A 3000 YEAR RECORD OF EXTREME FLOODS IN THE LLOBREGAT BASIN, NORTH EAST SPAIN

Slackwater palaeoflood deposits were found along two reaches of the Llobregat River at Pont de Vilomara and Monistrol de Montserrat. A total of 56 individual flood units have been preserved in rock alcoves located on the valley margins of these bedrock gorge reaches. The highest elevation modern slackwater flood deposits, from the 1971 flood, indicate that the instrumental gauging station records underestimate the magnitude of extreme flood events. At Pont de Vilomara a discharge estimate of 2300 m³s^-1 was obtained from slackwater flood deposits from the 1971 event, compared to a discharge of 1500 m³s^-1 recorded at the gauge station. In terms of flood risk estimation the relative magnitude of the 1971 flood, the largest modern event, in comparison to earlier flood events is of particular relevance. The Llobregat palaeoflood record shows the occurrence of at least 8 palaeofloods of greater magnitude than the 1971 flood, these being radiocarbon dated to two distinct periods: a) the Late Bronze Age (ca. 2700 yrs BP and b) the Little Ice Age (ca. 16-17th centuries AD). The minimum discharge estimates of these extreme palaeofloods are 3700 m³s^-1 at Pont de Vilomara and 4680 m³s^-1 at Monistrol de Montserrat, respectively 38% and 46.5% greater than the discharge of the 1971 event at these reaches. The data indicate that the most extreme Llobregat palaeofloods exceeded in magnitude the largest floods observed in the instrumental record and provide valuable additional information for flood risk assessment.