This paper discusses how flow conditions and consequent scour of channel bed induces the failure of short-span, masonry arch bridges over small to medium sized rivers. Such bridges are common in Ireland, the U.K. and Europe. A factor frequently complicating failure is that such bridges typically are deemed historic structures and, therefore, are expensive to repair. A significant hydraulic-engineering factor is that, these bridges readily choke approaching flows, especially under large return period flooding conditions. The bridge flow area quickly decreases once flow stage reaches the crown of the arch barrel. Flume experiments, undertaken with choked flow condition, showed that three locations of masonry arch bridges are particularly prone to scour-induced erosion: piers (multi-span only), abutment corners and the channel bed immediately downstream of the bridge (in the case of a paved or protected bridge inverts). Scour can be reduced only modestly by streamlining abutments by means of wing walls, and cutwaters on piers, because of flow choking. The use of stone pitching to armour the riverbed around abutments and piers prevents scour at the front of, and beneath, the bridge. However, supercritical flow through the bridge opening may scour the channel bed immediately downstream of a bridge. Alternative scour-mitigation measures are suggested. The increased failure incidence of these bridges has raised concerns in the U.K. and Ireland that many masonry arch structures will be imperilled by the effects of greater flow rates associated with climate change and watershed urbanization. The situation is complicated by the age of the structures and the lack of accurate bridge records.
|Title of host publication||E-proceedings of the 38th IAHR World Congress|
|Publication status||Published (in print/issue) - 6 Sept 2019|
- alluvial river; arch bridges, flow contraction, asset failure