V4 11 Stormwater Report Rev 3 1 26 2023

Table 3 — Target Runoff Rates Storm Existing | Percent Offsite Runoff Total Post- Event Runoff | Reduction | Reduction Development (Yr.) Rates (efs) Target Runoff (cfs) (cfs) 2 Year 20.08 cfs 26.0% 20.08 (0.260) = 20.08 — 5.221 = 5.221 cfs 14.859 cfs 10 Year | 45.76 cfs 13.0% 45.76 (0.130) = 45.76 — 5.949 = 5.949 cfs 39.811 cfs 100 Year | 96.75 cfs 10.4% 96.75 (0.104) = 96.75 — 10.062 = 10.062 cfs 86.688 cfs PROPOSED CONDITIONS The same overall drainage area used in existing conditions was used for proposed conditions. This drainage area was modeled as 5 drainage areas totaling 20.16 acres. (See Fig.-2 Proposed Drainage Area Map.) DRAINAGE AREA 1A (DA-1A) Drainage Area 1A consists of the portion of the building to be constructed under Phase I and the associated interior courtyard. The roof area consists of 1.03 acres of mostly impervious surfaces with green roof areas throughout (See Green Roof Calcs below). The interior courtyard has been assumed to be 50% impervious for this analysis, consisting of 0.03 acres of impervious surfaces and 0.03 acres of pervious surfaces, which will be landscape and grass areas. Drainage Area 1A drains to proposed Basin #1A, a subsurface detention basin. See below for a detailed description of the basin. A time of concentration of 6 minutes was used for the roof area, which is the minimum time that can be used. Meanwhile, a time of concentration of 7.46 minutes was calculated for the courtyard area. DRAINAGE AREA 1B (DA-1B) Drainage Area 1B consists of the portion of the building to be constructed under Phase II and III. The roof area consists of 1.75 acres of mostly impervious surfaces with green roof areas throughout (See Green Roof Calcs below). This roofed area drains to proposed Basin #1B, a subsurface detention basin. See below for a detailed description of the basin. A time of concentration of 6 minutes was used for this drainage area, which is the minimum time that can be used. DRAINAGE AREA 2 (DA-2) Drainage Area 2 consists of both pervious and impervious surfaces, which will bypass the proposed basins onsite and drain into the Pleasant Valley Way ROW. The pervious area, 3.18 acres, consists of mostly landscape and grass areas along with some existing wooded areas, which are to remain. The impervious area, 1.30 acres, consists of parking areas, drive aisles, and sidewalks. A time of concentration of

andscape and grass areas along with some existing wooded areas, which are to remain. The impervious area, 1.30 acres, consists of parking areas, drive aisles, and sidewalks. A time of concentration of 10.14 minutes was calculated for this drainage area. ® Xo ;