3003 W CASINO RD BOEING STORMWATER MGT SYSTEM 2023-12-28

Kennedy Jenks Technical Memorandum Kyle Bringedahl, The Boeing Company 13 April 2021 KJ 2096009.00 Page 2 spillway to the detention basin. The detention basin provides additional retention and settling before the water is discharged to Power Mill Creek. The primary outlet from the detention basin is a riser with an outlet orifice at 8-feet of basin depth. This maintains depth in the basin to prevent discharges of any spilled floatables that may enter the basin. Two manually operated sluice gates are located below the 8-foot mark on the riser for maintenance use; these gates are closed during normal operations. Flow from the riser is conveyed by pipe to a concrete stilling basin for energy dissipation before discharging to Powder Mill Creek. Flow out of the detention basin is controlled by an actuated gate to maintain a maximum discharge limit of 49 cfs to the stilling basin. Storm events greater than the 25-year, 24-hour storm enter the stilling basin via the detention basin spillway. 1.3 Proposed Stormwater System The proposed stormwater retrofit will increase treatment efficiency through infrastructure and changes in operation. The scope includes: • Replacement of the existing peat filter with an enhanced media filter • Retrofitting the existing gravity-based conveyance system to route flows from the detention basin and sedimentation basin to the proposed enhanced media filter. • Installation of an above-ground stormwater lift station for improved treatment efficiency • Associated site improvements including temporary erosion and sediment controls, electrical upgrades, actuated valves, grading, slope stabilization, and resurfacing. As in the existing system, Facility runoff will enter the sedimentation basin for settling. Up to 8 cfs will exit the sedimentation basin via the existing outlet pipe, and excess flows will enter the detention basin. The existing sedimentation basin outlet pipe will tie into the proposed storm drain system to convey flows to the enhanced media filter. The outflow pipe from the existing detention basin riser will tie into the proposed storm drain system downstream of the detention basin. Detention basin flows will be conveyed into the proposed gravity-based conveyance system and comingle with flows from the sedimentation basin. The proposed conveyance system will convey up to 24 cfs to the enhanced media filter and bypass the remaining flows to the stilling basin. The enhanced media filter discharges a maximum of 8 cfs to the existing wetland via orifice-controlled outlet structures. The remainder of the treated water will gravity drain to the stilling basin. 1\kjc.locallkjc-rootlkj-projectslfederalwayl202012096009.00 boeing everett sw treatment109-reports19.08-reports-tmslcity of everelt design tmlboeing everett sw design tech memo.docx ©Kennedy/Jenks Consultants, Inc.