1029 52ND ST SW 2022-10-31 - Laserfiche WebLink

• 3. OFF-SITE ANALYSIS AND MITIGATION This residential development is expected to add an assumed 14,595 sf (0.34 ac) for the total project. 11,695 sf (0.27 ac) on-site for future driveways and roofs. An additional 2,900 sf(0.07 ac) is expected for off-site improvements including curb and sidewalks. Total Site Areas The following table summarizes the total impervious surfaces for the entire project, off-site and on-site in each threshold discharge area. Proposed Proposed Total Disturbed Impervious (ac) Pervious (ac) Area (ac) Basin A 0.02 0 0.02 Basin B 0.32 0.21 0.53 In the developed conditions it was assumed that each lot will be covered with the maximum 55% total impervious coverage. The runoff from the roof surfaces will be collected and infiltrated on each lot within an infiltration trench. Runoff from the driveways will infiltrate via the permeable pavement surfacing proposed. New landscaping runoff will be allowed to leave the site as sheet flow. The infiltration trenches have been designed to infiltrate 100% of the roof runoff for each lot. Per Volume I, page G-13 of the SWMMWW, impervious surfaces are considered ineffective if residential roof runoff is infiltrated in accordance with Downspout Infiltration Systems in BMP 5.10A Volume III or approved continuous runoff modeling methods indicate that the entire runoff file is infiltrated. Since this project is proposing to utilize downspout infiltration trenches and is shown to 100% infiltrate runoff using the VVWHM, these surfaces (0.18 ac roof area) are considered ineffective impervious surfaces. Likewise, the permeable pavement section has been designed so that the entire runoff file is infiltrated and therefore also considered ineffective impervious surfaces as noted above (0.09 ac). Refer to Section 4 of this report, as well as Appendix A for WWHM•calculations. The proposed infiltration systems will be sized using the WWHM2012 software and shown to fully infiltrate up to the 100-year event. Refer to Section 9 of this report for more details. • "Seahurst'SP 18-0727Page 6 September 2019