Cascading Hazards of Land Subsidence and Relative Sea‑Level Rise: Flooding Risks in the Coastal Towns of Messolonghi and Aitolikon

This study investigates surface deformation phenomena in the towns of Messolonghi and Aitolikon, located in Aitoloakarnania, western Greece, by applying Persistent Scatterer Interferometry (PSI) techniques. No systematic remote sensing–based investigation of land subsidence has been conducted in these areas, despite recurring reports of flooding, ground deformation, and structural damage.

Multitemporal Interferometric Synthetic Aperture Radar (MT-InSAR) analysis was performed using Sentinel-1A and Sentinel-1B satellite data covering the period 2015–2022. Persistent Scatterer (PS) time series were extracted to quantify Line-of-Sight (LOS) deformation rates and were cross-validated with results from the European Ground Motion Service (EGMS) of the Copernicus. Geological, geotechnical, and hydrogeological data were also acquired from the Hellenic Survey of Geology and Mineral Exploration, the Central Laboratory of Public Works archives, and private geotechnical consultants’ reports. These findings enabled a more robust interpretation of the observed deformation patterns and their controlling mechanisms.

The InSAR analysis results reveal ongoing subsidence in both towns, with spatially variable deformation rates. In Messolonghi, LOS deformation rates reach up to −5 mm/yr, particularly in the eastern and southern sectors of the town, while northern areas exhibit stability (0.3 to −1.3 mm/yr). Subsidence rates increase towards the coastline, reflecting the presence of younger, unconsolidated alluvial deposits. In Aitolikon, mean deformation rates reach −4.5 mm/yr, with pronounced subsidence observed in both the southern and northern parts of the island, where significant structural damage has been reported. These areas coincide with zones of artificial fillings.

Geological and geotechnical data indicate the presence of laterally continuous Quaternary deposits, consisting of clay, clayey silt to silt, and clayey sand to sand horizons, with a thickness of 80-100m. The observed deformations are primarily attributed to the natural compaction and consolidation of these sediments, further intensified by anthropogenic interventions such as river diversion in Messolonghi.

Beyond the subsidence processes identified, the long‑term impacts of climate change further intensify the vulnerability of Messolonghi and Aitolikon. Continuous sea level rise, documented at rates of 2–4 mm/yr in the wider Mediterranean, combined with coastal erosion and increasingly frequent flooding events, places additional stress on both cities. The combination of the formations’ compaction with rising water levels and extreme precipitation has led to recurrent inundations of the low‑lying areas. Both towns have already been declared in a state of emergency during major flood events in recent years, underscoring the severity of the hazard. Projections under high‑emission scenarios (SSP5–8.5) suggest that by 2100, sea level rise could exceed 0.8 m in the Messolonghi lagoon, significantly expanding the existing flood‑prone zones.

Overall, the study demonstrates that land subsidence in Messolonghi and Aitolikon is an ongoing process with steady deformation rates, posing a risk to the infrastructure and buildings in both areas. In addition, the continuous rise in sea level, combined with coastal erosion and increasingly frequent flooding events driven by climate change, further exacerbates the vulnerability of these towns.