5900 36TH AVE W 2025-01-06 - Laserfiche WebLink

SOUNDVIEW TECHNOLOGY CENTER EVERETT, WA Basin B This drainage Basin B is conveyed to a steep ravine that intersects the property along the northern property line. The ravine has signs of surface flow at the low point.The stormwater is then conveyed along the forested swale, where it intersects a home-built wall and asphalt pavement over the flow path. The pavement is associated with an adjacent residence. The wall and asphalt pavement are shown in the photo 4.1 and 4.2 below. Just below this wall, a homemade 2-foot wide swale was constructed in the natural low point of the Swale to convey stormwater.This swale was rimmed on both sides with concrete test cylinders (see photo 5) with minor tree debris was observed in the swale. The stormwater is conveyed within this swale for approximately 60', and then discharges back to a more natural forest swale. The approximate slope of this swale is 20%. The stormwater then continues down in the natural swale until entering a 15" diameter culvert on the south side of Debrelon Lane. The culvert opening was clear at the time of the downstream analysis. The storm water is conveyed through catch basins for approximately 300 feet, and connects to a plastic storm pipe outlet. This pipe outlets to a catch basin with grate. See Picture 6.1 below. This catch basin connects to the existing 18-inch culvert under Lamar Drive where it discharges into an "Unnamed Stream," as documented by the Snohomish County GIS, located in the ravine above Edgewater Creek to the northeast.The unnamed stream discharges to Edgewater Creek. Navix attempted to locate the outfall from the pipe that heads northwest from the CB in Lamar Drive through private property, but was unable to obtain permission from the property owner to follow the pipe's path and locate the outfall. However, the angle of the pipe leaving the CB appears to head directly towards the unnamed stream and the contours of the land match what would be expected of a stream channel. See pictures 6.2, 6.3, and 6.4 below. Refer to Figures 1, 2, and 3 on pages 30-31 for the maps and photos of the drainage path. One potential issue of this basin outfall is the private wall and asphalt pavement constructed across the forest swale. Per an older downstream analysis prepared by Datum Pacific, there is a 12" CSP culvert underneath this wall connecting the forest swale with the 2' wide created swale.This could not be verified in the field. There are two 4" concrete pipes that outfall at the base of the wall, but since no inlet for these pipes could be found it can be assumed that they are there for sub-drainage behind the wall. There was evidence of water traveling around the eastern side of the wall to get to the ditch. There is also evidence of surface flow across the pavement, which will continue in the future. Since this project will detain the release rates to the predeveloped release rates in excess of the release rates required for the project (0.5 CFS max. discharge to this basin), no additional measures are proposed and the release flows will be dispersed on-site at the low point of the Swale. SEPTEMBER 23, 2022 STORMWATER DRAINAGE REPORT PAGE 11