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HomeMy WebLinkAbout17_Stormwater Calculations RetentionWEST BABCOCK APARTMENTSWEIGHTED C-VALUESImpervious 0.90Gravel 0.30Lawn/Landscaped 0.10Undisturbed 0.20Pre-Development: Existing ConditionWeighted C-Value (Cw)Area Runoff (AC) Coefficient CalculationsImpervious 0.21 0.9 0.19Gravel0.04 0.30 0.01Undisturbed0.35 0.20 0.07Totals0.60 0.27Cw =0.46Post-Development: ON-SITE STORAGEWeighted C-Value (Cw)Area Runoff (AC) Coefficient CalculationsImpervious0.41 0.90 0.37Totals0.41 0.90 0.37Cw =0.90Post-Development: RUNOFFWeighted C-Value (Cw)Area Runoff (AC) Coefficient CalculationsImpervious0.15 0.90 0.13Lawn0.05 0.10 0.00Totals0.19 1.00 0.14Cw =0.70 WEST BABCOCK APARTMENTS PRE-DEVELOPMENT RUNOFF Page 2 of 4 10/27/2022 By Miguel Ley, EI PRE-DEVELOPMENT CONDITIONS: EXHIBIT A Design Storm: 10-Year, 2 Hour (City of Bozeman) Basin Area (ac): 0.60 Pre-Dev C (weighted): 0.46 Storm Duration (min)120.00 Runoff Rate (Rational Method): Storm Event (yr): 2-yr, 2-hr 10-yr, 2-hr 25-yr, 2-hr 100-yr, 2-hr 100-yr, 24-hr *Storm Coeff. A: 0.36 0.64 0.78 1.01 1.01 *Storm Coeff. B:0.60 0.65 0.64 0.67 0.67 Composite C: 0.46 0.46 0.46 0.46 0.46 Storm Intensity (in/hr): 0.24 0.41 0.50 0.63 0.12 Pre-Dev Qp (cfs): 0.06 0.11 0.14 0.17 0.03 Pre-Dev V (cubic feet): 467 801 983 1247 236 Pre-Development Runoff: 10-Year, 2 Hour Storm Duration Time: 7200.00 sec V10YR/2HR : 801 cf *Values are from COB Design Standards Figure 2 - Rainfall Intensity Druation in Minutes L:\213611\Computations\STORMWATER\Stormwater Calculations Retention WEST BABCOCK APARTMENTS ON-SITE RETENTION SIZING Page 3 of 4 10/27/2022 By Miguel Ley, EI RETENTION CHAMBER: EXHIBIT B Design Storm: 10-Year, 2 Hour (City of Bozeman) Basin Area (ac): 0.41 Post-Dev C (weighted): 0.90 Storm Duration (min)120.00 Runoff Rate (Rational Method): Storm Event (yr): 2-yr, 2-hr 10-yr, 2-hr 25-yr, 2-hr 100-yr, 2-hr 100-yr, 24-hr *Storm Coeff. A: 0.36 0.64 0.78 1.01 1.01 *Storm Coeff. B:0.60 0.65 0.64 0.67 0.67 Composite C: 0.90 0.90 0.90 0.90 0.90 Storm Intensity (in/hr): 0.24 0.41 0.50 0.63 0.12 Post-Dev Qp (cfs): 0.09 0.15 0.18 0.23 0.04 Pond Size: 10-Year, 2 Hour Storm 100 Year Storm (Includes Infiltration) Duration Time: 7200.00 sec V100YR/2HR : 1687 cf V100YR/24HR : 3829 cf V10YR/2HR : 1084 cf Infiltration Area: 552 sf Infiltration Area Area:552 sf Infiltation Rate: 2.6 in/hr Infiltation Rate: 2.6 in/hr Volume Infltrated : 239 cf Volume Infltrated : 2870 cf Max CMP Storage: 1557 cf Max CMP Storage: 1557 cf Max CMP Capacity 1796 cf Max CMP Capacity 4427 cf *Values are from COB Design Standards Figure 2 - Rainfall Intensity Druation in Minutes Notes: 30% Void space used in CMP Storage Infiltration rate from DEQ 8 L:\213611\Computations\STORMWATER\Stormwater Calculations Retention WEST BABCOCK APARTMENTS POST-DEVELOPMENT RUNOFF Page 4 of 4 10/27/2022 By Miguel Ley, EI POST DEVELOPMENT CONDITIONS: EXHIBIT C Design Storm: 10-Year, 2 Hour (City of Bozeman) Basin Area (ac): 0.19 Pre-Dev C (weighted): 0.70 Storm Duration (min)120.00 Runoff Rate (Rational Method): Storm Event (yr): 2-yr, 2-hr 10-yr, 2-hr 25-yr, 2-hr 100-yr, 2-hr 100-yr, 24-hr *Storm Coeff. A: 0.36 0.64 0.78 1.01 1.01 *Storm Coeff. B:0.60 0.65 0.64 0.67 0.67 Composite C: 0.70 0.70 0.70 0.70 0.70 Storm Intensity (in/hr): 0.24 0.41 0.50 0.63 0.12 Pre-Dev Qp (cfs): 0.03 0.06 0.07 0.09 0.02 Pre-Dev V (cubic feet): 233 400 491 623 118 Pre-Development Runoff: 10-Year, 2 Hour Storm Duration Time: 7200.00 sec V10YR/2HR : 400 cf *Values are from COB Design Standards Figure 2 - Rainfall Intensity Druation in Minutes L:\213611\Computations\STORMWATER\Stormwater Calculations Retention