Scour and the Stability of Short-Span, Masonry Arch Bridge: findings from a diagnostic, flume study

This paper presents findings from diagnostic, flume experiments conducted to investigate how local scour development de-stabilizes short-span masonry arch bridges. A review conducted by the writers indicates that a significant proportion of the existing bridge stock in Ireland, the UK, Europe and North-Eastern USA are this bridge type. To complicate matters, these bridges often are deemed historic structures, and thus are difficult to simply replace. In recent years, spreading urbanization and increased frequency of extreme rainfall events in the U.K. and Ireland have increased the failure incidence of these old bridges. In addition to their inadequate flow opening, these bridges are usually founded on shallow footings of uncertain condition. The footings of some bridges supported on timber piles have weakened with age. The flume experiments involved flow and scour with the upstream face of arch openings fully submerged, creating an orifice-flow condition through the arch opening. The experiments showed that two locations of masonry arch bridges are particularly prone to settlement cracking owing to clearwater scour-induced undermining. These locations are at (i) the footings at the upstream corners of arch-bridge abutments, and (ii) footings beneath piers between arches (in multi-span bridges). The findings further show that greatest scour depth normally developed at the center-pier linking adjoining arches, however the addition of cutwaters shifted the location of the maximum scour so as to be more central in the arch opening. Additionally, the findings show that scour depth and consequent undermining can be reduced somewhat by streamlining abutments by means of wing walls, and piers by means of cutwaters. Where these remedial works are impracticable, the use of stone pitching to armour the riverbed around abutments and piers is a very effective way to prevent scour. Yet, once the scour is mitigated at the entrance of a short-span arch, flow contraction and acceleration through the opening may scour the channel bed immediately downstream of an arch. Subsequent erosion and head-cutting of the downstream bed can then migrate upstream to adversely affect the stability of the bridge. Ireland and the UK face substantial issues in determining how to ensure the long-term stability of historic, short-span, masonry arch bridges