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CIVIL WORK SUMMARY.pdfDoug's Lynnwood Mazda Preliminary Design Narrative Civil Work Summary Introduction 12.15.16 The Project The Doug's Lynnwood Mazda project is located at 22130 Hwy 99 in the City of Edmonds, Washington, at the site of the existing Mazda Dealership. The site, including the existing dealership and associated surface parking, includes eight parcels in total; though, site improvements will not occur on all eight parcels. Construction will occur in phases. This phase, Phase 1, will leave the existing Mazda sales and service building untouched to the extent feasible and construct a new sales and service building, including re -configured paved parking and vehicle display areas. Future site improvements have not been shown in the plans. They are anticipated to provide site improvements to western portions of the site adjacent to 76th Avenue W that are not currently shown to be improved. The total parcel area anticipated to have construction on them for Phase 1 is 2.534 acres, while the area of Phase 1 disturbance is approximately 1.289 acres. Site improvements will require utility upgrades including new fire, domestic water, and sewer services. Also, stormwater management facilities that comply with Edmonds most current stormwater code. More discussion of the site and related utility improvements are contained in the sections below. The site is bound by Highway 99 to the east, 223rd Street SW and a row of multi -family housing units to the south, 76th Avenue W to the west, and single-family residential housing and a Honda dealership (Lynwood Honda) to the north. It is east of the old Esperance School site, currently owned by Snohomish County Parks. The current site is developed with multiple automotive use buildings, including the existing Lynnwood Mazda sales and service building, surface (display) parking, and associated utilities. Records show that approval for the existing Lynwood Mazda building was granted in 1988, The assumption is that construction and site occupancy immediately followed. Record plans show stormwater pipes including flow control facilities that would have been designed to comply with earlier versions of Edmond's code for portions of the existing site. Topography The topography is mildly to moderately sloping from the west-northwest side of the site down to the southeast corner of the site where it abuts Highway 99. The highest elevation is approximately 379 feet near the connection with 761h Avenue W and the lowest elevation is approximately 346 feet near the connection with Highway 99 in the southeast corner of the site, a drop of 33 feet across the site. soils A draft soils report has been prepared by Golder and Associates, dated August 7, 2015, to document portions of the site that are planned to be impacted during construction. Golder completed nine borings across the site to provide preliminary construction recommendations, characterize soil stratigraphy, soil permeability, and groundwater conditions. From this, they concluded that the site is vastly overlain by impermeable till soil; however, a layer of permeable outwash sand located approximately 15 feet to 19 feet below grade was found south of the existing Mazda sales and service building. However deep, the proposed stormwater mitigation 1 of 4 12.15.16 plan is to utilize limited infiltration in the outwash soil to minimize detention volumes and comply with the new stormwater management regulations. No other stormwater BMPs located near the surface are anticipated to be feasible based on the findings due to the presence of low permeability glacial till at shallow depths. Water Distribution The water purveyor for the property is the City of Edmonds. A water availability certificate will be obtained to verify new service installations. Domestic Service An existing domestic water meter was observed, fed from Highway 99, that will be re -used or replaced as part of construction. Domestic water service will be required for the new sales and service building. Fire Service The new building will be sprinklered and require a fire service connection. A new fire service connection is anticipated to be served from the water main located on the west side of Highway 99. Sanitary_r r System The sewer purveyor for the property is the City of Edmonds. A sewer availability certificate will be obtained to verify new service installations. A new side sewer connection is anticipated to connect with the sewer main located on the west side of Highway 99. The new sales and service building will contain restrooms for occupants and auto service and detailing bays which will be drained to the sewer. Areas that might have incidental oil and grease or detergents getting in the drain will be equipped with necessary pre-treatment including oil/water separators, at a minimum. Stormwater MggggeMent. The developed stormwater management system will comply with the City of Edmonds code, anticipated to be updated in January of 2017. We anticipate the City will adopt the 2014 Amendments to the 2012 Stormwater Management Manual for Western Washington published by the Washington State Department of Ecology. The project will trigger large project drainage review, requiring a stormwater management design that addresses Minimum Requirements 1-9. Surface water will be collected from the project area in catch basins for stormwater from parking and landscape surfaces and roof leaders from the building areas. Stormwater will be conveyed in new stormwater piping to a centralized water quality treatment system. Then, flow into a combined infiltration/detention vault. Ultimately, the detention system will overflow to the public storm main located on the west side of Highway 99. A small area, the fire lane, is anticipated to be too low to be connected to the new stormwater facilities. Stormwater from this area is 2 of 4 12.15.16 anticipated to be treated as bypass for flow control and will have a stand-alone water quality treatment facility to treat stormwater prior to discharging from the site. The stormwater system associated with the existing Mazda sales and service building will be remain undisturbed to the extent feasible. Additionally, the project does not propose any improvements to the WSDOT right-of-way, SR 99; therefore, no stormwater management is anticipated in the right-of-way. Flow Control Facility Flow control will be provided by a combined infiltration/detention vault (vault). The vault will be strategically located where infiltration feasibility testing determined was optimal. Preliminary soils testing indicated a deep layer of outwash sand underlying a portion of the site, anticipated to be approximately 19 feet to 20 feet deep. For preliminary sizing, a 0.5 inch per hour infiltration rate has been assumed. Based on this, limited infiltration for the site is anticipated. A flow control structure will aid the limited infiltration by providing a fluctuating live storage volume in the top of the vault to guarantee adequate flow control has been provided. The vault has been sized, preliminarily, for future phases of construction. Although, the future improvements have not been identified the future site area and percent impervious can be guidelines for future planning. The flow control facility has preliminarily been sized for a future site area of 2.95 acres with 90% imperious cover. Stormwater stubs, one for stormwater requiring mitigation and one bypass line, will be provided to the future work area so that a connection can be made with minimal disturbance to the occupied site. Water Quality The site contains different surfaces, some are pollution generating and others are not. Pollution generating surfaces consist primarily of surfaces that have vehicle traffic. Heavy metals, dirt, and grease are categorized as total suspended solids (TSS) and are the primary pollutants of concern. Vehicle service areas will be located inside buildings and have containment for potential spills to prevent contaminating stormwater. Interior drains will be routed to the building's sewer connections. Runoff from the pollution generating surfaces will be treated to the Enhanced Water Quality standard, typically required for commercial projects. To provide Enhanced Water Quality system the site will provide proprietary filters. A modular wetland filter with GULD approval for Enhanced Water Quality is currently shown for runoff tributary to the vault. A Filterra unit, also with GULD approval for Enhanced Water Quality, will be a stand-alone filter for the fire lane located on the south portion of the site, too low to drain to the vault. It should be noted that for runoff draining to the vault if more than 91% of the volume of runoff infiltrates into the ground then the level of water quality treatment preceding the vault can be reduced to Basic Water Quality. The combined Basic Water Quality treatment followed by infiltration meets the Enhanced Water Quality requirements. The stormwater infiltration volume 3 of 4 12.15.16 and cost analysis will be performed to finalize the most efficient and effective stormwater management facilities. WilIOZOTISIM89UIFFAM 4 of 4