9530 HOLLY DR 2025-08-05 - Laserfiche WebLink

Due to the fact that ample open space on lots 1, 2, 4, 5 and 6 does not exist down - gradient of the proposed homes while still maintaining the required setbacks from property lines and structures, bioretention is infeasible on these lots. Downspout Dispersion: Infeasible, as the required vegetated flow paths do not exist. Perforated Stub -Out Connections: Infeasible. Instead of sending roof runoff from Lots 1, 2, 4, 5 and 6 through 2'x10' perforated stub -out trenches before connection to the stormwater conveyance system, a direct tightline is being proposed due to the low infiltration capacity of the site soils, and the mass -use of infiltration BMPs throughout the site. Perforated stub -out trenches would have to be located directly adjacent to proposed permeable concrete, which could undermine the functionality of the permeable concrete. Therefore, roof runoff from lots 1, 2, 4, 5 and 6 will be tightlined directly to the proposed stormwater conveyance system that will drain to the stormwater main within Holly Drive. BMP's for Other Hard Surfaces: 1. Full Dispersion: Not feasible due to not having 65% preserved vegetation. 2. Permeable Pavement: Feasible and selected for all individual driveways. Using the design infiltration rate of 0.30 inches per hour, permeable pavement is shown to infiltrate 100 percent with a 6-inch surfacing layer of permeable concrete underlain by a 6-inch storage course stone layer (See Appendix A for "infiltration sizing" VVVVHM printout). Per the infeasibility criteria listed under BMP T5.15 in the SVVMMVVVV, permeable pavement can be deemed 'infeasible' where fill soils are used that can become unstable when saturated. Because the entire private road will be constructed on top of at least a foot of fill, permeable pavement is infeasible for the private road. 3. Bioretention/Rain Gardens: Feasible and selected for the private asphalt road. The private road has been designed so that runoff from nearly it's entire surface will drain to bioretention cell #1. The VVVVHM was used to show that a bioretention cell with a 550 square foot bottom area will achieve 100 percent infiltration of the tributary road runoff. 4. Sheet Flow Dispersion: Higher priority BMP selected. 5. Concentrated Flow Dispersion: Higher priority BMP selected. The way the new road is graded, a small portion of the new road (approximately 300 so runoff near the intersection with Holly Drive will sheet flow in a southeastern direction and flow directly into the conveyance system within Holly Drive, and cannot be mitigated for on -site. Finally, lawn and landscaped areas must implement post - construction soil quality and depth in accordance with BMP T5.13. Therefore, BMP T5.13 will be applied to all areas of lawn/landscaping on this site to further provide filtration and some evapotranspiration of site pervious runoff. Perez SP March 2022 Revised: June%4ugust2022 Page12