SW5231001_Response To Comments_20240404

Subject: Request for Additional Information
Stormwater Permit No. SW5231001
Raleigh-Durham International Airport Park Economy 3 Expansion
Wake County
RESPONSE TO COMMENTS:
1. As designed, SCMs 1 and 7 are insufficiently sized. Please revise as needed so that
at least the minimum required treatment volume is being provided within each
SCM.
a. SCM 1: Per the calculations, the minimum required treatment volume of this
SCM is 40,670 cf. Per the SCM design, the SCM has a design volume of
approximately 36,500 cf. The design volume of an SCM is the volume that can
be stored between the bottom of the SCM (in this case, the top of the planting
surface at elevation 323.0') and the invert of the lowest bypass device (in this
case, the weir with an invert at elevation 324.25'). Per the stage-storage table,
the amount of storage volume between these two elevations is approximately
equal to 36,500 cf which is less than the minimum required treatment volume
of 40,670 cf. Please revise the design so that at least the minimum required
treatment volume is provided in this SCM.
RESPONSE: The weir elevation has been revised to accommodate the
minimum treatment volume calculated. The new weir elevation allows for
the design ponding depths to remain within the 15-18"lower range we
previously discussed in our last in person meeting. Dr. Hunt stated that 18-
in is acceptable to support plant life with greater amounts being possible
once established. Minutes have been uploaded for reference if needed.
Our revised design will allow for 3 SGWs to be in the 15-18"range, 2 SGWs
to be in the 20-30"range, and 2 SGWs to be in the 30-40"range which will
allow for the additional research through NCSU's monitoring program.As
previously submitted, RDU is committed to monitoring the SGWs beginning
in construction through the first two years of operation and will provide
results to NCDEQ.
b. SCM 7: Per the calculations, the minimum required treatment volume of this
SCM is 56,270 cf. Per the SCM design, the SCM has a design volume of
approximately 51,100 cf. The design volume of an SCM is the volume that can
be stored between the bottom of the SCM (in this case, the top of the planting
surface at elevation 340.0') and the invert of the lowest bypass device (in this
case, the weir with an invert at elevation 341.25'). Per the stage-storage table,
the amount of storage volume between these two elevations is approximately
equal to 51,100 cf which is less than the minimum required treatment volume
of 56,270 cf. Please revise the design so that at least the minimum required
treatment volume is provided in this SCM.
RESPONSE: The weir elevation has been revised to accommodate the
minimum treatment volume calculated. The new weir elevation allows for

the design ponding depths to remain within the 15-18"lower range we
previously discussed in our last in person meeting. Dr. Hunt stated that 18-
in is acceptable to support plant life with greater amounts being possible
once established. Minutes have been uploaded for reference if needed.
Our revised design will allow for 3 SGWs to be in the 15-18"range, 2 SGWs
to be in the 20-30"range, and 2 SGWs to be in the 30-40"range which will
allow for the additional research through NCSU's monitoring program.As
previously submitted, RDU is committed to monitoring the SGWs beginning
in construction through the first two years of operation and will provide
results to NCDEQ.A small amount of water will also be released during the
1-year 24-hr storm in order to accommodate the design ponding depth
requested during our last meeting. Releasing a portion of the 1-yr 24-hr
event was discussed during our last meeting and found to be acceptable
provided it is less than the pre-construction conditions. Minutes have been
uploaded for reference if needed.
2. Please update the Supplement-EZ form to account for the SCM design changes
that were made per the February 21, 2024 submission. Please revise the following
issues with the Supplement-EZ Form:
a. Drainage Areas Page:
i. Line 19-The design storm for the SCMs for this project are defined in 15A
NCAC 02H .1017(5). Please revise.
RESPONSE: This line has been revised to show design storm of 1"for all
SCMs per statute as requested, copied below:
"For high density projects designed to achieve runoff treatment, the
required storm depth shall be one inch"
ii. Line 20-See other comments with regard to the design volumes of the SCMs
RESPONSE: This line has been revised to reflect the design volumes
calculated based on the lower weir elevation to the planting surface for
each SCM as requested.
b. Submerged Gravel Wetland Page:
i. Line 2- Please revise these values so that they correspond to the minimum
required treatment volumes as shown in the calculations (Ex: SCM 1 =40,670
cf, SCM 2 = 45,571 cf, etc...). Please revise as needed.
RESPONSE: This row has been updated with the calculated minimum
treatment values.
ii. Line 19- Please revise these values so that they correspond to the minimum
required treatment volume ponding depth.These values should not exceed
the values of Line 20.
RESPONSE: This row has been updated with the calculated minimum
treatment values.

iii. Line 20-See below clarification for"design volume".The ponding depth of
the design volume for these SCMs is the distance from the planting surface
to the invert of the lowest bypass device (Ex: SCM 1 = 15", SCM 2 = 21",
etc...).
RESPONSE: This line has been revised to reflect the correct depths as
requested.
iv. Line 22-The design volume of an SCM is the volume that can be stored
between the bottom of the SCM (the top of the planting surface) and the
invert of the lowest bypass device (the lowest weir in the outlet structure).
Please revise as needed.
RESPONSE: This line has been revised to be the volume between the
lower weir and planting surface as requested.
v. Line 24-Please revise these values as needed so that they correspond to the
100-yr surface elevation (Ex: SCM 1 = 48", etc...).
RESPONSE: This line has been revised to match the 100-yr staging
elevations as shown in the plans.
vi. Lines 35 &36- Please revise these values so that they are consistent with
the percentages as listed in 15A NCAC 02H .1052(6)(b) &(c). NOTE: There is
a typo for the bioretention media mixture values shown in the Manual.
RESPONSE: The percentages of the mix have been corrected to match
the statue and previously submitted plan sheet.
As a general note, we have highlighted the cells in the spreadsheet in yellow
where changes have been made for ease of review. We have also added in
additional rows in the spreadsheet for ease of review.
3. Plan sheet P1.007.01(and elsewhere as needed)-Please revise the soil mixture
percentages so that they correspond to 15A NCAC 02H .1052(6)(b) &(c). This was
partially addressed as part of the February 21, 2024 submission, however there
were some submittal items that were not updated. Please revise as needed.
RESPONSE: This sheet has been modified. The previously submitted
percentages were correct but the subsequent note noting tolerances has
been removed.
Additionally, RDU is committed to pursuing water quality treatment and monitoring beyond
the State requirements. The language below is the commitment RDU has made to RDU per
previous correspondence:
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.