Modelling tsunami sedimentation associated with the AD 1755 event in Algarve (Portugal)

Abstract:

Numerical models of tsunami inundation and sedimentation can provide useful insights into the dynamics of palaeotsunamis. We applied a coupled field data and numerical modelling approach for the AD1755 tsunami, the most destructive tsunami to affect the Atlantic coast of Europe in historical times. At Salgados, a lowland on the south coast of Portugal, tsunami deposits from AD1755 mostly consist of massive or normally-graded, landward thinning layers of shell-rich sand with an erosive base within the mud-dominated lowlands. Landward of the foredune, the AD1755 deposit is roughly 10cm thick and thins in the landward and alongshore directions. It is possible to ascribe the sediment source of this deposit to the dune and/or beach based on mineralogical and grain-size comparisons with modern surface samples. The present dune crest height is 6 m above MSL (mean sea level) near the seasonally-closed inlet of the lagoon, and rises alongshore towards the west up to 17m above MSL. From the combination of the spatial distribution of the deposit thickness landward of the sloping dune, and GPR data, which shows an erosional surface at approximately 6m above MSL, we infer that the maximum tsunami water level at the coast was between 6 and 10m. Regional tsunami historical records, however, suggest higher heights, up to 12m above MSL at the coast.

We simulated tsunami inundation and sediment transport using Delft3D to examine these discrepancies. A 1D cross shore model was used to test flow height controls on deposit thickness and also to identify the sediment source of the AD1755 deposit. Four possible sediment sources were tested (nearshore, beach, dune and lagoon) using synthetic, long-period waves to simulate the AD1755 tsunami. The combination of geological studies with numerical modeling of inundation and sediment transport produces a better description of the AD1755 tsunami and its effects in coastal areas in the Algarve that will contribute to better hazard assessments.