9600 SHARON DR EXPLORER MIDDLE SCHOOL 2016-05-27

P engineering p lc C. Developed Hydrology Drainage patterns in the developed condition will match existing drainage patterns,maintaining two separate discharge points, one that connects to the existing onsite storm network and another sheet flowing northwest from the site. Runoff from the proposed loading and dumpster area will sheet flow northwest and disperse into the adjacent lawn area.Downspouts for the west half of the western building addition will be dispersed via splash blocks into the adjacent lawn area. A proposed 6-inch storm line will convey runoff from an area drain at a low point in the northern lawn area and a downspout connection from the western addition.The line will flow northeast to connect to the existing northern pipe network described above which will remain. The majority of the southern pipe network will be removed and rerouted around the proposed east addition. Downspouts along the southwest edge of the existing building, southeast edge of the western addition and southwest edge of the east addition will flow to this system.Area drains collecting runoff south of the proposed patio will also be located in-line with this re-routed system.The storm line will flow northeast of the proposed eastern addition to connect to the existing southern pipe network which will remain. The southern pipe network flows east to a point of discharge at a storm manhole located east of House 3. Refer to sheet C2.O1 Grading&Drainage,in Appendix A for additional information. D. Stormwater Flow Control Plan Due to the presence of two threshold dischargeareas onsite,the project has less than 5,000 SF of net new impervious surface in each threshold discharge area,and therefore flow control facilities are not required per table 2.2 of section 2.2.7 of the 2010 City of Everett Stormwater Management Manual. The replacement of the roof of the existing building is considered a replaced impervious surface since it will not alter the existing distribution of runoff from roofs and internal demolition of the existing building will be minimal. Refer to Figure 3-Existing Conditions,Figure 5-Proposed Conditions, and Table 1,Table 2,and Table 3 below for additional information. Table 1-Effective Pre-developed Areas '" E v Pie !,velo ed e Nz x . aree� ►c e1 Eastern TDA Existing Impervious Area 30,374 0.70 Western TDA Existing Impervious Area 1,857 .04 Total Area 60,486 1.39 LPD Engineering, PLC Page 8 Mukilteo School District Teaching and Learning Center Stormwater Site Plan,July 22, 2014; Revised August 25, 2014