Arch Bridges under flood conditions, a study of the velocity distributions, and the resulting bed scour.

In recent years, the increased frequency of extreme rainfall events has led to many bridge failures throughout the UK and Ireland. Particularly vulnerable are short span bridges, which typically include arch structures, where abutments and piers are quickly scoured at their foundations. Several bridges have collapsed under flood conditions and, recently, such events have occurred in Northern England during the floods during the latter part of 2015. This paper is the result of an on-going experimental investigation, funded by The Royal Society, where arch bridge scale models are subjected to pressurised flow. Results demonstrate the nature of the upstream and downstream velocity distributions, and resulting scour profiles. The study provides an insight into the significance of pressurised flow on the depth and extent of scour when compared to normal open channel conditions. Measurements of velocities were taken using Acoustic Doppler Velocimeter (ADV), where significant increases in bed velocities are experienced as the flow passes under the arch. Scour depth was measured using a 3D laser scanner, which verified the shape of the resulting scour hole. The outcome of this paper is the demonstration of the severity of scour within the vicinity of an inundated arch bridge structure. It highlights the likely risks to long term stability, identifying depth locations to consider in relation to protection against local scour.