Flood fragility assessment of bridges—Unified framework

This paper provides an analytical step-by-step probabilistic framework for undertaking a flood fragility assessment of bridges considering a spectrum of foundation scour severity scenarios. Currently, and despite the fact that bridge fragilities to natural hazards (e.g., earthquakes) are at a relatively mature stage−constituting the state-of-the-art in undertaking pertinent risk assessment studies−bridge flood fragilities and the relevant methodologies are scarce in the literature. This knowledge gap prevents us from delivering either quantitative flood risk assessments for existing bridge portfolios or risk-aware flood performance evaluations for new bridge designs. To address this gap, this paper proposes, for the first time, a unified probabilistic framework, that is showcased for a typical road bridge with piers on shallow foundations, which is not bridge-specific. The framework is applicable, with appropriate adjustments, to other bridge components and typologies. By means of numerical simulations on reduced-order Monte Carlo generated bridge pier samples, the response statistics of the considered bridge pier are being evaluated at increasing flood intensity levels through incremental flood-relevant static analyses by applying equivalent hydrodynamic forces. The flood intensity accounts for both the flow velocity and inundation depth, based on a new vector intensity measure. Different sets of flood fragility curves are produced for variable scour severity scenarios, whereas intra-scour severity scenario randomness is captured by considering a spectrum of plausible scour patterns. It is demonstrated that scour has a detrimental impact on the flood fragility of bridges, whereas if minimised by taking appropriate proactive measures, the probability of the bridge being severely damaged becomes very low even under the most severe realistic flood intensities. The proposed framework could serve a first-order flood risk assessment tool for large bridge inventories, offering to engineers, asset managers and network operators quantitative means for resource allocation and the implementation of adaptation measures.