CHAPTER 1

Introduction

Background

The treatment of stormwater runoff on bridges is a challenge for state Departments of Transportation (DOTs), primarily due to physical and safety constraints associated with implementing, operating, and maintaining treatment devices. As a result, stormwater runoff from bridges is most often routed to abutments for treatment, addressed through off-site mitigation, or simply not treated. This guide follows from NCHRP Report 474: Assessing the Impacts of Bridge Deck Runoff Contaminants in Receiving Waters, and NCHRP Report 778: Bridge Stormwater Runoff Analysis and Treatment Options.

NCHRP Report 474 reviewed scientific and technical literature addressing bridge deck runoff and highway runoff, focusing on the identification and quantification of pollutants in bridge deck runoff and how to identify the impacts of bridge deck runoff pollutants to receiving waters using a weight-of-evidence approach. Although undiluted highway runoff can exceed federal and state ambient water quality criteria, this alone does not automatically result in negative effects to receiving waters. Report 474 found no clear link between bridge deck runoff and biological impairment in the receiving water, though noted that salt from deicing could be a concern. Report 474 concluded that long-term untreated bridge deck discharges do not have an adverse impact on aquatic toxicity or sediment quality in the vast majority of cases. Still, in waterways that are subject to total maximum daily load (TMDL) allocations, source water protection requirements for drinking water, or other stringent water quality criteria, treatment of bridge deck stormwater runoff could be required.

NCHRP Report 778 found that wherever possible, the best option for treating bridge deck stormwater is to route it to land or provide offsetting mitigation via a treatment project elsewhere. This report highlighted many of the key constraints that were explored as part the current research project, including design and operations and maintenance (O&M) complexity, safety and structural risks, limited benefit, and very high cost. However, where offsite mitigation is unacceptable due to water quality restrictions placed on the receiving water body or site-specific conditions making the piping of bridge runoff to bridge ends for offsite treatment infeasible or undesirable, stormwater designers may need to evaluate on-bridge stormwater treatment. Report 778 report includes a set of spreadsheet-based best management practice (BMP) evaluation tools to evaluate stormwater volume, pollutant load removal, and cost implications for five types of treatment BMPs that could be used for treating stormwater runoff from bridges. Four of the BMPs included in the tool are intended for use at bridge abutments (swales, dry detention basins, bioretention, and media filters), and one is intended for use on the bridge deck (permeable friction course, or PFC). Stormwater designers have very few proven options for effective on-bridge treatment of stormwater and extremely limited experience applying these BMPs on bridges.

Scope of Research Efforts and Distinction from Previous Projects

The findings from NCHRP Report 474 and NCHRP Report 778 describe the extreme challenges faced and limited benefit that can be gained from bridge runoff treatment, particularly using on-bridge BMPs. This research project was intended to support cases where other options for stormwater control from bridges are exhausted and there is a need to further evaluate on-bridge BMPs. The focus of this research is on bridge and hydraulic designers working at the conceptual or preliminary design phase on a particular bridge that has been identified as requiring on-bridge stormwater treatment.

The primary objectives of this project were to recommend and test practical on-bridge placement and conceptual designs of stormwater treatment best management practices (BMPs), as well as develop guidelines for BMP selection, design, constructability, and maintenance. The BMP options developed through this research were directed to be specific to the target pollutants and constraints encountered on bridges. Key constraints include structural integrity; drainage system function (including lateral spread width in on the bridge deck); litter and sediment loads; maintenance; constructability; whole lifecycle costs; freeze-thaw cycles; and work zone safety.

As part of scoping this research project, the investigators and Project Panel worked to define the scope and focus of research and the primary audience for the research findings. These are summarized as follows:

• As discussed above, the research project is focused on circumstances where on-bridge stormwater treatment is required. This could arise from a combination of regulatory requirements, design goals, and physical or operational constraints where the more practical approaches recommended in NCHRP Reports 474 and NCHRP Report 778 are not applicable or not adequate. This research did not revisit the decision process of whether on-bridge stormwater treatment is needed or whether other options are available. Readers should refer to NCHRP Report 474 and NCHRP Report 778 for additional guides on those topics.
• On-bridge treatment could be required as part of a new bridge or as a retrofit to treat runoff from an existing bridge. Because existing bridges are far more numerous and more challenging, this research project focused on retrofits of existing bridges. However, similar concepts could be applied to new bridges.
• Research was intended to support technology selection, preliminary design, alternatives analysis, and feasibility assessment of BMPs. The research did not seek to provide detailed design guidelines, which will inherently be site-specific and DOT-specific.
• While structural capacity of bridges to bear additional weight is a key consideration and is addressed in general, each bridge is unique. Structural engineers should be engaged early in the design process when considering any on-bridge BMP. Structural engineers were not part of this research team.
• Research focused on providing non-proprietary stormwater treatment options. The potential role of proprietary technologies was evaluated but the goal of this research was to provide options that do not depend on proprietary treatment technologies.
• In defining an on-bridge stormwater treatment system, this research project included the BMP itself as well as the broader system of stormwater collection, conveyance, structural support, O&M elements, and other design features necessary to capture and treat stormwater.

• Permeable friction coarse (PFC) is a potential candidate for bridge decks that has been considered as part of previous research on bridge deck runoff (NCHRP Report 778). This could complement other treatment by reducing influent loads and potentially delaying clogging. However, the purpose of this project is specifically to review additional treatment practices beyond PFC. Therefore, PFC is not considered further.

Purpose of Report and Relationship to Guide

The Guide, published as NCHRP Research Report 1117: On-Bridge Stormwater Treatment Practices: A Guide, is the primary work product of this research project. The main findings of this research project are presented in the Guide. We assume the intended audience of this research can primarily rely on the Guide for the key findings and applicability of these findings.

This project summary report is intended to accompany the Guide and primarily document the conduct of research. This report summarizes the research approach, summarizes the intermediate and overall findings, and provides access to the detailed interim deliverables, which are included as appendices to this report. It also presents a summary of conclusions and opportunities for additional research. In discussing the findings of the research, this report intentionally does not replicate the same information contained in the Guide. It instead provides references to the sections of the Guide where relevant findings are presented.