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The paper describes the recent devastating landslide of Kotrupi, Himachal Pradesh in India which had claimed many human lives and properties along its course on 13 August 2017. This event was the third reactivation of the slide as per data available. The study for this landslide involves a preliminary investigation of the site and comparison of pre- and post-event slope stability analysis. Other than rainfall, the combined effect of differential weathering rate of the debris and soil layers, low material strength, and the presence of a fault zone was found to be the primary cause of instability. The analysis was carried out with the geotechnical investigation and numerical modeling with the help of finite element model-based Phase 2 simulator. The significant findings of this paper include numerical simulation of pre- and post-event slide as stated under unsaturated and saturated condition. The study finds a low factor of safety of the present slope. The unstable condition of the slope may trigger another event of the same intensity shortly. [ABSTRACT FROM AUTHOR]

The history of glacier length fluctuations serves as a reliable indicator of the past climate. In this paper, a numerical flowline model has been used to study the relationship between length variations of Chhota Shigri glacier and local climate since 1876. The simulated front positions of Chhota Shigri glacier are in agreement with those observed. After a successful simulation of the past retreat, the model was also used to predict future evolution of the glacier for the next 100 years under different climatic scenarios. These simulations indicate that the Chhota Shigri glacier may lose ~90% of its present volume by 2100 if the local temperature increases by 2.4 K, and for a temperature rise of 5.5 K, the glacier loses almost all its volume. [ABSTRACT FROM AUTHOR]

Infrequent, large‐magnitude discharge (>106 m3/s) outburst floods—megafloods—can play a major role in landscape evolution. Prehistoric glacial lake outburst megafloods transported and deposited large boulders (≥4 m), yet few studies consider their potential lasting impact on river processes and form. We use a numerical model, constrained by observed boulder size distributions, to investigate the fluvial response to boulder deposition by megaflooding in the Yarlung‐Siang River, eastern Himalaya. Results show that boulder deposition changes local channel steepness (ksn) up to ∼180% compared to simulations without boulder bars, introducing >100 meter‐scale knickpoints to the channel that can be sustained for >20 kyr. Simulations demonstrate that deposition of boulders in a single megaflood can have a greater influence on ksn than another common source of fluvial boulders: incision‐rate‐dependent delivery of boulders from hillslopes. Through widespread boulder deposition, megafloods leave a lasting legacy of channel disequilibrium that compounds over multiple floods and persists for millennia. Plain Language Summary: Megafloods (discharge equivalent to ≥400 instantaneously draining Olympic‐sized swimming pools per second) can transport a lot of material, including car‐ to house‐sized boulders. Because these boulders are so big, they remain in the channel until the next megaflood or until they weather into smaller pieces. We use a computer model to understand the impact of these megaflood deposited boulders on mountain river processes. We find that megaflood boulders can protect the river from being eroded, causing other processes, like tectonic uplift, to outcompete erosion. Megaflood boulders cause small steps to form within the river. Our modeling shows that these effects can be felt for >20,000 yrs after a single flood. We suggest that megaflood deposition (in addition to erosion) can cause a significant, unique change in mountain river processes. Key Points: We studied channel response to megaflood boulder deposition in the eastern Himalaya using a numerical modelMegaflood boulder deposition locally changes channel steepness up to 180%, creating >100 knickpointsThe compounding effect from multiple megaflood boulder deposits in rapid succession perturbs channel form for many thousands of years [ABSTRACT FROM AUTHOR]

Zhangmu Town in Tibet of China, which lies in the southern piedmont of the median Himalayas, is a small but strategically important port of trade exchange between China and Nepal. Many rockfall events have occurred in Zhangmu since 1970, resulting in huge economic losses and serious influence on the bilateral trade. We conducted a detailed field investigation on the high and steep slope in Zhangmu Town, and analyzed the distribution features, stability, failure modes and evolution of dangerous rocks of potential rockfalls. Then we numerically simulated the movement path, velocity and accumulation forms of the rockfall with PFC3D program. The results indicated that the dangerous rock belt could be divided into three sections, namely, unstable section, slightly stable section and basically stable section. It was estimated that the rock debris and single dangerous rock would be unstable in the case of earthquakes or rainstorms. Due to the terrain constraints, the fallen rocks would scatter near the mouth of the Zhangmu ditch and in the Buqu River through multiple times of rolling, collision-induced diversion and bouncing. Without reinforcement, the rockfall could cause serious damage to the car parks, gas stations and National Highway 318 along the line from Zhangmu Town to Zhangmu ditch. Based on the field survey and numerical simulation, we recommended rockfall removal and interception as the major prevention measures, and protective sheds as auxiliary measure. [ABSTRACT FROM AUTHOR]

Ground observation of precipitation over complex terrain is subject to large uncertainties due to inadequate sampling. This study explores a method that combines limited gauge data and a high‐resolution numerical simulation to quantify the precipitation uncertainties in central Himalaya. Specifically, the Coefficient of spatial Variability (CV) of precipitation and the minimum Number of Required Stations (NRS) to obtain areal‐mean precipitation ground truth values within a 0.25° area are investigated using fine‐scale meteorological simulation at 1.5 km grid spacing. Evaluation over a densely‐gauged area demonstrates comparable CV and NRS values between station observations and simulations. The simulated CV and NRS values in a larger area show a strong and positive dependence on each other and an expected positive (negative) correlation with topographic complexity (temporal scale). The proposed method sheds lights on evaluating precipitation products and holds promise for informing the layout of rain gauge networks in complex terrain. Plain Language Summary: A challenging issue in evaluating gridded precipitation products for regions of complex terrain is to understand the uncertainties in the ground "truth," which usually requires a dense observing network. In this study we show that data from a high‐resolution numerical simulation can be used to quantify the uncertainties in the ground "truth" even though precipitation amount at individual rain gauge stations may not be accurately simulated. This method is validated in a valley of central Himalaya with high terrain complexity, in terms of the spatial variability of precipitation and the minimum number of stations needed to estimate the area‐averaged precipitation. We further show that uncertainties in the ground "truth" can be considerable even in flat areas. Key Points: A high‐resolution numerical simulation can be used to quantify the uncertainties in precipitation ground truth over complex terrainThe minimum number of stations to represent the area‐averaged precipitation highly depends on the spatial variability of precipitationAlthough topography strongly influences precipitation, it does not fully determine precipitation spatial variation even at monthly scale [ABSTRACT FROM AUTHOR]

A recent heavy precipitation event on 13 September 2012 and the associated landslide on 14 September 2012 is one of the most severe calamities that occurred over the Rudraprayag region in Uttarakhand, India. This heavy precipitation event is also emblematic of the natural hazards occuring in the Himalayan region. Study objectives are to present dynamical fields associated with this event, and understand the processes related to the severe storm event, using the Weather Research and Forecasting (WRF ver 3.4) model. A triple-nested WRF model is configured over the Uttarakhand region centered over Ukhimath (3030′N; 79 15′E), where the heavy precipitation event is reported. Model simulation of the intense storm on 13 September 2012 is with parameterized and then with explicit convection are examined for the 3 km grid spacing domain. The event was better simulated without the consideration of convection parameterization for the innermost domain. The role of steep orography forcings is notable in rapid dynamical lifting as revealed by the positive vorticity and high reflectivity values and the intensification of the monsoonal storm. Incursion of moist air, in the lower levels, converges at the foothills of the mountains and rise along the orography to form the updraft zone of the storm. Such rapid unstable ascent leads to deep convection and increases the condensation rate of the water vapour forming clouds at a swift rate. This culminates into high intensity precipitation which leads to high amount of surface runoff over regions of susceptible geomorphology causing the landslide. Even for this intense and potentially unsual rainfall event, the processes involved appear to be the 'classic' enhanced convective activity by orographic lifting of the moist air, as an important driver of the event. [ABSTRACT FROM AUTHOR]

Observations and model simulations were conducted in a typical Himalayan valley to investigate the role of snow/ice cover in the formation of the local diurnal wind. An unusual local circulation was observed in the Himalayas with a strong down-valley flow dominant from noon to midnight, greatly differing from those in other mountainous regions. Two experiments with snow/ice cover included/excluded were performed using the Regional Atmospheric Modeling System (RAMS) to reconstruct the Himalayan circulation, and to reveal the role of snow/ice in this circulation. The results show that the wind system in the Himalayas is composed of both glacier winds driven by the snow/ice cover and classical mountain-valley winds. In particular, the glacier winds establish the distinctive feature of the Himalayan local circulation, i.e., the strong down-valley flow in the afternoon. [ABSTRACT FROM AUTHOR]

Considering the different uplifting time of different subregions of the Himalaya-Tibetan Plateau (TP), a series of numerical simulations have been conducted with the Community Atmosphere Model (CAM4) developed at the National Center for Atmospheric Research to explore the effects of the phased tectonic uplift of the Himalaya-TP on the evolution of Asian summer monsoons. The results show that the uplifts of the Himalaya and northern TP significantly affect the evolutions of South Asian summer monsoon and northern East Asian summer monsoon respectively. That is, the tectonic uplift of the Himalaya intensifies the South Asian summer monsoon circulation and increases the precipitation in South Asia, whereas the uplift of the northern TP intensifies the northern East Asian summer monsoon circulation and increases the precipitation in northern East Asia. Compared with previous simulations, current comparative analyses of modeling results for different subregional uplifts within the Himalaya-TP help deepen our understanding of the evolutionary history of Asian monsoons. [ABSTRACT FROM AUTHOR]