On 22 May 2025, a large bedding rock landslide occurred in Guowa Township, Dafang County, Guizhou, China, burying 19 people. This high-speed, long-runout landslide had a volume of 2.4 × 106 m3, a horizontal travel distance of 858 m, and a height difference of 350 m. Based on field investigations, rock mechanics tests, SBAS-InSAR time-series analysis, and UDEC numerical simulation, we reveal its geological features, deformation history, and failure mechanism. The landslide developed in argillaceous siltstone of the Lower Triassic Yelang Formation, with bedding dip direction parallel to the slope aspect and two conjugate joint sets controlling the boundaries, forming a typical dip-slope. SBAS-InSAR detected accelerated deformation starting in March 2025, with 18 mm of cumulative displacement before failure. The direct trigger was an extreme local rainstorm, delivering 224.2 mm of rainfall in 5 h (max intensity 115.4 mm/h). The slip surface shows an “upper-steep, middle-gentle, lower-steep” geometry; the middle gentle slope acted as both a catchment area and a locked segment. Rainfall infiltration increased pore-water pressure and softened the rock, causing the locked segment to fail first and provoking retrogressive chain sliding. The peak velocity reached 37.08 m/s, and the entire movement lasted 84 s. This work elucidates the locked-segment failure and chain-instability mechanism under the coupled action of extreme rainfall and structural planes, offering insights for early warning and risk reduction in similar mountainous areas of southwestern China.
Landslide dams are landslides that obstruct the river’s flow, forming a dammed lake upstream. While landslides pose significant risks, the unstable dammed lakes increase the likelihood of dam outburst and flood hazard, which can have disastrous consequences downstream. Here, we present the spatio-temporal evolution of an active landslide in Kuwari, Uttarakhand, India, which has significantly expanded over the past 12 years, obstructing the Shambhu River, a tributary of the Pindar River in the upper Ganges catchment. High-resolution satellite imagery reveals that the landslide has evolved from three smaller slope instabilities into a large, complex mass movement measuring approximately 2 km in length and 0.6 km in width, covering an area of 918,275 m2. The resulting dammed lake exceeds 54,200 m2 in area with an estimated capacity of 178,000 m3. A small baseline subset–derived InSAR results reveals an accelerated trend in ground displacement of 20 cm within the landslide body. Correlation analysis reveals a very strong positive relationship between landslide and dammed lake areas, but weak, statistically insignificant correlations with rainfall and seismic activity, suggesting that internal geomorphic processes, rather than external triggers, predominantly control the evolution of this landslide, for which a detailed investigation of internal dynamics is necessary. These findings provide valuable insights for monitoring, hazard, and risk assessment related to landslide dams in fragile Himalayan catchments.
On 30 July 2024, an episode of extreme monsoonal rainfall ( 572.6 mm in 48 h) triggered a catastrophic landslide in Wayanad, on the eastern flank of the Western Ghats in India. The event resulted in approximately 298 fatalities and 32 persons missing, making it one of the most destructive landslides recorded in this region. The unusual nature of this disaster is reflected in its deep-seated origin, large failure volume, unusually long runout, transport of very large boulders, and extensive geomorphological modification of the landscape. Irrespective of other studies, this highly sheared metamorphic terrain demands an integrated structural and petrographic investigation, with geochemical analyses providing complementary lithological confirmation, to resolve how lithology and deformation architecture govern the high-magnitude landslide dynamics. Field observations indicate that the terrain is underlain by multiple lithologies, including biotite gneiss, garnet–biotite gneiss, charnockite, metagabbro, and granite gneiss. The region has experienced intense ductile shearing followed by brittle deformation, resulting in well-developed foliations parallel to a major E–W-trending shear zone and three principal joint sets (NNW–SSE, NW–SE, and E–W). Analysis of the field data using stereonet projections and rose diagrams, together with geochemical classification based on XRF data, further supports these field observations. Microstructural features such as grain rotation, grain-size reduction, mineral kinking, mica fish, pressure shadows, microfractures, and joints provide additional insights into the intensity of deformation and the tectonic evolution of the region. During extreme rainfall, upstream-dipping joint sets (NE), together with fractures and shear zones, likely facilitated water infiltration and may have promoted localized pore-pressure build-up, whereas downstream-dipping joints (SW) acted as preferential sliding planes. Lithology-dependent deformation and weathering contrasts, combined with the structural framework, control valley confinement, temporary dam formation, the generation and transport of large boulders, deep channel scouring, and high debris mobility were associated with this landslide. The results demonstrate that pre-existing geological and structural conditions contributed to the evolution and destructive behavior of the rainfall-triggered 2024 Wayanad landslide event.
Extreme rainfall has become one of the primary triggers of catastrophic geological hazards in the mountainous regions of northern China. In July 2025, the Yanshan Mountains experienced a prolonged extreme rainfall event that triggered widespread landslides and debris flows throughout the region. Based on field investigations, remote sensing interpretation, and multi-source spatial analysis, this study systematically investigated seven representative geological hazard sites in the central Yanshan Mountains to elucidate their formation mechanisms under extreme rainfall conditions. The results showed that the investigated landslides and debris flows were primarily concentrated within the major rainfall centers of the July 2025 event and predominantly developed in Carboniferous carbonate-rock strata. Steep gullies, fractured bedrock, shallow weathered deposits, and abundant loose materials jointly controlled the development of geological hazards. Most debris-flow catchments exhibited strong coupling between shallow landslides and channelized debris flows. Catastrophic geological hazards also occurred in areas with relatively high vegetation coverage, suggesting that the reinforcing effect of vegetation may become insufficient under prolonged extreme rainfall conditions. Field investigations further revealed that terrace-modified hillslopes significantly increased sediment supply and intensified debris-flow activity. Overall, the formation of extreme rainfall-induced geological hazards in the Yanshan Mountains is governed by the combined effects of geological conditions, geomorphology, sediment availability, and anthropogenic slope modification.
The SAR pixel offset tracking (POT) technique, which exploits synthetic aperture radar (SAR) intensity images, is an effective tool for measuring large-gradient displacements by overcoming phase decorrelation and phase unwrapping errors that limit interferometric synthetic aperture radar (InSAR) measurements. However, the accuracy of POT depends on image resolution, and its performance degrades significantly when using medium-resolution SAR data such as Sentinel-1, often leading to mismatches, noisy displacement fields, and reduced reliability in capturing localized landslide deformation. To address this challenge, we propose an autofocusing-based adaptive time-series POT (AF-TSPOT), which integrates autofocusing-based temporal stacking into a time-series POT framework for landslide deformation retrieval from medium-resolution SAR image sequences. By refocusing temporally stacked SAR intensity images at the pixel level and integrating displacement estimates from multiple temporal baselines, AF-TSPOT suppresses matching noise and reconstructs a more stable displacement time series. Application to the pre-failure deformation of the 2018 Baige landslide along the Jinsha River demonstrates that AF-TSPOT retrieves deformation patterns comparable to those derived from high-resolution ALOS-2 data, reducing the root mean square error (RMSE) in stable areas by 54.16
Landslides are a recurrent hazard in the Western Ghats, India, driven by steep terrain, intense monsoonal rainfall, and increasing anthropogenic disturbances. This study delineates landslide-prone areas in Sindhudurg district, western Maharashtra, by integrating stakeholder perception with geospatial susceptibility modelling. Perception data are collected from 322 stakeholders using a structured questionnaire based on a criterion-based purposive sampling approach targeting landslide-prone villages. Statistical analyses are used to identify dominant triggering factors, while eleven causative factors are integrated using the analytical hierarchy process (AHP) within a Geographic Information System (GIS) framework. Relative weights are derived through pairwise comparison with acceptable consistency ratio (CR = 0.08). The resulting landslide susceptibility map (LSM) indicates that approximately 40
Loess landslides are major hazards in Northwest China and often result in severe casualties and property losses. The southern Jingyang loess tableland in Xianyang City, Shaanxi Province, China, is one of the most active areas for loess landslides. Rapid long-runout flowslides and relatively short-runout slides are the two representative landslide types in this region. Both of them exhibit clear landslide-terrace interaction, yet their entrainment characteristics differ, and the governing mechanisms remain poorly understood. Therefore, this study investigates two well-documented landslides that are representative of these two types, respectively, the Dongfeng landslide (rapid long-runout flowslide) and the Shutangwang landslide (relatively short-runout slide), using a material point method (MPM) model that incorporates entrainment effects. The results indicate that the entrainment mode of the Dongfeng landslide involves shearing of terrace materials by the loess sliding mass, whereas the entrainment mode of the Shutangwang landslide is characterized by thrusting of terrace materials. These distinct entrainment patterns should be primarily attributed to differences in the liquefaction susceptibility of the terrace materials, which are controlled by pre‑landslide topographic and hydrogeological conditions. Prior to failure, the Dongfeng landslide site featured a longer terrace with poor drainage and a relatively shallow groundwater level. As a result, the sand and gravel in the terrace are prone to liquefaction under the impact of the loess slide, which reduces sliding resistance and promotes a longer runout distance. In contrast, the Shutangwang landslide site has a shorter terrace with better drainage and a deeper groundwater level. Consequently, the liquefaction potential of the sand and gravel in the terrace is relatively low, leading to a shorter sliding distance.
Colluvial landslides along gently inclined interfaces between deposits and red beds represent a significant, yet mechanically poorly understood, hazard in the Sichuan Basin, China. This study investigates the failure mechanism of the 2022 Aiziping landslide in Tongjiang County by integrating site reconnaissance, laboratory testing, finite element seepage analysis, and stability assessment. The results show that toe excavation for roadway construction preconditioned the landslide by creating preferential infiltration pathways, while the excavated free face impeded groundwater drainage. Subsequent rainfall generated a distinct pore-water pressure distribution, characterized by sustained suction in the mid-rear section and rising positive pressure at the slope toe. This hydraulic heterogeneity drove spatially differential soil softening and strength loss, progressively lowering the factor of safety below unity along two sliding surfaces, and leading to a two-stage retrogressive failure. The failure mechanism is, therefore, governed by excavation-induced hydrogeological change followed by rainfall-triggered soil softening and pore-pressure increase at the poorly drained toe, suggesting that the post-excavation groundwater table at the slope toe can serve as a critical factor controlling slope stability, and that appropriate reinforcement and drainage measures after excavation are essential for risk mitigation in similar geological settings.
Stabilizing piles are widely used in landslide governance, yet their interaction with multi-sliding zones reservoir landslides (MSZRL) remains poorly understood, often leading to unexpected failures. Specifically, the mechanism by which piles influence the force transmission path between adjacent slide masses during reservoir water level (RWL) fluctuations has not been fully elucidated. This study investigates the coupled deformation evolution of MSZRL reinforced by stabilizing piles, using a 3D fluid–solid coupling numerical model based on the prototype of the Majiagou MSZRL in the Three Gorges Reservoir area. The results reveal a critical “bridging effect” of improperly anchored piles: rather than stabilizing the slope, the piles transfer the residual sliding force from the active shallow slide mass to the metastable middle slide mass. This alters the stress distribution, causing a concentration of shear stress that triggers the penetration of shallow (S1) and middle (S2) slip surfaces, thereby accelerating the failure of the deeper strata. Furthermore, by analyzing the evolution of displacement, seepage fields, and unbalanced thrust under varying pile depths, this study classifies the deformation of pile-reinforced MSZRL into four distinct modes: shallow sliding (no piles), middle sliding (short piles), deep sliding (long piles), and minimal sliding (ultra-deep piles). These findings provide new theoretical insights into the interlayer shear coupling mechanism and offer practical guidelines for optimizing pile design in complex multi-layered reservoir slopes.
The reliability of limit equilibrium slope stability analysis depends critically on the construction of an appropriate slip surface model. Conventional regular-geometry surfaces often fail to represent complex failure mechanisms, whereas fully random surfaces can produce kinematically inadmissible shapes and low computational efficiency. To balance physical realism and practicality, this study proposes a slip surface construction method based on a Taylor-series-derived random polar angle increment function, with morphological variability constrained by physically motivated end conditions. An improved tensile-shear strength criterion is incorporated into a stress-based limit equilibrium framework, thereby coupling slip surface morphology, stress distribution, and strength parameters. A genetic algorithm is then employed to optimize the polar angle increment function and efficiently identify the critical slip surface. Numerical examples and engineering case studies demonstrate that the proposed method achieves high geometric flexibility with a small number of controlling parameters, captures complex slope failure mechanisms, and provides a reliable and practical tool for slope engineering applications.
Landslide processes in the Carpathian Mountains represent a major hazard threatening both human lives and the socio-economic development and infrastructure of the region. The Carpathians are vulnerable to landslide hazards due to several factors related to regional geology and tectonics. The main goal of this study is to examine the main factors influencing landslides in the Outer Carpathian Mountains in Ukraine, to understand which factor(s) have the most impact on landslide susceptibility, and finally to demonstrate the potential of the integrated technique for assessment of landslide hazards at the regional and local scale. This analysis confirms the essential role of tectonics in landslide formation. Genetic classification and formation mechanisms of landslides are spatially associated with fracture zones. Fracture zones are generally subparallel to river valleys but locally are orthogonal or diagonal. This article examines the conditions of landslide formation associated with fracture zones orthogonal to river valleys. Landslide occurrence and the corresponding damage to infrastructure within such zones exhibit a localized character dictated by the width of fracture zones. These zones are clearly expressed in the topography. Therefore, in the regional classification of landslides in the Outer Carpathians it is necessary to add the landslides associated with a tectonic structure of various nature and orientations. This study also provides valuable insights for hazard preparedness and landslide risk reduction in the region.
Subaqueous gravity-driven mass movements and their resulting mass transport deposits (MTDs) are primary agents in shaping continental margins and volcanic edifices. This study provides a high-resolution geomorphological and geophysical characterization of MTDs in the Graham Bank, a complex setting in the Sicily Channel (Central Mediterranean) where active rifting, anorogenic volcanism, and vigorous oceanographic circulation converge. High-resolution multibeam bathymetry (5 m grid) integrated with sub-bottom CHIRP seismic profiles documents a complex seafloor shaped by the interaction of tectonics, volcanism, and oceanographic processes at water depths between 10 and 350 m. We mapped and classified a diverse spectrum of deposits into three thematic categories: (i) large-scale volcaniclastic debris avalanches (up to 2.2 km2 and thicknesses exceeding 20–30 m) characterized by blocky topography and amphitheater scars; (ii) tectonically-controlled rockfalls at the foot of steep escarpments; and (iii) soft-sediment deformation structures (slumps and channelized-fills) showing mounded hummocky facies and basal erosional unconformities. These features are variably distributed across three main physiographic domains: the volcanic sector, a fault-bounded erosive channel, and an adjacent flat outer-shelf area. Our findings suggest a conceptual "cascading" morphogenetic framework where tectonic rifting with NW–SE-oriented normal faulting and high regional heat flow (75–100 mW/m2) pre-condition the slopes, while regional seismicity (ML ≥ 5) and bottom-current erosion (Atlantic Ionian Stream) act as possible transient destabilizing mechanisms. Volcanic-driven instabilities may be related to volcanic activity; the 1831 eruption illustrates the system’s capacity for rapid morphodynamic change, though direct temporal links to individual MTDs cannot be established without sediment dating. The Graham Bank thus represents a high-instability environment where MTDs record the combined effects of internal geodynamic forcing and external oceanographic modulation, providing a key natural laboratory for assessing gravity-driven processes. The proximity of these features to submarine cables and pipelines, coupled with the recent volcanic history of the area (e.g., the 1831 eruption), underscores a preliminary geohazard concern warranting further investigation in this sector of the Mediterranean.
Woody debris flows (WDFs) are among the most destructive compound geohazards in forested mountainous regions. The large volume of woody debris can rapidly induce blockage and sedimentation in Open-type check dams, significantly increasing the risk of structural failure. Slit check dams (SCDs) are a type of open-check dam reported to mitigate WDFs, but their regulatory mechanisms remain unclear. In this study, a series of physical model experiments was conducted to investigate the effects of relative opening ratio, woody debris length and content, and debris flow bulk density on opening clogging characteristics and mitigation performance. The results indicate that the interception process of WDFs by SCDs can be divided into five dynamic evolutionary stages: initial interception, flow runup and climbing, temporary clogging by woody debris, crest overflow, and impoundment with gradual drainage. Woody debris interception significantly increases the probability of opening blockage inducing four distinct blockage states: unobstructed, partially obstructed, fully obstructed, and supercritical blockage. Based on the relative opening ratio (b/dmax), three functional mitigation zones are identified: interception-dominated zone (b/dmax ≤ 1.5), interception–retention cooperative regulation zone (1.5 < b/dmax < 4.0), and flow-regulation zone (b/dmax ≥ 4.0). The blockage ratio and mitigation indices decrease progressively with increasing opening ratio. Under different experimental conditions, the woody debris interception ratio (W) ranged from 40.1
Landslides that occur on hillslopes within reservoir watersheds can generate tsunami-like impulsive waves when the mobilized debris flows into the reservoir. Such waves may overtop the dam, particularly during normal high-water-level periods such as the flood season, and are of special concern for earth-fill dams, for which overtopping alone can trigger failure and lead to catastrophic downstream flooding. Although several numerical studies have addressed these phenomena, few adequately represent the erosion and entrainment processes that critically govern debris flow volume, momentum, and the resulting wave amplitude. This study develops Deb2L, a two-dimensional, two-layer numerical model based on the shallow-water equations and discretized using the finite volume method, which explicitly accounts for erosion, entrainment, and deposition. The model was verified against analytical benchmark solutions and validated with laboratory experiments: the benchmark tests reproduced the analytical solutions with R2 values above 0.93, and the debris flow-influx experiments were reproduced with RMSE values of 0.004–0.06 m, confirming the accuracy of the model. Its applicability to field-scale events was demonstrated by simulating the 2020 Sanyang Reservoir event in Icheon, South Korea. To assess the model’s capability for scenario-based analysis, the landslide-prediction model TiVaSS was coupled with Deb2L by using the TiVaSS outputs as inputs for the debris flow simulation, confirming the feasibility of the coupled framework. The results further show that neglecting erosion and entrainment leads to a substantial underestimation of the scale of debris flow-induced impulsive waves and the associated compound hazards in reservoir basins.
Distributed acoustic sensing (DAS) offers unprecedented spatial resolution and long-range coverage for rockfall hazard monitoring. However, in acoustically complex field environments, strong ambient wavefields heavily mask transient rockfall impact signatures. This interference hinders the accurate extraction of signal onset times, severely degrading spatial localization accuracy. To overcome this operational bottleneck, this study proposes a high-precision rockfall localization framework that integrates adaptive signal isolation with a time-difference-of-arrival algorithm. Using DAS data acquired from controlled artificial rockfall experiments, we systematically evaluated the signal-enhancement performance of several advanced modal decomposition and wavelet-based techniques to address the non-stationary characteristics of DAS recordings. Results demonstrate that an optimized variational mode decomposition approach maximizes kinematic signal fidelity. These isolated rockfall signatures achieved an average signal-to-noise ratio of up to 26.94 dB, representing a 58
This study investigates instability mechanisms in soft rocky coastal cliffs through an integrated numerical framework that couples computational fluid dynamics (CFD) with the finite-discrete element method (FDEM), applied to the tuffaceous cliffs of Ventotene Island (Italy). The modelling strategy systematically explores the role of structural predisposition, marine erosion, wave-induced loading, thermal forcing, and time-dependent strength degradation in controlling cliff stability and failure evolution. The results show that neither gravity loading nor basal undercutting alone can reproduce realistic failure mechanisms, even in the presence of a DFN. Progressive rock failure (PRF) emerges only when basal erosion is explicitly coupled with inward-migrating strength degradation, implemented using a simplified fatigue-based S–N approach. Under the considered lithology and the present Mediterranean climatic conditions, wave impacts and thermal forcing induce predominantly elastic or sub-critical mechanical responses and do not directly trigger failure. However, wave-induced dynamic perturbations concentrate displacements in fractured cliff sectors, suggesting a preparatory role through fatigue and stress redistribution, with potential triggering effects once critical degradation thresholds are approached. Thermal forcing becomes mechanically effective only under amplified temperature excursions or in lithologies with higher thermal sensitivity. The coupled CFD–FDEM approach is therefore interpreted as a process-based, exploratory framework that captures the dynamic response of coastal cliffs to realistic environmental boundary conditions, rather than as a direct predictor of wave-driven collapse. The methodology provides a physically grounded and transferable framework for investigating multi-forcing interactions, damage accumulation, and PRF in coastal cliffs, offering new insights into the mechanisms governing coastal instability and long-term cliff evolution.
On March 14–16, 2025, a sequence of two distinct landslide events affected the Poggio Baldi landslide (PBL) area (43° 54′ 26″ N, 11° 48′ 34″ E, 441 m a.s.l., Northern Apennines, Italy): an earth flow on March 14 involving the debris talus accumulated since 2010, followed by a rock slide on March 16 affecting the right shoulder of the main scarp of the former PBL. Both events were triggered by exceptional rainfall (117.8 mm/day, the second-highest recorded since 2021) and comprehensively documented through the site’s integrated multi-sensor monitoring infrastructure. The earthflow, with an estimated volume of 30,000 m3, occurred 3 h after peak rainfall intensity, demonstrating a rapid response to the meteorological trigger. Pre-, syn-, and post-failure evolution was framed by a combination of remote sensing approaches. Optical Digital Image Correlation and thermal image analyses revealed average velocities of 18.3 cm/h with peaks reaching 100 cm/h. The rockslide (35,000 m3) was preceded by precursory rockfall activity detected by motion-triggered optical cameras and an InfraRed camera, as well as progressively monitored through optical monitoring. Acoustic analysis characterised the collapse as a three-phase event lasting 4 min and 30 s with dominant frequencies at 315 Hz. An Interferometric Arc-SAR monitoring system documented the post-failure evolution of the rockslide debris, showing maximum displacements of 1.8 m, and revealing sudden reactivations to the following rainfall events. An optical satellite analysis confirms the area of the event, approximately 3 ha, and highlights the reactivated part with respect to the older PBL. The combined events mobilised approximately 65,000 m3 of material, with the slope achieving near-stable conditions after 2 months. This case study demonstrates the effectiveness of integrated remote sensing–based monitoring for capturing the complete landslide sequence, from its precursors, triggers, up to progressive stabilisation, providing crucial quantitative data to advance the understanding of complex slope deformation processes that anticipate the ultimate failure. Furthermore, this event proved the applicability of low-budget image-based monitoring systems in early warning applications.
In this paper, the authors assess a small rock cut failure in a rather jointed slate rock mass in northwestern Spain occurred after a rainfall event. A non-standard failure mechanism was observed to produce a toppling failure partially induced by a sliding failure in a lateral zone of the slope. The failure generated a small landslide mobilizing about 300 m3 of rock that closed the access road to a hydropower plant underground facility for a few days. To investigate this failure mechanism, a detailed field survey was conducted to characterize the regional geology, discontinuities, and photogrammetry, alongside a series of laboratory tests to determine intact rock and joint geomechanical properties. Based on these observations and tests, a wedge sliding failure was initially identified; however, further kinematic and limit equilibrium analyses revealed a more complex wedge-toppling combined failure mechanism. The mechanism is also noteworthy in that the kink-banding associated with the slate in the rock mass plays a non-negligible role in the development of the rock slope failure. Additionally, the analysis is presented in the context of combined failure mechanism analysis, specifically, wedge and toppling failures, which can contribute to a better understanding of rock-mass slope failure phenomena and could be also useful for designing future protection measures.
This study examines the tectonic origin, hydrodynamic behavior, and morphometric characteristics of three rockslide-dammed lakes—Oyu, Nomin, and Binderya—formed by the 1957 Mw 8.1 Gobi-Altay Range (GAR) earthquake in southern Mongolia. The earthquake generated large rockslides that blocked narrow valleys, producing rockslide dams and disrupting hydrological connectivity across this arid mountain region. Remote sensing data, morphometric analysis, and field measurements identify three major rockslide deposits, the largest near Binderya Lake (2.49 km2; 0.417 km3), with relatively smaller deposits (0.12–0.71 km2) at Oyu and Nomin Lakes. Steep valley slopes (avg. 35–43°, max 65–68°) combined with strong tectonic control govern rockslide initiation and downslope mobility. From 2016 to 2025, all lakes show declining area and volume, most notably Nomin and Oyu. However, the modest R2 values (0.16–0.26) indicate strong interannual variability, and the 10-year observation period is likely too short to distinguish long-term trends from short-term fluctuations. The lake depressions show distinct morphometric patterns. These patterns result from tectonic deformation and slope failure, and they control sediment storage and downstream flow. Terrain classification further reveals high surface heterogeneity, providing essential constraints for realistic modeling of rockslide dynamics. The findings demonstrate that earthquake-triggered rockslides are key geomorphic agents that reorganize sediment fluxes and modify depression morphology in arid, seismically active mountain environments, with important implications for seismic hazard assessment.
