Paleo-landscape evolution and abrupt changes in Spanish coastal settings: insights into hazard analyses and risk management.

Southern and southeastern Spain has experienced an outstanding coastal landscape change since the middle Holocene, when most of the lower reaches of fluvial valleys were drowned by the mid-Holocene sea level highstand. Since then, many of the main rivers have seen how their mouths evolved from open estuaries discharging into wide embayments to broad wetlands isolated from the open sea by the growths of spit bar systems.

The silting up of these estuaries led to the development of flat lowland areas where geological processes such as floodings, earthquakes and tsunamis affected historic and prehistoric coastal settlements inducing severe landscape changes, that triggered abrupt abandonments of sites and important population changes. Multiple geoarchaeological data support this complex interaction among coastal changes and ancient populations from at least Late Bronze times including Phoenician and Roman settlements in the Atlantic and Mediterranean Iberian littorals.

Good examples of this evolution are, among many others, the present marshlands at the Guadalquivir (Atlantic) and Segura (Mediterranean) river mouths. Historical descriptions show that during roman times (1^st century BCE) the present Guadalquivir marshland (Doñana National Park) was an ancient embayment (Lacus Ligustinus) that was progressively closed by the growth of large spit-bars since roman times. Geological and historical data indicate the occurrence of several islets downstream Sevilla and the initial filling of the ancient bay by a prograding delta-like environment. A severe tsunami event (218-90 BCE) reshaped the geometry of the littoral spit bar system inducing important coastal changes and site abandonments. Progressive growth of the closing spit bars, and coeval fluvial dynamics accelerated by the strong deforestation suffered in Spain in the 19^th century, caused the continuous infilling of these embayments.

The evolution of the coastal landscape at the Segura River mouth (Lower Segura Depression) is quite similar, with a Segura River mouth evolving from a mid-Holocene estuary to a delta environment that grew into a broad brackish lagoon isolated from the open sea by spit bar systems (Sinus Ilicitanus) during Phoenician to Roman times. This lagoon environment persisted till the dawn of the 17^th century CE when it was anthropically infilled for the agricultural improvement of the zone. These reclaimed lowlands suffered destructive earthquakes during Phoenician, Roman, Muslim and modern times. The first documented event affected a littoral Phoenician settlement (7^th century BCE) and the last destructive one was the 1829 AD Torrevieja earthquake with multiple liquefaction cases inland and coastal uplift of centimetric scale (10-15 cm). Both events have triggered the destruction, abandonment and relocation of several localities.

In both cases, these lowland coastal areas have been recurrently affected by severe flash-flood events, storm-surges, tsunamis and large earthquakes. The massive urbanization and tourism improvement of these zones, before modern practices of land planning and management, imprint to these coastal zones an important multi-hazard nature to be considered by policy makers for future land-use planning.

Acknowledgements: This is a contribution to IAG Working Group on Coastal Geoarchaeology; supported by the Spanish Research Project I+D+i PID2021-123510OB-I00 (QTECIBERIA-USAL) funded by the MICIN AEI/10.13039/501100011033/.