Effective On-Bridge Treatment of Stormwater

Focus Area

Water Quality/Wetlands


Natural Resources






Over 3 years

Research Idea Scope

Stormwater treatment of bridge deck runoff has always been a tough design challenge. Traditional methods of treating bridge deck runoff suggest piping stormwater off the bridge and treating it on land. NCHRP Report 778 “Bridge Stormwater Runoff Analysis and Treatment Options” says, “Treatment of runoff from a comparable section of highway on land is preferable to treatment of runoff from the bridge deck…” In situations where local regulations or site-specific conditions make piping bridge runoff to the bridge ends for off-site compensatory treatment is infeasible or undesirable, stormwater designers have very few options for effective on-bridge treatment of stormwater. There is an urgent need to add more tools into the toolbox for on-bridge treatment of stormwater. NCHRP 474, “Bridge Stormwater Runoff Analysis and Treatment Options,” gives a step-by-step analysis to aid stormwater designers in selecting BMPs for treating bridge runoff. However, the only on-bridge BMP suggested in NCHRP 474 is Permeable Friction Course (PFC). PFC will not work over rivers, streams, or lakes where there are stringent discharge requirements and water quality or endangered species concerns. NCHRP Report 767, “Measuring and Removing Dissolved Metals from Stormwater in Highly Urbanized Areas,” presents a conceptual BMP design that includes an inlet scupper and filtration media to treat metals discharge in stormwater runoff directly from a bridge. Unfortunately, this on-bridge BMP still needs testing and may not work in situations where bridge infrastructure limits installation. Overall, there are not many tools in the toolbox for providing on-bridge treatment of stormwater. On-bridge stormwater treatment BMPs need another look. Treating stormwater on-bridge has many challenges to overcome. There is a balance between the added weight of the on-bridge stormwater BMP that needs to be accounted for in the bridge design as well as the potential added maintenance burden when compared to stormwater that is routed to the bridge ends and treated on land. Any on-bridge stormwater BMP must also be easy to maintain and have a relatively long life. The on-bridge stormwater BMP must be able to remove the pollutants of concern from bridge deck runoff. Today’s current treatment technologies utilize media filtration to remove many pollutants of concern. Many DOTs utilize media filtration for rapid passive filtering of stormwater along the roadway. The same concept could be applied to on-bridge stormwater treatment. This research would identify filter media and mixes that would be successful in an on-bridge application. It would give stormwater designers a realistic idea of what the options truly are for on-bridge stormwater treatment. The objectives of this research are to: 1) Develop media filter mixes that effectively remove pollutants of concern in bridge deck runoff using rapid passive filtration techniques. 2) Develop guidance on how and where to install the media filter mixes for on-bridge treatment. Issues surrounding the installation, long term maintenance, removal, and life cycle of the media filter mix installations would also be explored and discussed. Achieving the research’s objectives involves the following tasks: 1) Conduct a survey and synthesis of existing literature related to bridge deck runoff, on-bridge treatment options, media filter mixes used around the country, and current bridge maintenance practices. 2) Design and implement testing of one or more media filter mix installations (including laboratory column tests and real world on-bridge testing). 3) Develop guidance for the use, design, and implementation of the media filter mix. This guidance would include design criteria, taking into account varying facility operations and climate.

Urgency and Payoff

On-bridge stormwater treatment may become the only option in certain bridge locations. The amount of on-bridge stormwater treatment BMPs is severely limited to the Permeable Friction Course (PFC) based on previous NCHRP studies. This study proposes to add more tools into the toolbox for on-bridge treatment using rapid passive filtration of bridge deck runoff through a media filter mix to remove pollutants of concern.

Suggested By

Alex Nguyen Washington State Department of Transportation 206-440-4537

[email protected]