PSI - Issue 62

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

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high rates of precipitation in the hydrographic basin. In particular, the area is 1 km downstream of the confluence between river Torre and its main tributary, river Natisone, see Fig. 1b. The Isonzo/Soča is a 140 km l ong alpine river that flows from north-east to south-west (100 km in Slovenia, upstream, and 40 km in Italy, downstream). The total area of the basin is about 3400 km 2 . One third of its basin is located in Italy, about 1150 km 2 , and two-thirds in Slovenia, about 2250 km 2 . The Torre is a torrential watercourse, which cuts through the Friulian plain starting from the Julian Prealps in a southerly direction, towards the Adriatic Sea; after a journey of approximately 67 km it enters the Isonzo river. The total catchment area of the Torre, calculated immediately upstream of its confluence with the Isonzo, is estimated at 1091 km 2 . At this closure section the highest flow rate observed was 1700 m 3 /s. The basin of river Natisone extends over a total area of 327 km², of which approximately 65 km² fall within Slovenian territory. The mountain basin, closed at Cividale del Friuli, measures approximately 285 km², while the part of the basin downstream, up to the confluence with the Torre, has a surface area of approximately 42 km². 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 working. The result is that there are many bridge piers in the riverbed, determining some problems due to interference effects and local pier scours. The presence of a check-dam 600 m downstream of the piers had a stabilizing effect in this sense, although it was originally designed as a grade control structure of the river.

Fig. 1. (a) General view of the Isonzo river basin; (b) Hydrographic setting of Torre and location of the two bridges and of the check-dam.

3. The morphodynamic numerical model and the simulation of the event of 4-6 June, 2020 As mentioned, numerical analyses have been carried out with a two-dimensional morphodynamic model based on the BASEMENT software. In order to avoid any influence of boundary conditions, the computational domain covers a stretch of the riverbed of approximately 18 km of the Torre and the last 6 km of the Natisone river (Fig. 2a,b). Fig. 2 depicts also the location of the two bridges, whose modeling required the description of the bridge piers.