A late Holocene palaeoflood record from Slackwater flood deposits of the Llobregat River, NE Spain

Abstract

The palaeofloods of the Llobregat River are the first to be reconstructed for a Spanish Mediterranean river basin. In total, 56 individual slackwater flood units were identified in eight valley-side alcoves located along two study reaches, Pont de Vilomara and Monistrol de Montserrat. The majority of the deposits are fine sands or very fine sandy-silts, with a variety of sedimentary structures identified, namely parallel laminations, climbing ripples (both in-phase and in-drift) and current ripples. The estimation of the palaeoflood discharges associated with these deposits, using the HEC-RAS one-dimensional hydraulic model, has provided long-term data regarding flood magnitude within the catchment. Palaeofloods at Pont de Vilomara, radiocarbon dated to 2640±55 yrs BP and 2580±75 yrs BP, have minimum estimated discharges of 3700-4300 m3s-1. The largest palaeoflood at Monistrol de Montserrat, dated to 305±50 yrs BP, has an estimated minimum discharge of 4700 m3s-1. The results indicate that the instrumental discharge series is of insufficient length to have witnessed the largest magnitude flood events within the Llobregat catchment and that the use of palaeoflood hydrology is a valuable means of improving the flood record of Mediterranean catchments.