Assessment of Riverine Scour for Design of a Major Bridge Crossing Using 2-Dimensional Hydraulic Modeling – A Methodologic Comparison Case Study

The American Society of Civil Engineers cites riverine scour as the leading cause of bridge failure. The design of mitigation measures to protect critical infrastructure requires integrated analyses of geomorphology and water resources engineering, and the use of sophisticated tools and analyses. The West Don Crossing long-span segmental bridge is being designed and constructed as part of the Ontario Line subway system, a 15.6-kilometre rapid transit line in the City of Toronto. The bridge spans more than 400 metres across the Don Valley supported by five piers, two of which are within the regulatory floodplain limits.  Scour analysis and countermeasure design was initially completed using the agency approved 1-Dimensional (1D) HEC-RAS model of the Don River Watershed, in accordance with current practice. While preliminary design submissions received approval without much comment from local regulators, it was determined a traditional 1D hydraulic model was not suitable for scour analysis due to the complex river geometry and the resulting highly variable depth and velocity distributions along the channel profile and section.  Ultimately, a 2D HEC-RAS hydraulic model was developed for the analysis and design of scour mitigation measures at the crossing.

This presentation provides an overview of the model selection criteria for the scour assessment, the approach applied in the development of the 2D hydraulic model, and the methodology applied for the design of the scour mitigation measures which integrated water resources engineering with fluvial geomorphology design principles.

The findings from this case study indicate that the traditional application of 1D modelling yields comparable results to 2D modelling for assessing scour, but different results regarding countermeasure design requirements.

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