9600 SHARON DR EXPLORER MIDDLE SCHOOL 2016-05-27

P engineering p is site slopes gradually downward towards the southeast corner of the site.The remaining lawn and landscaped area slopes northwest towards a grass amphitheater. No critical areas exist on the site. Existing soil conditions on site were investigated by GeoTest Services,Inc. and summarized in the Geotechnical Engineering Memo dated April 10,2014.A copy of this report is available under a separate cover upon request. Existing fill was encountered in all three exploration borings, underlying a thin layer of top soil. Below the fill material,the exploration borings encountered "medium dense to very dense gravelly,very silty sands and sandy silts will occasional cobbles". The native soils are interpreted as glacial till. Refer to Figure 2—Soils Map. The majority of the existing site is generally flat with slopes down to the south,northwest and northeast and a slight incline along the western perimeter.The site is divided into two threshold discharge areas(TDA),an eastern TDA and a western TDA.The gravel path along the west side of the site roughly demarcates the boundary of the two threshold discharge areas.Drainage courses from the two TDAs combine approximately 1,375 feet downstream of their respective discharge points. Refer to Figure 3—Existing Conditions for a layout of the existing site. For a detailed description of the existing site hydrology,refer to Pre-Developed Hydrology in Section VIII of this report. III. SITE DEVELOPMENT PLAN W/ ON-SITE STORMWATER MANAGEMENT Developed Conditions On-site stormwater management BMPs provided for this project include BMP 12.20 Soil Amendments, Quality and Depth,BMP 12.50 Downspout Dispersion Systems and BMP 12.70 Sheet Flow Dispersion. In addition,the effective impervious area of the site has been limited by removing existing impervious surfaces within the project limits. The western building addition will occupy areas of existing lawn, gravel road and paved play area. The loading area serving the western building addition will be converted from existing gravel to asphalt pavement.An existing asphalt path along the western edge of the building will be removed and converted to lawn.Much of the existing covered play and paved play area located southwest of the building will be removed and replaced with a concrete plaza and lawn.The eastern building addition will occupy an area currently used as an enclosed paved play area and lawn. One accessible parking stall and one"20-minute" stall will be located northeast of House 1, adjacent to the concrete walk. The gravel road along the western edge of the site will be removed and converted to lawn. Stormwater from the developed site will be discharged matching existing onsite drainage patterns with sheet flow discharge to the northwest and a piped discharge point connecting to an existing storm manhole near the northwest corner of House 3. LPD Engineering, PLLC Page 2 Mukilteo School District Teaching and Learning Center Stormwater Site Plan,July 22, 2014; Revised August 25, 2014