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HomeMy WebLinkAboutWPO201500057 Calculations 2018-01-18 AVINITY II STORMWATER MANAGEMENT CALCULATIONS PACKET (VSMP Amendment #1) Date of Originally Approved WPO 201600004 Calculations October 26, 2016 Date of WPO Amendment #1 Calculations January 18, 2018 PREPARED BY: CE COLLINS ESNGtNEERING 200 GARRETT STREET, SUITE K CHARLOTTESVILLE, VA 22902 434.293.3719 PH 434.293.2813 FX www.collins-engineering.com Amendment #1 Water Quality Calculations Summary: The proposed amendment modifies the development's impervious areas, includes newly added preservation areas and utilizes an updated Virginia Runoff Reduction Method Spreadsheet (3rd version). The following calculations also remove the previously approved two bioretention basins and two dry swales. The removal of these facilities is offset by the best management practice of purchasing nutrient credits, preserving opening spaces and a manufactured BMP. Soils Mapping (Source: NRCS Web Soil Survey Online Database) Soils Table (Source: NRCS Web Soil Survey Online Database) Albemarle County,Virginia(VA003) Albemarle County, Virginia (VAQQ3) Map Unit _..._ Percent of Symbol Map Unit Name AOI 716 Rabun clay 35.7% loam, 2to7 percent slopes 72C3 Rabun clay, 7 to 49.9%' 15 percent slopes, severely eroded 7203 Rabun clay, 15 13.5% to 25 percent slopes, severely eroded 79B Starr silt loam, 0.9% 2 to 7 percent slopes Totals for Area of 100.0% Interest Note:The NRCS web soil survey has catorgized this parcel's soil types as predominantly consisting of hydrologic group types B. Precipitation Data (Source: NOAA's National Weather Service, Hydrometerorological Design Studies Center, Atlas 14 Point Precipitation Frequency Estimates) Map et Terrain - F ,% v r u 4 ' �; 5 - .14aDT " ,,'`, ri 0 r C„ 4.6 14 „s O 46rr 41',. _ I .21112 POINT PRECIPITATION FREQUENCY (PF) ESTIMATES WITH 90%CONFIDENCE INTERVALS AND SUPPLEMENTARY INFORMATION NOAA Atlas 14,Volume 2,Version 3 PDS-based precipitation frequency estimates with 90%confidence intervals(in inches)1 1 hvera e rectmeme 021erv_,ai(years} Dur I i _ 2—II 5 �II 10 It I( so II 100 Il 290 "11 soct 11 ; ___1 24.9r 3,04 3.67 4.70 5.55 6.83 7.92 9.13 10.5 12.5 142 (273-341) (3,30-4.12) ( (4.21-5.27) �� (496.6.21) (6i06-780) (6.988.80) (7.97-10.1) (904-116) �� (10.6-13.8) (119-157) n.e 050 vino,*Runoff Reduction Methad7 eOlDeNelilgtl>nt - r ®20aa 8MP.Standards and SpeNRcatbana 72013 Draft aMP standards and SpedRixtfo n 4 1 Project Name: Avinity II VSMP Amendment Rl 1/18/2018 CLEAR ALL date input mns Date. constant values BMP Design Specifications List.2011 Stds&Specs aituletteene ;Site Information,Site It, Post-Development Project(Treatment Volume and Loads) land Cover(acres) A Soils B Soils C Soils D Soils Totals forest/Open Space(acres)—undisturbed. 0.32 Protected forest/open space or reforested land 0.32 Managed Turf(acres)—disturbed,graded for .yards or other turf to be mowed/managed 5.68 5b8 !impervious Cover(acres) 5.89 5.89 •Forest/Open Space areas must be protected in accordance with the Virginia Runoff Reduction Method ;11.89 ;Constants Runoff Coefficients(Rv) ;Annual Rainfall(inches) 43 A Soils B Soils C Soils D Soils 'Target Rainfall Event(inches) 1.00 Forest/Open Space 0.02 0.03 0.04 0.05 Total Phosphorus)TP)EMC(mg/L) 0.26 Managed Turf 0.15 0.20 0.22 0.25 )Total Nitrogen(TN)EMC(mg/0 1.86 Impervious Cover 0.95 0.95 0.95 0.95 Target TP Load(lb/acre/yr) 0.41 ?Pl lunitless correction(actor) 0.90 Post-Development Requirement for Site Area TP Load Reduction Required(Ib/yr) 10.50 LAND COVER SUMMARY—POST DEVELOPMENT Land Cover Summary Treatment Volume and Nutrient Loads Treatment Volume Forest/Open Space Cover(acres) 0.32 115618 (acre-ft) Weighted Rv(forest) OD 3 Treatment Volume(cubic teet) 24.470 %Forest 3% TP load(Ib/yr) 1537 Managed Turf Cover(acres) 5.58IIIIPNRNIPIIM 109.99 Weighted Rv(turf) 020 %Managed Turf 48%, Impervious Cover(acres) 5419 Rv(impervious) 0.95_. %Impervious 50% Site Area(acres) 1189 057 • • Drainage Area A wrRtan Am A Im1 Cpm I.mn) IIEAR BMPAIIEAS.. 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Osolk .w 0 k La.Corer Alwer..luelwm/ 0.13 -9.tP ...wwktw.r)n.l 0AS 0.45 0.95 lu.lvM.pl.ru.Av.R.b4lor Remw.l In DA.AII0/M 5A8 'Total US vw.D.r.bpm.m Trw.m.m Yaunw bDA.AIh'I I,R9 Stormwater Bess Management Rractices(RR=RunoH Reduction) -vbnr.9merepee.nn.n- rrtna A. wr.e'T.n rnwn„w. r In R.m.)Iw Tpe) r' rMrro.oho.u. pbr'" rrInRrNVRr M.w Mo.pl.r w rRkewwp.M—rI op.r..r.m rr..Bp.mr b� Credit Corer Crack Upstraurn y ..+n raw c..0056) 0.. aew095 R.p� YauY.lh` vaun. 7(l) r; b La.Ibw C'" • x.v...w.e Room I.e.vgwwm Rwrnlsow nl 50 P p I R°°heplY..nmueen4RR) I...imwe Dwwn.twm 411 solk m 0 0 P 0 0 am 0.00 11).5.wp o.moxnnn.owsar xs 0 0 0 0 o Dm OW Mw rat 0m 0m ..wr.wrm.Arm* OD wrllswTikl s0 0 0 0 u 0 0.m ace 0.00 am To On,Weil f..e m 0 P o 0 n 0m 0m Dm am nl%w wl - To "n`k 0i""x,rswr�) 0 e 0 0 n am om am 0.m 21 To Pain �I MR nI PO 0 0 o Ts ¢m 0m ¢m am Mlicro-Gaoretnotiwin�x lsw�nl m 0 0 0 0 w 0.00 am 0.m ram x.k.TeRw....R..arwop..e) 0 0 0 o o 0 om o.m om o.m wkmbwwm.:np.n,Aw.nekn n 0 0 P u am 0.00 Pro 600 3....emuwk rwemm�.l Ism.n) .s o o o Ts ram ¢m m Dm t,t x.b v.m..de r..eme .x lsp«nl TS a . ..k.er..erw.n tab sa.Is,er.R Io 0 0 0 II IN 0w raw 000 •t 666.per w nr.aa.wa pompon Amended �D 0 0 0 Is DPP : ¢m o00 DrY.L+IRIRR) a�lSa.nl 'I°, a o 0 P P n am a.m .......ram 0.00 urea,.Bowl en sb Sweeten too Isp.si'm.wmm.n n 0 0. SO O. a. 9® >.rnfab.000lwp .. , p.k.nr raw.+n Dv.nl m o 0 r ° ° om Dn ( ..a. 011 71 •E.Imeea Dmennon YoM@Ri BA cDn Dv.c n¢) o P 0 e 0.... 15 Pm om uaD U.m e.b.mnlsp.nsl is 0 u 0 0 Is 0.00 Dm 0m OM -.Tl.enb.m 0Ym/Drly 0mlo lml molar° w ..N. ., P P P 0 D600 OW 000 000 9.0.sn..eb. ...,w506 pxl1pi m 0 0 0 0 PM am 0m am .. to Mated C sr. so 0 0 o 0 0 0.00 0m OM o.m Isµ..iAN) roru lrepERvrous eovER TRUTSD Iace am c0U.):: oro ax.ncneEx oA. roauuRwpREwrnon IR D.A.Alnh ss.;,B A.AIb/krl roru n.o5vho0US RErurmNSu0ER APPLYING Rueopp Rcoucrr0R VRACncuwo.AAlb/vr) v SEE WATER IIOALITY COMPLIANCE TAB FOR SRE COMPLIANCE CALCULATIONS 10.Wn Swae,no RR) 1a.e.wN 5...rzlsaw nD 0 a a 0 n 001 0.00 Gm . DAG 11.EneLOOT Panw 0w PT/ u.Fw,a.PracOon u1 0 40 0 a Fo R 11 e.Fur,n.vaa.nlsPw P111 0 0 0 t3 Fs GAG VAD oaG: 0m 11 Penman.WRNane InR RRI 11..Ce..w.Na Wn..a Al DAN.RF1 D o G o. - G b Gm OCO em 0.0 VA. .N.O. WNS.a.211.c 023/ P G a a o TS 0. LU.Wet POSE(no RR) TR..W.Pend n11gacm1 0 a 0 0 x am am r.. Ya Wet Pone nrywR.PW.11S0e•n0 G G c G. a . .s Do11 G Y...MFann W..ul G P c n a n AGO om 400 O. 2.1.41 wRPwdnlOrn 1.1v.11low 02.1 a o b o a ES 4m -PAD 4110 0,10 ElyOrodynernk 0 0^:: ID Gm 4.G0 .'GAIT aW TOTAL IMPERVIOUS mvERTNEATED lac) 445:' MGCNECK OR TOTAL MANAGED TURF AREA TREATED(Lc) ':0]];;'::.., AREA CHECK:OA TOTAL PHOSPHORUS REMOVAL REQUIRED ON ATE11b/yr 1030 TOTAL PHOSPHORUS AVAILABLE FOR REMOVAL W 0.A A(Ib/Lr 1OS TOTAL PHOSPHORUS AEMOVED WITHOUT RUNOFF REDUCTION PRACTICES IN DA.A 11b/yr OSA TOTAL PHOSPHORUS REMOVED WITH RUNOFF REDUCTION PRACTICES IN DA.A(I0/yr 000 TOTAL PHOSPHORUS LOAD REDUCTION ACHIEVED IN D.A.A 11b/yr 034 TOTAL PHOSPHORUS REMAINING AFTER APPLYING RMP LOAD REDUCTIONS I E.G.AIlb/yr a5G SEE WATER QUALITY COMPLIANCE TAB FOR SITE COMPLIANCE CALCULATION! NITROGEN REMOVED WITH RUNOFF REDUCTION PEUCMES M C.A.A 1113/yr1 SOB NITROGEN REMOVED WITHOUT RUNOFF REDUCTION PRACTICES E D.A.A IIb/T.l D W TOTAL NITROGEN REMOVED IN O.A A(Ib/yrl^ 0.CO Amendment #1 Water Quantity Calculations Summary: The amended plan continues to include a dry detention cell, which attenuates and decreases outflows for the majority of the development. This SWM facility treats subarea 'A'. The amended plan also includes a plunge pool at the outlet of subarea 'B's storm sewer. The plunge pool dissipates the runoff prior to it exiting the parcel, and provides minor attenuation. This amendment plan and the attached calculations show the proposed modifications result in post-development amendment flows less than or equal to the previously approved post-development 1-, 2- and 10- year flows. Additionally, County engineering previously evaluated and approved the channel and flood protection calculations with the originally approved WPO plan, so this amendment's reduced flows ensure the amendment is still in compliance with County and State minimum requirements. As a result, no further analyses are required for quantity compliance with this amendment. U S Department of Agriculture FL-ENG-21A Natural Resources Conservation Service 06/04 TR 55 Worksheet 2:Runoff Curve Number and Runoff Project Avinity II Designed By FGM,PE Date• 1/18/2018 Location. South of Avinity I Checked By:SRC,PE Date: 1/18/2018 Check One. Present X Developed X 1 Runoff curve Number(CN) Soil name and Cover description CN(weighted)= Drainage Area Area Product of CN Calculated Description hydrologic group (Cover type,treatment,and hydrologic condition,percent CN ) total product/ (Appendix A) impervious;unconnected/connected impervious area ratio) (Acres x Area total area 'S'Value DA A Impervious Areas 98 4.32 423.3 (Amended) B Woods in Good Condition(Offsite) 55 0.76 41.8 80 5 2 42 Lawns in Good Condition(75%+Groundcover) 61 2.87 175 1 Impervious Areas 98 1.44 141.3 DA B B Woods in Good Condition 55 0 00 0 0 90.0 1.11 (Amended) Lawns in Good Condition(75%+Groundcover) 61 0.40 24.3 2 Runoff 1-Year Storm 2-Year Storm 10-Year Storm Drainage Area Description Frequency-years 1 2 10 n/a Rainfall,P(24 hour)-inches 3.04 3.67 5.55 n/a Runoff,Q-inches 1 31 1.81 3 43 DA A(Amended) Runoff,Q-inches 2 02 2 61 4 41 DAB(Amended) U.S. Department of Agriculture FL-ENG-21A Natural Resources Conservation Service 06/04 TR 55 Worksheet 3:Time of Concentration(TO or Travel Time(Tt) Project: Avinity II Designed By: FGM,PE Date: 1/18/2018 Location: South of Avinity I Checked By: SRC, PE Date: 1/18/2018 Check One: Present X Developed X Check One: Tt X Tt Through subarea n/a DA A DA B Segment ID: (Amended) (Amended) Sheet Flow:(Applicable to T,only) 1 Surface description(Table 3-1) 2 Manning's roughness coeff., n(Table 3-1 3 Flow length, L(total L<100)(ft) 4 Two-year 24-hour rainfall, P2(in.) 5 Land slope,s(ft/ft) 6 Compute Tt=[0.007(n*L)o e)/p205 s0.4 Shallow Concentrated Flow: 7 Surface description(paved or unpaved) 8 Flow Length, L(ft) 9 Watercourse slope,s(ft/ft) 10 Average velocity,V(Figure 3-1)(ft/s) 11 Tt=L/3600*V Channel Flow: 12 Cross sectional flow area,a(ft2) 13 Wetted perimeter,PW(ft) 14 Hydraulic radius,r=a/PW(ft) 15 Channel Slope,s(ft/ft) 16 Manning's Roughness Coeff,n 17 V=[1.49r213s05 I/n 18 Flow length, L(ft) 19 Tt=L/3600*V 20 Watershed or subarea Tc or Tt (Add Tt in steps 6, 11 and 19) 0.21 0.10 (Approved Tc) U.S.Department of Agriculture FL-ENG-21C Natural Resources Conservation Service 06/04 TR 55 Worksheet 4:Graphical Peak Discharge Method Project: Avinity II Designed By: FGM,PE Date: 1/18/2018 Location: South of Avinity I Checked By: SRC,PE Date: 1/18/2018 Check One: Present X Developed X Drainage Area Drainage Area Drainage Area Drainage Area 1.Data Description Description Description Description DA A DA B (Amended) (Amended) Drainage Area(Am)in miles2= 0.0124 0.0029 Runoff curve number CN= 80.5 90.0 Time of concentration(Tc)= 0.21 0.10 Rainfall distribution type= II II Pond and swamp areas spread 0 0 throughout the watershed= 2.Frequency-years 1 2 10 1 2 10 3.Rainfall,P(24 hour)-inches 3 04 3.67 5.55 3.04 3.67 5.55 4 Initial Abstraction,la-inches 0.48 0.48 0.48 0.22 0.22 0.22 5.Compute la/P 0.16 0.13 0 09 0.07 0.06 0.04 6.Unit peak discharge,Qu-csm/in 1000 1000 1000 1000 1000 1000 7.Runoff,Qfrom Worksheet 2-inches 1.31 1.81 3.43 2.02 2.61 4.41 8.Pond and Swamp adjustment factor,Fp 1 1 1 1 1 1 9.Peak Discharge,Qp-cfs See Routing where Qp=Qu Am Q Fp Calculations See Routing Calculations Avinity II Detention Basin A BasinFlow printout INPUT: Basin: Avinity II Detention Basin A 6 Contour Areas Elevation(ft) Area(sf) Computed Vol. (cy) 489.00 3425.00 0.0 490.00 4009.00 137.5 492.00 5260.00 479.8 494.00 6630.00 919.2 496.00 8130.00 1464.9 497.75 9340.00 2030.6 Start_Elevation(ft) 489.00 Vol. (cy) 0.00 5 Outlet Structures Outlet structure 0 Orifice name: Barrel area (sf) 0.785 diameter or depth (in) 12.000 width for rect. (in) 0.000 coefficient 0.500 invert (ft) 488.440 multiple 1 discharge out of riser Outlet structure 1 Orifice name: Low-Fow Orifice area (sf) 0.136 diameter or depth (in) 5.000 width for rect. (in) 0.000 coefficient 0.500 invert (ft) 489.000 multiple 1 discharge into riser Outlet structure 2 Orifice name: Low-Fow Orifice area (sf) 0.196 diameter or depth (in) 6.000 width for rect. (in) 0.000 coefficient 0.500 invert (ft) 493.500 multiple 3 discharge into riser Page 1 Avinity II Detention Basin A Outlet structure 3 Weir name: Riser diameter (in) 24.000 side angle 0.000 coefficient 3.300 invert (ft) 495.330 multiple 1 discharge into riser transition at (ft) 0.608 orifice coef. 0.500 orifice area (sf) 3.142 Outlet structure 4 Weir name: Emergency Spillway length (ft) 55.000 side angle 0.000 coefficient 3.300 invert (ft) 496.330 multiple 1 discharge through dam 4 Inflow Hydrographs Hydrograph 0 SCS name: 1-yr SCS 24-hr Design Storm Area (acres) 7.950 CN 80.500 Type 2 rainfall, P (in) 3.040 time of conc. (hrs) 0.2100 time increment (hrs) 0.0200 time limit (hrs) 30.000 fudge factor 1.00 routed true peak flow (cfs) 11.181 peak time (hrs) 11.982 volume (cy) 1400.926 Page 2 Avinity II Detention Basin A Hydrograph 1 SCS name: 2-yr SCS 24-hr Design Storm Area (acres) 7.950 CN 80.500 Type 2 rainfall, P (in) 3.670 time of conc. (hrs) 0.2100 time increment (hrs) 0.0200 time limit (hrs) 30.000 fudge factor 1.00 routed true peak flow (cfs) 15.421 peak time (hrs) 11.982 volume (cy) 1932.256 Hydrograph 2 SCS name: 10-yr SCS 24-hr Design Storm Area (acres) 7.950 CN 80.500 Type 2 rainfall, P (in) 5.550 time of conc. (hrs) 0.2100 time increment (hrs) 0.0200 time limit (hrs) 30.000 fudge factor 1.00 routed true peak flow (cfs) 29.204 peak time (hrs) 11.982 volume (cy) 3659.258 Hydrograph 3 SCS name: 100-yr SCS 24-hr Design Storm Area (acres) 7.950 CN 80.500 Type 2 rainfall, P (in) 9.130 time of conc. (hrs) 0.2100 time increment (hrs) 0.0200 time limit (hrs) 30.000 fudge factor 1.00 routed true peak flow (cfs) 57.554 peak time (hrs) 11.982 volume (cy) 7211.424 Page 3 Avinity II Detention Basin A OUTPUT: Routing Method: storage-indication Hydrograph 0 Routing Summary of Peaks: 1-yr SCS 24-hr Design Storm inflow (cfs) 11.173 at 11.98 (hrs) discharge (cfs) 1.042 at 12.54 (hrs) water level (ft) 492.840 at 12.60 (hrs) storage (cy) 651.910 Hydrograph 1 Routing Summary of Peaks: 2-yr SCS 24-hr Design Storm inflow (cfs) 15.410 at 11.98 (hrs) discharge (cfs) 2.093 at 12.42 (hrs) water level (ft) 493.893 at 12.44 (hrs) storage (cy) 892.990 Hydrograph 2 Routing Summary of Peaks: 10-yr SCS 24-hr Design Storm inflow (cfs) 29.183 at 11.98 (hrs) discharge (cfs) 8.498 at 12.20 (hrs) water level (ft) 496.217 at 12.20 (hrs) storage (cy) 1530.914 Hydrograph 3 Routing Summary of Peaks: 100-yr SCS 24-hr Design Storm inflow (cfs) 57.512 at 11.98 (hrs) discharge (cfs) 55.866 at 11.98 (hrs) water level (ft) 496.737 at 11.98 (hrs) storage (cy) 1693.487 Thu Jan 11 16:05:10 EST 2018 Page 4 Avinity II Plunge Pool BasinFlow printout INPUT: Basin: Avinity II Plunge Pool B 5 Contour Areas Elevation(ft) Area(sf) Computed Vol. (cy) 491.00 156.00 0.0 492.00 335.00 8.9 494.00 783.00 49.1 496.00 1344.00 127.0 498.00 2005.00 250.2 Start_Elevation(ft) 491.00 Vol. (cy) 0.00 4 Outlet Structures Outlet structure 0 Orifice name: Barrel area (sf) 0.196 diameter or depth (in) 6.000 width for rect. (in) 0.000 coefficient 0.500 invert (ft) 490.000 multiple 1 discharge out of riser Outlet structure 1 Orifice name: Low-Fow Orifice area (sf) 0.087 diameter or depth (in) 4.000 width for rect. (in) 0.000 coefficient 0.500 invert (ft) 491.000 multiple 1 discharge into riser Outlet structure 2 Weir name: Riser diameter (in) 6.000 side angle 0.000 coefficient 3.300 invert (ft) 496.750 multiple 1 discharge into riser transition at (ft) 0.152 orifice coef. 0.500 orifice area (sf) 0.196 Page 1 Avinity II Plunge Pool Outlet structure 3 Weir name: Emergency Spillway length (ft) 30.000 side angle 0.000 coefficient 3.300 invert (ft) 497.750 multiple 1 discharge through dam 3 Inflow Hydrographs Hydrograph 0 SCS name: 1-yr SCS 24-hr Design Storm Area (acres) 1.840 CN 90.000 Type 2 rainfall, P (in) 3.040 time of conc. (hrs) 0.1000 time increment (hrs) 0.0200 time limit (hrs) 30.000 fudge factor 1.00 routed true peak flow (cfs) 4.718 peak time (hrs) 11.917 volume (cy) 499.516 Hydrograph 1 SCS name: 2-yr SCS 24-hr Design Storm Area (acres) 1.840 CN 90.000 Type 2 rainfall, P (in) 3.670 time of conc. (hrs) 0.1000 time increment (hrs) 0.0200 time limit (hrs) 30.000 fudge factor 1.00 routed true peak flow (cfs) 6.088 peak time (hrs) 11.917 volume (cy) 644.523 Page 2 Avinity II Plunge Pool Hydrograph 2 SCS name: 10-yr SCS 24-hr Design Storm Area (acres) 1.840 CN 90.000 Type 2 rainfall, P (in) 5.550 time of conc. (hrs) 0.1000 time increment (hrs) 0.0200 time limit (hrs) 30.000 fudge factor 1.00 routed true peak flow (cfs) 10.293 peak time (hrs) 11.917 volume (cy) 1089.689 OUTPUT: Routing Method: storage-indication Hydrograph 0 Routing Summary of Peaks: 1-yr SCS 24-hr Design Storm inflow (cfs) 4.696 at 11.92 (hrs) discharge (cfs) 0.816 at 12.12 (hrs) water level (ft) 496.608 at 12.16 (hrs) storage (cy) 159.330 Hydrograph 1 Routing Summary of Peaks: 2-yr SCS 24-hr Design Storm inflow (cfs) 6.059 at 11.92 (hrs) discharge (cfs) 1.491 at 12.10 (hrs) water level (ft) 497.369 at 12.10 (hrs) storage (cy) 205.960 Hydrograph 2 Routing Summary of Peaks: 10-yr SCS 24-hr Design Storm inflow (cfs) 10.244 at 11.92 (hrs) discharge (cfs) 9.835 at 11.94 (hrs) water level (ft) 497.938 at 11.94 (hrs) storage (cy) 245.623 Thu Jan 11 16:11:12 EST 2018 Page 3