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HomeMy WebLinkAbout012 Updated Detention Pond 2 CalculationsGran Cielo Subdivision Detention Pond 2 Calculations Calculation of Required Volume for Storm Detention Pond (Reference: Bozeman Stormwater Master Plan - 1982) Design Rainfall Freq.10 year (see page III - 5 of master plan) IDF coefficient a 0.64 IDF coefficient b IDF coefficient n 0.65 Pre-development Calculations Post-development Calculations C Area (AC)C Areas (ft2):open space 853,178 0.20 Areas (AC): Basin 3 4.46 0.52 Basin 4 6.11 0.49 Basin 5 4.51 0.40 Basin 14 2.05 0.41 Basin 15 0.33 0.73 Total:857,696 Total:17.45 total area:19.69 acres total area:17.45 acres composite C:0.20 composite C:0.47 Overland tc Overland tc average slope:1.66 percent average slope:1.66 percent travel distance:1315 feet travel distance:1830 feet tc:52 minutes tc:43 minutes Channel tc Channel tcchannel tc:minutes channel tc:minutes Total tc:52 minutes Total tc:43 minutes intensity at tc (fig 23):0.71 in/hr intensity at tc (fig 23):0.80 in/hr pre-devel peak runoff:2.77 cfs post-devel peak runoff:6.58 cfs Storm Duration Intensity Future Runoff Runoff Release Required (minutes)(in/hr)Rate (cfs)Volume (cf)Volume (cf)Storage (cf) 25 1.13 9.29 13931 4148 9784 27 1.08 8.83 14312 4479 9832 29 1.03 8.43 14674 4811 9863 31 0.98 8.08 15021 5143 9878 33 0.94 7.75 15353 5475 9878 35 0.91 7.46 15673 5807 9866 37 0.88 7.20 15980 6139 9842 39 0.85 6.96 16278 6470 9807 41 0.82 6.73 16565 6802 9763 43 0.79 6.53 16843 7134 9710 45 0.77 6.34 17114 7466 9648 47 0.75 6.16 17376 7798 9578 49 0.73 6.00 17631 8129 9502 51 0.71 5.84 17880 8461 9419 53 0.69 5.70 18122 8793 9329 55 0.68 5.56 18359 9125 9234 57 0.66 5.44 18590 9457 9133 required detention storage (ft3) =9,878 Detention Pond Calculations:0.5 Inch Stormwater in Roadways Calculations: C design depth of pond 1.50 feet Areas (ft2):asphalt 217,364 0.90 max side slope 4.00 horizontal to 1.00 vertical length/width ratio 3.00 A =4.99 acres min. particle removed 40 microns (1 micron = 1 x 10-6 meters)I = 0.021 in/hr (0.5" in 24 hrs) settling velocity of particle 0.0069 feet/second C =0.90 Q = 0.09 cfs min. pond to settle particle 953 square feet Volume = 8,084 cf bottom weir h =1.08 ft pond dimentions assuming vertical side slopes (actual pond footprint will be larger) width 47 length 141 Volume held between contours: Cumulative Contour Area (ft2)Delta V (ft3)Volume (ft3)Overflow Retention Pond Calculations: 4955.0 7,4704955.5 8,425 3,974 3,974 Q = CIA4956.0 9,404 4,457 8,431 C =0.44 (post-development)4956.5 10,410 4,954 13,385 I = 0.41 in/hr (10-yr, 2-hr storm) A = 17.45 acres Q =3.15 cfs Design storage at 1.5' depth (ft3) =13,385 required retention storage (ft3) =22,665 ft3 minus detention volume 13,385 ft3 Overflow Design storage at 1.5' depth (ft3) =9,281 ft3 Retention Gran Cielo Subdivision Post-Constructed Peak Flow Calculations (cfs) Appendix B BASIN 1 Area (acre) C 90' ROW 0.775 0.67 Weighted C 0.67 Low-Med. Res. 0 0.35 Average slope:1.86 percent Open Space 0 0.2 Travel Distance 780.00 feet Total area 0.775 0.67 Design Rainfall Freq.100 25 10 C*Cf 0.84 0.74 0.67 (Shall not exceed 1.00) Total tc:11.15 15.42 18.26 minutes intensity at tc 3.12 1.86 1.39 in/hr peak runoff:1.62 0.97 0.72 cfs BASIN 2 Area (acre) C 90' ROW 0.547 0.67 Weighted C 0.67 Low-Med. Res. 0 0.35 Average slope:1.86 percent Open Space 0 0.2 Travel Distance 780.00 feet Total area 0.547 0.67 Design Rainfall Freq.100 25 10 C*Cf 0.84 0.74 0.67 (Shall not exceed 1.00) Total tc:11.15 15.42 18.26 minutes intensity at tc 3.12 1.86 1.39 in/hr peak runoff:1.14 0.68 0.51 cfs BASIN 3 Area (acre) C Total Area:4.457 acres 60' ROW 0.358 0.73 C:0.52 Low-Med. Res. 2.349 0.35 Average slope:1.50 percent Res. (Multi-Units, Attached) 1.75 0.7 Open Space 0 0.2 Travel Distance 1323.00 feet Total area 4.457 0.52 Design Rainfall Freq.100 25 10 C*Cf 0.65 0.57 0.52 (Shall not exceed 1.00) Total tc:26.89 31.51 34.59 minutes intensity at tc 1.73 1.18 0.92 in/hr peak runoff:3.99 2.72 2.11 cfs Peak Q Values Page 2 of 7