3214 BROADWAY BROADWAY KINECT APARTMENTS 2021-08-11

• • CITY OF EVERETT STORMWATER MANAGEMENT MANUAL The 10-year recurrence interval peak flow shall be used if the project size,expected timing and i duration of construction, or downstream conditions warrant a higher level of protection. For sediment traps having a contributing area with a time of concentration less than or equal to 60 minutes,the rational method may be used to determine the design flow. Otherwise, a continuous 411, runoff model(WWHM or an approved alternative, see Volume I, Chapter 5) shall be used to • determine the design flow. The developed(unmitigated) land use condition shall be used in the hydrologic analysis. IR VS =The settling velocity of the soil particle of interest. The 0.02 mm(medium silt)particle • with an assumed density of 2.65 g/cm3 has been selected as the particle of interest and ID has a settling velocity(Vs)of 0.00096 ft/sec. • FS=A safety factor of 2 to account for non-ideal settling. - The equation for computing surface area becomes: fb SA =2 x Q/0.00096 This is equal to 2,083 square feet per cubic feet per second of inflow,measured at the invert of • the weir. . Note: Even if permanent facilities are used,they must still have a surface area that is at least as • large as that derived from the above formula. If they do not,the pond must be enlarged or additional surface area must be provided in a temporary sediment control facility. • 4. To aid in determining sediment depth,all sediment traps shall have a staff gauge with a • prominent labeled mark each 1-foot interval above the bottom of the trap. 5 5. Sediment traps may not be feasible on utility projects due to the limited work space or the short- • term nature of the work. Portable tanks may be used in place of sediment traps for utility • projects. Maintenance Standards • I. Sediment shall be removed from the trap when it reaches 1-foot in depth 2. Any damage to the pond embankments or slopes shall be repaired. 3.3.18 BMP C241: Temporary Sediment Pond • Purpose • Sediment ponds remove sediment from runoff originating from disturbed areas of the site. Sediment S ponds are typically designed to remove sediment no smaller than medium silt(0.02 mm). Consequently, • they usually reduce turbidity only slightly. It Conditions of Use - • Prior to leaving a construction site, stormwater runoff must pass through a sediment pond or other •, appropriate sediment removal BMP. • 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 Installation Specifications • 1. Sediment ponds shall not be installed on sites where failure of the structure would result in loss of • life, damage to homes or buildings, or interruption of use or service of public roads or utilities. S Volume 11— Construction Storrawater Pollution Prevention • 3-61 a