Rethinking Hydrodynamic Assessments for River Infrastructures: Are Simplified Methods Leaving Bridges Exposed?

Bridge owners and regulatory agencies have a duty to assess risks derived from hydraulic actions including scour, uplift, drag, debris impact, deck displacement, and other consequences that can lead to a loss in the load carrying capacity of a bridge. In the UK, the CS469 (Management of scour and other hydraulic actions at highway structures) is the standard for the assessment of hydraulic actions to highway bridges. The methodology in CS469 for the calculation of the hydraulic characteristics of the flow at critical cross-sections within the channel and the bridge crossing, although simplistic by design to minimize computational effort, is intrinsically inaccurate since it makes use of unrealistic (i.e., non-physically based) approximations. This results in estimations of risk and vulnerability levels that could include high levels of uncertainty. In this paper, we propose to bypass these approximated hydraulic calculations by harnessing the computational power of 2D hydraulic models, which would not require any additional field data collection than needed for the original CS469 method. We recommend a fully 2D HEC-RAS model with the inclusion of bridges as 1D elements within the flow areas and only requiring publicly available data or data obtained from existing assessments in order to future-proof the approaches and adhere to an open-source/open-access philosophy, but also imposing only a marginal increase in cost for bridge management teams. Results from the two models—2D HEC-RAS and the existing approach in CS469, are compared for a number of real-world bridges. The comparisons show that the estimations by HEC-RAS are substantially higher for water depth (up to 138%) and lower for flow velocity (down by 58%). When these values are applied to the estimation of hydraulic vulnerability and scour risk, the differences are significant. Scour depths with the use of HEC-RAS models are typically much lower (up to 3.9 m, and on average 1.7 m) than with simplified hydraulic equations, and this translates into lower (yet, more appropriate) scour risk levels. Hydraulic vulnerability to submergence of the assessed bridges is also assessed very differently, typically higher by the 2D model method. Overall, the results show that 2D numerical hydraulic simulations present a much more accurate estimation than existing methods, better balancing risks deriving from scour and hydrodynamic actions and with comparable effort and data requirements. The model displays consistency across an exhaustive set of simulations for a range of variables and bridges, showing limited variability and proneness to errors, whilst values estimated by CS469 are in most cases significantly different. Future versions of CS469 and similar documents should prioritize this methodology to provide a more accurate and realistic risk estimation.