SW1240301_Response To Comments_20240710

Genesis Engineering
(111111r..".. Collaborative
July 9,2024
Ms.Brianna Holland
Sate Stormwater Engineer
DEVILRPost-Construction Stormwater Program
512 North Salisbury Street,Office 625Y
Raleigh,NC 27604
Re: Application for Stormwater Management Permit
Response to Stormwater Request for Additional Information
Marketplace at Mills River
Stormwater Permit No. SW1240301
GEC Project No.21-118
Dear Ms.Holland,
We are in receipt of the DFIVILRPost-Construction Stormwater Program review comments for
the above referenced project dated 6/11/2024. The construction plans, stormwater
management report,and application forms have been revised to address said comments and
below is a summary of the comments received and GEC's response to how each was addressed.
DFMLRcomment is BOLD and GEC response is iTALICIZED.
1. As designed, it is unclear if this project meets Runoff Treatment (required per 15A
NCAC 02H.1003(3)(a)). For a project to meet Runoff Treatment, the net increase in
BUAmust be captured and treated in one or more primary SCMs(see later note with
regard to BUAaccounting for this project). Please note that the net increase in BUAis
measured from existing conditions to proposed conditions where existing conditions
refer to what existed on-site when the rules went into effect in this area(not
necessarily what currently exists on-site). Please revise as needed.
GEC Response:Per email from Jim Farkas received on 6/27/2024, the existing conditions
of the site for the year2007have been estimated using aerial imagery available from
Google Earth. See Exhibit E1,2007Fxisting Conditions Map,in the Stormwater
Management Report. The treatment volume calculations have been revised using the net
increase in BUA from these 2007 existing conditions. See Appendix Ain the Stormwater
Management Report for updated calculations.
2. Please provide deed restrictions for the proposed Lots(2-5). This form can be found at
the following web address: http s://www.deq.nc.gov/about/divisions/energy-
mineral-and-land-resources/stormwater/stormwater-program/post-construct ion-
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program/post-construction-forms and is required per 15ANCAC 02H.1042(2)(k).
NOTE: Any lot whose ownership is not maintained by the permittee will need to get
their own permit (as a condition of the finalpermit language).
GEC Response:Deed restrictions form has been submitted to the property owner for
signature and will be provided with the next submittal.
3. SCM Design/Plan s
a) Per 15ANCAC 02H.1042(2)(h),please provide a plan view of the SCM, indicating,
among other things,the location and dimensions ofthe riprap energy dissipators for
the pipes entering the SCM, the extent of the riprap berm separating the chambers,
and the surface area/cross-sectional area of the sand layer.
GEC Response:Plan view ofSCM shown on C-3.4, Grading&Drainage Plan has been
revised and additional labels added.Riprap energy dissipator dimensions are provided on
Sheet C-9, Details, on the Stone Rip-Rap Detail
b) Please indicate the thickness ofthe gravel layer located over the sand layer in the
cross-sectional view of the SCM and the minimum depth of the sand layer provided
over the underdrain(see later comment with regard to this). These items are also
required per 15ANCAC 02H.1042(2)(h).
GEC Response:Cross section detail of the storm water management facility(SCM)has
been revised to specifya 6-inch gravel depth cover and a 12-inch minimum depth of the
sand layer above the underdrain on Sheet C-9 and Exhibit Ein Storm water Management
Report.
c) Please provide calculations demonstrating that the underdrain lines are sufficiently
large and numerous to adequately drawdown the stormwater in the sand filter.
Calculations are required per 15ANCAC 02H.1042(2)(f).
GEC Response:Drawdown calculations have been provided with the Water Quality
Calculations of the Storm water Management Report(page 30 of 161 of the PDF file
provided).
d) General MDC 3—Please ensure that the side slopes of the SCM embankment are at
least 3:1 (or demonstrate that the provided 2:1 side slopes will remain stable in perthe
MDC). Please revise as needed.
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GEC Response: The internal side slopes of the SCM embankment are 3:1 maximum. The
external slopes of the SCM are 2:1 and will be stabilized with rolled erosion control
product and sod upon reaching final grades to prevent anyerosion from taking place. On
the western side of the pond, the 2:1 side slope is a maximum of8 ft in height. The
limited height of the 2:1 side slope along with the immediate stabilization ofit should,in
my engineering opinion,yield the side slopes remaining stable after construction.
e) General MDC 4—Please provide calculations demonstrating that the inlet and outlet
of the SCM are stable in accordance with the MDC. Since this SCM is discharging to an
existing sewer system,please demonstrate that the ultimate outfall will b e stable in
proposed conditions. Please revise as needed.
GEC Response:Further topographic survey information is being obtained to confirm
these conditions.Additional calculations will be provided with the next submittal.
f) General MDC 8&9—Please clearly show the stormwater maintenance and access
easement in the main set of plans. Please ensure that the easement meets all of the
applicable requirements outlined in General MDC 8&9.
i.NOTE: The minimum width of the maintenance access must be at least 10 ft (the
plans appear to indicate 8 ft).
ii.NOTE: Slopes that are steeper than 3:1 cannot be included within the maintenance
access.
iii.NOTE: The maintenance access must extend to the nearest public right-of-way.
iv.NOTE: The entire footprint of the SCM system must be included in the access and
maintenance easement,plus an additional ten or more feet around the SCM to
provide enough room to complete maintenance tasks. This SCM system includes
the side slopes, forebay,riser structure, SCM device, and basin outlet, dam
embankment, outlet, and emergency spillway.
GEC Response: Astormwatermaintenance and access easement meeting MDC
requirements has been added to Sheets C-1.1, C-1.2, C-1.4, C-3.1, C-3.2,and C-3.4.
g) Sand Filter MDC 1 —Please provide the SHWT boring information within the
footprint of the SCM. It is noted that a SHWT is assumed for the southern portion of
the site based on boring information from borings B-30&13-31 (which are located
somewhat near the proposed SCM),however,the SHWT elevation needs to be
determined within the footprint of the SCM.NOTE: The boring logs for borings I 1,
I 2, &I-3 were not provided within the soils report.
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GEC Response: Full geotechnicalreport for this project has been included with this
submittal as Appendix B2 in the Storm water Management Report.
h) Sand Filter MDC 7—Please specify on the cross-sectional view of the sand filter the
depth of the sand layer over the underdrain pipe so the MDC can be verified.NOTE:
Do not forget to account for the slope of the underdrain line when determining the
thickness of the sand layer and invert of the underdrain line. Acommon mistake is to
only provide 12"of sand above the underdrain line at the outlet structure (and not
over the far end of the underdrain line).
GEC Response: Cross section detail of the stonnwatermanagement facility(SCM)has
been revised to specifya 6-inch gravel depth cover and a 12-inch minimum depth of the
sand layer above the underdrain on Sheet C-9 and Exhibit Ein StormwaterManagement
Report.
4. Stormwater Application
a) Section IV, 10, Receiving Water—Per Mills River's water supply watershed map,the
proposed SCM does not drain to Mills River(WS-Ili). Please correct this information
on the form. NOTE: If the site does actually drain to Mills River(WS-IQ)and does
not drain to a non-water supply watershed stream,the project will need to be
reviewed by Mills River(as water supply watershed areas are reviewed locally).
GEC Response: Application has been revised to correct the receiving water information
to unnamed tributary of the French Broad River.
b) Section IV, 10—In accordance with 15ANCAC 02H.1003(3)(b), off-site areas that are
not bypassed around the SCM need to be accounted for at their full build-out
potential. Full build-out potential can be determined by having the property owners
enter into a legal agreement limiting the amount of area and BUAdraining from the
off-site area to the SCM or by assuming that the off-site area is 100%BUA(its full
build-out potential). It is recommended to bypass off-site areas around the SCM.
Please revise as needed.
GEC Response: hnpervious area calculations have been revised to assume off-site areas
draining to the SCM are treated as 100%impervious to account for their full build-out
potential.Areas are noted on Exhibit C,Post-Developed Conditions Map, and within the
calculations in Appendix A of the Storm water Management Report.
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c) Section IV, 10—Please ensure that the information listed in this table only refers to Bua
draining to the proposed SCM and ensure that the sum of the BUAtypes equals the
totalBUAshown(61,125 sf+173,649 sf+195,856 sf+ 12,025 sf+4,141 sf+ 160,344
sf=607,140 sf*435,600 sf). Please revise as needed.
GEC Response: Information listed in this table has been revised to accommodate
necessary design changes. We have confirmed that the sum ofthe BUAtypes equals the
total BUA
d) While not required at this time, it is strongly recommended to revise the application
form to indicate that Publix is the property owner, as the property has been sold as of
5/21/2024(doing so will make the permit simpler as the"purchaser"language and
permitting conditions will not need to be included in the fmalpermit). If you are doing
this,please revise the Application Form, and provide the current property deed.
GEC Response: Revised application form has been sent to Publix for signature. The
updated application form and current propertydeed will be included in the next
submittal.
5. Supplement-FZForm
a) Cover Page
i. Line 2—The provided value, 16.43 ac, does not correspond to the value shown in
Section IV,7 of the Application. Revise either as needed.
GEC Response: Line 2 has been revised to match the application form.
ii. Lines 7-10—Please complete these items. It is noted that there are no surface
waters located on-site, however there are surface waters located within the vicinity
ofthe site. Please include the required vegetated setback width(See 15ANCAC
02H.1017(10))for Line 7 and answer the other lines(either"yes", "no"or"N/a")
based on the proposed project.
GEC Response: Per the USGS map provided via email by.k m Farkas on 6/27/2024, the
indicated blue streamline that is located near the project site is a roadside drainage ditch
within the NCDOTright-of-way, and should not require a vegetated buffer since it is not
a perennial waterbody,perennial stream, or intermittent stream as specified in 15ANCAC
02H.1017(10).As indicated in the follow up email from Tim Farkas on 6/27/2024, we will
be scheduling a site visit with a Asheville Regional Office representative to make a on-site
determination as to the classification of this roadside drainage ditch.
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b) Drainage Area Page
i. Please complete the entire site column. The entire site column is an accounting of the
entire project area, not necessarily the sum of the other columns. When this column
asks for the"drainage area",please list the entire site/project area(see Section IV, 7 of
the Application).NOTE: Typically, there are no off-site components to the entire site
column as off-site areas are not located within the site.
GECResponse: The form has been revised such that the `Entire Site"column has been
filled out.
ii. Please ensure that the drainage area 1 column reflects only the area that is draining to
the SCM. Any BUAthat is not draining to the SCM should not be included in this
column. See earlier comment with regard to Section IV, 10 of the Application.
GEC Response: The form has been revised such that the Drainage Area 1 column only
reflects the area that drains to the SCM.
iii. Lines 5-7—Please ensure that these values are indicated in square feet (and not acres).
GECResponse: Values have been updated and listed in square feet.
iv. DA1 Column, Line 20—This value should correspond to the design volume of the SCM
(volume that can be stored between the bottom of the sediment chamber/top of the
sand layer and below the invert of the lowest bypass device(i.e.,the sum of Lines 23 &
30 on the Sand Filter Page).
GEC Response: Value has been revised.
c) Sand Filter Page
i. Line 15—This item is required per General MDC 12.
GEC Response: Operations and Maintenance plan is being developed and will be
provided with the next submittal.
ii. Line 26—Per the provided stage-storage table,this value should be 2,307 sf(2,516 sf
is shown).
GEC Response: Value has been revised.
iii. Line 27—This item refers to the distance from the excavated bottom of the
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sediment chamber(Line 25)to the invert of the lowest bypass orifice/weir(Line 35).
Please revise as needed.
GEC Response: Value has been revised.
iv. Line 28—This item is Line 27 minus the depth of any permanent pool provided in the
sediment chamber(If there is no permanent pool in the sediment chamber,this value
will be the same as Line 27). Please revise as needed.
GEC Response: Value has been revised.
v. Line 32—Per the provided stage-storage table,this value should be 2,516 sf(2,838 sf
is shown).
GEC Response: Value has been revised.
vi. Line 33—This item refers to the distance from the excavated bottom of the sand
chamber(Line 31)to the invert of the lowest bypass orifice/weir(Line 35).
GEC Response: Value has been revised.
vii. Line 34—This item refers to the distance from the top of the sand layer to the
invert of the lowest bypass orifice/weir(Line 35).
GEC Response: Value has been revised.
viii. Line 37—Per the provided plans, the dispersal mechanism appears to be a pipe
network(if that is not an option,please indicate it in the additional information
section, Line 43).
GEC Response: Line 37has been updated to specify`,bipe network"per the included
plans.
If you have any questions, comments or need additional information, please feel free to contact
me at 678.984.4683 or [e-mail].
Sincerely,
Je erLo• rdo,PE
Managing Partner
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