This study investigates the spatial and temporal climate variability, particularly focusing on extreme hydrometeorological events, over Uttarakhand, India. Located within the Central Himalaya, the region is characterised by frequent extreme rainfall events during the Indian Summer Monsoon (ISM) and winter western disturbances (WDs). To understand the relationship between climate variability and global teleconnections (North Atlantic Oscillation (NAO), El Ni & ntilde;o-Southern Oscillation (ENSO), Dipole Mode Index (DMI), we analysed four decades (1982-2020) of observational data, including rainfall, surface radiative temperature, and surface runoff, alongside teleconnection indices. A composite analysis of 140 hydro-meteorological events is performed to correlate climate variations. The results demonstrate significant spatial variability in rainfall and surface runoff between western and eastern Uttarakhand. A marked increase in extreme events, particularly during the monsoon season, is observed after 2010. Notably, the period 1998-2009 shows increased annual temperatures and decreased precipitation and surface runoff, coinciding with periods of low amplitudes in the considered global teleconnections.

Flash floods have emerged as one of the major hazards in the Himalayas, particularly due to the anthro-pogenic encroachment on the flood plains and increasing extreme weather events caused by global temperature rise (climate change). In comparison, mitigation measures are disproportionate, partly because the documented history of flood magnitude and frequencies are limited both in space and time, and the mitigation strategies rely more on compensatory approaches rather than having any long-term proactive planning. Because topographical and climatic elements influence flash floods, it is possible to identify hazardous land areas and model the potential impact of flash floods in a given region based on observations of extreme flood events. In the Himalayas, the hazard zone can be roughly marked between 2500 and 3500 m elevations. In this study, we model three different flash flood scenarios for a tourist destination, Gangotri town, in the northwest Himalayas, based on the flood discharge of three events in similar topography and climatic conditions. To model the flood hydrograph along the Bhagirathi River near Gangotri town, we utilised the open-source software HEC-RAS (1D). Our calculations demonstrated that the floods having discharges equal to the 2013 Gangotri, 2013 Kedarnath, and 2021 Rishi Ganga flash floods would cause water surface elevations of 1.34, 4.16, and 28.66 meters, respectively. The results suggest that a more signifi-cant part of Gangotri town is exposed to a discharge scenario similar to the Kedarnath flood of 2013. According to our preliminary assessment, the minimal loss to buildings in such a scenario would be more than Rs. 2,000 lakh. The study contributes to the most feasible and cost-effective flash flood disaster risk reduction strategy in the Himalayas.

The present work focuses on the role of topographic factors, sediment sources, and their connectivity in determining the flash flood hazard in mountain terrain. The study highlights the climatic and topographic configuration of the Dhauli Ganga Valley of the NW Himalaya that have contributed to the vulnerability of the area. We employed the digital elevation model, satellite data, and field observations to produce the stream profile, stream power index, land use land cover map, and connectivity index to better understand the interaction between topographic parameters and sediment transport dynamics in the high-altitude region. Furthermore, the 2D hydrodynamic model in HAC-RAS software was used to analyze the risk of flash flooding caused by a potential glacier lake outburst. The sedimentary landforms of the valley show evidence of high-magnitude paleo-flood, suggesting enormous potential for the valley to generate high-magnitude floods during extreme events and climate change. The analyses show that the channel slope and sediment availability vary from the headwater glaciated region to the trunk river. These factors, along with connectivity, determine the nature of the hazards associated with the flash flood. The upper reaches have considerable sediment availability, but due to low stream power, the sediments reside there for a considerable time. The middle section has high stream power because of the optimization of the slope and stream discharge, which plays a significant role in sediment entrainment. The trunk river in the lower reaches works like the local sink and, accordingly, leads to sedimentation during flood events. The study highlights the need for a quaternary geological approach in assessing long-term hazard potential and risk reduction in the Himalayan terrain.

The present study aimed to understand the debris flow characteristics in view of frequent extreme rainfall events, expansion of road networks, tourist influx, and population pressure in the NW Central Himalaya. Notably, majority of the human settlements, roads, bridges, buildings, and even protection measures in the NW Central Himalaya do not take into consideration such debris flow impact scenario despite a history of debris flow disasters. The Voellmy-Salm rheology dependent dynamic runout simulation method was used to determine the debris flow pressure and velocity regime in 9 debris flow locations belonging to different litho-tectonic conditions. Results revealed that the debris flow pressure and velocity in these 9 studied debris flows might reach up to 3000 kPa and 20 m/s, respectively. The debris flow pressure and velocity of these orders have the potential to damage the protection measures and infrastructures, which have also been observed in other hilly terrains. The sensitivity analysis was carried out at a range of input parameters by considering 729 possible simulations and debris flow pressure and velocity are found to follow relatively better corelation until 250 kPa flow pressure and 15 m/s velocity thresholds. The influence of slope topography on the debris flow characteristics is also observed in the form of amplification of flow pressure and velocity at concave portions. The rapid development of road network in the NW Central Himalayan region and its subjectivity to potential debris flow risk is also discussed.

Landslides have become more frequent in the last decade in the NW Himalayan region, threatening people and damaging infrastructure. The study presented here aims to identify landslide hazards and risk hotspots in the NW Himalaya. The area is strategically important due the many holy pilgrimage sites and major hydropower projects. A Multilayer Perceptron (MLP) was used to generate the landslide susceptibility map, which was combined with the rainfall intensity map to create the hazard map. To determine the spatial landslide vulnerability, land use and land cover components were taken into account with their respective monetary values. The landslide risk map is the combination of landslide hazard and vulnerability maps, showing that ∼5% of the area falls in high and very high-risk zones, ∼6% in moderate, ∼47% in low, and ∼42% in very low landslide risk zones. High to very high landslide risk zones are mainly confined to Uttarkashi and its surroundings in the southwestern part of the study area, as well as the Tehri, Karanprayag, and Pithoragarh regions in the southern area. Societal risk was also analyzed and revealed that ∼53% of the human population resides in high to very high landslide risk-prone areas. The findings of this study will be beneficial for promoting sustainable development and safe urbanization in the Himalayan region, if used for planning.

The present study is done in the Beas river (Manali to Mandi town) after the July 2023 flood to assess the geomorphic and geological vulnerability of the terrain and understand the role of human intervention in the disaster. The study is based on preliminary field observations and morphometric analysis that indicates that the surfaces proximal to the trunk riverbed and the ephemeral tributary channels suffered maximum damage. Along the upper Beas river, the lateral erosion caused by hyperconcentrated flows saturated with paraglacial sediments partly obstructed the river and also increased the erosivity that also led to remobilization of midchannel bars along with the uprooted trees. In the downstream (southern mountain front), maximum damage was caused by activating seemingly dormant ephemeral tributary channels. The slope-channel coupling locally bulked the sediment supply of the trunk river. The disaster was force amplified when the river was temporarily obstructed by the manmade structures (e.g., suspension bridges), and most importantly, urban settlements (largely hotels) on the flood plain. Also, many public buildings suffered as these were constructed in/along the ephemeral tributary channels, which were temporarily blocked by these structures, as well as by logged tree trunks. The Beas flood is yet another brutal reminder to change our perception towards the projected to increase in the Himalaya.

Landslides are one of the most destructive geological hazards in the Himalaya, and their frequency has increased in the last few decades. Therefore, it is essential to evaluate the landslide risk in such mountains. The present study provides a comprehensive landslide hazard, vulnerability, and risk assessment along the Alaknanda valley, NW Himalaya, India. The investigated area is selected because of its importance for holy pilgrimage sites in India. A machine learning technique, the MLP (Multilayer Perceptron) approach, has been used for preparing the landslide susceptibility map, which has provided efficient results with SRC (success rate curve) of 0.95 and PRC (prediction rate curve) of 0.87. The landslide hazard map was prepared using the maximum rainfall intensity from the last two decades of precipitation data. The economic vulnerability was derived by using the the monetary value of the components of LULC (Land Use Land Cover). It has been observed that dam sites, settlements, and roads are highly vulnerable to landslide hazards. Finally, the risk map was generated by integrating the landslide hazard map with the vulnerability map that covered the 4

The Indo-Gangetic Plain is one of the most densely populated regions of the world, and the seismo-tectonics of the Himalaya has a profound impact on the seismic hazards on this area. We analysed various seismic and lithological parameters of one of the oldest and most densely populated cities of the Indo-Gangetic Plain, called Varanasi, to assess the seismic risk for the city. This study revealed that the surface geology of this region has a considerable impact on the Peak Ground Acceleration (PGA), which determines the degree of damage in any area after an earthquake. The various factors, such as the population density, thickness of sediment, and building structure pattern of the city, make it highly vulnerable to seismic hazards. In view of this, we have used multiple data sets to assess the seismic amplification for the city. Our results suggest that the surface geology of the area may amplify the signal from 1.7 to 3.2 times in the study area. Thus, the seismic hazard of the area is higher than the expected hazard for seismic zone III.

Growing human population along the river valleys in hilly terrain particularly on the fluvial sediments poses increasing risk of terrace instability and subsequent failure. Such instability and failure result in frequent loss of settlement, agricultural lands, and often lives. Alaknanda River valley in Uttarakhand comprises many fluvial terrace slopes that accommodate human settlement and hence one such fluvial terrace slope was taken as a case study area. On Feb. 28, 2022, Saari (or Sari) village, situated on the fluvial sequence, witnessed a slope failure collapsing 3 houses but no casualties. The hillslope with a total disturbed area of ∼3889±5.0 m 2 and failed material volume of ∼16858±4.3 m 3 partially dammed a tributary of Alaknanda River, which passes through the toe of the failed slope. The present study is an attempt to understand the instability that led to such failure because there was no extreme rainfall or earthquake prior to this failure. Pre- and post-failure topography of slope was used to demarcate the detachment and deposition zones. Pre-failure topography was used to perform the Finite Element Method (FEM) based slope stability simulation. In order to evaluate the possible contribution of exposed rockmass in failure, kinematic analysis was also performed. Results revealed the development of displacement pattern, particularly due to anthropogenic loads that must have initiated this failure. Such studies are primary requisites for an effective disaster mitigation in the NW Himalaya where growing human population on fragile hillslopes are at risk.

This study analyses the causes and consequences of slope instability around the historical Himalatral Thrust. The lithology constitutes fissile, shattered and sheared gneiss. Consequently, the slopes are prone to land subsidence and mass wasting. In the last few decades rise in population has led to a surge of infrastructure development, thus causing immense pressure on the finite resources and limited accommodation space on precariously balanced vulnerable slopes. Particularly, the unplanned infrastructure development, lack of adequate drainage and excavation of roads through unstable debris slopes are some of the reasons that seem to have accelerated the ongoing slope instability and land subsidence.

The present study focuses on the reconstruction of the pattern of late Quaternary climate variability through sediment-landform assemblages in the monsoon-dominated Dhauli Ganga valley. The South Tibet Detachment System (STDS) is a major litho-tectonic boundary that divides the Dhauli Ganga valley into two broad geomorphic entities. Towards the north of STDS, the valley is wide and "U" shaped, and the rivers have a braid-meandering channel, implying that the valley was carved by glacial sculpturing in the past. Whereas in the south, deep gorges indicate the dominance of fluvial processes. Based on the stratigraphic position and optical chro-nologies, the lithified moraines were assigned to Marine Isotopic Stage-3 (MIS-3) they were followed by a major deglaciation event represented by moderately lithified outwash gravels. Following this, a second glacier advance of lesser magnitude is dated 21.3 +/- 2.2 ka, corresponding to the Last Glacial Maximum (LGM). A gradual recession of the LGM moraines led to the formation of a proglacial lake which probably persisted until around the onset of a pulsating deglaciation stage represented by outwash gravels dated between 13.4 +/- 1.6 and 9.4 +/- 0.8 ka. This was also the period when the valleys were overwhelmed by a high sediment water ratio, as indicated by temporary impoundments dated between 15.0 +/- 1 ka and 10.0 +/- 1 ka. Alluvial fan and debris flow sedimen-tation overwhelm the valley after around 9 ka and continues till the present.Climatically, the older lithified moraines indicate that glaciers advanced during the cooler MIS-4 or MIS-3. The first major deglaciation event seems to represent the pluvial phase of MIS-3. The presence of LGM moraines indicate that the valley responded to the global cooling and associated enhanced westerlies. In contrast, the chronology of outwash gravels suggests that the valley witnessed insolation driven by the early Holocene strengthened Indian Summer Monsoon (ISM). The undated youngest alluvial fans/debris flows are assigned the mid to late Holocene age (<9.5 ka) and seem to have deposited during the declining phase of ISM (low solar insolation). The study suggests that the landform evolution responded to both the global and regional climate variability indicating the sensitivity of the paraglacial valleys in the monsoon-dominated region to the late Quaternary climate variability.

Floods are becoming more frequent in Himalaya, particularly in the NW Himalaya, and have been related to the increasing impact of changing climate. Uttarakhand in the NW Himalaya has witnessed 2 major flood events in the last decade that killed more than 6000 people. This study is an attempt to explore the impact of potential flood on a riverbank slope in Uttarakhand, NW Himalaya. The response of this riverbank slope during extreme rainfall is also explored in terms of stability and debris flow runout. Therefore, we evaluated the riverbank slope stability and the runout extent of its material to understand the slope response during extreme rainfall. Flood simulation was also performed to determine the potential flood impact on the riverbank slope. Results revealed that the slope material at the exposed fluvial sequence and slope toe might displace forward ~0.12–0.4 m. The potential debris flow from the slope may impact the retaining wall supporting the slope with a pressure up to 150 k Pa. The potential flood may strike the riverbank with a velocity and stream power of 10 ± 2 m/s and 0.2 ± 0.1 M N/m-s, respectively, which is about three times higher than the approximated resistance of the retaining wall.

The present study attempts to understand the geomorphic response in the upper Ganga catchment to the mid-late Holocene (neoglacial) climate variability. The study infers five major phases of millennial-scale climate variability with centennial-scale inversions using geochemical and magnetic proxies from relict Lesser Himalayan Lake sediments. Phase-1 (6–4 ka) is marked by enhanced precipitation/runoff (increased allochthonous contribution) under a stronger Indian Summer Monsoon (ISM). The prominent reversal in the trend between ∼5 and 4 ka includes global arid events such as 4.2 ka. Phase-2 (4–2.2 ka) shows a declining precipitation/runoff (decreased allochthonous input) under declining ISM with a prominent dip after ∼3 ka. After phase-2 the climate reversals are distinct and of shorter (centennial) duration. For example, in Phase-3 (2.2–1.4 ka) improved ISM is inferred; Phase-4 (1.4–1.0 ka) is marked by a sharp decline in the ISM, and Phase-5 (<1.0 ka) includes centennial-scale events of Medieval Climate Anomaly (MCA) and the onset of Little Ice Age (LIA). The relative increase (decrease) in the concentration of geochemical and magnetic proxies is indicative of strengthened (weakened) ISM where relatively drier phases are in sync with the North Atlantic climate perturbations. We observed clustering of optically dated flood events around 6.5, 4.5, 2.6, 1.4, 0.8, and 0.4 ka which corresponds to periods of moderate ISM thus, suggesting a coupling between warm-humid monsoon and relatively dry westerlies. The relatively higher concentration of micro-charcoal in the lake sediments indicates widespread forest fires around 5.9–5.3, 4.5–4.3, 3.4–3.0, 2.0–1.5 and ∼1 ka. Given the archaeological evidence of sedentary settlements since ∼3 ka in the upper Ganga catchment, the study speculatively argues anthropogenic forcing for forest fires after 3 ka. Further, the highest probability flood phases succeed the fire events and may be indicative of enhanced vulnerability of the catchment to floods due to vegetation loss (enhanced erosion and surface runoff).

We have mapped more than 400 major landslides (debris slides, rockfalls, and rock avalanches) in 5 fluvial valleys in Himalaya (India) between 77.3° E - 80.5° E longitudes. Field/high- resolution satellite imagery based landslide area mapping and field based landslide thickness approximation were used to determine landslide area and volume. Area-volume scaling exponents of these landslides revealed a lateral variation in the study area implying that landslide slopes in the eastern part of the study area retain relatively less volume that increases towards western part of the study area. We have hypothesized that such lateral variation is possibly caused by lateral variation in the landslide occurrence that in turn is mostly caused by lateral variation in the seismic-climatic regimes. Following the hypothesis, we noted that rainfall, surface runoff, soil moisture, and air moisture (climatic variables) data of years 1982-2020 represent a general decrease laterally from east to west in the study area. Further, the role of topography on the climate variables is also noted as it increases from east to west. Earthquake (Mw=>4) distribution (1960-2020), Arc Parallel Gravity Anomaly (APGA), cumulative seismic moment, shear stress accumulation rate, and convergence (India-Eurasia) rate (Seismic variables) also represent a general decrease laterally from east to west in the study area. The climatic variability is attributed to the spatial variability of the Indian Summer Monsoon (ISM), whereas seismic variability is referred to the spatial variability in the subsurface pattern of the Main Himalayan Thrust (MHT). Thus, such variability in the seismic-climatic regimes is noted to support our hypothesis.

Uttarakhand region in the NW Himalaya has experienced two extreme climatic-geomorphic events within last 10 years that killed more than 6000 people. Though these events, like many others in the Himalaya, have been attributed to climate-change and anthropogenic disturbances, identification of potential hotspots of land use/land cover change is rarely attempted to make future inferences for disaster risk reduction. An evaluation of spatio-temporal changes in land use/land cover can be used to identify such hotspots. Therefore, we analysed the spatio-temporal changes in a climatically sensitive and natural disaster-prone area (~28856 km2) of Uttarakhand (NW Himalaya), India, by comparing the satellite data of years 1991-2020 for ten land use/land cover elements to track the spatio-temporal changes over these years. Results revealed the formation of two hotspots exhibiting relatively more changes in land use/land cover pattern. Though the anthropogenic influence is observed in both hotspots, the influence of spatio-temporally changing climatic parametres is also noted. In view of frequent extreme climatic-geomorphic events, temporally increasing population and tourist pressure, and temporally changing climatic conditions, it is vital to identify hotspots having dominant changes in land use/land cover to understand the possible source of potential disasters.

Roads are the most critical means of connectivity in Himalayan villages. However, the terrain is inherently fragile with varied geological, geomorphological, ecological, and climate regimes, that result in frequent slope failure and disruption in connectivity. The risk is further to be increased by extreme events-generated hazards, which are expected to rise in frequency and magnitude with ongoing climate change. Critical scientific intervention, however, can improve the sustainability of road networks. The present study attempts to analyse and quantify the impacts of a major road widening project initiated in 2018 in the upper Ganga catchment, Uttarakhand Himalaya which has destabilised valley slopes along the widened segments. Also, a large quantity of excavated sediments is dumped down slopes, which is posing a threat to aquatic biodiversity. The estimates are based on Google Earth imagery of a few representative road segments recently widened in the upper Ganga catchment, which indicate a substantial increase in the landslide and unstable slope area following the road widening. The increase in unstable slope area is attributed to improper road widening approaches and poor slope management in seismically active Himalayan terrain. Further, the mean velocity plots of Persistent Scatterer Interferometric Synthetic Aperture Radar (PSInSAR) indicate that the segments undergoing road widening are coherent with areas of significant earth surface change. A broad correlation between the road width and sediment yield indicates that even a slight increase in road width can result in a large-scale mass removal from the toe of the hillslope, inflicting cascading impact on hillslopes. The study recommends a more flexible road construction approach based on the environmental and geological aspects of the terrain for sustainable road networks. Further, the impact of climate change is looming over the Himalayas, and the relation between climate change and its potential effects on the stability of slopes remains an open issue.

A flash flood that originated from Raunthi Gad-a tributary of the Rishi Ganga river, in Garhwal Himalaya, caused unprecedented loss to lives and damaged two hydropower projects on 7th Februray 2021. In order to asses the flood magnitude, the flow parameters of the flood were calculated using the super-elevation of the flood marks preserved in the flood affected valleys. The textural characteristics of the flood deposits in the upper reaches of the valleys indicate dominance of debris flows. The peak discharge upstream of the confluence of Rishi Ganga and Dhauli Ganga was around 1.1×105 m3/s, which was four order of magnitiude higher than the normal peak discharge (∼ 3 m3/s). The flow achieved a velocity of 30±3 m/s. An exponential reduction in the flow velocity (from ∼37 to 2 m/s) with distance is observed. For which the river gradient and increase in sediment load is implied flow that along its entrained way downstream between Raini and Tapovan. Considering the sensitivity of paraglacial zones to climate change, the paper calls for detailed studies pertaining to the response of paraglacial zones to extreme weather events. Importantly, it is necessary to have more hydrological data covering multiple valleys for predictive model simulation of the nature and magnitude of such disasters in future.

A short-lived flashflood in Rishi and Dhauli Ganga rivers on 7th February 2021, Uttarakhand Himalaya, killed 65 people with 141 reported missing (official estimate) and devastated two hydropower projects. Geomorphological observations supported by meteorological data suggest that the flood was triggered by a combination of avalanche and debris flow. The Dhauli Ganga valley has preserved ponded sedimentary sequences (laminated sand and silty-clay), suggesting that the valley is prone to episodic mega foods in the recent geological past. Considering that the receding glaciers in the higher Himalaya have left behind enormous sediment, unusual weather events are likely to generate such disasters more frequently as the climate becomes warmer. Thus, the study calls for not only incorporating the disaster risk assessment in the developmental planning of the Himalayan region but also recommends routine monitoring of the potential areas of structural failures in the glaciated valleys along with supra-glacial lakes.

In November 2018 a low magnitude earthquake swarm started on the west coast of central India which is continuing into 2020. The swarm started much after the monsoon season and was on decline in May-June 2019 but the earthquake frequency again increased during monsoonal rainfall which started in June 2019. So far it has produced more than 16,000 earthquakes of magnitude -0.5 to 3.8 with an equivalent single earthquake magnitude of 4.5. All the earthquakes are tightly clustered in a region of 10 x 6 km(2) and occur through normal slip on north-south oriented east dipping steep fault(s) which extends up to a depth of 6-7 km only. The InSAR analysis reveals a subsidence of similar to 3 cm between November 2018 to May 2019 in the swarm region and confirms that the resulting deformation occurred through normal slip. The occurrence of tightly clustered shallow focus earthquakes causing subsidence due to normal slip in the overall compressive regime of stable continental region of the Indian plate, imply some shallow subsurface process of precipitated water migration leading to collapse of subsurface cavities and may not be linked with the tectonics of the region. We propose that aseismic slip driven by the fluid migration at shallow depth is responsible for the swarm.