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

SOUNDVIEW TECHNOLOGY CENTER EVERETT, WA As can be seen in the WWHM output, modeled vault with dimensions of 9' x 444' x 4.75' = 18,981 CF meets and exceeds the detention requirements for Basin D. The arched pipe detention system utilizes the void rock space between the arches as well as the rock void space above the arched pipes as detention volume. A 40%void stone (0.75"-2.0" angular/crushed rock) space is assumed. The chamber system has 3 rows for a total of 99 chambers and 26 end caps. The designed vault correlates to an overall area of 741 feet and a storage volume of 19,021 CF, exceeding the required volume. Each pipe is 45" tall x 77"wide with 9" of rock bedding and 12" of rock above as shown above. WATER QUALITY SYSTEM The proposed water quality facilities were designed in accordance with the 2019 manual.The process of selection is as follows: • An oil control facility is not required. • Infiltration is not feasible. • Phosphorus control is not required. • Basic treatment is not required. • An Enhanced treatment facility was selected. An Oldcastle BioPod was selected for the discharges of vault#3 and 4 because they contain Pollution Generating Surface (PGS). The BioPod system has GUILD designation for Basic, Phosphorus, and Enhanced treatment. A BioPod system is an underground water quality vault utilizing biofiltration for treatment. They are located downstream of the proposed detention facilities and sized for the full 2-year release rate and have the capacity to flow-through the 100-year. See Appendix E for BioPod detail sheets and GULD pages. Vault#3 • Water quality treatment required (2-year) = 0.094 CFS • Peak vault release rate (100-year) = 0.436 CFS A BioPod BPU-612IB-8 unit has been selected to provide the water quality treatment for vault#3 outlet flows. The BioPod is 6'x12' and has a treatment flowrate of 0.213 CFS. Vault#4 • Water quality treatment required (2-year) = 0.053 CFS • Peak vault release rate (100-year) = 0.306 CFS A BioPod BPU-612IB-8 has been selected to provide the water quality treatment for vault#4 outlet flows. The BioPod is 6'X12' and has a treatment flowrate of 0.213 CFS. Per the Trip Generation and Mitigation memo prepared by Jake Traffic Engineering, Inc., dated February 28, 2022, it is anticipated the project will generate 745 net new average daily trips. With a total of 486,383 SF of gross building area, the average per 1,000 SF of building is 1.5, which is below the 100 trips/1,000 SF threshold for oil control. Therefore, the site will not exceed the threshold as a "High-Use" site and oil/water separators are not required. Refer the Trip Generation and Mitigation memo in Appendix B for additional information. The proposed development will not provide parking, storage, or maintenance of 25 or more vehicles that are over 10 tons gross weight. Therefore oil/water separators are not required. APPLICABILITY OF MINIMUM REQUIREMENT#8 WETLANDS PROTECTION Per coordination with the City, the at-grade site is vested under the approved 2017 stormwater management design. However, roof areas are not vested per code. Therefore, Basin A and Basin B must be SEPTEMBER 23, 2022 STORMWATER DRAINAGE REPORT PAGE 28