9600 SHARON DR EXPLORER MIDDLE SCHOOL 2016-05-27

P ! engineering p c where it is assumed to be collected within the golf course drainage system. Information from the City of Everett indicates that the drainage system for the golf course ultimately outfalls at a tributary to Swamp Creek. Eastern Threshold Discharge Area There are existing storm drainage systems located throughout the site that collect and convey surface runoff from the existing impervious and pervious surfaces to a storm drainage manhole located at the southeast corner of the project area. See Section VIII.B Predeveloped Hydrology for a detailed description of the onsite drainage systems to the point of discharge from the site. From the storm manhole near the northwest corner of House 3 (southeast corner of project area): 1. Stormwater flows south in a 12 inch pipe for approximately 205 feet to a manhole between the southeast corner of House 2 and the southwest corner of House 4. 2. Drainage flows from the manhole southwest through approximately 375 feet of 16-inch pipe to a storm manhole near the southwestern corner of the asphalt paved play area south of House 2. 3. From this storm manhole, site records show the system flowing southwest for approximately 160 feet to daylight at an outfall along the western property line,adjacent to the baseball field.Though the outfall was not found during the site visit,evidence of an overgrown ditch flowing south along the west property line was observed. Information from the City of Everett indicates that flows can be presumed to continue southwest,and ultimately outfall at a tributary to Swamp Creek. B. Pre-Developed Hydrology Refer to the offsite analysis Section VIII.A of this report for a detailed description of the existing downstream drainage conditions.In general, all runoff from the project area eventually discharges from the site via two main discharge points. One discharge point is via an existing 16-inch concrete pipe on the west side of the school property and the other discharge point is via sheet flow to the northwest. The two discharge points from the site are located within separate threshold discharge areas. Drainage tributary to the western TDA sheet flows from gravel and lawn areas to the northwest towards a grass amphitheater. Flow continues to sheet flow northwest, exiting the property. The existing drainage system tributary to the eastern TDA(within the project limits)is comprised of two pipe networks,which combine at a storm manhole west of House 3.The northern pipe network contains four catch basins and collects runoff from Aradia Ave and the drop-off loop,as well as downspout connections along the northern perimeter of House 1.The southern pipe network contains three catch basins and collects runoff from the paved areas south and east of the building as well as existing downspouts along the southern and eastern perimeter. The two pipe networks combine in the storm manhole at the northwest corner of House 3,which also collects drainage from nearby parking lots.The area tributary to the western TDA includes the majority of the gravel road and lawn areas on the west side of the project. LPD Engineering, PLLC Page 1 Mukilteo School District Teaching and Learning Center Stormwater Site Plan,July 22, 2014; Revised August 25, 2014