HomeMy WebLinkAboutWPO201500090 Calculations Minor Amendment 2015-07-28 f err/ 'val/
Glenn Brooks
From: Glenn Brooks
Sent: Tuesday, July 28, 2015 10:31 AM
To: 'Graham Murray'
Cc: John Anderson; Max Greene; Justin Deel
Subject: Glenmore K2B
Graham,
Getting back to you regarding our meeting last Thursday and the plan and 2C worksheets you submitted for Glenmore
K2C. I have a few points that may need work. Your computations assumed an impervious area per lot of 3000sf. I
compute more like 7000sf using our GIS system, and noting that patios,walks and other features will add to this. A
sample is shown below. This changes the computations somewhat. I have included an abbreviated version of the
comp. I have also included the other lane of the road in the overall load computation, but not in the load to the
facilities. The facilities themselves seem to make less sense now, with the largest facility not in the largest drainage
area.
\ A5
4A�-K2-C6-5
4- •
93A5-K2-013.4 hh
1 O
4.16'hscre.2 /11543
Virginia 2C Method computations
Total Pollutant Load and treatment
A= 16.34
Iw=0.16
Lpre= [0.05+0.9*Iw]*A*2.28 Lpre = 7.2
1p= (16*7000+2830*20)/43560/A 1p=_0.2369
Lpost= [0.05+0.9*Ip]*A*2.28 Lpost= 9.8
RR= Lpost-Lpre
RR=_2.6 lbs/yr
Required Effeciency
1
PERFORMANCE-BASED WATER QUALITY CALCULATIONS APPENDIX 5D
1-RC `7r
swN Fnc:trt .
Worksheet 1 17.1A,
SRC-1!'•�•�wf
Page 1 of 3 5, ,fntauTti Brx�
TNALill' (DA
STEP I Determine the applicable area(A) and the 4t_developd imper ions cov r v.N(rwucro)
("post"' �a
Applicable area(A)* = 16;)11 acres4-- �a 8 N WIO iv. 3.69x. 6.VIAe.
Post-development impervious cover:
LcriS
a
►etttres• = ?tag o v n aefes-4— Lam, 9. cors \o
•
3 K 3a31,CO) (3.jai: {° t9►Boa 4"1.5)
parking lot = NI ill acres a t,
1,oa> 4,
(GA(mcu-(kV!,t.;;;) 1,a Y
roadway = 3O,ow -aere - — '150'x 1v{�; � gtio',uh„,
other:
crc.tar!a 8,�co �1
CONS) Age A= "1I,l35 Beres--4---
4Ain=rG AakA
=sGarsS 4 a/..1t`p__ 0.66 acres 4-- �.�KBY. ta.66�•.)82 (3a'1rr)�
Q,ao P. p' .
0.
Total = 3.cq acres
"post=(total post-development impervious cover±A) x 100= ad.'7 %
* 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 (lwatenned) the existing
impervious cover(lesr;og).
Average land cover condition(Iwatershedl:
If the locality has determined land cover conditions for individual watersheds within its
jurisdiction,use the watershed specific value determined by the locality as 'watershed.
'watershed
Otherwise, use the Chesapeake Bay default value:
"watershed = 16%
5D-5
REMQJ AJ( 74-+C S).r,J,`eSic;45,10syffics 1,tiaittxN L.cc
—CkcAjEU RuNo , -11.(C vCLQPMEr'fSTXL Y:
VRcrtY RC--p u.4R.:-/iG,,rtt TrvNG�i,
SWM FACS"11 5 SkASARCP
1771“) i N.i41v(
r -TILE4C- &tssn+s 1)c-cx6e. cgM4r1AN�.
C Swti Ftwit3,.cs Z t 5 S‘4 )4KA% so) a.1+r ;rL7y -ft{Eij�.t4,144,. Th. nAi,S' ,toe atb IYl� �3K (OMPLzA►JT.
i A Qv�NTzTs. •ANALYri.5 Mrrt� ec 4Q6z0c/} t"AKArsr. .r, Tx -4r Art s STati =n, cOAAArlaAi\JCC".
Nue
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{I ) is less than or equal to the average land cover condition
(lwate,hed) and the proposed improvements will create a total percent
impervious cover(1,„,,,) which is greater than the average land cover
condition (1„ater,iled).
'existing CJ % ~Watershed O %; and
"post ao�.6 %> Watershed Ido
Situation 3: This consists of land development where the existing percent impervious
cover(1,„,,,171x) is ereater than the average land cover condition (l,,,atershed)•
0'
existing O�U > Watershed /0
Situation 4: This consists of land development where the existing percent impervious
cover('existing) 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 1
Page 2 of 3
Existing impervious cover(1tX1,,;,, :
Determine the existing impervious cover of the development site if present.
Existing impervious cover:
structures = .- acres
parking lot =, i ) acres
roadway = '-' acres !t ssvrtc c.6 f,�,.c.
other:
acres 1
4
acres K�F. .v A'� `t:r'. . S:
Fot,.
Total = U acres K v, rte
'CX1";ng=(total existing impervious cover_ A*) x 100= 0 %
* 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 O. j the appropriate development situation as follows:
Situation 1: This consists of land development where the existing percent impervious
cover(Iex;,,,,,g) is less than or equal to the average land cover condition
('watershed) and the proposed improvements will create a total percent
impervious cover(11„,,,) which is less than or equal to the average land
cover condition (Iwu1trthCd).
Ipost % • •Watershed
5D-6
EFF= (RR/Lpost) err=_0'529 'veer'
BMP 1A
bmpEFF=0.35
DA= 2.88 acres
Ida = [3*7000+750*10]/(DA*43560) Ida = 0.23
Lbmp = [0.05+0.9*Ida]*DA*2.28 Lbmp= 1.7
Lrem1 = bmpEFF*Lbmp Lrem1= 0.5848
BMP 2
bmpEFF=0.35
DA= 3.64 acres
Ida = [4*7000+800*10]/(DA*43560) Ida = 0.23
Lbmp= [0.05+0.9*Ida]*DA*2.28 Lbmp= 2.1
Lrem2 = bmpEFF*Lbmp Lrem2= 0.7388
BMP 3
bmpEFF= 0.35
DA= 6.82 acres
Ida = [6*7000+1280*10]/(DA*43560) Ida= 0.18
Lbmp= [0.05+0.9*Ida]*DA*2.28 Lbmp= 3.4
Lrem3 = bmpEFF*Lbmp Lrem3 = 1.1756
Lreml+Lrem2+Lrem3 = 2.4992
Glenn Brooks, P.E.
County Engineer
VSMP Administrator
Albemarle County
2