Rainfall-Induced Landslides in Flysch-Dominated Terrains of the Tzoumerka Mountains (NW Greece): A Post-Event Engineering Geological and Hydrogeological Assessment

Abstract

An extreme rainfall event affected the mountainous area of the Tzoumerka Mountains (NW Greece) during 18–22 November 2025, with cumulative precipitation exceeding 1000 mm on a monthly basis. This exceptional hydrometeorological episode was characterized by prolonged and high-intensity rainfall and triggered widespread slope instabilities across a geomorphologically complex and tectonically active region. Numerous failures were recorded, impacting settlements, road infrastructure, and retaining structures within the Municipality of North Tzoumerka, causing significant disruption to local communities and transport networks.

This study presents the results of detailed post-event field surveys and an integrated engineering geological assessment of rainfall-induced landslides developed primarily within flysch formations and their associated weathered mantles. The investigated area is characterized by steep slopes, thick weathering profiles, and heterogeneous lithological conditions, which strongly influence slope stability under extreme hydrological loading. The observed failure mechanisms include shallow translational landslides, debris and mud flows, surface erosion phenomena, and failures of retaining structures, often occurring in close spatial association.

Particular emphasis is placed on the hydrogeological conditions governing slope instability. Field evidence indicates the development of temporary perched groundwater within the weathered mantle and along permeability contrasts between permeable colluvial deposits and the underlying low-permeability flysch formations. These conditions promoted rapid infiltration, accumulation of subsurface water, and limited drainage capacity, leading to critical pore-water pressure build-up during the rainfall event.

Field observations and qualitative assessments suggest that prolonged and intense rainfall resulted in a rapid increase in pore-water pressures, reduction of effective shear strength, and progressive degradation of slope stability. In several locations, anthropogenic factors significantly aggravated slope instability, including inadequate surface drainage systems, road excavations that altered natural slope geometry, and retaining structures founded on weathered or poorly characterized materials.

The results highlight the high susceptibility of flysch-dominated terrains to extreme precipitation events and underline the critical role of coupled hydrogeological and engineering geological processes in landslide initiation and evolution. The study emphasizes the importance of post-event field investigations for understanding failure mechanisms and supports the need for integrated hazard assessment, improved drainage design, and targeted mitigation strategies in mountainous regions increasingly exposed to extreme rainfall conditions under a changing climate.