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' r 4 Stormwater Technical MemorandumptAc),- Perteet 2707 Colby Avenue,Suite 900,Everett,WA 98201 I P 425.252.7700 I F 425339.6018 Ekt To: Kelli Smith,AIA, Dykeman Architects �SO' From: Erik Emerson,PE . 4. Date: April 21,2015 Arry . 4► 84 Re: New Portable Classrooms at Silver Lake Elementary School re • ; ' Stormwater Impacts and Mitigation Measures /0NAL The Everett School District proposes to place six new portable classrooms and one new portable restroom within an existing playfield at Silver Lake Elementary. The portables will be accessed by a raised walkway through which rainfall and sunlight will be able to penetrate. Other improvements will be limited to utility connections to the temporary structures and minor clearing of the footprint of each temporary structure. The new portables will create approximately 5868 square feet of new impervious area. As such, stormwater impacts of the project must be addressed. This memorandum documents the project's compliance with the City of Everett's stormwater standards as defined in the City's 2010 Stormwater Management Manual (Stormwater Manual). Previous Studies A 2003 report (Targeted Drainage Report for Silver Lake Elementary Parking Lot Expansion, Perteet, 2003) examined the drainage patterns and facilities within the entire Silver Lake Elementary site(see attached). The associated project constructed a new parking lot in the northwest corner of the site and other paved areas within the site. The increase in runoff was mitigated by construction of stormwater facilities under the new parking lot. A subsequent modernization project was constructed in 2007 and included removal of some structures and additional building construction. A Schematic Design Narrative (Pace, 2006)documented that the project caused a net reduction in impervious area and that new pollution generating areas were routed to an existing bioswale located north of where the current project proposed to install new portables. Both reports are attached to this memorandum. The 2003 report contains an offsite analysis and basin maps which are still applicable to the site. Design Approach Construction of the new portables will add a small amount of new impervious area, but will not add any new pollution generating surfaces. Therefore,water quality mitigation is not required. The new portable structures will be installed on an existing grassed play field which will remain as a grassed area. Immediately below each portable structure the manufacturer requires that topsoil be removed and replaced with a layer of gravel, matching the structure footprint.To mitigate the impacts of adding additional impervious surface to the site the roof drains will be routed to the gravel zone and allowed to infiltrate in much the same manner as does in the current condition. Water will be stored in the gravel zone below the structure and allowed to slowly infiltrate over time. This approach follows the intent of the low impact development methodology by retaining stormwater at the point of generation and providing an opportunity to infiltrate rather than collecting and routing to a single facility. The gravel depth will be increased to a nine inch minimum depth which will allow full infiltration in all storms up to the 100 year recurrence(see attached WWHM calculations.) The gravel layer will be specified as gravel backfill for drains to ensure that sufficient void space is available to store the runoff until it infiltrates.The following assumptions have been made in the design of this facility: Page 1 File location:x:\Everett Public Schools\Projects\20150040-Silver Lake Elementary Portables\Design\Drainage\Silver Lake Elementary Drainage Memorandum-Everettdocx