The increase in frequency and severity of flooding events jeopardises the integrity of fundamental infrastructures, such as bridges. These events can lead bridge decks to be submerged and exposed to drag and lift forces for which the structure might not have been previously design for. The intensity of these forces depends on several parameters, such as water velocity, water height, girder depth, and girder spacing. However, existing literature limited to evaluating the effect of water height on bridge decks. Consequently, design guidelines follow simplistic approaches to analyse submerged bridge structures based on a few typologies and do not explicitly take into account these parameters except for water height and proximity ratio.

This thesis addresses this gap by evaluating the effect of water velocity, water height, girder depth, and girder spacing on drag and lift coefficients based on the outcomes of more than 500 numerical simulations. These simulations were performed through Computational Fluid Dynamics (CFD) using the ANSYS Fluent software and run on the High-Performance Computer at University of Surrey. A novel regression expression to estimate drag and lift coefficients is proposed in this work based on the numerical analyses performed. A further novel equation is proposed to estimate drag forces on isolated bridge components to recognise the uneven distribution of forces on a bridge deck, which is overlooked in the literature and design guidelines. The accuracy of design guidelines for estimating drag and lift forces is further analysed by comparing the results using these documents with those from CFD simulations.

This thesis assesses the susceptibility of bridge structures to hydraulic actions by evaluating limit states resulting from models using the Finite Element Method. Given that design guidelines do not specify how hydrodynamic forces should be applied when conducting structural analyses, several distribution patterns were proposed and evaluated to define a technique for load application that provides reasonable structural results without the need of running CFD simulations.

Recommendations are given in this thesis for design guidelines to consider the effect that bridge cross-section configuration and hydraulic and structural parameters have on hydrodynamic coefficients to avoid inaccurate estimation of hydrodynamic forces on submerged decks. The findings of this research allow bridge designers to choose more suitable deck typologies when designing new bridges and support bridge owners in ranking the vulnerability of their bridges against different hydraulic events.