CHAPTER 2

Literature Review

This chapter gathers and summarizes information regarding stormwater retrofits and partnerships. The sources of the information include academic literature and guidance documents from federal and state environmental and transportation agencies. Much of the information was identified using web-based searches through the Prospector search engine of the Colorado Alliance of Research Libraries. The majority of the items found using Prospector were available online and were accessed through the Morgan Library at Colorado State University in Fort Collins or the Arthur Lakes Library at the Colorado School of Mines in Golden. Secondary searches were undertaken at various websites operated by the EPA, TRB, the Mile High Flood District, and the National Transportation Library.

As there is considerable literature on water resources and water quality, specific search terms were necessary for identifying pertinent work on stormwater retrofit partnerships. Obvious terms, such as retrofit, partnership, and water quality agreements, were used to find published material. Next, themes that might drive partnerships were explored. These included the watershed approach, offsite flow, water quality financing, stormwater credits, and regional water quality. Sometimes, references included in publications identified through a search engine led to further material for review.

As part of the survey questionnaire, state DOT respondents were provided the opportunity to submit unpublished guidance and policy documentation, and these materials were included in the literature review. Several state DOT websites with considerable water resource material were reviewed for pertinent material beyond what was submitted with survey responses.

2.1 Watershed Approach

There is limited literature on the use of third-party partnerships for permanent stormwater management facility installation and maintenance. Partnerships are often based on one or both of two factors: facilities that treat shared flows within watersheds and shared financing or operation of facilities. Often, the shared treatment of stormwater is articulated as the watershed approach. Notable papers regarding the watershed approach published by the EPA include Watershed Approach Framework (EPA 1996) and Handbook for Developing Watershed Plans to Restore and Protect Our Waters (EPA 2008). The 1996 work provides the guiding principles for managing stormwater quality on a watershed basis, whereas Chapter 3 in the 2008 work details techniques to build partnerships for watershed planning. Both works include a discussion of low-impact development and green infrastructure practices. These practices decentralize stormwater features and attempt to mimic natural processes by having the runoff infiltrate the groundwater high in the drainage basin, rather than “at the end of the pipe.”

The EPA, in their guidance for MS4 permit improvements, provides the following recommendations for both retrofits and partnerships among permit holders (EPA 2010):

Given the potential for overlapping activities in close proximity, EPA encourages permittees in a geographic area to establish cooperative agreements in implementing their stormwater programs. Partnerships and agreements between permittees and/or other agencies can minimize unnecessarily repeating activities and result in using available resources as efficiently as possible. Using existing tools and programs instead of creating new ones can allow permittees to focus resources on high priority program components instead. In addition, by forming partnerships, water quality can be examined and improved on a larger, consolidated scale rather than on a piece-meal, site-by-site basis.

It is possible and reasonable to significantly improve water quality in many urban receiving waters. This requires more than just a new development and redeveloped sites program, however, which at best can only hold the line. To actually improve the quality of receiving waters it is necessary to mitigate discharges from existing developed sites, which generally means implementation of measures to bring about the retrofit the stormwater control measures at existing sites to retain most stormwater on site. Such programs are a recommendation of EPA’s MS4 Permit Improvement Guide.

The EPA also hosts a website titled Alternative Site Stormwater Management (https://www.epa.gov/npdes/alternative-site-stormwater-management), which contains considerable information regarding the treatment, crediting, and permitting of offsite or regional permanent stormwater facilities.

Poresky et al. (2019) discusses offsite flow treatment and the potential it provides for partnerships, as well as the challenges it poses to infiltration practices. Offsite flows by both total volume and pollutant level can overwhelm infiltration best management practices (BMPs). Guidance is provided for partnership elements that ensure offsite flows are controlled by the owner of the offsite land such that they do not damage the stormwater facility, through either too much flow or contamination “hot spots.” The report also discusses the necessary regulatory framework to initiate watershed-based solutions or stormwater credit trading programs, and that those regulatory frameworks vary greatly through jurisdictions.

Within the transportation industry–focused literature, there exist several notable publications regarding the watershed approach. Chapter 7 in Connecting the DOTs through Collaboration in Stormwater Management (AASHTO 2012) provides details on the watershed approach and several case studies. The case studies include a mitigation fund approach in Colorado, where funds from the DOT’s construction budget are diverted from pure transportation use and instead directed toward water quality projects; “Tributary Action Teams” in Delaware, whereby the DOT participates with stakeholders to assess total maximum daily load (TMDL) watersheds; and a Massachusetts retrofit program through which the DOT constructed BMPs to address impaired waters downstream of highway runoff.

In NCHRP Research Report 840: A Watershed Approach to Mitigating Hydrologic Impacts of Transportation Projects (Weinstein et al. 2022), the watershed approach is viewed from the perspective of linear transportation infrastructure. The publication introduces the terms of art “in-kind” and “out-of-kind” facilities. In-kind facilities are those constructed off site but in the same watershed and are of the same type that would be constructed on site to address runoff pollutants generated by the highway. Out-of-kind facilities are watershed improvements like wetland reconstruction, stream improvements, or upland stabilization that will improve the condition of the watershed but are not directly linked to the runoff pollutants generated by the highway.

NCHRP Synthesis 602: Resilient Design with Distributed Rainfall-Runoff Modeling (Vasconcelos et al. 2023) explores current practices with the use of watershed-scale hydrological modeling by state DOTs. A distributed rainfall-runoff model subdivides large watersheds into sub-basins that can be studied more discretely. Rather than assuming hydrological processes are averaged throughout the larger watershed, the sub-basins are modeled using hydrological parameters

specifically crafted for each sub-basin. With the subdivision of the watershed into discrete components, water quantity and quality impacts can be examined at the sub-basin level.

In the fall of 2020, TR News published a special edition that contains six articles pertaining to highway stormwater. Four of the articles are of particular interest to this synthesis. Currier (2020) presents a history of stormwater regulation from the establishment of the Clean Water Act in 1972 to the present. Harmon (2020) describes how to craft an MS4 permit that considers the impact on TMDL watersheds on a DOT MS4 permit. Swanson and O’Bannon (2020) describes Virginia’s Chesapeake Bay Nutrient Credit Exchange program and how it results in stormwater quality partnerships. In Keeping the Water Clean: How Transportation Agencies Manage Roadway Stormwater Runoff, Lantin (2020) describes the challenges of offsite flows interacting and degrading DOT runoff, as well as the difficulty DOTs face in making measurable progress in reducing pollutant loads in a watershed when the DOT portion of the watershed is small.

2.2 Financing Approaches

There exists considerable guidance regarding innovative financing of stormwater infrastructure. The Sustainable Financing Forum for Faster, Cheaper, Greener Urban Stormwater Retrofits summary notes (EPA 2014) recap a forum held in Washington, DC, with various alternative financing techniques described. Notable among these techniques are stormwater credit trading and public-private partnerships, commonly referred to as P3s.

The State of Vermont published a guide on P3s titled Public-Private Partnership for Stormwater Management (Torizzo 2021). The guide details a pilot program to demonstrate the feasibility of P3s to address a change in Vermont’s stormwater permitting rules. The rule change, known as the 3-acre rule, mandates that parcels within the Lake Champlain watershed with more than 3 acres of impervious surface provide retrofit permanent stormwater facilities to address several phosphorus TMDLs within the Lake Champlain watershed. Seven hundred such parcels exist, and the study selects 10 for inclusion in the pilot study. The partnerships include residential, commercial, industrial, and municipal partners. Each of the 10 projects is detailed with the types of partners and facilities constructed, as well as the challenges met on the project. An analysis is also included for each project of the amount of phosphorus reduced per year and the cost per kilogram of that removal, presuming a 25-year project life. The guide concludes that P3s are often able to cost-effectively help sites meet the new Lake Champlain watershed requirements, as well as other environmental goals. P3 partnerships, however, are not always useful for every project. Onsite retrofits sometimes provide a lower-cost solution and are simpler to implement than partnership projects. The guide is focused on partnerships for private development, but it includes some useful information that could be translated to transportation.

Financing Integrated Green Stormwater Infrastructure to Improve Community Health, Resiliency: Getting the Best Deal for the Money (Lueckenhoff and Brown 2016) provides a summary of P3 partnership-financing options with discussions of private equity, stormwater utilities, state revolving funds, and socially responsible investing.

Washington, DC, has operated a stormwater credit trading program for over a decade, and its most recent annual report, Stormwater Retention Credit Program Fiscal Year 2020–2021 Summary Report (District of Columbia Department of Energy and Environment 2023) offers some valuable insights for state DOTs that might wish to participate in such a program. The stormwater credits traded in this program appear to be solely for private development, rather than public transportation projects, but the report does describe an innovative technique for constructing and accounting for offsite stormwater infrastructure.

There are multiple publications regarding community-based public-private partnerships (CBP3s) in the stormwater management world. CBP3 is a form of project partnership for which both water quality and quality of life are goals. In these partnerships, a government agency and a private entity seek to provide benefits in addition to stormwater improvements and permit compliance. These publications are generally aimed at municipalities seeking to achieve their MS4 requirements.

Publications on CBP3s include Meeting Regulatory Stormwater Management Requirements Using a Community-Based Public-Private Partnership Business Model (Prince George’s County Department of the Environment 2016) and Is a Community-Based Public-Private Partnership Right for Your Community? A Guide for Municipal Stormwater Managers in Washington (Washington State Department of Commerce 2019). In the Prince George’s County work, Chapter 6 outlines several partnership techniques not commonly considered by DOTs, such as rebates and stormwater credits. The Washington State work outlines performance-based contracts and other methods CBP3s can use to better deliver both stormwater and community goals.

Establishing a Stormwater Volume Credit Trading Program (Odefey et al. 2019) presents a case study for a stormwater credit trading program under establishment in Grand Rapids, Michigan. The paper provides background information on stormwater credit trading and prerequisites for establishing such a program. It also examines technical and economic feasibility, and strategies for implementation and administration.

Clean Water State Revolving Funds (CWSRFs) are an EPA-developed mechanism for states to fund water quality infrastructure. Historically, CWSRFs have been used mostly to construct point source treatment facilities, but they are allowed to be used for non-point source treatment. CWSRF 101: An Introduction to EPA’s Clean Water State Revolving Fund (EPA 2015) is a presentation on the EPA website that is a good introduction to CWSRF.

The EPA has published two papers regarding the use of CWSRFs to construct green infrastructure. CWSRF Best Practices Guide for Financing Nonpoint Source Solutions: Building Successful Project Funding Partnerships (EPA 2021) is a handbook on construction of green infrastructure through Clean Water State Revolving Funds, and Environmental Benefits of Clean Water State Revolving Fund Green Infrastructure Projects (EPA, n.d.) presents five case studies in which the CWSRF helped construct green infrastructure projects in various regions. In the course of this synthesis, the Rhode Island DOT indicated that it has been advantageous for them to use DOT funding to guarantee loans from the Rhode Island Infrastructure Bank to advance permanent stormwater facilities. That is the only example known in which a state DOT has used a similar approach to the CWSRF financing techniques described in these publications.

In the Denver metropolitan area of Colorado, the Mile High Flood District (formerly known as the Urban Drainage and Flood Control District) has published their Urban Storm Drainage Criteria Manual since 1969 (Mile High Flood Control District 2010). Before 1992, the two-volume criteria manual was focused on flood control policy and practice for the region. In 1992, the Mile High Flood District added a third volume, which expanded the focus to stormwater quality.

Several points important to this synthesis are included in Denver’s criteria manual. In the policy section, the manual describes the relative magnitudes of regional, subregional, and onsite treatment. Regional facilities are defined as those treating tributary areas between 130 and 640 acres, while subregional facilities are those that treat two or more development parcels totaling less than 130 acres. The district operates a maintenance program responsible for maintaining regional flood control and any water quality facilities constructed to Denver’s standards.

Mile High Flood Control District (2012) policy describes the types of facilities accepted for maintenance, the types of maintenance efforts performed, and a protocol for getting those facilities approved. This policy is a template for maintenance partnerships that involve the district.

In differentiating between flood control and stormwater quality infrastructure, the criteria manual states that “Whereas flood control is best handled on a regional basis, stormwater quality is best managed when stormwater is viewed as a resource and distributed throughout the site.” Though Denver allows regional stormwater quality infrastructure on a regional and subregional basis, they view smaller distributed techniques as more effective for stormwater quality. For flow control, their preference is the opposite—larger regional facilities. Through the policy, they accept maintenance partnerships for regional flow control facilities but discourage construction of facilities at that scale for stormwater quality purposes.

In the development of this synthesis, it was noted that some states rely on TMDL pollutant loading to develop their stormwater treatment programs, and in some cases to assign various partnership stakeholders’ pollutant load responsibilities within shared facilities. Techniques that can be used to quantify pollutant loading from stormwater runoff are described in Stochastic Empirical Loading and Dilution Model (SELDM) Version 1.0.0 (Granato 2013), as well as Pollutant Loadings and Impacts from Highway Stormwater Runoff, Volumes I, II, and III (Driscoll, Shelley, and Strecka 1990).

2.3 Survey Literature

As part of the synthesis, survey respondents were invited to submit their written stormwater guidance; 20 state DOTs submitted documents. Though many state DOTs have detailed manuals on how to design stormwater management facilities, none submitted documents that describe how to develop third-party partnerships for off-site or shared stormwater management facilities.

A listing of all documents provided is shown in Table 2.1. Hyperlinks to the documents are provided in Appendix D. Documents that do not include any discussion of partnerships, offsite treatment, or retrofit practices receive no further discussion. Details of documents that do include this information are described in the narrative that follows the table.

Arizona provided their Post Construction Best Management Practices Manual for Water Quality (Arizona DOT 2016). The manual is a comprehensive guide to selection, design, and construction of permanent stormwater facilities. It does not include information about inspection or maintenance. The manual contains a short discussion of Arizona DOT partnerships with other agencies (including flood control districts) using intergovernmental agreements but includes no information specific to their development or appropriate use.

California submitted both their Stormwater Quality Handbook Maintenance Staff Guide (Caltrans 2018) and a trifold brochure that describes their partnership programs for the construction of stormwater infrastructure. The stormwater maintenance manual is targeted at maintenance staff’s day-to-day activities that could result in discharges to surface water, along with work practices to minimize those discharges. Sections in the body and the appendix of the manual pertain to the inspection and maintenance of stormwater facilities and appropriate practices for the various types of devices. The partnership programs described in the trifold brochure are discussed more fully in Chapter 4, and the trifold is included in Appendix E.

Colorado submitted their Permanent Water Quality Program Manual (Colorado DOT 2021), a comprehensive manual for the design, construction, operation, and maintenance of their permanent water quality program. The manual includes a discussion of their Permanent Water Quality Mitigation Fund. This fund provides $6.5 million per year and is available to construct permanent stormwater quality control measures on both CDOT and local agency projects on a competitive grant basis. Since 2014, the fund has spent $57 million on 31 CDOT and 10 local agency projects.

Colorado also submitted an excerpt from their Drainage Design Manual (Colorado DOT 2019) which includes a description of responsibilities of local agencies when the stormwater

Table 2.1. DOT-provided stormwater facility documentation.

infrastructure is shared. Colorado has a statute commonly referred to as the division of authority in which state highway maintenance responsibilities within incorporated cities and towns are defined. This statute mandates that improvements outside the edge of asphalt or face of curb but within state highway right-of-way must be maintained by the city or town. These improvements include sidewalks, streetlights, and permanent stormwater facilities. These division of authority maintenance responsibilities are often documented in an intergovernmental agreement. The Permanent Water Quality Program Manual (Colorado DOT 2021) includes a checklist for acceptance of a stormwater facility, and one of the items on that checklist is the execution of the intergovernmental agreement. The Drainage Design Manual also notes that an intergovernmental agreement is required for maintenance of any stormwater control measure (SCM) outside DOT right-of-way, but no example agreements were provided.

Delaware provided their BMP Field Inventory/Inspection Standard Operational Procedure Guide (Delaware DOT 2023), as well as a template for shared use agreements for BMPs. The template is included in Appendix E. Aside from the template, the material contains no direction on partnerships or offsite facilities.

Florida submitted a process guidebook (Florida DOT 2021) for their Watershed Approach to Evaluate Regional Stormwater Solutions (WATERSS). The WATERSS process screens alternatives for both traditional stand-alone DOT projects and watershed-based partnership opportunities. The intent is to initiate these screening steps at the conceptual stage of a project. The WATERSS program is described more completely in Chapter 4.

Illinois submitted their Bureau of Design and Environment Manual (Illinois DOT 2024). Chapter 5 in this manual codifies an extensive policy for local agency agreements. It uses as one of its examples a storm sewer agreement. It also outlines maintenance obligations and division of cost-sharing requirements for storm sewers and appurtenances.

Maryland submitted their Sediment and Stormwater Guidelines and Procedures (Maryland DOT 2017). The document is a comprehensive guide to design, construction, and operations. Sections 5.4 and 5.5 describe the allowance of alternative stormwater measures, including offsite treatment and compensatory stormwater management. Compensatory stormwater management means that stormwater from an offsite area is managed, rather than stormwater from the area being developed. When standard techniques will not meet the treatment requirements for the

site, MDOT’s guidelines and procedures allow alternate management measures and lists them in the following priority order:

1. Onsite structural BMPs
2. Offsite structural BMPs located within the project watershed
3. Retrofitting an existing BMP in the project watershed
4. Stream restoration of a stream within the project watershed
5. Innovative technologies as approved by the Maryland Department of the Environment

Alternative B, and in cases with alternatives C and D improvements off site, could lead to partnerships for compensatory stormwater management. The level of treatment for offsite compensatory stormwater must meet the level required for any onsite treatment. These agreements must be documented in one of three ways: within the project’s stormwater management plan, within a memorandum of agreement between the Maryland Department of the Environment and MDOT for their water quality bank, or by the local stormwater management authority approving a watershed management plan that includes the agreement. There is no guidance for how those partnership agreements are specifically structured, only that they are documented in one of the three manners prescribed.

Nebraska provided their Stormwater Treatment Facility Maintenance Guide (Nebraska DOT 2020), as well as an excerpted chapter from their Drainage Design and Erosion Control Manual (Nebraska DOT 2013). The excerpted Chapter 3, pertaining to stormwater treatment, discusses the possibility of a local public agency constructing permanent stormwater facility projects within DOT right-of-way and the precedence of permit requirements that would occur in those situations. The chapter also discusses potential partnerships when DOT flows are commingled with local public agency flows and the need for agreements to manage those facilities. The manual also notably discusses offsite treatment, with the following verbiage found in section 7.A4 of Chapter 3 (Nebraska DOT 2013):

In some cases, it may not be practicable to provide stormwater treatment within the project limits due to various constraints such as site limitations (available right-of-way), costs, or other obstacles. If onsite mitigation is not feasible, offsite mitigation may be an option at other locations within the local watershed or MS4.

The manual further discusses the need for maintenance agreements with the underlying MS4 but aside from that, the discussion does not list requirements or provide templates.

Nevada submitted both their Stormwater Planning Design Guide (Nevada DOT 2023) and their Stormwater Operation and Maintenance Plan (Nevada DOT 2017). The design guide includes considerable discussion of low-impact development of stormwater quality features, with both the philosophical underpinnings and the design details of such non-centralized features. In some of those discussions, the need for partnerships with cooperating agencies is described but no specific method or policy is included to finalize these agreements.

North Carolina provided considerable documentation for their program. NCDOT Post-Construction Stormwater Program: Post-Construction Stormwater Controls for Roadway and Non-Roadway Projects (North Carolina DOT 2022) describes their program in depth. In Chapter 1, the guiding principles for the North Carolina highway stormwater program include “develop[ing] solutions that improve program delivery, are proactive, form partnerships, have technical merit, and are fiscally responsible.” The document does not appear to contain specific policies or techniques to establish these partnership agreements.

North Carolina also provided several supporting documents to their primary manual. These include a BMP toolbox, a BMP decision support matrix, a stormwater inspection and maintenance guide, a field guide for post-construction BMPs and a spreadsheet and supporting information regarding the NC-SELDM Catalog model. The model establishes goals for stormwater

treatment at each waterbody crossing along the project corridor. Links to these materials are included in Appendix D.

Oregon submitted their comprehensive Hydraulics Design Manual (Oregon DOT 2014). Chapter 14 focuses on stormwater quality. There is no specific discussion of agreements, but the need for cooperative agreements is noted in Chapter 13 with regard to stream bank stabilization. Oregon also submitted both general operations guidance for common stormwater facilities and templates to generate operations, inspection, and maintenance guidance for specific facility installations.

Wisconsin submitted the stormwater quality section excerpted from their Facilities Development Manual (Wisconsin DOT 2012). In the design workflow for stormwater control practices, the project manager is instructed to use the stormwater report to negotiate cost sharing with local communities. The report, properly prepared, includes quantification of onsite and offsite flows and pollutant loads being conveyed or treated, to aid in the establishment of partnership agreements.