Landslide and flood hazards on mountain roads: a reconnaissance survey from eastern Nepal

A rapid reconnaissance survey of selected roads in east Nepal is described. The survey was undertaken in December 2025, following extreme rainfall, floods and landslides in September 2024 and October 2025. Road damage was recorded and summarised as a percentage of alignment length and presented according to the terrain zones of the Fookes et al. (1985) mountain model. The most significant damage was observed along alignments located on active flood plain and flood plain terraces of Zone 5, followed by the steep lower-valley side slopes of Zone 4, thus vindicating the mountain model. The results cast severe doubt on whether new alignments should be constructed in Zone 5 unless advantage can be taken of high-level terraces distant from active flood plains. Road improvement schemes should also seriously consider realignment away from vulnerable Zone 5 areas, including relocation into Zone 4 or preferably Zone 3 terrain. The lesson from the floods of 2024 and 2025 is the fluvial dynamism of Zone 5 and the fact that large volumes of transported debris, from natural and construction sources, can transform river valley floors with the potential to destroy any built infrastructure located on them. Poor land management practices can also lead to increased runoff and the contribution of debris to the flood plain. Some of the landslides impacting the surveyed roads are illustrated and discussed. The need to integrate engineering geological and geomorphological assessment into route selection, design, construction, operational maintenance and risk management is demonstrated.