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HomeMy WebLinkAboutWPO201500068 Calculations 2015-10-05 HOLLYMEAD TOWN CENTER STORMWATER MANAGEMENT CALCULATIONS PACKET Date of Calculations AUGUST 3, 2015 Revised on SEPTEMBER 11, 2015 Revised on SEPTEMBER 17, 2015 Revised on SEPTEMBER 28, 2015 Revised on OCTOBER 5, 2015 (Revisions noted in Red) PREPARED BY: COLLINS 'ENGINEERING 200 GARRETT STREET, SUITE K CHARLOTTESVILLE,VA 22902 434.293.3719 PH 434.293.2813 FX www.collins-engineering.com SCS TR-55 Calculations U.S.Department of Agriculture FL-ENG-21A Natural Resources Conservation Service 06/04 TR 55 Worksheet 2:Runoff Curve Number and Runoff Project: WPO Plan for SWM Facility#1(SB conversion) Designed By: FGM,PE Date: 9/10/201t Location: Hollymead Town Center Checked By: SRC,PE Date: 9/10/2015 Check One: Present X Devi 1.Runoff curve Number(CN) Cover description Drainage Soil name and CN(weighted)= (Cover type,treatment,and hydrologic condition; Area Product of Calculated Area hydrologic group CN total product/ Description percent impervious;unconnected/connected (Acres) CN x Area 'S'Value p (Appendix A) total area impervious area ratio) DA A Impervious Areas(Dickerson Dr+Willow Glen) 98 0.63 61.9 (Present) B Woods in Good Condition(Remainder of watershed) 55 12.31 677.0 57.1 7.51 Lawns in Good Condition 61 0.00 0.0 DA A Impervious Areas(Exist+Future Dev+Towncenter Dr) 98 7.25 710.8 (Developed) B Woods in Good Condition 55 0.00 0.0 81.7 2.23 Lawns in Good Condition 61 5.69 346.9 2.Runoff 1-Year Storm 2-Year Storm Drainage Area Description Frequency-years 1 2 n/a Rainfall,P(24 hour)-inches 3.5 3.7 n/a Runoff,Q-inches 0.42 0.50 DA A(Present) Runoff,Q-inches 1.76 1.93 DA A(Developed) Runoff,Q-inches Runoff,Q-inches U.S.Department of Agriculture FL-ENG-21A Natural Resources Conservation Service 06/04 TR 55 Worksheet 3:Time of Concentration(T,)or Travel Time(Ti) Project: WPO Plan for SWM Facility#1(SB conversion) Designed By: FGM,PE Date: 8/3/2015 Location: Hollymead Town Center Checked By: SRC,PE Date: 8/3/2015 Check One: Present X Developed X Check One: T, X Tt Through subarea n/a DAA DA A Segment ID: (Present) (Developed) Sheet Flow:(Applicable to T,only) 1 Surface description(Table 3-1) Dense Grasses Dense Grasses 2 Manning's roughness coeff.,n(Table 3-1 0.24 0.24 3 Flow length,L(total L<100)(ft) 100 100 4 Two-year 24-hour rainfall,P2(in.) 3.7 3.7 5 Land slope,s(ft/ft) 0.02 0.02 6 Compute Tt=[0.007(n*L)°S]/P2(15 s°'4 0.22 0.22 Shallow Concentrated Flow: 7 Surface description(paved or unpaved) unpaved 8 Flow Length,L(ft) 1170 9 Watercourse slope,s(ft/ft) 0.06 10 Average velocity,V(Figure 3-1)(ft/s) 4.0 Estimated 11 Tt=L/3600*V 0.08 0.03 Channel Flow: 12 Cross sectional flow area,a(ft 2) 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=[1.49r213s°5]/n 18 Flow length,L(ft) 19 Tt=L/3600*V 20 Watershed or subarea T,or Tt (Add Tt in steps 6,11 and 19) 0.30 0.25 U.S. Department of Agriculture FL-ENG-21C Natural Resources Conservation Service 06/04 TR 55 Worksheet 4:Graphical Peak Discharge Method Project: WPO Plan for SWM Facility#1(SB conversion) Designed By: FGM,PE Date: 9/28/2015 Location: Hollymead Town Center Checked By: SRC, PE Date: 9/28/2015 Check One: Present X Developed X Drainage Area Drainage Area Drainage Area Drainage Area 1. Data Description Description Description Description DAA DA A (Present) (Developed) Drainage Area (Am) in miles2= 0.0202 0.0202 Runoff curve number CN= 57 82 Time of concentration (Tc)= 0.30 0.25 Rainfall distribution type= II II Pond and swamp areas spread 0 0 throughout the watershed= 2.Frequency-years 1 2 1 2 3. Rainfall,P(24 hour)-inches 3.50 3.7 3.50 3.7 4. Initial Abstraction, la-inches 1.503 1.503 0.447 0.447 5.Compute Ia/P 0.43 0.41 0.13 0.12 6. Unit peak discharge,Qu-csm/in 425 450 700 700 7. Runoff,Q from Worksheet 2-inches 0.42 0.50 1.76 1.93 8. Pond and Swamp adjustment factor, Fp 1 1 1 1 9. Peak Discharge,Qp-cfs Routed Through where Qp=Qu Am Q Fp 3.60 4.52 SWM Facility Watershed Summary Pre-Development Future Post-Development Area 1-Year Outfall 2-Year Outfall Area 1-Year Outfall 2-Year Outfall Description (acres) CN (cfs) (cfs) (acres) CN (cfs) (cfs) DA A 12.94 57.1 3.60 4.52 12.94 81.7 0.72 4780 0.67 1.71 Post-Development Routings (Following 3 Sheets were Revised) & Channel Protection Calculations HTC_TCdr_basin_1_ ADDED 1 YR STORM AND CHANGED SOME CONTOUR AREAS BasinFlow printout INPUT: Basin: HTC basin for future dev adn TC Dr.- ADDED 1 YR STORM AND CHANGED SOME CONTOUR AREAS 6 Contour Areas Elevation(ft) Area(sf) Computed vol . (cy) 521.00 18214.00 0.0 522.00 21493.00 734.5 523.00 23703.00 1571.1 524.00 25839.00 2488.3 525.00 27775.00 3480.9 526.00 30136.00 4553.0 Start_Elevation(ft) 521.00 vol . (cy) 0.00 6 Outlet Structures Outlet structure 0 Orifice name: riser outlet area (sf) 7.069 diameter or depth (in) 36.000 width for rect. (in) 0.000 coefficient 0. 500 invert (ft) 518.000 multiple 1 discharge out of riser Outlet structure 1 Orifice name: riser inlet 3in- INTO PROPOSED RISER area (sf) 0.049 diameter or depth (in) 3.000 width for rect. (in) 0.000 coefficient 0. 500 invert (ft) 521.000 multiple 1 discharge into riser outlet structure 2 Orifice name: riser inlet 3in- INTO PROPOSED RISER area (sf) 0.049 diameter or depth (in) 3.000 width for rect. (in) 0.000 coefficient 0.500 invert (ft) 522.600 multiple 1 discharge into riser outlet structure 3 Weir name: Existing Riser top- WITH SEALED ORIFICES diameter (in) 60.000 side angle 0.000 coefficient 3.300 invert (ft) 523.740 multiple 1 discharge into riser transition at (ft) 1.519 orifice coef. 0.500 orifice area (sf) 19.635 Page 1 HTC_TCdr_basin_1_ ADDED 1 YR STORM AND CHANGED SOME CONTOUR AREAS Outlet structure 4 weir name: Proposed Riser top diameter (in) 36.000 side angle 0.000 coefficient 3.300 invert (ft) 523.600 multiple 1 discharge into riser transition at (ft) 0.912 orifice coef. 0.500 orifice area (sf) 7.069 Outlet structure 5 Weir name: emergency spillway length (ft) 35.000 side angle 71.600 coefficient 3.300 invert (ft) 524.400 multiple 1 discharge through dam 4 Inflow Hydrographs Hydrograph 0 SCS name: lyr post dev captured Area (acres) 12.940 CN 81.700 Type 2 rainfall , P (in) 3.500 time of conc. (hrs) 0.2500 time increment (hrs) 0.0200 time limit (hrs) 30.000 fudge factor 1.00 routed true peak flow (cfs) 23.153 peak time (hrs) 12.003 volume (cy) 3058.579 Hydrograph 1 SCS name: 2yr post dev captured Area (acres) 12.940 CN 81.700 Type 2 rainfall , P (in) 3.700 time of conc. (hrs) 0.2500 time increment (hrs) 0.0200 time limit (hrs) 30.000 fudge factor 1.00 routed true peak flow (cfs) 25.330 peak time (hrs) 12.003 volume (cy) 3346.101 Page 2 HTC_TCdr_basin_1_ ADDED 1 YR STORM AND CHANGED SOME CONTOUR AREAS Hydrograph 2 SCS name: l0yr post dev captured Area (acres) 12.940 CN 81.700 Type 2 rainfall , P (in) 5.600 time of conc. (hrs) 0.2500 time increment (hrs) 0.0200 time limit (hrs) 30.000 fudge factor 1.00 routed true peak flow (cfs) 47.233 peak time (hrs) 12.003 volume (cy) 6239.581 Hydrograph 3 SCS name: 100yr post dev captured Area (acres) 12.940 CN 81.700 Type 2 rainfall , P (in) 9.100 time of conc. (hrs) 0.2500 time increment (hrs) 0.0200 time limit (hrs) 30.000 fudge factor 1.00 routed true peak flow (cfs) 90.402 peak time (hrs) 12.003 volume (cy) 11942.307 OUTPUT: Routing Method: storage-indication Hydrograph 0 Routing Summary of Peaks: lyr post dev captured inflow (cfs) 23.125 at 12.00 (hrs) discharge (cfs) 0.673 at 13.56 (hrs) water level (ft) 523.631 at 13.58 (hrs) storage (cy) 2141.067 Hydrograph 1 Routing Summary of Peaks: 2yr post dev captured inflow (cfs) 25.299 at 12.00 (hrs) discharge (cfs) 1.707 at 12.78 (hrs) water level (ft) 523.714 at 12.76 (hrs) storage (cy) 2217.698 Hydrograph 2 Routing Summary of Peaks: l0yr post dev captured inflow (cfs) 47.175 at 12.00 (hrs) discharge (cfs) 37.961 at 12.08 (hrs) water level (ft) 524.273 at 12.08 (hrs) storage (cy) 2752.407 Hydrograph 3 Routing summary of Peaks: 100yr post dev captured inflow (cfs) 90.291 at 12.00 (hrs) discharge (cfs) 82.490 at 12.06 (hrs) water level (ft) 524.685 at 12.06 (hrs) storage (cy) 3160.624 Thu Oct 01 09:27:31 EDT 2015 Page 3 gia.4 5 .�.►,� �. ? . \JA . . 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Tt/itA �k'J� �, T15 0 O, 6'7 1999 Grandfathered VSMH Calculations PERFORMANCE-BA )WATER QUALITY CALCULATIO APPENDIX 5D Worksheet 1 Page 1 of 3 STEP 1 Determine the applicable area(A) and the post-developed impervious cover (I post). Applicable area(A)* = ; .` acres ( (4., 0161,. Y) Post-development impervious cover: rwigi k)Ev. strustsrdes = •r . ';r Q acres 4a C./ I�':mo&J fc A, -paeking-lot _ 61'3 i l j aces ' '}' i r, feadway _ a owes has+ other: acres Total = "T AS acres Ipon=(total post-development impervious cover=A) x 100= % * The area subject to the criteria may vary from locality to locality. Therefore, consult the locality for proper determination of this value. STEP 2 Determine the average land cover condition(I„,t,,,,,1ed)IE the existing impervious cover(I, ). Average land cover condition(Iwatershed): If the locality has determined land cover conditions for individual watersheds within its jurisdiction,use the watershed specific value determined by the locality as Iwaterahed o watershed _ o Otherwise,use the Chesapeake Bay default value: V7Iwaterahed = 16% 5D-5 PERFORMANCE-BASED WATER QUALITY CALCULATIONS APPENDIX 5D Worksheet 1 Page 2 of 3 Existing impervious cover(Listing).: Determine the existing impervious cover of the development site if present. Existing impervious cover: VII Via uk.m.4�yGr:•� r structures-& = i f1L` tlt1 acres. 'T Gi, pta: = "1 t.aOO awes-, roadway = acres other: acres acres Total = 0.63 acres Ie„,sti�e=(total existing impervious cover_A*) x 100= 101,. * The area should be the same as used in STEP 1. STEP 3 Determine the appropriate development situation. The site information determined in STEP 1 and STEP 2 provide enough information to determine the appropriate development situation under which the performance criteria will apply. Check(. )the appropriate development situation as follows: Situation 1: This consists of land development where the existing percent impervious cover(I ;s,;2,8)is less than or equal to the average land cover condition (watershed)and the proposed improvements will create a total percent impervious cover(I.)which is less than or equal to the average land cover condition(Iwatershed)• 'post %•gwatershed SD-6 PERFORMANCE-BASED WATER QUALITY CALCULATIONS APPENDIX 5D Worksheet 1 Page 3 of 3 / Situation 2: This consists of land development where the existing percent impervious cover(Ie,tstir,g)is less than or equal to the average land cover condition ('watershed) and the proposed improvements will create a total percent impervious cover(I post) which is greater than the average land cover condition(Iwatershed)• 'existing . %"'watershed «' %; and 'post ' -" %>'watershed 16 /o Situation 3: This consists of land development where the existing percent impervious cover(Ie„,stia$)is greater than the average land cover condition (Iwatershed)• 'existing %>'watershed % Situation 4: This consists of land development where the existing percent impervious cover(Ie7 ,,„g)is served by an existing stormwater management BMP(s) that addresses water quality. If the proposed development meets the criteria for development Situation 1,than the low density development is considered to be the BMP and no pollutant removal is required. The calculation procedure for Situation 1 stops here. If the proposed development meets the criteria for development Situations 2,3,or 4,then proceed to STEP 4 on the appropriate worksheet. 5D-7 PERFORMANCE-BASED WATER QUALITY CALCULATIONS APPENDIX 5D Worksheet 2: Situation 2 Page 1 of 4 Summary of Situation 2 criteria: from calculation procedure STEP 1 thru STEP 3, Worksheet 1: Applicable area(A)* _ 1`e:).,`11 acres I�st =(total post-development impervious cover_A) x 100= `56.0 % 'watershed = % Or Iwatern = 16% 'existing =(total existing impervious cover_A*) x 100= % 'existing 41 % htwatershed %; and 'post -:'V.3 et� %>'watershed 16 % STEP 4 Determine the relative pre-development pollutant load (Lpre). Lpre(watershed)= [0.05 +(0.009 x 'watershed)] x A x 2.28 (Equation 5-16) where: Lpre(watershed) = relative pre-development total phosphorous load (pounds per year) 'watershed = average land cover condition for specific watershed or locality or the Chesapeake Bay default value of 16%(percent expressed in whole numbers) A = applicable area(acres) Lpre(watershed)=[0.05 +(0.009 x 16 )] x 19.0'14 x 2.28 = `'a."T ik pounds per year 5D-9 PERFORMANCE-BASED WATER QUALITY CALCULATIONS APPENDIX 5D Worksheet 2 : Situation 2 Page 2 of 4 STEP 5 Determine the relative post-development pollutant load (Lpo,t). Lpost = [0.05 + (0.009 x Ipost)] x A x 2.28 (Equation 5-21) where: Lp5t = relative post-development total phosphorous load (pounds per year) I t = post-development percent impervious cover(percent expressed in whole numbers) A = applicable area(acres) Lpust = [0.05 +(0.009 x 56.0 )] x ( ..`!Li x 2.28 = •.3"', pounds per year STEP 6 Determine the relative pollutant removal requirement(RR). RR = Lpost ~I-'pre(watershed) RR = s,r1a = ` pounds per year STEP 7 Identify best management practice (BMP)for the site. 1. Determine the required pollutant removal efficiency for the site: EFF = (RR-Lpost) x 100 (Equation 5-22) where: EFF = required pollutant removal efficiency (percent expressed in whole numbers) RR = pollutant removal requirement(pounds per year) Lpost = relative post-development total phosphorous load(pounds per year) EFF = ( ‘0. x 100 - 65 % 5D-10 PERFORMANCE-BASED WATER QUALITY CALCULATIONS APPENDIX 5D Worksheet 2 : Situation 2 Page 3 of 4 2. Select BMP(s)from Table 5-15 and locate on the site: BMP 1: N1' tai,..! j' - a^r ( 4yr: =') BMP 2: BMP 3: 3. Determine the pollutant load entering the proposed BMP(s): LBmp = [0.05+(0.009 x Inmp)] x A x 2.28 (Equation 5-23) where: LB,,,g, = relative post-development total phosphorous load entering proposed BMP(pounds per year) IBmp = post-development percent impervious cover of BMP drainage area (percent expressed in whole numbers) A = drainage area of proposed BMP(acres) LBmpi = [0.05+(0.009 x 5 .c )] x x 2.28 = 1 6.3 LS pounds per year LBMPZ = [0.05 +(0.009 x )] x x 2.28 pounds per year Lam, = [0.05+(0.009 x )] x x 2.28 pounds per year 5D-11 PERFORMANCE-BASED WATER QUALITY CALCULATIONS APPENDIX 5D Worksheet 2 : Situation 2 Page 4 of 4 4. Calculate the pollutant load removed by the proposed BMP(s): "'removed = EffBMP X LBMP (Equation 5-24) where: Lremoved = Post-development pollutant load removed by proposed BMP (pounds per year) EffB1,,11, = pollutant removal efficiency of BMP(expressed in decimal form) LB1„. = relative post-development total phosphorous load entering proposed BMP(pounds per year) c Lremoved/BMP1 = 0. 5 x 16°'`�1-` = \O.Ga. pounds per year X = pounds per year Lremoved/BMP2 = X = pounds per year Lremoved/s11IP3 = 5. Calculate the total pollutant load removed by the BMP(s): Lremoved/total = Lremoved/BMP1 +Lremoved/BMP2+Lremoved/BMP3+ • • • (Equation 5-25) where: Lremoved/total =total pollutant load removed by proposed BMPs Lremoved/BMPi =pollutant load removed by proposed BMP No. 1 =pollutant load removed by proposed BMP No. 2 LremovedlBMP2 LremovecU13MP3=pollutant load removed by proposed BMP No. 3 Lremoved/total = 19,63. + ',,/A + !,i/A + = pounds per year e 6. Verify compliance: Lremoved/total 411 19,6a. = ko.ga 5D-12