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HomeMy WebLinkAboutStormwater Run-off and Basin Calculations 06-25-19 Nexus Point Lot 1 Minor Sub 235 Stonmwater Detention/Retention Calculations Calculation of Required Volume for Storm Detention Pond (Reference: Bozeman Stormwater Master Plan-1982) Design Rainfall Freq. 0; :::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 C Areas(ft): open space 692,330 0.20 Areas(ft): open space 0.20 med.res. 0.35 med.res. 0.35 dense res. 0.50 dense res. 692,330 0.50 Total: 692,330 Total: 692,330 total area: 15.89 acres total area: 15.89 acres composite C: 0.20 composite C: 0.50 Overland tc Overland tt average slope: 0.9 percent average slope: 1.5 percent travel distance: 1185 feet travel distance: 1185 feet tc: 60 minutes tc: 34 minutes Channel t� Channel tc channel tc. minutes channel t.. minutes Total t�: 60 minutes Total tt: 34 minutes intensity at tc(fig 23): 0.64 in/hr intensity at tc(fig 23): 0.93 in/hr pre-devel peak runoff: 2.03 cis post-level peak runoff: 7.39 cis Stone Duration Intensity Future Runoff Runoff Release Required (minutes) (in/hr) Rate(cfs) Volume(cf) Volume(cf) Storage(cf) 34 0.93 7.39 14969 4118 10850 36 0.90 7.12 15273 4362 10911 38 0.87 6.87 15567 4607 10961 40 0.84 6.65 15851 4851 11000 42 0.81 6.44 16126 5095 11031 44 0.79 6.25 16392 5339 11053 46 0.76 6.07 16651 5583 11068 48 0.74 5.90 16902 5827 11075 50 0.72 5.75 17147 6071 11075 52 0.70 5.60 17385 6315 11069 54 0.69 5.46 17617 6559 11058 56 0.67 5.34 17844 6804 11040 58 0.66 5.21 18065 7048 11018 60 0.64 5.10 18282 7292 10990 62 0.63 4.99 1 B494 7536 10958 64 0.62 4.89 18701 7780 10921 66 0.60 4.79 18905 8024 10881 required detention storage(ft)= 11,075 Nexus Point-Stormwater Calculations.xls Nexus Point Lot 1 Minor Sub 235 Stormwater Detention/Retention Calculations Detention Pond Calculations: design depth of pond 2.30 feet max side slope 4.00 horizontal to 1.00 vertical length/Width ratio 300 min.particle removed 40 microns(1 micron=1 x 10-6 meters) settling velocity of particle 0.0069 feet/second min.pond to settle particle 1072 square feet Volume held between contours: Cumulative Contour Area(ft) Delta V(ft) Volume(ft3) 4952,00 1,091 4953.0 5,637 3,364 3,364 4954.0 7,278 6,458 9,822 4954.30 4,548 1,774 11,595 Design storage at 2.3'depth(ft)= 11,595 Nexus Point-Stormwater Calculations.xls Nexus Point Lot 1 Minor Sub 235 3r Detention/Retention Calculations Detention Pond Outlet Calculations (Reference: City of Bozeman, Design Standards and Specifications Policy, March 2004, II.D.2, page 24) Note: see Figure A-2 in above reference. Rectangular weir-Q=3.331-1-132 Determine Outlet Slot Width needed. Pre-development flow rate= 2.03 cfs Vertical Slot Height= 15 inches Req'd Outlet Slot Width= 0.44 7feet or 5 8/32 of an inch Determine Outlet Flow: Outlet Slot Width= 5.50 inches stage ft Q (cfs) Q (qpm) 0.25 0.191 86 0.50 0.540 242 0.75 0.991 445 1.00 1.526 685 1.25 2.133 957 1.50 2.804 1258 Flow through outlet Stage vs. Discharge 3.00 2.50 2.00 1.50 �a 1.00 0 0.50 0.00 0 0.5 1 1.5 Stage (ft)