SUMMARY

On-Bridge Stormwater Treatment Practices: A Guide

This Guide is a primary work product of NCHRP Project 25-61, “Development of On-Bridge Stormwater Treatment Practices.” This project built upon findings of previous NCHRP research projects that investigated questions about the management of stormwater runoff from bridge decks. NCHRP Report 474: Assessing the Impacts of Bridge Deck Runoff Contaminants in Receiving Waters (Dupuis 2002a, 2002b) provided a process that practitioners can use to analyze the characteristics of a particular bridge deck and receiving water body, decide whether mitigation is needed, and, if necessary, choose a mitigation strategy. NCHRP Report 474 found that long-term, untreated bridge-deck discharges do not have adverse impacts to aquatic toxicity or sediment quality in most cases. NCHRP Report 778: Bridge Stormwater Runoff Analysis and Treatment Options provided an assessment framework; a description of stormwater management options, including source controls and treatment controls; and a best management practice (BMP) calculation tool that can guide decisions for a specific bridge. NCHRP Report 778 concluded that treatment of runoff from a comparable section of highway on land is preferable to treatment of runoff from the bridge deck. Where off-site mitigation is unacceptable due to (1) water quality restrictions placed on the receiving water body or (2) site-specific conditions making the piping of bridge runoff to bridge ends for off-site treatment infeasible or undesirable, stormwater designers have few options for effective on-bridge treatment of stormwater.

This Guide supports practitioners in answering new questions focused specifically on the design and configuration of on-bridge treatment BMPs and associated elements. Prior research has provided guidance for answering the questions: In what cases does bridge runoff need to be treated to mitigate water-quality impacts? What combination of source controls and treatment strategies should be used? This Guide starts with the assumption that on-bridge treatment has been determined to be necessary for a particular bridge and receiving water body. It supports practitioners in addressing the questions: Is it feasible to provide on-bridge treatment? If so, what design, operation, and maintenance approaches are necessary to mitigate risks and balance costs and environmental protection? The information in this Guide provides a practical approach to assist the practitioner in answering these questions and developing clear documentation of the decision process for a particular bridge. This Guide is focused on retrofitting existing bridges with stormwater treatment BMPs.

Research Process and Outcomes

While there has been extensive research and development for stormwater BMPs, there has been little focus on designs specific to the bridge environment. This Guide is the product of a multifaceted research process aimed at the development of BMPs specific to the bridge environment and providing a preliminary design and decision-making framework for the

selection, design, placement, and maintenance of BMPs as part of the overall on-bridge stormwater system for a particular bridge. Key steps and outcomes of this research project included:

• Characterization of the highway bridge environment for on-bridge stormwater treatment, including assessment of a broad range of conditions and constraints that exist in the highway bridge environment to define the requirements for the development of BMPs.
• Development of a high-performance, non-proprietary filtration media blend via review and selection of treatment technologies, literature review of field studies, and laboratory confirmation of key variables that are needed to support design, sizing, and operations and maintenance (O&M) planning.
• Development of a prototype on-bridge BMP via risk-based assessment of stormwater treatment design alternatives and O&M paradigms.
• Development of a stepwise design and decision-making approach, supporting practitioners with project-specific decisions regarding the feasibility of on-bridge treatment and how the necessary elements of a stormwater treatment system and O&M strategy can be configured to meet site-specific objectives and constraints.
• Vetting, refinement, and demonstration of guidelines via case study applications, five of which are included in the Appendix: On-Bridge Stormwater Treatment Case Studies.

This process resulted in advancements in technologies and approaches specific to the bridge environment that can provide practitioners with more suitable options for stormwater management. Guidelines on applying and configuring these approaches are provided. However, the findings of this research align with the findings of prior reports in that many of the challenges of on-bridge stormwater treatment are independent of treatment technology and, in many cases, cannot be reasonably overcome. The resulting decision-making process provides practitioners with guidelines for when constraints are prohibitive to on-bridge treatment.

Prototype On-Bridge BMP Proposals

The primary design requirements for on-bridge BMPs are to minimize size and weight, offer versatility in placement within irregular geometries, provide effective treatment targeting highway pollutants, allow for manageable O&M cycles, and enable adaptation to various O&M paradigms. BMPs should be reasonably simple to design, construct, and rehabilitate and should be available nationwide without sole-source procurement.

The proposed BMP is a high-rate media filtration system. The most important design variables for high-rate filtration BMPs are the media filtration rate, required media depth, and the tolerance for sediment accumulation before clogging occurs. Each of these variables influences size and weight requirements and O&M intervals. Filtration rate and media depth are key factors in the removal of dissolved pollutants. In addition to filtration media, a complete on-bridge BMP design includes the pre-treatment system, filtration media box, inflow and outflow control systems, and other elements. Chapter 3: “Stormwater Treatment Practice Design Options” describes the proposed on-bridge BMP and the filtration media evaluated in this study:

between sediment load and permeability to support sizing and O&M forecasting. The treatment media is also effective in removing the tire wear compound 6PPD-quinone.

Prototype On-Bridge BMP Container and Hydraulic Controls Design (i.e., the “box”). This Guide includes the conceptual design of a prototype on-bridge BMP demonstrating how design elements can be configured to provide treatment performance and also support O&M. The system is intended to be relatively simple to construct and maintain, providing a modular design that allows the BMP to be configured to specific available space and enables off-site construction and rehabilitation to reduce the work that must be performed on a bridge.

The Guide describes approaches for sizing the prototype BMP to balance performance, size, weight, and O&M requirements.

Additionally, flexibility is needed to adapt designs to project-specific objectives. Design guidelines identify equivalent adaptations that can be used within the overall prototype suggestions. Guidelines are provided on how practitioners can leverage proprietary technologies to provide additional on-bridge design options without requiring sole source procurement.

Design Development and Decision-Making Framework

Design development and decision-making regarding on-bridge stormwater treatment systems for a particular bridge requires efficient integration of many project-specific factors. This guide defines a design, development, and decision-making process organized based on the classical phases of the engineering design cycle:

1. problem definition,
2. development of alternatives,
3. evaluation of alternatives, and
4. iterative refinement of the design.

This process is intended to be aligned with typical department of transportation (DOT) planning and design processes. The scope of this process encompasses a wide range of factors, including structural assessment, drainage and lateral spread risk, O&M access and worker safety, integration with existing O&M activities, traffic impacts, constructability, cold weather issues, lifecycle costs, and additional factors.

This process is intended to support DOT practitioners in the following areas:

1. Setting project-specific objectives for on-bridge stormwater treatment, including minimum thresholds to mitigate risks and determine feasibility.
2. Assessing bridge characteristics to determine opportunities and constraints, including identification of fatal flaws that could prohibit further development of an on-bridge option.
3. Selecting, siting, sizing and designing on-bridge BMPs and associated capture and conveyance elements, and determining O&M needs.
4. Evaluating preliminary design alternatives using a multi-objective, risk-based framework, including if there is any option that adequately mitigates risks and meets thresholds to be carried forward.
5. Determining the scope of additional studies necessary to make decisions to abandon, refine, or advance a design.

A critical component of this process is the identification of decision gates where fatal flaws are assessed, and decisions are made with the information available at that phase. This approach helps structure the process to specific questions. It also helps avoid wasted effort in cases where there are clear fatal flaws that cannot be overcome with reasonable design approaches.

General Findings

This project included an extensive review of highway bridge conditions, a risk-based assessment of critical design elements, and the completion of several case studies, five of which are documented in the Appendix. While the ability to draw general conclusions is limited by the extreme variability in bridge conditions, this research has yielded general findings that apply in many cases.

First, there are several principal factors that can render a project physically infeasible or present risks that cannot be mitigated by reasonable approaches. The most common of these are structural limitations on adding additional load or attachments, the ability to reach potential BMP locations practically and safely for construction and O&M, and unreasonable risks posed by lane closures to ensure that BMPs function and drainage pathways are clear. For example, except in rare cases, mounting BMPs below the bridge deck without clear overhead access will present unavoidable risks. Similarly, most edge barrier designs lack the strength to support BMPs.

Where on-bridge treatment retrofit projects can mitigate risks, they are still anticipated to be extremely expensive. The underlying issue is that existing bridges were not designed for this purpose. Adding the full suite of new functions needed for on-bridge treatment (drainage modifications, structural mounting, O&M access) without impacts to existing functions requires complicated engineering analysis, extensive design customization, specialized construction methods, and a measure of good fortune to find suitable opportunities. The Guide identifies conditions and design approaches that are most likely to reduce risks and balance costs. However, even where conditions are relatively favorable, costs likely exceed the cost of on-land BMPs by a multiple of five or more. This finding is based on case studies in the Appendix and general costing guidance in Chapter 5.

BMP technology enhancements may be beneficial in reducing the size and weight of BMPs. Proprietary technologies could enable a reduction in size and weight of approximately 50 percent. However, many of the challenges associated with extensive drainage modifications, structural attachments, specialized design, and challenging work conditions apply regardless of whether size could be reduced in this range.

Overall, practitioners should follow a structured process to reach efficient, defensible, and technically based findings about when on-bridge BMPs are feasible and how designs should be advanced. Applying the process in this Guide, or a local agency adaptation thereof, to a particular bridge can support an informed and data-driven conversation about managing risks, benefits, and costs.