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Home My WebLink About2 - App. B. - Stormwater Detention Pond #1 Calculations 10-16-2020DETENTION POND #1 REQUIRED VOLUME 1. Calculate Weighted C Factor for Right-of-Way Component Width C ROW Hardscape 43 0.95 ROW Landscape 17 0.2 Weighted C Factor = 0.74 2. Calculate Area and Weighted C Factor (Post-Development) Contributing Area C Area (ft2)C * Area Composite ROW 0.74 358934 264714 Hardscape 0.95 0 0 OS 0.2 206332 41266 Low-Med Residential 0.35 720728 252255 Dense Residential 0.5 0 0 Norton Phase 3 0.49 1474393 722453 *From existing Norton Ranch Ph 3 Total 2760387 1280688 (see Appendix F) A = Area (acres) 63.3698 C = Weighted C Factor 0.46 3. Calculate T c (Pre-Development) Tc Overland Flow Tc = 1.87 (1.1-CCf)D1/2/S1/3 Storm S = Slope of Basin (%) 1.49% Return (yrs)Cf C = Rational Method Runoff Coefficient 0.2 2 to 10 1 Cf = Frequency Adjustment Factor 1 11 to 25 1.1 D = Length of Basin (ft) 1168 26 to 50 1.2 51 to 100 1.25 Tc (Pre-Development) (minutes) 50 4. 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 5. 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) 63.37 (calculated above) Q = Runoff Rate (Pre-Development) (cfs) 9.09 6. Calculate Required Pond Volume Total Area (acres) = 63.37 acres Weighted C = 0.46 Discharge Rate (cfs) = 9.09 cfs (Equal to Pre-Development Runoff Rate) Groundwater Influence System Infiltration: 300 gal/ac/day Flow Rate into System: 19010.93 gal/day 13.20 gal/minute 0.03 cfs (1.77 cfm) Duration(min) Duration(hrs)Intensity (in/hr)Qin (cfs) Qin (cfs) (GW Inflow) Qin (cfs) (Total) Runoff Volume (cf) Release Volume (cf) Required Storage (ft3) 31 0.52 0.98 28.90 0.03 28.93 53,815 16,906 36,909 32 0.53 0.96 28.31 0.03 28.34 54,418 17,452 36,966 33 0.55 0.94 27.75 0.03 27.78 55,008 17,997 37,011 34 0.57 0.93 27.22 0.03 27.25 55,587 18,543 37,044 35 0.58 0.91 26.71 0.03 26.74 56,155 19,088 37,067 36 0.60 0.89 26.23 0.03 26.26 56,712 19,633 37,079 37 0.62 0.88 25.76 0.03 25.79 57,260 20,179 37,081 38 0.63 0.86 25.32 0.03 25.35 57,798 20,724 37,074 39 0.65 0.85 24.90 0.03 24.93 58,327 21,269 37,058 40 0.67 0.83 24.49 0.03 24.52 58,848 21,815 37,033 OUTLET STRUCTURE SLOT Q=CLH3/2 Q = Discharge (cfs) 9.09 (calculated above) C = Weir Coefficient 3.33 (per COB Design Standards) H = Head (ft) 1.5 L = Horizontal Length (ft) 1.49 L = Slot Width (inches) 17.8