10521 19TH AVE SE 2022-05-23 - Laserfiche WebLink

Silver Lake Plaza Upstream Analysis Runoff from 19th Avenue SE is intercepted by existing public storm drainage systems along the existing street frontage and road runoff does not appear to enter the site. Approximately 0.16 acres of adjacent lawn and landscaped areas to the east of the site appear to sheet flow toward the site. Portions of single family residence roof area may also flow in the same direction, but the location of adjacent residence splash blocks was not determined. No erosion or concentrated flows were observed from adjacent runoff. The project proposes to receive upstream runoff through sheet flow into the site and covey it to the new on -site detention system. Project detention system designed to accommodate this additional volume of runoff and convey it to the existing public drainage system within 19th Avenue SE. The project also proposes a lower finished grade than adjacent upstream properties and is not expected to create any backwater effects; therefore, further upstream analysis is not required. All approximated upstream runoff entering into the site is to sheet flow into the new on -site drainage and detention facility and then drain into the existing storm system along the east side of 19t" Ave. Refer to the following pages for depiction of upstream basin Downstream Analysis A downstream analysis was provided by Timothy Sarkela, E.I.T. of Western Engineers on January 17, 2017. Refer to the aerial map in the next section of this report for a depiction of runoff paths. Due to traffic volumes in the area, downstream photos were taken from Google Street View. The site tends to sheet flow westerly toward 191h Avenue SE west of the site. The site runoff is then intercepted by existing public storm drainage catch basins along the east site of 19th Avenue SE (refer to photos 1 & 2). According to City of Everett interactive maps, the storm drainage line continues approximately %2 mile south along the east side of 19th Avenue SE, eventually discharging into Silver Lake. As this is the closest receiving water, further downstream analysis is not required. The existing downstream drainage system consists of City of Everett documented conveyance systems. Due to the fact that the entire downstream system is documented and maintained by the City, there appears to be no current issues with the existing system. Proposed runoff from the site will be managed with a designed on -site detention system and is not expected to have any significant impact on the existing downstream system. Note that there are no known geographical features or sensitive critical areas know between the site and Silver Lake. Refer to the project Grading and SWPPP plan for more descriptions of existing site hydrology. Refer to the project Drainage plan for more detailed description of developed site hydrology. Refer to Appendix II of this report for general soil descriptions along with Geotechnical Engineering Report by Robinson Noble, Inc. dated March 23, 2016 for more detailed soil information. Drainage Complaints There are no known drainage complaints for this property. WESTERN ENGINEERS, INC. 6 PROJECT: 15-1415-A