Cascading Events Following the Roggia Morlana Barrage Failure and Multi-Risk Assessment

Barrages are low-head dams built to divert a prefixed amount of flow rate from the river. They set the water level upstream, for instance to ensure the diversion of water toward a hydroelectric plant or for irrigation purposes, affecting the riverbed elevation and the topography of the surrounding area. The Roggia Morlana barrage is an ancient structure located along the Serio river in the province of Bergamo, northern Italy. Since 1200, it was used for irrigation purposes thanks to the several artificial canals that derive water from it. Nowadays, it has an important role also for hydroelectric power production. Furthermore, various fundamental infrastructures are located 200 m upstream of the barrage, that are a gas pipeline, a water main and a bridge. In October 2020, a flood event led to the collapse of a part of the barrage and to the subsequent lowering of the river bed and destabilization of the banks. In addition to the stop of the hydropower production and the lack in satisfying the irrigation demand, the retrogressive erosion threatened the upstream infrastructures, which possible failure induces remarkable issues to surrounding villages. Hence, to ensure the safety condition of all the infrastructures, a proper rehabilitation of the barrage was needed. The stability of the restored barrage depends on the magnitude of the flood event and on the scouring phenomena that develop downstream of it. To analyse the physical processes involved, a physical model was developed reproducing several riverbed protection configurations and different flood events. By this way, the exposure and vulnerability of the critical infrastructures are assessed. The multi-risk of failure of the infrastructures and the subsequent impact is evaluated through a combination of probabilities and consequences, according to a combination of the exceedance probability of the flood event and the vulnerability of the infrastructures. The consequences, instead, are defined according to the importance of each infrastructure for the society and the impact caused by its failure. The application of the multi-risk concept aims to highlight how the interaction among hazards increases the risk respect to that for independent events. Other possible flood risk management strategies for these interrelated critical infrastructures are also described.