Scour effects on an unconventional bridge foundation

Reliable prediction of scour at bridge piers supported by shallow foundations of unusual geometry remains challenging using conventional empirical formulations, particularly in confined river sections where foundation–flow interaction is significant. Inspired by an existing historic bridge with a non-standard shallow foundation, this study investigates scour in the presence of buttressed and unbuttressed retaining walls, originally conceived for slope stabilisation rather than scour protection. A series of 1 g physical model tests is conducted under steady, near clear-water flow conditions. Three configurations are comparatively assessed: (a) a single foundation with an unbuttressed retaining wall; (b) the same foundation with a buttressed wall; and (c) two foundations with buttressed walls, reproducing the prototype arrangement. Scour evolution is monitored through time-resolved imaging and high-resolution bed-elevation measurements. The results show that retaining walls modify the approaching flow and promote sediment redistribution, thereby limiting direct foundation exposure. Buttresses further enhance this effect by altering local erosion patterns and delaying scour development at the foundation level. However, when two foundation–wall systems interact within a confined channel, these beneficial mechanisms are partly reduced by mutual interaction effects. The findings clarify geometry-induced scour mechanisms relevant to assessing shallow bridge foundations in confined fluvial environments.