SWA000227_Response To Comments_20231031

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610 East Morehead Street P 704.602.8600
T I M M O N S GROUP Suite 250 F 704.376.1076
YOUR VISION ACHIEVED THROUGH OURS. Charlotte, NC 28202 www.timmons.com
October 30, 2023
NCDEQ Stormwater Comments
Subject: Response to Comments—Waxhaw Volunteer Fire Department
NCDEQ Project# SWA000227
Timmons Group received comments from Union County on June 23rd, 2023 regarding the
Waxhaw Volunteer Fire Department project. Timmons has modified the construction documents
to address the below comments and offers the following responses. The initial comments are
black italic while Timmons responses are shown in bold type.
Stormwater - Jim Farkas
[e-mail]
Review Comments
1. This project is located within the Waxhaw Creek Watershed and is subject to the Goose Creek
Rules. The Goose Creek Rules require the control and treatment of the difference in the pre-&post-
development 1-year,24-hour storm (instead of the 1.0"storm as in other parts of the State)and
requires infiltrating SCMs(such as infiltration trenches, bioretention cells(in A or B soils),
infiltrating permeable pavement, and Silva Cell unless it is demonstrated that these SCMs are not
practicable for use on-site. Please ensure that your design will meet the Goose Creek Rules upon
completion.
Response: The sand filter has been revised to treat the 1-year,24-hour storm. Pre-
development and post-development project area runoff calculations have been added to the
calculation book and the sand filter design spreadsheet has been revised.Infiltrating BMPs
were not considered for this site due to the significant presence of Type C and D soils.
2. Please clearly delineate the project area on the plans. This item is required per 15A NCAC
02H.1043(3)(f).Per the plans, the property area (4.0 ac) and clearing limits(3.35 ac) area shown,
but it is unclear where the project area boundary is.NOTE: While not required, it is typical for the
project area to correspond with the property area, that way, the entire project area is covered by
the permit. Please revise as needed.
Response: The project area boundary has been added to the Sand Filter No.1 Detail Sheet
(C604) and is 4.15 ac.
3. Application,Section E5- The project density does not appear to be calculated correctly. For a
project without existing BUA,project density is calculated as Proposed New BUA (49,361 sf)
divided by the Total Project Area (145,926 sf). 49,361 sf/145,926 sf 34.2%. Please revise as
needed.
Response: The BUA has been recalculated and revised both on the application and the Site
Plan sheet(C200).
CIVIL ENGINEERING I ENVIRONMENTAL I SURVEYING I GIS I LANDSCAPE ARCHITECTURE I CONSTRUCTION SERVICES

Design
1. From the States perspective, the proposed SCM is just considered a sand filter(which includes
both the layer of sand used for filtering and the detention area provided above the sand layer)not
as both a sand filter and dry detention pond.Please refer to Part C-6 of the Manual for more
information.
Response: Notations have been revised to"Sand Filter".
2. Sand Filter MDC 1 - You will need to provide at least one SHWT boring within the footprint of the
proposed sand filter. NOTE: When getting the SHWT boring,please ensure that either the existing
ground surface elevation or elevation of the SHWT is provided. Merely stating the depth from the
existing ground surface to the SHWT is insufficient to determine the SHWT elevation.
Response: The geotechnical report,including SHWT investigation,has been included with
this submittal and the elevation has been included.
3. Sand Filter MDC 2-Per the MDC, the storage volume provided in the sand chamber and sediment
chamber should be equivalent. The provided information appears to indicate the volume that can
be stored in the sediment chamber is larger than the volume that can be stored in the sand
chamber. NOTE: The sediment chamber can be oversized(please refer to the Manual). If doing so,
you must demonstrate that at least half of the design volume of the SCM can be stored in each
chamber(for example, if the design volume was 10,000 cf, a sand chamber that can hold 5,000 cf
and a sediment chamber that can hold 9,000 cf would be acceptable since each chamber holds at
least half of the design volume(5,000 cf)). NOTE: The design volume must be stored between the
"bottom"of the sand filter(elevation 577.0'per the plans)and the lowest bypass weir/orifice
(elevation 577.67'per the plans),storage provided within the sand layer and storage provided
above the bypass invert do not count towards the treatment volume provided by the SCM.
Response: The sediment chamber and sand chamber have been revised to hold equivalent
volume.
4. Sand Filter MDC 3- The treatment volume of this SCM will need to be based on the difference
between the pre-and post-development runoff from the 1-year,24-hour storm (since this project
is subject to the Goose Creek Rules).
Response: The sand filter has been revised to treat the 1-year,24-hour storm. Pre-
development and post-development project area runoff calculations have been added to the
calculation book and the sand filter design spreadsheet has been revised.
5. Sand Filter MDC 6- You will need to specify the type of sand that is to be used in the SCM(if this
was provided, I did not see it).
Response: A sand filter cross section has been added to the Sand Filter No.1 Detail Sheet
(C604).
6. It is recommended to reduce the amount of pervious areas draining to the sand filter as pervious
areas produce fines which clog the sand filter and necessitate more frequent maintenance which
will increase the cost of ownership for the SCM.
Response: The drainage area has been reduced in post-development conditions to primarily
contain the new development.
End of Comments&Responses
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