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 these rules. 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 these rules. 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 in 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) 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 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.
|
(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 MTD 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 in subparagraph (d) of 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 width 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 a part of the stormwater management
program.
(e) Water quality riparian
buffer widths are measured from the top of 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 initial
permit coverage, 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 coverage under this
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
re-submittal 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;
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 coverage.
(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.
