Unsteady two-dimensional paleohydraulic reconstruction of extreme floods over the last 4000 yr in Segura River, southeast Spain

SummaryFloods in the Almadenes Canyon (Segura River, southeast Spain) have been reconstructed using unsteady two-dimensional shallow water modelling. Slackwater deposits, archaeological age dating and hydraulic modelling allowed us to reconstruct some of the oldest extreme floods reported in Spain. The paleoreconstruction was performed for the first time in three steps: retrodicted flood discharges for the most recent flood (AD 900) were quantified comparing the simulated Shields and Froude numbers with respect to characteristic values for the existence of ripples and upper-stage plane bed at a cave that served as a shelter for humans; ensuing velocity, Shields and Froude maps were then used as a forward predictor allowing the finding of additional paleostage indicators far from the shelter; finally, new PSIs were employed to verify the reliability of the reconstruction. In doing so, better insight into the paleoflood landscape was derived from consideration of the association of bedforms and modelled hydraulic conditions. The present study, assuming a stable geometry of the bedrock canyon during the Late Holocene, extends the estimation of flow discharge magnitudes to ca. 4000 yr BP ranging from 500 m3/s in the oldest floods (3700 yr BP) up to 700–1000 m3/s in the most recent (AD 900).

► Slackwater deposits dated from 3700 yr BP to after AD 900 are found in Segura River. ► Unsteady 2D shallow water modelling is employed to fit Shields and Froude numbers. ► Retrodicted discharges increased from 500 m3/s (3700 yr BP) to 1000 m3/s (AD 900). ► Retrodicted flood maps are used as a forward predictor in a second phase of fieldwork. ► New sediment records corroborate the suitability of the paleoflood reconstruction.