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HomeMy WebLinkAboutSDP201200056 Calculations 2012-09-05 PROJECT MANAGEMENT SHIMP CIVIL ENGINEERING LAND PLANNING ENGINEERING v� September 5, 2012 Albemarle County Department of Community Development Regarding: Broadway Street Amendment Stormwater Quality and Quantity Plan Dear Plan Reviewer, Enclosed are the calculations and details compliance with the applicable regulations for stormwater management for this project. Stormwater detention for the 2-and 10-year storms is addressed using underground detention pipe. Water quality is addressed using filterras, as documented on sheet C7 of the plan set.A summary of the run-off from the site follows: Summary Table: Pre-Development Post-Development 2-Yr Flow(cfs) 3.09 3.03 .06 10-Yr Flow(cfs) 5.42 5.35 If you have any questions about these calculations please do not hesitate to contact me at your earliest convenience. I am available to meet you at your office to discuss if that would be helpful for your review. Justin Shimp, P.E. Contents: Pre-Development Stormwater Management Calculations: . Pre-Development Drainage Area Map Pre-Development HydroCAD Analysis Post-Development Stormwater Management Calculations: Post-Development Drainage Area Map Post-Development HydroCAD Analysis Independent Reports: NOAA Precipitation Report .. .. - Pre-Development Stormwater Management Calculations Pre-Development Drainage Area Map Pre-Development HydroCAD Analysis Note: The NRCS Report for the site did not list a Hydrologic Soil Group for the on-site Udorthents soils, so all calculations are made assuming Hydrologic Soil Group D. a .p i" .A 00 w gl as 0 C < m m -s CU = -„ir 0- law 0 N 0 0 tn 0 CD D -0 : 0 a D• N N • 0 N ---h 'A O rt W 1 - el to n CD I 7 o d4 1 m o ,s* 5NM — M � MOM � — fD m 0 1 1 142;11111;1111i I I 1 N N ` U l0 N V N 1 1 1 1 I L11 / 1 1 1 1 I I LO In p , I I I I ! 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Parking a a Subcat Reach 'on• Drainage Diagram for Broadway-Pre-Development Prepared by Shimp Engineering, P.C., Printed 7/20/2012 HydroCAD®9.10 s/n 07054 ©2011 HydroCAD Software Solutions LLC a Broadway-Pre-Development Type ll 24-hreier Rainfall-3.66''= Prepared by Shimp Engineering, P.C. Printed 7/20/2012 HydroCAD®9.10 s/n 07054 ©2011 HydroCAD Software Solutions LLC Page 2 Summary for Subcatchment 1: Ex. Parking Runoff = 3.09 cfs © 12.02 hrs, Volume= 0.167 af, Depth> 2.15" ... Runoff by SCS TR-20 method, UH=SCS, Time Span= 1.00-24.00 hrs, dt= 0.01 hrs Type II 24-hr 2yr Rainfall=3.66" Area (sf) CN Description 28,726 80 >75% Grass cover, Good, HSG D 11,923 98 Impervious 40,649 85 Weighted Average 28,726 70.67% Pervious Area 11,923 29.33% Impervious Area Tc Length Slope Velocity Capacity Description (min) (feet) (ft/ft) (ft/sec) (cfs) 9.9 Direct Entry, Summary for Link 2: Point of Analysis Inflow Area = 0.933 ac, 29.33% Impervious, Inflow Depth > 2.15" for 2yr event Inflow = 3.09 cfs @ 12.02 hrs, Volume= 0.167 af Primary = ROg tom@ 12.02 hrs, Volume= 0.167 af, Atten= 0%, Lag= 0.0 min Primary outflow = Inflow, Time Span= 1.00-24.00 hrs, dt= 0.01 hrs Broadway-Pre-Development Type II 24-hr, 'Ramal)=5:54H Prepared by Shimp Engineering, P.C. Printed 7/20/2012 .r HydroCAD®9.10 s/n 07054 ©2011 HydroCAD Software Solutions LLC Page 3 Summary for Subcatchment 1: Ex. Parking Runoff = 5.42 cfs @ 12.01 hrs, Volume= 0.300 af, Depth> 3.86" Runoff by SCS TR-20 method, UH=SCS, Time Span= 1.00-24.00 hrs, dt= 0.01 hrs Type II 24-hr 10yr Rainfall=5.54" Area (sf) CN Description 28,726 80 >75% Grass cover, Good, HSG D 11,923 98 Impervious 40,649 85 Weighted Average 28,726 70.67% Pervious Area 11,923 29.33% Impervious Area Tc Length Slope Velocity Capacity Description (min) (feet) (ft/ft) (ft/sec) (cfs) 9.9 Direct Entry, Summary for Link 2: Point of Analysis Inflow Area = 0.933 ac, 29.33% Impervious, Inflow Depth > 3.86" for 10yr event Inflow = 5.42 cfs • 12.01 hrs, Volume= 0.300 of Primary = .` •• 12.01 hrs, Volume= 0.300 af, Allen= 0%, Lag= 0.0 min Primary outflow = Inflow, Time Span= 1.00-24.00 hrs, dt= 0.01 hrs AS ale a a a . Post-Development Stormwater Management Calculations Post-Development Drainage Area Map Post-Development HydroCAD Analysis aaaa a .a. sra .a +aa a aa. a a a a aaW a a 1- t-4 ID LID . . = N -+ 00 O O C < 'wont C±,16) 0 r* < -I ID (1) n. = 0 T_ CO CG N N N -t m O - Q _. Q (1) r-r -- \ �• O A F-4 1 r* O o a) g' W to 9 v �. r+ C %"." ' ''"4° N. ,...."iiiiir..,............................\_42. (..1 lk *oft F/ •• ' 1 i cu N ` p ,� illi , 0 I O N A 00 tl ilifgliiI I IoI I I II 1 - 3 5 3 . , , 1 ' 1 1 1 1 1 , I O -. 1,, ii I 1 Ii I n o i 11 � I11 � ` CD n Q. ,-) 1 I I I I 1 n 13 M I II -a --- t- J _- tf I r r n n- roC-_ FT-I `1 cn _ ,1 I I -r. c-, c_ ff I I I 1 r �, m U+ 1 ,f 1- I I 1 I / I 1 (I) -z Cr) - y, r* -f ,�� I I I y 1j I , I I T 0 0- / / r - jilV o I II /l iAri V / l/l// / , T\v/ I n. ft. . f) , �, 0 ,� t�OC \ ! I / / v \ , co , ,, 4 A .„ ii: , __., / / I N.-:,‘,1.- i/ 4faL \ / kl I c 3S N /I a ' I o II F) 14 \ 1110CI *411 4 0 CO 0 iND0 rr 4 3 )'a Point of Analyse Ex. Parking K ) > New Road Underground Detention err a (Subca) Reach 'on. Drainage Diagram for Broadway-Post-Development Prepared by Shimp Engineering, P.C., Printed 7/20/2012 HydroCAD®9.10 s/n 07054 ©2011 HydroCAD Software Solutions LLC Assa Broadway-Post-Development Type II 24-h ,RR8infaii=3.86"4 Prepared by Shimp Engineering, P.C. Printed 7/20/2012 HydroCAD®9.10 s/n 07054 ©2011 HydroCAD Software Solutions LLC Page 2 Summary for Subcatchment 1: New Road Runoff = 0.78 cfs @ 12.03 hrs, Volume= 0.044 af, Depth> 2.32" •• Runoff by SCS TR-20 method, UH=SCS, Time Span= 0.00-24.00 hrs, dt= 0.01 hrs Type II 24-hr 2yr Rainfall=3.66" Area (sf) CN Description 5,992 80 >75% Grass cover, Good, HSG D 4,004 98 Impervious 9,996 87 Weighted Average 5,992 59.94% Pervious Area 4,004 40.06% Impervious Area Tc Length Slope Velocity Capacity Description (min) (feet) (ft/ft) (ft/sec) (cfs) 11.2 Direct Entry, Summary for Pond 2: Underground Detention •■ Inflow Area = 0.229 ac, 40.06% Impervious, Inflow Depth > 2.32" for 2yr event Inflow = 0.78 cfs @ 12.03 hrs, Volume= 0.044 of Outflow = 0.53 cfs @ 12.12 hrs, Volume= 0.044 af, Atten= 33%, Lag= 5.4 min Primary = 0.53 cfs @ 12.12 hrs, Volume= 0.044 af R_o ; b ,. _.r-Ind method, Time Span= 0.00-24.00 hrs, dt= 0.01 hrs ¢t . 12.12 hrs Surf.Area= 175 sf Storage= 226 cf Plug-Flow detention time= 2.8 min calculated for 0.044 af(100% of inflow) Center-of-Mass det. time= 2.7 min ( 816.1 - 813.4 ) a Volume Invert Avail.Storage Storage Description #1 386.00' 481 cf 42.0" D x 50.0'L Pipe Storage S=0.0025'/' Device Routing Invert Outlet Devices #1 Primary 386.00' 15.0" Round Culvert L= 55.0' Ke= 0.400 Inlet/ Outlet Invert= 386.00' / 385.50' S= 0.0091 '/' Cc= 0.900 n= 0.010 #2 Device 1 386.00' 4.0"Vert. Orifice/Grate C= 0.600 #3 Device 1 388.70' 15.0" Horiz. Orifice/Grate C= 0.600 Limited to weir flow at low heads Primary OutFlow Max=0.52 cfs @ 12.12 hrs HW=387.73' (Free Discharge) =Culvert (Passes 0.52 cfs of 6.63 cfs potential flow) 2=Orifice/Grate (Orifice Controls 0.52 cfs @ 6.02 fps) 3=Orifice/Grate ( Controls 0.00 cfs) a a a Broadway-Post-Development Type II 24-42yis Rainfell=3,4rI Prepared by Shimp Engineering, P.C. Printed 7/20/2012 HydroCAD®9.10 s/n 07054 ©2011 HydroCAD Software Solutions LLC Page 3 Summary for Subcatchment 3: Ex. Parking Runoff = 2.57 cfs @ 12.01 hrs, Volume= 0.141 af, Depth> 2.41" �• Runoff by SCS TR-20 method, UH=SCS, Time Span= 0.00-24.00 hrs, dt= 0.01 hrs Type II 24-hr 2yr Rainfall=3.66" Area (sf) CN Description 17,556 80 >75% Grass cover, Good, HSG D 13,097 98 Impervious 30,653 88 Weighted Average 17,556 57.27% Pervious Area 13,097 42.73% Impervious Area "-' Tc Length Slope Velocity Capacity Description (min) (feet) (ft/ft) (ft/sec) (cfs) 9.9 Direct Entry, Summary for Link 4: Point of Analysis -• Inflow Area = 0.933 ac, 42.07% Impervious, Inflow Depth > 2.39" for 2yr event Inflow = 3.03 cfs @12.02 hrs, Volume= 0.186 of Primary = , ;, @ 12.02 hrs, Volume= 0.186 af, Atten= 0%, Lag= 0.0 min Primary outflow= Inflow, Time Span= 0.00-24.00 hrs, dt= 0.01 hrs Broadway-Post-Development Type II 24-hr '.-RIOnfallm.544 Prepared by Shimp Engineering, P.C. Printed 7/20/2012 HydroCAD®9.10 s/n 07054 ©2011 HydroCAD Software Solutions LLC Page 4 Summary for Subcatchment 1: New Road Runoff = 1.33 cfs @ 12.03 hrs, Volume= 0.078 af, Depth> 4.07" Runoff by SCS TR-20 method, UH=SCS, Time Span= 0.00-24.00 hrs, dt= 0.01 hrs Type II 24-hr 10yr Rainfall=5.54" Area (sf) CN Description 5,992 80 >75% Grass cover, Good, HSG D 4,004 98 Impervious 9,996 87 Weighted Average 5,992 59.94% Pervious Area 4,004 40.06% Impervious Area Tc Length Slope Velocity Capacity Description (min) (feet) (ft/ft) (ft/sec) (cfs) 11.2 Direct Entry, Summary for Pond 2: Underground Detention Inflow Area = 0.229 ac, 40.06% Impervious, Inflow Depth > 4.07" for 10yr event Inflow = 1.33 cfs @ 12.03 hrs, Volume= 0.078 of Outflow = 1.30 cfs @ 12.05 hrs, Volume= 0.078 af, Atten= 2%, Lag= 1.6 min Primary = 1.30 cfs @ 12.05 hrs, Volume= 0.078 of Routin. b ,_.,tor-Ind method, Time Span= 0.00-24.00 hrs, dt= 0.01 hrs >{*y}. @ 12.05 hrs Surf.Area= 142 sf Storage= 408 cf Plug-Flow detention time= 3.8 min calculated for 0.078 af(100% of inflow) Center-of-Mass det. time= 3.6 min ( 801.2 - 797.6 ) Volume Invert Avail.Storage Storage Description #1 386.00' 481 cf 42.0" D x 50.0'L Pipe Storage S=0.0025 '/' Device Routing Invert Outlet Devices #1 Primary 386.00' 15.0" Round Culvert L= 55.0' Ke= 0.400 Inlet/ Outlet Invert= 386.00'/ 385.50' S= 0.0091 '/' Cc= 0.900 n= 0.010 #2 Device 1 386.00' 4.0"Vert. Orifice/Grate C= 0.600 #3 Device 1 388.70' 15.0" Horiz. Orifice/Grate C= 0.600 Limited to weir flow at low heads Primary OutFlow Max=1.28 cfs @ 12.05 hrs HW=388.83' (Free Discharge) r- t =Culvert (Passes 1.28 cfs of 9.40 cfs potential flow) 2=Orifice/Grate (Orifice Controls 0.69 cfs @ 7.86 fps) 3 Orifice/Grate (Weir Controls 0.60 cfs @ 1.18 fps) Broadway-Post-Development Type 1124-hr Mir:Atinfa1104.541,1 Prepared by Shimp Engineering, P.C. Printed 7/20/2012 .. HydroCAD®9.10 sin 07054 ©2011 HydroCAD Software Solutions LLC Page 5 Summary for Subcatchment 3: Ex. Parking Runoff = 4.34 cfs @ 12.01 hrs, Volume= 0.245 af, Depth> 4.18" a Runoff by SCS TR-20 method, UH=SCS, Time Span= 0.00-24.00 hrs, dt= 0.01 hrs Type II 24-hr 10yr Rainfall=5.54" aArea (sf) CN Description 17,556 80 >75% Grass cover, Good, HSG D 13,097 98 Impervious a30,653 88 Weighted Average 17,556 57.27% Pervious Area 13,097 42.73% Impervious Area Tc Length Slope Velocity Capacity Description (min) (feet) (ft/ft) (ft/sec) (cfs) 9.9 Direct Entry, Summary for Link 4: Point of Analysis Inflow Area = 0.933 ac, 42.07% Impervious, Inflow Depth > 4.15" for 10yr event Inflow = 5.35 cfs @ 12.04 hrs, Volume= 0.323 af Primary = r ► 12.04 hrs, Volume= 0.323 af, Atten= 0%, Lag= 0.0 min Primary outflow = Inflow, Time Span= 0.00-24.00 hrs, dt= 0.01 hrs a a a a a Broadway-Post-Development Type 1124-hr <a #; ,. 6` Prepared by Shimp Engineering, P.C. Printed 7/20/2012 HydroCAD®9.10 s/n 07054 ©2011 HydroCAD Software Solutions LLC Page 6 Summary for Pond 2: Underground Detention Inflow Area = 0.229 ac, 40.06% Impervious, Inflow Depth > 7.51" for 100yr event Inflow = 2.37 cfs @ 12.02 hrs, Volume= 0.144 of Outflow = 2.37 cfs @ 12.03 hrs, Volume= 0.144 af, Atten= 0%, Lag= 0.2 min Primary = 2.37 cfs @ 12.03 hrs, Volume= 0.144 af Routing_ _ by Stor-Ind method, Time Span= 0.00-24.00 hrs, dt= 0.01 hrs ' @ 12.03 hrs Surf.Area= 132 sf Storage= 425 cf Plug-Flow detention time= 3.3 min calculated for 0.143 af(100% of inflow) "" Center-of-Mass det. time= 3.1 min ( 784.0 - 780.9 ) Volume Invert Avail.Storage Storage Description "" #1 386.00' 481 cf 42.0" D x 50.0'L Pipe Storage S=0.0025 '/' Device Routing Invert Outlet Devices ••. #1 Primary 386.00' 15.0" Round Culvert L= 55.0' Ke= 0.400 Inlet/ Outlet Invert= 386.00'/ 385.50' S= 0.0091 '/' Cc= 0.900 n= 0.010 #2 Device 1 386.00' 4.0"Vert. Orifice/Grate C= 0.600 .. #3 Device 1 388.70' 15.0" Horiz. Orifice/Grate C= 0.600 Limited to weir flow at low heads "" Primary OutFlow Max=2.36 cfs @ 12.03 hrs HW=388.96' (Free Discharge) L =Culvert (Passes 2.36 cfs of 9.67 cfs potential flow) 2 Orifice/Grate (Orifice Controls 0.70 cfs @ 8.04 fps) .. 3=Orifice/Grate (Weir Controls 1.66 cfs @ 1.65 fps) .. f Independent Reports NOAA Precipitation Report .. .. .. — .r 7/19/12 Precipitation Frequency Data Server ® NOAA Atlas 14,Volume 2,Version 3 N\ Location name:Charlottesville,Virginia, US* I non k Coordinates:38.0213, -78.4644 V.0 Elevation:381 ft* s *source:Google Maps „T or POINT PRECIPITATION FREQUENCY ESTIMATES G.M.Bonnin,D.Martin,B.Lin,T.Parzybok M.Yelda,and D.Riley gm NOAA,National Weather Service,Silver Spring,Maryland PF tabular I PF graphical I Maps & aerials imi PF tabular PDS-based point precipitation frequency estimates with 90% confidence intervals (in inches)1 'r" Average recurrence interval (years) Duration;._-_' ..__. • 1 I 2 5 I 10 25 50 100 200 ; 500 1000 ' 5-min dile 0.353 i 0.419 0.491 0.552 0.622 0.676 ? 0.726 ' 0.773 0.829 0.875 r (0.318.0.392)5(0.378-0.466):(0.442-0.544);(0.496-0.612);(0.556-0.686)'(0.602-0.745) (0.643-0;800)'(0.680,0.852)'(0.722-0.916);(0.755-0.970) 10-min 0.563 € 0.671 0.786 0.883 0.991 1.08 1.15 • 1.23 1.31 1.38 (0.507-0.626):(0.605-0.745) (0.708-0.871) (0.794-0.978) (0.886-1.09)' (0.959-1.19)`: (1.02-1.27) (1.08-1.35) ' (1.14-1.45) (1.19-1.53) ... 15-min 0.704 ; 0.843 ; 0.995 1.12 1.26 1.36 1.46 1.55 1.65 1.73 . (0.634-0.783)3(0.760-0.936) (0.895-1.10)5 (1.00-1.24) (1.12-1.39) ' (1.21-1.50) (1.29-1.61) (1.36-1.70) (1.44-1.82) (1.49-1.92) 30-min 0.966 1.16 1.41 1.62 1.86 2.05 2.23 2.41 2.63 , 2.80 (0.869-1.07)• (1.05-1.29) (1.27-1.57),5 (1.46-1.79) (1.66-2.05) i (1.83-2.26) , (1;98-2.46) ; (2.12-2.65) i (2.29-2.90) (2.42-3.10) . "' 1.81 2.11 2.48 2.78 ; 3.08 3.38 3.77 4.09 60-min (1082.34) € (1.32-1.62) (1.63-2.01) i (1.89-2.33) (2.22-2.73) (2.48-3.07) (2.72-3.39) F (2.97-3.72) (3.28-4.16) (3.53-4.53) 1.45 - ..... ._..... 2-hr ? 1.75 , 2.19 2.58 3.07 , 3.48 3.90 4.34 4.94 5.45 (1,27,1.66) i (1.54-2.00) i J1:92-2.49).) (2.26-2.92) (2.67-3.47) . (3:02-3.94)., (3.36-4.41) (3.71-4.89) i (4.18-5.58) (4.57-6.17) MO .. 3-hr 1.58 ! 1.92 2.39 2.81 3.35 3.80 4.26 4.75 5.41 5.97 ; (1.39-1.82) ; (1.68-2.20) (2.09-2.75) j (2,45-3.22) • (2.90-3.83) (3.28-4.33) (3;66-4.86) J-(4.04-5.41) ; (4;54-6.17) (4.97-6.82) 6-hr 2.02 2.43 3.02 i 3.56 4.27 4.90 5.56 1 6.26 7.26 8.15 AM (1,78-2 29) 3 (2.15-2.76) (2.65-3.42) . (3.12-4.03) (3.72-4.83) > (4.24-5.52) (4.76-6.26) ' (5.31-7.07) (6.07-8.19) : (6.71-9.20) 3 ' 12-hr ; 2.53 =. 3.05 3.80 i 4.50 5.48 6.35 7.29 8.32 ; 9.84 11.2 - , .. ; (2.23-2.90) € (2.69-3.49) , 5(3.34-4.35) ! (3.95-5.14) (4.76-6.23) i (5.46-7.21) ° (6.20-8.27) , (6.98-9.42) " (8.09,11.1) (9.07-12.7) 24-hr 3.03 t, 4.68 O 6.81 7.90 t;i 10.4 12.4 14.1 a. _f (2.72-3.40)_ (3.29-4.11) [ (4.19-5.25) i (494-6.20) (6.04-7.59) (6.95-8.79) (7.95-10.1) (9.01-11.6) •' (10.6-13.8) (11.8-15.6 2-day 3.57 j 4.33 5.50 6.48 7.88 > 9.06 10.3 11.7 13.8 15.5 (3.20-3.99) (3.89-4.84) , (4.93-6.15) ` (5.78-7.22) (6.99-8.77) , (7.98-10.1) ; (9.04-11.5) ' (10.2-13.1) , (11.8-15.4) (13.1-17.3) .r 3-day ' 3.80 4.61 5.85 6.88 8.37 9.62 11.0 12.4 14.6 16.4 (3.45,4.21) i (4.18-5.10) (5.30-6.48) : (6.21-7.61) (7.52-9.24) (8.58-10.6) , (9.71-12.1) (10.9-13.7) (12.7-16.2) (14.1-18.2) 4-day ; 4.03 1 4.88 6.20 7.29 8.86 10.2 11.6 13.1 15.4 17.3 (3.70 4.43) (4.47-5.37) (5.67-6.81) ° (6.65-7.99) (8.05-9.71) (9.19-11.2) ; (10.4-12.7) • (11.7-14.4) ; (13.5-16.9) (15.0-19.1) a 7-day 4.68 5.64 7.05 8.21 9.88 11.3 12.8 14.4 ; 16.7 18.6 ; - , (4.31-5.11) (5.19-6.15) ' (6,47 -7.69) (7.52-8.95) (9.00-10.8) ,(10.2-12.3)s (11.5-13.9) ,,,(12.8-15.7),.: (14.7-18.2) (16.2-20.4) 10-da 5.31 6.37 7.87 9.09 10.8 12.2 13.7 ; 15.3 17.5 19.4 y ' (4.91.5.74) i (5.89-6.89) (7.27-8.51) i (8.37-9.82) (9.91-11.7) (11.1-13.2) : (12.4-14.8) , (13.8-16.5) (15.6-19.0) ; ,(17.1-21.1) 20-da 6.97 ' 8.32 10.1 11.4 13.3 ' 14.7 16.2 ; 17.7 19.8 21.4 ,y • (6.53-7.46).2 (7.80-8.90) ' (9.41-10.8),, (10.7-12.2) (12.3-14.2) ; (13.6-15.7) ; (15.0-17.3),,,; (16.3-19.0) (18.1-21.3) (19.4-23.1) 30-da 8.56 10.1 ' 12.0 ; 13.4 15.3 16.7 18.0 19.4 21.1 22.5 i , y ; (8.05-9.11) 5 (9.55-10.8) (11.3-12.8) 3 (12.6-14.3) ' (14.3-16.2) (15.6-17.7) ; (16.8-19.2) , (18.0-20.6) (19.5-22.6) (20.7-24.1) 10.7 12.6 14.8 16.3 18.4 19.9 21.4 22.8 • 24.6 26.0 45-day (10.1-11.3) ` (11.9-13.4) (13.9-15.6) (15.4-17.3) (17.3-19.5) (18.7-21.1) ' (20.0-22.7) (21.3-24.2) 5 (22.9-26.2) (24.0-27.7) ma 12.5 • 14.7 17.0 18.7 20.9 22.5 24.0 . ^.._25.5 27.3 28.7 _ . 60-day ; (11.9-13.2),^( (14.0-15.6) f� (16.1-18.0 17.7-19.8) ) (19.8-22.1) (21.2-23.7 i_.. .. ... ) 3 ( ) ; (22.6-25.4) (23.9-27.0) ; (25.6-29.0) � (26.7-30.5) ' I Precipitation frequency(PF)estimates in this table are based on frequency analysis of partial duration series(PDS). ", Numbers in parenthesis are PF estimates at lower and upper bounds of the 90%confidence interval.The probability that precipitation frequency estimates(for a given duration and average recurrence interval)w ill be greater than the upper bound(or less than the lower bound)is 5%.Estimates at upper bounds are not checked against probable maximum precipitation(PNP)estimates and may be higher than currently valid PIvPvalues. Please refer to NOAA Atlas 14 document for more information. mil Back to Top PF graphical di.hdsc.nws.noaa.gov/hdsc/pfds/pfds printpage.html?lat=38.0213&Ion=-78.4644&data=depth&units=eng... 1/4