 square feet per cfs of inflow See BMP C240: Sediment Trap for more information on the above equation. The basic geometry of the pond can now be determined using the following design criteria: . Required surface area SA (from the equation above) at the top of the riser. . Minimum 3.5-foot depth from the top of the riser to the bottom of the pond. 2019 Stormwater Management Manual for Western Washington Volume 11 - Chapter 3 - Page 389