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HomeMy WebLinkAboutTown&CountryStormReport-Revised011421 STORMWATER MANAGEMENT DESIGN REPORT for LOT 4, SPRING CREEK VILLAGE RESORT SUBDIVISION (MINOR SUBDIVISION NO. 295A) Prepared for: Delaney & Company, Inc. 101 East Main Street, Bozeman, MT 59715 Prepared by: C & H Engineering and Surveying, Inc. 1091 Stoneridge Drive Bozeman, MT 59718 (406) 587-1115 Project No.: 11019 MARCH 2014 Page 1 of 25 INTRODUCTION Lot 4, Spring Creek Village Resort Subdivision is located north of Huffine Lane (U.S. Highway 191), south of Fallon Street, west of Resort Drive and east of Cottonwood Road in Bozeman, Montana. The 19.9621-acre development will consist of 20 commercial lots. This project will include construction of all public improvements within the subdivision and will tie into existing City of Bozeman water, sewer, and road networks. Construction of the public improvements will take place first, which includes the construction of public streets, water, sanitary sewer, and storm sewer facilities. The individual lots will be developed after all improvements are in place. STORMWATER MANAGEMENT Stormwater runoff from the site will be directed by a storm sewer system and gutter flow to retention ponds. These retention ponds will filter sediment and oils from the stormwater runoff. The retention ponds are sized for the 10-year, 2-hour storm event while all storm sewer piping is designed for the 25-year storm. STORMWATER RUNOFF (POST DEVELOPMENT) In accordance with City of Bozeman policy, the stormwater retention ponds must be sized for a storm intensity having a 10-year frequency and a 2-hour duration. The equation used to model the intensity of the 10-year frequency storm is given by the City of Bozeman Design Standards and Specifications Policy (COB DSSP). The design intensity for this report was calculated as follows: i = 0.64(t-0.65) i = rainfall intensity (in/hr) t = 2 hrs t = storm duration (hours) i = 0.64(2-0.65) = 0.408 in/hr = 0.034 ft/hr d = 2 hr water depth = 0.068 ft The stormwater runoff for all drainage areas on the attached drainage exhibit were calculated as follows: The stormwater runoff rate was calculated with the rational formula as shown. A runoff coefficient (C) of 0.85 was applied to roof areas. A runoff coefficient (C) of 0.90 was applied to hardscaping. A runoff coefficient (C) of 0.20 was applied to landscaping. The stormwater runoff rate for each respective drainage area was calculated as follows: Q = CiA Q = stormwater runoff rate (cfs) i = (0.408 in/hr)x(1 ft/12 in)x( 2 hr) C = weighted runoff coefficient d = 0.068 ft i = stormwater intensity (in/hr) A = surface area of drainage area (ft2) d = depth V = CdA V = volume (ft3) Page 2 of 25 Drainage Area 1: Roof Area = 0 ft2 Hardscape Area = 89,996 ft2 Landscape Area = 32,141 ft2 __________ TOTAL = 122,137 ft2 Weighted Runoff Coefficient: C = (0 ft2 x 0.85)+(89,996 ft2x0.90)+(32,141 ft2x0.20) 122,137 ft2 C = 0.72 Runoff Volume: V = 0.068x0.72x122,137 ft2 = 5,945 ft3 Drainage Area 2: Roof Area = 5,705 ft2 Hardscape Area = 0 ft2 Landscape Area = 0 ft2 __________ TOTAL = 5,705 ft2 Weighted Runoff Coefficient: C = (5,705 ft2 x 0.85)+(0 ft2x0.90)+(0 ft2x0.20) 5,705 ft2 C = 0.85 Runoff Volume V = 0.068x0.85x5,705ft2 = 419 ft3 Drainage Area 3: Roof Area = 8,072 ft2 Hardscape Area = 0 ft2 Landscape Area = 0 ft2 __________ TOTAL = 8,072 ft2 Page 3 of 25 Weighted Runoff Coefficient: C = (8,072 ft2 x 0.85)+(0 ft2x0.90)+(0 ft2x0.20) 8,072 ft2 C = 0.85 Runoff Volume V = 0.068x0.85x8,072 ft2 = 192 ft3 Drainage Area 4: Roof Area = 13,281 ft2 Hardscape Area = 0 ft2 Landscape Area = 0 ft2 __________ TOTAL = 13,281 ft2 Weighted Runoff Coefficient: C = (13,281 ft2 x 0.85)+(0 ft2x0.90)+(0 ft2x0.20) 13,281 ft2 C = 0.85 Runoff Volume V = 0.068x0.85x13,281 ft2 = 768 ft3 Drainage Area 5: Roof Area = 4,355 ft2 Hardscape Area = 0 ft2 Landscape Area = 0 ft2 __________ TOTAL = 4,355 ft2 Weighted Runoff Coefficient: C = (4,355 ft2 x 0.85)+(0 ft2x0.90)+(0 ft2x0.20) 4,355 ft2 C = 0.85 Page 4 of 25 Runoff Volume V = 0.068x0.85x4,355 ft2 = 314 ft3 Drainage Area 6: Roof Area = 0 ft2 Hardscape Area = 40,647 ft2 Landscape Area = 15,369 ft2 __________ TOTAL = 56,015 ft2 Weighted Runoff Coefficient: C = (0 ft2 x 0.85)+(40,647 ft2x0.90)+(15,369 ft2x0.20) 56,015 ft2 C = 0.71 Runoff Volume V = 0.068x0.71x56,015 ft2 = 2,697 ft3 Drainage Area 7: Roof Area = 8,792 ft2 Hardscape Area = 0 ft2 Landscape Area = 0 ft2 __________ TOTAL = 8,792 ft2 Weighted Runoff Coefficient: C = (8,792 ft2 x 0.85)+(0 ft2x0.90)+(0 ft2x0.20) 8,792 ft2 C = 0.85 Runoff Volume V = 0.068x0.85x8,792 ft2 = 508 ft3 Drainage Area 8: Roof Area = 9,219 ft2 Hardscape Area = 19,005 ft2 Landscape Area = 17,958 ft2 Page 5 of 25 __________ TOTAL = 46,182 ft2 Weighted Runoff Coefficient: C = (9,219 ft2 x 0.85)+(19,005 ft2x0.90)+(17,958 ft2x0.20) 46,182 ft2 C = 0.62 Runoff Volume V = 0.068x0.62x46,182 ft2 = 1,940 ft3 Drainage Area 9: Roof Area = 0 ft2 Hardscape Area = 20,788 ft2 Landscape Area = 6,553 ft2 __________ TOTAL = 27,341 ft2 Weighted Runoff Coefficient: C = (0 ft2 x 0.85)+(20,788 ft2x0.90)+(6,553 ft2x0.20) 27,341 ft2 C = 0.73 Runoff Volume V = 0.068x0.73x27,341 ft2 = 1,361 ft3 Drainage Area 10: Roof Area = 5,718 ft2 Hardscape Area = 0 ft2 Landscape Area = 0 ft2 __________ TOTAL = 5,718 ft2 Weighted Runoff Coefficient: C = (5,718 ft2 x 0.85)+(0 ft2x0.90)+(0 ft2x0.20) 5,718 ft2 Page 6 of 25 C = 0.85 Runoff Volume V = 0.068x0.85x5,718 ft2 = 331 ft3 Drainage Area 11: Roof Area = 8,170 ft2 Hardscape Area = 31,465 ft2 Landscape Area = 10,122 ft2 __________ TOTAL = 49,757 ft2 Weighted Runoff Coefficient: C = (8,170 ft2 x 0.85)+(31,465 ft2x0.90)+(10,122 ft2x0.20) 49,757 ft2 C = 0.75 Runoff Volume V = 0.068x0.75x49,757 ft2 = 2,536 ft3 Drainage Area 12: Roof Area = 0 ft2 Hardscape Area = 22,326 ft2 Landscape Area = 2,385 ft2 __________ TOTAL = 24,712 ft2 Weighted Runoff Coefficient: C = (0 ft2 x 0.85)+(22,326 ft2x0.90)+(2,385 ft2x0.20) 24,712 ft2 C = 0.83 Runoff Volume V = 0.068x0.83x24,712 ft2 = 1,399 ft3 Page 7 of 25 Drainage Area 13: Roof Area = 6,548 ft2 Hardscape Area = 0 ft2 Landscape Area = 0 ft2 __________ TOTAL = 6,548 ft2 Weighted Runoff Coefficient: C = (6,548 ft2 x 0.85)+(0 ft2x0.90)+(0 ft2x0.20) 6,548 ft2 C = 0.85 Runoff Volume V = 0.068x0.85x6,548 ft2 = 378 ft3 Drainage Area 14: Roof Area = 6,560 ft2 Hardscape Area = 0 ft2 Landscape Area = 0 ft2 __________ TOTAL = 6,560 ft2 Weighted Runoff Coefficient: C = (6,560 ft2 x 0.85)+(0 ft2x0.90)+(0 ft2x0.20) 6,560 ft2 C = 0.85 Runoff Volume V = 0.068x0.85x6,560 ft2 = 379 ft3 Drainage Area 15: Roof Area = 8,812 ft2 Hardscape Area = 0 ft2 Landscape Area = 0 ft2 __________ TOTAL = 8,812 ft2 Page 8 of 25 Weighted Runoff Coefficient: C = (8,812 ft2 x 0.85)+(0 ft2x0.90)+(0 ft2x0.20) 8,812 ft2 C = 0.85 Runoff Volume V = 0.068x0.85x8,812 ft2 = 509 ft3 Drainage Area 16: Roof Area = 0 ft2 Hardscape Area = 22,242 ft2 Landscape Area = 2,470 ft2 __________ TOTAL = 24,712 ft2 Weighted Runoff Coefficient: C = (0 ft2 x 0.85)+(22,242 ft2x0.90)+(2,470 ft2x0.20) 24,712 ft2 C = 0.83 Runoff Volume V = 0.068x0.83x24,712 ft2 = 1,395 ft3 Drainage Area 17: Roof Area = 0 ft2 Hardscape Area = 27,266 ft2 Landscape Area = 12,456 ft2 __________ TOTAL = 39,722 ft2 Weighted Runoff Coefficient: C = (0 ft2 x 0.85)+(27,266 ft2x0.90)+(12,456 ft2x0.20) 39,722 ft2 C = 0.68 Page 9 of 25 Runoff Volume V = 0.068x0.68x39,722 ft2 = 1,838 ft3 Drainage Area 18: Roof Area = 8,812 ft2 Hardscape Area = 0 ft2 Landscape Area = 0 ft2 __________ TOTAL = 8,812 ft2 Weighted Runoff Coefficient: C = (8,812 ft2 x 0.85)+(0 ft2x0.90)+(0 ft2x0.20) 8,812 ft2 C = 0.85 Runoff Volume V = 0.068x0.85x8,812 ft2 = 509 ft3 Drainage Area 19: Roof Area = 6,560 ft2 Hardscape Area = 0 ft2 Landscape Area = 0 ft2 __________ TOTAL = 6,560 ft2 Weighted Runoff Coefficient: C = (6,560 ft2 x 0.85)+(0 ft2x0.90)+(0 ft2x0.20) 6,560 ft2 C = 0.85 Runoff Volume V = 0.068x0.85x6,560 ft2 = 379 ft3 Drainage Area 20: Roof Area = 6,548 ft2 Hardscape Area = 0 ft2 Page 10 of 25 Landscape Area = 0 ft2 __________ TOTAL = 6,548 ft2 Weighted Runoff Coefficient: C = (6,548 ft2 x 0.85)+(0 ft2x0.90)+(0 ft2x0.20) 6,548 ft2 C = 0.85 Runoff Volume V = 0.068x0.85x6,548 ft2 = 378 ft3 Drainage Area 21: Roof Area = 0 ft2 Hardscape Area = 132,173 ft2 Landscape Area = 88,238 ft2 __________ TOTAL = 220,411 ft2 Weighted Runoff Coefficient: C = (0 ft2 x 0.85)+( 132,173 ft2x0.90)+( 88,238 ft2x0.20) 220,411 ft2 C = 0.62 Runoff Volume V = 0.068x0.62x220,411ft2 = 9,293 ft3 Drainage Area 21A: Roof Area = 0 ft2 Hardscape Area = 32,654 ft2 Landscape Area = 11,381 ft2 __________ TOTAL = 44,036 ft2 Weighted Runoff Coefficient: C = (0 ft2 x 0.85)+( 32,654 ft2x0.90)+( 11,381 ft2x0.20) 44,036 ft2 Page 11 of 25 C = 0.72 Runoff Volume V = 0.068x0.72x44,036 ft2 = 2,153 ft3 Drainage Area 21B: Roof Area = 0 ft2 Hardscape Area = 40,648 ft2 Landscape Area = 1,022 ft2 __________ TOTAL = 41,669 ft2 Weighted Runoff Coefficient: C = (0 ft2 x 0.85)+( 40,648 ft2x0.90)+( 1,022 ft2x0.20) 41,669 ft2 C = 0.88 Runoff Volume V = 0.068x0.88x41,669ft2 = 2,502 ft3 Drainage Area 21C: Roof Area = 0 ft2 Hardscape Area = 22,410 ft2 Landscape Area = 4,490 ft2 __________ TOTAL = 26,900 ft2 Weighted Runoff Coefficient: C = (0 ft2 x 0.85)+( 22,410 ft2x0.90)+( 4,490 ft2x0.20) 26,900 ft2 C = 0.78 Runoff Volume V = 0.068x0.78x26,900 ft2 = 1,433 ft3 Page 12 of 25 Drainage Area 22: Roof Area = 19,620 ft2 Hardscape Area = 0 ft2 Landscape Area = 0 ft2 __________ TOTAL = 19,620 ft2 Weighted Runoff Coefficient: C = (19,620 ft2 x 0.85)+(0 ft2x0.90)+(0 ft2x0.20) 19,620 ft2 C = 0.85 Runoff Volume V = 0.068x0.85x 19,620 ft2 = 1,134 ft3 Drainage Area 23: Roof Area = 3,299 ft2 Hardscape Area = 0 ft2 Landscape Area = 0 ft2 __________ TOTAL = 3,299 ft2 Weighted Runoff Coefficient: C = (3,299 ft2 x 0.85)+(0 ft2x0.90)+(0 ft2x0.20) 3,299 ft2 C = 0.85 Runoff Volume V = 0.068x0.85x3,299 ft2 = 191 ft3 Drainage Area 24: Roof Area = 4,000 ft2 Hardscape Area = 0 ft2 Landscape Area = 0 ft2 __________ TOTAL = 4,000 ft2 Page 13 of 25 Weighted Runoff Coefficient: C = (4,000 ft2 x 0.85)+(0 ft2x0.90)+(0 ft2x0.20) 4,000 ft2 C = 0.85 Runoff Volume V = 0.068x0.85x4,000 ft2 = 231 ft3 Drainage Area 25: Roof Area = 3,299 ft2 Hardscape Area = 0 ft2 Landscape Area = 0 ft2 __________ TOTAL = 3,299 ft2 Weighted Runoff Coefficient: C = (3,299 ft2 x 0.85)+(0 ft2x0.90)+(0 ft2x0.20) 3,299 ft2 C = 0.85 Runoff Volume V = 0.068x0.85x3,299 ft2 = 191 ft3 Drainage Area 26: Roof Area = 15,186 ft2 Hardscape Area = 0 ft2 Landscape Area = 0 ft2 __________ TOTAL = 15,186 ft2 Weighted Runoff Coefficient: C = (15,186 ft2 x 0.85)+(0 ft2x0.90)+(0 ft2x0.20) 15,186 ft2 C = 0.85 Page 14 of 25 Runoff Volume V = 0.068x0.85x15,186 ft2 = 878 ft3 Drainage Area 27: Roof Area = 0 ft2 Hardscape Area = 22,586 ft2 Landscape Area = 33,650 ft2 __________ TOTAL = 56,236 ft2 Weighted Runoff Coefficient: C = (0 ft2 x 0.85)+(22,586 ft2x0.90)+(33,650 ft2x0.20) 56,236 ft2 C = 0.48 Runoff Volume V = 0.068x0.48x56,236 ft2 = 1,840 ft3 Drainage Area 28: Roof Area = 4,193 ft2 Hardscape Area = 0 ft2 Landscape Area = 0 ft2 __________ TOTAL = 4,193 ft2 Weighted Runoff Coefficient: C = (4,193 ft2 x 0.85)+(0 ft2x0.90)+(0 ft2x0.20) 4,193 ft2 C = 0.85 Runoff Volume V = 0.068x0.85x4,193 ft2 = 242 ft3 Drainage Area 29: Roof Area = 6,401 ft2 Hardscape Area = 0 ft2 Page 15 of 25 Landscape Area = 0 ft2 __________ TOTAL = 6,401 ft2 Weighted Runoff Coefficient: C = (6,401 ft2 x 0.85)+(0 ft2x0.90)+(0 ft2x0.20) 6,401 ft2 C = 0.85 Runoff Volume V = 0.068x0.85x6,401 ft2 = 370 ft3 Drainage Area 30: Roof Area = 8,824 ft2 Hardscape Area = 0 ft2 Landscape Area = 0 ft2 __________ TOTAL = 8,824 ft2 Weighted Runoff Coefficient: C = (8,824 ft2 x 0.85)+(0 ft2x0.90)+(0 ft2x0.20) 8,824 ft2 C = 0.85 Runoff Volume V = 0.068x0.85x8,824 ft2 = 510 ft3 Drainage Area 31: Roof Area = 0 ft2 Hardscape Area = 23,657 ft2 Landscape Area = 6,307 ft2 __________ TOTAL = 29,964 ft2 Weighted Runoff Coefficient: C = (0 ft2 x 0.85)+(23,657 ft2x0.90)+(6,307 ft2x0.20) 29,964 ft2 Page 16 of 25 C = 0.75 Runoff Volume V = 0.068x0.75x29,964 ft2 = 1,534 ft3 Drainage Area 32: Roof Area = 0 ft2 Hardscape Area = 4,877 ft2 Landscape Area = 748 ft2 __________ TOTAL = 5,625 ft2 Weighted Runoff Coefficient: C = (0 ft2 x 0.85)+(4,877 ft2x0.90)+(748 ft2x0.20) 5,625 ft2 C = 0.81 Runoff Volume V = 0.068x0.81x5,625 ft2 = 309 ft3 STORMWATER ROUTING (POST DEVELOPMENT) Stormwater runoff from the drainage areas will be conveyed, via overland, pipe and gutter flow, to retention ponds. The drainage area distribution is detailed below: Retention Ponds 1A and 1B: Drainage Areas flowing into Ponds 1A and 1B include: 1, 6, 12, 21, 21A, 21B, and 21C. Runoff from Resort Drive and Fallon Street (portions of Drainage Area 21) will follow existing patterns and discharge to the existing storm drain inlet on the south side of Fallon Street, directly north of Retention Pond 1A. The existing retention pond will be re-shaped as shown on the associated construction plans. Drainage Areas 1, 6, and 21A will discharge to a storm drain inlet (Inlet 1) on the south side of the intersection of Valley Commons Drive and Boardwalk Avenue. A 15-inch PVC pipe will convey water from Inlet 1 to Inlet 2, located on the north side of the same intersection. Inlet 2 will collect all runoff from Valley Commons Drive (Drainage Area 21B). The combined flows from Inlets 1 and 2 will then flow in an 18-inch PVC pipe to Inlet 3, located on the south side of the Field Street and Boardwalk Avenue intersection. The combined flows from all inlets will then flow via a 21- inch PVC pipe to discharge at the bottom of Retention Pond 1B. The rest of Drainage Area 21 will Page 17 of 25 discharge to the ponds via surface flow. Retention Pond 1B will overflow to Retention Pond 1A if runoff volumes ever exceed Pond 1B’s capacity. Total Volume Required: 25,420 ft3 (sum of runoff volumes from Drainage Areas: 1, 6, 12, 21, 21A, 21B, and 21C) Total Volume Supplied: 20,660 ft3 Deficient Volume: 4,760 ft3 A concrete overflow structure with a 21-inch PVC outlet pipe will be constructed in Pond 1A to convey any overflow from Pond 1A to the existing retention pond in Lot 3A on the north side of Fallon Street. The current capacity of the existing pond on the north side of Fallon Street is 1,350 ft3. The proposed pond expansion will result in a total capacity of 6,387 ft3 – enough to manage existing drainage and the addition of 4,760 ft3. Retention Ponds 2A and 2B: Drainage areas flowing into Ponds 2A and 2B include: 16, 17, 27, and 31. Runoff from Drainage Areas 16, 17, a portion of 27, and 31 will discharge to a new stormwater chase (Stormwater Chase 1 – sizing calculations below) located north of the Field Street and Resort Drive intersection. The chase will route runoff into Retention Pond 2B. Pond 2B will overflow to Pond 2A if runoff volumes ever exceed Pond 2B’s capacity. Volume Required: 6,607 ft3 (sum of runoff volumes from Drainage Areas: 16, 17, 27, and 31) Volume Supplied: 11,642 ft3 Surplus Volume: 5,017 ft3 Retention Pond 3: 803 ft3 of Drainage Area 8. Volume Required: 803 ft3 Volume Supplied: 887 ft3 Retention Pond 4: 1,137 ft3 of Drainage Area 8. Volume Required: 1,137 ft3 Volume Supplied: 2,720 ft3 Retention Pond 5: Drainage Area 9. Volume Required: 1,361 ft3 Volume Supplied: 2,457 ft3 Retention Pond 6: Drainage Area 11. Volume Required: 2,536 ft3 Volume Supplied: 2,720 ft3 Page 18 of 25 Retention Pond 7: Drainage Area 32. Volume Required: 309 ft3 Volume Supplied: 338 ft3 Drywells: Drainage Areas 2, 3, 4, 5, 7, 10, 13, 14, 15, 18, 19, 20, 22, 23, 24, 25, 26, 27, 28, 29, and 30. All drainage areas flowing to drywells are purely roof drainage areas. See Attachment 1 for an example drywell calculation. STORMWATER CHASE 1 Drainage Areas 16, 17, 31, and the east 206 lf of Field Street (portion of Drainage Area 27) will drain to a stormwater chase which will then convey water to Retention Pond 2B via swale. The drainage area is 2.4530 acres, with a composite runoff coefficient calculated as follows: Weighted Runoff Coefficient: C =(12,456 ft2x0.88)+(24,712 ft2x0.83)+(39,722 ft2x0.68)+( 29,964 ft2x0.75) 106,854 ft2 C = 0.76 Runoff Volume V = 0.068x0.76x106,854 ft2 = 11,277 ft3 The stormwater chase will be a rectangular concrete channel as detailed in City of Bozeman Standard Drawing No. 02529-14. The chase is designed to handle a 25-year storm event. The post- development time of concentration from the most hydraulically remote section of the drainage area is first calculated to determine the peak discharge: Overland flow (614 ft @ 1.029% avg. slope, C=0.90) = 10.0 min (see Figure I-1, COB DSSP) Gutter flow (543 ft @ 1.394% avg. slope, C=0.90) V= (1.486/n)R2/3S1/2 (n=0.013, A=1.24 ft2, P=9.23ft, R2/3=0.2623, S1/2=0.1181) V= 3.54 ft/s T= 543 ft/3.54 ft/s/60s/min = 2.56 min Page 19 of 25 Total Time of Concentration = 12.56 minutes (0.2093 hours) For a 25-year storm event, I25 = 0.78X-0.64 = 0.78 (0.2093)-0.64 = 2.12 in/hr Q25 = CIA = 0.76 (2.12 in/hr) (2.4530 acres) = 3.95 cfs The chase will be 2.0 ft wide by 0.5 ft high. The capacity of a 2.0 ft x 0.5 ft rectangular channel at the proposed slope of 2.5% is checked using Manning’s equation: Qfull = (1.486/n)AR2/3S1/2 Manning's n = 0.013 for concrete Slope = 0.025 ft/ft A = area = w x h = 2.0 ft x 0.5 ft = 1.0 ft2 P = wetted perimeter = w + 2h = 2.0 ft + 1.0 ft = 3.0 ft R = hydraulic radius = A/P = 1.0 / 3.0 = 0.3333 ft R2/3 = 0.4808 ft S1/2 = 0.1581 ft/ft Qfull = (1.486/0.013)(1.0)(0.4808)(0.1581) = 8.69 cfs 3.95 cfs  8.69 cfs  2.0ft x0.5ft chase @ 2.5% slope is adequate. STORM SEWER PIPE NETWORK DESIGN – BOARDWALK AVE. & FIELD ST. Three storm drain inlets will be installed on Boardwalk Avenue to collect surface runoff from Drainage Areas 1, 6, 21A, 21B, and 21C. Storm sewer manholes will be installed at the intersection of Boardwalk Avenue and Field Street and at the west end of Field St to direct stormwater to Retention Pond 1B. All storm water pipes are sized to handle the 25-year storm (COB DSSP). The pipe sizing calculations are presented below: Inlet 1 First, it is necessary to calculate the time of concentration for stormwater runoff contributing to Inlet 1 from Drainage Areas 1, 6, and 21A. Overland flow - Landscape (133 ft @ 0.5%, C=0.2) = 21.0 min (see Figure I-1, COB DSSP) Overland flow – Pavement (482 ft @ 2.4%. C=0.9) = 7.5 min (see Figure I-1, COB DSSP) Gutter flow along Boardwalk Ave. (82 ft @ 2.81% avg. slope) Page 20 of 25 V= (1.486/n)R2/3S1/2 (n=0.013, A=1.24 ft2, P=9.23ft, R2/3=0.2623, S1/2=0.1676) V= 5.03 ft/s Tc= 82 ft/5.03ft/s/60s/min = 0.27 min Total Time of Concentration = 21.0+7.5+0.27= 28.8 minutes (0.4795 hours) Next, the rainfall intensity of a 25-year storm with duration equal to the time of concentration is calculated. For a 25-year storm event: I25 = 0.78X-.64 = 0.78(0.4795)-.64 = 1.25 in/hr (COB DSSP) The composite runoff coefficient for the contributing Drainage Areas 1, 6, and 21A is calculated as follows: C = (122,137 ft2 x 0.72)+(56,015 ft2 x 0.71)+(44,036 ft2x0.72) 222,191 ft2 C = 0.72 The flow rate anticipated from Drainage Areas 1, 6, and 21A during a 25-year storm event is calculated as follows: Q25 = CIA = (0.72)(1.25 in/hr)(5.1008 acres) = 4.57 cfs Finally, a 15-inch PVC pipe is checked for adequacy. The slope of the 72 ft section of pipe from Inlet 1 to Inlet 2 is 0.80%. The capacity of the 15-inch PVC pipe is calculated as follows using Manning’s equation: Manning's n = 0.013 for PVC Pipe Slope = 0.008 ft/ft A = area = (3.14/4)d 2 = (3.14/4)(15/12)2 = 1.2272 ft2 P = perimeter = 2(3.14)r = 2(3.14)(7.5/12) = 3.9270 ft R = hydraulic radius = A/P = 1.2272 /3.9270 = 0.3125 ft R2/3 = 0.4605 ft S1/2 = 0.0894 ft/ft Qfull = (1.486/0.013)(1.2272)(0.4605)(0.0894) = 5.78 cfs > 4.57 cfs (Q25) adequate Page 21 of 25 The calculations show that a 15-inch PVC pipe will have adequate capacity to handle the runoff from a 25-year storm. Inlet 2 The time of concentration for stormwater runoff contributing to Inlet 2 from Drainage Area 21B is calculated as follows: Overland flow (61.5 ft @ 0.93%, C=0.9) = 2.0 min (see Figure I-1, COB DSSP) Valley gutter flow along Valley Commons Dr. (307.6 ft @ 0.93% avg. slope) V= (1.486/n)R2/3S1/2 (n=0.013, A=0.12 ft2, P=8.01ft, R2/3=0.0608, S1/2=0.0964) V= 0.67 ft/s Tc= 307.6 ft/0.67ft/s/60s/min = 7.65 min Total Time of Concentration = 2+7.65 = 9.65 minutes (0.1608 hours) Next, the rainfall intensity of a 25-year storm with duration equal to the time of concentration is calculated. For a 25-year storm event: I25 = 0.78X-.64 = 0.78(0.1608)-.64 = 2.51 in/hr (COB DSSP) Summary of given information for Drainage Area 21B that contributes to Inlet 2: Weighted Runoff Coefficient, C = 0.88 Rainfall intensity for 25-year storm, I25 = 2.51 in/hr Area= 41,669ft2 = 0.9566acres The flow rate anticipated from Drainage Area 21B during a 25-year storm event is calculated as follows: Q25 = CIA = (0.88)(2.51 in/hr)(0.9566acres) = 2.11 cfs Because Inlet 2 is downstream of Inlet 1, the runoff contributing to Inlet 1 must be added to the runoff contributing to Inlet 2 in order to determine an adequate pipe size at Inlet 2. Q25_cumulative= 4.57+2.11 = 6.68 cfs Page 22 of 25 An 18-inch PVC pipe is checked for adequacy. The slope of the 296 ft section of pipe from Inlet 2 to Inlet 3 is 0.84%. The capacity of the 18-inch PVC pipe is calculated as follows using Manning’s equation: Manning's n = 0.013 for PVC Pipe Slope = 0.0084 ft/ft A = area = (3.14/4)d 2 = (3.14/4)(18/12)2 = 1.7671 ft2 P = perimeter = 2(3.14)r = 2(3.14)(9/12) = 4.7124 ft R = hydraulic radius = A/P = 1.7671 /4.7124= 0.3750 ft R2/3 = 0.5200 ft S1/2 = 0.0917 ft/ft Qfull = (1.486/0.013)(1.7671)(0.5200)(0.0917) = 9.63 cfs > 6.68 cfs (Q25) adequate The calculations show that an 18-inch PVC pipe will have adequate capacity to handle the runoff from a 25-year storm. Inlet 3 The time of concentration for stormwater runoff contributing to Inlet 3 from Drainage Area 21C is calculated as follows: Gutter flow along Boardwalk Ave. (307.9 ft @ 0.77% avg. slope) V= (1.486/n)R2/3S1/2 (n=0.013, A=1.24 ft2, P=9.23ft, R2/3=0.2623, S1/2=0.0877) V= 2.63 ft/s Tc= 307.9 ft/2.63ft/s/60s/min = 1.95 min (0.0325 hours) Next, the rainfall intensity of a 25-year storm with duration equal to the time of concentration is calculated. For a 25-year storm event: I25 = 0.78X-.64 = 0.78(0.0325)-.64 = 6.99 in/hr (COB DSSP) Summary of given information for Drainage Area 21C that contributes to Inlet 3: Weighted Runoff Coefficient, C = 0.78 Rainfall intensity for 25-year storm, I25=6.99 in/hr Area= 26,900ft2 =0.6175acres Page 23 of 25 The flow rate anticipated from Drainage Area 21C during a 25-year storm event is calculated as follows: Q25 = CIA = (0.78)(6.99in/hr)(0.6175acres) = 3.37 cfs Because Inlet 3 is downstream of Inlets 1 and 2, the runoff contributing to Inlets 1 and 2 must be added to the runoff contributing to Inlet 3 in order to determine an adequate pipe size at Inlet 3. Q25_cumulative= 4.57+2.11+3.37 = 10.05 cfs Finally, a 21-inch PVC pipe is checked for adequacy. There are three sections of pipe that must carry the 10.42 cfs of runoff. The critical section is the 339-ft run from STMH 1 to STMH 2 at a slope of 0.44%. The capacity of the 21-inch PVC pipe at this critical section is calculated as follows using Manning’s equation: Manning's n = 0.013 for PVC Pipe Slope = 0.0044 ft/ft A = area = (3.14/4)d 2 = (3.14/4)(21/12)2 = 2.4053ft2 P = perimeter = 2(3.14)r = 2(3.14)(10.5/12) = 5.4978 ft R = hydraulic radius = A/P = 2.40553 /5.4978= 0.4375ft R2/3 = 0.5763 ft S1/2 = 0.0663 ft/ft Qfull = (1.486/0.013)(2.4053)(0.5763)(0.0663) = 10.51 cfs > 10.05 cfs (Q25) adequate The calculations show that a 21-inch PVC pipe will have adequate capacity to handle the runoff from a 25-year storm from Inlet 3 to STMH 1, STMH 1 to STMH 2, and from STMH 2 to Retention Pond 1B. OVERFLOW STRUCTURE – RETENTION POND 1A An overflow structure will be constructed in the south embankment of Retention Pond 1A. The top of the structure elevation will be set to the top of pond elevation (4801.00) and will have a Page 24 of 25 slotted grate cover to allow any overflow to drain. A 21-inch PVC pipe will convey water from the overflow structure to the existing Retention Pond Lot 3A on the north side of Fallon Street. The pipe is sized to handle the calculated peak runoff for a 25-year storm (10.05 cfs from calculations for flow into Retention Pond 1B). Previous calculations show that the capacity of a 21-inch pipe at minimum slope is adequate to manage the peak flow rate. This outlet pipe will have a steeper slope at 0.82%, providing additional capacity.