SW6190503_2017.0643 Comment Response Letter_6/27/2019

INTERPLANJ
Over 40 years and 40,000 projects
June 27, 2019
North Carolina Department of Environmental Quality-
DEMLR
Corey Anen
512 North Salisbury Street -1612 Mail Service Center
Raleigh, NC 27699-1612
919.707.3649
[e-mail]
Reference: Rainbow Childcare - Spout Springs
Stormwater Permit # SW6190503
I P # 2017.0643
Response to: Request for Additional Information
Dear Mr. Anen,
Please accept the following in response to comments dated June 12, 2019:
604 COURTLAND STREET
SUITE 1DO
ORLANDO, FL 32804
ph: 407,645.5008
fx: 407.629.9124
AA 003420 1 CA 8660
www.interplantic.com
1. Comment: The stormwater narrative incorrectly states that the requirements for
stormwater treatment are to retain the 1-year storm, attenuate the 2- and
10- year storm, reduce total suspended solids by 85%, and reduce nutrient
loading rates. The Stormwater Management Regulations at 15A NCAC
02H.1000 require treatment of 1-inch rain event in a Primary SCM in
accordance with the Minimum Design Criteria laid out in 15A NCAC
02H.1050-.1062.
Response: The stormwater narrative and design are revised to comply with the NCAC
code, the NCDEP Stormwater Design Manual, and the Erosion and
Sediment Control Planning and Design Manual to provide treatment of the
runoff of 1 " of rainfall, reduction of the TSS runoff of the site by 857o and
erosion control for the 10-year storm event.
ORLANDO CHICAGO DALLA5IFOAT WORTH
ARCHITECTURE ENGINEERING INTERIOR DESIGN PERMITTING
PHILADELPHIA MIAMI
PERMITTING ENTITLEMENTS DUE DILIGENCE

INTERPLANJ
Rainbow Childcare- Spout Springs
June 27, 2019
Page 2of5
2. Comment: The stormwater narrative and supplemental forms use a variety of terms to
describe the stormwater control measures (SCM), such as a bio-retention
pond, dry pond, rain garden, bio- retention swale, and rain swale. It is
unclear what each supplement form refers to and which SCMs are
intended to provide treatment. For example, the supplemental form for a
dry pond is presumably referring to the system labelled "Bio-retention
Pond" in the northwest corner of the site. A supplemental fir was submitted
for a treatment swale, but no treatment swale is indicated on the plans.
Please use consistent, clear terminology that references the SCMs named
in the Stormwater Design Manual. Also note that dry ponds and treatment
swales are not primary SCMs and thus cannot be used alone to provide
treatment for runoff.
Response: The plans and stormwater narrative are revised to more clearly delineate
the method of stormwater treatment and erosion control. The proposed
design eliminates the small "bioretention" areas and, instead, the runoff of
all built -upon areas is directed to the forebay of an underground
detention system which is used for pre-treatment and stormwater
attenuation. The runoff is then directed into a large infiltration basin that is
designed to infiltrate the entire stormwater treatment design volume as
well as provide additional attenuation.
3. Comment: Please provide calculations for the water quality volume based on a
1-inch rain event using one of the allowable methods in Part B of the
Stormwater Design Manual.
Response: Stormwater treatment and erosion control computations are provided in
Appendix F of the attached stormwater narrative. The infiltration basin is
sized to treat over 100% of the DV and provide 85% TSS removal.
4. Comment: Please provide pre -development and post -development drainage area
maps for the site. The post -development maps should break down the
areas draining to each SCM.
Response: Revised pre -development and post -development drainage basin maps
are presented in Appendix D of the attached stormwater narrative.
5. Comment: The invert of both outlet pipes in Inlet #1 1 is the same. Is there a weir or
other diversion device that routs runoff in a particular direction?
Response: The design is revised to direct all runoff through the forebay of the
underground detention system which is a pre-treatment system for the
downstream infiltration basin.

INTERPLAN.
Rainbow Childcare- Spout Springs
June 27, 2019
Page 3 of 5
6. Comment: Many applicable fields in the Supplement EZ forms were left blank. Please
complete all of the fields in the Supplemental EZ forms.
Response: Revised Supplemental EZ Forms are attached documenting the Infiltration
Basin design.
7. Comment: For the infiltration system:
a.) The Supplemental EZ form indicates that the hydraulic conductivity of
the soil is 5-inches per hour; however, there is no mention of soil testing in
the geotechnical report. Soil testing is to verify the hydraulic conductivity
is required for all infiltration systems (Infiltration MDC 5).
b.) Pretreatment is required for all infiltration systems (Infiltration MDC 4).
c.) Calculations are required to show that the water quality volume will
drain within 72 hours (Infiltration MDC 5).
Response: The attached Supplemental EZ for is revised to include the infiltration rate
listed in the geotechnical engineering report. The infiltration rates in the
area of the infiltration basin ranged from 5 to 7 in/hr. Therefore, a design
rate of 6 in/hr is used. Pretreatment for the infiltration basin is provided in
the upstream underground detention system and forebay. The proposed
infiltration basin infiltrates the water quality design volume within 4 hours.
Water Quality computations are included in Appendix F of the attached
stormwater narrative.
8. Comment: For the bio-retention system:
a.) The detail on Sheet C4.1 shows the bio-retention media depth as 30
inches with mulch on the surface and the stormwater narrative indicates
that there will be trees in the bio-retention cell. The minimum media depth
for any bio-retention cell with trees or shrubs is 36 inches (Bio-retention
MCD 5). In addition, the landscape plan on Sheet Ll shows that the
bio-retention are will be Bermuda sod and does not show any trees.
Please revise whichever is incorrect.
b.) The Supplemental EZ form indicates that the hydraulic conductivity of
the soil is 5-inches per hour; however, there is no mention of the soil testing
in the geotechnical report. Soil testing is required to show that the in -situ
infiltration rate is at least 2-inches per hour. Otherwise, an underdrain with
internal water storage (IWS) is required for the bio-retention cell
(Bio-retention MDC 4). This is shown on the detail on Sheet C4.1 but it is not
shown on the plan and the Supplemental EZ form that indicates that there
is no IWS.
c.) Please specify on the plans that the media P-index shall not exceed 50
(Bio-retention MDC 7).

INTERPLANJ
Rainbow Childcare- Spout Springs
June 27, 2019
Page 4of5
d.) The detail on Sheet C4.1 indicates that the bio-retention cell surface is
at elevation 335.5, whereas the plans and Supplemental EZ form show
elevation 333.5.
Response: Bio-retention areas are removed from the design.
9. Comment: If the pond in the northwest corner is intended to be designed as a dry
pond, it must be designed with an outlet at the surface to allow it to fully
drain between rain events. If it is to be designed as a bio-retention pond
the peak attenuation orifice must be lowered to no more than 18 inches
from the bottom of the pond, and routing calculations must show that the
peak attenuation elevation from the design storm is no greater than 24
inches above the planting surface.
Response: The pond to the northwest of the parking area is designed as an infiltration
basin and supplemental stormwater attenuation meeting all requirements
of the Stormwater Design Manual. Water Quality computations are
included in Appendix F of the attached stormwater narrative.
10. Comment: Level spreaders are not required prior to discharging stormwater from an
SCM. In addition, runoff should not be discharged perpendicular to the
level spreader to avoid short-circuiting the level spreader. A rip rap apron
or other form of erosion control can be used in lieu of the level spreader;
however if a level spreader is to be used please provide calculations and
additional details to show that it meets the requirements of Chapter C-9 of
the Stormwater Design Manual.
Response: Riprap aprons are designed at the inflow to the infiltration basin / pond as
well as the pond outlet. The flow rates and tailwaters of the 10-year storm
event are used to size the aprons. Erosion Control computations are
provided in Appendix F of the attached stormwater narrative.
11. Comment: Please provide calculations that show that the inlet to the pond in the
northwest corner is designed to prevent erosion during peak flow from the
10-year storm. If a rip rap apron or other erosion control device will be
used for the outlet, please also provide calculations to show that it is also
sufficient (General MDC 4).
Response: See response to comment 10. Erosion Control computations are provided
in Appendix F of the attached stormwater narrative.

INTERPLAN.
Rainbow Childcare- Spout Springs
June 27, 2019
Page 5 of 5
12. Comment: A maintenance access must be shown on the plans. The maintenance
access must be at least 10 feet wide, have no greater than 3:1 slopes,
and extend to the nearest right of way. The maintenance access and
SCMs must also be shown in a permanent recorded easement
(General MDCs 8 and 9).
Response: Maintenance access is depicted on Sheet C1 of the revised construction
plans as requested.
13. Comment: Provide pdfs of all revisions, 2 hardcopies of revised plans, and 1 hardcopy
of other documents. PDFs must be uploaded using the form at:
https://edocs.deg.nc.gov/Forms/SW_Proiect_Submittal
Response: Hardcopies of plans and referenced attachments are included with this
submittal and uploaded electronically to above referenced website.
Please feel free to contact me if you have any questions or require additional information. If I
am not available, George Georgy is the Project Manager and will be able to answer your
questions.
Sincerely,
INTERPLAN LLC
Stuart Anderson, PE, LEED AP
Director of Civil Engineering