7805 40TH AVE W 2018 Plans 2022-11-30 (2)

•/b Design Build Sterviceso Inc. Engineering &ConstructionEfianagernent ovNag 060 7P:Aal To:City of Everett Public Works; Erik Emerson, PE From: CC: Date: July 20,2021 SUBJECT: Drainage Revision Justification at 7805 40th Ave.W, Permit No.PW1803-007 The attached redline markup of drawing C3.0 shows the proposed drainage revision for the project at 7805 40th Ave.W,under active permit No. PW1803-007.The proposed revision moves catch basin(CB) #7 from behind the new sidewalk to the inside of the curb line and changes to an open grate instead of a solid locking lid. This memo provides(a)a description of the factors driving the need for this revision and(b)an explanation and supporting calculations showing that this revision meets the intent of the City of Everett code requirements. Purpose for the Revision The basis of this proposed revision are on site observations of the actual flow path of the existing street drainage. Field observations by the owner during rainfall events over the recent rainy season have revealed that it appears that the flow path assumptions used as the basis for the stormwater flow control analysis presented in the final Stormwater Site Plan(SSP),signed and dated 12/03/2018,are not consistent with what is being observed on-site. Figure 4 from the SSP,and copied below as Figure 1,presents the upstream analysis,showing 2.03 acres of upstream stormwater flow that drains through the site.The SSP states that the drainage system is designed such that this area bypasses the proposed detention system. Drawing C3.0 shows that this is intended to be achieved via catch basin(CB)#6,which collects the upstream flow and sends it through the storm drain pipe that discharges into the stream at the east of the site. Per the SSP Section D,the detention system is sized to convey and treat stormwater flow coming from the frontage improvements in addition to the on-site roof and road areas. Included in the detention analysis is an area of 0.03 acres, or 1.588SF,from the existing road along 40th Ave W,which is also taken through the detention system, see Figure 2 below. Drawing C3.0 shows that CB#2 is positioned to collect the stormwater at the frontage and stormwater from the existing road,conveying the water through the detention system before it empties out into the stream.