4620 SEAHURST AVE 2020-05-20 - Laserfiche WebLink

8. STORMWATER MANAGEMENT Minimum Requirement#5: On-site Stormwater Management Multiple BMPs contained in Volume III, Chapter 12 will be utilized to mitigate the project improvements and protect the downstream system. For this project BMPs 12.10, 12.20, 12.30 and 12.40 will be utilized to infiltrate, retain and disperse stormwater on-site to the maximum extent feasible without creating flooding or erosion problems downslope. The existing vegetation contained west of the proposed level spreader will be retained to protect the existing slopes. In addition, this slope was evaluated by the project Geotech and found that if the vegetation were retained that dispersion of runoff via a properly designed level spreader would not affect the slope stability and the native vegetation and soil conditions would provide attenuation of the dispersed flows prior to leaving the site. The roof areas on Lots 1 & 2 and all the proposed pavement areas will be infiltrated up to the 100-year storm event which means they will not be considered effective impervious surfaces. This willremove the runoff from these surfaces from the project surface flow discharge rates. Any area of the site that is graded or compacted and proposed to remain as landscaping or a pervious condition will be treated with soil amendments to meet or exceed BMP 12.20. This is shown and detailed on the construction plans submitted with the Public Works/construction permit submittal set. Minimum Requirement#7: Flow Control This project does not meet any of the three discharge requirements provided in Volume I, Section 2.2.7. They are as follows: Project in which the total of effective impervious surfaces is 5,000 sf or more in a threshold discharge area. Though this project does propose to construct more than 5,000 of impervious surfaces the roof areas on Lots 9 and 2 will be infiltrated in a common infiltration trench located between the two lots within a future easement. Therefore, the roof areas are not considered effective impervious surfaces and are removed from that area calculation. In addition, the proposed private road and driveway surfaces have been designed with pervious asphalt with a storage rock layer beneath and no under drain or outfall. The depth of the storage rock layer was sized using WWHM3 to ensure that the pervious pavement section was adequate to fully infiltrate all the runoff from the pervious pavement surface up to the 100-year storm event. Therefore, this area can also be removed from the effective impervious surface calculation. Finally, the only impervious surface that cannot be fully infiltrated is the roof on future lot 3. At this time the proposed roof area for the house and garage along with the proposed walkways is 3,230 sf. So the total effective impervious surface area for this project is 3,230 sf which is less than the 5,000 sf threshold for this minimum requirement. Thompson Short Plat 11-012 May 2011 Page 13