Himalayan sediment bombs – generalizing from the 2021 Melamchi Khola disaster

The Himalayas experience frequent fluvial disasters due to precipitation-driven floods, as well as events such as glacial lake outburst floods and landslide lake outburst floods. In the past years, these events have received much attention in order to understand and estimate their likelihoods and potential magnitudes. In addition to inundation-related impacts, geomorphic processes such as sediment transport, bank erosion, and hillslope feedbacks have been addressed by numerous studies at different timescales, demonstrating that sediment mobilization, transport, and deposition can become a dominant driver of fluvial hazard and risk.

The combined impact of hydrological and geomorphological processes is exemplified by the 2021 disaster on the Melamchi River in central Nepal. A flood event on 15 June caused wide-spread aggradation along tens of km of river corridor, with up to 15 meters of deposition in the town of Melamchi Bazar, destruction of the intake of the Melamchi-Kathmandu drinking water tunnel, and extensive modification of the river channel. This modification led to increased terrace erosion, slope destabilization, and amplified impacts of later floods. A second flood event on 31 July caused further aggradation and extensive damage. Overall, sediment deposition in these events was on the order of 50-75 million m³. One source of these deposits can be identified in the upper headwaters of the Melamchi Khola at an elevation of ~3600m, where ~100 million m³ of sediment was stored behind a paleo-landslide dam.  Destabilization of the landslide deposit during the June event has led to ~1 km of headward incision and the rapid evacuation of both landslide and paleo-lake fill sediment.

The Melamchi event highlights the importance of understanding controls on sediment storage and mobility in mountain catchments, and of recognizing the potential hazards involved once these sediments are entrained and routed downstream. In particular, situations where a large amount of relatively fine-grained mobile sediment is trapped behind a landslide dam, moraine, or other temporary dam have the potential for catastrophic sediment transport events. To better understand these potential hazards and their spatial patterns, we have conducted a Himalaya-wide assessment to identify, classify, and predict the occurrence of such features, which we term sediment bombs. A statistical model of sediment bomb locations identifies mean hillslope gradient and glacier extent during the last glacial maximum as predictors of their occurrence, suggesting that post-glacial adjustment may promote their formation in steeply incised valleys. Our analysis suggests that the conditions that led to the Melamchi disaster are more widespread than previously recognized, and that these upstream reservoirs of potentially mobile sediment should be considered in assessments of fluvial hazards.