SW5231001_Response To Comments_20240119

1. Please clearly indicate the extents of the project area in the main set of plans, including
bearings and distances.This item is required per 15A NCAC 02H .1042(2)(g)(ii).
NOTE: For projects like this that are located on a single tract of land that is significantly
larger than the area that the project will be located in, the entire property area is typically
not used as the project area (the project area must include all of the proposed
development and corresponds to the area covered by the permit). Please also do the
following:
a. Revise Section IV 4-7 of the Application Form to use the project area instead of the
entire airport area (this form was not set up with projects like this in mind).
RESPONSE:Form has been revised and uploaded. Original copy will be delivered. Grading and
drainage plans show project area outline and bearings.
b. Indicate the total amount of existing BUA located within the project area in existing
conditions.
RESPONSE:BUA value has been updated accordingly.
c. Recalculate the percent BUA for the project area in Section IV, 8 of the Application
Form. For a project with existing BUA, the percent BUA for the project area is
calculated as follows: [Net Increase in BUA] /([Total Project Area] —[Existing
BUA])
RESPONSE:BUA value has been updated accordingly
2. When determining the minimum required treatment volume of the SCMs and showing
the drainage area delineations in the main set of plans, please ensure that the entire
drainage area to the SCM is included.The provided maps and calculations appear to only
include the areas draining to the storm drain system and do not include other areas (such
as the SCM itself and the adjacent pervious areas) draining to the SCM. Per 15A NCAC
02H .1050(1),the SCM must be sized based on all areas draining to the SCM (not just the
areas draining to the storm drain system). Please revise as needed. Calculations are
required per 15A NCAC 02H .1042(2)(f) and drainage area plans are required per 15A
NCAC 02H .1042(2)(g)(iv) &Section Vl.8.o. of the Application Form. Please also show
the proposed site conditions on the proposed drainage area map in the main set of plans
(The current version only appears to include the proposed grading but does not include
the proposed site layout/BUA). If doing so would make the sheet "too busy", you can just
include the extents of the proposed BUA.
RESPONSE:Drainage area map and associated numbers have been updated. BUA layout has
been added to post construction drainage map(shaded or otherwise hatched).Application
form and other calculation documents have been updated based on new site drainage areas.
Note: The original submitted value for SCM 4 included all of the SCM and surrounding pervious
embankment in the calculations as impervious. This has been corrected to show the depicted
pervious and impervious areas.
3. Please correct the following issues with the Application Form (in addition to those
mentioned in earlier comments):

a. While not required, if you would like to designate someone, such as a project
manager within RDUAA, to also receive these request for additional information
letters and to serve as the main point of contact with regard to technical project
matters,then please list their name and contact information in Section VII of the form
(they do not need to be a consultant to be listed in this section, although that is the
most common use of this section of the form).
RESPONSE:At this time, RDUAA does not wish to add any additional designating individuals.
b. Section IV, 10— Please update the drainage area information in this table in
accordance with the earlier comment with regard to accounting for the entire drainage
area to the SCM.
RESPONSE:Drainage area information updated.
4. Please correct the following issues with the Supplement-EZ Form (in addition to those
mentioned in earlier comments):
a. Cover Page:
i. Line 2—Please revise as needed.This item should correspond to Section IV, 7 of
the Application Form.
ii. Line 261/2 - Please indicate the type and number of SCMs that are being proposed
for this project.You can manually add an additional line to the form below Line 26
or cross out and repurpose the line of one of the existing SCMs.
RESPONSE:EZ form has been updated and uploaded. Original signed copy will be delivered.
b. Drainage Area Page:
i. Please complete the entire site column.The entire site column is an accounting of
the entire project area (similar to how the drainage area columns are an accounting
of the entire drainage area to the SCM). When the entire site column asks for the
"drainage area" please enter the project area.
ii. Drainage area columns—Please update the drainage area information in this table
in accordance with the earlier comment with regard to accounting for the entire
drainage area to the SCM.
iii. Line 20—The design volume of the SCM is the volume that can be stored above
the top of the planting surface and below the invert of the lowest bypass elevation.
The design volume must be at least as large as the minimum required treatment
volume and is typically close to the minimum required treatment volume. See later
comment with regard to the design volume ponding depth.
RESPONSE:Drainage area information has been updated. In regard to the design volume,
please see comment responses to design volume ponding depth.
c. Submerged Gravel Wetland Page:
i. Revise as needed in accordance with other comments (the provided values appear
to correspond to the design as shown).

RESPONSE:Plans and calculations have been revised in accordance with the other comments.
5. Since this proposed design is being permitted as an alternative design (in accordance with
15A NCAC 02H .1003(6)) and does not have predefined MDCs associated with it, we
request that the following changes be made to the proposed design. If further discussion
is needed, we can schedule a meeting.As indicated below, MDCs for stormwater
wetlands and bioretention cells (the closest, currently available SCMs) are referenced as
the basis for these comments:
a. Revise the forebay designs so that they are larger/more substantially sized.
i. Forebays are required for stormwater wetlands (Stormwater Wetland MDC 6) and
are strongly recommended for bioretention cells (Bioretention Cell
Recommendation 2).
ii. Including more substantial forebays would help reduce sedimentation
accumulation issues on top of the soil layer as noted in some of the included case
studies and will help reduce maintenance frequency, especially given the size of
the drainage areas to the SCMs.
RESPONSE:Forebays were designed to be approximately 5%of the total basin area. Forebay
sizes have been doubled to meet a 10%minimum per Constructed Wetland MDC 6. The
forebay sizes also now meet the Gravel Wetland MDC 2 recommendations of the uploaded
Mitchell et al.paper developed by Dr. Hunt's colleagues. Forebay size by percent now included
in the plans.
b. Specify a soil media mixture that meets the bioretention mixture requirements in lieu
of the provided wetland soils mixture.
i. Bioretention media mixture (specified in 15A NCAC 02H .1052(6)) has been
studied and approved for use in North Carolina.
ii.The provided wetland soil mixture is similar to bioretention media mixture so the
requested change should not negatively impact the function of the SCM or ease of
construction.
RESPONSE:Plans have been revised to show the soil mixture on P1.007.01 meeting
bioretention media mixture requirements per Bioretention MDC 6.
c. Demonstrate that the inverts of the underdrain outfalls are located above the SHWT.
i.This ensures that the SCMs are not draining the water table since the underdrain
inverts will be located below grade and there is no proposed liner under the SCM.
SCMs with underdrains have SHWT separation requirements (Ex. Bioretention
MDC 1).
RESPONSE: There is a proposed impervious liner included in the plans. See sheet P1.007.01,
Detail 1 and General SCM Notes. The Mitchell et. al paper, MDC 3 also state that a liner is
acceptable.
d. Limit the ponding depth of the design volume to no more than 15" above the top of
the soil layer.
i.The MDCs for bioretention cells require that the design volume be ponded at a
depth of no greater than 12 inches above the planting surface (Bioretention MDC
2).

ii.The MDCs for stormwater wetlands require that the design volume be ponded at a
depth of no greater than 15 inches above the permanent pool surface (Stormwater
Wetland MDC 1).
iii. Since the planting conditions for this SCM are closer to that of a stormwater
wetland than a bioretention cell,the maximum design volume ponding depth from
the stormwater wetland MDC can be used and should not be exceeded for the
health of the plants.
iv. Please ensue that the design includes some form of bypass device (orifice, weir,
etc...)to establish the temporary pool surface elevation at or below 15" above the
top of the planting surface. A bypass device with an adjustable bypass invert is
recommended (see guidance under MDC 1 in Part C-4 of the Manual).
RESPONSE: Wetland MDC 1 limits ponding depth to 15-in above the permanent pool. This
implies that the wetland vegetation is partially submerged as a permanent condition and that
15-in above the permanent pool elevation is recommended for the maximum temporary
inundation over vegetation. Based on C-4 Wetland Figure 2, the depth of the permanent pool
has a depth of 0-in to 9-in in shallow water zones and a minimum of 18-in in deep pool areas
meaning that the allowable inundation for the design storm event is between a depth of 15-in
to 33-in above the soil layer, with up to 24-in covering the majority of the vegetation in the
shallow water zones.
The proposed submerged gravel wetlands do not inundate any plants except for rain events as
the "permanent pool"elevation is below the surface. Per Dr. Hunt's colleagues in the Mitchell
et al.paper, MDC 1 allows for a maximum depth of 36-in above the planting surface for
temporary inundation for the design volume. When reviewing the 1-in event across each
drainage area, the depth of inundation is between 12"-36"and the 1-year storm is under 36".
These depths have been depicted on the plans and revised in the associated calculation
spreadsheets. Additionally, careful consideration for plant selection has been given to select
plants that can withstand the anticipated temporary inundation.
e. Limit the ponding depth for peak attenuation above the top of the soil layer.
i. The MDCs for bioretention cells require that the peak attenuation volume be
ponded at a depth of no greater than 24 inches above the planting surface with the
invert of the peak attenuation outlet no more than 18 inches above the planting
surface (Bioretention MDC 3).
ii.The MDCs for stormwater wetlands don't specify a maximum peak attenuation
ponding depth, but extended submersion is not good for the health of the plants or
soil.
RESPONSE:See the uploaded Mitchell et. al. paper, Gravel Wetland MDC 4. For the gravel
wetland, Dr. Hunt's colleagues recommended a maximum depth of 5-ft for peak attenuation.
We have revised the proposed weirs to limit the attenuation to 5-ft. We are attenuating for up
to the 100-year storm, 24-hr in this design to significantly exceed attenuation requirements set
forth by the State (1-yr, 24-hr)in order to improve overall water quality by treated more
runoff through the proposed media. The drawdown time for the wetlands is still under the
recommended 5 days so the plant life should not be negatively impacted by this.
f. Please provide a draft of the language that you would like included in the permit to

reflect that RDUAA will engage a local university or consultant in monitoring the
installation and observing the efficiency of the SCM for research use for the Division
(language subject to revision by the Division prior to approval).These items will be
added as special permitting conditions to the permit. If the SCMs fail to perform
satisfactorily or meet the intent of the stormwater rules, the Division will require that
the permit be modified to either meet the requirements or propose another variation
from the rules. If you would like to design the SCMs so that certain design parameters
are varied across the proposed SCMs (so that the effectiveness of the design
parameters may be studied) we can discuss this further.
RESPONSE:RDUAA will engage a local university or consultant to provide installation
oversight and monitoring services for the proposed SCMs.All proposed SCMs will have their
water levels recorded to determine the fraction of inflow that percolates through the media
versus that which bypasses media treatment(overflow)via water level recording devices.
Drawdown rates through the media will also be determined. Two SCMs(to be field
determined)shall have each inlet and outlet equipped with monitoring stations including an
automated water sampler. Inflow and outflow volumes will also be calculated. Samples will be
collected from 18 storm events distributed throughout various seasons. Samples will be tested
for Total Suspended Solids, Total Kjeldahl Nitrogen,Ammoniacal Nitrogen, Nitrate+Nitrite
Nitrogen, Total Phosphorus and Ortho Phosphorus. Testing will begin after the SCMs have had
one growing season to establish. It is anticipated that all data collection will conclude within 5
years of the permit issuance and within 2 years of construction concluding. RDUAA (or it's
university/consultant liaison) will provide the research results to NCDEQ for use in developing
new approved SCMs for the State of North Carolina upon the conclusion of the monitoring
period. Results will be provided in NCDEQ NEST format as described in the SCM Credit
Document.
g. While not required, it is recommended to increase the depth of the soil layer to 8
inches. The depth of the#78 aggregate layer can be reduced to no less than 3 inches if
you wish.
i. Per the NCSU submerged gravel wetland presentation by Dr. Hunt and colleagues
(https://www.bae.ncsu.edu/workshops-conferences/wpcontent/
uploads/sites/3/2020/06/Subsurface-Gravel-Wetlands.pdf, slide 27).
RESPONSE:Dr. Hunt's colleagues recommend a minimum of 0.5-ft of wetland soil, 0.3-ft of#78
stone, 1-ft of#57 stone. We are currently using 0.5-ft of wetland soil, 0.5-ft of#78 stone, and
3-ft of#57 stone. Refer to the highlighted section of the uploaded Mitchell et. al. paper, MDC
7. The wetland soil depth has been revised to an 8-in depth and 4-in depth for#78 stone.