Issue 46

M. L. Puppio et alii, Frattura ed Integrità Strutturale, 46 (2016) 190-202; DOI: 10.3221/IGF-ESIS.46.18 191 alternate to longer dry periods as well [5]. This scenario, quite common in tropical areas, is becoming more and more frequent even in the Mediterranean one. Its negative consequences are worsened by the improper transformation of the territory and of ecosystems, such as [6]: (1) river filling, (2) waterproofing of wide urban areas, (3) infrastructures in areas with a high hydrogeological risk, (4) inadequate secondary network of rain water discharge. This paper deals with this last aspect, analysing the risk mitigation on reduced span bridges [7-8]. This topic involves several possible failure scenarios associated to the possibility of a hydraulic overlap of the bridge. These phenomena affect catching basins characterized by a reduced extension, which are challenged by intense and short term rains. The study case examined occurred in Livorno. During the flood in September 2017, several people were injured, while buildings, enterprises and three road bridges were damaged. The failure mechanisms will be further highlighted, emphasizing the recurring elements of vulnerability. T HE L IVORNO CASE ivorno (Wgs_84: 43°33′0″N; 10°19′0″E) is a town along the Tyrrhenian coast of the Italian peninsula, located on the sea. Its territory has a surface of 105 km 2 with about 160.000 inhabitants. It develops lengthways along the seacoast, with a main north-southern direction. Some short watercourses, flowing from the inland to the sea towards east west, cross it (see Tab. 1). Number Denomination Total Length* [km] Catching basin extension* [km 2 ] 1 Rio Ardenza 7.31 21.20 2 Rio Cignolo 3.96 0.46 3 Rio Dei Poggi 1.13 0.31 4 Rio Maggiore 9.30 11.29 5 Rio Paganello 4.82 4.92 6 Rio Popogna 2.31 4.01 7 Torrente Chioma 9.05 18.68 8 Torrente Ugione 12.17 11.52 Table 1: river of Livorno. The port of Livorno is one of the characterizing elements of the town (Fig. 1). It is classified as a first level port inside the Tyrrhenian Corridor [9], remarkably equipped with infrastructures and means to host fast displacement ships. Infrastructures allow the direct connection with the national road and railway networks and with the airport areas of Pisa and Florence. Due to its significant urbanization, Livorno has several intersections between the road network and the hydrographical one (including moats and canals as well as minor watercourses). While other public works strategic for the territory (schools, hospitals, etc.) are easily identified and qualified, reduced span bridge are dealt with differently. They are widely spread on the territory (about 1 infrastructure per km of road development). The identification, evaluation and study of their vulnerability is discussed in [9-13]. The collapse of short span bridges is a frequent scenarios but is not so common in scientific literature as for example the failure of long span bridges [14-17]. The survey of infrastructures in this area has been carried out through Gis software (Q-GIS rel. 2.18 Las Palmas) intersecting the road network with the hydrographical one [17-18]. This instrument allows a classification of bridges according to the importance of the networks generating the intersection. In Tuscany there are about 50.000 small bridges, whose classification in the Tuscan provinces is given in Fig. 2. An average density of 2.92 intersections per km 2 and a linear density of 1.06 intersections per km are recorded; the crossing density in the single provinces is given in Fig. 3. L