Current through September 24, 2024
Permits issued to entities that operate a municipal separate
storm sewer system (MS4) shall include the following effluent limitations to
manage post-construction stormwater at all new development and redevelopment
projects that disturb one or more acres of land, or less than one acre if part
of a larger common plan of development, and discharge into the permittee's
MS4:
(1) Permanent Stormwater
Management Program.
(a) The permittee shall
develop and implement a permanent stormwater management program to reduce
pollutants in stormwater discharges through management practices, control
techniques, and systems, design, and engineering practices implemented to the
maximum extent practicable (MEP), as set forth herein.
(b) The permanent stormwater management
program shall include plans review, site inspections, and a means to ensure
that permanent stormwater control measures (SCMs) are adequately operated and
maintained.
(c) The permittee must
develop and implement, and modify as necessary, an ordinance or other
regulatory mechanism to address permanent stormwater management at new
development and redevelopment projects.
(d) The permittee must submit an
implementation plan for its permanent stormwater management program not later
than 90 days after the effective date of the first new or revised permit issued
after the effective date of this rule. The implementation plan shall include a
brief description of the main components of the permittee's permanent
stormwater management program, which should include: codes and ordinance
development and implementation; procedures for plans review and criteria for
approval; procedures for conducting and tracking site inspections; and SCM
operation and maintenance policies. The implementation plan shall also include
a timeline to develop and implement the program. If the permittee has
implemented a permanent stormwater management program that complies with all
requirements of the new or revised permit, the permittee may submit an
implementation plan explaining how its program complies and identifying any new
or modified elements of its program. The schedule must indicate completion as
soon as feasible but no later than 24 months from the effective date of the
first permit issued after the effective date of this rule. Further, if
implementation will take longer than 12 months, the plan must include interim
milestones. Implementation plans must be submitted to the Division.
(2) Permanent Stormwater
Standards.
(a) The permanent stormwater
management program must require new development and redevelopment projects to
be designed to reduce pollutants to the MEP, as set forth herein. Compliance
with permanent stormwater standards for new development and redevelopment
projects is determined by designing and installing SCMs as established by this
rule and complying with other requirements of this rule. For design purposes,
total suspended solids (TSS) may be used as the indicator for the reduction of
pollutants.
(b) SCMs must be
designed to provide full treatment capacity within 72 hours following the end
of the preceding rain event for the life of the new development or
redevelopment project. The permittee shall identify a suite of SCMs to be used
in various situations. Information relevant to identified SCMs should be made
readily available. Application of innovative SCMs is encouraged. If the
permittee decides to significantly limit the number of SCM options, it must be
documented as part of the stormwater management program how the performance
standards of this rule can be met with the limited set of control measures that
are allowed.
(c) For the purposes
of this paragraph, the water quality treatment design storm is a 1-year,
24-hour storm event as defined by Precipitation-Frequency Atlas of the United
States. Atlas 14. Volume 2. Version 3.0. U.S. Department of Commerce. National
Oceanic and Atmospheric Administration (NOAA), National Weather Service,
Hydrometeorological Design Studies Center, Silver Springs, Maryland or its
digital product equivalent. The water quality treatment volume (WQTV) is a
portion of the runoff generated from impervious surfaces at a new development
or redevelopment project by the design storm, as set forth below. SCMs must be
designed, at a minimum, to achieve an overall treatment efficiency of 80% TSS
removal from the WQTV. The quantity of the WQTV depends on the type of
treatment provided, as established in the following table:
|
Water Quality Treatment Volume and
the Corresponding SCM Treatment Type for the 1-year, 24-hour design
storm
|
|
SCM Treatment Type
|
WQTV
|
Notes
|
|
infiltration, evaporation, transpiration, and/or
reuse
|
runoff generated from the first 1 inch of the
design storm
|
Examples include, but are not limited to,
bioretention, stormwater wetlands, and infiltration systems.
|
|
biologically active filtration, with an
underdrain
|
runoff generated from the first 1.25 inches of the
design storm
|
To achieve biologically active filtration, SCMs
must provide a minimum of 12 inches of internal water storage.
|
|
sand or gravel filtration, settling ponds, extended
detention ponds, and wet ponds
|
runoff generated from the first 2.5 inches of the
design storm or the first 75% of the design storm, whichever is less
|
Examples include, but are not limited to, sand
filters, permeable pavers, and underground gravel detention systems. Ponds must
provide forebays comprising a minimum of 10% of the total design volume.
Existing regional detention ponds are not subject to the forebay
requirement.
|
|
hydrodynamic separation, baffle box settling, other
flow-through manufactured treatment devices (MTDs), and treatment trains using
MTDs
|
maximum runoff generated from the entire design
storm
|
Flow-through MTDs must provide an overall treatment
efficiency of at least 80% TSS reduction. Refer to subparagraph (2)(d) of this
rule.
|
|
Alternative permanent stormwater standards that
provide equal or equivalent reduction of pollutants to the above may be
submitted to the Division for approval.
|
(d)
Treatment Train Calculations.
1. Treatment
trains using MTDs.
Treatment trains using MTDs must provide an overall treatment
efficiency of at least 80% TSS reduction utilizing the following
formula:
The calculation:
R = A + B - (A x B)/100
Where:
R = total TSS percent removal from application of both
SCMs,
A = the TSS percent removal rate applicable to the first
SCM, and
B = the TSS percent removal rate applicable to the second
SCM.
TSS removal rates for MTDs must be evaluated using
industry-wide standards.
TSS removal rates for other SCMs must be from published
reference literature.
2.
Treatment trains not using MTDs.
Treatment trains using infiltration, evaporation,
transpiration, reuse, or biologically active filtration followed by sand or
gravel filtration, settling ponds, extended detention ponds, or wet ponds may
subtract the treated WQTV of the upstream SCMs from the WQTV of the downstream
SCMs.
(e) The
permittee may also develop a mitigation program and/or system of payment into a
public stormwater fund as described in paragraph (3) of this rule.
(f) The permanent stormwater management
program may allow for a reduction of the WQTV for a new development or
redevelopment project up to 20% for any one of the following conditions, and up
to a total maximum of 50% for a combination of the following conditions:
1. Redevelopment projects (including, but not
limited to, brownfield redevelopment);
2. Vertical density (floor to area ratio of
at least 2, or at least 18 units per acre); and
3. Incentives as identified by the permittee,
submitted to the Division and approved by the Division in writing, and
documented as part of the stormwater management program.
(3) Stormwater Mitigation and
Public Stormwater Fund.
(a) A permittee may
choose to develop an offsite mitigation program or payment in lieu into a
public stormwater fund, or both, to offset the portion of the WQTV that cannot
be treated on site to the MEP. The program must ensure that off-site stormwater
mitigation will be accomplished within the same USGS 12-digit hydrologic unit
code watershed as the new development or redevelopment project, if practicable,
and will treat a minimum of 1.5 times the portion of the WQTV not treated on
site. The permittee may identify priority areas within the watershed in which
stormwater mitigation projects are to be completed. The program must have a
mitigation project approval procedure, and all projects must meet all
requirements in this permit. Procedures and requirements in the offsite
mitigation and payment in lieu programs should be documented as part of the
stormwater management program and available for review.
(b) If the permittee allows payment into a
public stormwater fund, the permittee assumes responsibility to provide the
required mitigation projects. The public stormwater fund should be used to fund
public mitigation projects. The payment amount into a public stormwater fund
must be sufficient to design, install, and maintain the stormwater mitigation
measures.
(4) Water
Quality Riparian Buffers.
Permittees shall develop and implement a set of requirements
to establish, protect, and maintain permanent water quality riparian buffers to
provide additional water quality treatment in riparian areas of new development
and redevelopment projects that contain streams, including wetlands, ponds, and
lakes. Riparian buffers must meet the following minimum standards:
(a) Stormwater discharges should enter the
water quality riparian buffer as sheet flow, not as concentrated flow, where
site conditions allow.
(b) Water
quality riparian buffers must have the following minimum widths, unless
site-specific conditions necessitate alternative widths, as described later in
this paragraph:
|
|
Average buffer width (feet)
|
Minimum buffer width (feet)
|
Notes
|
|
Waters with available parameters for siltation or
habitat alteration or unassessed waters
|
30
|
15
|
The criteria for the width of the buffer zone can
be established on an average width basis at a project, as long as the minimum
width of the buffer zone is more than the required minimum width at any
measured location. If the new development or redevelopment site encompasses
both sides of a stream, buffer averaging can be applied to both sides, but must
be applied independently.
|
|
Exceptional Tennessee Waters or waters with
unavailable parameters for siltation or habitat alteration
|
60
|
30
|
The predominant vegetation within the minimum buffer area
should be trees. The remaining riparian buffers may be composed of herbaceous
cover or infiltration-based SCMs.
(c) Permittees may establish permissible land
uses or activities within the buffer, such as biking and walking trails,
infiltration-based SCMs, selective landscaping, habitat improvement, road and
utility crossings, or other limited uses as determined by the permittee. The
permittee must have a process to review proposed activities within buffers to
ensure the pollutant removal function of the buffer will be retained. Trails
constructed within the buffer should prevent or minimize the generation of
pollutants. If trails are constructed from impervious materials, runoff must
either be directed to infiltration-based SCMs or the buffer width must be
increased by the width of the trail.
(d) Permittees may authorize alternative
buffer widths for new development and redevelopment projects where averaged
water quality riparian buffers cannot be fully implemented on-site. In order to
allow alternative widths, the permittee must develop and apply criteria for
determining the circumstances under which required buffer widths cannot be
achieved based on the type of project, existing land use, and physical
conditions that restrict the use of water quality riparian buffers. Any such
procedures and criteria for alternative buffer widths must ensure that
implementing full buffer widths would be impracticable and that the maximum
practicable buffer widths are required. Procedures and criteria for alternative
buffer widths must be submitted to the Division, approved by the Division in
writing, and documented as part of the stormwater management program.
(e) Water quality riparian buffer widths are
measured from the top of the bank also referred to as the "ordinary high-water
mark."
(f) Ordinances and local
requirements adopted prior to November 13, 2018, and that mandate minimum
30-foot water quality riparian buffers for drainage areas less than one square
mile, and minimum 60-foot water quality riparian buffers for drainage areas of
greater than one square mile (with provisions for buffer averaging down to a
minimum 30-foot width), are deemed to satisfy the conditions of this
paragraph.
(5) Codes and
Ordinances Review and Update.
(a) Within one
year of obtaining an initial permit, newly permitted programs shall review
local codes and ordinances using the EPA Water Quality Scorecard. A completed
copy of the Scorecard shall be submitted with the subsequent annual report.
Permittees who have completed and submitted the Scorecard in the past are not
required to repeat this review.
(b)
Newly permitted programs shall update codes and ordinances or other legal
instruments as necessary to comply with the permit within 24 months of the
effective date of the permit. Current permittees shall continue to implement
the existing permanent stormwater management program and update legal
instruments according to the compliance schedule in subparagraph (1)(d) of this
rule.
(6) Development
Project Plan Review, Approval, and Enforcement.
The permittee shall develop and implement project plan
review, approval, and enforcement procedures applicable, at a minimum, to all
new development and redevelopment projects, which shall include:
(a) Procedures for review and approval of
site plans, including inter-departmental consultations and a resubmittal
process when modifications to the project require changes to an approved site
design plan;
(b) A plans review
process that requires SCMs to be properly designed, installed, and maintained
to meet the performance standards established in this rule. The process must
also include incentives adopted by the permittee as authorized by paragraph (2)
of this rule, if any, along with water quality buffers as required by paragraph
(4) of this rule; and
(c) A
verification process to document that SCMs have been installed per design
specifications within 90 days of installation. Verification shall include
submission of as-built plans to the permittee, permittee inspection, or
inspection by a qualified design professional. The verification process shall
include enforcement procedures to bring noncompliant projects into compliance,
which shall be detailed in the enforcement response plan.
(7) Maintenance of Permanent Stormwater
Control Measure Assets.
(a) Permanent SCMs,
including SCMs used at mitigation projects, must be installed, implemented, and
maintained to meet the performance standards of paragraph (2) of this rule, and
provide full treatment capacity within 72 hours following the end of the
preceding rain event.
(b) The
permittee must develop and implement a program to require implementation of
appropriate SCM maintenance procedures to sustain pollutant reduction
efficiency for the life of the new development or redevelopment project. All
procedures, reports, and documentation must be maintained as part of the
stormwater management program. The program must include at a minimum:
1. The development and documentation of
maintenance and inspection procedures and frequencies for approved SCMs, which
shall require all SCMs to be inspected at least once every five years by the
permittee, a licensed professional engineer, a licensed landscape architect, or
other qualified professional familiar with applicable SCM design and
maintenance requirements or submit an alternative schedule to the Division for
approval;
2. The development and
documentation of the procedure the permittee will use to verify that SCMs are
being inspected and maintained including any written reports from the
responsible party;
3. A clear,
documented, legally binding agreement assigning SCM maintenance responsibility
to the owner/operator, a third party, or the permittee as appropriate. For SCMs
designed to manage stormwater from multiple properties, appropriate deed
restrictions shall be recorded; and
4. An allowance or agreement for permittee
personnel to access the SCMs for inspections and provide for enforcement action
for failure to maintain SCMs according to agreement.
(8) Inventory and Tracking of
Permanent Stormwater Control Measure Assets.
(a) Existing permittees must continue to
implement and maintain a system to inventory and track the status of all public
and private SCMs installed on new development and redevelopment projects. New
permittees must implement the system within 24 months of the effective date of
the permit.
(b) The inventory and
tracking system must be a searchable database, either paper or electronic, that
retrieves SCM information by location or other similar identification. The
system must be made available to the Division or to members of the public upon
request. Other than the basic information of location and project
identification, the system should include information and records the permittee
will use to demonstrate that SCMs are properly maintained, including but not
limited to:
1. A brief description of the type
of SCM and basic design characteristics;
2. The responsible party contact
information;
3. Inspection
schedules (both permittee and responsible party);
4. A brief description of or reference to
maintenance procedures and frequency;
5. Photographs of the installed SCMs;
and
6. Maintenance and inspection
records.
Authority: T.C.A. §§
4-5-201, et seq., and 69-3-101, et
seq.
