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INUNDATIONS IN LISBON DURING THE 20TH CENTURY AND THEIR URBANAND PHYSICAL CAUSES. Lisbon often suffers inundations, which must be classified as urban inundations, because the flooded areas involved are impermeable due to urban development. These inundations are not related to the overflow of watercourses, but can be classified as flash inundations because of the speed with which they form and spread. The present study covers a time period from 1918/19 to 1997/98, during which 420 occurrences were recorded in two leading daily newspapers, «O Seculo» and «Diario de Noticias». The research based on these newspapers takes into account daily rainfalls above 10 l/m2 recorded by the Instituto Geofisico Infante D. Luis, as well as daily rainfall below this level but that have also caused inundations. The analysis of the frequency and evolution of these inundations has showed that they occur more often during the autumn (specially in November) and that from the seventies onwards, they have decreased considerably, while at the same time the total rainfall volume has increased, as well as the maximum hourly rainfall that bring them about. Fieldwork was very important in the definition of flooded sites. Research was carried out to specify the physical factors (relief and drainage basins) and urban network (squares and streets). The relationship between these urban factors and the old drainage network was also taken into account in order to distinguish increasing inundation factors. Apart from the description of the physical characteristics of these sites, they were also typified through factor analysis. A hazard factor was attributed to each site based on inundation frequency. The most endangered areas and sites are located mainly in the «Frente Ribeirinha» (along the river bank) built upon the terminal sectors of the natural watercourses, which consists of the receptor streets and receptor squares of the flow. These streets and squares lie perpendicular to the ancient watercourses, and in places where artificial barriers and micro depressions exist. The latter result from the artificial surface of the riverside embankment. Other critical sites exist throughout the town mainly because of obstruction problems or deficient drainage systems, often aggravated, by newly built up impervious areas.

Inundações na Cidade de Lisboa durante o Século XX e os seus factores agraventes