15 FERN RD 2017-10-30 - Laserfiche WebLink

6. PRESERVATION OF NATURAL DRAINAGE SYSTEM The runoff from the NPGIS improvements will be dispersed on-site. By not grading or compacting the majority of the pervious surfaces to remain after construction in the area of disturbance a maximum amount of site runoff from the developed project will be absorbed and infiltrated into the existing on-site soils. This will mimic the existing conditions by maintaining the existing drainage path on-site. 7. ON-SITE STORMWATER MANAGEMENT To address the requirements for managing stormwater from roof surfaces, the applicability and feasibility of on-site infiltration areas were considered first. As previously mentioned, this site has already been developed and appears to have been graded. Therefore it is assumed that the site contains fill material and has been compacted in several locations on-site. Furthermore, soil logs provided by Ryan Kramer, indicate hard pan or water table at a depth of 31-43 inches below grade. Since the site appears to contain fill material and the depth to restrictive layer/water table appears to be less than 3 feet below grade, infiltration is not feasible for this site. Rain gardens were considered next. However since the soils are considered sandy loam, per the licensed septic designer, the depth to restrictive layer/water table appears to be less than 3 feet below grade and that the parcel is less than 22,000 sf, a rain garden is not a practical choice for managing stormwater for this site. This allowed for the selection of downspout dispersion as the stormwater management BMP of choice for application to the roof surfaces proposed for this project. The dispersal system will allow for roof runoff to disperse over vegetated surfaces for the maximum length of flow path on-site before being collected by a proposed on-site vegetated swale. To implement downspout dispersion, the roof downspouts of the proposed SFR will be dispersed via splash blocks. The proposed roof surfaces total approximately 3,035 sf. A maximum of 700 sf of roof area may be directed to each splash block. Therefore, a minimum of 5 splash blocks are required. Splash blocks must discharge to a vegetated flow path of at least 10 feet in width and 50 feet in length. Due to site constraints, the minimum required splash blocks would not fit on the site while having a vegetated flow path of 50 feet. Therefore, only 3 splash blocks will be used. This will allow for the attenuation of flows to the maximum extent feasible and the remainder of roof runoff will be connected to a perforated stub-out connection located south of the proposed duplex. To keep runoff from leaving the site and to protect downstream neighbors a shallow swale will be located along the northeast property corner. This swale will collect runoff from the proposed splash. Any runoff that does not naturally infiltrate into the soils of the swale will be collected by a yard drain. This yard drain will direct runoff south along the eastern property line and connect to the existing stormwater pipe system within the right-of-way of Fern Rd. As mentioned, the remainder of roof area that will not be dispersed will be tightlined and connected to a perforated stub-out connection. This will allow for some infiltration before flows outfall to the existing stormwater pipe system within the right-of-way of Fern Rd. Refer to sheet 2 of the drainage plans for more details. 15 Fern Rd Duplex 16-0302 REVISED: June 2016 Page 9