4620 SEAHURST AVE 2020-05-20 - Laserfiche WebLink

Projects that convert 3/4 of an acre or more of native vegetation to lawn or landscape... • The total area to be cleared/disturbed for this project is less than % of an acre so this threshold does not apply. Projects that through a combination of effective impervious surfaces and converted pervious surfaces cause a 0.1 cfs increase in the 100-year flow frequency from a threshold discharge area. In order to analyze this project three different basins were used and the existing site was modeled as it is in the current condition as allowed in the definition of Predeveloped Condition contained on page G-28 in the Glossary. It is clear from my site visit and the photos provided with this report that the area of the site proposed to be cleared/worked is landscaped and contains some impervious surfaces. As noted above the existing land cover today is lawn/landscape with 4,155 sf of existing dispersed impervious surfaces. The developed site was modeled with the roof area of 3,230 sf on lot 3 along with the remaining landscaping areas on Lots 1-3. However, the roof areas on lots 1 and 2 and the pervious pavement areas were not included since they are fully infiltrated and are not considered effective impervious surfaces. The infiltration system analysis is discussed below. Based on the above site assumptions and using the WWHM3 software the existing and developed surface runoff conditions were calculated. For the existing condition the peak 100-year flow rate is 0.14 cfs and for the developed condition 0.12 cfs. That is an actual decrease of 0.02 cfs which is less than the 0.1 cfs increase threshold so this project is exempt from flow control requirements. Refer to the attached WWMH3 printouts labeled "Site". For the sizing the of the infiltration trench needed to serve the roof areas on Lots 1 and 2 a roof area of 2,440 sf for lot 1 and 1,800 sf for lot 2 was assumed and that is what is shown on the preliminary plans as well. Therefore, a total of 4,240 sf of roof will be tributary to the proposed infiltration trench between the two future SFRs. The Geotechnical Engineer for the project found the soils in this location of the site to be silty sand over fine to medium sand with trace gravel at a depth ranging from 3-5.5 feet. Refer to test pits 5 & 6. Therefore, the trench design will require the bottom of the trench to be excavated a minimum of 6-feet below grade to ensure that the bottom of the trench fullyextends into the sand layer. The Geotech also provided a recommended long-term infiltration rate of 1.5"/hr in which he has already applied the necessary factors of safety. This rate was used in the sizing of the proposed trench. Refer to the attached WWMH3 printouts labeled "Roof Trench" for the documentation showing the trench as designed (5.5'W, 60'L, 4'D) will fully infiltrate the roof runoff up to the 100-year storm event. The pervious pavement section was also analyzed and designed so that the sizing of the storage rock layer will allow for the full infiltration of the runoff from the private road and Lot 3 driveway up to the 100-year storm event. This was also done using the WWHM3 software. The bottom area for the facility is the same as the area of pavement which for this project is 5,300 sf. It was determined through trial and error that the storage rock layer needs to be 12-inches thick with a void ratio or 35%. The WWMH3 printout labeled Thompson Short Plat 11-012 May 2011 Page 14