V4 11 Stormwater Report Rev 3 1 26 2023

DRAINAGE AREA 3 (DA-3) Drainage Area 3 consists of 4.54 acres of both pervious and impervious surfaces that drain to via onsite storm sewers directly into the storm sewer located in Pleasant Valley Way. The pervious area, 2.44 acres, consists of a large wooded area along with onsite landscape islands, lawns, and planting beds. The impervious area, 2.10 acres, consists of parking lots, drive aisles, and sidewalks. A time of concentration of 10.47 minutes was calculated for this drainage area. DRAINAGE AREA 4 (DA-4) Drainage Area 4 consists of 1.25 acres of mostly pervious surfaces which drain directly onto Pleasant Valley Way, where it will ultimately flow into the storm sewers in the road. The pervious area, 1.22 acres, consists primarily of lawn and landscaped areas, with some wooded sections as well. The impervious area, 0.03 acres, consists of a small portion of the existing driveway that bypasses the onsite storm sewers. A time of concentration of 9.59 minutes was calculated for this drainage area. DRAINAGE AREA 5 (DA-5) Drainage Area 5 consists of 3.13 acres of both pervious and impervious surfaces, which are collected through a series of storm sewers that ultimately connect directly to the existing storm sewers within Pleasant Valley Way. The pervious area, 2.59 acres, consists predominately of wooded areas and along with some landscaped and lawn areas. The impervious surfaces, 0.54 acres, include portions of the existing parking lot and sidewalks. A time of concentration of 13.63 minutes was calculated for this drainage area. DRAINAGE AREA 6 (DA-6) Drainage Area 6 consists entirely of the existing roof area, 3.43 acres. The existing roof appears to be tied directly into the existing storm sewers within Pleasant Valley Way. A time of concentration of 6 minutes was used in the analysis, which is the minimum time used for this analysis. EXISTING BASIN Currently, the subject property contains a detention basin. This detention basin is served by a series of storm sewers that collect runoff from the existing parking lot and wooded areas in the rear of the site. It should be noted that based on record documents, this basin had existed on site since at least 1981, before the current stormwater regulations were in effect. Nevertheless, we have included the basin routing into our existing conditions model. The existing detention basin has 14,340 cubic

east 1981, before the current stormwater regulations were in effect. Nevertheless, we have included the basin routing into our existing conditions model. The existing detention basin has 14,340 cubic feet of storage. Over the years, the basin has become severely degraded and filled with debris and overgrowth, which has resulted in the outlet structure silting up. Based on historic mapping, the outlset structure has an orifice at the bottom, and a wier at the top. Our model reflects this same setup however the size of the bottom orifice is unknown. As such, we have assumed, conservatively, that the existing basin is served only by a 3" orifice located at the bottom. This assumption results in the least amount of runoff in the pre-construction conditions. ) Se 3