20090259 Ver 1_More Info Received_20090420

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Response to Comments Dated 4/7/09 A PR 2 o zD
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1. Wet Ponds:
a. The average depths of both ponds are inadequate; North Carolina regulations?RBHCk
establish 3.0 feet as the minimum average depth for wet pond. Please redesign
each pond by increasing the nominal pond depth in order to increase the average
depth.
Response: The average depth calculations have been revised. The pond
bottom elevations were adjusted accordingly.
b. Please also note that, the computation of the average depth, and hence required
surface area of a wet pond, is an iterative (often a 2-step) process. For instance,
if assigned initial depth is 4.0 feet (i.e. assumed average depth), and the average
depth is computed as 3.22 feet, then a second SA/DA value shall be read
corresponding to the new average depth of 3.0 feet and the percent
imperviousness.
Response: The average depth calculations have been revised. The pond
bottom elevations were adjusted accordingly.
c. The orifice size for wet pond 1 is inadequate. For both ponds, please select an
orifice size in the range of 1.5 - 2.0 inches.
Response: The orifice size for Wet Pond #1 has been changed to 1.5"
d. This office recommends the use of a submerged orifice with a typical
configuration as shown in Figure 10.4 in the NC DWQ BMP Manual.
Response: The submerged low flow orifice detail has been added.
e. Since no soil report and seasonal high water table information were included in
the application, please specify on the construction plans that the soil material at
the bottom and sides of each wet pond shall be verified by a geotechnical
engineer to be a silty-clay or clayey material, and shall be compacted to minimize
infiltration and thereby maintain the water at the permanent pool level.
Response: The requested note has been added to the wet pond details. The
geotechnical report has been forwarded via email.
f. Please revise the anti-flotation calculations using the outer dimensions of the
riser structure for the computation of volume, and hence of water displaced.
Response: The anti-flotation calculations have been revised as requested.
Bioretention Areas:
a. Please revise the cross sections to depict the underdrains and cleanout pipes in
the plan views as close as possible. Please extend the underdrains along the
base of the cell as long as they could possibly be (to the left/west side) to clearly
mimic how they are shown in the plan view.
Response: The bioretention area sections have been revised as requested.
Some cleanouts have been left out on for clarity on the bioretention area 3
section.
Construction plans should describe the installation or construction sequence of the
BMP's. In particular, clearly describe how and when the Sediment Basins (S& EC
structures) would be converted to Stormwater BMP's.
Response: The construction sequence has been revised to describe the
conversion of sediment traps/basins over to permanent BMP's. The sequence has
been also been approved by Land Quality.
4. For FES 42 and FES 48, please design the outlet structure as a triangular or trapezoidal
riprap channel to dissipate as well as convey the water. Please show cross sections of
channels.
Response: Rip rap channels have been designed as requested. A cross section
has also been added.
5. Please provide an enlarged circular scour hole at the confluence of FES 21 and 48 to
prevent scouring in the receiving draw.
Response: A circular scour hole has been added in the existing draw as
requested.
6. Planting Plans:
a. Bioretention - Grass bioretention areas must be planted with sod as discussed in
the pre-application meeting. Prepared seed mixes are not acceptable.
Centipede grass sod is highly recommended.
Response: The groundcover for the bioretention areas has been changed to
centipede sod.
b. Wet Pond - Please identify the nursery source(s) of your proposed littoral shelf
plants. Provide sizing information for each plant (4" container, etc.). Additionally,
although the following plants are listed in the BMP manual, it is recommended
that you choose other options:
i. Zizonia aquatica (Wild rice) - bird fodder
ii. Helianthus angustifolia (Swamp sunflower) - anaerobic intolerant, poor
survivability
iii. Hibiscus moscheutus - no drought tolerance (recommend Hibiscus
coccineus or other choice)
iv. Hymenocallis euloe (spider lily) - does not grow in NC, also no local
nursery source
Response: The plants have been revised and planting sizes have been added.
Three potential nursery sources have been added to the plans.
Riparian Buffer Impacts - Please enumerate riparian buffer impacts on plan sheet C1201
in square feet. Converting submitted acreage to square feet does not match the square
footage listed on the PCN form. Also, in Stream Impact #1, 0.036 acres of Zone 2 is
mislabeled as Zone 1.
Response: The labels have been revised as requested. Acreages have been
converted to square feet.