GREEN INFRASTRUCTURE PERMITTING AND ENFORCEMENT SERIES: FACTSMEET 2
United States
Environmental Protection
Agency
EPA832F12013
Combined Sewer
Overflows
This factsheet is the second in a series of six on integrating green
infrastructure concepts into permitting, enforcement, and
water quality standards actions.
Introduction
Evaluating the Potential of
Green Infrastructure for CSO Control
Developing Quantitative
Implementation Targets
Incorporating Green Infrastructure Page 5
Approaches into Long-Term Control Plans
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Integrating Green Infrastructure Concepts into
Permitting, Enforcement, and Water Quality
Standards Actions
This factsheet is the second in a series
of six factsheets in the U.S. EPA Green
Infrastructure Permitting and Enforcement
Series (http://water.epa.gov/infrastructure/
greeninfrastructure/gi_regulatory.
cfm#permittingseriesA This series
describes how EPA and state permitting and
enforcement professionals can incorporate
green infrastructure practices and approaches
into National Pollutant Discharge Elimination
System (NPDES) wet weather programs,
including stormwater permits, Total
Maximum Daily Loads (TMDLs), combined
sewer overflow (CSO) long-term control
plans (LTCPs), and enforcement actions.
This series builds upon EPA's continued
investment in green infrastructure and low
impact development. Existing EPA authority,
guidance, and agreements enable EPA
Regions and state agencies to work with
permittees to include green infrastructure
measures as part of control programs.
For additional resources on green infrastructure,
go to the EPA Green Infrastructure Web
page: http://water.epa.gov/infrastructure/
greeninfrastructure/index.cfm.
Key green infrastructure guidance issued
to date can be found at: http://water.epa.
gov/infrastructure/greeninfrastructure/
gLpolicy.cfm.
Combined Sewer Overflows (CSOs)
Introduction
Green infrastructure can reduce the volume of water going
into combined systems during precipitation events, which
may reduce numbers and volumes of overflows. Green
infrastructure can also slow the delivery of wet weather
flows to sewer systems, helping to mitigate peak flows
while providing filtration through soil for some portion of the
release into the sewer system, thereby reducing pollutant
= loads. The implementation of
green infrastructure practices
may allow communities
to downsize certain grey
infrastructure components of
their CSO control plans. This may
provide some CSO communities
with significant cost savings.
EPA GUIDANCE: CONSIDER
SOURCE CONTROLS
Existing EPA guidance
states that, as part of the
"Identification Control
Alternatives" for inclusion in
CSO LTCPs, CSO communities
must consider source controls,
which are defined specifically
to include green infrastructure
approaches (Combined Sewer
Overflows Guidance for Long-
Term Control Plan, EPA832-B-
95-002, at pp. 3-31 -3-33).
Under the Clean Water Act
and EPA's 1994 CSO Control
Policy, most CSO communities
are required to develop and
implement a Long-Term Control
Plan(LTCP)to restore and
protect water quality. National
Pollutant Discharge Elimination System (NPDES) permits
and administrative or judicial orders establish requirements
for developing and implementing LTCPs. There is also
existing guidance on development and implementation of
LTCPs (see sidebar below).
Existing Guidance on Development and Implementation of LTCPs
PERMITTING: http://cfpub.epa.gov/npdes/home.cfm?program_id=5
ENFORCEMENT: http://www.epa.gov/compliance/resources/policies/civil/cwa/
csosso-guidelines-enf.pdf
CSO POLICY: http://cfpub.epa.gov/npdes/cso/cpolicy.cfm
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GREEN INFRASTRUCTURE PERMITTING AND ENFORCEMENT SERIES: FACTSMEET 2
Evaluating the Potential of Green Infrastructure for CSO Control
In many cases planning for the use of green and grey
infrastructure will be most effective if both elements
are integrated throughout the planning and engineering
design processes. Therefore, it is recommended that
communities carry out integrated green/grey planning
to identify opportunities to use green infrastructure in
cost-effective combinations with grey infrastructure. This
can help lower upfront and/or operational costs. If, for
example, a community does engineering analyses to plan
grey infrastructure, sized to achieve high levels of control,
and then adds green infrastructure as a layer near the
end of the planning process, the community may conclude
that green infrastructure does not appreciably increase
the level of control. However, if planning specifically
encompasses green and grey infrastructure together
throughout the process, it is likely the planning will reveal
many opportunities to use green infrastructure to keep
water out of the system in some or all sewersheds. By
capitalizing on opportunities to place green infrastructure in
sewersheds, communities may be able to reduce the size of
grey infrastructure controls.
This is not meant to imply that grey infrastructure controls
are not needed; in most communities green infrastructure
alone will not resolve CSO problems for large storms.
Depending on land uses, land owners, and other variables,
some sewersheds are well-suited for green solutions
whereas others may provide less opportunity. Therefore,
stormwater reduction analyses typically should be
considered sewershed by sewershed. Estimating the
maximum or optimal amount of green infrastructure that
can be implemented in a sewershed requires an analysis
of land use and technical/environmental factors such
as soil types and topography, as well as institutional
considerations, such as the need to develop incentives to
facilitate implementation of green infrastructure features
on private property.
Development of CSO LTCPs involves analysis of the
financial capability of the community and analysis of
alternatives for reducing CSO frequencies, volumes, and
pollutant loads. Historically, grey infrastructure approaches
and operational enhancements have been the key
components of LTCPs. Recently, there has been greater
interest in using green infrastructure approaches, often
in combination with grey infrastructure and operational
enhancements, to meet CSO control needs. This approach
may have the advantage of distributing the cost of control
more broadly, rather than relying solely on utility ratepayers.
For example, if a green streetscapes project is implemented
it may be possible to cost-share between the stormwater
or CSO authority and a transportation organization. In
other cases a school or park district may cost-share with
the local stormwater/CSO authority. Additionally, several
recent CSO consent decrees have required the retrofitting
of sizeable areas with green infrastructure as part of holistic
approaches to CSO reduction.
(See Supplement 1).
Case Study of the Impacts of Trees and Green Roofs on Stormwater Runoff
Various organizations and communities have recently conducted studies to estimate the potential for
reducing flows into combined sewer systems through systematic use of green infrastructure practices.
In 2007, Casey Trees and LimnoTech, with funding from EPA, conducted a modeling study of the impacts
of trees and green roofs on stormwater runoff in the Washington, DC area (http://caseytrees.org/
programs/policyadvocacy/). The Casey Trees modeling estimated, upon completion of implementation
of green infrastructure projects:
• For an average year, the intensive greening scenario would prevent over 1.2 billion gallons of stormwater
from entering the sewer systems, resulting in a reduction of over 1 billion gallons in discharges to local
rivers.
• For an average year, the moderate greening scenario would prevent over 311 million gallons of
stormwater from entering the sewer systems, resulting in a reduction of 282 million gallons in discharges to local waterways.
• With the intensive greening scenario, installing 55 million square feet of green roofs in the Combined Sewer System (CSS) area
would reduce CSO discharges by 435 million gallons, or 19%, each year.
The initial round of modeling focused onlyongreen roofs and enhancing the urban tree canopy. Further work was then done to model
the effects of other green infrastructure components in the Washington D.C. service area. Other communities and regional sewer
authorities that have incorporated green infrastructure controls in their CSO planning include New York, Cincinnati, Louisville,, Omaha,
San Francisco, Kansas City, and Cleveland.
Figure 1: A bioretention cell
absorbs runoff.
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Developing Quantitative Implementation Targets
Once a community has evaluated the potential of green
infrastructure practices for CSO control, and determined
green infrastructure practices can be a cost-effective
component of an LTCP, it is important to identify the
locations for green infrastructure implementation and
to quantify the projected level of green infrastructure
implementation. A community can identify what green
infrastructure of what size/capacity can be put where in a
sewershed, and can then determine what level of reduction
that will achieve in terms of wet weather flows entering the
sewer system. The new flow information can then be used
in the sizing of grey infrastructure. See Supplement 3 for
a summary of tools and calculators that are available to
help quantify the impacts of green infrastructure.
Once a community has completed a desktop analysis
identifying priority sewersheds for green infrastructure
implementation, a more detailed analysis must be
completed to establish a quantitative green infrastructure
implementation target. A discussion of alternative analysis
methodologies is beyond the scope of this document. In
general, however, the methodology should first develop a
set of green infrastructure scenarios, and then assess the
outcomes associated with each scenario. The scenario
that best meets the community's needs may be adopted as
an implementation target. Ideally, the methodology should
allow the community to compare the cost-effectiveness
of each alternative in meeting CSO control targets, and
the range of environmental benefits provided by each
alternative. The checklist on Page 5 provides a general
methodology for establishing a quantitative green
infrastructure implementation target. Note that this is only
one of many approaches that a community might take.
The implementation target identified may call for many
decentralized green infrastructure practices. In a permit
or enforcement action, it will be important to include
appropriate provisions to ensure the decentralized
practices (many of which will not be on land owned/
controlled by the sewer authority) are properly installed,
preserved overtime, and maintained.
Many communities have identified municipally-owned
properties and road right-of-ways, and other parcels that
may be well-suited for green infrastructure practices, (e.g.,
corporate campuses, school campuses, and vacant parcels
where there is no near-term demand for redevelopment).
These communities have quantified the flow volumes that
could be managed at these sites, and then incorporated
the results into planning of the complementary grey
infrastructure controls.
Figure 2: Stormwater park at Saylor
Grove in Philadelphia
Also, important factors in some sub-watersheds may
be the preservation or enhancement of natural green
infrastructure, including features like riparian buffers, forest
preserves, floodplains, wetlands, and parks. In estimating
flows coming out of a sewershed, the capacity of such
areas to absorb stormwater flows needs to be considered. It
may be appropriate to incorporate the need to preserve, and
in some cases enhance such areas in a LTCP.
In some urban areas, a
city or sewer authority
may determine that it will
focus on relatively larger
green infrastructure
practices, perhaps at
the block scale, and will
setup ownership and
operation of the sites
and practices underthe
direct control of the city
or sewer authority. An example of this would be where a city
constructs "stormwater parks" to store and infiltrate wet
weather flows (see Figure 2). With an approach like this, the
capacity of the practice can be readily determined, much
like a detention pond, and green infrastructure plans and
commitments can reflect the number, locations, and sizing
of the larger-scale green practices. Stormwater parks can
be planned at strategic locations in the sewer network, and
where they fit well into the fabric of the community area.
Using larger scale green infrastructure practices, where the
city or sewer authority retains control over the practices,
may be advantageous for a community in terms of assuring
the practices are properly built, preserved, and maintained.
Adaptive management approaches can be used during LTCP
implementation to ensure green infrastructure measures
are being implemented and are working to the degree
expected (see further discussion below). Closely monitoring
green infrastructure implementation and performance is
important to ensure the projected levels of storage and
control are being achieved. Mid-course adjustments can be
made if necessary. The monitoring of implementation and
performance coupled with the use of adaptive management
approaches — making adjustments to future efforts
based on lessons learned — can help alleviate possible
uncertainty or perceived risks about implementing green
solutions as part of a CSO control program.
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GREEN INFRASTRUCTURE PERMITTING AND ENFORCEMENT SERIES: FACTSMEET 2
A General Methodology for Establishing a Quantitative Green Infrastructure Implementation Target
] Select a sample set of sewershedsthatare generally representative of the service area as a whole, in terms of land uses, land
ownership, soils, and topography.
] Characterize existing land use/land cover in the subwatersheds; this can often be done using aerial photographs and/or a
community's geographic information system (GIS) coverages.
Q Create templates for the various land uses in the sewersheds (e.g., typical single family residential lot, typical commercial/office
site). Estimate the pervious and impervious areas for the templates.
] Identify green infrastructure opportunities for the different land use categories (templates) in the sewersheds, taking into account
space needs, soil types, and slopes.
] Estimate the total green infrastructure that could be implemented in the sewershed by extrapolating from the templates to the
sewershed as a whole. This estimate should take into account current and future zoning and institutional considerations, such as
acceptance by property owners of green infrastructure features on private property. The level of buy-in to the green infrastructure
program on the part of local property owners is an important variable, and needs to be explicitly considered in CSO planning. The
estimate should also consider public properties and parks that may be good candidates for green infrastructure practices.
] Examine the cost-effectiveness of green infrastructure approaches. Will the green solutions reduce upfront or operational costs?
Experiment with various combinations of green and grey infrastructure to determine what combination results in the lowest costs.
] Estimate the green infrastructure opportunities for the CSO service area as a whole by extrapolating from the sample set of
sewersheds studied.
] Estimate the stormwater volumes that can be kept out of the system by the green infrastructure, taking into account the level of
estimated implementation and the size of the practices. Also consider if there should be a margin of safety to reflect actual green
implementation that may vary from projections, especially for sites not underthe direct control of the sewer authority.
Incorporating Green Infrastructure Approaches into Long-Term Control Plans
Green infrastructure components should be explicitly
identified and accompanied by compliance schedules in
LTCPs along with grey infrastructure components. A list of
the items that should be included in a LTCP if a community
chooses to utilize green infrastructure measures is
provided in the checklist on Page 6.
The timing for green infrastructure implementation should
be expressly considered in CSO planning. Some green
infrastructure benefits will probably be realized sooner than
those for grey solutions, while others maytake longer. It is
important to achieve a reasonable balance while keeping
in mind the overall environmental objectives. Discussion
of these items and how they will be addressed in the LTCP
should be done jointly between the community carrying out
implementation and the permitting/enforcement authority.
As a companion to LTCP implementation, CSO
communities planning for significant green infrastructure
implementation should:
• Develop strategies or standard operating procedures
(SOPs) for green infrastructure implementation;
• Consider approaches for dealing with legal and
institutional issues including updating codes and
ordinances;
• Consider changes to fee structures to incentivize green
infrastructure;
• Consider how they will work to systematically install
green infrastructure on different types of sites, e.g.,
municipally-owned public sites, schools, park district
sites, corporate sites, and residential properties. The
issuesthatwill be encountered in putting rain gardens
in parks or schools will be very different from the issues
to be dealt with in getting green roofs on public and
private buildings.
SOPs can help communities plan for and implement
effective approaches to place green infrastructure at
different types of sites within their service area.
Preservation of green infrastructure sites and practices
In addition to including provisions for operation and
maintenance of green infrastructure practices, permits, and
enforcement actions also need to consider mechanisms
to assure green infrastructure is preserved (i.e., that
a site or green infrastructure practice is not changed
or removed at some point in the future). For example,
language in a general permit issued by Ohio EPA specifies
that protection (preservation) of infiltration areas shall be
by binding conservation easements that identify a third
party management agency, such as a homeowner or
condominium association, political jurisdiction, orthird
party land trust. See: http://www.epa.state.oh.us/dsw/
permits/GP_ConstructionSiteStormWater_Darby.aspx.
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Including Green Infrastructure in LTCPs
Green infrastructure components should be explicitly identified and accompanied by compliance schedules in LTCPs along
with grey infrastructure components.
The following should be included in an LTCP with green infrastructure:
Q The planned (and quantified) level of green infrastructure implementation (what will be installed where, e.g.,
number of infiltration practices to be installed and associated sizes/capacity);
Q Key implementation steps (actions);
Q Sequencing (ensure green and grey elements fit together; also in many cases it may work well to start in upstream
areas and work toward downstream areas);
Q Schedule;
Q Methods and milestones for tracking and reporting on green infrastructure implementation (are the green
infrastructure practices going in as planned and scheduled);
QJ Requirements to assure appropriate operation and management (O&M) of the green infrastructure;
n Methods for monitoring the performance and effects of green infrastructure implementation (e.g., are individual
practices working as planned, are collections of practices in a sewershed keeping flows out of the sewer system
as projected);
Q Provisions for adaptive management/corrective actions if green infrastructure performance (at the site scale and/
orthe sewershed scale) does not meet expectations
Green for Grey Substitutions
In some cases much of the foundational planning and
engineering work on CSO controls may have focused on
grey infrastructure practices, but well into CSO planning
work the idea of incorporating green infrastructure
into the LTCP may have been raised. In these types of
situations it may be appropriate in a permit or enforcement
action to include provisionsthatwould govern a possible
substitution of green infrastructure control measures
for grey infrastructure control measures. The Consent
Decrees dealing with CSOs in the Kansas City, Missouri
and Cleveland, Ohio areas are examples of agreements
that include provisions for green for grey substitutions.
Supplement 2 provides example language which
addresses some of the issues that may be associated with
green for grey substitutions.
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GREEN INFRASTRUCTURE PERMITTING AND ENFORCEMENT SERIES: FACTSMEET 2
Monitoring and Evaluating Green Infrastructure Performance
Permits and enforcement actions that include green infrastructure measures should include provisions for evaluating
the performance and effects of installed green infrastructure control measures. These provisions would be an essential
component of post-construction monitoring required for CSO control practices. It may also be appropriate to include
requirements for corrective action implementation if green infrastructure practices do not perform as projected.
Following is example language to address post-construction monitoring for green infrastructure practices: [http://www.
ohioenvironmentallawblog.com/uploads/file/NEORSD%20Green%20infrastructure%20CO.pdf]
"The Sewer District shall submit a plan for performing
green infrastructure post-construction monitoring
("GIPCM") at two scales: (a) site or practice scale; and
(b) sewershed scale. The monitoring shall be planned
to evaluate the performance and effectiveness of
the green infrastructure control measures, as further
defined below. Once approved by EPA and the State,
the District shall implement the GIPCM program in
accordance with the approved GIPCM plan. The
District shall submit green infrastructure post-
construction monitoring reports providing the results of
the GIPCM programs to EPA and the State.
a. The site or practice scale GIPCM program
shall evaluate the effectiveness of the green
infrastructure control measures on a site-specific
scale. The GIPCM plan shall set forth the ways
the various types of green infrastructure control
measures to be implemented (e.g., constructed
wetland, etc.) will function to control wet weather
flows (e.g., through storage, infiltration, and/
or evapotranspiration), and the monitoring/
assessment methods that will be used to evaluate
the performance and effectiveness of the various
types of practices. The GIPCM plan shall set
forth the District's methods and procedures for
evaluating the performance of green infrastructure
control measures on a site-specific scale, such as
monitoring practices during and after rain events
to gauge storage and/or infiltration performance.
The GIPCM plan shall establish procedures for
conducting performance evaluations on the fully
constructed and operating green infrastructure
control measures. Underthe site-specific
program, performance evaluations shall assess
the effectiveness of the practices in terms of the
functions the green infrastructure control measure
was intended to fulfill (e.g., storage, infiltration).
Each site-specific green infrastructure control
measure (or a representative sample if similar
practices are installed at similar sites) shall be
monitored for a minimum of 12-months immediately
following implementation.
b. The sewershed-specific GIPCM program shall
set forth the steps the District shall take to
evaluate the performance and effectiveness of
green infrastructure measures on a sewershed
scale. Examples of such methods and procedures
include collecting rainfall and wet weather flow
data sufficient in scope and detail to allow: (i)
characterization of the performance of the green
infrastructure measures in a sewershed, and (ii)
hydrologic adjustment of the sewershed portion
of the collection system model to determine the
impacts of the green infrastructure measures
on system performance within the subject
sewershed. The District shall adjust the hydrologic
model parameters directly related to the green
infrastructure control measures as necessary to
accommodate changes in model parameterization
caused by shifts in runoff hydrology from the green
infrastructure measures. The District shall then
use both the appropriate CSO model without the
green infrastructure measures, and the model that
includes the green infrastructure measures, to
simulate the sewershed's typical year performance
both with and without the green infrastructure
measures in order to demonstrate the CSO volume
reduction.
If the green infrastructure post-construction
monitoring report submitted by the District fails to
demonstrate that the green infrastructure control
measures have met the performance criteria
specified for such control measures, then within
180 days of submission of the report, the District
shall submit to EPA and the State a corrective
action proposal. The corrective action proposal
shall define the green or grey infrastructure
enhancements/expansions to be carried out to
address performance shortcomings and ensure the
performance criteria are met. The proposal shall
include a schedule for completion of all corrective
action measures and an updated post-construction
monitoring plan to evaluate whether the corrective
actions have resulted in the performance criteria
being met. The performance criteria forthe green
infrastructure sites/practices must be achieved
within [XX] years of entry of the Consent Decree."
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Green Infrastructure Permitting and Enforcement Series
This series on integrating green infrastructure concepts into
permitting, enforcement, and water quality standards actions
contains six factsheets plus four supplemental materials
that can be found at http://water.epa.gov/infrastructure/
greeninfrastructure/gi_regulatory.cfm#permittingseries.
Factsheets
1. Potential Challenges and Accountability Considerations
2. Combined Sewer Overflows
3. Sanitary Sewer Overflows
b. Stormwater
5. Total Maximum Daily Loads
6. Water Quality Standards
Supplemental Materials
1. Consent Decrees that Include Green Infrastructure Provisions
2. Consent Decree Language Addressing Green for Grey
Substitutions
3. Green Infrastructure Models and Calculators
b. Green Infrastructure in Total Maximum Daily Loads (TMDLs)
United States
Environmental Protection
Agency
For additional resources on green infrastructure,
goto the EPA Green Infrastructure Web page:
http://www.epa.gov/greeninfrastructure/.
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