20090063 Ver 2_Triage Comments_20100319

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Triage Comment Sheet
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DATE MAR-19-2010 FRI 16:27
NAME DWQ--WETLANDS
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North Carolina Department of Environment and Natural Resources
Pivieion of Wear hre1lY bse Freeman
gevery Ewes %rdue Coieen H. Sulft Secretary
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Division of Water Quality
Department of Environmental and Natural Resources
401 Oversight/Express Review, Program Development and
Transportation Permitting Units
Street Address: 2321 Crabtree Blvd. Suke 25% Raleigh, NC 276042250
Malang Address: 1650 man service Center Raleigh, NC 27699-1650
C01ktact100Mation: Phonetk919-733-1786
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Project Name: Rolesville Middle School
DWQ Project #: 09-0063v2
Date Received: 02/05/2010
Date of Comments: 03/19/2010
SMP & Diffuse Flow Comments:
Stormwater Comments:
1. Wet Pond:
a. Please note that wet detention pond alone does not achieve 30% Total
Nitrogen removal efficiency as required by the Neuse (NSW) stormwater
requirements. You may do one of the following:
i. Redesign the pond for 85% TSS and combine it with other
treatment device such as grassed swale (meeting swale seeking
credit design guidelines) or level spreader-vegetative filter strip
system (minimum 30 feet to wetland).
ii. Since the pond has been designed for 90% TSS (being larger than
85% TSS pond but less than area required for a stormwater
wetland), you may choose to redesign it as a pond-wetland hybrid
system. This can be achieved by creating a pond (large deep pool
of water) in the middle, widening the 10-ft shelf to about 15-20 ft
around the pool of water with the inner two-thirds as shallow water
and the outer one-third as shallow land.
b. Appropriate plants for the zones MUST be selected, and a detail planting plan
shall be provided. We believe if designed and constructed properly, this could
achieve the required 30% nitrogen treatment. Please refer to the stormwater
wetland section of the NC DWQ BMP Manual for plant selection.
c. Due to the close proximity of the wet pond to the wetland, coupled with the
type of soils at the pond location, this office recommends that a clay liner or
other engineering measure must be provided to avoid impacting the adjacent
wetland.
d. The 3" orifice appears to drawdown too quickly (1.75 days) by our
calculations. Please use a 2.5" orifice unless design changes. It is
recommended that you use a larger pipe with the orifice hole drilled in a cap
on the pipe for easy maintenance.
e. Please provide anti-flotation calculation for the outlet structure.
f. Please show the emergency spillway on the plan view
2. Level Spreader:
a. Please provide level spreader detail having all required components such
as forebay, blind swale/channel, level spreader lip (concrete) and a 3-ft of
#57 stone underlain by a filter fabric immediately downslope of the LS-
lip.
Bioretention Areas:
a. Please note that the required surface area at the top of the bioretention cells
shall be provided and measured at the top of the planting media and not the
top of ponded water surface (berm). This area shall be equal to the design
volume divided by the ponding depth.
b. Please provide a depressed type of dissipation device rather than a flat apron
at the pipe outlets into the bioretention cells.
c. Please specify the soil media mix, source of the media, phosphorus index (P-
Index) and they type of grass (Centipede or fescue/blue grass mix) on the
plans.
d. Please provide direct discharge for the larger storms to the stream. This can be
achieved with a pipe or riprap channel from an overflow device, and the flow
from the underdrain pipes must be connected to the bypass pipe/channel.
4. Operation and Maintenance (O & M) Agreement Forms: All sections and pages of
the form for each device must be provided. Only the notarized pages were
received for each device.