SW1220701_Response To Comments_20230329

February, 28, 2023
North Carolina Department of Environmental Quality
Division of Energy, Mineral and Land Resources U
512 North Salisbury Street
Raleigh, NC 27699
919-707-9200
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Attention: Mr. Jim Farkas
Q
RE: All About Plumbing Building Addition c'
Henderson County a)
Stormwater Permit No. SWSW1220701 c
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Mr. Farkas: _
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Thank you for your review comments regarding above referenced project. 3
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Below is an itemized list of the Stormwater Management Permit Application review v
comments and how we have addressed them.
1) Swale Design:
a. The provided actual 10-year design storm velocities for the vegetated
conveyances do not appear to be correct. The provided calculations show this
velocity value as being determined based on the 10-year design flow (4.48
cfs for swale # 1) and the available cross -sectional area of the Swale (9.75 sf,
derived from a trapezoidal cross-section with bottom width of 2', side slopes
of 3:1, and a depth of 1.5'). This velocity, 0.46 fps, is significantly less than
what would actually occur as a flow rate of 4.48 cfs would produce a much
shallower for this swale (which would have a smaller cross -sectional area
and, therefore, a higher velocity). Instead of assuming that the swale will
flow full (depth = 1.5') Manning's equation should be iteratively solved for
depth of flow based on the design flowrate (4.48 cfs). Using the provided
swale geometry, a design flow of 4.48 cfs, and a Manning's n value of 0.15
would result in an actual velocity closer to 1.36 fps (A = 3.287 sf, Wp =
6.840', depth = 0.767'). Please revise as needed.
Response: The drainage swale calculations have been revised to utilize a
Manning's roughness of 0.035 for grass channels and the velocity check has
been revised to check the velocity for the swale flowing at design flowrate's
channel depth. See drainage Swale calculations in Appendix D of the revised
S WPPP for further information.
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QT 864 582 0585
V) www.mcc-ea.com

a) The provided Manning's n value (0.15) appears toe on the high side (we
typically expect n values in the 0,03-0.04 range for grass lined channels. Please
either provide a source for the n values used in this calculation or revise the
calculation.
Response: The drainage swale calculations have been revised to utilize a
Manning's roughness of 0.035 for grass channels. See drainage swale
calculations in Appendix D of the revised SWPPP for further information.
2) Application, Section IV, 10:
a) I apologize in advance for the seemingly contradictory comments that I will be
making for this section. High -density and low -density projects get handled
differently in this section of the Application and the prior comments were under
the assumption that the project was a high -density project. (Also, this form
predates the latest version of the rules so there are some
inconsistencies/confusions about how to fill out the table).
Response: Understood.
b) For a low -density project, the "drainage area" for the project is equal to the
project area (see information in Section IV, 9 of the Application). Please revise
the values in the table as follows.
• Total & On -site Drainage Area — This value should equal the project area
(3.15 ac per the plans), expressed in square feet.
• Proposed Impervious Area — This should equal the net increase in BUA
(0.33 ac per the plans), expressed in square feet.
® % Impervious Area — This value should correspond to the value in
Section IV, 8.
® On -site Building/Lots — This should equal the net increase in building
roof area (0.15 ae per the plans), expressed in square feet.
® On -site Parking — This should equal the net increase in gravel parking
area (0.18 ae per the plans), expressed in square feet.
• Existing BUA — This should be equal to the total amount of existing
BUA (1.00 ac per the plans), expressed in square feet.
• Total — This should be the sum of the items in the lower half of the table
and should equal the total amount of BUA that will exist in proposed
conditions (1.33 ae per the plans), expressed in square feet.
Response: Application has been revised and resubmitted.
3) Supplement-EZ Form:
a) Cover Page:
Line 7 — "Phis item refers to the width of the vegetated
setback from surface waters (which is 30 ft in this part of the
State). Please revise. It is noted that there are no surface
waters located in the project area, we just ask that this value
be included on the form.
Response: Cover page has been revised.

b) Drainage Area Page:
i) Line 4 — Similar to the Application, this project only has I
"drainage area" which is equal to the entire project area.
ii) Entire Site Column:
o Line 10 — This item is the sum of the items of Line
12 and should equal the net increase in BUA (0.33
ac per the plans), expressed ins square feet.
o Line 12, Parking— This should equal the net increase
in gravel parking area (0.18 ac per the plans),
expressed in square feet.
o Line 15 —This should be equal to the total amount of
existing BUA (1.00 ac per the plans), expressed in
square feet. NOTE: BUA that is relocated within a
project area or changed from one type to another is
still considered to be existing BUA that will remain
and does not need to be treated.
o Line 18 — This value should correspond to the value
in Section IV, 8.
Response: Drainage area page has been revised.
c) Low Density Page:
i) Line 5 — Vactual, and Flow Depth — Please revise as needed
ii) Vallowable — This is the maximum allowable velocity of
runoff in the vegetated conveyances before it starts eroding.
Maximum permissible velocityes can be found in Part C-I I
of the Manual or from another acceptable source.
Response: Low Density page has been revised. Revised Supplement
EZ-Form has been resubmitted.

4) Please provide the following:
• Ix hard copy & Ix electronic copy of the revised
calculations.
• Ix hard copy & Ix electronic copy of the revised pages of
the Application
• Ix hard copy & Ix electronic copy of the revised pages of
the Supplement -EX Form.
• I hard copy (2x hard copy for plan sheets) & I electronic
copy of any other items needed to address these comments.
Response: Attached you will find a digital copy of the revised plans, SWPPP, and
supplemental documents and applications as well as 2 hardcopies of the revised plans
and 1 hardcopy of the supplemental documents.
Sincere Regards,
McCutchen Engineering Associates, PC
Tommy Karnes, P.E.
Project Manager