20221200 Ver 1_Diffuse Flow Provisions_20220902

Options for Meeting Diffuse Flow Provisions of the
Stormwater and Riparian Buffer Protection Programs
March 7, 2014
Approvals: Bradley Bennett, DEMLR Stormwater Program Supervisor
Toby Vinson, DEMLR Acting Land Quality Section Chief
Matt Matthews, DWR Water Quality Permitting Section Chief
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Initials Date
4 3/7//V
.37»1
3/ 7/:_Q/
Effective immediately, the Division of Energy, Mineral and Land Resources and the Division of
Water Resources will consider Options 1-4 as meeting the diffuse flow provisions of the BOTH
the stormwater rules AND the various riparian buffer rules. Option 5 will be allowed for
meeting the diffuse flow provisions of the stormwater rules but is not currently allowed for
meeting the diffuse flow provisions of the buffer rules
The water quality design storm intensity for diffuse flow is the one inch/hour storm for
stormwater conveyed directly from the drainage area. If a diffuse flow device receives flow
from an upslope BMP, then it should be designed based on the peak discharge from the BMP
during the water quality design storm depth.
Flows in excess of the discharge rate from either the drainage area or the BMP should not be
directed to the diffuse flow device. Instead, excess flows should be discharged through the
buffer provided that the flows are non -erosive for the 10-year storm. Please note that any new
stormwater bypasses from a level spreader through a protected riparian buffer require a buffer
authorization from the Division of Water Resources or the local buffer protection authority.
Please see the options listed in the table below.
Any questions regarding the use of these diffuse flow options can be directed to Annette Lucas
at [phone].

March 7, 2014
Page 2 of 3
Options for Meeting Diffuse Flow Provisions of the
Stormwater and Riparian Buffer Protection Programs
Option
Explanation
1
Design a Level Spreader -Vegetated Filter Strip (LS-VFS) in accordance with
Chapter 8 of the BMP Manual. Per the BMP Manual, the LS-VFS may be
designed to handle the one inch/hour storm intensity and larger storms may
be discharged non-erosively. Also per the BMP Manual, alternate designs
that create uniform shallow flow across a vegetated filter strip in an
equivalent or better manner will be approved.
_
Provide a BMP that removes a minimum of 30% of TN removal outside of the
vegetated or riparian buffer to treat the design storm before it is discharged
through the buffer.
(For lower SHWT and soils with good infiltration rates) Design the vegetated
conveyance to infiltrate the design storm.
+4
(For high SHWT and/or soils with poor infiltration rates) Design the vegetated
conveyance as a wetland swale with an appropriately designed outlet system.
5
Swales that discharge less than 0.5 cfs during the design storm shall be
allowed to discharge through a stormwater buffer outside of SA waters
without any further requirements. This option is NOT currently allowed if the
buffer is protected by a state riparian buffer rule (Neuse, Tar -Pamlico, Jordan,
Randleman, Catawba, and Goose Creek).
Explanation
Diffuse flow is not defined in the 2H .1000 rules; however, the following explanation of diffuse
flow is provided in each of the Riparian Buffer Protection rules found in 15A NCAC 2B .0233
(Neuse River Basin), .0259 (Tar -Pamlico River Basin), .0243 (Catawba Mainstem), .0250
(Randleman Lake), .0605 (Goose Creek), .0267 (Jordan Lake).
(5) DIFFUSE FLOW REQUIREMENT. Diffuse flow of runoff shall be maintained in the
riparian buffer by dispersing concentrated flow and reestablishing vegetation.
(a) Concentrated runoff from new ditches or manmade conveyances shall be
converted to diffuse flow before the runoff enters the Zone 2 of the riparian
buffer.
(b) Periodic corrective action to restore diffuse flow shall be taken if necessary
to impede the formation of erosion gullies.
(6) TABLE OF USES. The following chart sets out the uses and their designation under
this Rule as exempt, allowable, allowable with mitigation, or prohibited. The
requirements for each category are given in Item (7) of this Rule.

March 7, 2014
Page 3 of 3
Options for Meeting Diffuse Flow Provisions of the
Stormwater and Riparian Buffer Protection Programs
Exempt
i Allowable
Allowable
with
Mitigation
[Prohibi1ted
Drainage ditches, roadside ditches and stormwater
outfalls through riparian buffers:
• Existing drainage ditches, roadside ditches, and
stormwater outfalls provided that they are
managed to minimize the sediment, nutrients
and other pollution that convey to waterbodies
■ New drainage ditches, roadside ditches and
stormwater outfalls provided that a stormwater
management facility is installed to control
nitrogen and attenuate flow before the
conveyance discharges through the riparian
buffer
New drainage ditches, roadside ditches and
stormwater outfalls that do not provide control
for nitrogen before discharging through the
riparian buffer
■ Excavation of the streambed in order to bring it
to the same elevation as the invert of a ditch
X
X
In the past, the state has interpreted the diffuse flow language in the buffer rules to mean that
designers have two primary options to provide diffuse flow:
▪ Provide a Level Spreader -Vegetated Filter Strip (LS-VFS) in accordance with Chapter 8 of
the BMP Manual. Per the BMP Manual, the LS-VFS may be designed to handle the one
inch/hour storm and larger storms may be discharged non-erosively. (Option 1 above)
• Provide a BMP that removes a minimum of 30% of TN removal outside of the vegetated
or riparian buffer to treat the design storm before it is discharged through the buffer.
(Option 2 above)
The above table provides three additional options. Options 3 and 4 are intended to provide
relatively low-cost approaches to providing treatment in the swale prior to discharge through a
buffer. Option 5 provides an exemption to the diffuse flow provision in the stormwater rules
for very low flows. As stated previously, the current wording in the riparian buffer rules does
not allow the Division of Water Resources to provide an exemption to the diffuse flow
requirements, even for low flows.