2007 CEDAR ST 2023-04-10 - Laserfiche WebLink

5. SOURCE CONTROL (MR #3) This project proposes to construct an addition to an existing SFR. Per Volume 1 of the 2019 DOE manual, residential development does not require source control. 6.• 'PRESE'RVATI'ON OF NATURAL DRAINAGE SYSTEM (MR #4) The runoff from the site improvements will either be infiltrated on site via a bioretention area. However, by not grading or compacting the majority of the pervious surfaces to remain after construction in the area of disturbance a maximum amount of site runoff from the developed project will be absorbed and infiltrated into the existing on-site soils. This will mimic the existing conditions by maintaining the existing direction and condition of flow from this site. 7. ON-SITE STORMWATER MANAGEMENT (MR #5) As documented in Section 1 of this report this project is required to meet Minimum Requirements 1-5. As a result, this project is required to use the On-Site Stormwater Management BMP's from the 2019 DOE SWMMWW. Due to the fact that the projectsite 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 %4 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, the proposed impervious surfaces will be infiltrated to the maximum extent feasible. Per the DOE SWMMWW the BMP's must be evaluated in the order they are listed. BMP's for roofs: 1. Full Dispersion: Not feasible due to the 65:35 requirement. 2. Downspout Infiltration Systems: As previously mentioned in this report, soil logs taken by the Rich's Septic Design LLC found sandy loams to hardpan at 35-45 inches below grade, and also possible seasonal water table at 28-37 inches below grade. However for this project, grade shall be raised where necessary to ensure a minimum of 12 inches between the potential seasonal water table and the bottom of the proposed bioretention area. This will make infiltration feasible, by providing at least 36 inches of permeable soils over hardpan/water table. Due to the combined storm-sewer area, infiltration of the entire proposed roof area is required. This will be accomplished by a proposed bioretention area.per'BMP T7:30. This bioretenion area was sized using the 2012 WWHM, using an assumed infiltration rate of 0.5 inches/hr for the sandy loams found with the soil logs. The resulting size for the bioretention bottom area came to 52 sf, which was laid out as a 26' long by 2' wide area. This area was shown to infiltrate 100% of the roof runoff proposed with this project (1,673 sf). Diedrich Addition 21-0224 Sept 2021 Page 10