HomeMy WebLinkAboutWPO201300002 Calculations 2020-05-04PHASE I NORTHTOWN CENTER VSMP AMENDMENT #1
STORMWATER MANAGEMENT CALCULATIONS PACKET
Date of Calculations
APRIL 29, 2020
PREPARED BY:
COLLINS ENSINFFPINC-7
200 GARRETT STREET, SUITE K
CHARLOTTESVILLE, VA 22902
434.293.3719 PH
434.293.2813 FX
www.collins-engineering.com
Part IIC Stormwater Quantity Calculations
DA 1 Calculations
(Treating TMP 45-111A, 45-111C, 45-110,
Rte. 29's Turn Lane & a Portion of Gander Drive)
1 r rr � � iREA�in►G 'iM4'45-114, N5-1\%Aj LISR�C a''s TW-" I� � A ('c -z" tF-
��r'
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VIA
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rlr'i AC. 0T 1p1ppAS P+t CN- SS
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~ro wCa 1.o�3aooal
_ �k TM�S �'�^1`5 'FRoPc�S�•c A�'^e,.rONt�n�i� Gh��S : a
G .�� AC. -Y 1i3O\0 pia DF 3r^��A�ti oys PXA CN = l
l
nF 4 "'S P
A %= to.g5hc.
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Sw AVJ a 4 w1c�S \0^(e Few. c� S
rA
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ou1�I Gs CAUut-jc o .
APPROVED Northtown IIA Biofilter Routings WITH UPDATED CN
BasinFlow printout
INPUT:
Basin: APPROVED Northtown IIA Biofilter Routings WITH UPDATED CN Value
APPROVED 6 Contour Areas
Elevation(ft)
Area(sf)
446.00
12512.00
448.00
14639.00
450.00
17031.00
452.00
19552.00
454.00
21891.00
455.00
23001.00
Start—Elevation(ft)
APPROVED 3 Outlet Structures
Outlet structure 0
Orifice
Computed Vol.(cy)
0.0
1004.6
2176.4
3530.3
5064.4
5895.6
446.00 Vol.(cy) 0.00
name: APPROVED Low -Flow Orifices
area (sf)
0.196
diameter or depth (in)
6.000
width for rect. (in)
0.000
coefficient
0.600
invert (ft)
447.000
multiple
4
discharge into
riser
Outlet structure 1
Culvert
name: APPROVED Barrel
multiple
1
discharge
out of riser
D (in)
54.000
h (in)
0.000
Length (ft)
80.000
Slope
0.006
Manning's n
0.013
Inlet coeff. Ke
0.500
Equation constant set
3
Invert (ft)
430.480
Page 1
APPROVED Northtown IIA Biofilter Routings WITH UPDATED CN
Outlet structure 2
Orifice
name: APPROVED Riser DI-7 Effective Opening
area (sf) 6.002
diameter or depth (in) 29.400
width for rect. (in) 29.400
coefficient 0.600
invert (ft) 450.000
multiple 1
discharge into riser
AMENDED 3 Inflow Hydrographs
Hydrograph 0
SCS
name: AMENDED 2-yr 24-hr Peak SCS Storm
Area (acres)
10.950
CN
84.200
Type 2
rainfall, P (in)
3.700
time of conc. (hrs)
0.2100
time increment (hrs)
0.0200
time limit (hrs)
300.000
fudge factor
1.00
routed
true
peak flow (cfs)
24.946
peak time (hrs)
11.982
volume (cy)
3125.736
Hydrograph 1
SCS
name: AMENDED 10-yr 24-hr Peak SCS Storm
Area (acres)
10.950
CN
84.200
Type 2
rainfall, P (in)
5.600
time of conc. (hrs)
0.2100
time increment (hrs)
0.0200
time limit (hrs)
300.000
fudge factor
1.00
routed
true
peak flow (cfs)
45.123
peak time (hrs)
11.982
volume (cy)
5653.810
Page 2
Hydrograph 2
SCS
APPROVED Northtown IIA Biofilter Routings WITH UPDATED CN
name: AMENDED 100-yr 24-hr Peak SCS Storm
Area (acres)
10.950
CN
84.200
Type 2
rainfall, P (in)
9.100
time of conc. (hrs)
0.2100
time increment (hrs)
0.0200
time limit (hrs)
300.000
fudge factor
1.00
routed
true
peak flow (cfs)
84.285
peak time (hrs)
11.982
volume (cy)
10560.790
AMENDED OUTPUT:
Routing Method: storage -indication
Hydrograph 0
Routing Summary
of
Peaks: AMENDED
2-yr 24-hr
Peak SCS Storm
inflow
(cfs)
24.928
at
11.98
(hrs)
discharge
(cfs)
5.151
at
12.30
(hrs)
water level
(ft)
449.107
at
12.30
(hrs)
storage
(cy)
1630.978
Hydrograph 1
Routing Summary
of
Peaks: AMENDED
10-yr 24-hr
Peak SCS Storm
inflow
(cfs)
45.090
at
11.98
(hrs)
discharge
(cfs)
10.905
at
12.26
(hrs)
water level
(ft)
450.658
at
12.26
(hrs)
storage
(cy)
2601.551
Hydrograph 2
Routing Summary
of
Peaks: AMENDED
100-yr 24-hr
Peak SCS Storm
inflow
(cfs)
84.224
at
11.98
(hrs)
discharge
(cfs)
40.960
at
12.14
(hrs)
water level
(ft)
452.476
at
12.14
(hrs)
storage
(cy)
3880.007
Fri Apr 24 13:53:56 EDT 2020
Page 3
DA 2 Calculations
(Treating Undeveloped TMP 45-111 & a Portion of Gander Drive)
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U.S. Department of Agriculture
Natural Resources Conservation Service
TR 55 Worksheet 3: Time of Concentration (TJ or Travel Time (Tt)
Project: Northtown IIA Designed By: FGM, PE
Location: TMP 45-110 Checked By: SRC, PE
Check One: Present X Developed X
Check One: Tc X Tt Through subarea
Segment ID;
Sheet Flow: (Applicable to Tc 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 $] / P2 .s s°.a
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 (ft)
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= [ 1.49r2/3so.5 ] / n
18 Flow length, L (ft)
19 Tt= L / 3600*V
20 Watershed or subarea TcorTt
(Add Tt in steps 6, 11 and 19)
Note: The shoretest SCS Tc is 0.10 hrs.
FL-ENG-21A
06/04
Date: 4/29/2020
Date: 4/29/2020
AMENDED DA 2
AMENDED DA 2
(Pre-Dev.)
(Post-Dev.)
Woods- Light
Dense
Underbrush
Grass
0.4
0.4
100
100
3.62
3.62
0.06
0.17
0.217
0.14
Unpaved
Unpaved
370
460
0.13
0.05
5.9
3.6
0.017
0.04
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1999 VSMH Part IIC Stormwater Quality Calculations
(Treating TMP 45-111A, 45-111C, 45-111, 45-110,
Rte. 29's Turn Lane & a Portion of Gander Drive)
PERFORMANCE -BASED WATER QUALITY CALCULATIONS APPENDIX 5D
Worksheet I
Page 1 of 3
STEP 1 Determine the applicable area (A) and the post -developed impervious cover
(1post)•
1a.53 acres �g�Asc 3.`10 nc. 4 �s S ti4
Applicable area (A)* = gq �,� , t�
Post -development impervious cover:
''
-asfes e-- -n-k
structures 'CrnP �1 -11�
�ACgJ\ apt' dca6-6!
A -a SC T SO, W-A {�vE2laG � oN
arkin lot = lt,�lo ass *x- AeeotN'CEd
FAR
p � TMF t'jS -iiu TMA'T wh.S
16aow µ l-heeraa„r'o woo aoV30000a
other:
roadway = acres
Oppoa �M�,Fpµ tvr IS 1MP, RrLSA
p,eeRDUErs w 0 ao13 u A TNIP Lt5- Ikkc
-{aka 'tMP t1S-110 t (sntt '�R ENO {etc a 15 ' q�LArlE,
'�1cRSE T�Mf,p AS fRoM NGC �2�c `�G TR�P�NFJ�'t
Total = `t • 37 acres �^' `�N $ `�� fh'
IFAoM 90GGWa 5 pQgpp� Cp OA ti
6 ►1 `i acres t�
st = (total post -development impervious cover - A) X 100 = `f N g %
The area subject to the criteria may vary from locality to Iocality. Therefore,
consult the Iocality for proper determination of this value.
STEP 2 Determine the average land cover condition (I,1mershed) or the existing
impervious cover (Icaisting)•
Averse land cover condition vaters3ted :
If the locality has determined land cover conditions for individual watersheds vdthin its
jurisdiction, use the watershed specific value determined by the locality as Itvatershed•
I`vatershed " P o
Otherwise, use the Chesapeake Bay default value:
Avatershed - 16%
5D-5
PERFORMANCE -BASED WATER QUALITY CALCULATIONS APPENDIX SD
Worksheet 1
Page 2 of 3
Existing impervious cover
Determine the existing impervious cover of the development site if present.
Existing impervious cover:
structures = acres
parking Iot = acres
roadway = acres
other:
1Z)PLa,i1CwA"f = 0o35 acres
5kr.�xx, _ 495 -acres
Total = 0.36 acres
1,,. _ (total existing impervious cover = A*) x 100 9 %
The area should be the same as used in STEP 1.
STEP 3 Determine the appropriate deveIopmment 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 (1e7;;s,j,$) is less than or equal to the average land cover condition
{I,,,at,,h,d} and the proposed improvements will create a total percent
impervious cover (ZPpsa) which is less than or equal to the average land
cover condition
I 0
post ° lwatcrshcd
5D-6
PERFORMANCE -BASED WATER QUALITY CALCULATIONS APPENDIX 5D
Worksheet I
Page 3 of 3
V Situation Z. This consists of land development where the existing percent impervious
cover (tC7151�g) is less than or equal to the average Iand cover condition
G vatcrsbcd and the proposed improvements will create a total percent
impervious cover (Jp,,51} which is greater than the average land cover
condition {'svatershea}
Iet�ne a %I,vaterstie8 l G %; and
1pnst -74 O % > INaterslted t ro
Situation 3: This consists of land development where the existing percent impervious
cover `'easfing} is Beater than the average land cover condition {Tnstershed}
IexhUng % > Iwatershed
Situation 4: This consists of land development where the existing percent impervious
cover {Tee) is served by an existing stormwater management BMW(s)
that addresses water guality.
If the proposed development meets the criteria for development Situation 1., than the low
density development is considered to be the SNIP 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 a . '�,3 acres
Ipast = (total post -development impervious cover - A) x 100 = 1 y • S %
o � o
Iw/n atcrshcd or Lawrshcd = 16 /o
Iua;sting = (total existing impervious cover � A") x 100 = a , 8 %
lesutin; a % "watershed k6 %; and
Ipost 1 L' 0 � � � �0 snicrshed 16 0 �0
STEP 4 Determine the relative pre-develop.-nerat pollutant load (Lpre)-
Lpre(rsntershed) — [0.05 + (0.009 x l,vntershed)l x A X 2.28 (Equation 5-16)
where: Lpre(watershed) = relative pre -development total phosphorous load (pounds per year)
ka,mUd = 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 Xa, 53 x 2.28
S• S4 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 (Li,t,j.
Lpost = [0.05 + (0.009 x Ip,, j] x A x 2.28 (Equation 5-21)
where: Lpost = relative post -development total phosphorous load (pounds per
year)
Ip.t = post -development percent impervious cover (percent expressed in
whole numbers)
A = applicable area (acres)
Lp..t = [0,05 + (0.009 x -7�] x Va.53 x 2.28
a.o . 66 pounds per year
STEP 6 Determine the relative pollutant removal requirement (RR).
RR = Lpast _Lpre(watershed)
RR 6b . 5,5 Lj
_ i 5 . la 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 . Lp,�t) x 100 (Equation 5-22)
where: EFF = required pollutant removal efficiency (percent expressed in whole
numbers)
RR = pollutant removal requirement (pounds per year)
Lpp,t = relative post -development total phosphorous load (pounds per
year)
EFF = { 1S 1� - �0.66 ) x 100
rj3, IS Rio
5D-10
PERFORMANCE -BASED WATER QUALITY CALCULATIONS APPENDIX 5ID
Worksheet 2: Situation 2
Page 3 of 4
2. Select BMP(s) from Table 5-15 and locate on the site:
smlP 1: Reec c, woo a0k30000a g�xUTA TRVh�i�1G Q P f � �rovAt R,
BMP 2: WIxT(L-In't Cam~- %,QCR se VEk fwv. %M?, N us orj 17A9 V5-\«gip = o!;
SOP aM6-6) A140 Faz IFu, 1144, WA CQVSAPG,-)cN 'VA45_111( NaN R-95 K AccDA'MOVOk
s,► A-,QA0,1C-0 w4o aa,.--?,a000a
3. Determine the pollutant load entering the proposed. BMP(s): -
LBnjp = [0.05 + (0.009 x lgh,,)] x A x 2.28 (Equation 5-23)
where: LBw = relative post -development total phosphorous load entering
proposed BMP (pounds per year)
lBw = post -development percent impervious cover of BMP drainage area
(percent expressed in whole numbers)
A = drainage area of proposed BNT (acres)
[0.05 + (0.009 x 6 t •6 x WAS x 2.28 6--lipte,' , 1oO
15 40g pounds per year
L13,,Im— [0.05 + (0.009 x )1 x x 2.28 Q"�a A^av� l2vx���^�Ni iRR
- — jaQc�.evC-� w4o'a613Qooa C� QRrr7Ja,�� C3►�"P1
pounds per year
LB�,Im = [0.05 + (0.009 x )] x x 2.28
pounds per year
5D-1I
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):
Lrenloved = EffB.. X
(Equation 5-24)
where: Lramoved = Post -development pollutant load removed by proposed BMP
(pounds per year)
EffBmp = pollutant removal efficiency of BMP (expressed in decimal form)
LB�,a, = relative post -development total, phosphorous load entering
proposed BMP (pounds per year)
LremovedIRNMI = Oo 65 X 15 , nQ — `[.Sl pounds per year
Lrea4aved/BMP2 = X = pounds per year
LremovedMiNIP3 = X = pounds per year
5. Calculate the total pollutant load removed by the BMP(s):
Lremovedrtatal "' Lrcm0vedMW1 + Lmm0v<daNp2 + Lmmoved/9W3 + (Equation 5-25)
where: Lrcmovedltutal = total[ pollutant load removed by proposed BMPs
Lremovedmw, = pollutant load removed by proposed BMP No. 1
Lremavea,BIvIP2 = pollutant load removed by proposed BMP No. 2
LramovealA?vCP3 = pollutant load removed by proposed BMP No. 3
rcmaved/lafal — yopa
6. Verify, compliance.
Lrcmavedllotal °";its`
\S.\a �-,- 15,\a
pounds per year
5D-12
¢ '[ectA� 4c+uan AMA- RV,,ovn L
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