23 - Operational Definition of MS4

memo

This memo provides operational guidance to City of Raleigh staff for interpreting “MS4”
as it is used in the City of Raleigh’s NPDES MS4 Permit No. NCS000245 and in the City’s
Stormwater Management Plan (SWMP). Many of the requirements of the City’s NPDES
MS4 permit apply only to the MS4 (mainly through the permit’s “minimum control
measures”). Because of the permit’s emphasis on the MS4, having a working definition
of Raleigh’s MS4 is important for planning and achieving operational compliance with
the City’s permit.
The City’s permit does not prohibit the City from having programs outside the MS4 that
are similar to permit-required programs. However, the only permit-required programs
that extend beyond Raleigh’s MS4 are Construction Site Runoff Controls and Post
Construction Site Runoff Controls, which apply to both Raleigh’s extra territorial
jurisdictional (ETJ) and the area within Raleigh’s corporate limits.
Further, this operational definition for Raleigh’s MS4 is not intended to apply to or limit
the City’s implementation of other programs outside its MS4 voluntarily or for
complying with other regulatory requirements. Such other City programs should not be
identified as being part of or required by the City’s NPDES MS4 permit.
Operational definition for Raleigh’s Municipal Separate Storm Sewer System (MS4)
Raleigh’s Municipal Separate Storm Sewer System (MS4) is the collection of stormwater1
conveyances2 that are:
a) within the City’s corporate limits, and
b) owned or operated by the City, and
c) under the City’s authority to control, and
d) designed or used for collecting or conveying stormwater, and

To Stormwater Management Division Staff
From S. Wayne Miles, PE, Stormwater Program Manager
Department Engineering Services
Date April 19, 2022
Subject Operational Definition for Raleigh’s Municipal Separate Storm Sewer
System (MS4) based on Raleigh’s NPDES MS4 Permit No. NCS000245
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e) any of the following3 City-owned or City-operated infrastructure:
o roads with drainage systems
o municipal streets
o catch basins
o curbs
o gutters
o ditches
o manmade channels
o storm drains
Notes:
1. Refer to the permit’s definition of “stormwater runoff.”
2. The permit does not define “conveyance(s)” per se, but uses this term in
defining “MS4” and other terms.
3. The permit wording is such that this list is not exclusive.

Basis for the Operational Definition for Raleigh’s MS4
1) Part I, Permit Coverage, Section D, of the City’s permit states that “the permit
covers activities … within the corporate limits of [the City of Raleigh]”:
This permit covers activities associated with the discharge of stormwater
from the MS4 that are under the permittee’ s authority to control within the
corporate limits of the permittee. Requirements under this permit for
Construction Site Runoff Controls and Post Construction Site Runoff Controls
also apply to the Permittee’ s Extra Territorial Jurisdictional (ETJ) to the extent
allowable under State and local law. The permit also applies to areas that
seek coverage under this permit through inter-local or other similar
agreements with permittee. Agreements for coverage under this permit must
be approved by the Division of Energy, Mineral and Land Resources, herein
referred to as the Division.
Interpretations:
• Raleigh’s MS4 exists only within Raleigh’s corporate limits (which change
with annexations into Raleigh’s corporate limits).
• Raleigh’s ETJ is not within or part of Raleigh’s MS4.
• City-owned land and City-operated facilities outside Raleigh’s corporate
limits are not within or part of Raleigh’s MS4.
• Stormwater conveyances on private properties in permanent easements
granted to the City are part of Raleigh’s MS4 because these conveyances
are owned and operated by the City and are under the City’s authority to
control.
• The City’s permit addresses Construction Site Runoff Controls and Post
Construction Site Runoff Controls as applicable to the area of Raleigh’s
ETJ in addition to the area within Raleigh’s corporate limits. In this
context, the area within Raleigh’s corporate limits sometimes is
informally referred to as “the MS4.” However, this informal use is not
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consistent with the permit definition of MS4 (see below) nor with the
operational definition for Raleigh’s MS4.
2) In Part VIII, Definitions, of the City’s permit, “MS4” is defined as:
Pursuant to 40 CFR 122.26(b)(8), means a conveyance or system of
conveyances (including roads with drainage systems, municipal streets, catch
basins, curbs, gutters, ditches, manmade channels, or storm drains):
a) Owned or operated by the city or town (created by or pursuant to
State law) having jurisdiction over disposal of sewage, industrial
wastes, stormwater, or other wastes, including special districts under
State law such as a sewer district, flood control district or drainage
district, or similar entity, or a designated and approved management
agency under Section 208 of the Clean Water Act (CWA) that
discharges to waters of the United States or waters of the State;
b) Designed or used for collecting or conveying stormwater;
c) Which is not a combined sewer; and
d) Which is not part of a Publicly Owned Treatment Works (POTW) as
defined in 40 CFR 122.2.
Interpretations:
• Not all stormwater conveyances within Raleigh’s corporate limits are part
of Raleigh’s MS4. Only conveyances that are within Raleigh’s corporate
limits and are under the City’s authority to control are part of the City’s
MS4. (The City’s Stormwater Management Plan, required by the permit,
defines “MS4” in this way.)
• Culverts that convey only stormwater runoff under streets within City-
owned rights-of-way or otherwise on City-owned property or within a
permanent easement granted to the City are stormwater conveyances
owned or operated by the City and are under the City’s authority to
control and therefore are part of Raleigh’s MS4.
• Culverts that convey stream or lake flows under streets within City-
owned rights-of-way or otherwise on City-owned property or within a
permanent easement granted to the City are not part of Raleigh’s MS4
because stream flow is not stormwater, and stream culverts are not
stormwater conveyances.

What is NOT part of Raleigh’s MS4?
1) City-owned or City-operated facilities that are outside Raleigh’s corporate limits
are not part of Raleigh’s MS4.
2) Stormwater conveyances (including infrastructure listed above) that are owned
or operated by NCDOT or by NCSU are not part of Raleigh’s MS4. (NCDOT and
NCSU have stormwater permits and MS4s of their own, which are not part of
Raleigh’s MS4.)
3) Stormwater conveyances on private properties are not part of Raleigh’s MS4,
except that stormwater conveyances in easements granted to the City are part
of Raleigh’s MS4.
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4) Streams and lakes that are mapped as solid or dashed blue lines on USGS 7½-
minute topographic maps are not part of Raleigh’s MS4 because:
a) For the most part, streams and lakes are not “manmade channels” and
therefore should not be considered stormwater conveyances, by
definition.
b) Water in streams and lakes should not be considered to be “stormwater
runoff.” Once stormwater enters a stream or lake, it becomes stream
flow or lake water and this water no longer should be considered to be
stormwater. Definitions of relevant terms in Part VIII of the City’s permit
support the conclusion that streams and lakes should not be considered
to be stormwater conveyances:
o “Stormwater runoff” is defined as:
The flow of water which results from precipitation and which
occurs immediately following rainfall or as a result of snowmelt.
[Note: Flow of water in streams and lakes occurs before and after
rainfall, not only “immediately following rainfall.”]
o Part I, Permit Coverage, Section A, of the City’s permit states:
…the City of Raleigh is authorized to discharge stormwater from
the municipal separate storm sewer system (MS4) to receiving
waters of the State within the Neuse River Basin.
o “Outfall” is defined as:
A point source as defined by 40 CFR 122.2 at the point where a
municipal separate storm sewer discharges to waters of the
United States and does not include open conveyances connecting
two municipal separate storm sewers, or pipes, tunnels or other
conveyances which connect segments of the same stream or other
waters of the United States and are used to convey waters of the
United States.
o “Point source discharge of stormwater” is defined as:
Any discernible, confined, and discrete conveyance including, but
not specifically limited to, any pipe, ditch, channel, tunnel, conduit,
well, or discrete fissure from which stormwater is or may be
discharged to waters of the state.
c) By definition of “outfall,” stormwater conveyances discharge into waters
of the State and therefore are exclusive of waters of the State.
Stormwater outfalls are the points at which drainage features cease
being stormwater conveyances. In general, drainage features
downstream of outfalls are waters of the State/US. A drainage feature
cannot be both a stormwater conveyance and waters of the State/US.
Therefore, streams and lakes are not part of Raleigh’s MS4.
d) Culverts that convey stream or lake flows under streets within City-
owned rights-of-way or otherwise on City-owned property or within a
permanent easement granted to the City are not part of Raleigh’s MS4
because stream and lake flows are not stormwater, and stream and lake
culverts are not stormwater conveyances.
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Document Revisions
The following table is a record of revisions to this document by the City of Raleigh
Stormwater Management Division.
Working Definition for Raleigh’s Municipal Separate Storm Sewer System (MS4)
Revision
Date
Description of
Revision(s) Reviser’s Name Reviser’s
Signature