2514 RAINIER AVE 2022-03-01 - Laserfiche WebLink

NORTH MIDDLE SCHOOL 11. SITE ASSESSMENT isSee Figures 2a and 2b — Existing Conditions for the current contour elevations, surface features, and utility infrastructure. TOPOGRAPHY Generally, the main site slopes from the northeast comer to the southwest corner and flattens out in the southwest corner of the site. The field to the north is almost flat and is currently underdrained, but the walks and areas surrounding the site still slope from the northeast to the southwest. The slopes on site range from 0%-6%. DRAINAGE Existing stormwater runoff from the site is either collected via building/canopy downspouts or sheet flows from paved areas of the site into a series of catch basins and conveyed to an existing 10" and 12" storm main located in the middle of the site between both existing school buildings before being conveyed to the public combined sewer in 26th Street. The stormwater continues to flow in a series of public combined sewer pipes south and southeast of the site and eventually discharges to the Snohomish River approximately 2 miles downstream of the project site. There are no on -site or adjacent surface water bodies within the North Middle School property. SOILS The site is predominantly classified as Map Unit "Alderwood-Urban Land complex, 2-8 percent slopes". See the Geotechnical report for more regarding these soils. According to Volume III, Chapter 12, of the COESWMM, the feasibility and sizing of infiltrating stormwater facilities with an Urban Land Map Unity should not be determined based on typical soil classification properties through the steps outlines in Chapter 12. Geotechnical recommendations for the specific site are instead required in order to determine feasibility. . As noted in VI, Special Reports and/or Studies, a geotechnical report has been provided for reference. See Appendix F. The current report summarizes the subsurface soil conditions in the area of the North Middle School project, which are expected to consist of loose fill with scattered organics over sandy silt in the field and fill over fine sandy silt over gravelly silty sand near the buildings. No groundwater was encountered in any of the borings, but at the drainage sand -soil subgrade in Bagshaw Field perched groundwater was encountered and appeared to be caused by the athletic field irrigation system. Based on results of subsurface exploration and lab testing, the geotechnical engineers have come to the conclusion that infiltration appears possible in the underlying Advance Outwash soils, depending on the planned location and depth of any proposed infiltration facilities. Infiltration would be less favorable in areas with fill. Due to the depth of the soils that have infiltration potential (10 ft to 17 ft in depth), it has been deemed infeasible to use infiltration facilities to provide flow control for this site. This site is proposing to install a synthetic turf field. Although artificial turf is considered a pollution -generating surface, the project geotechnical engineer has issued a letter stating that the recessional lacustrine silt in the field is a confining layer that will not infiltrate surface stormwater (See this Infiltration Infeasibility Letter at the end of the Geotechnical Report in Appendix F). GROUND COVER The existing site is covered by a variety of materials. Existing impervious areas include the three buildings onsite: west and south classroom buildings and the gymnasium, an asphalt parking lot, various concrete and asphalt walks, and an asphalt play area in the northwest comer of the site. Existing pervious areas consisting of primarily landscaped areas with a variety of groundcover and trees are also onsite. • Existing pervious and impervious surfaces are discussed in Section Vill, Permanent Stormwater Control Plan. COUG HL,INPORTERLUNDEEN 3TRii4 ,;RAi, C: V!1., SE: 5",..' ctite i Nl ERjNC