5900 36TH AVE W PRE APP 015 - 032 - SOUNDVIEW BUSINESS CAMPUS - VERITAS Drainage Report 2025-04-22

SOUNDVIEW TECHNOLOGY CENTER EVERETT, WA OFFSITE ANALYSIS REPORT Navix Engineering conducted a downstream analysis for the Soundview project October 8, 2016 and January 18, 2017. Downstream analysis included visual, quantitative and qualitative review as required by the City of Everett. The Soundview project will release storm water to four (4) separate sub -basins. Refer to the Downstream Map in Appendix A for each basin analysis. rDownstream Analysis Basin A The western portion of the site drains into Japanese Gulch. The natural drainage flows down to the forest valley which continues to the bottom of Japanese Gulch, where it intersects the Burlington Northern Railroad Spur. The average slope of this large forest valley is approximately 10-20% with portions near the top greater than 40%. Existing flows intersect the railroad bed and are conveyed underneath the railroad bed by a 24" reinforced concrete pipe. See photo 1 below. The flows are then conveyed to west underneath the railroad track and into a catch basin on the west side. The outfall from this catch basin was not found, but during the downstream walk with Gary Gray, BNSF, it was assumed this stormwater outfalled into the drainage swale along the west side of the railroad tracks. This drainage system then continues north into regional detention ponds. The outfalls of this pond system was not reviewed at this time. The flows from the regional pond are piped north, underneath 5th Avenue and exit into the stream channel to the north. This system was not investigated at this time. The emergency outfall of the regional detention pond would travel overland through a riprap channel and into the 54"x36" CMP culvert. The culvert inlet is within a concrete headwall (see photo 2 below) with a debris cage over the inlet. An 18" emergency overflow pipe exists approximately 48" above the invert of the CMP culvert, flows northeast underneath the railroad bed, to an open channel CMP ditch which parallels the railroad bed along the east side. This was the end of the field reconnaissance. This location is approximately'/a mile from Possession Sound. See photo 3 below. In conclusion, the downstream conveyance system is not limited by the 24" pipe traveling underneath the railroad spur. The developed basin flows are designed to match existing forested conditions therefore it is concluded that there are no downstream restrictions to the western conveyance system. I '7 I IMAY 23, 2017 STORMWATER DRAINAGE REPORT PAGE 9