HomeMy WebLinkAboutWPO201900018 Calculations Stormwater Management Plan 2019-06-06 PEABODY VSMP PLAN STORMWATER
MANAGEMENT CALCULATIONS PACKET
(WPO 201900018)
Date of Calculations
APRIL 22, 2019
PREPARED BY:
COLLINS ENGINEERING
200 GARRETT STREET, SUITE K
CHARLOTTESVILLE, VA 22902 VISA1 rR Op`
434.293.3719 PH �r�•
434.293.2813 FX SCOT rY
www.collins-engineering.com Z Lip. No 035797NS
c?i
APPROVED �%NAL „
by the Albemarle County
Community Dev IuprlentDepartment
Date
File Zo rq- ON
Virginia Runoff Reduction Method
Water Quality Calculations for ReDevelopments
The following VRRM water quality calculations for ReDevelopments are for
the proposed amendment. The areas shown in the following calculations are
defined by the limits of disturbance. These calculations show the REQUIRED
phosphorous removal rate for the amendment.
DEQ Virginia Runoff Reduction Method Re-Development Compliance Spreadsheet-Version 3.0
0'2011 BMP Standards and Specifications 02013 Draft BMP Standards and Specifications
cells'
input
Project Name: Peabody-Amendment's Required Water Quality Treatment
constantr•Mes
_. _.
Date: 4/22/2019 _
_ Linear Development Project? No calculation cells '
Site Information final result.
Post-Development Project(Treatment Volume and Loads)
Enter Total Disturbed Area(acres) _/I®, Check:
_ BMP Dealg Specifications
icoons List:2011 Stds&Specs
Maximum reduction required: , Linear project? No
the site's net increase in impervious cover(acres)is:gorcom Land cover areas entered correctly?, se
Post-DevelopmentI 7P Load Reduction for Site(!b/yrJr� Total disturbed area entered?i ...I
•
Pre-Reoevelopment Land Cover(acres) ___I
A Solis 8tolls CSoils I)Soils Totals
forest/Open Space lanes)-undlstumed, 0.00
protected forest/open span or reforested lend _
Mamged Turf(sore)-disturbed,graded for
yards or other turf to be mowed/managed 0.55 0.55
Inmarvbua Cosa(urn) 0 39 0.39
0.94
Post-Development Land Cover(acres)
A Salle 8Solls CSolb USoils ( Totals
Faert/Open Spate(acres)-undisturbed, 0.00
protected forest/open spaceor reforested land
Managed Turf(aoeas)-disturbed,graded for
yards or other turf to be mowed/managed 0.46 0.46
• 1 .Impervious Cow 0.48 0.48 I. ......wr
Area Check OK, OK. OK OR 0.94
Constants Runoff CaaTfidante(Rv)
Annual Rainfall(inches) 43 ASoh BSelis CSolis 0Solis
Target Rainfall Event(Inches) too _forest/DP.5We 0.02 0A3 0.04 0.05 ill _---.
Total Phosphorus DTP)EMC(mg/LI 0.26 Managed Turf 0.15 0.20 0.22 0.25 --
Total Nitrogen MO HNC inWL) 1.86 Impervious Cover 0.95 0.95 0.95 0.95 = _
Target Tv Load llb/screM( 0.41 __
Pjluntess correction factor) D.90 •
LAND COVER SUMMARY--PRE-REDEVELOPMENT LAND COVER SUMMARY-POST DEVELOPMENT
I_
Land Cover Summary-Pre Land Cover Summery-Post(Final) Land Cover Summary-Post Land Cover Summary-Post
Pre-ReDevebpment Listed Adjusted' Post ReDev.B New Impervious Post-ReDevelopment Post-Development New Impervious
Pores/Open Span Cover lanes) 0.00 0.00 Forest/Open Span 000 Forest/Open Space 000
Cover(acres) <over Octet)
Weighted Rvlforestl 0.00 0.00 Weighted ev(forea) 0.00 Weighted Rvllorestl 0.00 •
%Fares) 0% 0% %forca 0% %Forest OS
Managed Turfeowrlaaesl 0.55 046 Managed TurfCover 0.46 Managed Turf Cover •
0.46 •
Dm.) .._)goes)...
Weighted Rulttn0 0.20 0.20 Weighted Rv(turf) 0.20 Weighted Rv ltuS? 0.20
-%Managed Turf 59% 54% %Managed Turf 49% %Managed Tud 54% 'V
Impervious Cover lames) 0.39 0.39 Impervious Cover(acres) 0.48 Re0lv.Impeeous 039 New impervious Covrr 009
Cover)gals) )acr.)
RNlmpervious) 0.95 0.95 Rvlmpervbusl 0.95 thlimpehous) 0.95 Relimperviousi 0.95
%Impervious 41% 46% %Impervious 51% %Impervious 46%
Total Site Mee(amasl 0.94 0,85 Flael mess,.lanes) 0.94 Total Rnoev.Ste Area 0.85 -
...._.__)ate) .ys
Site Re 0.51 0.54 Fkel Past NW She Re 0.51 Ridley Site Rv 0.54
Treatment Volume and Nutrient Load Treatment Volume and Nutrient Load
Pre-0 or Ispmeet Treatment Volume Final Post-DevelopmentP t Re0evelapmenl r•Developmrnt I'.
lam-h) 0.0400 0.0385 Treatment Volume 0.0457 Treatment Volume 0.0385 T eatmem Volume 0.0071
)acre-ffl latre.hI I last.-n)
Pre-ReDevebpment Treatment Volume Final Post-Development PoshoeDevebpment Post.Develoament I..
(cubic leer) 1,744 1,679 Treatment Volume 1,989 Treatment Volume 1,679 l Treatment Volume(cubic 310 _
(cask feet) (cubic feed feet)
Final Post- Post-Reoeveb meet
Pre-Re Development TP Load I LC I Post-Development TP
(Ib/yr) 1.10 1.05 Development TP Load 1.25 lb/frre) 1.0S f 0.20
(Ib/Yr) Ilb/Yrl' Load OWvrl
weReDevebPmem TP toad per acre •
N rot0 parapmeni lr PmI.RaDevri,ar,enr r:.
(proem/vrl IV 124 load per acre 1.33 load per u:e 1.24
(16/..6/1 (le/aaekr)
BaselinelP Load(lb/yrI 1 Max.Reduction Required
(S41lba/0ae/nr applied topre-redetebpment area exhelingcm,,'rn. 0.35 (Below Pre- t0%
land proposed to!new Impervious cover) Re Development mad)
'Ad/ushdlono Lover lo:nmory TP Load reduction TP Load Reduction
Pre ReDevelopment fond cover minus pervious lord Dover(/aresVoven saace or Required for 0.11 Required for New 0.36
monoaedroriloceege proposed for new Impervious cover. Redeveloped Area Impervious Area(Ib/yr)
iiehid
Adjusted total acreage is wnsishnt MO Post-ReOevNapment wedge Imnnus Doreoue ------ '-
of new impervious Dover).
Column I oomvs load reduction reporienrenl for new impecviovs cover(bused on new •
developmenr load limit,0.41 Ibs/ure/year)
Post-Development Requirement for Site Area
TP Load Reduction Required(Ib/yr) 0.26
Site Results (Water Quality Compliance)
Area Checks D.A.A D.A.B D.A.C D.A.D D.A.E AREA CHECK
FOREST/OPEN SPACE(ac) 0.00 0.00 0.00 0.00 0.00 OK.
IMPERVIOUS COVER(ac) 0.00 0.00 0.00 0.00 0.00 OK.
IMPERVIOUS COVER TREATED(ac)_ 0.00 0.00 0.00 0.00 0.00 OK.
MANAGED TURF AREA(ac) 0.00 0.00 0.00 0.00 0.00 OK.
MANAGED TURF AREA TREATED(ac) 0.00 0.00 0.00 0.00 0.00 OK.
AREA CHECK I OK. OK. I OK. OK. OK. 1
Site Treatment Volume(ft3) 1,989
Runoff Reduction Volume and TP By Drainage Area
D.A.A D.A.B D.A.C I D.A.D l D.A.E I TOTAL
RUNOFF REDUCTION VOLUME ACHIEVED(ft3) 0 0 0 0 0 0
TP LOAD AVAILABLE FOR REMOVAL (Ib/yr) 0.00 0.00 0.00 0.00 0.00 0.00
TP LOAD REDUCTION ACHIEVED (Ib/yr) 0.00 0.00 0.00 0.00 0.00 0.00
TP LOAD REMAINING (Ib/yr) 0.00 0.00 0.00 0.00 j 0.00 ft 0.00
NITROGEN LOAD REDUCTION ACHIEVED (Ib/yr) 0.00 1 0.00 0.00 0.00 0.00 0.00
Total Phosphorus
FINAL POST-DEVELOPMENT TP LOAD(Ib/yr)1 1.25 _
TP LOAD REDUCTION REQUIRED(Ib/yr) 0.26
TP LOAD REDUCTION ACHIEVED (Ib/yr) 0.00
TP LOAD REMAINING(Ib/yr): 1.25
REMAINING TP LOAD REDUCTION REQUIRED(Ib/yr): 0.26
Total Nitrogen(For Information Purposes)
POST-DEVELOPMENT LOAD(Ib/yr) 8.94
NITROGEN LOAD REDUCTION ACHIEVED (Ib/yr) 0.00
REMAINING POST-DEVELOPMENT NITROGEN LOAD(Ib/yr) 8.94 _ -
Virginia Runoff Reduction Method
Water Quality Calculations for ReDevelopments
The following VRRM water quality calculations for ReDevelopments are for
the proposed amendment. These calculations show the PROVIDED
phosphorous removal rate. The credit shown in the following VRRM
calculations is achieved through the proposed best management practice of
preserving open spaces. This credit exceeds the minimum REQUIRED
phosphorous removal rate previously shown within this report.
DEQ Virginia Runoff Reduction Method Re-Development Compliance Spreadsheet-Version 3.0
E 2011 BMP Standards and Specifications 02013 Draft IMP Standards and Specifications
•
.......................__.. - data Input cells
Project Name: Peabody-Amendment's Provided Water Quality Treatment CLEAR ALL
_ Date: 4/22/2019 constant values
Linear Development Project? No calculation calk
Site Information • Real re5ula .---
Post-Development Project(Treatment Volume and Loads)
Enter Total Disturbed Area(acres) - 0.86 Check: _
IMP Design Specifications List:2011 Stds&Specs •__
Maximum reduction required: rdi �_ Linear project? No
The site's net increase in impervious cover(acres)is:r '0. Land cover areas entered correctly? d
Post-Development TP LOW Reduction for Site((b/yr):Il Total disturbed area entered? ve
TP LOAD REDUCTION NOT REQUIRED
Pre-ReDevelopment Land Cover(acres)
Atolls B Sous C Sons 05oils Toteb
Forest/Open Span leans)-undisturbed, -- -
protected forest/open space or reforested land 0'00
Managed Tod leans)-disturbed,graded for .
yards or other turf to be mowed/managed 0.75 0.75 I _ _
Impervious Cover(saes) 0.00
0.75 _
Post-Development Land Cover(acres
A Soils BSails CSob Dsalk Totals
Forest/Open Stsace hires)-undisturbed. •
protectedforest/open space or reforested land 0.75 0.75
Meneged Turf(scree)-disturbed,graded for
yards or other write
be owed/managed 0'00
-..._. ... _ III - _........._..._ ........_._._ ..._..._.-..-...
impervious Cover(wee) 0.00 _... - -_
Area Check OK OK OK OK. 0.75
•ion Jopen;pan areas must be protected In accordance with the Virginia euneff Reduction Methodd
Constants Runoff Coeffldents(RA)
Annual Rainfall(inches) 43 Agana B Soils C Soils D SOUS
Target Rainfall Even[Instill 1.00 Forest/Open Span 0.02 0.03 0.04 0.05
Total Phosphorus(TP)EMClnsg/L) 0.26 _ Managed Turf 0.15 0.20 0.22 0.25
Total Nitrogen(TN)ERIC(mg/L) 1.86 impeMous Cover 0.95 0.95 0.95 0.95
Target TP Load lib/acre/Tr) OAI
Nlunxkss Lorremon factor) 0.90 II
•
LAND COVER SUMMARY--PRE-REDEVELOPMENT LAND COVER SUMMARY--POST DEVELOPMENT
Land Cover Summary-Pre Land Cover Summery-Past(Final) Land Laver Summary-Post Lond Cover Summary-Post
Pre-ReDevelopment Usted Adjusted' Post ReOev.a New impervious Post-ReDevebpmem Post.Devrlopment New Impervious
- -
ForesVOpen Space Cover(acres) 0.00 0.00 Forest/Open Space 0.75 Forest/Open Space 0.75 -
Cover(acres) Cover(acres)
Weighted Rvllarest) 0A0 0,00 Weighted Rvl/orest) 0.05 Weighted Rvltorest) 0.05
%Forest 0% 0% %Forest 100% %forest 100%
Managed Turf Cover(acres) 0.75 0.75 Managed Turf Cover 0.00 Managetl Tod Cover 0.00
lures) ]acres)
Weighted Rvlturq 0.25 0.25 Weighted Re(turfi 0.00 Weighted Rvlturf) 0.00
%Managed Turf 100% 100% � %Managed Turf 0% _-- %Managed Turf 0% '
ImpeMous Cover(acres) 0.00 0.00 Impervious Cover(acres) 0.00 e0000tmpervous 0.00 New lmPervoas Cover 0.00
Cover(zaesl laces;
ev(ImpeMous) 0.95 0.95 Rs1impervlous) 0.95
N./impervious) 0.95 .(m -nenloazl
ppervious 0% %Impervious 0%
%Ito eMous 0% 0% %Ito
- --Tat]Site A lacteal 0.75 0.75 al be Arta keres) 0.75 Trial ReDev.She Area
Ana We
0.75
_ (acres)
Sk.M 0.25 0.25 Motel Post Oev She M 0.05 Remy Pt.Rv 0.05
Treatment Volume and Nutrient Load Treatment Volume and Nutrient Load
Pre-ReDevelopmenlTreatmem VolumeFinal Post-Development Post-ReDevelopment Post-Development
lucre-fc) 0.0156 0.0156 Treatment Volume 0.0031 Treatment volume 0.0031 Trat m enr Volue -
laae-nl atrc-nl (ass-n)
Pre-ReDovebpment Treatment Volume Final Post-DevelopmentPost-ReDevelopment Post-Development
lade feet) 581 681 Treatment Volume 136 Treatment volume 136 Treatment Volume(cubic -
(eubk feet) (cubic feet] & feet/
Pre-ReDevelopment TP load final Post- Post ReDevebpment P
0.43 0.43 Devebpment TP toad 0.09 Load(TP) 0.09 o0eperopment TP -
(Ib/yr) & I Load lb/yri
(Ib/Yr) (Ib/yri'
pre-Re TP Load nr Mre Finalost-Development Tr n rot .t
�f1 Ala/aae/yrl 0.57 0.57 toad per acre 0.11 toad 0..,ar, 0.11
(b/.../ril go/acre/0
Baseline TP Load Ilb/yrl Max.Reduction Required
IoA11bs/acre/rr aretsd to pre-redevelopment area excluding toc::_ 031 •
(Below Pre- SP%
tend proposed for new Impenco n coven Renevelopmem Leedl
r Adjusted fond Cover Summary: TP Load Reduction
Pre Osoro lopmenirab cover m,nvs pervious loop cover gprnVopenspore or Required for TP Load Reduction
'noncom,toll acreage proposed for new Impervious cover. Redeveloped Area -0.30 Required for NBW 0
lib/yri Impervious Area(Ib/yr)
Adjusted royal acreage is consistent with Post-000eoe10pmeni orient/minus acreage -
of new impervious cover).
Column I thaws load reduction requriement for new MOP/WO.cove.(based M new
devebement bad limit,DA1 lbs/orie/year!
1
Post-Development Requirement for Site Area
TP Load Reduction Required(Ib/Yr) -0.30 '•
Site Results (Water Quality Compliance)
Area Checks D.A.A D.A.B p D.A.C D.A.D D.A.E AREA CHECK
FOREST/OPEN SPACE(ac) 0.00 0.00 0.00 0.00 0.00 OK.
IMPERVIOUS COVER(ac) 0.00 0.00 0.00 0.00 0.00 OK.
IMPERVIOUS COVER TREATED(ac) 0.00 0.00 0.00 0.00 0.00 OK.
MANAGED TURF AREA(ac) 0.00 0.00 0.00 0.00 0.00 OK.
MANAGED TURF AREA TREATED(ac) 0.00 0.00 0.00 0.00 0.00 OK.
AREA CHECK L OK. OK. OK. OK. OK. J
Site Treatment Volume(ft3)l 136
Runoff Reduction Volume and TP By Drainage Area
I D.A.A I D.A.B D.A.C D.A.D D.A.E TOTAL
RUNOFF REDUCTION VOLUME ACHIEVED(ft3) 0 0 0 _ 0 0 0
TP LOAD AVAILABLE FOR REMOVAL (Ib/yr) 0.00 0.00 0.00 0.00 0.00 0.00
TP LOAD REDUCTION ACHIEVED (Ib/yr) 0.00 0.00 0.00 0.00 0.00 0.00
TP LOAD REMAINING (Ib/yr) 0.00 0.00 0.00 0.00 0.00 0.00
NITROGEN LOAD REDUCTION ACHIEVED (Ib/yr) 0.00 I 0.00 0.00 0.00 I 0.00 I 0.00
Total Phosphorus
FINAL POST-DEVELOPMENT TP LOAD(Ib/yr) 0.09
TP LOAD REDUCTION REQUIRED(Ib/yr) -0.30
TP LOAD REDUCTION ACHIEVED (Ib/yr) 0.00
TP LOAD REMAINING(Ib/yr): 0.09
REMAINING TP LOAD REDUCTION REQUIRED(Ib/yr):I 0.00 I**
**TARGET TP REDUCTION EXCEEDED BY 0.3 LB/YEAR**
Total Nitrogen(For Information Purposes)
POST-DEVELOPMENT LOAD(Ib/yr)1 0.61
NITROGEN LOAD REDUCTION ACHIEVED (Ib/yr)_ 0.00
REMAINING POST-DEVELOPMENT NITROGEN LOAD(Ib/yr) 0.61
I
SCS TR-55 Calculations
(Treatment Provided via an Underground Detention System)
Soils Mapping (Source: NRCS Web Soil Survey Online Database)
is
VIA
�•7 1 I 1 I I t.i,
Soils Table (Source: NRCS Web Soil Survey Online Database)
Albemarle County, Virginia (VA003)
Albemarle County, Virginia (VA003)
Map Unit Map Unit Name Acres Percent of
Symbol in AOI AOI
21C Culpeper fine 0.3 100.0%
sandy loam, 7
to 15 percent
slopes
Totals for Area of 0.3 100.0%
Interest
Note: The soils' properties witin this area are predominantly composed of hydrologic type B soils.
Precipitation Data
(Source: NOAA's National Weather Service, Hydrometerorological Design Studies Center, Atlas 14
Point Precipitation Frequency Estimates)
Map v x�a�
f p CR
EI Terrain 6.
'Plih 1!i
h!'
fi
o+
t h
a+ C+
R �
vl d
Ia°j
01'
$Utlf�6r Gf
r
'lain
n
a
1r00m
N
300ft
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)'
Duratio Average recurrence interval(years) 1
n I
I 1 II 2 II 5 II 10 j� 25 II SO II 100 JI 200 JI 500 IL 1000 I
24-hr 3.04 3.68 4.71 5.57 6.84 7.94 9.15 10.5 12.5 14.2
(2.73-3.41) (3.31-4.13) (4.22-5.27) (4.97-6.22) (6.07-7.61) (6.99.8.82) (7.99-10.1) (9.05-116) (10.6-13.8) (11.9-15.7)
U.S.Department of Agriculture FL-ENG-21A
Natural Resources Conservation Service 06/04
TR 55 Worksheet 2:Runoff Curve Number and Runoff
Project: Peabody Amendment Designed By:FGM,PE Date: 4/22/2019
Location: 1232 Stoney Ridge Rd. Checked By:SRC,PE Date: 4/22/2019
Check One: Present X Developed X
1.Runoff curve Number(CN)
Cover description CN(weighted)_
Drainage Area Soil name and hydrologic Area Product of CN Calculated'S'
(Cover type,treatment,and hydrologic condition;percent CN total product/
Description group(Appendix A) (Acres) x Area Value
impervious;unconnected/connected impervious area ratio) total area
DA A Impervious Areas 98 0.11 10.9
(Present) 8 Woods in Good Condition 55 0.00 0.0 80 2.56
Lawns in Good Condition(75%+Groundcover) 61 0.11 6.7
DA A Impervious Areas 98 0.19 18.5
(Developed)
8 Woods in Good Condition 55 0.00 0.0 93 0.78
Lawns in Good Condition(75%+Groundcover) 61 0.03 1.9
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 3.68 5.57 n/a
Runoff,Q-inches 1.26 1.75 3.36 DA A(Present)
Runoff,Q-inches 2.27 2.88 4.73 DA A(Developed)
U.S. Department of Agriculture FL-ENG-21A
Natural Resources Conservation Service 06/04
TR 55 Worksheet 3:Time of Concentration(Tt)or Travel Time(Tt)
Project: Peabody Amendment Designed By: FGM, PE Date: 4/22/2019
Location: 1232 Stoney Ridge Rd. Checked By: SRC,PE Date: 4/22/2019
Check One: Present X Developed X
Check One: T, X Tt Through subarea
DAA DA A
Segment ID: (Present) (Developed)
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)0.8]/Pz•s 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, Pb, (ft)
14 Hydraulic radius,r=a/P,„,(ft)
15 Channel Slope,s(ft/ft)
16 Manning's Roughness Coeff, n
17 V=[1.49r213s05]/n
18 Flow length,L(ft)
19 Tt=L/3600*V
20 Watershed or subarea Tt or Tt
(Add Tt in steps 6,11 and 19) 0.10 0.10
U.S. Department of Agriculture
Natural Resources Conservation Service
TR 55 Worksheet 4:Graphical Peak Discharge Method
Project: Peabody Amendment Designed By: FGM, PE Date: 4/22/2019
Location: 1232 Stoney Ridge Rd. Checked By: SRC, PE Date: 4/22/2019
Check One: Present X Developed X
Drainage Area Drainage Area Drainage Area
1. Data Description Description Description
DAA DA A
(Present) (Developed)
Drainage Area (Am) in miles2= 0.0003 0.0003
1 Runoff curve number CN= 80 93
Time of concentration(Tc)= 0.10 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 3.68 5.57 3.04 3.68 5.57
4. Initial Abstraction, la-inches 0,51 0.51 0.51 0.16 0.16 0.16
5. Compute la/P 0.17 0.14 0.09 0.05 0.04 0.03
6. Unit peak discharge,Qu-csm/in 975 1000 1000 1000 1000 1000
7. Runoff,Q from Worksheet 2-inches 1.26 1.75 3.36 2,27 2.88 4.73
8. Pond and Swamp adjustment factor, Fp 1 1 1 1 1 1
9. Peak Discharge,Qp-cfs Routed Through the
where Qp=Qu Am Q Fp 0.42 0.60 1.15 UGD System
WATERSHED SUMMARY
(Using a 3" Low-Flow Orifce)
1-year Flow, 10-year Flow,
CN Area,ac. cfs 2-year Flow,cfs cfs
DA A(Present) 80 0.22 0.42 0.60 1.15
DA A(Developed) 93 0.22 0.28 0.52 1.10
WATERSHED SUMMARY
(Using a 2- 1/3" Low-Flow Orifce Meeting Minimum 9VAC25-870-66 Requirements)
CN Area,ac. 1-year Flow, 2-year Flow,cfs 10-year Flow,
DA A(Present) 80 0.22 0.42 0.60 1.15
DA A(Developed) 93 0.22 0.19 0.22 0.97
Post-Development Routing Calculations
(Using a 2-1/3" Low-Flow Orifice)
The following routing calculations show compliance with 9VAC25-870-66
minimum requirements. These routing calculations are for a 115' long 36"
Underground Detention System and meet minimum channel protection
requirements via subsection B.3.a (Energy Balance) with a limits of analysis set
via subdivision 3 (Energy Balance). The flood protection requirements are met
for this system via subsection C.2.b (10yr. Post-ReDev.< 10yr. Pre-ReDev.) with a
limits of analysis set via subdivision C.2.b (10yr. Post-ReDev.< 10yr. Pre-ReDev.).
Peabody- Underground Detention System Routings- EB
BasinFlow printout
INPUT:
Basin: Peabody- Underground Detention System Routings Meeting Energy Balance Flow
0 Contour Areas
Elevation(ft) Area(sf) Computed Vol. (cy)
1 Storage Pipes
Storage pipe 0
name: UGD Piping
diameter (in) 36.000
length (ft) 115.000
invert (ft) 444.000
angle 0.287
volume (cy) 30.107
Start_Elevation(ft) 444.00 Vol. (cy) 0.00
5 Outlet Structures
Outlet structure 0
Orifice
name: Low-Flow Orifice
area (sf) 0.030
diameter or depth (in) 2.330
width for rect. (in) 0.000
coefficient 0.600
invert (ft) 444.000
multiple 1
discharge into riser
Page 1
Peabody- Underground Detention System Routings- EB
Outlet structure 1
Orifice
name: Barrel
area (sf) 0.196
diameter or depth (in) 6.000
width for rect. (in) 0.000
coefficient 0.600
invert (ft) 443.900
multiple 1
discharge out of riser
Outlet structure 2
Orifice
name: Emergency Spillway Simulation
area (sf) 0.423
diameter or depth (in) 8.810
width for rect. (in) 0.000
coefficient 0.600
invert (ft) 446.670
multiple 1
discharge into riser
Outlet structure 3
Orifice
name: Emergency Bypass Pipe
area (sf) 0.196
diameter or depth (in) 6.000
width for rect. (in) 0.000
coefficient 0.600
invert (ft) 445.500
multiple 1
discharge out of riser
Outlet structure 4
Weir
name: Proposed MH-1
diameter (in) 48.000
side angle 0.000
coefficient 3.000
invert (ft) 453.750
multiple 1
discharge into riser
Page 2
Peabody- Underground Detention System Routings- EB
3 Inflow Hydrographs
Hydrograph 0
SCS
name: 1-yr. 24-hr Peak SCS Storm Event
Area (acres) 0.220
CN 93.000
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) 0.640
peak time (hrs) 11.917
volume (cy) 67.757
Hydrograph 1
SCS
name: 2-yr. 24-hr Peak SCS Storm Event
Area (acres) 0.220
CN 93.000
Type 2
rainfall, P (in) 3.680
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) 0.812
peak time (hrs) 11.917
volume (cy) 85.978
Hydrograph 2
SCS
name: 10-yr. 24-hr Peak SCS Storm Event
Area (acres) 0.220
CN 93.000
Type 2
rainfall, P (in) 5.570
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) 1.328
peak time (hrs) 11.917
volume (cy) 140.631
Page 3
Peabody- Underground Detention System Routings- EB
OUTPUT:
Routing Method: storage-indication
Hydrograph 0
Routing Summary of Peaks: 1-yr. 24-hr Peak SCS Storm Event
inflow (cfs) 0.637 at 11.92 (hrs)
discharge (cfs) 0.192 at 12.08 (hrs)
water level (ft) 445.914 at 12.10 (hrs)
storage (cy) 16.644
Hydrograph 1
Routing Summary of Peaks: 2-yr. 24-hr Peak SCS Storm Event
inflow (cfs) 0.808 at 11.92 (hrs)
discharge (cfs) 0.217 at 12.10 (hrs)
water level (ft) 446.410 at 12.10 (hrs)
storage (cy) 22.714
Hydrograph 2
Routing Summary of Peaks: 10-yr. 24-hr Peak SCS Storm Event
inflow (cfs) 1.322 at 11.92 (hrs)
discharge (cfs) 0.973 at 11.98 (hrs)
water level (ft) 447.168 at 11.98 (hrs)
storage (cy) 29.398
Wed Apr 24 15:18:31 EDT 2019
Page 4
9 VAC 25-870-66
Section B: Channel Protection
"Concentrated stormwater flow shall be released into a stormwater conveyance system and shall meet the
criteria in subdivision 1, 2 or 3 of this subsection..."
Section B.1: Manmade stormwater conveyance systems
"When stormwater from a development is discharged to a manmade stormwater conveyance system,
following the land-disturbing activity, either..." a. or b. shall be met:
Section B.1.b.
"The peak discharge requirements for concentrated stormwater flow to natural stormwater conveyance
systems in subdivision 3 of this subsection shall be met."
Section B.3
"...the maximum peak flow rate from the one-year 24-hour storm following the land-disturbing activity
shall be calculated either.." a. or b."
Section B.3.a.
In accordance with the following methodology:
Q < I.F. * (Q Pre-Developed * RV Pre-Developed)/RV Developed
Q Allowed = 0.21 cfs
Where the following inputs are used:
CN Pre-ReDev.= 80 CN Post-ReDev.= 93
S Pre-ReDev.= 2.56 S Post-ReDev.= 0.78
RV Pre-ReDev.= 1.26 RV Post-ReDev.= 2.27
Q Pre-Developed = 0.42 I.F. = 0.9
Q Developed= 0.19 cfs
(Using the smaller theoritical 2-1/3" Low-Flow Orifice)
NOTE:
Q Pre-ReDeveloped = 0.42 cfs
Q Developed = 0.28 cfs
(Using a 3" Low-Flow Orifice)
Section B.4 Limits of Analysis
The analysis terminates at the proposed SWM facility's outfall since subsection B.3.a is met when a smaller
orifice is designed.
9 VAC 25-870-66
Section C: Flood Protection
"Concentrated stormwater flow shall be released into a stormwater conveyance system
and shall meet one of the following criteria as demonstrated by use of acceptable
hydrologic and hydraulic methodologies:"
Section C.2.b.:
The point of discharge "releases a postdevelopment peak flow rate for the 10-year 24-
hour storm event that is less than the predevelopment peak flow rate from the 10-year 24-
hour storm event. Downstream stormwater conveyance systems do not require any
additional analysis to show compliance with flood protection criteria if this option is
utilized."
Section C.3 Limits of Analysis
The analysis terminates at the proposed SWM facility's outfall since subsection C.2.b is
met.
Post-Development Routing Calculations
(Using a 3" Low-Flow Orifice)
The previously shown routing calculations meet 9VAC25-870-66 requirements.
This is a problematic design though due to its small low-flow orifice diameter.
That system is prone to standing water, advanced degradation of the pipe, an
increased likelihood of joint and subsurface failure and presents an unhealthy
breeding ground for disease infested mosquitos. Even with a trash rack the small
orifice diameter increases the risk of clogging. To resolve this concern the
following routing calculations and the proposed design provides a system with a
3" low-flow orifice. This design is in accordance with the Virginia Stormwater
BMP Clearinghouse, where it states "Orifices less than 3 inches in diameter may
require extra attention during design to minimize the potential for clogging."
The following routing calculations are also for a 115' long 36" Underground
Detention System and continue to release Post-ReDevelopment subarea A flows
less than Pre-ReDevelopment subarea A flows.
Peabody- Underground Detention System Routings
BasinFlow printout
INPUT:
Basin: Peabody- Underground Detention System Routings
0 Contour Areas
Elevation(ft) Area(sf) Computed Vol. (cy)
1 Storage Pipes
Storage pipe 0
name: UGD Piping
diameter (in) 36.000
length (ft) 115.000
invert (ft) 444.000
angle 0.287
volume (cy) 30.107
Start_Elevation(ft) 444.00 Vol. (cy) 0.00
6 Outlet Structures
Outlet structure 0
Orifice
name: Low-Flow Orifice
area (sf) 0.049
diameter or depth (in) 3.000
width for rect. (in) 0.000
coefficient 0.600
invert (ft) 444.000
multiple 1
discharge into riser
Outlet structure 1
Orifice
name: Mid-Flow Orifices
area (sf) 0.087
diameter or depth (in) 4.000
width for rect. (in) 0.000
coefficient 0.600
invert (ft) 445.670
multiple 3
discharge into riser
Page 1
Peabody- Underground Detention System Routings
Outlet structure 2
Orifice
name: Barrel
area (sf) 0.196
diameter or depth (in) 6.000
width for rect. (in) 0.000
coefficient 0.600
invert (ft) 443.900
multiple 1
discharge out of riser
Outlet structure 3
Orifice
name: Emergency Spillway Simulation
area (sf) 1.382
diameter or depth (in) 15.920
width for rect. (in) 0.000
coefficient 0.600
invert (ft) 446.250
multiple 1
discharge into riser
Outlet structure 4
Orifice
name: Emergency Bypass Pipe
area (sf) 0.196
diameter or depth (in) 6.000
width for rect. (in) 0.000
coefficient 0.600
invert (ft) 446.250
multiple 1
discharge out of riser
Outlet structure 5
Weir
name: Proposed MH-1
diameter (in) 48.000
side angle 0.000
coefficient 3.000
invert (ft) 453.750
multiple 1
discharge into riser
Page 2
Peabody- Underground Detention System Routings
3 Inflow Hydrographs
Hydrograph 0
SCS
name: 1-yr. 24-hr Peak SCS Storm Event
Area (acres) 0.220
CN 93.000
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) 0.640
peak time (hrs) 11.917
volume (cy) 67.757
Hydrograph 1
SCS
name: 2-yr. 24-hr Peak SCS Storm Event
Area (acres) 0.220
CN 93.000
Type 2
rainfall, P (in) 3.680
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) 0.812
peak time (hrs) 11.917
volume (cy) 85.978
Hydrograph 2
SCS
name: 10-yr. 24-hr Peak SCS Storm Event
Area (acres) 0.220
CN 93.000
Type 2
rainfall, P (in) 5.570
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) 1.328
peak time (hrs) 11.917
volume (cy) 140.631
Page 3
Peabody- Underground Detention System Routings
OUTPUT:
Routing Method: storage-indication
Hydrograph 0
Routing Summary of Peaks: 1-yr. 24-hr Peak SCS Storm Event
inflow (cfs) 0.637 at 11.92 (hrs)
discharge (cfs) 0.284 at 12.06 (hrs)
water level (ft) 445.573 at 12.06 (hrs)
storage (cy) 12.335
Hydrograph 1
Routing Summary of Peaks: 2-yr. 24-hr Peak SCS Storm Event
inflow (cfs) 0.808 at 11.92 (hrs)
discharge (cfs) 0.524 at 12.04 (hrs)
water level (ft) 445.842 at 12.04 (hrs)
storage (cy) 15.741
Hydrograph 2
Routing Summary of Peaks: 10-yr. 24-hr Peak SCS Storm Event
inflow (cfs) 1.322 at 11.92 (hrs)
discharge (cfs) 1.098 at 11.98 (hrs)
water level (ft) 446.198 at 11.98 (hrs)
storage (cy) 20.194
Wed Apr 24 15:17:39 EDT 2019
Page 4
RECEIVED
.0M1 3 7 1°11
COMMUNITY
DEVELOPMENT