20221191 Ver 1_2021-04-29 StormwaterReport_20220831

Stormwater Management Report
Cash Corporate Center
Town of Apex, North Carolina
Prepared By:
f4_
ADVANCED
CEVII. DES I G N
51 Kilrnayne Drive
Suite 105
Cary, North Carolina 27511
ph 919.481.6290
ENGINEER. S S U R V E Y O R S
Prepared For:
Beacon Partners
April 29, 2021

Table of Contents:
1. Stormwater Narrative
Appendixes
A. Pre & Post Development Drainage Area Maps
B. 1, 10, & 100-yr Stormwater Routing Calculations
C. Water Quality Volume Calculations
D. BMP Sizing Calculations
E. Storm Sewer & Ditch Calculations
F. Sediment Basin Sizing
G. FEMA Map
H. Soils Map

Stormwater Narrative
Summary:
The subject project consists of a newly platted lot containing approximately 26.0 acres. The project
is located Southeast of Burma Drive and North of Pristine Water Drive. Ultimately, the project site
will be located just east of an extension of Burma Drive. One warehouse building with associated
parking and infrastructure will be proposed as part of this development. The site is located within
the Town of Apex's Secondary Watershed Protection Overlay District and located just east of the
Neuse/Cape Fear River Basin line.
Existing Soils:
Existing site soils are made up of mostly Georgeville Silt Loam (GeB & GeC), Nanford Silt Loam
(NaD) centrally east and Chewacla and Wehadkee soils (ChA) along the east property line adjacent
to the stream. The soils are a mixture of hydraulic Class C and B soils. Detailed information for each
of these soil types can be found in Appendix E.
Pre -developed Conditions:
The subject site is currently undeveloped and wooded. A gravel access road that runs south of
Burma Drive is the only impervious area currently in the subject watershed. A buffered stream that
runs from the northeast to southeast portion of the site has been delineated. Wetlands have also
been delineated adjacent to the east side of the buffered stream. The site is located in the Middle
Creek Watershed and Upper Neuse River Basin. The site does not contain floodplain per the most
recent FIRM map 3720075100J dated May 2, 2006.
The site contains three main drainage areas. Drainage area #2 & #3 begin at the southeast end of
Burma Drive and drain to their respective analysis points in mainly undefined swales. The drainage
area for analysis point #1 also begins at the southeast end of Bruma drive and follows the northern
property line and empties into the above mentioned buffered steam. Existing contours range in
elevation from 437 to 379 feet. A pre -developed weighted curve number based on wooded
conditions and of Class C soils has been used to reflect the existing runoff conditions.
Analysis point #1 has a weighted pre -developed runoff coefficient of 73. The time of concentration
used for the pre -developed condition was determined using TR-55 methodology using a
combination of sheet, shallow concentrated and channel flow. Reference the Pre -developed
Drainage Area Map for the lengths and flow route used to calculate the pre -developed time of
concentration. The time of concentration for Outlet 1 is 26.5 minutes.
Analysis point #2 has a weighted pre -developed runoff coefficient of 73. The time of concentration
used for the pre -developed condition was determined using TR-55 methodology using a
combination of sheet, shallow concentrated and channel flow. Reference the Pre -developed
Drainage Area Map for the lengths and flow route used to calculate the pre -developed time of
concentration. The time of concentration for Outlet 2 is 34.1 minutes.
Analysis point #3 has a weighted pre -developed runoff coefficient of 75. The time of concentration
used for the pre -developed condition was determined using TR-55 methodology using a
combination of sheet, shallow concentrated and channel flow. Reference the Pre -developed
Drainage Area Map for the lengths and flow route used to calculate the pre -developed time of
concentration. The time of concentration for Outlet 3 is 22.7 minutes.

The following table shows the pre -development peak flow for the corresponding design storms at
each analysis point followed by the total leaving the site:
Storm
Event (yr)
Pre -Developed Peak Runoff Rate (cfs)
Outlet 1
Outlet 2
Outlet 3
Total
1
6.23
3.38
5.80
15.41
10
20.91
11.45
18.07
50.43
100
36.89
20.25
31.10
88.24
*See Appendix B for detailed Pre -developed hydrographs
Post -developed Conditions:
The project will consist of a 300,600 sf warehouse type building with a loading dock, and a
supporting network of utilities, sidewalk, and parking. Town of Apex development requirements
state 10% of the subject site must be dedicated to Resource Conservation Area (RCA). These areas
are located along the north and east portion of the site and will be preserved. The project will
contain one wet detention pond that has been designed to meet the stormwater quantity controls
and TSS removal required by the Town of Apex. Good conditions were used for grass cover along
with proposed impervious area to develop a post -development composite curve number, which can
be found in the Appendix B. The time of concentration used for the post -developed condition is
the based on the pipe flow calculations draining to the basin.
Town of Apex Stormwater Regulations
The following stormwater regulations for quantity and quality are required for projects in the Town
of Apex per the UDO (Section 6.1.)
1. Stormwater Quantity
(a) Control the post -development stormwater peak flow leaving the project site for the 1-
yr & 10-yr 24-hour storm events. The post -development peak flow cannot exceed the
pre -development flow at each outlet point.
2. Stormwater Quality
(a) Total suspended solids (TSS) must be reduced by 85% with the use of a North
Carolina DWQ approved BMP.
(b) Capture the stormwater runoff volume generated from 1-inch of rainfall in the post -
development condition and release this volume between 48 and 120 hours.
3. Nitrogen —
As required by the Neuse Stormwater Rule 15A NCAC 2B.0235 the nutrient load contributed by
new development activities is limited to 3.6 pounds per acre per year (lbs/ac/yr) of nitrogen loading.
Development shall have the option of partially offsetting projected nitrogen loads by funding
wetland or riparian area restoration through the North Carolina Wetland Restoration Program.
However, the total nitrogen loading rate cannot exceed 6.0 lbs/ac/yr for residential development or
10.0 lbs/ac/yr for nonresidential development. Best management practices (bmps) provided for in
the stormwater program must be used to reduce nitrogen loading to the 6.0 and 10.0 limits and may
be used to reduce nitrogen loading to the 3.6 limit. Maintenance of any bmps installed will be the
responsibility of the development.

Stormwater Quantity
The proposed site is graded so impervious areas are captured by a storm drainage network system
that then discharges to the proposed BMP. Some undisturbed areas and proposed grass areas
bypass the proposed BMP's. Since the runoff curve number for these areas is less than or the same
as the predeveloped curve numbers these areas meet the Town of Apex post -to -pre requirement
without going to a proposed BMP. These areas that bypass the proposed BMP's area not included in
the pre or post developed drainage areas. The ponds have been sized and designed to meet or
exceed Town of Apex's stormwater regulations. Refer to the table below for pre development and
post development routing for the site.
Storm Event
(yr)
Pre -Developed
Peak Runoff Rate
(cfs)
Post -Development
Peak Run-off Rate
w/ BMP (cfs)
Total Pre -Developed
Peak Runoff Rate
(cfs)
Total Post -Developed
Peak Runoff Rate
(cfs)
Analysis Point #1
Analysis Point #1
All Analysis Points
All Analysis Points
1
6.23
4.50
15.41
8.25
10
20.91
20.66
50.43
31.78
100
36.89
25.21
88.24
44.04
*The results in the tables above demonstrate that the proposed BMPs reduce the post development
runoff in accordance with Town standards
Conclusion:
The proposed development will utilize a wet detention pond for storage volume and stormwater
quality control of the post -developed stormwater runoff. Water quality control will reduce
environmental impacts from the improvements on downstream tributary systems at level
sufficient with Town of Apex standards. Stormwater management calculations included in this
report are in compliance with the Town of Apex Design Manual and the NCDENR Stormwater
BMP Manual.

APPENDIX A
PRE & POST DEVELOPMENT DRAINAGE AREA MAPS