HomeMy WebLinkAboutSDP201800054 Certified Engineer's Report 2018-08-28BROOKHILL INITIAL STORMWATER
MANAGEMENT CALCULATIONS PACKET
(Block 8B)
Date of Calculations
July 9, 2018
PREPARED BY:
COLLINS
200 GARRETT STREET, SUITE K
CHARLOTTESVILLE, VA 22902
434.293.3719 PH
434.293.2813 FX
www.collins-engineering.com
PROJECT SUMMARY
This project is part of the Brookhill Development, and stormwater treatment is provided to mitigate the impacts
from the proposed improvements. The stormwater management plan will comply with part 1113 SWM requirements.
WATER QUALITY SUMMARY
The stormwater quality requirements for this development are met through the use of a Level II wet pond. In
addition to treating the Level II wet pond, the facility treats the upland area where an existing home is located. This
existing home will be used as a feature and gathering place for the Brookhill Development.
WATER QUANTITY SUMMARY
The proposed development outfalls immediately upland of an existing channel. A proposed level II wet pond is
located in the south-east corner of Block 8B. Please note, the wet pond's contributing drainage area is less than 10
acres., however a water balance calculation was completed to ensure the pond is a viable design. This subarea,
which is less than 10 acres, is acceptable in accordance with VA DEQ Specification No. 14, section 6.2. This facility is
also designed to provide detention to mitigate the impacts from the proposed development. Channel and flood
protection requirements set forth in 9 VAC 25-870-66 will be met.
SCS TR-55 Calculations
Sods Mapping (Source: NRCS Web Soil Survey Online Database)
Soils Table (Source: NRCS Web Soil Survey Online Database)
Albemarle County, Virginia (VA003)
Albernarie County, Virginia
(VA003)
Map Unit
Map Unit Name
Percent of
Symbol
AOI
27B
Elloak loam, 2
6.90/0
to 7 percent
slopes
27C
Elloak loam, 7
32.90/c
to 15 percent
slopes
27D
Elloak loam,
11.40/c
15to25
percent slopes
39D
Hazel loam, 15
46.80Ic
to 25 percent
slopes
Totals for Area of
100.00J0
Interest
Note: Soils' properties witin this property 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
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POINT PRECIPITATION! FREQUENCY (PF) ESTIMATES
WITH 90"A CONFIDENCE INTERVALS AND SUPPLEMENTARY INFORMATION
NOAA Atlas 14, Volume 2, Version 3
PDS-based precipitation frequency estimates with 90% confidence intervals gin inches),
Average recurrence i nlerval(years)
Duration
100 �10D0
1 2 5 16 25 50 2D0 50D
���
24-hr
3.00
(2.69-3.38)
3.63
[3.28-4.09]
4.64
[4.15-5.21]
5.49
(4.89-6.15)
6.74
[5.97-7.53]
7.81
(637-3.71)
8.99
(7.85-10.D)
10.3
[8.89-11.4]
12.3
(10.4-13.fi]
13.9
{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: Brookhill Development Designed By: FGM, PE Date: 7/9/2018
Location: Block 8B Checked By: SRC, PE Date: 7/9/2018
Check One: Present X Developed
1. Runoff curve Number (CN)
Soil name and
Cover description
CN (weighted) _
Drainage Area
hydrologic group
(Cover type, treatment, and hydrologic condition; percent
CN
Area
Product of CN
total product/
Calculated
Description
(Appendix A)
impervious; unconnected/ connected impervious area ratio)
(Acres)
xArea
total area
S'Value
DA A & B
Impervious Areas
98
0.54
52.6
(Present)
B
Woods in Good Condition
55
4.37
240.4
60.3
6.58
Lawns in Good Condition (75%+Groundcover)
61
4.25
259.4
DA A
Impervious Areas
98
3.29
322.0
(Developed)
B
Woods in Good Condition
55
1.20
66.0
76.2
3.12
Lawns in Good Condition (75%+Groundcover)
61
3.03
185.1
Impervious Areas
98
0.58
56.7
DA B
B
Woods in Good Condition
55
0.00
0.0
83.5
1.97
(Developed)
Lawns in Good Condition (75%+Groundcover)
61
0.37
22.6
2. Runoff
1-Year Storm
I 2-Year Storm
10-Year Storm
Drainage Area Description
Frequency -years
1
2
10
n/a
Rainfall, P (24 hour)- inches
3.00
3.63
5.49
n/a
Runoff, Q-inches
0.34
0.60
1.62
DA A & B(Present)
Runoff, Q- inches
1.03
1.47
2.96
DA A (Developed)
Runoff, Q- inches
1.48
2.01
3.68
DA B (Developed)
U.S. Department of Agriculture
Natural Resources Conservation Service
TR 55 Worksheet 3: Time of Concentration (TJ or Travel Time (Tt)
Project: Brookhill Development
Location: Block 8B
Check One: Present X Developed X
Check One: Tc X Tt
Segment ID
Sheet Flow: (Applicable to Tc only)
Designed By: FGM, PE
Checked By: SRC, PE
Through subarea n/a
FL-ENG-21A
06/04
Date: 7/9/2018
Date: 7/9/2018
DAA&B
DAA
DAB
(Present)
I (Developed)
(Developed)
1 Surface description (Table 3-1)
Dense Grass
Dense Grass
Dense Grass
2 Manning's roughness coeff., n (Table 3-1
0.24
0.24
0.24
3 Flow length, L (total L < 100) (ft)
100
100
65
4 Two-year 24-hour rainfall, Pz (in.)
3.6
3.6
3.6
5 Land slope, s (ft/ft)
0.05
0.05
0.09
6 Compute Tt = [0.007(n*L)08] / Pz .5 s04
0.15
0.15
0.09
Shallow Concentrated Flow:
7 Surface description (paved or unpaved)
Unpaved
Unpaved
Unpaved
8 Flow Length, L (ft)
170
170
170
9 Watercourse slope, s (ft/ft)
0.112
0.141
0.141
10 Average velocity, V (Figure 3-1) (ft/s)
5.5
6.0
6.0
11 Tt = L / 3600*V
0.01
0.01
0.01
Channel Flow:
12 Cross sectional flow area, a (ft)
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= [ 1.49r2/3s05 ] / n
18 Flow length, L (ft)
19 Tt= L / 3600*V
20 Watershed or subarea Tc or Tt
(Add Tt in steps 6, 11 and 19)
t
3
3
u
o
0
Q N
w
d
w
ai
v
a \
a \
C o
c
C6 w
o2S
U 3
w
U
U
o
0
0
Ln
in
N
0
.D
�
Q1
M
0.02
0.02
0.01
0.18 1 0.18 1 0.11
U.S. Department of Agriculture
Natural Resources Conservation Service
TR 55 Worksheet 4: Graphical Peak Discharge Method
Project: Brookhill Development Designed By: FGM, PE
Location: Block 8B Checked By: SRC, PE
Check One: Present X Developed X
FL-ENG-21C
06/04
Date: 7/9/2018
Date: 7/9/2018
1. Data
Drainage Area Description
Drainage Area Description
Drainage Area Description
Drainage Area
Description
DAA&B
(Present)
DAA
(Developed)
DAB
(Developed)
Drainage Area (Am) in milesZ =
0.0143
0.0118
0.0015
Runoff curve number CN=
60.3
76.2
83.5
Time of concentration (Tc)=
0.18
0.18
0.11
Rainfall distribution type=
II
II
II
Pond and swamp areas spread
throughout the watershed=
0
0
0
2. Frequency -years
1
2
10
1
2 10
1
2
10
3. Rainfall, P (24 hour)- inches
3.00
3.63
5.49
3.00
3.63 5.49
3.00
3.63
5.49
4. Initial Abstraction, la- inches
1.32
1.32
1.32
0.62
0.62 0.62
0.39
0.39
0.39
5. Compute la/P
0.44
0.36
0.24
0.21
0.17 0.11
0.13
0.11
0.07
6. Unit peak discharge, Qu- csm/in
550
675
775
800
825 850
950
975
975
7. Runoff, Qfrom Worksheet 2- inches
0.34
0.60
1.62
1.03
1.47 2.96
1.48
2.01
3.68
8. Pond and Swamp adjustment factor, Fp
1 1
1
1
1 1
1 1
1 1
1
1
9. Peak Discharge, Qp- cfs
where Qp=Qu Am Q Fp
1 2.70
5.81
17.96
I To be Routed in the SWM
Facility w/ WPO
1 2.09
2.91
5.32
WATERSHED SUMMARY
CN Area, ac. 1-year Flow, cfs 2-year Flow, cfs 10-year Flow, cfs
DA A & B (Present) 60.3 9.16 2.70 5.81 17.96
DA A (Developed) 76.2 7.52 (To be Routed in the SWM Facility w/ WPO)
DA B )Developed) 83.5 0.95 2.09 2.91 5.32
WET POND'A' DESIGN & REQUIREMENTS, AS SHOWN
Treatment Volume, Tv (VRRM) 14,417 cf
Minimum Treatment Volume Required for Level II Basin 21,626 cf
Treatment Volume Provided at MWSE
17,219 cf
Minimum Percentage of Tv Provided at MWSE
50 %
Percentage of Tv Provided at MWSE
80 %
Ponding Depth Corresponding to Minimum Tv Required
ft
Maximum Ponding Depth allowed for Minimum Tv
4 ft
Maximum Percentage of Tv Provided above MWSE
50 %
Percentage of Tv Provided above MWSE
20 %
Minimum Forebay Volume Required (15%of Tv)
3,244 cf
Forebay Volume Provided
5,073 cf
Minimum Upper Wetland Shelf Area (10%of Pond Area)
750 sf
Upper Wetland Shelf Provided
837 sf
STORMWATER MANAGEMENT FACILITY GRADING
Elevation
(ft)
Area of Forebay
Cell Contours (sf)
Volume (cf)
366
73
0
368
438
460
370
1,103
1,951
371
1,549
3,270
372
2,069
5,073
374
-
-
376
378
Elevation
(ft)
Total Facility Area
Contours (sf)
Volume (cf)
366
579
0
368
1,610
2,103
370
3,467
7,063
371
4,744
11,152
372
7,495
17,219
374
9,977
34,632
376
12,456
57,019
378
15,161
84,592
Elevation
(ft)
Area of Wetland
Cell Contours (sf)
Volume
(cf)
366
368
-
-
370
249
0
371
504
369
372
837
1,033
374
-
-
376
378
Elevation
(ft)
Area of Main Cell
Contours (sf)
Volume
(cf)
366
506
0
368
1,172
1,632
370
2,115
4,873
371
2,691
7,27C
372
4,589
10,868
374
-
-
376
378
Notes:
1 A minimum forebay volume depth of 4' with an aquatic bench 4' wide, 2' deep has been provided.
2 A minimum forebay volume of 15%of the total Tv has been provided.
3 A wetland maximum treatment volume depth of 12" has been provided.
4 A 10' wide aquatic bench ranging from 12"-18" in depth has been provided.
5 The minimum treatment volume required has been provided.
Virginia Runoff Reduction Method Water Quality
Calculations
DEQ Virginia Runoff Reduction Method New Development Compliance Spreadsheet - Version 3.0
El 2011 BMP Standards and Specifications 112013 Draft BMP Standards and Specifications
Project Na.:: BrookhilI Development -Block 8B
Date: 7/9/2018
BMP Design Specifications List: 2011 Stds & Specs
Site Information
(Post -Development Project (Treatment Volume and Loads)
Land Cover (acres)
A Soils I BSoils I CSoils DSoils Totals
-®--
dh
yards or other turf to be mowed/managed
-®--
Constants
Annual Ralnfall(inches)
43
Target Rainfall Event(inches)
1.00
Total Phosphoms(TPI EMC(mg/L)
0.26
Total Nitrogen (TN) EMC(mg/L)
1.86
Target TP Load (Ib/acre/yr)
0.41
Pj(unitless correction factor)
0.90
Land Cover Summary
Forest/Open Space Cover (acres)
0.00
Weighted Rv (forest)
0.00
%Forest
0%
Managed Turf Cover (acres)
3.80
Weighted Rv(turf)
0.20
%Managed Turf
51%
Impervious Cover (acres)
3.61
Rv (impervious)
0.95
%Impervious
49%
Site Area (acres)
7A1
Site Rv
0.S7
Runoff Coefficients (Rv)
A Soils BSoils CSoils DSoils
Forest/Open Space 0.02 0.03 0.04 0.05
Managed Turt 1 0.15 0.20 0.22 1 0.25
Impervious Cover 1 0.95 0.95 0.95 1 0.95
Treatment Volume and Nutrient Loads
Treatment Volume
(acre-k) 0.3491
Treatment Volume (cubic feet) 15,208
TP bad (lb/yr) 9.56
TN bad (Ib/yr)
final results
Drainage Area A
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.TOTAIIM I c== ,, F2. ]A OK.
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ximooex.emoveo 1., 'IT .10, IT 1. 1.