NCHRP Research Report 1117 presents a guide 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 answering these questions 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, “Effective On-Bridge Treatment of Stormwater,” 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 a (1) high-performance filtration media suited to highway pollutants and design conditions that can be formulated from available materials; and (2) 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, three deliverables are not included in the published report but are available on the TRB website at trb.org by searching for NCHRP Research Report 1117. The deliverables are as follows: (1) a conduct of research report summarizing the entire effort, available as NCHRP Web-Only Document 401, Developing a Guide for On-Bridge Stormwater Treatment Practices; (2) a plan that identifies mechanisms and channels for communicating and implementing this research; and (3) a PowerPoint presentation introducing NCHRP Research Report 1117.
