HomeMy WebLinkAboutSW3190301_Apprentice Academy - SWM Report_20190513STORMWATER MANAGEMENT CALCULATIONS
FOR THE
APPRENTICE ACADEMY HIGH SCHOOL
LOCATED IN
UNION COUNTY, NORTH CAROLINA
BCCG
March 2019
Revised (April 29, 2019)
Revised (May, 13, 2019)
Comprehensive-Stormwater Pollution Prevention Plan (C-S WPPP)
Apprentice Academy High School
TABLE OF CONTENTS
1.0
Project Overview.............................................................................................................4
1.1
Narrative.....................................................................................................................................................4
1.2
Stormwater Management and Sediment Control........................................................................................6
1.3
Sequence of Construction.........................................................................................................................11
1.4
Non -Numeric Effluent Limits..................................................................................................................12
1.5
Management of Non-Stormwater Discharge............................................................................................12
1.6
Buffer Zone Management.........................................................................................................................12
1.7
Certification Statement.............................................................................................................................13
2.0
Site Features and Sensitive Areas.................................................................................15
2.1
Sources of Pollution.................................................................................................................................15
2.2
Surface Waters..........................................................................................................................................16
3.0
Compliance Requirements............................................................................................16
3.1
SWPPP Availability.................................................................................................................................16
3.2
Pre -Construction Conference...................................................................................................................16
3.3
Inspection Requirements..........................................................................................................................17
3.4
Maintenance Policies................................................................................................................................17
3.5
Record Keeping........................................................................................................................................18
3.6
Final Stabilization.....................................................................................................................................18
EXHIBITS
Exhibit A Erosion and Sediment Control Plans.............................Error! Bookmark not defined.
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Comprehensive-Stormwater Pollution Prevention Plan (C-S WPPP)
Apprentice Academy High School
LIST OF TABLES
Table 1- Post Developed Sand Filter Peak Discharges and Storage Elevations................................6
Table 2 - Locations of Potential Sources of Stormwater Contaminations.................................................10
Table 6 - Potential Construction Site Stormwater Pollutants....................................................................15
APPENDICES
APPENDIXA Site Maps.......................................................................................................................19
SiteLocation Map......................................................................................................................................20
AerialMap.......................................................................................................................................21
TopographicMap.......................................................................................................................................22
SoilsMap.......................................................................................................................................23
FEMAFlood Map......................................................................................................................................24
National Wetlands Inventory (NWI) Map.................................................................................................25
APPENDIX B Storm Pipe, Sediment Basin, Diversion Ditch Drainage Area Maps ............................26
APPENDIX C Rainfall Data..................................................................................................................27
APPENDIX D Hydraulic Engineering Report .......................................................................................28
APPENDIX E Erosion Control Calculations........................................................................................29
APPENDIX F Sand Filter Calculations................................................................................................30
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March 2019 (Revised May 13, 2019)
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CON5VLTING ENGINEERS
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Comprehensive-Stormwater Pollution Prevention Plan (C-S WPPP)
Apprentice Academy High School
1.0 Proiect Overview
1.1 Narrative
Apprentice Academy High School is proposing a new school, which consists of a new
approximately 32,513 SF, 2-story classroom building located at 2505 Weddington Road (NC-84)
in unincorporated Union County, North Carolina. The property (TMS # 09316009C) is located
approximately at the southeast corner of the intersection of Weddington Road (NC-84) and Clark
Road as illustrated in the Site Location Map in Appendix A. The proposed development will
include approximately 16.90 acres of land disturbance on the 30.32-acre site.
The Mecklenburg County Digital Ortho Quarter Quadrangle (DOQQ) imagery provided
by the United States Depart of Agriculture Farm Service Agency (USDA-FSA) (See Aerial Map
in Appendix A) was utilized to provide a general overview of the site and its surroundings. The
United States Geological Survey's (USGS) 7.5 Minute Union County Quad Map (See Topographic
Map in Appendix A) was utilized to provide a general overview of the site and surrounding area
drainage patterns. Utilizing a topographic survey conducted by R.B. Pharr & Associates, P.A.
dated January 9, 2019, it appears that the stormwater runoff drains generally south -to -north. Most
of the stormwater runs into a stream along the western side of the property. The remainder of the
on -site stormwater runoff drains to the northeast. Based on a review of the North Carolina Soil
Survey for Union County, it is apparent Badin channery silty clay loam (BdC2) (HSG C), Cid
channery silt loam (CmB) (HSG D), and Tarrus gravelly silty clay loam (TbB2) (HSG B) soils
comprise the project site (See Soils Map in Appendix A). The Federal Emergency Management
Agency (FEMA) Flood Map Panels 3710542500J (See FEMA Flood Map in Appendix A)
identifies that the project site is located outside of any floodzone, which indicates the site is in an
area of minimal flooding and is outside of the 100-year Flood Plain. Based on the provided base
flood elevations shown on the FEMA Map, our project site should not disturb this flood zone area.
The National Wetlands Inventory (NWI) Map provided by the US Fish and Wildlife Service (See
NWI Map in Appendix A), indicates there is an approximately 1.7-acre wetland in the northwest
corner of the site. However, Wetland Delineation performed by ECS Southeast, LLC indicates
potential wetlands exist along the bottom of the existing drainage feature draining the site. These
wetlands have been delineated and are shown on the Construction Plans.
The project will consist of the construction of the approximately 32,513 SF, 2-story
building, one (1) roadway loop, parking lot, and associated facilities. The post development
impervious coverage of the lot will only be 9.5% (14.02% for future expected expansion), below
the 24% maximum coverage limit for low density development, however, the parking lot and
drives located in front of the school building will create a point discharge so a sand filter is
proposed to treat run-off from the discharge of the storm drain system. Areas upstream of the storm
drain system are still designed to sheet flow into a vegetative swale before draining into the storm
drain system which is consistent with low density development. The remaining developed areas of
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Comprehensive-Stormwater Pollution Prevention Plan (C-SWPPP)
Apprentice Academy High School
the site are proposed to sheet flow into lawn and wooded areas which is also consistent with low
density development. All earth moving activities will tie into existing grade and will maintain the
existing flow patterns of stormwater on -site.
The remaining portion of the site will be left wooded except portions necessary for the
erosion control measures during construction and installation of utilities and facilities. The
following are the calculations for the proposed Sand Filter.
Sand Filter
Drainage Area: 3.96 Acres
Design Volume Required: 3.96 Acres x 43,560 sf / acre x (1 in / 12in/ft) = 14,375 cf
Discounted Design Volume: 14,375 cf x 0.75 = 10,782 cf
Design Volume Provided: 16,405 cf at elevation 639.5
Surface Area Required: 10,782 cf / 2'ponding depth = 5,391 sf
Sand Chamber Surface Area Required: 2,696 sf
Surface Area Provided: 6,640 SF at elevation 637.5
Sand Chamber Surface Area Provided: 2,777 sf at elevation 637.5
Sediment Chamber Surface Area Provided: 3,863 sf at elevation 637.5
The sand filter is designed with a 12" sand filter media depth with a 6" underdrain encased
in stone underneath that. The bottom of the detention is proposed at 637.5 with a sediment chamber
provided to elevation 639.5 with a volume of 9,538 and a sand chamber to elevation 639.5 and
volume of 6,867 as shown on the attached pond reports contained in the Sand Filter Appendix.
The area draining to the sand filter is 3.96 acres containing 2.16 acres of impervious coverage (CN
= 61) which includes 0.47 acres of future parking and 1.80 acres of grass, good condition (CN =
61), see attached drainage map (Appendix B). Resulting in a composite CN of 81. The area
draining to the sand filter contains soils with Hydrologic Soil Group B. The assumed time of
concentration is assumed the minimum of 6 minutes.
For the modeling of the system the media filtration rate is conservatively assumed 2 in/hr.
This will result in a flow rate input in the attached Hydraflow Model (Appendix F) of:
2 in/hr x 1 ft/12in x 3,600 s/hr x 2,777 sf (sand chamber bottom area) = 0.129 cfs
For comparison to pre -development the same area (3.96 acres) is calculated as woods, good
condition (CN = 55) with a time of concentration calculated to be 18.7 minutes (see attached
drainage map - Appendix B). The following are the peak flow rates to the sand filter, routed thru
the sand filter with storage elevations and the predevelopment rates calculated utilizing Autodesk
® Hydraflow Hydrographs. As shown below the sand filter will attenuated the 2-yr and 10-yr
storm events to pre -development peak flow rates and pass the 100-yr storm event thru the
emergency spillway with 0.70' of freeboard.
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14
Hydrograph Report
Hydraflow Hydrographs Extension for Autodesk® Civil 3D® 2019 by Autodesk, Inc. v2020
Monday, 05 / 13 / 2019
Hyd. No. 1
To Sand Filter
Hydrograph type
= SCS Runoff
Peak discharge
= 36.90 cfs
Storm frequency
= 100 yrs
Time to peak
= 716 min
Time interval
= 2 min
Hyd. volume
= 77,512 cuft
Drainage area
= 3.960 ac
Curve number
= 81 *
Basin Slope
= 0.0 %
Hydraulic length
= 0 ft
Tc method
= User
Time of conc. (Tc)
= 6.00 min
Total precip.
= 8.01 in
Distribution
= Type II
Storm duration
= 24 hrs
Shape factor
= 484
Composite (Area/CN) = [(2.160 x 98) + (1.800 x 61)] / 3.960
Q (cfs)
40.00
30.00
20.00
10.00
0.00
0 120 240
— Hyd No. 1
To Sand Filter
Hyd. No. 1 -- 100 Year
360 480 600 720 840 960
Q (cfs)
40.00
30.00
20.00
10.00
0.00
1080 1200 1320
Time (min)
Hydrograph
Report
15
Hydraflow Hydrographs Extension for Autodesk® Civil 3D® 2019 by Autodesk,
Inc. v2020
Monday, 05 / 13 / 2019
Hyd. No. 2
Routed thru Sand Filte
Hydrograph type
= Reservoir
Peak discharge
= 34.27 cfs
Storm frequency
= 100 yrs
Time to peak
= 718 min
Time interval
= 2 min
Hyd. volume
= 77,509 cuft
Inflow hyd. No.
= 1 - To Sand Filter
Max. Elevation
= 640.30 ft
Reservoir name
= Sand Filter
Max. Storage
= 24,994 cuft
Storage Indication method used
Q (cfs)
40.00
30.00
20.00
10.00
Routed thru Sand Filte
Hyd. No. 2 -- 100 Year
0.00 1
0 120 240 360 480 600 720 840 960 1080 1200 1320
— Hyd No. 2 Hyd No. 1 Total storage used = 24,994 cuft
Q (cfs)
40.00
30.00
20.00
10.00
— 0.00
1440
Time (min)
16
Hydrograph Report
Hydraflow Hydrographs Extension for Autodesk® Civil 3D® 2019 by Autodesk, Inc. v2020
Monday, 05 / 13 / 2019
Hyd. No. 4
Pre -Development
Hydrograph type
= SCS Runoff
Peak discharge
= 12.33 cfs
Storm frequency
= 100 yrs
Time to peak
= 726 min
Time interval
= 2 min
Hyd. volume
= 40,119 cuft
Drainage area
= 3.960 ac
Curve number
= 55
Basin Slope
= 0.0 %
Hydraulic length
= 0 ft
Tc method
= TR55
Time of conc. (Tc)
= 18.70 min
Total precip.
= 8.01 in
Distribution
= Type II
Storm duration
= 24 hrs
Shape factor
= 484
Q (cfs)
14.00
12.00
10.00
MIX
N. We
4.00
2.00
Pre -Development
Hyd. No. 4 -- 100 Year
Q (cfs)
14.00
12.00
10.00
01
4.00
2.00
0.00
120 240 360 480 600 720 840 960 1080 1200 1320 1440 1560
Hyd No. 4 Time (min)
Hydraflow Rainfall Report
17
Hydraflow Hydrographs Extension for Autodesk0 Civil 3DO 2019 by Autodesk, Inc. v2020
Return
Period
Intensity -Duration -Frequency Equation Coefficients (FHA)
(Yrs)
B
D
E
(N/A)
1
0.0000
0.0000
0.0000
--------
2
54.6434
10.3000
0.8092
--------
3
0.0000
0.0000
0.0000
--------
5
52.5793
10.3000
0.7554
--------
10
53.4352
10.3000
0.7315
--------
25
57.1938
10.5000
0.7117
--------
50
60.4231
10.6000
0.7000
--------
100
63.8015
10.7000
0.6906
--------
File name: OF - Union Co. NC.IDF
Intensity = B / (Tc + D)^E
Monday, 05 / 13 / 2019
Return
Period
Intensity Values (in/hr)
(Yrs)
6 min
10
16
20
26
30
36
40
46
60
66
60
1
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
2
6.01
4.78
4.00
3.46
3.06
2.75
2.50
2.29
2.13
1.98
1.86
1.75
3
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
5
6.70
5.41
4.58
4.00
3.56
3.22
2.95
2.73
2.54
2.38
2.24
2.12
10
7.26
5.91
5.03
4.41
3.94
3.58
3.28
3.04
2.84
2.66
2.51
2.38
25
8.13
6.67
5.71
5.02
4.51
4.11
3.78
3.51
3.28
3.09
2.92
2.77
50
8.83
7.27
6.24
5.51
4.96
4.52
4.17
3.88
3.63
3.42
3.23
3.07
100
9.53
7.87
6.78
6.00
5.40
4.93
4.56
4.24
3.97
3.74
3.55
3.37
Tc = time in minutes. Values may exceed 60.
9-1090 New Apprentice Academv Hiah School Weddinaton Rd Union Co NC\Enaineerina Calculations\Monroe-NC.DCD
Storm
Rainfall Precipitation Table (in)
Distribution
1-yr
2-yr
3-yr
6-yr
10-yr
26-yr
60-yr
100-yr
SCS 24-hour
2.99
3.61
0.00
4.54
5.28
6.31
7.14
8.01
SCS 6-Hr
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
Huff-1 st
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
Huff-2nd
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
Huff-3rd
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
Huff-4th
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
Huff-Indy
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
Custom
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00