1815 13TH ST SEATTLE CHILDRENS NORTH CLINIC 2019-07-08

MAGNUSSON KLEMENCIC ASSOCIATES%_. Both halves of the existing site direct runoff to on-site conveyance systems through area drains, catch basins, perforated underdrains, and roof drains. There are a number of existing landscape depressions that help collect overland flow within the landscape areas and convey it through perforated underdrains underneath, while runoff from the landscape buffer specifically is collected through the underdrain systems at the foothill of each slope. The western half of the existing site,which includes the CUP building and the southwest parking lot, connects its on-site conveyance system directly into the on-site combined sewer system, before ultimately tying into the existing public combined sewer system in 12th Street. No detention or flow control was required for the construction of the CUP project because at the time of construction the impervious area was reduced. The eastern half of the existing site,which includes the temporary parking and surrounding landscape, directs its conveyance system to a two-cell detention pond at the northeast corner of the site. The pond detains the runoff collected in the on-site conveyance system and a flow control structure on the south side of the eastern cell, then discharges flow into the existing public combined sewer system in 12th Street. Construction of the CUP Parking project triggered the need for detention and flow control, hence the existing infrastructure supporting the drainage for this side of the site. The existing public combined sewer system comes down Rockefeller Avenue, turns east along the north edge of the site, and then turns south at Oakes Avenue where it cuts through the site property. Where Oakes Avenue and 12th Street would intersect (if the streets were extended),the sewer turns east again and heads back off of the site as it continues down 12th Street. Both the western and eastern halves of the existing site discharge to the public combined sewer system along the stretch that it is within the site property. There are no on-site or adjacent surface waters, critical areas, flood plains, or Shoreline Management boundaries within the North Site. SOILS National Resource Conservation Service (NRCS) soil information for the North Site project location is shown on the Soil Map included as Figure 3. The site is predominantly classified as Map Unit"Urban land". Immediately surrounding the site is soil classified as Map Unit"Alderwood-Urban land complex, 2 to 8 percent slopes". There is a small amount of adjacent soil to the southeast of the site that is classified as Map Unit"Bellingham silty clay loam". According to Volume III, Chapter 12, of the COESWMM,the feasibility and sizing of infiltrating stormwater facilities with an Urban land Map Unit should not be determined based on typical soil classification properties through the steps outlined 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, two geotechnical reports have been provided for reference. See Appendices B.1 and B.2. The current (2016) report summarizes the subsurface soil conditions in the area of the North Site project,which are expected to consist of fill or organic soils overlying glacially consolidated soils. The fill was encountered 3 to 5 feet below existing grade and consists of very loose to medium density silty sand with varying gravel and organics. At only a few borings, a thin layer of organic soil was observed approximately 3 to 6 feet below existing grade. The glacially consolidated soils were encountered 0 to 5 feet below existing grade and extended to the Storm Drainage Report 2 Providence Regional Medical Center Everett North Site, Everett, Washington