825 N BROADWAY 2017-01-13 - Laserfiche WebLink

li�npervious area on the Index Site will decrease from the existing condition, therefore site runoff will decrease and the flow control standard is achieved without a detention system. Calculation of sufficient downstream capacity in the onsite storm-only line is discussed in the Stormwater Conveyance section below. WATER QUALITY REQUIREMENTS Per the requirements of the City of Everett Public Works Policy No. 2-11, Minimum Requirement No. 6, Runoff Treatment, is not applicable to this project because the site discharges to a combined sewer system. ON-SITE STORMWATER MANAGEMENT BMPS The City of EveretYs Stormwater Manual requires on-site BMPs to be implemented to the maximum extent feasible, per Minimum Requirement No. 5, On-Site Stormwater Management. Site soils are considered unsuitable for infiltration which limits the feasibility of on-site BMP's. Additionally, because the project is parking lots with very limited contiguous space available for non-parking uses, on-site stormwater BMP's are not applicable to the project. STORMWATER CONVEYANCE The proposed development includes drainage infrastructure to intercept on-site flows and route them through the site. The systems on each site were sized to safely convey the 25-year, 24-hour peak flow using the Santa Barbra Unit Hydrograph Method. The storm drainage design is shown on the project plans and the capacity calculations are included in Appendix C. The Index Site requires additional downstream analysis due to known capacity issues. Records indicate the Index Hall building storm drain previously connected to the onsite combined sewer main off the north edge of the site. A storm drain-only line was previously installed off the south edge of the site to support development on the campus as well as reduce demand on the sewer which was experiencing overflows during large storms. The storm-only main was subsequently extended offsite by the City of Everett in 2001 to bypass the 8-inch combined sewer in Little Broadway and connect to the 42-inch combined sewer in WaverlyAvenue. This project proposes to connect to the onsite storm-only line to avoid adding flows to the 8-inch combined sewer. To determine downstream system capacity we reviewed the onsite and offsite storm drain construction plans and the offsite drainage reports (see references below). According to Reference 3 below, the most restrictive downstream pipe has a capacity of 9.70 cubic feet per second. The peak runoff from the proposed site plus the existing flow is 9.22 cubic feet per second. This uses 78 percent of the pipe's capacity. Therefore, the existing storm system has capacity for the full 25-year, 24-hour event. References: 1. Onsite: Instructional Technology Center and Site Improvements (PW0206-001, September 1997) 2. Offsite: Storm Drainage Improvements Broadway to Waverly (PW0103-030, March 2001); Stormwater Hydraulics Report (February 2001) 3. Offsite: Preliminary Downstream Conveyance CapacityAnalysis (Public Works, November 2010) Stormwater Site Plan — City of Everett 3