SW6231102_Response To Comments_20240711

_HILL•IARp
ENGINEERING, PLLC
3417 Winterwind Circle,Sanford,NC 27330
PO Box 249,Sanford,NC 27331
Phone:[phone]
e-mail:[e-mail]
NC License#:P-0836
July 8, 2024
Stormwater Program
NCDEQ Raleigh Regional Office
1628 Mail Service Center
Raleigh, North Carolina 27699
Reference: Stormwater Management Permit Application
Buffalo Creek Residential Subdivision
Hoke County, North Carolina
Dear Ms. Holland:
Please find enclosed the following revised documents for the above referenced project:
1. Prior Comment 1.b.—"As designed, the project does not appear to meet the
definition of a low-density project.The project appears to have excessive piping
(prohibited per 15A NCACO2H .1003(2)(c))... Please reduce the lengths of the following
pipes..." Please reduce the lengths of the following pipes as they are excessively long
for a low-density project:
a. Pipe between CB101 & FES100.
b. Pipe between CB204& FES200.
c. Pipe between CB301 & FES300.
d. Pipe between CB401 & FES400.
e. Pipe between CB501 & FES500.
f. Pipe between CB601 & FES600.
g. Pipe between CB701 & FES700.
h. Pipe between CB801 & FES800.
i. Pipe between CB1001 & FES1000.
Response: Per phone conversation with Brianna Holland on July 8, 2024 it was discussed
the proposed pipe layout was designed as shown since the construction of the storm
drainage had to be at the proposed depths per NCDOT requirements.The lots will not
have enough septic area if the ditches are extended to the right-of-way. Since the state
requirements are written to allow for variations due to site conditions the pipes in these
areas needed to be extended to provide daylight without excessive ditch depths.The
propose ditches (Curb outlet Swales) meet the state requirements of min 100 ft in length
and less than 5.0%slope with 3:1 side slopes. Per the state requirements reduction of
erosion in the channels are also a concern without the pipes extended into the lots. Each
one of these outlets with exception of FES 100, 200, 300,and 1000 discharge into a larger

curb outlet swale that is picked up by FES 103 and conveyed under the road system to the
final discharge points named as FES 100, 200, 300,and 1000.These discharge points
provide 100 feet of minimum grass swale at less than 5.0%slope and 3:1 side slopes
which discharge as dispersed flow. A variance is attached requesting the piping as shown
and explaining that the requirements of the state requirements have been met.
2. Prior Comment 2.a.—"Please correct the following issues with the swale designs...
Please ensure that the design information shown in the plans, design calculations and
Supplement-EZ Form are consistent." The swale slope values shown in the
Supplement-EZ Form for Swales 11-13 (PD7, PD8, & PD9) do not match the
information shown in the other submittal items and the Vactual value calculated for
PD9 does not appear to be calculated correctly based on the swale geometry. Please
also clarify why the Vallowable for PD100 is shown as 18.0 fps (while the other
swales lined with P300 matting are shown to have Vallowable values of 9.0 fps.
NOTE:The spec sheet for P300 matting indicates a maximum allowable velocity of
16 fps for vegetated conditions (https://nagreen.com/sites/default/files/2017-
03/EC_RMX_MPDS_P300.pdf). Please revise as needed.
Response: The EC form has been updated to show the 18.0 fps as a vegetative swale.The 9.0 fps
was with the lining only without vegetation. But the permanent swale will be vegetated which
would allow for the 18.0 fps.
3. Prior Comment 7.a.i.—"Drainage Areas Page... Line 4—Low-density projects
typically only have 1 drainage area that is equal to the project area. Please revise."
This item was not addressed (see picture below).
Response: The EZ form has been updated to show one drainage area.
4. Prior Comment 7.a.i.—"... LD Column 1— For this project, the low-density drainage
area is equal to the project area.This column will be the same as the "Entire Site"
column."This item was not addressed (see picture below).
Response: The EZ form has been updated to fill out column 1.
5. Prior Comment 7.b.vi. —"Line 12—This item is required per 15A NCAC
nVu 1nnzIn\(o\ This item was not addressed (see picture below).
Response: The EZ has been updated to say yes to dispersed flow at the setback.
6. Prior Comment 8— 'Please upload the plan sheets either as individual sheets or in
groups of sheets that are all of the same type, not as one whole set.This is required
since our repository has a difficult time handling large files. Please also ensure that all
of the files (such as the property deed, Secretary of State information, etc...) have been
uploaded for this project. Electronic submissions are required per 15A NCAC
02H .1042(21" While the files were uploaded separately, they were all uploaded
under the same file type "Plansheet- Other". Please ensure that the files are
uploaded under their appropriate file type. See example below, on the left is the files
that were uploaded for this project (all under the same file type) and on the right
are files that were uploaded for a different project (each file under the appropriate
file type). Please revise as needed.
Response: Understood.The sheets have been uploaded correctly.
7. Please remove the piping shown in PD 9 (DI 404 to FES 403). Excessive piping is
prohibited per 15A NCACO2H .1003(2)(c). Please revise as needed.
Response: DI 404 to FES 403 has been removed from PD 9.The ditch slope has been recalculated
and regraded to remove piping.

8. Please revise the curb outlet swale drainage area map in the main set of plans.This
item is required per 15A NCAC 02H .1042(2)(g)(iv) and Section Vl,8o of the
Application. It is noted that plansheet 13 is labeled as being the swale drainage area
map, however a number of swale drainage areas are missing from this sheet and the
provided drainage areas do not appear to correspond to the swales. Please revise as
needed.
Response: Curb outlet swales has been added to the plan sheet to indicate drainage areas to
each swale. Initially the intent was to provide 100, 200, 300 and 1000 as the low density
vegetative discharge and the individual curb outlet swales as conveyance only.
9. As shown,there are issues with the time of concentration (Tc) calculations for
drainage areas 1A& 1B. Per the NOAA Atlas 14 data, the 2-year, 24 hour rainfall
amount is 3.68 inches (0.1 inches is shown in the calculations).This results in a Tc
for the sheetflow section of approximately 0.037 hours (0.18 hours shown). For the
shallow concentrated flow portion of the Tc calculations,the average velocity
determined from Figure 3-1 does not appear to be correct. Based on a watercourse
slope of 0.013 ft/ft,the velocity for an unpaved area would be approximately 1.85
fps (1.5 fps is shown and would more closely correspond to a watercourse slope of
about 0.0085 ft/ft).This results in a Tc for the shallow concentrated flow section of
approximately 0.135 hours (0.17 hours shown).These changes reduce the total Tc
to approximately 0.172 hours, which is equal to 10 minutes (21 minutes shown).
This change in Tc changes the intensity of the swales it was used for and appears to
result in Swale PD1B being undersized. Please revise as needed.
Response: The time of concentration was corrected and the calculations for the ditches were
updated.
If after review of the submittal package you have any questions, comments or if you require any
additional information please do not hesitate to contact me at (919) 499-8759 or
[e-mail]. Thank you for your assistance.
Sincerely,
Hilliard Engineering, PLLC.
Michael Blakley