1130 90TH ST SW 2025-07-29 - Laserfiche WebLink

1. PROJECT OVERVIEW This document is intended to provide engineering information necessary to support the 4 condo building permit applications to the City of Everett for the 4 new condos and 4 driveways proposed on these units. The site is located at 1130 90th Street SW, Everett, WA 98204, refer to Figure 1 for a vicinity map. The site area is approximately 0.39 acres. The existing site consists of pasture/landscaping. The slope of the site is flat with a slight slope to the west that does not exceed 2 percent. The new condos will be evenly spaced. The condos on units 1 and 2 will be and set back approximately 20-feet from the northern property line and their driveways will take access from the existing road to the north. The condos on units 3 and 4 will be set back approximately 20-feet from the western property line and their driveways will take access from the existing road to the west. Each unit of this parcel proposes to construct less than 2,500 sf of new impervious surfaces per unit. Since this parcel is less than 1 acre in size per the COE 2010 stormwater manual the existing conditions will be modeled as they were pre 1997, which was landscaped with a paved/gravel driveway and at two on-site buildings, refer to Appendix A for aerial photographs of pre 1997 conditions. Therefore, minimum requirements 1-5 apply to all of the new impervious surfaces for this project along with all the disturbed pervious surfaces. This proposal does not meet any of the land-use criteria that require specific source control for the final use and a SWPPP has been prepared to address the runoff from the site during construction. Refer to Sections 4 and 5 for a detailed discussion. Figures 2 & 3 are provided to show the existing and developed basins and runoff conditions for all 4 units. The runoff in the existing conditions will spread over the pervious surfaces of the 4 units as sheet flow and drain to the west. In the developed conditions all of the runoff from the new roofs will be directed to downspout slash blocks, which will disperse the runoff toward on-site swales. Runoff from the new driveway surfaces will be dispersed by use of driveway transition zones. The general soil conditions for this property as mapped by the NCSC web-soil survey were found to be Alderwood — Urban Land Complex type soils. The hard pan for the Alderwood soil ranges between 20 and 40 inches below grade. Therefore, dispersion will be used for the PGIS on this project. The drainage design has been laid out per the COE 2010 Stormwater Management Manual. The downstream system for this project is comprised of on-site swales, yard drains, catch basins, pipes/culverts and a detention pond. The collecting water body is the detention pond located at the end (west side) of the cul-de-sac on 11th Place W, less than 500-feet south from the building site. During the site visit there were no signs of flooding or capacity issues within the observed downstream systems. Due to the size of this project only Minimum requirements 1-5 apply to the proposed drainage design'as noted above. Included on the following page is a copy of the MR flow chart out of the COE Drainage Manual showing how the applicable minimum requirements were determined. OLYMPIC TERRACE CONDOS 13-0605 June 2013 Page 3