Hydrodynamic modelling of glacial lake outburst flood in lower Barun lake: Fid-Bau Portal

Hydrodynamic modelling of glacial lake outburst flood in lower Barun lake

Mandal, Aakash / Adhikari, Abiral / Shakya, Abisha / Dwivedi, Aaditya / Bhusal, Amrit / Shrestha, Anish / Kafle, Mukesh Raj

Abstract

Due to climate change, the frequency of Glacial Lake Outburst Floods (GLOFs) is expected to increase, posing significant risks downstream. This study aims to assess the potential risks associated with GLOFs in Lower Barun Lake and their impacts on downstream settlements and the Arun-3 Hydroelectric Power Plant (HEP). Various breach scenarios were modeled using FLDWAV and HEC-RAS, incorporating avalanche events and the potential complete breach of Upper Barun Lake as triggering factors. Avalanche zones were identified and simulated using the Analytical Hierarchy Process (AHP) within a Geographic Information System (GIS) platform, and the Rapid Mass Movement Simulation (RAMMS) model was used for dynamic flow analysis. A breach hydrograph was generated with FLDWAV and routed through HEC-RAS to assess the downstream impacts. Satellite imagery and GIS were employed to map vulnerable sites and identify high-risk areas. The simulations predicted peak discharge, timing, and potential floodwater depth from Lower Barun Lake to the Arun-3 HEP. The results indicate that the Lower Barun Lake moraine dam is highly vulnerable to overtopping from avalanche-induced waves, with a 5.4 m high impulse wave potentially causing dam failure. A complete moraine dam failure results in significant downstream impacts, contrasting with minimal or negligible effects from a 25% breach. Peak discharge values ranged from 51,231 m³/s for the full dam breach scenario (100%) to 9,014 m³/s for the partial breach scenario (25%). Following a 100% breach, flooding at the Arun-3 HEP would occur after approximately 2 h, reaching a maximum discharge of 13,403.08 m³/s and a peak flood depth of 49.12 m.