2007 CEDAR ST 2023-04-10 - Laserfiche WebLink

3. Downspout Dispersion Systems: Higher priority BMP selected. 4. Perforated Stub-out Connections Higher priority BMP selected. A total of approximately 191 sf of patio area is proposed for this project. Because all other runoff proposed with this project is being infiltrated (and is thus non-effective), the patio area is below the 200 sf of effective impervious area which would require flow control. Finally, any area of the site not proposed to be covered with impervious surfaces but graded or compacted will be treated to meet the criteria for BMP 12.20. Please refer to the engineering plans for basic soil amendment notes. 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 1,864 sf of new and replaced hard surfaces. 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 WWHM2012 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 (WWHM2012), shall be performed using a 5-minute timestep. Therefore the 5-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 bioretention area. Diedrich Addition 21-0224 Sept 2021 Page 11