405 RIVERSIDE RD 2023-01-30 - Laserfiche WebLink

BMP C241: Temporary Sediment Pond Purpose Sediment ponds remove sediment from runoff originating from disturbed I I I I I I I I I I I I I areas of the site. Sediment ponds are typically designed to remove sediment no smaller than medium silt(0.02 mm). Consequently, they usually reduce turbidity only slightly. Conditions of Use Prior to leaving a construction site, stormwater runoff must pass through a sediment pond or other appropriate sediment removal best management practice. A sediment pond shall be used where the contributing drainage area is 3 acres or more. Ponds must be used in conjunction with erosion control practices to reduce the amount of sediment flowing into the basin. Design and • Sediment basins must be installed only on sites where failure of the Installation structure would not result in loss of life, damage to homes or Specifications buildings, or interruption of use or service of public roads or utilities. Also, sediment traps and ponds are attractive to children and can be very dangerous. Compliance with local ordinances regarding health and safety must be addressed. If fencing of the pond is required,the type of fence and its location shall be shown on the ESC plan. • Structures having a maximum storage capacity at the top of the dam of 10 acre-ft(435,600 ft3) or more are subject to the Washington Dam Safety Regulations (Chapter 173-175 WAC). • See Figure 4.24, Figure 4.25, and Figure 4.26 for details. • If permanent runoff control facilities are part of the project, they should be used for sediment retention. The surface area requirements of the sediment basin must be met. This may require enlarging the permanent basin to comply with the surface area requirements. If a permanent control structure is used, it may be advisable to partially restrict the lower orifice with gravel to increase residence time while still allowing dewatering of the basin. • Use of infiltration facilities for sedimentation basins during construction tends to clog the soils and reduce their capacity to infiltrate. If infiltration facilities are to be used, the sides and bottom of the facility must only be rough excavated to a minimum of 2 feet above final grade. Final grading of the infiltration facility shall occur only when all contributing drainage areas are fully stabilized. The infiltration pretreatment facility should be fully constructed and used with the sedimentation basin to help prevent clogging. I • Determining Pond Geometry Obtain the discharge from the hydrologic calculations of the peak flow for the 2-year runoff event(Q2). The 10-year peak flow shall be used if the project size, expected timing and duration of construction, or downstream conditions warrant a higher level of protection. If no hydrologic analysis is required, the Rational Method may be used. February 2005 Volume II— Construction Stormwater Pollution Prevention 4-105