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HomeMy WebLinkAboutAppendix D - Stormwater Design Report - Flanders Mill DESIGN REPORT STORMWATER MANAGEMENT FLANDERS MILL SUBDIVISION Prepared for: Flanders Mill, LLC 235 Greenhills Ranch Road, Bozeman, MT 59718 Prepared by: C&H Engineering and Surveying, Inc. 1091 Stoneridge Drive, Bozeman, MT 59718 (406) 587-1115 Project Number: 13067 April 2014 Design Report - Page 2 of 8 INTRODUCTION The proposed Flanders Mill Subdivision is a major subdivision located on a 136.75-acre parcel legally described as Tract 1 of Certificate of Survey No. 2834 located in the East Half of the West Half of Section 3, Township 2 South, Range 5 East of P.M.M., City of Bozeman, Gallatin County, Montana. It is bordered by Baxter Lane to the north, Ferguson Avenue to the east, and Flanders Mill Road to the west. The proposed subdivision includes 292 single family lots on 55.33 acres (zoned R-3) and two multi-family lots on 5.42 acres (zoned R-4). STORMWATER MANAGEMENT Stormwater runoff from the subdivision will be conveyed to numerous retention and detention facilities on site via surface flow in catch curb and gutter to various storm sewer curb inlets. The site is split up into 31 drainage areas, which are identified on the Stormwater Facilities Plan found in Appendix B. Drainage Areas 1-11 contribute to the part of the site located south of Oak St. Drainage Areas 12-31 contribute to the part of the site located north of Oak Street. There is a large groundwater lake proposed to be built just south of Oak St. This lake will provide detention volume for nearly the entire southern part of the site. The remainder (Drainage Area #6) of runoff from the south part of the site will be detained in an existing pond that has an outlet structure. The outlet structure will be routed to discharge into the new creek. There are four proposed detention ponds and two proposed retention ponds to control stormwater runoff in the north part of the site. Existing Detention Pond – South of Oak St. A 16,931 ft3 detention pond was constructed in 2006 near the southwest intersection of Ferguson Ave. and Oak St. during the construction of Phases II and III of Diamond Estates Subdivision No. 2. Phase III of the subdivision, which included over 100 residential lots within the proposed Flanders Mill Subdivision (south of Oak and west of Ferguson), was never constructed after the economic recession. The original design of the detention pond included a contributing area within the proposed Flanders Mill Subdivision of 15.35 acres with a composite runoff coefficient of 0.33 (Water, Design Report - Page 3 of 8 Sewer, Storm Drainage, & Street Improvements Design Report; 2006, Gaston Engineering). The new proposal reduces the contributing area from the development to 3.87 acres with a runoff coefficient of 0.61 (Drainage Area #6). The existing detention pond will not require modifications as the proposed runoff volume into the pond has been reduced from the original design. The Diamond Estates Storm Drainage Area exhibit for the existing detention pond is provided in Appendix C (2006, Gaston Engineering). Detention Pond – Man-Made Lake Detention Facility – South of Oak St. A groundwater lake (Man-Made Lake) is proposed within Park #1. The Man-Made Lake has been designed to have adequate capacity above groundwater table elevations to manage stormwater flows from the south part of the development. Collected runoff will discharge to the Lake at two locations: from Tanzanite Drive (contributing Drainage Areas 1, 2, 3, 4, 5, 7 and 8), and from Oak Street (contributing Drainage Areas 9, 10 and 11). The pre-development runoff rate for this 50.65-acre area was calculated to be 6.21 cfs for the 10-year storm. The required detention storage volume for the Lake was calculated as 26,676 ft3, which limits the discharge of post-development stormwater runoff to the pre-development rate of 6.21 cfs. The footprint of the proposed lake is 112,762 ft2. The stormwater additions will cause the Lake water level to rise 0.24 ft conservatively assuming that the embankment slopes rise straight up. The Lake will have a minimum of 0.5 ft of freeboard to account for the stormwater additions and wind/wave action. Supporting stormwater calculations can be found in Appendix A. Detention Pond #1 – North of Oak St. Detention Pond #1 is located to the southeast of Park #2 (the center oval park), as shown on the Stormwater Facilities Plan in Appendix B. It receives runoff from Drainage Areas 12 and 13, totaling 16.99 acres. The pre-development runoff rate for this 16.99-acre area was calculated to be 2.15 cfs for the 10-year storm. The required storage volume for Detention Pond #1 was calculated as 11,277 ft3, which limits the discharge of post-development stormwater runoff to the pre-development rate of 2.15 cfs. Due to the preliminary nature of this report, Detention Pond #1 is shown as 21,233 ft3 on the Stormwater Facilities Plan. This oversizing is to account for potential changes in the stormwater management plan. Supporting stormwater calculations can be found in Design Report - Page 4 of 8 Appendix A. Detention Pond #2 – North of Oak St. Detention Pond #2 is located in the northwest corner of the site, immediately east of Flanders Mill Rd., as shown on the Stormwater Facilities Plan in Appendix B. It receives runoff from Drainage Areas 14, 15, 16, 17, 29, 30 and 31, totaling 19.47 acres. The pre-development runoff rate for this 19.47-acre area was calculated to be 2.19 cfs for the 10-year storm. The required storage volume for Detention Pond #2 was calculated as 13,096 ft3, which limits the discharge of post-development stormwater runoff to the pre-development rate of 2.19 cfs. Due to the preliminary nature of this report, Detention Pond #2 is shown as 15,591 ft3 on the Stormwater Facilities Plan. This oversizing is to account for potential changes in the stormwater management plan. Supporting stormwater calculations can be found in Appendix A. Detention Pond #3 – North of Oak St. Detention Pond #3 is located in the northwest corner of the site, immediately to the south of Baxter Lane, as shown on the Stormwater Facilities Plan in Appendix B. It receives runoff from Drainage Areas 18, 26, 27, and 28, totaling 11.63 acres. The pre-development runoff rate for this 11.63-acre area was calculated to be 1.46 cfs for the 10-year storm. The required storage volume for Detention Pond #3 was calculated as 7,147 ft3, which limits the discharge of post-development stormwater runoff to the pre-development rate of 1.46 cfs. Due to the preliminary nature of this report, Detention Pond #3 is shown as 11,709 ft3 on the Stormwater Facilities Plan. This oversizing is to account for potential changes in the stormwater management plan. Supporting stormwater calculations can be found in Appendix A. Detention Pond #4 – North of Oak St. Detention Pond #4 is located in the northeast corner of the site, immediately south of Baxter Lane, as shown on the Stormwater Facilities Plan in Appendix B. It receives runoff from Drainage Areas 20, 21, 22, and 25, totaling 10.25 acres. The pre-development runoff rate for this 10.25-acre area was calculated to be 1.34 cfs for the 10-year storm. The required storage volume for Detention Pond #4 was calculated as 6,613 ft3, which limits the discharge of post-development stormwater runoff to the pre-development rate of 1.34 cfs. Due to the preliminary nature of this report, Design Report - Page 5 of 8 Detention Pond #4 is shown as 7,509 ft3 on the Stormwater Facilities Plan. This oversizing is to account for potential changes in the stormwater management plan. Supporting stormwater calculations can be found in Appendix A. Retention Pond #1 – North of Oak St. Retention Pond #1 is located in the northeast corner of the site, immediately to the west of Ferguson Avenue, as shown on the Stormwater Facilities Plan in Appendix B. It receives runoff from Drainage Area 23, totaling 1.99 acres. The runoff volume for the 10-year, 2-hour storm was calculated to be 2,726 ft3. Due to the preliminary nature of this report, Retention Pond #1 is shown as 16,347 ft3 on the Stormwater Facilities Plan. This oversizing is to account for potential changes in the stormwater management plan. Supporting stormwater calculations can be found in Appendix A. Retention Pond #2 – North of Oak St. Retention Pond #2 is located within Park #2 (the center oval park), as shown on the Stormwater Facilities Plan in Appendix B. It receives runoff from Drainage Area 19, totaling 2.53 acres. The runoff volume for the 10-year, 2-hour storm was calculated to be 2,624 ft3. The pond is sized to hold 2,663 ft3 of stormwater. Supporting stormwater calculations can be found in Appendix A. Design Report - Page 6 of 8 APPENDIX A SUPPORTING CALCULATIONS EXISTING DETENTION POND - SOUTH OF OAK Stormwater Design Calculations Calculate required volume Q = CiA V=7200Q Calculate Weighted C - Post Development condition ROW Component Width (ft)C FACTOR Road/C&G/Sidewalk 41 0.95 Landscaping 19 0.2 Total 60 ROW Weighted C=0.71 Drainage Area 6 Area (acre)C FACTOR Low-Den Residential 0.80 0.35 High-Den Residential 0.00 0.50 Open Space/Park 0.20 0.20 ROW 2.87 0.71 Total 3.87 Weighted C6=0.61 Retention Pond Summary Total Area = 3.87 acres Weighted C = 0.61 Rainfall intensity = 0.41 in./hr. (C.O.B Design Standards 10-yr, 2-hr storm) Runoff, Q = 1.0 cfs Volume, V = 6983 ft2 MAN-MADE LAKE DETENTION FACILITY Stormwater Design Calculations Calculate T c - Pre Development condition Overland Flow Tc = 1.87 (1.1-CCf)D1/2/S1/3 S = Slope of Basin (%) 1.20 Storm Return C = Rational Method Runoff Coefficient 0.2 2 to 10 Cf = Frequency Adjustment Factor 1 11 to 25 D = Length of Basin (ft) 1639.5 26 to 50 51 to 100 Tc = Time of concentration (minutes)64.10 Calculate Rainfall intensity (in./hr.) - Pre Development condition i = 0.64x-0.65 (See Figure I-3, Bozeman Design Standards and Specs - 10 yr storm) x = storm duration (a.k.a. Tc)1.07 hrs i = rainfall intensity (in./hr.) 0.613 Calculate Runoff (cfs) - Pre Development condition Q = CiA C = Rational Method Runoff Coefficient 0.2 i = rainfall intensity (in./hr.) 0.613 A = Area (acres) 50.65 Q = Runoff (cfs) 6.21 Calculate Weighted C - Post Development condition ROW Component Width (ft)C FACTOR Road/C&G/Sidewalk 41 0.95 Landscaping 19 0.2 Total 60 ROW Weighted C=0.71 Drainage Area 1 Area (acre)C FACTOR Low-Den Residential 12.49 0.35 High-Den Residential 0.00 0.50 Open Space/Park 0.76 0.20 ROW 4.77 0.71 Total 18.01 Weighted C1=0.44 Drainage Area 2 Area (acre)C FACTOR Low-Den Residential 0.00 0.35 High-Den Residential 0.00 0.50 Open Space/Park 0.00 0.20 MAN-MADE LAKE DETENTION FACILITY Stormwater Design Calculations ROW 0.55 0.71 Total 0.55 Weighted C2=0.71 Drainage Area 3 Area (acre)C FACTOR Low-Den Residential 0.00 0.35 High-Den Residential 0.00 0.50 Open Space/Park 0.00 0.20 ROW 0.39 0.71 Total 0.39 Weighted C3=0.71 Drainage Area 4 Area (acre)C FACTOR Low-Den Residential 7.74 0.35 High-Den Residential 0.00 0.50 Open Space/Park 0.56 0.20 ROW 2.65 0.71 Total 10.96 Weighted C4=0.43 Drainage Area 5 Area (acre)C FACTOR Low-Den Residential 2.45 0.35 High-Den Residential 0.00 0.50 Open Space/Park 0.00 0.20 ROW 1.00 0.71 Total 3.45 Weighted C5=0.46 Drainage Area 7 Area (acre)C FACTOR Low-Den Residential 0.00 0.35 High-Den Residential 0.00 0.50 Open Space/Park 0.00 0.20 ROW 0.43 0.71 Total 0.43 Weighted C7=0.71 Drainage Area 8 Area (acre)C FACTOR Low-Den Residential 0.00 0.35 High-Den Residential 0.00 0.50 Open Space/Park 0.00 0.20 ROW 0.32 0.71 Total 0.32 MAN-MADE LAKE DETENTION FACILITY Stormwater Design Calculations Weighted C8=0.71 Drainage Area 9 Area (acre)C FACTOR Low-Den Residential 0.00 0.35 High-Den Residential 0.00 0.50 Open Space/Park 10.31 0.20 ROW 0.00 0.71 Total 10.31 Weighted C9=0.20 Drainage Area 10 Area (acre)C FACTOR Low-Den Residential 2.01 0.35 High-Den Residential 0.00 0.50 Open Space/Park 0.74 0.20 ROW 2.03 0.71 Total 4.78 Weighted C10=0.48 Drainage Area 11 Area (acre)C FACTOR Low-Den Residential 0.00 0.35 High-Den Residential 0.00 0.50 Open Space/Park 0.00 0.20 ROW 1.43 0.71 Total 1.43 Weighted C11=0.71 Lake Detention (from south) Summary Total Area = 50.65 acres Weighted C = 0.41 Release = 6.21 cfs Determine required storage volume (ft 3 ) in detention pond Duration(min) Duration(hrs) Intensity (in/hr)Qin (cfs)Runoff Volume Release Volume Required Storage (ft3) 35 0.58 0.91 18.89 39676 13041 26635 36 0.60 0.89 18.55 40070 13414 26656 37 0.62 0.88 18.22 40456 13786 26669 38 0.63 0.86 17.91 40835 14159 26676 39 0.65 0.85 17.61 41208 14532 26676 40 0.67 0.83 17.32 41575 14904 26671 41 0.68 0.82 17.05 41936 15277 26659 42 0.70 0.81 16.78 42291 15649 26641 43 0.72 0.79 16.53 42641 16022 26618 44 0.73 0.78 16.28 42985 16395 26590 MAN-MADE LAKE DETENTION FACILITY Stormwater Design Calculations Determine Lake water level rise Lake Footprint= 112762 ft2 Embankment Slopes= 1V:0H conservative assumption Lake Water Level Rise= 0.24 ft DETENTION POND #1 Stormwater Design Calculations Calculate T c - Pre Development condition Overland Flow Tc = 1.87 (1.1-CCf)D1/2/S1/3 S = Slope of Basin (%) 1.26 Storm Return 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) 1537 26 to 50 1.2 51 to 100 1.25 Tc = Time of concentration (minutes)61.16 Calculate Rainfall intensity (in./hr.) - Pre Development condition i = 0.64x-0.65 (See Figure I-3, Bozeman Design Standards and Specs - 10 yr storm) x = storm duration (a.k.a. Tc)1.02 hrs i = rainfall intensity (in./hr.) 0.632 Calculate Runoff (cfs) - Pre Development condition Q = CiA C = Rational Method Runoff Coefficient 0.2 i = rainfall intensity (in./hr.) 0.632 A = Area (acres) 16.99 Q = Runoff (cfs) 2.15 Calculate Weighted C - Post Development condition ROW Component Width (ft)C FACTOR Road/C&G/Sidewalk 41 0.95 Landscaping 19 0.2 Total 60 ROW Weighted C=0.71 Drainage Area 12 Area (acre)C FACTOR Low-Den Residential 9.33 0.35 High-Den Residential 0.00 0.50 Open Space/Park 1.03 0.20 ROW 5.20 0.71 Total 15.55 Weighted C12=0.46 Drainage Area 13 Area (acre)C FACTOR Low-Den Residential 0.00 0.35 High-Den Residential 0.00 0.50 Open Space/Park 0.00 0.20 DETENTION POND #1 Stormwater Design Calculations ROW 1.43 0.71 Total 1.43 Weighted C13=0.71 Detention Pond #1 Summary Total Area = 16.99 acres Weighted C = 0.48 Release = 2.15 cfs Determine required storage volume (ft 3 ) in detention pond Duration(min) Duration(hrs) Intensity (in/hr)Qin (cfs)Runoff Volume Release Volume Required Storage (ft3) 44 0.73 0.78 6.42 16937 5669 11268 45 0.75 0.77 6.32 17071 5798 11273 46 0.77 0.76 6.23 17202 5927 11276 47 0.78 0.75 6.15 17332 6055 11277 48 0.80 0.74 6.06 17461 6184 11276 49 0.82 0.73 5.98 17587 6313 11274 50 0.83 0.72 5.90 17712 6442 11270 51 0.85 0.71 5.83 17835 6571 11264 52 0.87 0.70 5.76 17957 6700 11257 53 0.88 0.69 5.68 18077 6829 11248 DETENTION POND #2 Stormwater Design Calculations Calculate T c - Pre Development condition Overland Flow Tc = 1.87 (1.1-CCf)D1/2/S1/3 S = Slope of Basin (%) 1.09 Storm Return 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) 2010 26 to 50 1.2 51 to 100 1.25 Tc = Time of concentration (minutes)73.22 Calculate Rainfall intensity (in./hr.) - Pre Development condition i = 0.64x-0.65 (See Figure I-3, Bozeman Design Standards and Specs - 10 yr storm) x = storm duration (a.k.a. Tc)1.22 hrs i = rainfall intensity (in./hr.) 0.562 Calculate Runoff (cfs) - Pre Development condition Q = CiA C = Rational Method Runoff Coefficient 0.2 i = rainfall intensity (in./hr.) 0.562 A = Area (acres)19.47 Q = Runoff (cfs)2.19 Calculate Weighted C - Post Development condition ROW Component Width (ft)C FACTOR Road/C&G/Sidewalk 41 0.95 Landscaping 19 0.2 Total 60 ROW Weighted C=0.71 Drainage Area 14 Area (acre)C FACTOR Low-Den Residential 0.83 0.35 High-Den Residential 5.50 0.50 Open Space/Park 0.00 0.20 ROW 1.68 0.71 Total 8.01 Weighted C14=0.53 Drainage Area 15 Area (acre)C FACTOR Low-Den Residential 1.98 0.35 High-Den Residential 0.00 0.50 Open Space/Park 0.00 0.20 DETENTION POND #2 Stormwater Design Calculations ROW 0.78 0.71 Total 2.76 Weighted C15=0.45 Drainage Area 16 Area (acre)C FACTOR Low-Den Residential 0.73 0.35 High-Den Residential 0.00 0.50 Open Space/Park 0.30 0.20 ROW 1.15 0.71 Total 2.18 Weighted C16=0.52 Drainage Area 17 Area (acre)C FACTOR Low-Den Residential 0.93 0.35 High-Den Residential 0.00 0.50 Open Space/Park 0.00 0.20 ROW 0.54 0.71 Total 1.47 Weighted C17=0.48 Drainage Area 29 Area (acre)C FACTOR Low-Den Residential 1.73 0.35 High-Den Residential 0.00 0.50 Open Space/Park 0.99 0.20 ROW 0.00 0.71 Total 2.71 Weighted C29=0.30 Drainage Area 30 Area (acre)C FACTOR Low-Den Residential 0.20 0.35 High-Den Residential 0.00 0.50 Open Space/Park 0.00 0.20 ROW 0.10 0.71 Total 0.30 Weighted C30=0.47 Drainage Area 31 Area (acre)C FACTOR Low-Den Residential 1.73 0.35 High-Den Residential 0.00 0.50 Open Space/Park 0.00 0.20 ROW 0.30 0.71 Total 2.02 DETENTION POND #2 Stormwater Design Calculations Weighted C31=0.40 Detention Pond #2 Summary Total Area = 19.47 acres Weighted C = 0.47 Release = 2.19 cfs Determine required storage volume (ft 3 ) in detention pond Duration(min) Duration(hrs) Intensity (in/hr)Qin (cfs)Runoff Volume Release Volume Required Storage (ft3) 50 0.83 0.72 6.55 19653 6568 13085 51 0.85 0.71 6.47 19790 6699 13090 52 0.87 0.70 6.39 19925 6831 13094 53 0.88 0.69 6.31 20058 6962 13096 54 0.90 0.69 6.23 20190 7094 13096 55 0.92 0.68 6.16 20320 7225 13095 56 0.93 0.67 6.09 20448 7356 13092 57 0.95 0.66 6.02 20575 7488 13088 58 0.97 0.65 5.95 20701 7619 13082 59 0.98 0.65 5.88 20825 7750 13075 DETENTION POND #3 Stormwater Design Calculations Calculate T c - Pre Development condition Overland Flow Tc = 1.87 (1.1-CCf)D1/2/S1/3 S = Slope of Basin (%) 1.09 Storm Return 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) 1426 26 to 50 1.2 51 to 100 1.25 Tc = Time of concentration (minutes)61.68 Calculate Rainfall intensity (in./hr.) - Pre Development condition i = 0.64x-0.65 (See Figure I-3, Bozeman Design Standards and Specs - 10 yr storm) x = storm duration (a.k.a. Tc)1.03 hrs i = rainfall intensity (in./hr.) 0.629 Calculate Runoff (cfs) - Pre Development condition Q = CiA C = Rational Method Runoff Coefficient 0.2 i = rainfall intensity (in./hr.) 0.629 A = Area (acres) 11.63 Q = Runoff (cfs) 1.46 Calculate Weighted C - Post Development condition ROW Component Width (ft)C FACTOR Road/C&G/Sidewalk 41 0.95 Landscaping 19 0.2 Total 60 ROW Weighted C=0.71 Drainage Area 18 Area (acre)C FACTOR Low-Den Residential 4.60 0.35 High-Den Residential 0.00 0.50 Open Space/Park 0.37 0.20 ROW 2.42 0.71 Total 7.39 Weighted C18=0.46 Drainage Area 26 Area (acre)C FACTOR Low-Den Residential 0.00 0.35 High-Den Residential 0.00 0.50 Open Space/Park 0.00 0.20 DETENTION POND #3 Stormwater Design Calculations ROW 0.73 0.71 Total 0.73 Weighted C26=0.71 Drainage Area 27 Area (acre)C FACTOR Low-Den Residential 0.91 0.35 High-Den Residential 0.00 0.50 Open Space/Park 0.47 0.20 ROW 0.00 0.71 Total 1.38 Weighted C27=0.30 Drainage Area 28 Area (acre)C FACTOR Low-Den Residential 0.71 0.35 High-Den Residential 0.00 0.50 Open Space/Park 0.54 0.20 ROW 0.88 0.71 Total 2.13 Weighted C28=0.46 Detention Pond #3 Summary Total Area = 11.63 acres Weighted C = 0.46 Release = 1.46 cfs Determine required storage volume (ft 3 ) in detention pond Duration(min) Duration(hrs) Intensity (in/hr)Qin (cfs)Runoff Volume Release Volume Required Storage (ft3) 40 0.67 0.83 4.44 10646 3509 7137 41 0.68 0.82 4.37 10738 3597 7141 42 0.70 0.81 4.30 10829 3684 7144 43 0.72 0.79 4.23 10918 3772 7146 44 0.73 0.78 4.17 11007 3860 7147 45 0.75 0.77 4.11 11094 3948 7146 46 0.77 0.76 4.05 11179 4035 7144 47 0.78 0.75 3.99 11264 4123 7141 48 0.80 0.74 3.94 11347 4211 7136 49 0.82 0.73 3.89 11429 4299 7131 DETENTION POND #4 Stormwater Design Calculations Calculate T c - Pre Development condition Overland Flow Tc = 1.87 (1.1-CCf)D1/2/S1/3 S = Slope of Basin (%) 1.21 Storm Return 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) 1365 26 to 50 1.2 51 to 100 1.25 Tc = Time of concentration (minutes)58.37 Calculate Rainfall intensity (in./hr.) - Pre Development condition i = 0.64x-0.65 (See Figure I-3, Bozeman Design Standards and Specs - 10 yr storm) x = storm duration (a.k.a. Tc)0.97 hrs i = rainfall intensity (in./hr.) 0.652 Calculate Runoff (cfs) - Pre Development condition Q = CiA C = Rational Method Runoff Coefficient 0.2 i = rainfall intensity (in./hr.) 0.652 A = Area (acres) 10.25 Q = Runoff (cfs) 1.34 Calculate Weighted C - Post Development condition ROW Component Width (ft)C FACTOR Road/C&G/Sidewalk 41 0.95 Landscaping 19 0.2 Total 60 ROW Weighted C=0.71 Drainage Area 21 Area (acre)C FACTOR Low-Den Residential 2.45 0.35 High-Den Residential 0.00 0.50 Open Space/Park 0.08 0.20 ROW 0.78 0.71 Total 3.31 Weighted C21=0.43 Drainage Area 22 Area (acre)C FACTOR Low-Den Residential 2.69 0.35 High-Den Residential 0.00 0.50 Open Space/Park 0.08 0.20 DETENTION POND #4 Stormwater Design Calculations ROW 2.94 0.71 Total 5.70 Weighted C22=0.53 Drainage Area 25 Area (acre)C FACTOR Low-Den Residential 0.71 0.35 High-Den Residential 0.00 0.50 Open Space/Park 0.39 0.20 ROW 0.00 0.71 Total 1.10 Weighted C25=0.30 Drainage Area 20 Area (acre)C FACTOR Low-Den Residential 0.00 0.35 High-Den Residential 0.00 0.50 Open Space/Park 0.00 0.20 ROW 0.14 0.71 Total 0.14 Weighted C20=0.71 Detention Pond #4 Summary Total Area = 10.25 acres Weighted C = 0.48 Release = 1.34 cfs Determine required storage volume (ft 3 ) in detention pond Duration(min) Duration(hrs) Intensity (in/hr)Qin (cfs)Runoff Volume Release Volume Required Storage (ft3) 40 0.67 0.83 4.09 9806 3206 6601 41 0.68 0.82 4.02 9892 3286 6606 42 0.70 0.81 3.96 9975 3366 6609 43 0.72 0.79 3.90 10058 3446 6612 44 0.73 0.78 3.84 10139 3526 6613 45 0.75 0.77 3.78 10219 3607 6613 46 0.77 0.76 3.73 10298 3687 6611 47 0.78 0.75 3.68 10376 3767 6609 48 0.80 0.74 3.63 10453 3847 6606 49 0.82 0.73 3.58 10528 3927 6601 RETENTION POND #1 Stormwater Design Calculations Calculate required volume Q = CiA V=7200Q Calculate Weighted C - Post Development condition ROW Component Width (ft)C FACTOR Road/C&G/Sidewalk 41 0.95 Landscaping 19 0.2 Total 60 ROW Weighted C=0.71 Drainage Area 23 Area (acre) C FACTOR Low-Den Residential 0.00 0.35 High-Den Residential 0.00 0.50 Open Space/Park 0.96 0.20 ROW 1.03 0.71 Total 1.99 Weighted C23=0.47 Retention Pond #1 Summary Total Area = 1.99 acres Weighted C = 0.47 Rainfall intensity = 0.41 in./hr. (C.O.B Design Standards 10-yr, 2-hr storm) Runoff, Q = 0.4 cfs Volume, V = 2726 ft3 RETENTION POND #2 Stormwater Design Calculations Calculate required volume Q = CiA V=7200Q Calculate Weighted C - Post Development condition ROW Component Width (ft)C FACTOR Road/C&G/Sidewalk 41 0.95 Landscaping 19 0.2 Total 60 ROW Weighted C=0.71 Drainage Area 19 Area (acre) C FACTOR Low-Den Residential 0.00 0.35 High-Den Residential 0.00 0.50 Open Space/Park 1.78 0.20 ROW 0.75 0.71 Total 2.53 Weighted C19=0.35 Retention Pond #2 Summary Total Area = 2.53 acres Weighted C = 0.35 Rainfall intensity = 0.41 in./hr. (C.O.B Design Standards 10-yr, 2-hr storm) Runoff, Q = 0.4 cfs Volume, V = 2624 ft3 Design Report - Page 7 of 8 APPENDIX B STORMWATER FACILITIES PLAN Design Report - Page 8 of 8 APPENDIX C STORM DRAINAGE AREA EXHIBIT –AREA 7 DIAMOND ESTATES SUBDIVISION PHASES II & III 2006 GASTON ENGINEERING