3003 W CASINO RD BLDG 45-61 2019-07-15

Jane Zimmerman, Sr. Engineer City of Everett Public Works Department phone:425-257-8885 fax: 425-257-8882 e-mail:Zimmerman@everettwa.gov From: Tyler, David D [mailto:david.d.tyler@aboeing.com] Sent: Wednesday, April 06, 2016 11:16 AM To: Jane Zimmerman Cc: Gomez, Martin H; Page, Gayle M Subject: FW: Preliminary Stormwater Design for 45-61 Building Importance: High Jane, I'm sending some preliminary stormwater design drawings I was hoping you could look over before we proceed to final design and permit application. This project received SEPA approval last month (SEPA 16-004). The first two buildings have been reviewed and approved for permit, but this one triggered stormwater controls,so it has taken us a little longer to design. Would you mind taking a look at the attached concept and providing some feedback as to whether this will be an acceptable approach? I have included some narrative info from our engineer below to provide additional context and background. Thanks and best regards, Dave Tyler, Boeing Everett Site Services/Facilities 425-239-7983 Section 2.2.7 of the City of Everett Stormwater Management Manual (Manual) requires flow control mitigation if any of the following are exceeded: 1. Projects in which the total of effective impervious surfaces is 5,000 square feet or more in a threshold discharge area, or 2. Projects that convert three-fourths of an acre or more of native vegetation to lawn or landscape, or convert 2.5 acres or more of native vegetation to pasture in a threshold discharge area, and from which there is a surface discharge in a natural or man-made conveyance system from the site, or 3. Projects that through a combination of effective impervious surfaces and converted pervious surfaces cause a 0.1 cubic feet per second increase in the 100-year flow frequency from a threshold discharge area. Due to Site limitations(small footprint needed, minimal vertical drop)and the high cost compared to the size of the project,we would like to pursue an innovative option utilizing supplemental guidance found in the Manual. In the Supplemental Information portion of Section 2.2.7 of the Manual, it states"Application of sufficient Low Impact Development techniques and On-Site Stormwater Management BMPs can result in reducing the effective impervious area and the converted pervious areas,thereby reducing or eliminating the need for flow control facilities..."With this in mind,we designed Option 4a to drop the increase of the 100 yr peak flow rate to less than 0.1cfs,which would eliminate 4/s