Morphodynamic numerical modelling of bridge pier erosion induced by downstream check-dam partial collapse

The paper presents a case study in which a severe bridge pier erosion effect was triggered by a partial collapse of a downstream check-dam, due to a flood event in June 2020. The area under study is about 30 km east from the city of Udine (Italy), along the river Torre, which is the main tributary of the river Isonzo. In particular, it is 1 km downstream of the confluence between river Torre and its main tributary, river Natisone. The hydrodynamic complexity of the reach is further exacerbated by the presence of two bridges, close to each other and almost parallel (the new one is just upstream and it is under construction, although almost finished, while the old one is still in service). The result is that there are many bridge piers in the riverbed, determining some interference effects also increasing local erosion. The presence of a check-dam 600 m downstream of the old bridge had a stabilizing effect in this sense, although it was originally designed as a grade control structure of the river. The flood event determined a partial collapse of such structure, whose central portion was progressively excised by tipping. The backward propagation of the erosion reached the area of the bridges, determining a progressive general lowering of the river bed. As a consequence, many bridge piers of the old bridge were excavated, determining severe conditions for the stability of the structure (the traffic was interrupted). The paper presents the methodology adopted and the main results obtained by hydrodynamic and morphodynamic numerical analyses, which were conducted through a two-dimensional, finite volume, shallow water model able to determine the hydrodynamic fields (water depth and flow velocity) and the evolutionary trends of the river bed, allowing to identify the best retrofitting strategy for the design phase.