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1. PROJECT OVERVIEW <br /> This document is intended to provide engineering information necessary to support the <br /> construction plat application to the City of Everett for the proposed 6-lot short plat on these <br /> parcels. The site is located at 5125 &"5133-Seahurst Ave, Everett, WA 98203; refer to <br /> Figure 1 for a vicinity map. The gross site area is approximately 0.86 acres. The existing <br /> parcels are developed and contain existing houses, garage and parking on lots 1 & 2. All <br /> of the existing impervious surfaces on lots 1 & 2 will remain. The slope of the site is <br /> moderate and averages approximately 10 percent down to the east. Lots 1 & 2 will <br /> continue to take access off of Seahurst Ave, which borders the west property line. Lot 3 <br /> will take access off of 52nd St SW and lots 4-6 will take access off of 51st PI SW, which <br /> borders the eastern property line. Work within the right-of-way of Seahurst, 52nd and 51st <br /> for utilities and frontage improvements will be required. Frontage improvements will <br /> include curb and sidewalk on all existing roads with some pavement widening also <br /> required at the corner of 51st PI SW and 52nd St SW. <br /> This project proposes to create six new lots and several driveway cuts. The existing <br /> SFRs, garage and driveways on lots 1 & 2 will remain. For this permit the proposed <br /> improvements will require the construction of public frontage improvements, <br /> stormwater/infiltration facilities and other utilities. Roof runoff from the future houses will <br /> be collected and infiltrated on each individual lot. New driveways will be constructed with <br /> permeable pavement. Due to the fact that all the new roof and driveway surfaces will be <br /> 100% infiltrate on-site, these surfaces are considered non-effective (see Section 7 for <br /> more details). <br /> This project has two threshold discharge areas (Basin 'A' and Basin B'). Approximately <br /> 800 sf of sidewalk will be added to Basin 'A' and approximately 13,795 sf of future roof, <br /> driveway and sidewalk will be added to Basin 'B': <br /> Figures 2 & 3 are provided to show the existing and developed basins and runoff <br /> conditions for the plat. The runoff in the existing conditions spreads over the pervious <br /> surfaces on-site and drains toward the eastern property line. The runoff is then collected <br /> within the 51st PI SW right-of-way. The downstream system for this project is comprised of <br /> catch basins and underground pipes. <br /> The drainage design has been laid out per the DOE 2014 Stormwater Management <br /> Manual for Western Washington (SWMMWW), as well as Chapter 4 of the City of <br /> Everett's current Design and Construction Standards and Specifications for Development <br /> manual. Due to the size of this project Minimum requirements 1-9 apply to all of the new <br /> plus replaced impervious surfaces, as well as any converted pervious surfaces. Flow <br /> control will be met using on-site infiltration systems as noted above, refer to Section 9 of <br /> this report for detailed design information. Water quality will be met using the,permeable <br /> pavement based on the project Geotech's analysis of the cation exchange capacity of the <br /> existing soil. <br /> A geotechnical investigation was performed on this site by GeoDesign, Inc. The general <br /> soil conditions for the site were found to be glacial till. Explorations encountered silty sand <br /> with gravel material underlain by silty fine to coarse sand with gravel. See Appendix B for <br /> geotechnical evaluation. <br /> Seahttrst°SP 18-07278 3 <br /> September 2019 <br />