PSI - Issue 62

Matteo Nicolini et al. / Procedia Structural Integrity 62 (2024) 601–608 Matteo Nicolini/ Structural Integrity Procedia 00 (2019) 000 – 000

602

2

1. Introduction The interaction between flowing water and bridge piers can result in severe erosion, compromising the structural integrity of the bridge and posing significant risks to public safety. Smith (1977) counted and analyzed the causes of 143 bridges that had major accidents from 1847 to 1975 and found that the damage to 70 bridges was determined by the displacement of piers during flooding. In one study of five hundred bridge failures that occurred in the United States between 1989 and 2000, flooding and scour were the cause of 53% of the recorded failures (Wardhana and Hadipriono, 2003). The same result was achieved by the investigation report by Lagasse et al. (2007), showing that about 60% of bridge failures had hydraulics-related issues, with scour the main reason. Beyond causing major operating disruption, bridge failure also generally implies financial losses (De Falco and Mele, 2002). The investigation of the US Federal Highway Administration (Arneson et al., 2012) showed that local scours are one of the primary issues to be considered in bridge design and protection. 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. As a consequence, a morphodynamic evolution of the river bed initiated: in particular, the material upstream of the check-dam gradually began to flow downstream, and the backward propagation of the erosion determined a progressive general lowering of the river bed reaching the area of the piers. Such serious situation led to the emergency design and programming of the restoration of the collapsed part of the structure, together with a reshaping of the bed (excavation and landfill). The activities were preceded by an in-depth hydrodynamic and morphodynamic analysis which served as a support tool for the design phase. In particular, the model has been implemented with the software BASEMENT 3.2 1 , which is a state-of-the-art modelling environment developed by ETH Zurich D-BAU (Department of Civil, Environmental and Geomatic Engineering), Laboratory of Hydraulics, Hydrology and Glaciology. One of the major strengths of the software is the possibility of conducting numerical simulations with GPUs, thus increasing enormously the speed of calculations and decreasing computational times, which may be of the order of several days for simulating 24 hours of flood with a computational mesh with some millions of nodes. For the case study the model is characterized by a triangular mesh with 1003906 nodes and with 2001791 cells. The extension of the modelled area is 57.11 square kilometers. The computational mesh is characterized by a very detailed resolution of the cells, in order to reproduce the geometry of the piers. The characteristics of the bed material were determined by ad-hoc surveys in the field, where measurements of the grains allowed to obtain the granulometric curve of the sediments. Hydrological data were available by two measurement stations, both along the Torre and the Natisone rivers, allowing to describe in terms of stage hydrographs the event of the 4-6 June 2020 flood. The simulations were performed with the aim of reproducing what happened: this has been possible due to the availability of a recent (2018) digital terrain model (DTM) with a resolution of 0.5 m, which represented the conditions ante -event, and which has been adopted as initial morphology of the mesh. A further, ad-hoc, topographic survey allowed to establish the morphodynamic conditions post -event, fundamental for the restoration project, and was partly useful for the calibration of the model, which has been subsequently adopted in the design phase. The paper is organized as follows: Section 2 introduces the general characteristics of the area under study, Section 3 describes the morphodynamic model and some results obtained by the simulation of the 4-6 June 2020 flood event, while Section 4 gives some details of the restoration phase, which resulted in the design of a new central portion of the check-dam. 2. General characteristics of the area under study 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 river Isonzo (Fig. 1a) and is characterized by dry bed most of the time and sudden flash floods due to the

1 BASEMENT version 3.2: System Manuals, Laboratory of Hydraulics, Glaciology and Hydrology (VAW), Swiss Federal Institute of Technology (ETH), Zurich (May 2022), http://www.basement.ethz.ch