Numerical Modelling of a Tsunami impinging Verde Lago, Algarve

Onshore lithostratigraphic surveys were used to validate tsunami inundation models in Verde Lago (Algarve, Portugal). To validate numerical modelling, filed data was gathered including: sediment samples collected in eight trenches and three cores retrieved within the Late Holocene stratigraphic sequence. Furthermore, textural analysis and age-estimation of the main lithostratigraphic units was obtained. Five sequential lithostratigraphic units were identified and defined in the different cores and trenches.

A tsunami-related depositional unit was described within the medium-sand dominated sequence. It presented features commonly associated with tsunami sedimentological imprints (e.g. coarser unit, thinning inland; erosional contact). However, dating results indicate the deposit to be originated from a yet-unknown evet at app. 4500 yrs BP. Four tsunami-earthquake sources were simulated (i.e. Gorringe Bank, the Horseshoe Fault, the Marquês de Pombal Fault and a combined scenario that is the result of the combination of Gorringe and Horseshoe Fault). Tsunami generation and propagation and sediment transport modelling were conducted using Delft-3D, a numerical model extensively used to simulate tsunamis and storm events.

For the Gorringe Bank seismic source, a maximum value 2.4 m of water column near the coastline was recorded and a velocity 0.7 m/s, the maximum inundation extension recorded was 32 m. The Horseshoe Fault scenario, presented the greatest inundation extent in Verde Lago with a maximum extent of 845 m, it was also possible to observe that the water column was about 5.5 m, finally, a maximum flood velocity of about 2.8 m/s. For the Marquês de Pombal Fault a maximum water level of 3.75 m, and a flood velocity of 2.2 m/s was recorded, in this scenario, a maximum length of 390 m was recorded. For the Scenario 1, a maximum water level of 2.5 m was recorded, as well as a maximum velocity of 1.3 m/s, a maximum inundation extent of 195 m was recorded. From the results obtained, it was evident that the Horseshoe Fault scenario showed the best correspondence between the morphological changes along the perpendicular coastal stratigraphic profiles in Verde Lago and the hydrodynamic results.

This work is supported by the Portuguese Fundação para a Ciência e Tecnologia, FCT, I.P./MCTES through national funds (PIDDAC): UID/50019/2025, UIDB/50019/2020 (https://doi.org/10.54499/UIDB/50019/2020) and LA/P/0068/2020 https://doi.org/10.54499/LA/P/0068/2020).