5901 23RD DR W SEAWAY CORP 3C 2025-07-29

I Stormwater Management Report Everett, WA IDesign Criteria: The proposed Stormwater Management System is designed per the City of Everett IStormwater Management Manual, January 1997, which complies with the 1992 Department of Ecology Stormwater Manual, for Washington State. Water quality will be I addressed in compliance to the City of Everett's Stormwater Manual via dead storage in the detention pond. i Table 1 Jurisdictional Requirements I Peak Run-off Control: 2-year: Reduce to predeveloped peak 10-year: Match predeveloped Ii 00-year: Match predeveloped Water Quality: Treat the 6-month, 24 hour design I storm Downstream Analysis: '/a Mile downstream IProposed Drainage System: Project proposes to introduce a network of underground pipe, catch basins, curbs and I gutter, to collect surface water runoff throughout the site and direct it to existing and proposed stormwater management facilities off site. Due to the topography of the site, the stormwater runoff generated is split into two separate basins. An existing stormwater management facility has been constructed off-site to treat and detain runoff generated by 3.15 acres of the subject property that drains to the East Fork of Powdermill Gulch Creek. The runoff generated by the remainder of the site will be routed to a detention pond in the I northern portion of the Northshore Christian Church (NCC) property. This wet pond will also be sized to detain and treat stormwater runoff from the 4.0 acres of Lot 3D in the NCC property that drains to Narbeck Creek. After detention, the stormwater will tie into the I existing drainage system that connects to the previously designed and constructed outfall into Narbeck Creek. III Conclusion: The proposed stormwater management system for this project has been designed in accordance with regulatory criteria described above and consistent with sound engineering I practice. This design has incorporated stormwater detention and stormwater quality BMP's, etc. Therefore, no significant adverse impacts to the stormwater management system are expected as a result of the proposed development. IThe site is split into two basins and the Powdermill Gulch Creek basin area has been accounted for in the design of the existing stormwater pond to the north by Trepanier IEngineering. Due to this division, the treatment and detention calculations in this report will only be performed for the Narbeck Creek basin. Conveyance will be analyzed for the runoff routed to the Powdermill Gulch Creek basin, but no treatment or detention is 1 necessary on-site since it drains to an existing detention/water quality pond. 1 PACLAND Job# 50108001 Page 3