HomeMy WebLinkAboutSDP201800052 Calculations 2018-07-10BROOKHILL INITIAL STORMWATER
MANAGEMENT CALCULATIONS PACKET
(Blocks 9-11)
Date of Calculations
July 9, 2018
PREPARED BY:
COLLINS " NGINErgING
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 IIB SWM requirements.
WATER QUALITY SUMMARY
The stormwater quality requirements for this development are met through three best management practices. The
best management practice of installing a centralized level II wet pond will be implemented. This BMP will treat the
bulk of the proposed residential development. Downstream from the development the plan proposes the best
management practice of preserving open spaces. A recorded easement will be provided protecting these areas
prior to the issuance of a land disturbance permit for this project. The remaining water quality requirements will be
met with the best management practice of purchasing nutrient credits.
WATER QUANTITY SUMMARY
The proposed development outfalls in multiple locations immediately upland of various existing channels. Where
road / infrastructure extensions are proposed leading to the new residential development, adequate outfalls are
provided with subareas less than 1% of the receiving channels' watersheds. Additionally, where the bulk of the
development occurs a proposed wet pond will be installed capturing the vast majority of the new residential area.
This facility will provide detention to mitigate the impacts from the majority of the proposed development. Channel
and flood protection requirements set forth in 9 VAC 25-870-66 will be met with the final VSMP plan.
SCS TR-55 Calculations
Soils Mapping (Source: NRCS Web Soil Survey Online Database)
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Soils Table (Source: NRCS Web Soil Survey Online Database)
Albemarle County, Virginia (VA003)
Albemarle County, Virginia (VA003)
Map Unit
Map Unit Name
Percent of
symbol
AOI
27B
Elioak loam, 2
22.10l0
to 7 percent
slopes
27C
Elioak loam, 7
13.1%
to 15 percent
slopes
34C
Glenelg loam, 7
6.4%
to 15 percent
slopes
34D
Glenelg loam,
6.811l0
15to25
percent slopes
39D
Hazel loam, 15
46.60l0
to 25 percent
slopes
95
Wehadkee silt
3.00l0
loam
Totals for Area of
1OO.00/O
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)
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POINT PRECIPITATION FREQUENCY (PF) ESTIMATES
WITH W/o CONFIDENCE INTERVALS AND SUPPLEMENTARY INFORMATION
NOAA Atlas 14, V011llne 2, Version 3
PDS-based precipitation frequency estimates with 90% confidence intervals (in inches)'
Duration
Average recurrence inWrval (years)
50
1 2 5 1D 25 100 2D0 50D 10D0
�����
3.00
3.62
4.63
5.47
6.72
7.79
8.97
10.3
12.2
13.9
24-hr
(2.69-3.37)
(3.25-4.08)
(4.14-5.20)
(4.88-6.14)
(5.95-7.51)
(6.36A.69)
(7.83-9.99)
[8.87-11.4)
{1D.4-13.6)
(71.6-15.4)
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: Blocks 9-11 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
Impervious Areas
98
0.00
0.0
(Present)
B
Woods in Good Condition
55
10.55
580.3
55.0
8.18
Lawns in Good Condition (75%+Groundcover)
61
0.00
0.0
DA A
Impervious Areas
98
6.22
609.3
(Developed)
B
Woods in Good Condition
55
0.14
7.7
82.7
2.09
Lawns in Good Condition (75%+Groundcover)
61
4.19
255.8
2. Runoff
1-Year Storm
I 2-Year Storm
I 10-Year Storm
JDrainage Area Description
Frequency -years
1
2
10
n/a
Rainfall, P (24 hour)- inches
3.00
3.62
5.47
n/a
Runoff, Q- inches
0.19
0.39
1.22
DA A (Present)
Runoff, Q- inches
1.43
1.94
3.57
DA A (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: Blocks 9-11
Check One: Present X Developed X
Check One: Tc X Tt
Segment ID
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, Pz (in.)
5 Land slope, s (ft/ft)
6 Compute Tt = [0.007(n*L)0.8] / Pz .s so.a
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
DAA
(Present)
(Developed)
Woods- Light
Woods- Light
Underbrush
Underbrush
0.4
0.4
100
100
3.62
3.62
0.03
0.03
0.29
0.29
Shallow Concentrated Flow:
7 Surface description (paved or unpaved)
Unpaved
Unpaved
8 Flow Length, L (ft)
330
90
9 Watercourse slope, s (ft/ft)
0.167
0.233
10 Average velocity, V (Figure 3-1) (ft/s)
7.6
7.8
11 Tt = L / 3600*V
0.01
0.00
Channel Flow:
12 Cross sectional flow area, a (ft)
13 Wetted perimeter, P, (ft)
14 Hydraulic radius, r = a/P, (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)
0
3
0
v
o
a
t -1 0
06
U @) ,
-0
Ln
0 0
o 0-
U
@)
o 0
-
0
N v
zn
r`
i+
v7
N
0.06
0.04
0.36 1 0.33
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: Blocks 9-11 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
DA A
(Present)
DA A
(Developed)
Drainage Area (Am) in mileS2 =
0.0165
0.0165
Runoff curve number CN=
55.0
82.7
Time of concentration (Tc)=
0.36
0.33
Rainfall distribution type=
II
II
Pond and swamp areas spread
throughout the watershed=
0
0
2. Frequency -years
1
2
10
1
2
10
3. Rainfall, P (24 hour)- inches
3.00
3.62
5.47
3.00
3.62
5.47
4. Initial Abstraction, la- inches
1.64
1.64
1.64
0.42
0.42
0.42
5. Compute la/P
0.55
0.45
0.30
0.14
0.12
0.08
6. Unit peak discharge, Qu- csm/in
230
375
525
625
625
650
7. Runoff, Qfrom Worksheet 2- inches
0.19
0.39
1.22
1.43
1.94
3.57
8. Pond and Swamp adjustment factor, Fp
1
1
1
1 1
1
1
9. Peak Discharge, Qp- cfs
where Qp=Qu Am Q Fp
0.74
2.39
10.59
I To be Routed in the SWM
Facility w/ WPO
WATERSHED SUMMARY
CN Area, ac. 1-year Flow, cfs 2-year Flow, cfs 10-year Flow, cfs
DA A (Present) 55.0 10.55 0.74 2.39 10.59
DA A (Developed) 82.7 10.55 (To be Routed in the SWM Facilitv w/ WPO)
WET POND'A' DESIGN & REQUIREMENTS, AS SHOWN
Treatment Volume, Tv (VRRM) 24,535 cf
Minimum Treatment Volume Required for Level II Basin 36,803 cf
Treatment Volume Provided at MWSE
23,288 cf
Minimum Percentage of Tv Provided at MWSE
50 %
Percentage of Tv Provided at MWSE
63 %
Ponding Depth Corresponding to Minimum Tv Required
ft
Maximum Fondling Depth allowed for Minimum Tv
4 ft
Maximum Percentage of Tv Provided above MWSE
50 %
Percentage of Tv Provided above MWSE
37 %
Minimum Forebay Volume Required (15%of Tv)
5,520 cf
Forebay Volume Provided
5,868 cf
Minimum Upper Wetland Shelf Area (10%of Pond Area)
1,030 sf
Upper Wetland Shelf Provided
1,488 sf
STORMWATER MANAGEMENT FACILITY GRADING
Elevation
(ft)
Area of Forebay
Cell Contours (sf)
Volume (cf)
372
84
0
374
540
558
376
1,282
2,327
377
1,759
3,842
378
2,306
5,868
380
-
-
382
384
Elevation
(ft)
Total Facility Area
Contours (sf)
Volume (cf)
372
614
0
374
2,019
2,498
376
4,895
9,203
377
6,578
14,919
378
10,300
23,288
380
13,435
46,954
382
16,527
76,863
384
19,848
113,187
Elevation
(ft)
Area of Wetland
Cell Contours (sf)
Volume
(cf)
372
374
-
-
376
700
0
377
1,059
873
378
1,488
2,141
380
-
-
382
384
Elevation
(ft)
Area of Main Cell
Contours (sf)
Volume
(cf)
372
530
0
374
1,479
1,930
376
2,913
6,241
377
3,760
9,569
378
6,506
14,640
380
-
-
382
384
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
® 2011 BMP Standards and Specifications P 2013 Draft IMP Standards and Specifications
Project Name: BrookhiR Development- Blocks 9-11
Date: 7/9/2018
BMP Design Specifications List: 2011 Stds & Specs
Site Information
Post-Develooment Proiect (Treatment Volume and Loads)
Land Cover (acres)
A Soils
BSolis
CSoils
DSoils
Totals
Fomst/Open Space (acres) — undisturbed,
protected forest/open space or reforested land
12.71
12.]1
Managed Turf (acres) — disturbed, graded for
12.52
ards or other turf to be owed/managed
12.52
Impervious Cover (acres)
10.11
10.11
Forest/Open Space ace as most be protected
in oc ..donce with
the Virginia RuneNReduction
Method
35.34
Constants
Annual Ralnfall(inches) 43
Target Rainfall Event linches) 1.00
Total Phosphorus(TP) EMC(mg/L) 0.26
Total Nitrogen(TN) EMC(mg/L) 1.86
Target TP Load (lb/acre/yr) 0.41
PJ(unitless correction factor) 0.90
Land Cover Summary
Forest/Open Space Cover (acres)
12.71
Weighted RV (forest)
0.03
%Forest
36%
Managed Turf Cover (acres)
12.52
Weighted Rv(turf)
0.20
%Managed Turf
35%
Impervious Cover (acres)
10.11
RV (impervious)
0.95
%Impervious
29%
Site Area (acres)
35.34
Site RV
0.35
Runoff Coefficients (Rv)
Treatment Volume and Nutrient Loads
Treatment Volume
1.0408
(acre-h)
Treatment Volume (cubic feet)
45,338
TP Load (lb/yr)
28.49
TN Load (Ib/yr)
final results
Drainage Area A
D.I..A—
vastorn .1 --.1 1.— m — A Wl
IAIII 11, ��l AIIA I"ICI 11
Toro auxorr arournox ix A
' DI bll
:b D �A hL::Z�
o
mom
—1 IIDIIII" I" D.1 A (a'j
aRvoDra —.— -1. D—I.. I—C. 1. D.1 A vwrl L— 0
AVIN DA. A (lNhIj