HomeMy WebLinkAboutWPO201900050 Calculations 2019-12-04BERKMAR OVERLOOK STORMWATER
MANAGEMENT CALCULATIONS PACKET
Date of Calculations
AUGUST 28, 2019
Revised on
October 4, 2019
PREPARED BY:
cover
COLLINS =NGINEEPING
200 GARRETT STREET, SUITE K
CHARLOTTESVILLE, VA 22902
434.293.3719 PH
434.293.2813 FX
www.collins-engineering.com
APPROVED
by the Albemarle County
Community Development Department
Date 12/4/2019
File WPO201900050
SCS TR-55 Calculations
Soils Map (Source: NRCS Web Soil Survey Online Database)
Soils Table (Source: NRCS Web Soil Survey Online Database)
Albemarle County, Virginia (VA003)
Albemarle County, Virginia (VA003) C3
Map Unit Map Unit Name Acres Percent of
Symbol in AOI AOI
27C Eiioak loam, 7 3.5 39.70/c
to 15 percent
slopes
34D Glenelg loam, 0.7 8.511/c
15to25
percent slopes
65B Pacolet sandy 4.6 51.80/0
loam, 2 to 7
percent slopes
Totals for Area of 8.8 100.00/0
Interest
Note: The NRCS web soil survey has catorgized this development's soils as predominately consisting of
hydrologic soil group type B.
Precipitation Data
(Source: NOAA's National Weather Service, Hydrometerorological Design Studies Center, Atlas 14
Point Precipitation Frequency Estimates)
map
❑� Terrain
South Fork
y1
Rivanna RIVer
Reservolr
a
c
c
104rn
INlft
POINT PRECIPITATION FREQUENCY (PF) ESTIMATES
WITH 90"/fl CONFIDENCE INTERVALS AND SUPPLEMENTARY INFORMATION
NOAA Atlas 14, VOIIlme 2, Version 3
PD5-based precipitation frequency estimates with 90% confidence intervals [in inches]'
Average recurrence interval [years]
1 2 5 7� 25 50 1Q0 2Q0 5l}0 7�116
23.01 3.fi4 4.fi5 5.50 fi.75 7.83 9.01 10.3 12.3 14.6
4hr
(2.70-3.38) (3.27-409) (4-165.22) (4.91-6.15) (5-99-7.53) (6.39-6.71) [7-87-10.0} 11 (8.91-11.4) 11 (10.413.6) (11.7-15.5)
U.S. Department of Agriculture
Natural Resources Conservation Service
TR 55 Worksheet 2: Runoff Curve Number and Runoff
FL-ENG-21A
06/04
Project: Berkmar Overlook Designed By: FGM, PE Date: 10/4/2019
Location: Berkmar Drive Checked By: SRC, PE Date: 10/4/2019
Check One: Present X Developed
1. Runoff curve Number (CN)
Cover description
CN (weighted) _Calculaued'S'
Drainage Area
Soil name and hydrologic
(Cover type, treatment, and hydrologic condition; percent
CN
Area
Product of CN x
total product/
Description
group (Appendix A)
(Acres)
Area
Value
impervious; unconnected/ connected impervious area ratio)
total area
Impervious Areas
98
0.42
41.1
DA A
Gravel Drive Areas
85
1.05
88.8
(Present)
B
Dirt Drive Areas
82
0.00
0.0
79'4
2.59
Lawns in Good Condition
61
0.74
44.9
Impervious Areas
98
0.35
34.1
DA B & C
B
Gravel Drive Areas
85
2.55
216.6
76.3
3.11
(Present)
Dirt Drive Areas
82
0.70
57.5
Lawns in Good Condition
61
2.20
134.4
DAD
Impervious Areas
98
0.18
17.2
(Present)
B
Gravel Drive Areas
85
0.51
43.7
81.9
2.21
Lawns in Good Condition
61
0.21
12.9
Impervious Areas
98
0.22
21.3
DAA
(Developed)
B
Woods in Good Condition
55
0.00
0.0
69.5
4.40
Lawns in Good Condition
61
0.73
44.7
Impervious Areas
98
3.65
358.2
DA B
(Developed)
B
Woods in Good Condition
55
0.00
0.0
80.9
2.35
Lawns in Good Condition
61
3.13
190.6
DAC
Impervious Areas
98
0.12
11.5
(Developed)
B
Woods in Good Condition
55
0.00
0.0
65.8
5.19
Lawns in Good Condition
61
0.78
47.7
DAD
Impervious Areas
98
0.00
0.0
(Developed)
B
Woods in Good Condition
55
0.00
0.0
61.0
6.39
Lawns in Good Condition
61
0.27
16.5
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
3.01
3.64
5.50
n/a
Runoff, Q-inches
1.22
1.71
3.28
DAA(Present)
Runoff, Q- inches
1.04
1.49
2.98
DA B & C (Present)
Runoff, Q- inches
1.38
1.89
3.52
DA D (Present)
Runoff, Q-inches
0.70
1.06
2.37
DA A (Developed)
Runoff, Q-inches
1.32
1.82
3.43
DAB (Developed)
Runoff, Q-inches
0.54
0.87
2.06
DAC(Developed)
Runoff, Q- inches
0.37
1 0.64
1 1.68
IDA D (Developed)
U.S. Department of Agriculture
Natural Resources Conservation Service
TR 55 Worksheet 3: Time of Concentration (TJ or Travel Time (Tt)
Project: Berkmar Overlook
Location: Berkmar Drive
Check One: Present X Developed X
Check One: Tc X Tt
Designed By: FGM, PE
Checked By: SRC, PE
Through subarea n/a
FL-ENG-21A
06/04
Date: 10/4/2019
Date: 10/4/2019
DAA
DAB&C
DAD
DAA
DAB
DAC
DAD
(Present)
(Present)
(Present)
(Developed)
(Developed)
(Developed)
(Developed)
Segment ID:
Sheet Flow: (Applicable to Tc only)
1 Surface description (Table 3-1)
Dense Grass
Dense Grass
Dense Grass
Dense Grass
Dense Grass
Dense Grass
Dense Grass
2 Manning's roughness coeff., n (Table 3-1
0.24
0.24
0.24
0.24
0.24
0.24
0.24
3 Flow length, L (total L < 100) (ft)
40
20
40
40
60
100
30
4 Two-year 24-hour rainfall, PZ (in.)
3.6
3.6
3.6
3.6
3.6
3.6
3.6
5 Land slope, s (ft/ft)
0.075
0.10
0.025
0.075
0.05
0.18
0.2
6 Compute Tt = [0.007(n*L)0 e] / P205 SOA
0.06
0.03
0.10
0.06
0.10
0.09
0.03
Shallow Concentrated Flow:
7 Surface description (paved or unpaved)
Unpaved
Unpaved
8 Flow Length, L (ft)
75
110
9 Watercourse slope, s (ft/ft)
0.013
0.082
10 Average velocity, V (Figure 3-1) (ft/s)
1.8
4.6
11 Tt=L/3600*V
0.01
0.01
Channel Flow:
12 Cross sectional flow area, a (ft)
3 Y
3
3 Y
o
3 v
13 Wetted perimeter, Pw (ft)
_O „
O m
_
" X
O m
_
" x
(O
LL c
v c E
o c
LL
E
c
14 Hydraulic radius, r= a/Pw (ft)
X
c a Y
a Y
cL Y
N X
E° Y
v
a)X
E° Y
15 Channel Slope, s (ft/ft)
r C,
M C ,�
m C ,�
u v a
m o
v a
o
16 Manning's Roughness Coeff, n
u
- m
- m
o d
d
o
17 V= [ 1.49r213s°'5 ]/ n
D u
u
o u
F, o a
o o a
18 Flow length, L (ft)
tom'+
L,
o
19 Tt= L / 3600*V
0.03
0.02
0.03
0.02
0.00
20 Watershed or subarea T,orTt
(Add Ttin steps 6, 11 and 19)
0.10
0.10
0.11
0.10
0.12
0.10
0.10
Time of Concentration Notes:
1. The most conservative post -development SCS Tc is 0.10 hrs.
2. The Post -Development DA B Tc is slightly longer because the ReDevelopment scenario has less impervious areas and promotes infiltration with more grass
chi c0-I
O O
m
d c
0 0
c V
p>
O
D
O
m m
p p
m
` C p
a ° w v
bOq Q O O V1 O= O
c vui p > O lD O
0 O Q O
m
` C 6
a ° w to
toOm Oa O Q N
_ a w 0 O
c vuO
i p > m O
0 O 0
o
L
v
m
m
d C
bmc0 a
G
p p
¢ o
Q;
aj
o
O'D
o
o=
O
L
d c
a w
¢
V
p
Ul
c-I
m V
m
b0
Q Ln
O
O
m
(Da
O
(7 0�
L.� to
c V
p
O
_
p
u5.
a
m coo m
u
p
t a a
U" V Y
m
pcp V
L
om u
Ul
�
d C
N
L
¢ O
U
c-I
Y
n
y
rn
m
o
b0
a,
p
OO
t0
c-I
= O
j
V
Q
1�
O
Ln
p
m
d c
O
w Q Q M O
to m V ¢ ` 0 m `""� _ O
c p O n O
p
X
N
O
N
O 4)
O O p
— �
V >
>
> x
v O p
m
H
m
Y Y
Y m�v
11
v u
_
3 � m co L a
—_
II
V N
E
u
H
Q
N N
a0
o
C
o
:T
v a
_
a`
o v
�
E
Q
E
7
m
o
Q 3
E O
N
u=
m
3
a c c
v
o
o
6N 0
p
m
pcO
m 0
N u i
O
'O O
n 4m Ou L
u
w
Yz
O
O
o0
M
Ln
a0
N
In
N
N
^
O
c-I
LD
Ln
c-I
O
o)
c-I
O
V
w
Ln
O
V
V
ry
l0
N
M
O
lO
to
c
O
o)
O
O
c-I
ci
O
00
N
N
'*
O
In
I,
M
N
to
c
O
n
O
O
O
On
O
ci
Ln
ID
c
W
N
li l
c-I
O
0) �
N
N
�
O
N
lO
N
LLn
00
iy
to
c
O
o)
O
v
v
c-I
o
o
m
p
In
c.l
tD
m
e
o
o
0
O
Ln
V
O
Lon
V
cy
Ln
o
o
-
rn
w Ln
C c
o
ry
lD
C
cm -I
Lon
00
c-I
0
m
O
Oo)
c
7
cu
Itr
tD
N
n u
c-Io
o
C
c-I
N
M
iy
to
o
O
o)
c
0
On
00
c-I
n
M
c-I
Ln
O
O
0)
N
M
v
oo
v
Ln
to
a
N
O
m
N
n
O
to
O
O
o)
c-I
iy
Cl
aO
N
In
r,
to
0
o
o)
o
O
o
0
o
Ln
v
o
Ln
c
rn
m
o
o
2
tm
V
ry
llo
cN-I
-
00
c-I
L61n
M
O
O
M
-,
N
c-I
V
Ln
O
o0
't
c-I
O
C
c-I
Ln
cn
c-I
o0
L
o
o
m
cj
c
o
O
Ln
N
D
ci
O
o
rn
r
N
o
~
Ln
O
O
N
N
Ln
ry
llo
llo
M
O
O
Dl
cy
In
c-I
ci
N
c
.0a-I
O
ID
N
^
O
to
O
O
c
m
O
n
O
Ln
Ln
O
O
N
cy
N
m
O
O
M
ry
to
Ln
c�-I
^
n
cy
n
m
o
o
Ql
c-I
Ln
c-I
N
I,
m
N
O
cy
O
Ln
c-I
-
N
cy
c-I
M
O
O
Ql
c-I
't
N
cL
O
L
c
C
w
N
r
u
N
E
V
c
jcucL
Y
U
`'o
o
�
a
Cf
.4u
a
L
E
m
m E
w
?'
a
m
LUVi
Q
o
a
m
?
m
cl
p
°
°
s
c
E
co
c
Li
C
U
7=
a
a t
N
C6
v
Ln
to
I,
oo
m 3
3
0
V N 00 Z
m V m N N O m
T
O
c-I
O
LL N N N
V rl c4 o
T
N
N
W
U
p00 N Dl N O
LL pl to lD c-I V
O M O O In
m
m
ey
N U m O N
Q m o to 0 0 00
z Ln Ol 00 O lD
U M O Vl -4 I�
tD a0 LD O n
3
_O
Ln N
O
m
O
c-I
0 N ti Ol D
m V Ln (M N c-I
N
N
3
O N
w N O 0000 c
m V V Ol c
QJ
T
c-1
cu m O vi o M
Q N CO
Z
v m m o
u
n n 000 n
CQ
C
D
N
0
W
2
H
W
Qa
m u p
a a a a
3
O p O p
Channel and Flood Protection Calculations & Summaries
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 = 12.13 cfs
Where the following inputs are used:
CN Pre-ReDev.=
77.6
CN Post-ReDev.=
77.6
S Pre-ReDev.=
2.88
S Post-ReDev.=
2.89
RV Pre-ReDev.=
1.12
RV Post-ReDev.=
1.11
Q Pre -Developed =
15.12
I.F. =
0.8
Notes:
CN represents the weighted Curve Numbers
(Shown in the Watershed Summary Table at the bottom of Worksheet 4.)
S was calculated by using the SCS TR-55 manual's equation 2-4.
(The Weighted CN values described above were used.)
RV was calculated by using the SCS TR-55 manual's equation 2-3.
(The Weighted CN values & RV values described above were used.)
Q Developed = 5.40 cfs
Section B.4 Limits of Analysis
The analysis terminates at subareas A, B, C & D since subsection B.3.a is met.
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 site "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 flood protection analysis terminates at subareas A, B, C & D since subsection C.2.b is met.
Proposed Routing Calculations
Berkmar Drive- SWM Facility Routings
BasinFlow printout
INPUT:
Basin: Berkmar Drive- SWM Facility Routings
5 Contour Areas
Elevation(ft)
Area(sf)
Computed Vol.(cy)
520.00
34.00
0.0
522.00
941.00
28.5
524.00
2596.00
154.4
526.00
4770.00
423.2
528.00
7355.00
868.8
Start—Elevation(ft)
5 Outlet Structures
Outlet structure 0
Orifice
520.00 Vol.(cy) 0.00
name: Barrel
area (sf) 1.767
diameter or depth (in) 18.000
width for rect. (in) 0.000
coefficient 0.500
invert (ft) 416.000
multiple 1
discharge out of riser
Outlet structure 1
Orifice
name: Low -Flow Orifice
area (sf) 0.196
diameter or depth (in) 6.000
width for rect. (in) 0.000
coefficient 0.500
invert (ft) 520.000
multiple 1
discharge into riser
Page 1
Berkmar Drive- SWM Facility Routings
Outlet structure 2
Orifice
name: Mid -Flow Orifice
area (sf) 0.442
diameter or depth (in) 9.000
width for rect. (in) 0.000
coefficient 0.500
invert (ft) 524.000
multiple 1
discharge into riser
Outlet structure 3
Weir
name: Riser
diameter (in)
48.000
side angle
0.000
coefficient
3.300
invert (ft)
525.500
multiple
1
discharge
into riser
transition at (ft)
1.215
orifice coef.
0.500
orifice area (sf)
12.566
Outlet structure 4
Weir
name: Emergency Spillway
length (ft)
25.000
side angle
75.960
coefficient
3.300
invert (ft)
526.500
multiple
1
discharge
through dam
Page 2
Berkmar Drive- SWM Facility Routings
4 Inflow Hydrographs
Hydrograph 0
SCS
name: 1-yr
24-hr. SCS Design Storm
Area (acres)
6.780
CN
80.900
Type 2
rainfall, P (in)
3.010
time of conc. (hrs)
0.1200
time increment (hrs)
0.0200
time limit (hrs)
30.000
fudge factor
1.00
routed
true
peak flow (cfs)
10.939
peak time (hrs)
11.922
volume (cy)
1197.384
Hydrograph 1
SCS
name: 2-yr
24-hr. SCS Design Storm
Area (acres)
6.780
CN
80.900
Type 2
rainfall, P (in)
3.640
time of conc. (hrs)
0.1200
time increment (hrs)
0.0200
time limit (hrs)
30.000
fudge factor
1.00
routed
true
peak flow (cfs)
15.102
peak time (hrs)
11.922
volume (cy)
1653.045
Hydrograph 2
SCS
name: 10-yr
24-hr. SCS Design Storm
Area (acres)
6.780
CN
80.900
Type 2
rainfall, P (in)
5.500
time of conc. (hrs)
0.1200
time increment (hrs)
0.0200
time limit (hrs)
30.000
fudge factor
1.00
routed
true
peak flow (cfs)
28.465
peak time (hrs)
11.922
volume (cy)
3115.907
Page 3
Hydrograph 3
SCS
name:
Area (acres)
CN
Type
rainfall, P (in)
time of conc. (hrs)
time increment (hrs)
time limit (hrs)
fudge factor
routed
peak flow (cfs)
peak time (hrs)
volume (cy)
OUTPUT:
Berkmar Drive- SWM Facility Routings
100-yr 24-hr. SCS Design Storm
6.780
80.900
2
9.010
0.1200
0.0200
30.000
1.00
true
55.647
11.922
6091.357
Routing Method: storage -indication
Hydrograph 0
Routing Summary
of
Peaks: 1-yr 24-hr.
SCS Design Storm
inflow
(cfs)
10.920
at
11.92
(hrs)
discharge
(cfs)
3.618
at
12.10
(hrs)
water level
(ft)
525.435
at
12.10
(hrs)
storage
(cy)
330.612
Hydrograph 1
Routing Summary
of
Peaks: 2-yr 24-hr.
SCS Design Storm
inflow
(cfs)
15.075
at
11.92
(hrs)
discharge
(cfs)
11.281
at
12.02
(hrs)
water level
(ft)
525.814
at
12.02
(hrs)
storage
(cy)
391.121
Hydrograph 2
Routing Summary
of
Peaks: 10-yr
24-hr.
SCS Design Storm
inflow
(cfs)
28.416
at
11.92
(hrs)
discharge
(cfs)
27.218
at
11.96
(hrs)
water level
(ft)
526.174
at
11.96
(hrs)
storage
(cy)
454.494
Hydrograph 3
Routing Summary
of
Peaks: 100-yr
24-hr.
SCS
Design Storm
inflow
(cfs)
55.550
at
11.92
(hrs)
discharge
(cfs)
54.441
at
11.94
(hrs)
water level
(ft)
526.594
at
11.94
(hrs)
storage
(cy)
535.757
Thu Oct 03 10:34:19
EDT 2019
Page 4
DEQ Virginia Runoff Reduction Method
Water Quality Calculations for ReDevelopments
(COMPLIANCE IS MET THROUGH THE BEST MANAGEMENT PRACTICE OF PURCHASING NUTRIENT CREDITS.)
CLEAR ALL --�
Linear Development ® --�
------- final resuks --■
Site Information
Post -Development Project (Treatment Volume and Loads)
Maximum
The site's net increase in impervious
Post -Development TP Load Reduction
reduction required:
cover (acres) is:
for Site (ib/yr):
_-__■-_,
®-
_---
BMP Design Specifications List.
2013 Draft
Stcls & Specs
20%
0Land
cow areas entered comedy?
m---
Total disturbed area entered?
-_.
Post -Development Land Cover (acres)
------------
Runoff Coefficients
(Rv)
------------
------------
LAND COVER SUMMARY--PRE-REDEVELOPMENT
LAND
COVER
SUMMARY-- POST DEVELOPMENT
■
Post -Development Requirement for Site
Area
Nitrogen Loads (informational Purposes
Only)
-_.
Y
WJ
m ]C Y Y Y Y
WO O O O O O p 0 O O O
F
a
W
a p 0 0 0 O
0 0 0 0
O o O O
a p o 0 0 0
0 0 0 0 0
GJ
f0
,CL a o 0 0 0
E a p 0 0 0 O
O � O O O O
V
a�
co
C m
a p o 0 0 0
e 0 0 0 0
GJ
a�
f0
H 00
W a p 0 0 0 O .0 rry 0 10
N O00
H0000� O O O O N �--i O a -I O 00
f+ Y v u u v v
Y m T T? T Vl > T T T T>
H yv c C Q C W a IA
W = Q 0 _
s a>Pix d >� c ii c L �mct� c c.coc
{A 0 W Q W W ba W Q W Z W Q Q uj W z W L Q W a
m c W C LU 3 m => > > ,� O > OC 3 O> O
W� F H Q C CJ O W Z W O J O W Z d J W J
Q 0. p\>Lu
> o c > L `+'c Q Q a Ica LL,� O W Q�
c 0 W Q j c Z c z W Z z c Z +� z 0
CA ...
a z y �` O 0 o O +�+ g O O a O m 0 0 Ix
m O z
02Op¢H a Zm = 0 H Oj OLU
>>~
p f0 H Lu
O a C W cZ
1= (7 L M O J ~ C W e ~ W 0LU
G
Q l0 >> C O pQ Oa p 0 0 Q CL
O
- +' dJ z� O 0 0 JOpJ 0 ao�
N a
1WL H W Q~ H F W C
O Oa O z O Op OF
> >~ z z 0 zO
z ._ � a
O z Q z Z 0
z
V W m Z
I= c a
a �
oc o
0
c
W
a
O
o
O
0
O
0
O
0
O
0
00000
c
�
o
0
0
0
0
Q
o
0
0
0
0
00000
0
v
o
0
0
0
0
Q
O
O
O
O
O
Y
0
0
0
0
0
�
�
Q
o
0
0
0
0
00000
0
a
o
0
0
0
0
Q
O
O
O
O
O
O
0
0
0
0
0
�