HomeMy WebLinkAboutSW6130501_HISTORICAL FILE_20130522STORMWATER DIVISION CODING SHEET
POST -CONSTRUCTION PERMITS
PERMIT NO.
SW Cphj
DOC TYPE
❑ CURRENT PERMIT
❑ APPROVED PLANS
IUD HISTORICAL FILE
C(❑,/ COMPLIANCE EVALUATION INSPECTION
DOC DATE
YYYYM M DD
Dewberry
Dewberry Engineers Inc.
2301 Rexwoods Drive, Suite 200
Raleigh, NC 27607-3366
919.881.9939
919.881.9923 fax
www.dewberry.com
May 22, 2013
Mr. Mike Randall
Stormwater Staff Engineer
NCDENR Division of Water Quality
512 N. Salisbury Street
Raleigh, North Carolina 27604
Tel.: (919) 807-6374
Reference: Conferencing and Catering Center
Fort Bragg, NC
SWM Submittal (Standard Review)
Dear Mr. Randall:
Please see attached the Stormwater Management Review package for your review. We have included the
following:
• 1 Stormwater Management Permit Application, signed
• 1 Bioretention Operation and Maintenance Agreement, signed
• 1 Dry Extended Detention Basin Operation and Maintenance Agreement, signed
• 1 Filter Strip, Riparian Buffer and Level Spreader Operation and Maintenance Agreement, signed
• 2 Sets of Design Drawings
• 2 Sets of Stormwater Management Reports
• 1 Check for Standard Review ($505.00)
Should you have any questions or need any additional information, please feel free to contact me via
phone (919) 424-3740 or email (fvnorton@dewberty.com).
Sincerely,
Dewberry Engineers Inc.
F. Virginia Norton, PE, LEED AP
Project Manager
P:\50057270\1)eliverables\Submittals\2013.05.09 - Permit Submittal (Slonmvater)\Documents
1. Stormwater Management Narrative
1.1. Project Description
The Government plans to construct a Conferencing and Catering Center off Reilly Road at Fort Bragg,
NC. The building is approximately 50,000 SF and will be built to support the growing requirements
for meetings, conferences, social events, and recreational needs at Fort Bragg. The building will be
constructed on a previously developed site bound by Reilly Road to the west, and woods on the
remaining sides.
1.2. Site Description
New work for this project will include construction of an approximately 50,000 SF building with
associated site work including parking area, sidewalk, access drives, new utilities and new stormwater
BMPs. New work will disturb approximately 10.5 acres.
1.3. Soils Characteristics
According to the USDA Web Soil Survey, the soils in the area are classified as WgB (Wagram-Urban
land complex, o to 8 percent slopes). See Appendix C for the USDA Web Soil Survey.
1.4. Stormwater Management
The project area has a total built upon area of 6o%. The site has been split up into three designated
drainage areas. The Upper Basin (1), the Lower Basin (2), and offsite flow (3). The offsite flow basin
includes area that could not be captured in the BMPs, based on the topography of the site, and the
overall layout of parking and building footprint. Every effort has been made to ensure the post
development conditions in terms of stornwater runoff are improved from the existing conditions of
the site.
The upper basin (1) is being routed to two bioretention cells that are placed within a parking island.
The bioretention cells have a pretreatment device that includes 8" of gravel and 3' of sod. The
bioretention area is designed to remove 85% of TSS. The cells have been sized with the ultimate build
out conditions in mind. Major design elements include:
The bottom of the bioretention cell is separated by 2' from the recorded seasonal high
water table
• The cell is located at least 30 feet from surface waters and 50 feet from class SA waters
• Ponding depth is 12 inches
• Soil media in cell is specified to be homogeneous soil mix of 85-88 percent by volume
sand, 8-12 percent fines, and 3-5 percent organic matter. The P-Index shall be between
to and 3o and permeability between 1 to 2 inches per hour
• 4 inch diameter perforated underdrains with sock have been provided with non
perforated standpipe and cleanout
The lower basin (2) is being routed to dual dry detention basins in series separated by a permanent
baffle. The flow from the dry detention basin is routed through an outlet structure to a level
® Dewberry Fort Bragg I Conferencing and Catering Center) Stormwater Narrative 11
spreader/vegetated filter strip. The combined removal efficiency for these devices is 85%. Again,
these devices are designed with ultimate build out in mind. Major design elements include:
• 3a vegetated side slopes
• 25% additional storage volume for sediment deposition
• Seasonal high water table is at least 2 feet below the bottom of the basin
• A sediment depth indicator
• Emergency drain
• Outlet structure with regular pipe out plus a normally closed 6 inch diameter emergency
drain
• Outlet structure is also equipped with a larger orifice at a higher elevation to serve as an
overflow drain
• A concrete level spreader with slope of zero across the surface
• A 30 feet long x 30' wide vegetated filter strip which will be vegetated with sod
1.5. Summary of Stormwater Flow Rates
Peak flow calculations (pre and post construction) were calculated using the Rational Method. The
inflow hydrographs and volumes were calculated using the SCS method. The volume of treatment
that must be controlled was calculated using the Simple Method. State -storage -discharge models
were developed using Chainsaw Routing. All calculations methods are described in Section 3 of the
North Carolina Division of Water Quality Best Management Practices Manual, latest edition.
A summary of the pre and post construction flow rates are as follows:
Summary of Flow Rates
t year
to year
too year
Pre Development Flow Rate (CFS)
32.6
49-5
62.
Post Development Flow Rate (CFS)
36.9
56.0
70.5
Post Development Flow Rate with BMPs
installed (CPS)
15.46
23.58
40.17
9 Dewberry Fort Bragg I Conferencing and Catering Center) Stormwater Narrative 12
Conferencing and Catering Center
Fort Bragg, INC
DISSIPATOR PAD A
Dissipator Pad Calculations
Objective: To ensure the flow velocity at the outlet of the crossing culvert is non -erosive for the receiving
ditch
Design Criteria: Dissipator Pad is selected based on Ofull from the culvert
Source: NCDENR E&SC Manual section 8.06
Values listed below are from FlowMaster Calculations
Culvert Size
18
INCHES.
Culvert Sloe
0.005
FT/FT
Ofull
7.43
US
Vfo
4.20
FT/S
Results from figure 8.06c
Zone 1
Apron Material
Class B
Minimum Thickness
18
INCHES
Minimum Apron Length
g
FT
Minimum To Width
4.5
FT
Minimum Bottom Width
10.5
FT
Conferencing and Catering Center
Fort Bragg, North Carolina
DISSIPATOR PAD B
Dissipator Pad Calculations
Objective: To ensure the flow velocity at the outlet of the crossing culvert is non -erosive for the receiving
ditch
Design Criteria: Dissipator Pad is selected based on Qfull from the culvert
Source: NCDENR EBSC Manual section 8.06
Values listed below are from FlowMaster Calculations
Culvert Size
24
INCHES.
Culvert Sloe
0.005
FT/FT
QfU11
16.00
CFS
All
5.09
FT/S
Results from figure 8.06c
Zone 1
Apron Material
Class B
Minimum Thickness
18
INCHES
Minimum Apron Length
121
FT
Minimum Top Width
6
FT
Minimum Bottom Width
1 141
FT
Conferencing and Catering Center
Fort Bragg, North Carolina
DISSIPATOR PAD C
Flume Information:
Approximate Length
Approximate Width
Approximate Depth
Mannings n coefficient
Equivalent Pipe Diameter
Drainage Area Information
Approximate Total Area
125
6 FT
6 FT
0.5 FT
0.013 for concrete
2.0 FT
0.13 AC
8.78 IN/HR
Rational C
Area ac
Open Space
0.40
0.02
Impervious
0.90
0.11
Woods
0.25
0.00
Composite C
0.84
Peak Flow Calculations
25= 0.93 CFS
25= 0.31 FPS
Dissipator Pad Calculations
Objective: To ensure the flow velocity at the outlet of the
crossing culvert is non -erosive for the receiving ditch
Design Criteria: Dissipator Pad is selected based on Qun
Source: NCDENR E&SC Manual section 8.06
Values listed below are from FlowMaster Calculations
mn
0.93
CIFS
mn
0.31
FT S
Results from figure 8.06c
ZONE 1
Apron Material
CLASS B
Minimum Thickness
18
INCHES
Minimum Apron Length
12
FT
Minimum Top Width
8
FT
Minimum Bottom Width
1 12
FT
Conferencing and Catering Center
Fort Bragg, North Carolina
DISSIPATOR PAD D
Flume Information:
Approximate Length
Approximate Width
Approximate Depth
Mannings n coefficient
Equivalent Pipe Diameter
Drainage Area Information
Approximate Total Area
125
6 FT
6 FT
0.5 FT
0.013 for concrete
2.0 FT
0.30 AC
8.78 IN/HR
Rational C
Area ac
Open Space
0.40
0.14
Impervious
0.90
0.16
Woods
0.25
0.00
Composite C
0.67
Peak Flow Calculations
2s= 1.78 CFS
25=1 0.59 FPS
Dissipator Pad Calculations
Objective: To ensure the flow velocity at the outlet of the
crossing culvert is non -erosive for the receiving ditch
Design Criteria: Dissipator Pad is selected based on Qw„
Source: NCDENR E&SC Manual section 8.06
Values listed below are from FlowMaster Calculations
f,n
1.78
CFS
n,o
0.59
T/S
Results from figure 8.06c
ZONE 1
Apron Material
CLASS B
Minimum Thickness
18
INCHES
Minimum Apron Length
12
FT
Minimum Top Width
8
FT
Minimum Bottom Width
1 121
FT
Conferencing and Catering Center
Fort Bragg, North Carolina
DISSIPATOR PAD E
Flume Information:
Approximate Length
Approximate Width
Approximate Depth
Mannings n coefficient
Equivalent Pipe Diameter
Drainage Area Information
Approximate Total Area
125
6 FT
6 FT
0.5 FT
0.013 for concrete
2.0 FT
0.07 AC
8.78 IN/HR
Rational C
Area (ac)
Open Space
0.40
0.01
Impervious
0.90
0.06
Woods
0.25
0.00
Composite C
0.80
Peak Flow Calculations
25-
25=
Dissioator Pad Calculations
Obiective: To ensure the flow velocity at the outlet of the
crossing culvert is non -erosive for the receiving ditch
Design Criteria: Dissipator Pad is selected based on Ctull
Source: NCDENR EBSC Manual section 8.06
Values listed below are from FlowMaster Calculations
ull
ull
Results from figure 8.06c
ZONE 1
Apron Material
CLASS B
Minimum Thickness
18
INCHES
Minimum Apron Length
12
FT
Minimum Top Width
8
FT
Minimum Bottom Width
1 12
FT