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

Sheet 5 9. Callout, provide sizing, and add a cross-section for the proposed French Drain near the eastern boundary of the project site. Sheet 6 10. The detail provided is for a Linear Sand Filter. Linear Sand Filters are approved for Oil Control, but not Enhanced Treatment. See Figure 2.1.1 (Treatment Facility Selection Flow Chart) in the SWMM for stand- alone BMP options for Enhanced Treatment or Table 3.4.1 (Treatment Trains for Dissolved Metals Removal) two -facility treatment train options. 11. Given the limited available space, likely high installation cost, and challenges with providing maintenance access for an underground sand filter vault in compliance with the SWMM requirements, consider other runoff treatment BMPs, such as bioretention with underdrain distributed in parking lot planter islands and/or perimeter landscaping upstream of the detention facility. In addition to providing enhanced treatment in an easy -to -maintain above -ground facility, this BMP may also help meet flow control sizing requirements when the above comments are considered in flow control sizing. 12. If a linear sand filter is to be used for oil control or as part of a two -facility treatment train for enhanced treatment, revise the designs for proper ventilation, maintenance access, and provision of all design requirements for BMP T8.30: Linear Sand Filter: o Slope underdrain min 0.5%. o Ensure that 8" gravel drain rock layer is sufficiently deep for underdrain bedding and cover. o Provide proper energy dissipation prior to flows entering the sand bed. o The inlet baffle wall is set to elevation 464.16, but the outlet baffle wall elevation must also be called out. Note that the SWMM design criteria include a maximum ponding depth of 1', but the inlet baffle is also set at 1'. This would cause short circuiting. Ensure that the designs both drain properly and do not short circuit treatment. o Provide inlet shut-off valve for maintenance. o Provide venting over the sand bed. o Provide cleanouts at both ends of the perforated pipe. o The cover over the sand bed must be removable for maintenance access. o Provide structural calculations for the vault design. r IF. A�: 1 1, �1" Iffa 94= P: 425.257.8995 Erik Emerson PE Senior Surface Water Engineer Public Works E: eemerson@everettwa.gov Office: 3200 Cedar Street, Everett, WA 98201 www.everettwa.gov MCI NOTICE: Emails and attachments sent to and from the City of Everett are public records and may be subject to disclosure pursuant to the Public Records Act.