Glacial Lake Inventory and Lake Outburst Flood/Debris Flow Hazard Assessment after the Gorkha Earthquake in the Bhote Koshi Basin

Glacial lake outburst floods threaten China-Nepal connectivity: Synergistic study of remote sensing, GIS and hydrodynamic modeling with regional implications

Holistic study of glacial lakes and glacial lake outburst floods (GLOFs) in the strategically important China-Nepal transportation corridors is imperative for regional connectivity and disaster risk reduction. This study focuses on four China-Nepal transportation corridors, namely Chentang-Kimathanka, Zhangmu-Kodari, Keyrung-Kathmandu and Taklakot-Hilsa from east to west in the Himalayan region. Within a remote integrated framework, we present the latest high-resolution inventory of glacial lakes, assess their decadal spatio-temporal changes (1992-2022), identify potentially dangerous glacial lakes, and apply hydrodynamic model to assess downstream impacts of possible GLOFs along the study area. The results show 2688 glacial lakes (≥0.001 km2) with a total area of 116.10 ± 8.53 km2 over the study area in 2022. Glacial lakes exhibited spatiotemporal heterogeneity in expansion, with overall expansion of 32 % during 30 years. Keyrung-Kathmandu corridor, among others, was assessed with high GLOF susceptibility. Furthermore, hydrodynamic modeling of four highly dangerous lakes in each transportation area reveals that GLOFs have cross-border effects, impacting ∼103 km of China-Nepal highway, 103 bridges, two major dry ports and 3301 buildings in both countries. Based on these findings, we emphasize the joint efforts of both countries for integrated disaster management for smooth connectivity between two countries and saving downstream population through joint cooperation from central to local government levels by initiating artificial lake lowering, developing cross-border early warning systems and cooperation. This study is valuable for presenting a synergistic study of glacial lakes and GLOF for informing decision- and policy-makers of both China and Nepal for a joint approach to disaster mitigation.

A novel framework for predicting glacial lake outburst debris flows in the Himalayas amidst climate change

The retreat of Himalayan glaciers and the expansion of glacial lakes due to global warming have increased the occurrence of glacial lake outburst debris flow (GLODF), posing a serious threat to downstream communities. However, there are gaps in understanding the changes in GLODF occurrence driven by climate change, which challenges disaster management and cross-border cooperation in the Himalayas. To consider this issue, our study presents a novel framework integrating environmental evolution, a process-driven indicator system, and a hybrid machine learning model to predict Himalayan GLODF occurrence in the 21st century. Our findings indicate ongoing temperature (0.27-0.60 °C/10a) and precipitation (1.30-5.00 %/10a) increases under both SSP245 and SSP585 scenarios. Meanwhile, Himalayan glaciers are projected to lose between 70 % and 86 % of their mass by 2100 compared to 2020. Additionally, 2722 ± 207 new glacial lakes are expected to emerge by 2100. GLODF occurrence probability index is anticipated to rise to 1.27-1.30 times the current levels, with the Western Himalayas and Indus basin as high-incidence areas. Currently and in the future, the China-Nepal border remains a hotspot for cross-border GLODF. Our framework offers valuable long-term insights into Himalayan GLODF occurrence trends in response to climate change.

Forecasting of flash flood susceptibility mapping using random forest regression model and geographic information systems

Flash floods, rapid and devastating inundations of water, are increasingly linked to the intensifying effects of climate change, posing significant challenges for both vulnerable communities and sustainable environmental management. The primary goal of this research is to investigate and predict a Flood Susceptibility Map (FSM) for the Ibaraki prefecture in Japan. This research utilizes a Random Forest (RF) regression model and GIS, incorporating 11 environmental variables (involving elevation, slope, aspect, distance to stream, distance to river, distance to road, land cover, topographic wetness index, stream power index, and plan and profile curvature), alongside a dataset comprising 224 instances of flooded and non-flooded locations. The data was randomly classified into a 70 % training set for model development, with the remaining 30 % used for model validation through Receiver Operating Characteristics (ROC) curve analysis. The resulting map indicated that approximately two-thirds of the prefecture as exhibiting low to very low flood susceptibility, while approximately one-fifth of the region is categorized as high to very high flood susceptibility. Furthermore, the RF model achieved a noteworthy validation with an area under the ROC curve of 99.56 %. Ultimately, this FSM serves as a crucial tool for policymakers in guiding appropriate spatial planning and flood mitigation strategies.

Transition of a small Himalayan glacier lake outburst flood to a giant transborder flood and debris flow

Glacial lake outburst floods (GLOFs) are a great concern for the Himalaya, as they can severely damage downstream populations and infrastructures. These floods originate at high altitudes and can flow down with enormous energy and change the terrain’s existing morphology. One such devastating event occurred on the night of 5 July 2016, from the inconspicuous Gongbatongsha Lake, located in the Poiqu basin, Eastern Himalaya. The Poiqu basin in the Tibetan Autonomous Region currently contains numerous big glacial lakes; however, this event originated from a small lake. The GLOF was triggered following heavy precipitation that led to a slope failure above the lake and deposition of debris into the lake, which breached the moraine dam and rapidly drained the entire lake. The flood damaged several downstream infrastructures, including the Arniko highway, the Upper Bhotekoshi hydropower plant, and several buildings as it made its way into the Bhotekoshi basin in Nepal. This study adopts a multi-model approach to reconstruct the GLOF trigger and the flood’s transformation into a severe debris flow. Proxies including flow discharge, flow velocity, runout distances were used to calibrate the model and validate the results. Results reveal that a debris flow of volume ranging between 3000 and 6000 m³ from the headwall must have led to lake overfill, eventually leading to the GLOF event. The GLOF showed a significant increase in peak discharge from 618 to 4123 m³ s⁻¹ at the Zhangzangbo-Bhotekoshi confluence. The average velocity of the flow is calculated to be ~ 5.5 m s⁻¹. Reconstruction of the erosion and deposition dynamics show that maximum erosion occurred in the first 6.5 km, with maximum deposition occurring near the Upper Bhotekoshi hydropower station. The modeling indicates that the availability of the entrainable debris along the channel, likely from the previous landslides, amplified the event by three orders of magnitude-additional water ingested from the river. Overall, we demonstrate how the small-scale Gongbatongsha GLOF amplified downstream by incorporating pre-existing sediment in the valley and triggered damaging secondary landslides leading to an economic loss of > 70 million USD.

Glacial Lake Evolution (1962–2018) and Outburst Susceptibility of Gurudongmar Lake Complex in the Tista Basin, Sikkim Himalaya (India)

The Sikkim Himalayan glaciers and glacial lakes are affected by climate change like other parts of the Himalayas. As a result of this climate variability in the Sikkim Himalaya, a detailed study of the Gurudongmar lake complex (GLC) evolution and outburst susceptibility assessment is required. Glacial lake volume estimation and lake outburst susceptibility assessment were carried out to reveal different characteristics for all four lakes (GL-1, GL-2, GL-3, and GL-4) from the lake complex. Each of these lakes has a moderate to very high potential to outburst. As the dam of GL-1 provides no retention capacity, there is a very high potential of a combined effect with the sudden failure of the moraine-dams of GL-2 or GL-3 located upstream. Temporal analysis of GLC using optical remote sensing data and in-field investigations revealed a rapidly increasing total lake area by ~74 ± 3%, with an expansion rate of +0.03 ± 0.002 km2 a−1 between 1962 and 2018 due to climate change and ongoing glacier retreat. The overall lake area expansion rates are dependent on climate-driven factors, and constantly increasing average air temperature is responsible for the enlargement of the lake areas. Simultaneously, changes in GLC expansion velocity are driven by changes in the total amount of precipitation. The deficit in precipitation probably triggered the initial higher rate from 1962 to 1988 during the winter and spring seasons. The post-1990s positive anomaly in precipitation might have reduced the rate of the glacial lake area expansion considerably.

Flash Flood Susceptibility Assessment and Zonation Using an Integrating Analytic Hierarchy Process and Frequency Ratio Model for the Chitral District, Khyber Pakhtunkhwa, Pakistan

Pakistan is a flood-prone country and almost every year, it is hit by floods of varying magnitudes. This study was conducted to generate a flash flood map using analytical hierarchy process (AHP) and frequency ratio (FR) models in the ArcGIS 10.6 environment. Eight flash-flood-causing physical parameters were considered for this study. Five parameters were based on the digital elevation model (DEM), Advanced Land Observation Satellite (ALOS), and Sentinel-2 satellite, including distance from the river and drainage density slope, elevation, and land cover, respectively. Two other parameters were geology and soil, consisting of different rock and soil formations, respectively, where both layers were classified based on their resistance against water percolation. One parameter was rainfall. Rainfall observation data obtained from five meteorological stations exist close to the Chitral District, Pakistan. According to its significant importance in the occurrence of a flash flood, each criterion was allotted an estimated weight with the help of AHP and FR. In the end, all the parameters were integrated using weighted overlay analysis in which the influence value of the drainage density was given the highest value. This gave the output in terms of five flood risk zones: very high risk, high risk, moderate risk, low risk, and very low risk. According to the results, 1168 km2, that is, 8% of the total area, showed a very high risk of flood occurrence. Reshun, Mastuj, Booni, Colony, and some other villages were identified as high-risk zones of the study area, which have been drastically damaged many times by flash floods. This study is pioneering in its field and provides policy guidelines for risk managers, emergency and disaster response services, urban and infrastructure planners, hydrologists, and climate scientists.

Climatic and tectonic significance of Taboche Lake, Khumbu Region, Nepal

The Everest region is characterized by its alpine glacial environment. In an effort to understand environmental change and tectonic activity, our team cored Taboche Lake, situated at 4,712 m along the western margin of the Ngozumpa Glacier. This research catalogs past earthquakes using geological records of the lake core that are important for the assessment of future earthquake hazards in the region and provides information for tectonic risk of glacial lake floods. Core grain size characteristics and internal sedimentary structures from computed tomographic scan were coupled with radiocarbon dating of organic matter preserved in the core to reconstruct the environmental history of the area. The 58-cm-long core consists of laminated silty sands and sandy silts with particle diameters <2 mm. The core records a syn-sedimentary deformational structure, folded sediments, rhythmically alternating dark- and light-colored laminations, and turbidites, which indicate coeval climatic and tectonic variations over the past ∼1,600 years.

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Sedimentology and dating are essential to identify environmental changes in lake cores

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Post-1807 AD ± 112 years flood turbidite record indicates warming trend in the Himalaya

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Earthquake-triggered turbidite and slump event in the core relate to historic tremors

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Earthquake records in glacial lake core highlight tectonic risk, which can cause GLFs

Simulation and Assessment of Future Glacial Lake Outburst Floods in the Poiqu River Basin, Central Himalayas

A glacial lake outburst flood (GLOF) is a typical glacier-related hazard in high mountain regions. In recent decades, glacial lakes in the Himalayas have expanded rapidly due to climate warming and glacial retreat. Some of these lakes are unstable, and may suddenly burst under different triggering factors, thus draining large amounts of water and impacting downstream social and economic development. Glacial lakes in the Poiqu River basin, Central Himalayas, have attracted great attention since GLOFs originating there could have a transboundary impact on both China and Nepal, as occurred during the Cirenmaco GLOF in 1981 and the Gongbatongshaco GLOF in 2016. Based on previous studies of this basin, we selected seven very high-risk moraine-dammed lakes (Gangxico, Galongco, Jialongco, Cirenmaco, Taraco, Beihu, and Cawuqudenco) to simulate GLOF propagation at different drainage percentage scenarios (i.e., 25%, 50%, 75%, and 100%), and to conduct hazard assessment. The results show that, when any glacial lake is drained completely or partly, most of the floods will enter Nepal after raging in China, and will continue to cause damage. In summary, 57.5 km of roads, 754 buildings, 3.3 km2 of farmland, and 25 bridges are at risk of damage due to GLOFs. The potentially inundated area within the Chinese part of the Poiqu River basin exceeds 45 km2. Due to the destructive impacts of GLOFs on downstream areas, appropriate and effective measures should be implemented to adapt to GLOF risk. We finally present a paradigm for conducting hazard assessment and risk management. It uses only freely available data and thus is easy to apply.

Assessing the Prospects of Transboundary Multihazard Dynamics: The Case of Bhotekoshi–Sunkoshi Watershed in Sino–Nepal Border Region

The impacts of multihazards have become more pronounced over the past few decades globally. Multiple hazards and their cascading impacts claim enormous losses of lives, livelihoods, and built environment. This paradigm prompts integrated and multidisciplinary perspectives to identify, characterize, and assess the occurrence of multihazards and subsequently design countermeasures considering impending multihazard scenarios at the local level. To this end, we considered one of the most egregious transboundary watersheds, which is regarded as a multihazard hotspot of Nepal, to analyze the underlying causes and cascade scenarios of multihazards, and their associated impacts. In this paper, geophysical, hydrometeorological, and socioeconomic perspectives are formulated to characterize the watershed from the dimension of susceptibility to multihazard occurrence. To characterize the complex dynamics of transboundary multihazard occurrence, insights have been presented from both the Nepali and the Chinese sides. Individual case studies and the interrelation matrix between various natural hazards are also presented so as to depict multihazard consequences in the transboundary region. The sum of the observations highlights that the watershed is highly vulnerable to a single as well as multiple natural hazards that often switch to disasters.

Flood Risk Assessments: Applications and Uncertainties

The present Special Issue brought together recent research findings in Flood Risk Assessments (FRA) and contains contributions on advanced techniques and real cases where FRA have been carried out. The 16 research contributions highlight various processes and related topics where FRA have been applied and the main benefits and improved knowledge derived from them, as well as their replicability in other study sites. The published papers can be classified into three major categories. (a) First, there are those papers focused on improving the methods and results of FRA over different scenarios of both flooding types (river flooding or flash flooding) and flooding areas (urban, non-urban, small mountain communities). (b) Second, there are studies that investigate the application of FRA to diverse topics such as “land urban planning” or “vulnerable infrastructure management (dams, power plants)”. (c) Finally, there is a third group of papers which are focused on the assessment of the sources of uncertainties in FRA, with the aim of improving the results and making it more consistent with the real world.