3003 W CASINO RD BOEING PARKING LOTS - E 6 2025-07-25

Drainage Report Boeing Everett-E-6 Parking Lot Expansion July 16,2013 5. SOURCE CONTROL PLAN A Source Control Plan was prepared and the standard City of Everett Source Control Activity Worksheet has been completed and included in APPENDIX G. Potential sources of pollutants in ' the stormwater are minimal on the site and only activity A7.12 (Maintenance of Stormwater Drainage and Treatment Facilities) was identified from the worksheet. Proper maintenance of catch basins is crucial to controlling sediment deposition in stormwater. Also, proper signage ' discouraging dumping of pollutants should be posted where practical on the project site. 6. SPECIAL REPORTS AND/OR STUDIES A geotechnical report and an offsite analysis were completed for this project. The 1991 Boeing Master Plan is also referenced within this report as the source of the flow control and water ' quality exemption. No other special reports or studies were conducted. See APPENDIX A for the geotechnical memorandum and Section 8.1 of this report for a summary of the offsite analysis. Correspondence confirming the flow control and water quality exemptions for the project can be found in APPENDIX F. 7. OTHER PERMITS No additional permits were applicable to this project. 8. PERMANENT STORMWATER CONTROL PLAN Total areas for impervious surfaces and pollution-generating impervious surfaces are shown in TABLE 1. Per the Stormwater Manual, the stormwater facility requirement thresholds are as follows: • On-site stormwater management BMPs shall be provided to infiltrate, retain and disperse stormwater runoff on-site to the maximum extent feasible without causing flooding or erosion impacts. o On-site stormwater management feasibility was evaluated and results of soils tests were not conducive to infiltration, and other LID BMPs are not feasible to site space constraints. • Flow Control: Projects in which the total of effective impervious surfaces is 5,000 square feet or more in a threshold discharge area. o Though the project falls within this threshold, flow control is not required for the site as the site is vested under the original 1991 Boeing Master Plan and is a tributary area to the Powder Mill Gulch detention/water quality basin. • Runoff Treatment (Water Quality): Projects in which the total of effective, new and/or replaced pollution-generating impervious surface (PGIS) is 5,000 square feet or more in a threshold discharge area of the project. o Though the project falls within this threshold, water quality treatment is not required for the site as the site is vested under the original 1991 Boeing Master Plan and is a tributary area to the Powder Mill Gulch detention/water quality basin. 1 20130715_Drainage Report Boeing Everett_Gate E-6 Parking Lot.doc Page 14