HomeMy WebLinkAboutWPO201600034 Calculations 2020-07-07GREENLOFT FARM VSMP AMENDMENT #1
STORMWATER MANAGEMENT CALCULATIONS PACKET
Date of Approved Calculations
OCTOBER 21, 2016
Date of Amendment Calculations
J U N E 16, 2020
PREPARED BY:
COLLINS FNSINE1701mr-
200 GARRETT STREET, SUITE K
CHARLOTTESVILLE, VA 22902
434.293.3719 PH
434.293.2813 FX
www.collins-engineering.com
APPROVED STORMWATER MANAGEMENT NARRATIVE
OVERALL:
This project utilizes Engineering Site Design (ESD) techniques outlined in the 2013 Draft VSMH. The
use of these ESD techniques, the vast acreage left undisturbed and the clustered low -impact
development proposed results in minimal environmental impacts.
STORMWATER QUALITY:
The considerable amount of acreage being preserved, and prohibited from disturbances and future
development, results in this clustered development's water quality compliance. The preservation of
over 45 acres of forested open space within the parcel's 148+ acre limits yields a minimum
phosphorus load reduction requirement of 0 pounds per year.
STORMWATER QUANTITY:
Subareas A and C are detained via proposed detention facilities. These facilities are not required to
adhere to DEQ's Post -Construction Virginia Stormwater BMP Clearinghouse's requirements due to
the site's previously noted water quality compliance through preservation. These facilities include a
multi -stage riser, barrel and an emergency spillway and detain the 1-year and 2-year storm events
below pre -development conditions. The routing of these facilities, and the final stormwater
management analysis, can be viewed within this report.
ENGINEERING SITE DESIGN PARCTICES IMPLEMENTED
This development utilizes environmental site design (ESD) techniques and practices in accordance
with the July 2013 draft VSMH. The stormwater management design concept was integrated early in
the site planning and design of this development. This is evident in the alignment of the roads
horizontally and vertically to reduce grading impacts and the low -impact fashion of the clustered
development. More detailed examples of ESD techniques and practices proposed are as follows:
ESD practice #1: The proposed preservation of undisturbed natural areas
This is accomplished through an easement dedication of the stream buffer. This is also achieved
through the significant preserved open spaces proposed to remain undisturbed outside the limits of
disturbance, and that are prohibited from future development. Please note the subdivision plat and
erosion and sediment control plans prohibit future disturbances and development of lots within the
open spaces shown to be preserved.
ESD practice #2: The proposed preservation of riparian buffers
This is accomplished through the easement dedication of the stream buffer noted above.
ESD practice #3: The proposed preservation or planting of native trees
This is accomplished through limiting the proposed disturbances in the erosion and sediment control
plan. The limits of disturbance remain outside the limits of existing native trees that are proposed to
remain within the stream buffer and preserved open spaces.
ESD practice #5: The proposed avoidance of disturbance within steep slopes where possible
This development limits disturbances to the Albemarle County defined critical slopes. This is evident
is the detention facilities' grading and the alignments of the roads and lots.
ESD practice #6: The proposed design fits the terrain
This development limits lot grading, has minimal disturbances with the proposed roads and places
the detention facilities in existing low areas outside of stream buffers and floodplain areas. This
development also maintains the general drainage patterns and does not excessively reroute runoff.
ESD practice #7: The proposed development is located in less sensitive areas
This development avoids disturbances to the more environmentally sensitive areas, inclusive of the
critical slopes, intermittent stream and stream buffers.
ESD practice #8: Reduced limits of disturbances
This project clusters its development, as is evident in the lot layout and roadways' grading plan. This
development also provides significant open spaces that are proposed to remain undisturbed outside
the limits of disturbance.
ESD practice #9: Proposed use of open space
This project clusters its development and utilizes open space outside of the limits of the right-of-way
and lots. The cluster development strategy yields extensive open spaces in excess of 30%.
ESD practice #10: Creative development design
The clustered development with significant open spaces, coupled with the road design resulting in
limited lot grading, provides for a creative development designed with limited disturbances in a low
impact fashion consistent with chapter 6 of the 2013 VSMH draft.
ESD practice #11: Reduced roadway widths and lengths
The proposed plan utilizes the shortest widths allowed by Albemarle County Fire and Rescue. Also,
the plan minimizes the lengths of roadways by having a cul-de-sac for lot frontage, and does not
propose single frontage lots. Meaning, all roadways have lots on either side of the road, hereby
minimizing roadway lengths.
ESD practice #12: Reduced impervious footprints
In accordance with ESD practice #10, the roadway impervious areas were minimized to the fullest
extent possible.
ESD practice #13: Reduced parking footprints
This development provides off street parking, and in many cases will provide homes with built-in
attached garages, hereby reducing the parking footprints.
ESD practice #17: Use of buffers & undisturbed filter areas
The natural site features of undisturbed vegetated areas throughout the parcel's limits reduce runoff.
These areas also provide infiltration and stormwater filtering of pollutants, sediment recycling
nutrients, and maximize on -site storage of stormwater.
ESD practice #18: Creative site grading, berming and terracing
The proposed detention facilities' grading utilizes berming in existing lot areas to minimize grading
disturbances for their installation.
ESD practice #19: Use of natural drainage ways & vegetated swales instead of storm sewers and curb
& gutter
The proposed plan utilizes vegetative grass -lined roadside swales in lieu of curb and gutter to convey
the roads' runoff. Storm sewer are proposed in select areas, where required. The use of existing
drainage ways is also incorporated into the design.
ESD practice #20: Drain runoff to pervious areas
The proposed grading plan and VRRM water quality calculations allow for the rear portions of specific
lots to drain away from the road towards pervious areas being preserved & left undisturbed. This
allows for groundwater recharge and recycling of nutrients.
APPROVED
DEQ Virginia Runott Keauction Method
Water Quality Calculations
Virginia Runoff Reduction Method New Development Worksheet - v2.8 - June 2014
To be used w/ 2011 BMP Standards and Specifications
Site Data
Project Name: Greenloft Farm VSMP Plan
Date: 10/21 /16
data input cells
calculation cells
constant values
1. Post -Development Project & Land Cover Information
Constants
Annual Rainfall (inches)
r 43
1.00
0.26
0.41
0.90
Target Rainfall Event (inches)
Phosphorus EMC (mg/L)
Nitrogen EMC (mg/L)
1.86
Target Phosphorus Target Load (lb/acre/yr)
Pj
Land Cover (acres)
A soils B Soils C Soils D Soils
Totals
Forest/Open Space (acres) -- undisturbed,
protected forest/open space or reforested land
0.00
45.48
0.00
0.00
45.48
Managed Turf (acres) -- disturbed, graded for
yards or other turf to be mowed/managed
0.00
17.80
81.39
0.00
99.19
Impervious Cover (acres)
0.00
1.93
2.16
0.00
1 4.09
Total
148.76
Rv Coefficients
A soils
B Soils
C Soils
D Soils
Forest/Open Space
0.02
0.03
0.04
0.05
Managed Turf
0.15
0.20
0.22
0.25
Impervious Cover
0.95
0.95
0.95
0.95
Land Cover Summa
Site Results
D.A. A
D.A. B
D.A. C D.A. D
D.A. E
AREA CHECK
IMPERVIOUS COVER
IMPERVIOUS -COVER TREATED
TURF AREA
TURF AREA TREATED
0.00
0.00
0.0
0.00
OK.
OK.
0.00
0.00
0.00
0.00
. 0
0.0
OK.
OK.
AREA CHECK
OK.
OK.
OK. OK.
OK.
-------------------------_-
Phosphorus
TOTAL TREATMENT VOLUME
,978
960.06
TOTAL PHOSPHORUS LOAD R DUCTION REQUIREDLBIYEAR
RUNOFF REDUCTION (cf)i
0
PHOSPHORUS LOAD REDUCTION ACHIEVED LB R
_
0.00
1
-+----------------
------
----------
----------------
ADJUSTED POST -DEVELOPMENT PHOSPHORUS LOAD TP Ib/r
__________-- -+-
60.93
--_--- —;
CONGRATULATIONS!! YOU EXCEEDED
REMAINING PHOSPHORUS LOAD REDUCTION LB/YR NEEDD
THE TARGET REDUCTION BY 0.1 LB/YEAR!
Nitrogen (for information purposes)
TOTAL TREATMENT VOLUME (cf)
RUNOFF REDUCTION
NITROGEN LOAD REDUCTION ACHIEVED (LBIYR)
______________
AMENDED SCS TR-55 Calculations
( The soils' properties witin the parcel's limits are predominantly composed of hydrologic types B & C.)
U.S. Department of Agriculture
Natural Resources Conservation Service
TR 55 Worksheet 2: Runoff Curve Number and Runoff
FL-ENG-21A
06/04
Project: Dudley Mountain Designed By: FGM, PE Date: 6/16/2020
Location: Dudley Mountain Road Checked By: SRC, PE Date: 6/16/2020
Check One: Present X Developed X
1. Runoff curve Number (CN)
Soil name and
Cover description
CN (weighted) _
Drainage Area
hydrologic group
(Cover type, treatment, and hydrologic condition; percent impervious;
CN
Area
Product of CN
total product/
Calculated 'S'
Description
(Appendix A)
unconnected/ connected impervious area ratio)
(Acres)
x Area
total area
Value
rvious Areas
9£
0.0
DA A
Woods in Good Condition (Approx. 1/2 of pre -development watershed)
7(
648.9
72.0
3.89
(Present)
Lawns in Good Condition (Approx. 1/2 of pre -development watershed)
7z
686.7
Impervious Areas
S
O.0
0.0
DAB
Woods in Good Condition (Approx. 1/4 of pre -development watershed)
7
1.07
74.9
7-
(Present)
Lawns in Good Condition (Approx. 3/4 of pre -development watershed)
3.23
239.0
Impervious Areas
0.00
0.0
Woods in Good Condition (Approx. 3/4 of pre -development watershed)
75.82
4170.0
Lawns in Good Condition (Approx. 1/4 of pre -development watershed)
25.27
1541.6
Impervious Areas
0.00
0.0
DAE
Woods in Good Condition (Approx. 1/2 of pre -development watershed)
11.43
800.1
7
3.89
(Present,
Lawns in Good Condition (Approx. 1/2 of pre -development watershed)
11.43
845.8
Impervious Areas
_.
1.18
115.4
DA A
C
Woods in Good Condition (Approx. 1/2 of areas not impervious)
10.76
753.2
73.3
3.63
(Developed)
Lawns in Good Condition (Approx. 1/2 of areas not impervious)
10.76
796.2
Impervious Areas
0.26
25.9
DA B
Woods in Good Condition (Approx. 1/4 of pre -development watershed)
0.75
52.5
75.0
3.33
(Developed)
Lawns in Good Condition (Approx. 3/4 of pre -development watershed)
2.27
168.0
Impervious Areas
1.44
140.9
DA C
Woods in Good Condition (Approx. 1/2 of areas not impervious)
9.27
509.9
E
Lawns in Good Condition (Approx. 1/2 of areas not impervious)
9.27
565.5
Impervious Areas
0.50
48.7
DA D
Woods in Good Condition (Approx. 3/4 of of areas not impervious)
59.66
3281.3
56.8
7.62
(Developed)
Lawns in Good Condition (Approx. 1/4 of of areas not impervious)
19.88
1212.7
Impervious Areas
2
70.4
DA E
L.
Woods in Good Condition (Approx. 1/2 of areas not impervious)
10.04
702.8
72.9
i (Approx. 1/2 of areas not impervious)
7z
10.04
743.0
2. Runoff
2-Year Storm
10-Year Storm
Drainage Area Description
Frequency -years
10
n/a
Rainfall, P (24 hou
5.6
n/a
Runoff, Q-ir
2.67
DAA(Present)
Runoff, Q- inches
2.76
DA B (Present)
Runoff, Q-inches
1.40
DA
Runoff, Q- inches
2.67
DA E (Present)
Runoff, Q-inches
2.79
DA A (Developed)
Runoff, Q- inches
1.30
2.95
DA B (Developed)
Runoff, Q- inches
0.57
1.73
DA C (Developed)
Runoff, Q- inches
0.41
1.42
DA D (Developed)
Runoff, Q- inches
1.17
2.75
DA E (Developed)
U.S. Department of Agriculture
Natural Resources Conservation Service
TR 55 Worksheet 3: Time of Concentration (TJ or Travel Time (T,)
Project: Dudley Mountain
Location: Dudley Mountain Road
Check One: Present X Developed X
Check One: Tc X Tt
Sheet Flow: (Applicable to T, only)
Designed By: FGM, PE
Checked By: SRC, PE
Through subarea n/a
FL-ENG-21A
06/04
Date: 10/21/2016
Date: 10/21/2016
DAA DAB DAC&D DAE DAA DAB DAC DAD DAE
Segment ID: (Present) (Present) (Present) (Present) (Developed) (Developed) (Developed) (Developed) (Developed)
Woods -Light
Woods -Light
Woods -Light
Woods -Light
1 Surface description (Table 3-1)
Underbrush
Dense Grass
Underbrush
Dense Grass
Underbrush
Dense Grass
Dense Grass
Underbrush
Dense Grass
2 Manning's roughness coeff., n (Table 3-1
0.4
0.4
0.4
0.24
0.4
0.4
0.24
0.4
0.24
3 Flow length, L (total L < 100) (ft)
100
100
100
100
100
100
100
100
100
4 Two-year 24-hour rainfall, Pz (in.)
3.7
3.7
3.7
3.7
3.7
3.7
3.7
3.7
3.7
5 Land slope, s (ft/ft)
0.04
0.07
0.07
0.05
0.04
0.10
0.02
0.07
0.05
6 Compute Tr = [0.007(n*L)° e] / Pz ,s s06
0.25
0.20
0.20
0.15
0.25
0.17
0.22
0.20 1
0.15
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 Tr= L / 3600*V
Channel Flow:
12 Cross sectional flow area, a (ft2)
13 Wetted perimeter, P. (ft)
14 Hydraulic radius, r = a/Pw (ft)
15 Channel Slope, s (ft/ft)
16 Manning's Roughness Coeff, n
17 V= [ 1.49r213s°.1 ] / n
18 Flow length, L (ft)
19 Tr= L / 3600*V
20 Watershed or subarea T, or Tr
(Add T, in steps 6, 11 and 19)
Unpaved
Unpaved
Unpaved
Unpaved
Unpaved
Unpaved
Unpaved
Unpaved
Unpaved
1015
93S
920
900
78S
725
140
920
900
0.128
0.070
0.177
0.107
0.143
0.077
0.100
0.177
0.107
S.8
4.3
6.8
5.4
6.1
4.4
5.2
6.8
5.4
0.05
0.06
0.04
0.05
0.04
0.05
0.01
0.04
1 0.05
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Detention Basin A
BasinFlow printout
INPUT:
Basin: Detention Basin A
9 Contour Areas
Elevation(ft)
Area(sf)
Computed Vol.(cy)
586.00
2194.00
0.0
588.00
2619.00
178.0
590.00
3406.00
400.5
592.00
4403.00
689.0
594.00
5500.00
1055.0
596.00
6698.00
1506.0
598.00
7997.00
2049.6
600.00
9396.00
2693.1
602.00
10661.00
3435.4
602.50
11040.00
3435.4
Start-Elevation(ft)
590.00 Vol.(cy)
4 Outlet Structures
Outlet structure 0
Orifice
name: ASBUILT
Barrel
area (sf)
1.767
diameter or depth (in)
18.000
width for rect. (in)
0.000
coefficient
0.500
invert (ft)
588.520
multiple
1
discharge out of riser
Outlet structure 1
Weir
name:
ASBUILT Riser Top
diameter (in)
48.000
side angle
0.000
coefficient
3.300
invert (ft)
597.830
multiple
1
discharge
into riser
transition at (ft)
1.215
orifice coef.
0.500
orifice area (sf)
12.566
Page 1
Detention Basin A
Outlet structure 2
Weir
name: ASBUILT Emergency Spillway
length (ft)
45.000
side angle
75.960
coefficient
3.300
invert (ft)
600.750
multiple
1
discharge
through dam
Outlet structure 3
Orifice
name: AMENDED Low -Flow Orifice
area (sf) 0.196
diameter or depth (in) 6.000
width for rect. (in) 0.000
coefficient 0.600
invert (ft) 590.000
multiple 1
discharge into riser
3 Inflow Hydrographs
Hydrograph 0
SCS
name: 1-Year 24 hr peak SCS Method Design Storm
Area (acres)
22.700
CN
73.300
Type 2
rainfall, P (in)
3.500
time of conc. (hrs)
0.2900
time increment (hrs)
0.0200
time limit (hrs)
30.000
fudge factor
1.00
routed
true
peak flow (cfs)
26.004
peak time (hrs)
11.995
volume (cy)
3650.932
Page 2
Hydrograph 1
SCS
Detention Basin A
name: 2-Year 24 hr peak SCS Method Design Storm
Area (acres)
22.700
CN
73.300
Type 2
rainfall, P (in)
3.700
time of conc. (hrs)
0.2900
time increment (hrs)
0.0200
time limit (hrs)
30.000
fudge factor
1.00
routed
true
peak flow (cfs)
28.988
peak time (hrs)
11.995
volume (cy)
4069.959
Hydrograph 2
SCS
name: 100-Year 24 hr peak SCS Method Design Storm
Area (acres)
22.700
CN
73.300
Type 2
rainfall, P (in)
9.100
time of conc. (hrs)
0.2900
time increment (hrs)
0.0200
time limit (hrs)
30.000
fudge factor
1.00
routed
true
peak flow (cfs)
126.665
peak time (hrs)
11.995
volume (cy)
17783.776
Page 3
Detention Basin A
OUTPUT:
Routing Method: storage -indication
Hydrograph 0
Routing Summary of
Peaks: 1-Year
24
hr peak
SCS Method Design Storm
inflow (cfs)
25.994
at
12.00
(hrs)
discharge (cfs)
2.600
at
12.50
(hrs)
water level (ft)
597.821
at
12.54
(hrs)
storage (cy)
1996.820
Hydrograph 1
Routing Summary of
Peaks: 2-Year
24
hr peak
SCS Method Design Storm
inflow (cfs)
28.977
at
12.00
(hrs)
discharge (cfs)
8.610
at
12.32
(hrs)
water level (ft)
598.104
at
12.32
(hrs)
storage (cy)
2080.656
Hydrograph 2
Routing Summary of
Peaks: 100-Year
24 hr peak SCS Method Design Storm
inflow (cfs)
126.616
at
12.00
(hrs)
discharge (cfs)
125.170
at
12.02
(hrs)
water level (ft)
601.494
at
12.02
(hrs)
storage (cy)
3238.817
Tue Jun 16 09:53:11 EDT 2020
Page 4
Detention Basin C
BasinFlow printout
INPUT:
Basin: Detention Basin C
9 Contour Areas
Elevation(ft)
Area(sf)
Computed Vol.(cy)
550.00
2486.00
0.0
552.00
2926.00
200.2
554.00
3401.00
434.3
556.00
4200.00
715.3
558.00
5350.00
1068.2
560.00
6603.00
1510.1
562.00
7955.00
2048.5
564.00
9408.00
2690.8
566.00
11336.00
3458.0
Start—Elevation(ft)
4 Outlet Structures
Outlet structure 0
Orifice
557.00 Vol.(cy) 881.42
name: ASBUILT
Barrel
area (sf)
1.767
diameter or depth (in)
18.000
width for rect. (in)
0.000
coefficient
0.500
invert (ft)
555.910
multiple
1
discharge out of riser
Outlet structure 1
Weir
name:
ASBUILT Riser Top
diameter (in)
36.000
side angle
0.000
coefficient
3.300
invert (ft)
562.940
multiple
1
discharge
into riser
transition at (ft)
0.912
orifice coef.
0.500
orifice area (sf)
7.069
Page 1
Detention Basin C
Outlet structure 2
Weir
name: ASBUILT Emergency Spillway
length (ft)
75.000
side angle
75.960
coefficient
3.300
invert (ft)
564.000
multiple
1
discharge
through dam
Outlet structure 3
Orifice
name: APPROVED Low -Flow Orifice
area (sf) 0.049
diameter or depth (in) 3.000
width for rect. (in) 0.000
coefficient 0.600
invert (ft) 557.000
multiple 1
discharge into riser
3 Inflow Hydrographs
Hydrograph 0
SCS
name: 1-Year 24 hr peak SCS Method Design Storm
Area (acres)
21.590
CN
60.900
Type 2
rainfall, P (in)
3.500
time of conc. (hrs)
0.2700
time increment (hrs)
0.0200
time limit (hrs)
30.000
fudge factor
1.00
routed
true
peak flow (cfs)
12.101
peak time (hrs)
11.994
volume (cy)
1648.602
Page 2
Hydrograph 1
SCS
Detention Basin C
name: 2-Year 24 hr peak SCS Method Design Storm
Area (acres)
21.590
CN
60.900
Type 2
rainfall, P (in)
3.700
time of conc. (hrs)
0.2700
time increment (hrs)
0.0200
time limit (hrs)
30.000
fudge factor
1.00
routed
true
peak flow (cfs)
14.058
peak time (hrs)
11.994
volume (cy)
1915.279
Hydrograph 2
SCS
name: 100-Year 24 hr peak SCS Method Design Storm
Area (acres)
21.590
CN
60.900
Type 2
rainfall, P (in)
9.100
time of conc. (hrs)
0.2700
time increment (hrs)
0.0200
time limit (hrs)
30.000
fudge factor
1.00
routed
true
peak flow (cfs)
91.328
peak time (hrs)
11.994
volume (cy)
12442.232
Page 3
Detention Basin C
OUTPUT:
Routing Method: storage -indication
Hydrograph 0
Routing Summary of
Peaks: 1-Year
24
hr peak
SCS Method Design Storm
inflow (cfs)
12.074
at
12.00
(hrs)
discharge (cfs)
0.493
at
14.00
(hrs)
water level (ft)
561.471
at
14.02
(hrs)
storage (cy)
1896.143
Hydrograph 1
Routing Summary of
Peaks: 2-Year
24
hr peak
SCS Method Design Storm
inflow (cfs)
14.027
at
12.00
(hrs)
discharge (cfs)
0.528
at
14.08
(hrs)
water level (ft)
562.112
at
14.24
(hrs)
storage (cy)
2081.553
Hydrograph 2
Routing Summary of
Peaks: 100-Year
24 hr peak SCS Method Design Storm
inflow (cfs)
91.126
at
12.00
(hrs)
discharge (cfs)
86.832
at
12.02
(hrs)
water level (ft)
564.414
at
12.02
(hrs)
storage (cy)
2837.999
Tue Jun 16 13:39:58 EDT 2020
Page 4
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