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Home My WebLink About15.0 Stormwater Design Report_06-12-2019 1 | P A G E INTRODUCTION The Bridger Vale Planned Unit Development (PUD) project is being proposed to annex and rezone to Residential Mixed Use (REMU) Tract GLR-1, COS No. 1221, which is located east of Manley Road at the East Gallatin Recreational Area (EGRA) access road. The project is planned to be developed as a residential subdivision PUD. This design report addresses stormwater management at the site based on the development plan being proposed. STORMWATER DETENTION PONDS The site runoff for Bridger Vale Subdivision PUD will be routed to Detention Pond 4 which is proposed south of Bridger Vale Drive on the west side of the site. The site is divided into two drainage areas as shown on the attached drainage area map included in Appendix A at the end of this report. Both drainage areas are routed to Detention Pond 4 through a series of ponds and overflow structures. There are 3 intermediate detention ponds (Detention Pond 1, Detention Pond 2 and Detention Pond 3) located in the center median strip of Bridger Vale Drive. The western-most Detention Pond 1 has a storage capacity of 1,610 cf, Detention Pond 2 has a storage capacity of 620 cf, and Detention Pond 3 has a storage capacity of 900 cf. Drainage Area 1 includes the majority of the alley along the rear of the residential lots which will be graded with an inverted crown cross section that will collect runoff from the rear of the lots, all of the buildings, the driveways and the commercial lot. The runoff collected by the alley will be routed to the west intersection of the Alley and Bridger Vale Drive and will drain into Detention Pond 1 through a curb cut in the western most median at the lowest grade point. Storm water then is routed through an outlet control structure to Detention Pond 4. The proposed outlet control structure in Detention Pond 1 is meant to hydraulically connect Detention Pond 1 and Detention Pond 4, as such it will not have a weir. Once in Detention Pond 4 another outlet control structure will transmit the runoff through the wetlands to the ditch along the east side of Manley Road. 2 | P A G E Drainage Area 2 consists of the landscaped front yards of four of the buildings, a portion of the Alley, open space, and the east end of Bridger Vale Drive. Catch curb along the center median will collect this drainage and route it to the appropriate median through four curb cuts. Drainage Area 2 will drain to Detention Ponds 2 & 3. During a storm event runoff held in Detention Pond 3 will be routed to Detention Pond 2 through a proposed culvert underneath the pedestrian crossing in the median of Bridger Vale Drive. The required detention volume for Drainage Area 2 is 700 cf; Detention Ponds 2 & 3 provide 1,520 cf of storage capacity. As an additional treatment measure, the runoff in Detention Pond 2 will be held and routed through an outlet control structure to Detention Pond 4, then another outlet control structure to the ditch along the east side of Manley Road. The outlet structure in Detention Pond 2 will have a 1.5’ headwall and a slot width of 0.4”. Due to the fact that Detention Pond 4 is detaining runoff for both Drainage Areas 1 & 2, sizing calculations for Pond 4 are provided for the entire site. Drainage Areas 1 & 2 have a combined required detention volume of 4,786 cf considering the 10-year 2-hour storm event. Detention Pond 4 has been sized to detain runoff from Drainage Areas 1 and 2, as such Detention Pond 4 has a volume of 5,238 cf. The outlet structure on Detention Pond 4 will have a 1.8” weir which will allow the pond to drain to the existing ditch at the predevelopment runoff rate of 0.93 cfs. As stated in the previous paragraphs Detention Ponds 1 & 2 will each have a drop detention pond outlet structure which will flow to Detention Pond 4. Detention Pond 4 will then outlet to the existing ditch located along the east side of Manley Road. There is a 30” corrugated metal culvert that currently runs under future Bridger Vale Drive and this culvert will be replaced by a 22” by 36” RCP arch culvert. For a full breakdown of the calculations used in sizing the detention pond please refer to the attached drainage spreadsheets found at the end of the report. 3 | P A G E APPENDIX A DRAINAGE AREA MAP 4 | P A G E APPENDIX B SUPPORTING CALCULATIONS DRAINAGE AREA 2 DETENTION PONDS 2 & 3 1. Calculate Area and Weighted C Factor (Post-Development) Contributing Area C Area (ft2 )C * Area Hardscape 0.95 17983 17084 Commercial 0.8 0 0 Landscape 0.2 41244 8249 Total 59227 25332 A = Area (acres) 1.36 Storm C = Weighted C Factor 0.43 Return (yrs)Cf 2 to 10 1 2. Calculate T c (Pre-Development)11 to 25 1.1 Tc Overland Flow 26 to 50 1.2 Tc = 1.87 (1.1-CCf)D1/2/S1/3 51 to 100 1.25 S = Slope of Basin (%) 0.8 C = Rational Method Runoff Coefficient 0.2 Cf = Frequency Adjustment Factor 1.1 D = Length of Basin (ft) 800 Tc (Pre-Development) (minutes) 50 3. Calculate Rainfall Intensity (Duration = Pre-Development Tc) i = 0.64x-0.65 (10-yr Storm, Fig. I-3, COB Design Standards) x = storm duration (hrs) 0.84 (Tc Pre-Development) i = rainfall intensity (in./hr.) 0.72 4. Calculate Runoff Rate (Pre-Development) Q = CiA C = Rational Method Runoff Coefficient 0.2 (open land) i = rainfall intensity (in./hr.) 0.72 (calculated above) A = Area (acres) 1.36 (calculated above) Q = Runoff Rate (Pre-Development) (cfs) 0.20 5. Calculate Required Pond Volume Total Area (acres) =1.36 acres Weighted C =0.43 Discharge Rate (cfs) =0.20 cfs (Equal to Pre-Development Runoff Rate) Duration(min)Duration(h rs) Intensity (in/hr)Qin (cfs)Runoff Volume Release Volume Required Storage (ft3) 28 0.47 1.05 0.61 1026 329 698 29 0.48 1.03 0.60 1039 340 699 30 0.50 1.00 0.58 1051 352 699 31 0.52 0.98 0.57 1063 364 700 32 0.53 0.96 0.56 1075 376 700 33 0.55 0.94 0.55 1087 387 700 34 0.57 0.93 0.54 1098 399 699 35 0.58 0.91 0.53 1110 411 699 36 0.60 0.89 0.52 1121 422 698 37 0.62 0.88 0.51 1131 434 697 PROVIDED VOLUME (ft3)1,520 OUTLET STRUCTURE SLOT - DETENTION POND 2 Q=CLH3/2 Q = Discharge (cfs)0.20 C = Weir Coefficient 3.33 (per COB Design Standards) H = Head (ft)1.5 L = Horizontal Length (ft) 0.03 L = Slot Width (inches) 0.4 DETENTION POND 4 DRAINAGE AREA 1 & 2 1. Calculate Area and Weighted C Factor (Post-Development) Contributing Area C Area (ft2 )C * Area Hardscape 0.95 109475 104002 Commercial 0.8 23540 18832 Landscape 0.2 149666 29933 Total 282681 152767 A = Area (acres) 6.49 Storm C = Weighted C Factor 0.54 Return (yrs)Cf 2 to 10 1 2. Calculate T c (Pre-Development)11 to 25 1.1 Tc Overland Flow 26 to 50 1.2 Tc = 1.87 (1.1-CCf)D1/2/S1/3 51 to 100 1.25 S = Slope of Basin (%) 0.8 C = Rational Method Runoff Coefficient 0.2 Cf = Frequency Adjustment Factor 1.1 D = Length of Basin (ft) 800 Tc (Pre-Development) (minutes) 50 3. Calculate Rainfall Intensity (Duration = Pre-Development Tc) i = 0.64x-0.65 (10-yr Storm, Fig. I-3, COB Design Standards) x = storm duration (hrs) 0.84 (Tc Pre-Development) i = rainfall intensity (in./hr.) 0.72 4. Calculate Runoff Rate (Pre-Development) Q = CiA C = Rational Method Runoff Coefficient 0.2 (open land) i = rainfall intensity (in./hr.) 0.72 (calculated above) A = Area (acres) 6.49 (calculated above) Q = Runoff Rate (Pre-Development) (cfs) 0.93 5. Calculate Required Pond Volume Total Area (acres) =6.49 acres Weighted C =0.54 Discharge Rate (cfs) =0.93 cfs (Equal to Pre-Development Runoff Rate) Duration(min)Duration(h rs) Intensity (in/hr)Qin (cfs)Runoff Volume Release Volume Required Storage (ft3) 42 0.70 0.81 2.83 7132 2352 4780 43 0.72 0.79 2.79 7191 2408 4783 44 0.73 0.78 2.75 7249 2464 4785 45 0.75 0.77 2.71 7306 2520 4786 46 0.77 0.76 2.67 7363 2576 4786 47 0.78 0.75 2.63 7418 2632 4786 48 0.80 0.74 2.59 7473 2688 4785 49 0.82 0.73 2.56 7527 2744 4783 50 0.83 0.72 2.53 7581 2800 4780 51 0.85 0.71 2.49 7633 2856 4777 PROVIDED VOLUME (ft3)5238 OUTLET STRUCTURE SLOT - DETENTION POND 1 & DETENTION POND 4 Q=CLH3/2 Q = Discharge (cfs)0.93 C = Weir Coefficient 3.33 (per COB Design Standards) H = Head (ft)1.5 L = Horizontal Length (ft) 0.15 L = Slot Width (inches) 1.8 MANNING'S EQUATION FOR PIPE FLOW Project:Bridger Vale PUD Location: By:AJC Date:6/10/2019 INPUT D=15 inches d=14.07 inches Mannings Formula n=0.013 mannings q=57.7 degrees Q=(1.486/n)ARh2/3S1/2 S=0.004 slope in/in R=A/P A=cross sectional area P=wetted perimeter V=(1.49/n)Rh2/3S1/2 S=slope of channel Q=V x A n=Manning's roughness coefficient Solution to Mannings Equation Area,ft2 Wetted Perimeter, ft Hydraulic Radius, ft velocity ft/s flow, cfs PVC 0.013 1.20 3.30 0.36 3.68 4.39 PE (<9"dia) 0.015 PE (>12"dia) 0.02 PE(9-12"dia) 0.017 CMP 0.025 ADS N12 0.012 HCMP 0.023 Conc 0.013 15" outlet pipe Manning's n-values d q D MANNING'S EQUATION FOR PIPE FLOW Project:Bridger Vale PUD Location: By:AJC Date:6/10/2019 INPUT D=8 inches d=7.50 inches Mannings Formula n=0.013 mannings q=57.7 degrees Q=(1.486/n)ARh2/3S1/2 S=0.004 slope in/in R=A/P A=cross sectional area P=wetted perimeter V=(1.49/n)Rh2/3S1/2 S=slope of channel Q=V x A n=Manning's roughness coefficient Solution to Mannings Equation Area,ft2 Wetted Perimeter, ft Hydraulic Radius, ft velocity ft/s flow, cfs PVC 0.013 0.34 1.76 0.19 2.42 0.82 PE (<9"dia) 0.015 PE (>12"dia) 0.02 PE(9-12"dia) 0.017 CMP 0.025 ADS N12 0.012 HCMP 0.023 Conc 0.013 8" PVC CULVERT Manning's n-values d q D