20100652 Ver 1_Buffer Determination_20080819

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Michael F. Easley
Governor
William G. Ross, Jr., Secretary
Department of Environment and Natural Resources
Coleen, tl. Sullins, Director
Division of water Quality
August 19, 2008
Buffer Interpretation/Clarification #2008-019
MEMORANDUM
RE: The Division of Water Quality's (DWQ's) stance on whether diffuse flow of stormwater through the newly restored buffers on
mitigation sites should be a requirement. Diffuse flow is a requirement for buffer restoration or enhancement in the Neuse River
Basin Buffer Rule 15A NCAC 02B.0242(9)(d)(iii), the Tar-Pamlico River Basin Buffer Rule 15A NCAC 02B.0260(9)(d)(iii), and tile
Catawba River Basin Buffer Rule 15A NCAC 02B.0244 (9)(d)(iii).
Diffuse flow is a requirement for all sites in a buffered basin for buffer mitigation and for for sites providing nutrient offset credit as
well.
Current Policy: According to the Mitigation rules in the Neuse, Tar-Pamlico and Catawba buffer rules, a grading plan must be
provided for buffer mitigation sites. In addition, those rules state that "The site shall be graded in a manner to ensure diffuse flow
through the riparian buffer",
Problem: The question has been raised as to whether stormwater carried by lateral ditches that enter buffered streams should provide
diffuse flow prior to that stormwater entering the restored buffers.
Solution: The Neuse, 'far-Pamlico and Catawba buffer rules with respect to buffer mitigation sites contain a very clear requirement
that states that diffuse flow of stormwater must be maintained through the buffer. Unless otherwise approved by DWQ, all buffer
mitigation sites must provide diffuse flow of stormwater from ditches and similar conveyances through the restored buffer.
Where such diffuse flow cannot be attained and where DWQ agrees that such treatment is not possible, deduction of butler credit will
be calculated as follows:
SCENARIO l
e mbrr DEDUCE
43,50 5F: 0.1 Ac
1
401 Wellands Certification Unit
1650 Mail Service Center, Raleigh, North Carolina 27699-1650
2321 Crabtree Boulevard. Suite 250. Raleigh, North Carolina 27604
Phone. [phone] / FAX [phone] / Internet: hllp;//h2o.enr.state.nc us/ncwetlands
Carolina
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Page 2 of 4
A, B and C are angles. a, b, and c are distances (lengths)
DWQ believes that using an immediate drainage area extending at a 60-degree angle from the point of discharge to the stream is a
reasonable approach to the issue of determining the area which is not draining through the restored buffer. To calculate the area of
buffer being "short-circuited" by the ditch, the area of the right triangles shown in the figure above must be determined.
a = 50'
A = 30°
13 = 60°
b=acotA
b = 50 (1.732)
b = 86.6' (87')
The area to be excluded from credit would be the area of the two right triangles:
Area = (a x b)/2
Area = (50 feet x 87feet)/2
Area = 2,175 SF
Total deducted area = 2,175 x 2 = 4,350 SF or 0.01 acres.
The example shown above assumes a buffer width of 50 feet from the top of bank (riparian buffer mitigation site). For nutrient offset
sites, credit can be generated out to 200 feet from the top of bank. The policy applies to sites with larger buffers as follows:
SCENARIO 2
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401 Wetlands Certification Unit
1650 Mail Service Center, Raleigh, North Carolina 27699-1650
2321 Crabtree Boulevard, Suite 250, Raleigh. North Carolina 27604
Phone: [phone] / FAX [phone] / Internet: hitt)•//h2o enr state nc us/ncwetlands
NorthCarolina
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Page 3 of 4
If a ditch leading to a buffered stream is buffered, then no credit is deducted from the stream buffer. If the upstream origin of the ditch
is within the buffer, no credit is deducted. If the upstream origin of the ditch is not buffered (e.g. if the ditch begins upstream offsite),
the credit deduction is applied to the most upstream portion of the ditch on the property.
SCENARIO 3
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Where a network of interconnecting ditches occurs on a site, and all of the ditches are buffered, the only credit deduction would be at
the point where an unbuffered ditch enters the project:
SCENARIO 4
401 Wetlands Certification Unit
1650 Mail Service Center, Raleigh, North Carolina 27699-1650
2321 Crabtree Boulevard, Suite 250. Raleigh, North Carolina 27604
Phone: [phone] / FAX [phone] / Internet: http://h2o,enr.stEte.nc.tis/ticwcllands
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NorthCarolina
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Where a natural stream enters the project site, no deduction of credit will occur. Also, when a natural stream or a modified natural
stream flow into a buffered stream, no deduction of credit will occur. The modified natural stream must be subject to the buffer rules,
and must be verified to be a modified natural stream (as opposed to a ditch) through an on-site determination by DWQ personnel.
SCENARIO 5
"DUN
1
NO"
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NO bebV DA
For any additional questions or clarifications on this issue, please contact Eric Kulz or Amy Chapman at [phone].
Signature: 4 Date:

Signature: Date: /-Zoo,S-Q

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401 Wetlands Certification Unitatli!ral[?
1650 Mail Service Center, Raleigh, North Carolina 27699-1650
2321 Crabtree Boulevard, Suite 250, Raleigh, North Carolina 27604
Phone: [phone] FAX [phone] / Internet: hI p•//h2o.enr.state.nc us/newetlands
An Equal Opportunity/Affirmative Action Employer- 50%Recycled/l0°/. Post Consumer Paper