Coupled Hydrogeological Controls on Long-Term Slope Displacement in a Coastal Landslide Site

Landslides are widespread geohazards worldwide, influenced by site-specific geological, geomorphological, hydrological, and vegetative conditions. Although rainfall, seismic activity, and human disturbances are the primary triggering factors, local environmental settings can substantially shape both the initiation mechanisms and movement patterns.

This study examines the role of hydrogeological characteristics in driving long-term, continuous slope displacement over several decades at a coastal landslide site. The 30-hectare catchment is mantled by colluvial deposits that vary in thickness from a few meters on the lower slope to several tens of meters on the middle slope, extending along an approximately 500-m-long hillslope. The colluvium overlies a thick mudstone formation, within which subsurface investigations identified a 5–10 m thick highly saturated zone near the coastline. A village is situated on the lower slope close to the coast, while the remainder of the site is covered by undeveloped forest.

Finite element analyses were performed to simulate hydrological evolution within the geological strata under rainfall conditions from 2013 to 2024. The results indicate that hydraulic head near the coast remains substantially higher than that in the upslope area, while within the mudstone layer it gradually decreases landward. This configuration restricts subsurface flow from the slope toward the sea during rainfall events, suggesting the presence of a hydraulic barrier along the coastal boundary. Precipitation in the catchment also appears to drive an upslope migration of the high-moisture zone in the shallow colluvium and mudstone layer, forming a freshwater aquifer. The gradual landward expansion of this moisture zone may induce long-term creep deformation within the highly saturated mudstone near the coastline, contributing to progressive slope destabilization—consistent with site instrumentation data showing horizontal displacements exceeding 25 meters into the underlying mudstone.

These findings highlight the critical role of site-specific interactions among geomorphological, geological, and hydrological processes in governing landslide mechanisms. They further underscore the importance of an integrated hydrogeological investigation and understanding for improving landslide assessment, prediction, and long-term hazard management.