AbstractThis paper explores flood-induced hydrodynamic and debris impact forces on masonry arch bridges using the numerical method smoothed particle hydrodynamics (SPH). Masonry arch bridges make up a significant proportion of the bridge stock in many parts of the world, including the UK and US. Although the masonry arch is a durable bridge form, those spanning watercourses can be vulnerable to the effects of flooding. Where fast flood flows impinge on the bridge superstructure, highly transient behavior is observed, which can lead to violent interactions, especially where debris is carried by the flow. This paper investigates flood-induced impact pressures on a typical single-span masonry arch bridge subject to hydrodynamic action and discrete floating debris. Different debris orientations relative to the bridge span are considered. Results revealed that the presence of the debris can lead to impact pressures an order of magnitude greater than the hydrodynamic conditions alone. Furthermore, the influence of debris orientation at impact was significant, with the 90° orientation resulting in higher peak impact pressures with shorter impact duration compared with the 0°.