Scenario-based HEC-RAS 2D unsteady flow analysis of Shisper Lake for GLOF risk assessment

Glacial lake outburst floods (GLOFs) are climate change induced natural disasters that can damage lives, economy, and infrastructure. Globally, 15 million people are exposed to GLOFs, and one-third of this population is from Pakistan and India. In Pakistan, Shisper glacier is surging rapidly leading to formation of Shisper lake fed by meltwater of Mochuwar glacier. In this study, four hypothetical flow scenarios (100%, 75%, 50%, and 25%) based on empirical volume estimation of the lake, are hydrologically modeled using 2D unsteady flow analysis approach on HEC-RAS 6.5. Maximum flood inundation boundary, mean depth, velocity, and depth times velocity are computed and compared for all the scenarios. Flood inundation boundary was highest for 100% scenario (0.48 km(2)), followed by 75% (0.46 km(2)), 50% (0.43 km(2)), and 25% (0.39 km(2)). For all scenarios, flood extent covered the croplands, Hassanabad bridge, and Karakoram highway (KKH). Mean depth exceeded threshold value (0.35 m) in all scenarios, whereas threshold value for velocity i.e., 1.5 ms(- 1) was exceeding in 100% (2.28 ms(- 1)), 75% (2.10 ms(- 1)), and 50% (1.85 ms(- 1)) flow scenarios. Depth * Velocity parameter which indicates the more realistic impact of flood highlighted that combined effect of depth and velocity is threatening in all scenarios i.e., exceeding the threshold value (0.52 m. m/s). Mean arrival time of the flood from Shisper lake to Hunza river was < 2 hours. Flood analysis and risk maps confirmed the devastating potential of all scenarios on downstream communities. Based on HEC-RAS simulations and risk mapping, potential GLOF mitigation strategies are recommended.