On-Bridge Stormwater Treatment Practices

A GUIDE

TRANSPORTATION RESEARCH BOARD 2024 EXECUTIVE COMMITTEE^*

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NATIONAL COOPERATIVE HIGHWAY RESEARCH PROGRAM

NCHRP RESEARCH REPORT 1117

On-Bridge Stormwater Treatment Practices

A GUIDE

Aaron Poresky
Ariel Mosbrucker
Jack Lisin
Maral Razmand
GEOSYNTEC CONSULTANTS, INC.
Portland, OR

Michael Barrett
Lynn Katz
UNIVERSITY OF TEXAS
Austin, TX

James Lenhart
STORMWATER NORTHWEST
Portland, OR

Subscriber Categories
Bridges and Other Structures • Design • Maintenance and Preservation

Research sponsored by the American Association of State Highway and Transportation Officials in cooperation with the Federal Highway Administration

NATIONAL COOPERATIVE HIGHWAY RESEARCH PROGRAM

Systematic, well-designed, and implementable research is the most effective way to solve many problems facing state departments of transportation (DOTs) administrators and engineers. Often, highway problems are of local or regional interest and can best be studied by state DOTs individually or in cooperation with their state universities and others. However, the accelerating growth of highway transportation results in increasingly complex problems of wide interest to highway authorities. These problems are best studied through a coordinated program of cooperative research.

Recognizing this need, the leadership of the American Association of State Highway and Transportation Officials (AASHTO) in 1962 initiated an objective national highway research program using modern scientific techniques—the National Cooperative Highway Research Program (NCHRP). NCHRP is supported on a continuing basis by funds from participating member states of AASHTO and receives the full cooperation and support of the Federal Highway Administration (FHWA), United States Department of Transportation, under Agreement No. 693JJ31950003.

The Transportation Research Board (TRB) of the National Academies of Sciences, Engineering, and Medicine was requested by AASHTO to administer the research program because of TRB’s recognized objectivity and understanding of modern research practices. TRB is uniquely suited for this purpose for many reasons: TRB maintains an extensive committee structure from which authorities on any highway transportation subject may be drawn; TRB possesses avenues of communications and cooperation with federal, state, and local governmental agencies, universities, and industry; TRB’s relationship to the National Academies is an insurance of objectivity; and TRB maintains a full-time staff of specialists in highway transportation matters to bring the findings of research directly to those in a position to use them.

The program is developed on the basis of research needs identified by chief administrators and other staff of the highway and transportation departments, by committees of AASHTO, and by the FHWA. Topics of the highest merit are selected by the AASHTO Special Committee on Research and Innovation (R&I), and each year R&I’s recommendations are proposed to the AASHTO Board of Directors and the National Academies. Research projects to address these topics are defined by NCHRP, and qualified research agencies are selected from submitted proposals. Administration and surveillance of research contracts are the responsibilities of the National Academies and TRB.

The needs for highway research are many, and NCHRP can make significant contributions to solving highway transportation problems of mutual concern to many responsible groups. The program, however, is intended to complement, rather than to substitute for or duplicate, other highway research programs.

NCHRP RESEARCH REPORT 1117

Project 25-61
ISSN 2572-3766 (Print)
ISSN 2572-3774 (Online)
ISBN 978-0-309-72432-6
DOI 10.17226/27904
Library of Congress Control Number 2024943198

© 2024 by the National Academy of Sciences. National Academies of Sciences, Engineering, and Medicine and the graphical logo are trademarks of the National Academy of Sciences. All rights reserved.

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COOPERATIVE RESEARCH PROGRAMS

CRP STAFF FOR NCHRP RESEARCH REPORT 1117

Monique R. Evans, Director, Cooperative Research Programs

Waseem Dekelbab, Deputy Director, Cooperative Research Programs, and Manager, National Cooperative Highway Research Program

David M. Jared, Senior Program Officer

Mazen Alsharif, Senior Program Assistant

Natalie Barnes, Director of Publications

Heather DiAngelis, Associate Director of Publications

Brendan Foht, Senior Editor

NCHRP PROJECT 25-61 PANEL

Field of Transportation Planning—Area of Human and Natural Environment

G. Scott McGowen, Michael Baker International, Inc., Rancho Cordova, CA (Chair)

Nicole A. Bartelt, Minnesota Department of Transportation, Oakdale, MN

Brian K. Currier, California State University, Sacramento, CA

Lewis N. Lloyd, Virginia Department of Transportation, Charlottesville, VA

Drew Andrew Martin, Georgia Department of Transportation, Atlanta, GA

Alex Nguyen, Washington State Department of Transportation, Seattle, WA

Alisa Diaz Richardson, Rhode Island Department of Transportation, Warwick, RI

Susan Jones, FHWA Liaison

AUTHOR ACKNOWLEDGMENTS

This report was developed under NCHRP Project 25-61, “Development of On-Bridge Stormwater Treatment Practices” by Geosyntec Consultants, with assistance from the University of Texas–Austin, Gresham Smith, Terraphase Engineering, and Stormwater Northwest. The co-principal investigators for the project were Aaron Poresky of Geosyntec Consultants and Dr. Michael Barrett of the University of Texas–Austin. The authors would like to thank Joel Prock and Myles Gray of Geosyntec Consultants, Tom Dietrich of Gresham Smith, Eric Strecker of Terraphase Engineering, and the team from SWI Group for their contributions to the project.

FOREWORD

By David M. Jared

Staff Officer

Transportation Research Board

NCHRP Research Report 1117: On-Bridge Stormwater Treatment Practices: A Guide provides guidance for determining the feasibility of on-bridge stormwater treatments for bridges and what design, construction, and operation and maintenance (O&M) approaches are necessary to mitigate risks and balance costs with environmental protection. This guide provides a practical approach to addressing these issues and developing clear documentation of the decision process for a particular bridge, with an overall focus on retrofitting existing bridges with stormwater treatment best management practices (BMPs). Case studies with summary findings are also included. The guide should be of interest to bridge and hydraulic designers at state departments of transportation seeking to develop effective designs and mitigate risks where possible.

Stormwater treatment of bridge deck runoff continues to be a design challenge. Traditional methods of treating this runoff suggest treating an equivalent area of roadway offsite that discharges to the same receiving water body or piping the bridge deck stormwater to an offsite treatment location. NCHRP Report 778: Bridge Stormwater Runoff Analysis and Treatment Options concluded that treatment of runoff from a comparable section of highway on land is preferable to treatment of runoff from the bridge deck. Where offsite 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 offsite treatment infeasible or undesirable, stormwater designers have few options for effective on-bridge treatment of stormwater. There is a growing need to develop more options for on-bridge treatment of stormwater that will address challenges, including, but not limited to, level of service, structural integrity, hydraulic function, litter and sediment loads, freeze-thaw cycles, maintenance operations, and work-zone safety.

Under NCHRP Project 25-61, “Development of On-Bridge Stormwater Treatment Practices,” Geosyntec Consultants was asked to develop (1) on-bridge stormwater treatment applications with a focus on retrofitting existing bridges utilizing proven stormwater treatment practices and (2) a guide for selection, design, placement, and maintenance of the developed on-bridge stormwater treatment applications. The focus of the guide is on retrofitting existing bridges with stormwater treatment BMPs, but the concepts can be applied to new bridges as well. The guide describes (1) a high-performance filtration media suited to highway pollutants and design conditions that can be formulated from available materials and (2) a prototype BMP design, incorporating the recommended filtration media and other design features and approaches adapted to construction and O&M in the highway bridge environment. The guide addresses BMP placement and sizing, drainage system modifications, development of O&M approaches to form preliminary design alternatives, and

design evaluation and decision-making at multiple points. Finally, the guide focuses on non-proprietary stormwater treatment options.

In addition to NCHRP Research Report 1117, the following deliverables can be found on the National Academies Press website (nap.nationalacademies.org) by searching for NCHRP Research Report 1117: On-Bridge Stormwater Treatment Practices: A Guide:

1. NCHRP Web-Only Document 401: Developing a Guide for On-Bridge Stormwater Treatment Practices, which is the research team’s report documenting the development of the guide and the entire research effort.
2. A plan that identifies mechanisms and channels for communicating and implementing this research.
3. A PowerPoint presentation introducing NCHRP Research Report 1117 and summarizing its main findings.

CONTENTS

Summary

Chapter 1 Introduction

Background

Purpose and Scope of This Guide

New Research Conducted

Template Planning and Design Process

Chapter 2 Characterization of the Bridge Environment for Stormwater Treatment

Chapter at a Glance

What Is the Bridge Environment?

Design Objectives and Feasibility Thresholds for On-Bridge Stormwater Treatment

Assessment of Existing Conditions Influencing Stormwater Treatment Design

Screening of Potential Fatal Flaws

Chapter 3 Stormwater Treatment Practice Design Options

Chapter at a Glance

Summary of Research Process and Outcomes

NCHRP 25-61 Filtration Media

NCHRP 25-61 Prototype BMP Design

Proprietary Stormwater Treatment Options

Chapter 4 Preliminary Design of On-Bridge Stormwater Treatment Systems

Chapter at a Glance

Preliminary Design and Decision-Making Process

Step 1: Screen Potential BMP Locations

Step 2: Develop Preliminary Design Alternatives

Step 2a: Determine Capture and Conveyance Approach

Step 2b: Select and Size BMPs

Step 2c: Preliminary Structural Review

Step 2d: Develop Conceptual O&M Approach

Step 2e: Integrate Preliminary Design Decisions and Document Preliminary Design Alternatives

Chapter 5 Decision-Making Framework and Supplemental Guidance

Chapter at a Glance

Step 3: Perform Multi-Factor Preliminary Design Evaluation

Step 4: Perform Supplemental Studies to Support Decisions

Guidance for Scoping of Structural Analysis

Guidance for Scoping Lifecycle Cost Analysis

References

Abbreviations and Acronyms

Glossary of Key Terms

Appendix On-Bridge Stormwater Treatment Case Studies