Flood hazards: Historical trends and perspectives in relation to Climate Change

Abstract

It is recognized that the most important impacts of climate change on society involve weather related disasters, in particular floods and droughts (IPCC, 2001). In this study, the relationships climate-floods are analysed through the study of instrumental, palaeoflood and documentary flood series. In Atlantic basins, the generation, duration and magnitude of floods are very much associated with changes in winter rainfall. Palaeoflood and documentary flood records show greater frequency of ordinary and extraordinary events during the initial and final phases of cold periods such as the Little Ice Age (AD 1550-1850) and the cold and wet phase around 2,650 years ago (van Geel et al., 1998). In the instrumental period (1910 to the present), Atlantic rivers underwent a decline in the frequency of extraordinary floods, although the magnitude of the most catastrophic floods has remained the same, despite the flood control effect of reservoirs. This upward trend of hydrological variability is expected to continue in the forthcoming decades if we take into account the intensification of the positive phase of the North Atlantic Oscillation (NAO). In Mediterranean basins, past flood series indicate that extreme floods occur during periods of high irregularity of both seasonal and annual rainfall. In recent periods (the seventies and eighties) an increase has been observed in intense rainfall episodes, some of which have caused extraordinary floods. These recent floods reached maximum discharges above those recorded in gauging stations in the first half of the 20th Century (prior to the construction of reservoirs). In this sense, existing data indicate that the temperature rise could increase the irregularity of the flood and drought regime and cause the generation of flash floods in Mediterranean basins and inland of the Iberian Peninsula.