A comparative analysis for assessment of hydrodynamic actions at bridges of 2D hydraulic models and traditional highway standards

Bridges worldwide face significant risks from flood-related hazards, with hydraulic actions being the primary cause of structural damage, service disruption, and catastrophic failure. Bridge owners and regulatory agencies must assess multiple hydraulic risks, including scour, uplift forces, drag effects, debris impact, and deck displacement, all of which can compromise the load-carrying capacity of a bridge. The assessment of hydraulic actions on highway bridges in many parts of the world still relies on simplified or qualitative methods, whereby hydraulic models are yet to be embedded within guidance.

In the United Kingdom, the CS469 standard governs the assessment of hydraulic actions on highway bridges, providing guidance for risk evaluation and management strategies. The CS469 methodology calculates hydraulic flow characteristics at critical cross-sections within channels and bridge crossings utilising Bernoulli theorem and specific energy. While computationally efficient, this simplified approach relies on non-physical approximations. This fundamental limitation introduces substantial uncertainty into risk and vulnerability assessments, potentially compromising the reliability of management decisions.

This study presents an alternative approach utilising 2D HEC-RAS hydraulic models with bridges modelled as 1D elements within flow areas. The proposed methodology crucially maintains compatibility with existing data requirements from CS469 while adhering to open-source principles, requiring only publicly available data or information from existing assessments. This approach ensures cost-effectiveness and accessibility for bridge management teams while providing significantly improved accuracy.

Comparative analyses between the 2D HEC-RAS model and traditional CS469 calculations for six case study bridges revealed substantial differences in hydraulic response. The 2D model showed water depths up to 138% higher and flow velocities 64% lower than CS469 estimates. These differences significantly impact scour risk assessments, with HEC-RAS models typically predicting scour depths up to 2.3m lower (averaging 1.5m reduction) compared to simplified equations, resulting in more realistic risk classifications.

While hydraulic vulnerability assessments showed limited variation, the CS469 approach only considers threshold values without quantifying effects. Our findings demonstrate that numerical hydraulic simulations provide more accurate risk estimations with comparable resource requirements, suggesting that future revisions of risk assessment guidelines should prioritise this methodology. This advancement would enhance the accuracy and reliability of bridge risk assessments, ultimately improving infrastructure resilience and safety management strategies.