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The stability of slope is usually characterized by many parameters which are mostly uncertain in nature. Deterministic approach is usually followed to calculate the factor of safety of a slope, but it does not depict the true state of the slope. Hence, a probabilistic approach is a better alternative, which can quantify the probability of failure of a slope under uncertain input parameters. In the present study, slope stability assessment of Pakhi landside is carried out using finite element modelling (FEM) to ascertain the stability conditions of the multilayer slope under both deterministic and probabilistic framework. The multilayer configurations of the profiles are established from the electrical resistivity tomography (ERT). The shear strength reduction (SSR) method is employed to determine the critical strength reduction factor of the slope considering four random variables, namely, cohesion (c), angle of internal friction (ϕ), Poisson’s ratio (ν) and elastic modulus (E) of each individual layers. The deterministic factor of safety values along two considered profiles namely, section X–X′ and Y–Y′ are calculated as 1.41 and 1.25, respectively. A data driven machine learning algorithm is used to build a computationally efficient surrogate model to perform Monte Carlo Simulations (MCS). MCS are performed for two different values of coefficient of variation, i.e., 5% and 15% for all the four random variables of all the layers. The proposed method has no idealization regarding the layering configuration and the failure surface. Probabilistic analysis has been made exhaustive and computationally efficient. The probabilistic analysis indicates good adherence with the recent landslide incident in the field. Further, the analysis indicates that the proposed methodology is favourable and useful tool for the system reliability analysis of landslide slopes.

Debris flows are frequently occurring natural processes in geologically complex terrains of the Indian Himalayas. Debris flow runout modelling leading to hazard assessment is essential for planning, designing, and execution of mitigation measures. In the present context, debris flow hazard assessment has been carried out for the Tangni debris flow in Garhwal Himalayas, India. Runout modelling was carried out using a Voellmy model-based 3D numerical simulation for estimation of flow intensity parameters such as runout distance, flow velocity, height and pressure along the propagation path. For calibration of the model inputs, back analysis of Tangni debris flow event that occurred in 2013, with its known runout length and deposition volume as the criteria, has been conducted. The best calibrated values of frictional parameters are obtained at μ = 0.10 and ξ = 400 m/s2. Using the best calibrated values of frictional parameters, hazard assessment was carried out for two potential release areas separately, with different initial volumes, and in combination to derive the probable runout distance along with other flow intensity parameters for different scenarios that may happen in the future. It has been observed that this debris flow scenario including the combination of two potential release areas will block the Alaknanda River, forming a landslide dam with a probable height of 6 m. Debris flow runout modelling-based hazard assessment will be helpful in determining quantitative information on flow intensity parameters, where complete data on past events are generally not available or were not possible to capture for the Indian Himalayan region.

The influence of rainfall on the Pakhi landslide has been examined using a pre‐ and post‐monsoon timelapse electrical resistivity imaging technique. The results obtained have been used to characterize the subsurface based on the depth and degree of water seepage and degree of weathering. The combined interpretation of pre‐ and post‐monsoon electrical resistivity tomography is not very common in landslide characterization. This study focuses on post‐monsoon electrical resistivity tomography investigation and its interpretation in conjunction with earlier research on pre‐monsoon investigations. The advancing water seepage across depth was reflected in the sharp decrease in the post‐monsoon resistivity value of the top (near‐surface) colluvial layer, mostly at the crown and toe portions of the investigated slope, as compared to that of weathered dolomites and fresh dolomites. Hence, the water‐saturated colluvium deposits could easily be delineated using a timelapse pre‐ and post‐monsoon electrical resistivity tomography technique. On the basis of degree of saturation, the materials were classified into four zones, namely: a highly saturated zone with resistivity values 4500 Ωm. Further, a twofold classification of dolomites based on the degree of weathering, namely highly weathered dolomites with resistivity values 1000–2500 Ωm and low weathered dolomites with resistivity values 2500–4500 Ωm, is presented. This type of information will add value to the design of landslide‐monitoring programmes and also for landslide stability analysis. Therefore, the method can be highly recommended as a cost‐ and time‐effective as well as an efficient way for characterization of potential landslide slopes.

Debris flows are one of the most frequently occurring and destructive hazards in Indian Himalayas which are often initiated by rainfall. To minimize the losses due to the destructive power of the debris flows, demarcation of debris flow risk zones is an effective practice for risk reduction. In the present study, site specific debris flow risk assessment has been carried out based upon runout behaviour modeling. Tangni debris flow is an active debris flow in the Chamoli district of Garhwal Himalayas, India which is responsible for disrupting the traffic by blocking the road for days. This debris flow is repetitive in nature and occurs many a times every year in the monsoon during the months between June to September. The Tangni debris flow is categorized as a hill slope debris flow and the failure is considered as a block failure. Runout modeling of Tangni debris flow has been carried out using a Voellmy approach based continuum model. Quantitative information on debris flow intensity parameters such as flow velocity, height and pressure was obtained from the numerical simulation. The calibration of model input parameters was done by back analysis of an event from a particular source area that took place in 2013. Depending upon the amount of materials present in different source areas in the entire source zone and using the calibrated model input parameters, several simulations were performed to assess the flow behaviour of at different possible scenarios. Thus, Tangni debris flow risk assessment has been carried out based on its runout effect modeling. This study revealed that there may be a possibility of damming of river as well as blocking of the National Highway which are located at the downstream of the debris flow.Key words: Debris flow, Risk assessment, Runout modeling, Garhwal Himalayas, Voellmy model

Pakhi landslide is one of the major debris slides of retrogressive nature along national highway (NH-58) corridor from Rishikesh to Badrinath in Garhwal Himalayas, India. The present work is the first such attempt in one of the landslides in Indian Himalayas and in the present case study, the subsurface characterization of the landslide was carried out with an aim to correlate the subsurface lithology, slip surface and geometry of the landslide using borehole investigation and electrical resistivity tomography (ERT) techniques. The results from ERT survey along and across the landslide could help to delineate different strata layers; thereby deciphering the probable slip surface of the landslide activity. Further, based on the landslide model drawn from the ERT interpretations, it can be stated that the crown portion of the right flank is more vulnerable to sliding phenomenon and that of the left flank is more prone to erosion. The results obtained from both the borehole investigation and ERT techniques are observed to be in great agreement and could complement each other for subsurface detailed investigation of landslide. The integration of both these techniques provides useful information about the landslide activities and defines the geometry of the landslide. Hence, it will be quite helpful in planning mitigation strategies as well as usefulness of each of these techniques.

Landslide disasters are known to be complex geological event with high socioeconomic effects in terms of loss of lives and properties. Landslide study involves the application of a multidisciplinary approach. Various techniques involving both surface and subsurface methods have been applied to investigate and understand the mechanism of its occurrence. Application of geophysical tools is one of such advanced techniques. In this paper, the recent trend in application of Electrical Resistivity Tomography (ERT) technique is being studied as it is a noninvasive method for investigation. ERT technique has been used widely for various landslides from rockfall to debris flow. The method has been applied to various geological conditions and environment to identify the slip surface, hydrological conditions, reconstruction of the slope body, estimation of the sliding materials volume/thickness, and depict the internal structures. Various array used in the measurement along with the algorithms for topographic inversion are also discussed. The results of 2D, 3D and 4D ERT techniques used in the recent times have been discussed. Future prospects and the limitations are also discussed.