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t tflI 1 1.0 PROJECT DESCRIPTION I The proposed Snohomish County PUD No. 1 Operations Center Administration Building Addition project, is located at 1802 75th Street SW Everett, WA 98203 within Sections 4 and 5, Township 29, Range 5 East. The Operations Center is located on a 52.67 acre site with a I corresponding parcel number 00392200000300. Due to the security requirements of the site access is limited to a single driveway located on 75th Street SW. The proposed Administration Building Addition is located south west of the existing Administration Building and consists of I an approximately 16,000 square foot building footprint. Construction activities include disturbing approximately 2.2 acres for construction of the addition, reconfiguration of an existing parking lot,the creation of 53 additional parking stalls, landscaping and utility improvements. i , The Addition will be completed in two phases. The first phase referred to as Site Preparation includes the construction of a geothermal well field, demolition and stabilization of the I construction area by application of gravel base. Construction of the first phase will take approximately 2-3 months. During the interim after phase 1 is completed before phase 2 (Building Addition) is constructed a temporary sediment and erosion control plan(see Appendix I D)will be implemented to prevent sediment from leaving the site. This report addresses the proposed improvements by examining the pre-developed and post- developed stormwater runoff characteristics of the site, and providing mitigating measures to Icompensate for potential impacts resulting from the development. 2.0 DESIGN CRITERIA AND ASSUMPTIONS I2.1 Governing Guidelines The City of Everett (COE) implemented the 1997 Stormwater Management Manual (as updated I in 1999 and 200) for identifying stormwater management requirements for development inside city limits. This manual is written to be technically equivalent to the 1992 Washington State Department of Ecology stormwater management manual. The proposed water quality and I detention stormwater facilities for this project are vested in the 1997 Manual and have been designed to meet the requirements within the 1997 COE Stormwater Management Manual. I The project is divided into two on-site drainage basins. The northern drainage basin consists of an existing parking lot west of the existing Administration Building (see Appendix A). This parking lot will be re-developed and will create less than 5,000 square feet of new impervious I surfacing. In accordance with Table 1-1.1 of the City of Everett Stormwater Management Manual runoff treatment is proposed for this area. Source control will be applied for the entire project area. Treated runoff from the re-developed parking area will continue to utilize the 1 existing conveyance system directing flow to the existing Operation Center stormwater facilities located in the north east corner of the 52 acre site. '' DAVID Evnvs AND ASSOCIATES,INC OPERATIONS CENTER ADDITION JUNE,2010 ' STORMWATER SITE PLAN PAGE 1