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

MAGNUSSON KLEMENCIC ASSOCIATES ■ ■ BMP 12.30— On-site Infiltration Areas and BMP 12.40— Raingardens. As explained above in Soils subsection of II, Site Assessment, the extremely low infiltration rates found on site make the use of infiltration facilities infeasible. Furthering that assessment, even if the recommended rate of 0.2 inches/hour was used to size a facility, the calculated drawdown rate would exceed the allowed 48- hour timeframe. ■ BMP 12.50— Downspout Dispersion Systems. There are no new roofs or downspouts to tie into, so this BMP is not applicable. ■ BMP 12.60—Concentrated Flow Dispersion and BMP 12.70— Sheet Flow Dispersion. Working with the existing topography of the site, the majority of the vegetation (proposed and existing) is uphill from the areas of new impervious surfaces. Other than the existing landscape areas to remain that are already depressed with underdrains,there are no new interior areas of vegetation downhill of significant runoff, of sufficient size, and without major utility facilities underneath, that would accommodate flow dispersion. Minimum Requirement #8: Wetlands Protection The project's stormwater will not discharge directly or indirectly to a wetland, so this requirement is met with no action required. IV. CONSTRUCTION STORMWATER POLLUTION PREVENTION PLAN There are two sub-requirements to be assessed under the Minimum Requirement#2: Construction Stormwater Pollution Prevention: ■ Department of Ecology (DOE) Construction Stormwater General Permit—apply for coverage if discharging stormwater either directly or indirectly to surface waters of the state ■ Construction Stormwater Pollution Prevention Plan (CSWPPP) —must be prepared if over 1 acre of land is disturbed CONSTRUCTION STORMWATER GENERAL PERMIT The North Site project will discharge stormwater to the existing public combined sewer and ultimately flow will be routed through the sewage treatment plant prior to being released to any surface waters. Thus,the project does not directly or indirectly discharge stormwater to the surface waters of the state, and so is not required to seek coverage under the DOE Construction Stormwater General Permit. This conclusion has been acceptable to COE reviewers for previous projects within the combined sewer area. LAND DISTURBING ACTIVITY Although discharging to the combined sewer system would typically render the project exempt from needing to address any of the Minimum Requirements, the Interim Policy requires that all projects within the combined sewer area to address all nine, including Minimum Requirement#2. The North Site Storm Drainage Report 6 Providence Regional Medical Center Everett North Site, Everett,Washington