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1 <br />1 <br />BMP C240: Sediment Trap <br />IPurpose <br />1 <br />L <br />1 <br />fl <br />1 <br />1 <br />1 <br />1 <br />1 <br />1 <br />1 <br />1 <br />1 <br />A sediment trap is a small temporary ponding area with a gravel outlet used to collect <br />and store sediment from sites cleared and/or graded during construction. Sediment <br />traps, along with other perimeter controls, shall be installed before any land disturbance <br />takes place in the drainage area. <br />Conditions of Use <br />Prior to leaving a construction site, stormwater runoff must pass through a sediment <br />pond or trap or other appropriate sediment removal best management practice. Non - <br />engineered sediment traps may be used on -site prior to an engineered sediment trap or <br />sediment pond to provide additional sediment removal capacity. <br />It is intended for use on sites where the tributary drainage area is less than 3 acres, with <br />no unusual drainage features, and a projected build -out time of six months or less. The <br />sediment trap is a temporary measure (with a design life of approximately 6 months) and <br />shall be maintained until the site area is permanently protected against erosion by veget- <br />ation and/or structures. <br />Sediment traps and ponds are only effective in removing sediment down to about the <br />medium silt size fraction. Runoff with sediment of finer grades (fine silt and clay) will <br />pass through untreated, emphasizing the need to control erosion to the maximum extent <br />fi rst. <br />Whenever possible, sediment -laden water shall be discharged into on -site, relatively <br />level, vegetated areas (see BMP C234: Vegetated Strip (p.375)). This is the only way to <br />effectively remove fine particles from runoff unless chemical treatment or filtration is <br />used. This can be particularly useful after initial treatment in a sediment trap or pond. <br />The areas of release must be evaluated on a site -by -site basis in order to determine <br />appropriate locations for and methods of releasing runoff. Vegetated wetlands shall not <br />be used for this purpose. Frequently, it may be possible to pump water from the col- <br />lection point at the downhill end of the site to an upslope vegetated area. Pumping shall <br />only augment the treatment system, not replace it, because of the possibility of pump fail- <br />ure or runoff volume in excess of pump capacity. <br />All projects that are constructing permanent facilities for runoff quantity control should <br />use the rough -graded or final -graded permanent facilities for traps and ponds. This <br />includes combined facilities and infiltration facilities. When permanent facilities are used <br />as temporary sedimentation facilities, the surface area requirement of a sediment trap or <br />pond must be met. If the surface area requirements are larger than the surface area of <br />the permanent facility, then the trap or pond shall be enlarged to comply with the surface <br />2014 Stormwater Management Manual for Western Washington <br />Volume 11 - Chapter 4 - Page 383 <br />