423 ROCKEFELLER AVE 2023-05-31 - Laserfiche WebLink

7. ON-SITE STORMWATER MANAGEMENT Due to the fact that the project site is located within the combined sewer area of the City of Everett, Section 6-10.1 of the City of Everett Design and Construction Standards and Specifications manual applies. According to this section, projects that have a total of 200 square feet or more of net new impervious surface or convert % acres or more of vegetation to lawn or landscape, the project is required to meet the Flow Control Performance Standard. This standard states "Stormwater discharges shall match developed discharge durations to existing site durations for the range of existing discharge rates from the 10-year peak flow through the full 100-year peak flow". Therefore, a detention pipe with control structure has been designed to collect and store all impervious surface runoff from this lot, and the five (5) other adjacent lots that will be developed concurrently to meet this standard (see Section 8 for details). 8. FLOW CONTROL (MR #7) The thresholds for flow control within the combined sewer area are as follows: • Projects that have a total of 200 square feet or more of net new impervious surface, or • Projects that convert % acres or more of vegetation to lawn or landscape. The site design for the improvements is for approximately 3,927 sf of new and replaced hard surfaces, including off-site improvements. This project will result in more than 200 square feet of net new impervious surface, so this project must meet the 'Flow Control Performance Standard" and all requirements stated in Section 6-10.1 of the City of Everett DCSS. The existing and proposed conditions were modeled and compared using WWHM12 software. Per Section 6-10.1(1) of the COE DCSS, stormwater discharges shall match developed discharge durations to existing site durations for the range of existing discharge rates from the 10-year peak flow through the full 100-year peak flow. Also, peak runoff rates from the developed site shall not exceed existing site runoff rates in the 25-year and 100-year return intervals. Finally, the existing condition to be matched shall be the site as it exists at the time of application, and a continuous simulation modeling program shall be used with 5-minute timesteps. As all six (6) Legion lots will be constructed concurrently and all drain in a southern direction, we found it most efficient and cost effective to utilize a combined detention system that will effectively meet the Flow Control standard for all six lots combined. Using the 2012 Western Washington Hydrology Model software, the basin was analyzed for the existing flow conditions for the disturbed area of all six lots. This data was then used as the matching criteria required in the sizing of the detention pipe. Legion Lots 12& 13 20-0215 April 2020 Page 9 Revised:April2021