The flood occurred at the beginning of november 1994 in north west Italy has emphasized the susceptibility of many urban areas towards river dynamics processes. In Piedmont 130 towns were flooded and a huge amount of lives were lost: 70 in all, 50 of which directly caused by the flooding. During the days that followed the flood event many surveys were carried out in the mostly damaged towns, in order to exactly define the flooded areas, measure the levels reached by the water and better understand the flood dynamics. The surveyed data were then compared with the aerial photographs available and were completed by the inspection of the historical records concerning past flood events, the analysis of the urban areas development and the survey of the man made changes of the river beds occurred during the last two centuries. This allowed in many cases to understand the reasons behind the apparent increase of the urban areas susceptibility towards hydrogeologic processes. In this paper the results are presented that were obtained from the detailed study of four urban areas (Alessandria, Asti, Clavesana, S. Stefano Belbo), the first two with a population of more than 70,000 people. These four towns were already flooded during past events and, in some cases, the flooded areas had been even larger. During the november 1994 event in Asti the 24% of the urban area was flooded; however, the level reached by the water within the town was 20 cm lower than in the 1948 flood. In Alessandria the 38% of the urban area was flooded, including zones, like the city, that were never reached by water before, according to the historical records available. Similar considerations can be done for the smaller towns examined: they were already flooded in the past (1926, 1948, 1951, 1968) and also in 1994 had large portions of the urban area flooded. In all the recently damaged towns, and in particular in the cases examined, the amount of the losses was much larger than in the preceding floods. This is due to the fact that the anthropic installations on the territory, especially those realized during the last two centuries, have heavily interfered with the flood dynamics. It appears clearly that the increased town susceptibility, compared to the past, is due to the lack of a correct land-use policy. Many towns, for instance, have expanded occupying river corridors, with scarce consideration for flood hazards. Large areas along the main channels have been used for poplar-growing, even though a law exists since last century that forbids it. This, together with the lack of an adequate natural drainage network maintenance and cleaning, has increased the amount of floating materials in the rivers during the flood, frequently causing the obstruction of those bridges that were not correctly designed and consequently the creation of lakes behind them held by the road and railway embankments. The sudden release of these ephemeral lakes, because of the bridge failure or the embankment removal, contributed to sensibly worsen the flood effects inside the towns, generating fast water flows and rapid increase of the water level. The formulation of effective projects for a correct land-use planning and, in particular, for a flood hazard reduction in heavily populated areas, will have to take into consideration the following aspects: 1) Precise definition of the river corridors, based on the morphological, hydrogeological, meteorological and hydraulic characteristics of the basin. 2) Reconstruction of the main anthropic interventions on the territory, occurred since last century, to evaluate their interactions with the river dynamics processes. 3) Investigation about past flood events through the examination of bibliographic documents or unpublished records, which often are the only source of information available about the historical changes of environmental conditions. 4) a review of the town planning tools on the basis of the knowledge obtained so far, with the individuation of zones at different risk degree where different rules will have to be applied, not only to the new buildings but also to those damaged and/or to be rebuilt. The introduction of a compulsory insurance can be an effective tool to correctly direct the urban development of areas already heavily populated, taking example from countries (France, Japan, United States) where this is a regular procedure since a long time. This measure, together with a correct information to the citizens and the local communities, would spare the government the expenses usually necessary to help the community in case of natural disasters.
E' stata svolta un'analisi dettagliata sull'inondazione degli abitati di Alessandria, Asti, Clavesana e S. Stefano Belbo per riconoscere quali siano state le ragioni all'origine dell'estrema vulnerabilità mostrata dagli insediamenti piemontesi nei confronti dei processi di dinamica fluviale. Lo studio della dinamica e degli effetti delle esondazioni è stato integrato con la ricostruzione delle trasformazioni, naturali ed antropiche, subite dal paesaggio negli ultimi due secoli. Il quadro che ne è emerso indica chiaramente come gli effetti dell'esondazione siano stati amplificati o, comunque, direttamente influenzati dagli interventi antropici e dalle strutture realizzate. La metodologia sviluppata può costituire un valido strumento per la definizione delle aree soggette al rischio di inondazione nei centri abitati e, in definitiva, per una corretta pianificazione del loro sviluppo territoriale.
Evento alluvionale del 5-6 novembre 1994 in Piemonte: considerazioni sulla vulnerabilità di alcuni centri abitati
ARATTANO M;LUINO F
1995
Abstract
The flood occurred at the beginning of november 1994 in north west Italy has emphasized the susceptibility of many urban areas towards river dynamics processes. In Piedmont 130 towns were flooded and a huge amount of lives were lost: 70 in all, 50 of which directly caused by the flooding. During the days that followed the flood event many surveys were carried out in the mostly damaged towns, in order to exactly define the flooded areas, measure the levels reached by the water and better understand the flood dynamics. The surveyed data were then compared with the aerial photographs available and were completed by the inspection of the historical records concerning past flood events, the analysis of the urban areas development and the survey of the man made changes of the river beds occurred during the last two centuries. This allowed in many cases to understand the reasons behind the apparent increase of the urban areas susceptibility towards hydrogeologic processes. In this paper the results are presented that were obtained from the detailed study of four urban areas (Alessandria, Asti, Clavesana, S. Stefano Belbo), the first two with a population of more than 70,000 people. These four towns were already flooded during past events and, in some cases, the flooded areas had been even larger. During the november 1994 event in Asti the 24% of the urban area was flooded; however, the level reached by the water within the town was 20 cm lower than in the 1948 flood. In Alessandria the 38% of the urban area was flooded, including zones, like the city, that were never reached by water before, according to the historical records available. Similar considerations can be done for the smaller towns examined: they were already flooded in the past (1926, 1948, 1951, 1968) and also in 1994 had large portions of the urban area flooded. In all the recently damaged towns, and in particular in the cases examined, the amount of the losses was much larger than in the preceding floods. This is due to the fact that the anthropic installations on the territory, especially those realized during the last two centuries, have heavily interfered with the flood dynamics. It appears clearly that the increased town susceptibility, compared to the past, is due to the lack of a correct land-use policy. Many towns, for instance, have expanded occupying river corridors, with scarce consideration for flood hazards. Large areas along the main channels have been used for poplar-growing, even though a law exists since last century that forbids it. This, together with the lack of an adequate natural drainage network maintenance and cleaning, has increased the amount of floating materials in the rivers during the flood, frequently causing the obstruction of those bridges that were not correctly designed and consequently the creation of lakes behind them held by the road and railway embankments. The sudden release of these ephemeral lakes, because of the bridge failure or the embankment removal, contributed to sensibly worsen the flood effects inside the towns, generating fast water flows and rapid increase of the water level. The formulation of effective projects for a correct land-use planning and, in particular, for a flood hazard reduction in heavily populated areas, will have to take into consideration the following aspects: 1) Precise definition of the river corridors, based on the morphological, hydrogeological, meteorological and hydraulic characteristics of the basin. 2) Reconstruction of the main anthropic interventions on the territory, occurred since last century, to evaluate their interactions with the river dynamics processes. 3) Investigation about past flood events through the examination of bibliographic documents or unpublished records, which often are the only source of information available about the historical changes of environmental conditions. 4) a review of the town planning tools on the basis of the knowledge obtained so far, with the individuation of zones at different risk degree where different rules will have to be applied, not only to the new buildings but also to those damaged and/or to be rebuilt. The introduction of a compulsory insurance can be an effective tool to correctly direct the urban development of areas already heavily populated, taking example from countries (France, Japan, United States) where this is a regular procedure since a long time. This measure, together with a correct information to the citizens and the local communities, would spare the government the expenses usually necessary to help the community in case of natural disasters.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
