2217 GRAND AVE 2025-05-29 - Laserfiche WebLink

o FLOW CONTROL 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 391 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". The existing and proposed conditions were modeled and compared using WWHM 12 software. Per the combined storm sewer design and construction standards and specifications chapter 6-10.1(1), the Washington Department of Ecology approved modelling program (WWHM), shall be performed using a 15-minute timestep. Therefore the 15-minute time step was used in this analysis. As per the interim Public Works Policy#2-11 the existing site will not be modeled as forested land cover but rather the existing site conditions. Using WWHM software the basin was analyzed for-the existing flow conditions for the disturbed basin only. This data was then used as the matching criteria required in the sizing of the detention pipe. The existing soils were found to be dark brown sandy loam to 10 inches, brown loamy fine sand to 30 inches, and tan fine sand to 48 inches. Due to the soil type, a typical infiltration trench is feasible. However, due to the extensive site grading in the past, no native soil conditions remain on the site. Therefore, a downspout dispersion system will be utilized. Since there is a vegetated flow path of at least 50 feet and the roof area of the proposed garage is 391 square feet, two splash blocks are sufficient to meet the requirements. L City of Everett EVERErr Reviewed for Permit 11/18/2021 Bedle Garage 21-0421 November 2021 Page 11