20061838 Ver 1_More Info Received_20061213

Mitchell Envit~onmental, P.A.
December 12, 2006
Ms. Lia Myott and Dr. C~•nthia Van Der Wiele
NC Division of Water Qualit<
401 Oversight /Express Review Permitting Unit
2321 Crabtree Boulevard
Suite 250
Raleigh, North Carolina 27604
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Re: Cedar Mountain Subdivision -Requested Additional Information
DWQ EXP No. 06-1838
Dear IVIs. M}'ott and Dr. Van Der VViele:
This letter is in response to your request for additional information for Cedar Mountain
Subdivision in Chatham Counh~, dated December 8, 2006.
1. Please provide fill-size nzups and site development plans as described in the PCN
Addendum.
Full-size maps are attached for your revietiv.
2. Stormwater Management: The impervious surface cover appears to i:ave been
calculated based on the ezztire 162-acre site. This project appears to be a phased
subdivision project and that the proposed development depicted or: the plans is
considerably smaller than 162 acres. Please recalculate the impervious surface area
based on the area of the tract you are developing. DWQ does require engineered
stor»:water managemezzt for pockets of l:iglt density even if the overall impervious
surface is below tl:e tlzreshofrl Please also include a breakdown of any lziglz-density
areas (e.g., the northeast corner of the site appears to be high densir)~).
Project Acreage and Phasing;
The proposed development area for Cedar Mountain Subdivision is 162.5 acres as
illustrated on the attached cover sheet and as confirmed by the survevor. The project will
be constructed on three separate but adjacent tracts that combined total the acreage listed
above. Please note that the west portion of one of these tracts (PIN 9746-85-6560. U00} is
currently being developed as Cedar Grove Subdivision by another owner. Additionally,
the same civil engineer is designing the infrastructure for both subdivisions. hence the
mislabeled title on the attached Plan & Profile sheet. This is likely causing the confusion
regarding phasing. To our knowledge, the owner does not intend to develop Cedar
Mountain Subdivision in phases. The vicinit<- map included on the attached Cover Sh et~~~
may help illustrate the actual boundaries of this project. ~-5 ~,~ ~ra~~
6\O2 East.4cademy Street, Suite 10~ Fuqu~•-i'arina, North Carolina 2?526
O~ce:919-55,%-,682 Fax: 919-~~?--1683 Rfobile:919-669-0329

Impervious Surface Area:
The proposed project imper\~ious surface area is correctly calculated based on the 162
acre project site. However. one mistake in our calculations was discovered while
preparing this response. The impervious surface area resulting from new roads in this
subdivision was incorrectly calculated using the right-of--way width instead of the actual
pavement width., hence. the proposed impervious surface area reported in the initial PCN
Application tivas higher than what is actually proposed. The corrected impervious surface
area calculations are:
Total Proposed Road Length = $,13 8 if
Pavement Width = 20 ft
Resultant Impervious Surface from Roads = 8.138 if x 20 ft = 162,760 ft2
Total Proposed Lots = 65
Estimated Impervious Surface Area Per Lot = 3.500 ft~
Resultant Impervious Surface from Lots = 65 lots x 3.500 ft'/lot = 227.500 ft'
Total Proposed Impervious Surface Area = 162,760 ft~ + 227.500 ft' = 390,260 ft'
or
8.96 acres
Percent Impervious Surface Area = ($.96 acres / 162.5 acres) x 100 = 5.5%
Hi~~h-Densiri~ Areas:
The areas of this project that appear to contain high-densit<~ lots are actually off-site
septic system nitrification fields. This was not clearly visible on the reduced plans that
were submitted on December 7`i'. However. the attached Cover Sheet will more clearly
illustrate the proposed use of these small lots. The shaded areas on the plan indicate soils
that are suitable for septic system nitrification fields. The soils in this area of Chatham
Count} are notoriously unsuitable for septic system nitrification fields, hence many
subdivisions in this area rely on off-site or remote septic system nitrification fields to
treat and dispose of residential waste. Note that each small lot is labeled with a number
and an `A". This label indicates the lot number it serves. For example, note that lot 3
has no shaded areas that are suitable for septic s}stem nitrification fields. The waste from
lot 3 will be pumped to lot 3-A for final treatment and disposal. These off-site lots are
tied b}' deed to the larger lot they serve. No impervious surfaces will be added to these
small lots.
3. The average stream widths indicated on the PCN application appear to be unusually
wide. Please recheck average stream width as compared with culvert design to ensure
adequate sizing.
The top of bank for both streams was located by Van R. Finch Land Surveys. PA. Both
streams are unusually wide and shallow due to a rock/boulder substrate. While the PCN
Application lists the small stream width as 7-feet, the average stream width in the vicinity
of the driveway crossing is closer to 4-feet. The 7-foot width indicates the maximum
stream width. in the vicinity of the driveway crossing. The engineer has confirmed that
pipe sizes are correct.

4. There appears to be a discrepancy between dre stream impact lengths on the PCNform
and file stream impact detail sheets. Please include any proposed rip-rap at the inlets
and outlets of the culverts in tlae stream impact length and that fire amount of impacts
noted and/or depicted on the plans match the amounts listed within the PCN
application.
Please note that the stream impact detail sheets include only the length of pipe proposed
for each stream crossing. The additional stream impact length listed in the PCN
Application accounts for stream impacts resulting from headwalls and endwalls
constructed at each crossing. The engineer did originall~~ include rip-rap outlet protection
for both of these stream crossings. Hot4•ever, at our request. the engineer has removed
these rip-rap pads in order to minimize stream impacts. Meadow Branch has a stone
bottom and should need no additional outlet protection. The smaller stream crossing is
for a drive«~ay that also does not warrant an outlet protection pad. Please note that there
is no full-size plan sheet attached for this drive«~av crossing, since none are typically
required for their construction.
I trust that this letter and the attached map adequately address your concerns. Please do not
hesitate to call me if you have amp questions or concerns about this project.
Sincerely,
uf.^~~~..~-__
Scott Mitchell, PE. LSS