Rethinking Hydropower Tunnel Design in the Nepal Himalaya after Extreme Flood, GLOF, and Debris-Flow Events: Lessons from the Rasuwa Disaster

Abstract

Hydropower development in the Nepal Himalaya faces significant engineering challenges due to steep terrain, active tectonics, fragile geological formations and increasing climatic variability. Natural hazards such as extreme floods, landslides, debris flows and glacial lake outburst floods (GLOFs) pose serious threats to hydropower infrastructure, particularly during the monsoon season. The August 2026 Rasuwa disaster highlights the potential impacts of cascading hazards, where multiple natural processes interact and produce severe downstream consequences. This paper examines the vulnerability of critical underground hydropower structures, including tunnel portals, headrace tunnels and underground powerhouses, to flooding, slope failures, river erosion and changing river channel conditions. It also discusses how conventional engineering design approaches, largely based on historical records and established hazard assessments, may not adequately address emerging and interconnected risks. The study emphasizes the importance of understanding geological, hydrological and climatic interactions when planning and designing hydropower projects in mountainous regions. Improved geological investigations, continuous monitoring, remote sensing and integrated multi-hazard assessments are essential for reducing infrastructure vulnerability. Adopting climate-resilient engineering practices and considering cascading disaster scenarios can strengthen the long-term safety, reliability and sustainability of hydropower development across the Nepal Himalaya under increasingly uncertain environmental conditions.