In-Stream Vegetation as a Nature-Based Approach to Scour Control at Bridge Crossings

"Scour and erosion are well-established as leading causes of bridge failures in North America. While bridge crossings often include countermeasures for scour control and mitigation, the majority of existing scour countermeasures are considered expensive, impractical, and ineffective. Although routinely commented on in state-of-practice reports and highly beneficial in building and maintaining sustainable communities and ecosystems, nature-based solutions have been overlooked as an approach to scour control. A knowledge base which provides scientific evidence of the efficacy of green infrastructure such as in-stream vegetation for scour control at bridge crossings is not currently available. Evaluation based on detailed bathymetric and flow field measurements is necessary for future development of practical guidelines. The proposed research will employ extensive physical modelling to explore the efficacy of in-stream vegetation for scour control at bridge crossings. Experiments will be conducted in the laboratory facilities of IIHR – Hydroscience & Engineering, which include a high-gradient sediment-capable tilting flume with a sediment recess. Robust flow measurement techniques, including particle image velocimetry (PIV) and acoustic Doppler velocimetry (ADV), will provide insight into distribution of velocity components, shear and normal stresses, and higher-order turbulence moments in the flow field of interest due to inclusion of vegetated sections in the channel. The results of the physical modelling efforts will enhance the severely limited understanding of the influence of green infrastructure elements on the scour mechanism. The primary anticipated product is the initiation of a knowledge base for the development of a framework of guidelines to be used in practice."

The proposed research will employ physical modelling to explore the efficacy of in-stream vegetation for scour control at bridge crossings. The results of the physical modelling efforts will improve severely limited understanding of the influence of green infrastructure elements on the scour mechanism. The primary anticipated product is the initiation of a knowledge base for the eventual development of a framework of guidelines in practice. The overarching purpose of this work is to provide recommendations for implementing combined green-grey approaches to scour control in riverine environments, supported by scientific evidence and justified by widely understood economic and environmental benefits.

"The primary interest of the proposed project is public safety. Scour and erosion are widely acknowledged as the leading cause of bridge collapses in North America. Scour control and mitigation at bridge crossings is paramount to public safety and well-being. Green infrastructure approaches prioritize sustainability in addition to preservation of aquatic ecosystems. Incorporation of green infrastructure elements in water resources design is known to improve water quality, reduce harmful emissions, and affect nutrient and pollutant dispersion in aquatic environments, in addition to influencing river morphodynamics (Roca et al. 2017). Furthermore, as changes in climate result in unprecedented flows in streams and rivers, design approaches to scour control are required to keep pace. The limitations of existing countermeasures are well known to the hydraulic engineer, and development of new approaches with new perspectives is paramount to building and maintaining safe and resilient communities."

"Damage to bridge crossings due to scour can range from minor erosion to complete failure of a bridge. Bridge restoration of any scale can require significant expenditure, cause disruption to local traffic and necessary community services, and pose appreciable risk to surrounding ecosystems. In addition to capital for reconstruction, costs include the rerouting of traffic and the potential erection of temporary service bridges, which can exceed the cost of replacement itself by 50 percent (Melville and Coleman 2000). It has also been estimated that indirect losses incurred by the general public, local business, and industry are five times greater than reconstruction costs alone (Lagasse et al. 1995). Furthermore, a recent social equity assessment of the National Bridge Inventory has indicated that bridges located in regions with lower-income and otherwise disadvantaged populations are less likely to be serviceable, and that racial equity should be a consideration in distribution of bridge rehabilitation and maintenance funds (Gandy et al. 2023). Bridge health assessment, including scour vulnerability, and prioritization of associated project funding must therefore be viewed through the lens of socio-economic equity in underserviced areas, thus factoring into the selection of conditions for the modelling efforts in the proposed project. The environmental impact of green infrastructure methods is widely known. Green and grey-green approaches to water resources design in streams and rivers provide habitat for endangered and protected species, affect nutrient and pollution dispersion in aquatic environments, and reduce harmful emissions. "