Developing a Guide for On-Bridge Stormwater Treatment Practices (2024)
Chapter: 4 Conclusions and Suggested Research
CHAPTER 4
Conclusions and Suggested Research
On-Bridge Stormwater BMP Development
This project performed an extensive characterization of the highway bridge environment to define the design requirements on-bridge stormwater treatment. The primary design requirements for on-bridge BMPs are to minimize space requirements and weight, offer versatility in placement within the irregular geometries, provide effective treatment targeting highway pollutants, provide 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 recommended BMP type 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 influence size and weight requirements and O&M intervals. Filtration rate and media depth are key factors in removal of dissolved pollutants. In addition to filtration media, a complete on-bridge BMP design the pre-treatment system, filtration media box, inflow and outflow control systems, and other elements.
This research project results in a recommended prototype on-bridge BMP, including the filtration media and the overall BMP system:
High-Performance Filtration Media.
The research project identified and adapted a proven filtration media and validated performance consistent with relevant benchmarks at 50 inches per hour filtration rate and 18-inch media depth. This research also developed relationships 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 Design (i.e., the “box”).
The research project results in 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 research project developed 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. The research project identifies equivalent adaptations that can used within the overall prototype recommendations. Additionally, guidelines are provided about how practitioners could 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 research project defined a design development and decision-making process organized based on 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. It is intended to be aligned with typical 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: (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, and (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 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.
Assessment of Key Feasibility Factors
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. Three case studies were prepared as interim deliverables, and five additional case studies are documented in the resulting Guide. 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 of adding additional load or attachments
- The ability to reach potential BMP locations practically and safely for construction and O&M
- 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 existing edge barrier designs lack strength to support BMPs.
Where on-bridge treatment retrofit projects are able to 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, BMPs, O&M access) without impacts to existing functions requires complicated engineering
analysis, extensive design customization, specialized construction methods, even if suitable siting opportunities are identified.
The Guide produced from this project 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.
BMP technology enhancements may offer benefit toward reducing the weight and size of BMPs and lessening the need for custom engineering. Proprietary technologies could enable reduction in size and weight on the order of 50 percent compared to the prototype recommended in this research and enable simpler design and procurement. 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-bridges are feasible and how designs should be advanced. Applying the process in the resulting 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.
Other Applications of Research Findings
The high-performance media and prototype BMP design developed through this research are relevant outside of the bridge environment. In cases where BMP footprints are limited, these solutions could provide effective treatment of roadway runoff in a smaller footprint than conventional roadside BMPs, such as bioretention, sand filters, or swales. This design will tend to require more frequent maintenance that conventional roadside BMPs. However, the tradeoff of more maintenance and smaller size could be justified in space constrained settings.
Potential Future Research
Through the conduct of research, several potential research topics were identified that could not be fully addressed within the scope of this project. These represent opportunities for future research.
Effect of mulch on clogging processes.
Media clogging processes were tested without mulch. The addition of a layer of mulch is common in filtration BMPs and could reduce the effect of solids loading on permeability reduction. Research on mulch would require a larger diameter column and more water. However, this could help improve understanding of clogging processes.
Pre-treatment optimization.
A simple pre-treatment system was recommended to settle coarse solids and retain floatable materials. The design of this was modeled after typical oil-water separator designs. However, the sizing and baffle design was not tested. Further research via numerical modeling or prototype testing could improve the design of this part of the system.
Design details, pilot fabrication, and system hydraulic testing.
This research would further detail the design elements of the BMP, including the structure design and materials, lifting features, hydraulic features, and other detailed components. The research would then involve contracting with a manufacturer to fabricate the system to demonstrate constructability and assess likely fabrication costs. Finally, this would involve full-scale hydraulic testing to confirm that the system provides the target treatment flowrates.
Pilot procurement of high-performance filtration media at regional sites.
The Guide included prototype specifications for a high-performance filtration media. DOTs seeking to implement the
recommendations of the guidelines to bridges or other space-constrained environments would benefit from region specific information on suppliers who can meet the recommended filtration media specifications. This would help improve confidence that these specialized materials can be obtained locally prior to specifying these designs as part of projects.
Standardized design and pursue performance certifications.
A DOT or group of DOTs could further standardize the design of the prototype BMP and obtain performance certifications such as through the Washington State Technology Acceptance Protocol Ecology (TAPE) program. The purpose of this task would be to provide further confirmation of performance results associated with a standard design specification, supporting more efficient specification of the BMP in projects and providing performance assurance that maybe required to demonstrate regulatory compliance.
Structural analysis of example on-bridge BMP case studies.
Two key uncertainties identified in case studies are (1) the necessary structural system to support the BMPs, which could weight between 15,000 and 30,000 lbs, and (2) the ability to attach these support systems to existing substructure elements or edge barriers. This project did not include these structural calculations. However, a focused evaluation of these questions for a range of common bridge types would be valuable to determine if planning level parameters or thresholds can be established. The fabrication and construction cost of support systems is also highly uncertain and could be a focus of future research.
Expanded sizing guidelines.
This project relied on modeling performed at 35 precipitation gages as part of NCHRP Report 778. These gages were selected because they had five-minute resolution precipitation data. Since publication of NCHRP Report 778, additional precipitation stations have adequate five-minute resolution data to perform a similar analysis. Expanding this analysis to a greater number of stations could improve the resolution of nationwide coverage.
Improved cost estimating for on-bridge stormwater retrofit projects.
Typical construction cost estimating resources are not representative of on-bridge retrofit project, which typically require complex and unusual construction methods. A focused review on bid tabulations from bridge retrofit projects and additional research with design and construction firms could improve the ability to develop reliable costs for on-bridge stormwater retrofit projects.
On-bridge BMP whole lifecycle cost and benefit tool.
Building on the two items above, this information could be incorporated into a whole lifecycle cost and benefit estimating tool adapted specifically to the components of on-bridge stormwater treatment projects. The tool be similar to the tools produced as part of NCHRP Report 778 but be adapted to the specific design elements needed for on-bridge treatment and incorporate the findings of the water quality and permeability studies performed as part of this project.
Economic feasibility benchmarking.
Even if on-bridge stormwater treatment is physically feasible, it is often much more expensive per unit of benefit than conventional on-land BMPs. DOTs may consider research to develop cost-benefit benchmarks for determining economic feasibility.
