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Home My WebLink AboutWPO200400100 Calculations 2004-10-06 LONGACRE FARM Albemarle County,Virginia STORMWATER/BIO-FILTER CALCULATION NARRATIVE October 6,2004 NP ENGINEERING 1850 BROWNS GAP TURNPIKE • CHARLOTTESVILLE, VA 22901 phone:434-823-1228•Fax:434-823-9746 LONGACRE FARM SUBDIVISION STORMWATER/BIO-FILTER CALCULATIONS INTENT: Perform calculations for the capacity of the bio-filter,size/design the stone weir at the bio-filter and the ditch lining from the bio-filter to the existing stream. HYDRAULICS: Pre-Development Given:Area is currently pasture(C=0.3) Area Number C Area(ac) CA BS l,rrite-- � S(cPJ1� .E 1 0.300 3.700 1.110 N 1�"-'�", 1'`t _ Weighted Totals: 0.300 ,3e706 1)(24fOlt LE rtD t11J G tik/ 0,33 Po s t.Tb1— `Ec�iA-t_ Time of concentration=5 min Period Intensity Peak Flow ` 1 � v- vc- ' -r- M.c (years) (in/hr) (cfs) 2 5.40 5.99 _ 5 6.35 7.05 10 �✓�. Z '� 10 7.10 7.88 25 8.00 8.88 y re. 1,4.7 50 8.90 9.88 100 9.60 10.66 Post Development Given: Total drainage area to bio-filter=3.7 ac Total impervious area= 17,167.77 sf Total impervious area draining to bio-filter=9,077.98 sf=0.21 ac. Area Number C Area(ac) CA 1 0.300 0.210 0.063 2 0.900 3.490 3.141 Weighted Totals: 0.866 3.700 3.204 Time of concentration=5 min Period Intensity Peak Flow (years) (in/hr) (cfs) 2 5.40 17.30 5 6.35 20.35 10 7.10 22.75 25 8.00 25.63 50 8.90 28.52 100 9.60 30.76 NP ENGINEERING 1 1850 BROWNS GAP TURNPIKE • CHARLOTTESVILLE, VA 22901 phone:434-823-1228•Fax:434-823-9746 DESIGN BIO-FILTER AND CHECK FOR CAPACITY DESIGN INTENT: Design the bio-filter capacity based on a surface area of 2.5%times the total impervious area.In addition, check the volume of the No-filter to handle the first'/2 inch of runoff from impervious areas. Total impervious area= 17,167.77 sf (see Drainage map) Calculated surface area required for the bio-filter= 17,167.77 x.025=429.19 sf Surface Area provided=475.8 sf Total impervious area draining to bio-filter=9,077.98 sf (see Drainage map) Required volume of bio-filter= 1/2"x 9,077.98 sf=378.25 cf= 14 cy Depth of water in basin= 1' Storage volume provided in bio-filter= 1'x 475.8 sf=478.5 cf= 17.72 cy > 14 cy(volume okay) CONCLUSION: The size of the bio-filter(@ 475.8 sf)satisfies the 2.5%rule for sizing and has sufficient capacity to handle the first'/2 inch of runoff from the impervious area draining to the structure. DESIGN OF STONE WEIR AT BIO-FILTER DESIGN INTENT: Design the stone weir to handle the 10-year post-development storm. Per the above hydraulic calculations,size the stone weir to handle a minimum of 22.75 cfs. DESIGN ASSUMPTIONS: Weir is trapezoidal with the following characteristics: Top Width=9' Bottom Width=4.5' Side Slope=2.25:1 (per VESCH Table 5-15—minimum side slope=2:1) Depth= 1' Cross Sectional Area=6.75 sq.ft. Hydraulic Radius=0.7162 Manning's Coefficient(based on d50) =0.0352 Slope(assume 1%) _.01 Based on Mannings Equation and Continuity Equation,the capacity of this weir is 22.87 cfs,which is greater than the 10-year storm requirement of 22.75 cfs. CONCLUSION: FINAL DESIGN: Weir is trapezoidal with the following characteristics: Top Width=9' Bottom Width=4.5' Side Slope=2.25:1 Depth= 1' NP ENGINEERING 2 1850 BROWNS GAP TURNPIKE • CHARLOTTESVILLE, VA 22901 phone:434-823-1228•Fax:434-823-9746 DESIGN OF THE DITCH LINING BELOW BIO-FILTER TO STREAM DESIGN INTENT: Design the ditch to convey 10-year peak storm and resist erosion from the 2 year peak storm velocity. Per the above hydraulic calculations, T - Qio=22.75 cfs Q2= 17.30 cfs c t D. 3 Verify ditch can convey the 10-year peak storm: V = U• is Assume a V-ditch with the following characteristics: 6'top width 1'deep 3:1 side slopes(per VESCH Table 5-15—minimum side slope=3:1) Grass lined(n=.05)with EC2 Slope of proposed ditch follows existing contours @ 0.23 ft/ft Cross sectional Area=3.0 sf f .. 1S Hydraulic Radius=0.474 o, Vdesign= 8.69 ft/sec " �o ' e-.4 3 /� Qd =AxV=3.0 x 8.69=26.07 cfs>Qio=22.75 cfs (Capacity okay) / eLSs Verify resistance to erosion for the 2-year peak velocity LZ= �• �G 1\s,j , Q2= 17.30 cfs V ;s V2=Q2/A= 17.30/3.0=5.77 ft/sec therefore use EC-3 T (V ,,=4.0-7.0 ft/sec) Z ^ 7. t? CONCLUSION: '1-1 C to s FINAL DESIGN: V-ditch with the following characteristics: Top Width=6' Depth= 1' Side Slope=3:1 Grass Lined with EC-3 Tgp ' ,ir-a• • NP ENGINEERING 3 1850 BROWNS GAP TURNPIKE • CHARLOTTESVILLE, VA 22901 phone:434-823-1228•Fax:434-823-9746 LONGACRE FARM Albemarle County,Virginia STORMWATER/BIO-FILTER CALCULATION NARRATIVE October 6,2004 NP ENGINEERING 1850 BROWNS GAP TURNPIKE • CHARLOTTESVILLE, VA 22901 phone:434-823-1228•Fax:434-823-9746 LONGACRE FARM SUBDIVISION STORMWATER/BIO-FILTER CALCULATIONS IN PENT: Perform calculations for the capacity of the bio-filter,size/design the stone weir at the bio-filter and the ditch lining from the bio-filter to the existing stream. HYDRAULICS: Pre-Development Given:Area is currently pasture(C =0.3) Area Number C Area(ac) CA 1 0.300 3.700 1.110 ✓ Weighted Totals: 0.300 3.700 1.110 / Time of concentration=5 min Period Intensity Peak Flow (years) (in/hr) (cfs) 2 5.40 5.99.f 5 6.35 7.05 ✓ 10 7.10 7.88 25 8.00 8.88 50 8.90 9.88 100 9.60 10.66 ✓ Post Development Given: Total drainage area to bio-filter= 3.7 ac Total impervious area= 17,167.77 sf Total impervious area draining to bio-filter=9,077.98 sf=0.21 ac. Area Number C Area(ac) CA 1 0.300 0.210 0.063 .(1) U0 c 0A i 2 0.900 3.490 3.141 . 3o 3,(11 : I, 0 I- Weighted Totals; 0.866'' 3.700' 3.204 1 f' 33 Time of concentration= 5 min Period Intensity Peak Flow (years) (in/hr) (cfs) 2 5.40 17.30✓ G,G'� 5 6.35 20.35✓ 7,ct.r 10 7.10 22.75✓ 8.1 25 8.00 25.63 Y.8 q 50 8.90 28.52 1(.06 100 9.60 30.76✓' 'BI" NP ENGINEERING 1 1850 BROWNS GAP TURNPIKE • CHARLOTTESVILLE, VA 22901 phone:434-823-1228•Fax:434-823-9746 DESIGN BIO-FILTER AND CHECK FOR CAPACITY DESIGN IN TENT: Design the bio-filter capacity based on a surface area of 2.5%times the total impervious area.In addition, check the volume of the bio-filter to handle the first'/z inch of runoff from impervious areas. Total impervious area= 17,167.77 sf (see Drainage map) Calculated surface area required for the bio-filter= 17,167.77 x.025=429.19 sf Surface Area provided=475.8 sf Total impervious area draining to bio-filter=9,077.98 sf (see Drainage map) Required volume of bio-filter= 1"x 9,077.98 sf=378.25 cf= 14 cy . { Depth of water in basin= 1' 11.Gv1-61. Storage volume provided in bio-filter= 1'x 475.8 sf=478.5 cf= 17.72 cy > 14 cy(volume okay) CONCLUSION: The size of the bio-filter(@ 475.8 sf)satisfies the 2.5%rule for sizing and has sufficient capacity to handle the first 1/2 inch of runoff from the impervious area draining to the structure. DESIGN OF STONE WEIR AT BIO-FILTER DESIGN INTENT: Design the stone weir to handle the 10-year post-development storm. Per the above hydraulic calculations,size the stone weir to handle a minimum of 22.75 cfs. DESIGN ASSUMPTIONS: Weir is trapezoidal with the following characteristics: Top Width=9' Bottom Width=4.5' Side Slope=2.25:1 (per VESCH Table 5-15—minimum side slope=2:1) Depth= 1' Cross Sectional Area=6.75 sq. ft. Hydraulic Radius=0.7162 Manning's Coefficient(based on d50) =0.0352 Slope(assume 1%) = .01 Based on Mannings Equation and Continuity Equation,the capacity of this weir is 22.87 cfs,which is greater than the 10-year storm requirement of 22.75 cfs. CONCLUSION: FINAL DESIGN: Weir is trapezoidal with the following characteristics: Top Width=9' Bottom Width=4.5' Side Slope=2.25:1 Depth= 1' NP ENGINEERING 2 1850 BROWNS GAP TURNPIKE • CHARLOTTESVILLE, VA 22901 phone:434-823-1228•Fax:434-823-9746 DESIGN OF THE DITCH LINING BELOW BIO-FILTER TO STREAM DESIGN IN 1'hNT: Design the ditch to convey 10-year peak storm and resist erosion from the 2 year peak storm velocity. Per the above hydraulic calculations, Qio= 22.75 cfs Q2= 17.30 cfs Verify ditch can convey the 10-year peak storm: Assume a V-ditch with the following characteristics: 6'top width 1'deep 3:1 side slopes(per VESCH Table 5-15—minimum side slope=3:1) Grass lined(n=.05)with EC2 Slope of proposed ditch follows existing contours @ 0.23 ft/ft Cross sectional Area=3.0 sf Hydraulic Radius=0.474 Vdesig,= 8.69 ft/sec Qdesign=AxV=3.0 x 8.69=26.07 cfs>Qio=22.75 cfs (Capacity okay) Verify resistance to erosion for the 2-year peak velocity Q2= 17.30 cfs V2=Q2/A= 17.30/3.0=5.77 ft/sec therefore use EC-3 Type A Lining(Vmpv=4.0-7.0 ft/sec) CONCLUSION: FINAL DESIGN: V-ditch with the following characteristics: Top Width=6' Depth= 1' Side Slope=3:1 Grass Lined with EC-3 Type A NP ENGINEERING 3 1850 BROWNS GAP TURNPIKE • CHARLOTTESVILLE, VA 22901 phone:434-823-1228•Fax:434-823-9746