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.

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

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.

The NW Himalaya (India) has been one of the most affected terrains of the Himalaya, subject to frequent disastrous landslides owing to active tectonics and multiple precipitation sources. This article focuses on two towns (Joshimath and Bhatwari) of Uttarakhand in the NW Himalaya, which have been witnessing subsidence for decades. Up until 9 January 2023, Joshimath had witnessed widespread cracks in more than 500 houses, which has prompted social unrest in the region. The hillslopes surrounding both towns comprise highly jointed gneisses with schistose interlayer rock mass. Both towns have subsidences and holes in the road, broken retaining walls, and displaced boulders, and the houses have cracks in the walls and holes. Recently, such slope instability phenomena have increased, which is leading to social movements in the region seeking government action such as possible evacuation and rehabilitation. The present study used a continuum-modelling-based slope stability simulation to determine the response of these hillslopes under various loading conditions: gravity, rainfall, building load, domestic discharge, and seismic load. Results revealed that the displacement in these hillslopes might reach up to 20–25 m, which will further aggravate the situation. The occurrence of frequent extreme rainfall in these towns and three major earthquakes, which occurred on 1 September 1803 (Mw 7.8), 20 October 1991 (Mw 6.8), and 29 March 1999 (Mw 6.6), having a hypocentral distance less than 30 km makes such a study more viable for decision making.

The present study aims at understanding the chronology and pattern of extreme flood events in the upper Ganga catchment. Towards this, we explored relict alluvial sequences, particularly the sand bodies overlying the fluvial gravels and underlain by the source proximal alluvial fans and debris flows, supported by geochemistry, and optical chronology. Based on the accretionary pattern of sand bodies, the progressive increase in the flood magnitude since the beginning of Marine Isotopic Phase-2 (MIS-2) has been identified. The oldest flood is dated to 30 & PLUSMN; 4 ka and the subsequent floods are dated to the post-glacial transitional climate (16 & PLUSMN; 4 ka) and during the early Holocene (between 14 & PLUSMN; 2 ka and 12 & PLUSMN; 2 ka), and the late mid-Holocene (7 & PLUSMN; 1 ka). Based on the temporal changes in the major element geochemistry (detrital and weathering proxies) the study indicate that during early to mid-MIS-2, the upper Ganga catchment witnessed high frequency-low magnitude floods. Further, the absence of flood sediment, the development of calcrete, and a sharp decrease in the detrital proxies, a significant reduction in high-magnitude floods during the Last Glacial Maximum (LGM) is inferred. The isotopic fingerprinting, based on 87Sr/86Sr and & epsilon;Nd(0), indicates that the majority of the floods during the MIS-2 originated from the Higher Himalayan lithologies, whereas the inner Lesser Himalayan lithology dominates the mid to late Holocene floods. Climatically, the floods were triggered neither during too-wet nor too-dry climatic conditions, but instead during the transitional climate caused due to the mesoscale atmospheric coupling between Indian Summer Monsoon (ISM) and mid-latitude westerlies.

Abstract. The NW Himalaya has been one of most affected terrains of Himalaya subjected to frequent disastrous landslides owing to active tectonics and multiple precipitation sources. This article aims at two towns (Joshimath and Bhatwari) of the Uttarakhand in the NW Himalaya (India), which have been witnessing subsidence for decades. In the last 1–2 weeks, Joshimath has witnessed widespread cracks in more than 500 houses that has created the social unrest. The hillslopes accommodating both the towns comprise highly jointed gneisses with schistose interlayers rockmass, subsidence in road, broken retaining wall, holes, displacing boulders, and cracks in the houses. Recently, such slope instability phenomena have increased that is leading to social movements in the region seeking government action for possible evacuation and rehabilitation. Present study has involved continuum modeling-based slope stability simulation to determine the response of these hillslopes under various loading conditions; gravity, rainfall, building load, domestic discharge, and seismic load. Results revealed that the displacement in these hillslopes might reach up to 20–25 m that will further aggravate the situation. Occurrence of frequent extreme rainfalls in these towns and three major earthquakes i.e., 1 Sep. 1803 (Mw7.8), 20 Oct. 1991 (Mw 6.8), and 29 Mar. 1999 (Mw 6.6) having hypocentral distance less than 30 km make such study more viable for decision making.

Earthquake-induced landslide hazard is the most serious threat in seismo-tectonically active mountains like the Himalayas. It has frequently been noted that the damage caused by earthquake-induced landslides is significantly greater than the earthquake itself. Therefore, assessing the susceptible zones of earthquake-induced landslides in seismically active areas is essential. In this study, the probabilistic hazard assessment of the earthquake-induced landslides has been conducted for the Goriganga Valley, Kumaun Himalaya. Numerous studies indicate that a great earthquake of magnitude 8 Mw or higher could strike this area at any time. Hence, mapping earthquake-induced landslides using an improved Newmark's model has been conducted for earthquakes of magnitude 8 Mw. The inclusion of arias intensity to estimate the permanent displacement of the slope for future scenario earthquakes make this work unique from others. The model provides the permanent displacements of potential slopes, which is the function of shear strength parameters of jointed rock mass, the inclination angle of valley slopes, and the arias intensity of the area. It provides the spatial distribution of possible slope failures in the area. It has been noted that ~25% of the study area is susceptible to earthquake-induced landslides when subjected to direct shaking of an earthquake of magnitude 8 Mw. The results of this work provide great insight to planners and civil engineers for hazard mitigation and assessment of the study region.

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.

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.

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.