Unstable Mountain Slope Research Articles

Rockfall from an increasingly unstable mountain slope driven by climate warming

Rockfall in high-mountain regions is thought to be changing due to accelerating climate warming and permafrost degradation, possibly resulting in enhanced activity and larger volumes involved in individual falls. Yet the systematic lack of long-term observations of rockfall largely hampers an in-depth assessment of how activity may have been altered by a warming climate. Here we compile a continuous time series from 1920 to 2020 of periglacial rockfall activity using growth-ring records from 375 trees damaged by past rockfall at Täschgufer (Swiss Alps). We show that the ongoing warming favours the release of rockfall and that changes in activity correlate significantly with summer air temperatures at interannual and decadal timescales. An initial increase in rockfall occurred in the late 1940s to early 1950s following early twentieth century warming. From the mid-1980s, activity reached new and hitherto unprecedented levels. This long-term record of rockfall activity can help to inform the design of vital mitigation and risk reduction measures in inhabited mountain environments.

Correlations Between Knowledge About Landslides and Preparedness for Disasters: Cross-Sectional Study

Landslide events in Indonesia are often caused by high rainfall and unstable mountain slopes. With these conditions, the risk of landslides is higher if the rainy season occurs. Landslide disasters cause loss of life and property and disrupt the order of life. Reducing the risk of disaster impacts can be reduced by increasing preparedness in dealing with disasters. This study aimed to determine the relationship between knowledge about landslides and community preparedness in Hargowilis village. This type of research is a quantitative study with a cross-sectional research design. The sampling technique used in this study was purposive sampling, with a sample size of 102 heads of households in Hargowilis Kokap Kulonprogo village. The measuring instrument used is a landslide disaster knowledge questionnaire and a disaster preparedness questionnaire—statistical data analysis using the Chi-Square test. Based on the characteristics of the respondents, most of them were aged 50-59 years (25.6%0, the majority were male (82.4%), most of them had high school education (42.2%), and most of them were farmers (51%). Most knowledge about landslides is in the moderate category (46.1), and the Preparedness Level is mostly in the high category (64.7). Landslide disaster with disaster preparedness in the village of Hargowilis Kokap Kulonprogo There is a significant relationship between knowledge of landslide disasters and disaster preparedness in the village of Hargowilis Kokap Kulonprogo. This research can provide information about the relationship between knowledge and preparedness for landslide disasters so that people can understand the importance of having sufficient knowledge and good preparedness in dealing with this disaster.

A GIS-Based Kinematic Analysis for Jointed Rock Slope Stability: An Application to Himalayan Slopes

GIS-based kinematic stability analysis in rock slopes is a rare practice in geological engineering despite its immense potential to delineate unstable zones in a mountainous region. In this article, we have used a GIS-based modified technique to assess the efficiency of kinematic analysis in predicting shallow landslides in the rock slopes of the Himalayan mountains on a regional scale. The limited use of this technique is primarily due to the complexities involved in its practical application. To make this technique more effective and convenient usability, we present modified methods and a new application, ‘GISMR’, that works with the aid of GIS software for the determination of kinematic susceptibility. A modified kinematic analysis method was implemented to define the stability in terms of failure susceptibility on a scale of 0 to 100 rather than a conservative result, such as failure or non-failure. We also present another functionality of the GISMR that provides optimised slope angles over a region. This functionality could aid the decision-making process when selecting a suitable location for a road path or other engineering constructions that are impacted by unstable mountain slopes. The applicability of this new method was demonstrated in a rock failure-prone region in the mountains of the Indian Himalayas. The outcomes delineate the unstable slopes in the region, which are intersected by a strategic National Highway 05 and have a long history of landslide-related hazards. It was found that 9.61% of the area is susceptible to failure. However, 2.28% is classified as a low susceptible region, and 2.58% of the area is very-low susceptible. The regions with moderately high, high, and very-high susceptibility cover 2.78%, 1.49%, and 0.46% of the whole area, respectively. The results were evaluated by receiver operating characteristic curve and a frequency ratio method to represent the association between kinematic susceptibility and the mass movement inventory in the area. It is concluded that kinematic susceptibility has a strong relationship with landslide activity in the rock slopes of the Himalayan region.

Hillslope-channel coupling in the Nepal Himalayas and threat to man-made structures: The middle Kali Gandaki valley

In mountain areas, the confinement of valleys favours landslide interaction with rivers, causing channel changes or short-lived dams and lakes that may threaten trails, roads and human settlements. Their impacts may occur successively in space and time, and they affect randomly the functioning of the sediment fluxes. The present study focuses on the interaction patterns between unstable mountain slopes and the Kali Gandaki River, in the Nepal Himalayas. In this valley, the deepest on earth, a road linking the Myagdi and Mustang districts has been under construction for the past 5 years, either cutting into the bedrock or crossing areas affected episodically by debris slides, earth flows, debris flows and rock slides. On the basis of the geomorphic evolution observed over the last three decades, we assess the potential threats that now arise following completion of the road. We mapped three areas of recurrent mass wasting features characteristic of the most frequent situations encountered in this valley. We analyzed the combination of the hydro-geomorphic processes involved. With the use of a DEM, we assessed the volume and spatial impact of temporary river dams on infrastructure located along the valley floor. We estimated hydraulic parameters to document the geomorphic efficiency of river flooding after dam breaching. We reconstructed the spatial extent of (1) areas threatened by backwater flooding upstream of the dams and (2) areas threatened by the collapse of the dams. We describe the current geomorphic and sedimentary adjustments still at work along the valley sides. Our findings confirm that in the High Himalaya, medium scale landslides (10 5–6 m 3) play a major role in the overall process of denudation and sediment transfer. They highly influence the transient nature of bedload transport in the channel. In reducing the residence time of sediments in temporary, spatially limited traps of the valley bottom, they enhance the vulnerability of land and people attracted by a roadside location.

Use of the simultaneous seismic, GPS and meteorological monitoring for the characterization of a large unstable mountain slope in the southern French Alps

SUMMARY The presence of unstable rock slopes in the mountainous regions makes these areas particularly prone to the development of sudden dramatic events, which may cause damage and injuries. The efficient disaster management requires a good understanding of the main causes of these mass movements, such as, increased fluid pressure and seismic shaking. In order to improve our understanding of the mechanics and dynamics of the mountain slope instabilities, we developed a monitoring system based on microseismic, geodetic and meteorological measurements. The ⎛ ⎝ � ⎞ �

Ambient vibration analysis of an unstable mountain slope

A field experiment with small aperture seismic arrays was performed on the unstable rock slope above the village of Randa in the southern Swiss Alps. The aim of this experiment was to constrain the seismic response of a potential future rockslide using ambient vibration recordings. Weak seismic events were identified on the recordings and site-to-reference spectral ratios were calculated using a reference site located on the stable part of the slope. Spectral ratios of up to 30 were observed at sites located within the unstable portion of the slope. A strong variation of spectral ratios with azimuth indicates a directional site effect. Neither amplification nor directionality were observed at sites located in the stable part of the slope. Furthermore, time-frequency polarization analysis of the ambient noise was performed to provide robust estimates of frequency dependent directions of the maximum polarization. It was found that the unstable part of the slope vibrates within a narrow range of directions (130 ± 10°) for the frequency range centred around 5 Hz. The polarization directions estimated from ambient seismic vibrations are in good agreement with the deformation directions obtained by geodetic and in situ measurements. No directionality of ambient vibrations was observed at sites within the stable part of the slope.

Microseismic investigation of an unstable mountain slope in the Swiss Alps

Risks associated with unstable rocky slopes are growing as a result of climate change and rapid expansions of human habitats and critical infrastructure in mountainous regions. To improve our understanding of mountain slope instability, we developed a microseismic monitoring system that operates autonomously in remote areas afflicted by harsh weather. Our microseismic system comprising 12 three‐component geophones was deployed across ∼60,000 m2 of rugged crystalline terrain above a huge (30 million m3) recent rockfall in the Swiss Alps. During its 31‐month lifetime, signals from 223 microearthquakes with approximate moment magnitudes ranging from −2 to 0 were recorded. Determining the hypocenters was challenging for several reasons: (1) P wave velocities were highly heterogeneous, varying abruptly from <1.5 km/s to >3.8 km/s. (2) First‐break picks were either inaccurate or lacking for some microearthquakes. (3) There were no reliable S wave picks. (4) Numerous microearthquakes occurred just outside the network boundaries. These issues were addressed by using a three‐dimensional (3‐D) P wave velocity model of the mountain slope determined from refraction tomography in a nonlinear inversion for hypocenter parameters and their probability density functions. Recordings from geophones at different altitudes and in boreholes constrained microearthquake depth estimates. Most microearthquakes were concentrated within 50–100 m of the surface in two zones, one that followed the recent rockslide scarp and one that spanned the volume of highest fracture zone/fault density. These two active zones delineated a mass of rock that according to geodetic measurements has moved toward the scarp at 1–2 cm/yr.

Characterizing an unstable mountain slope using shallow 2D and 3D seismic tomography

As transport routes and population centers in mountainous areas expand, risks associated with rockfalls and rockslides grow at an alarming rate. As a consequence, there is an urgent need to delineate mountain slopes susceptible to catastrophic collapse in a safe and noninvasive manner. For this purpose, we have developed a 3D tomographic seismic refraction technique and applied it to an unstable alpine mountain slope, a significant segment of which is moving at [Formula: see text] toward the adjacent valley floor. First arrivals recorded across an extensive region of the exposed gneissic rock mass have extraordinarily low apparent velocities at short [Formula: see text] to long [Formula: see text] shot-receiver offsets. Inversion of the first-arrival traveltimes produces a 3D tomogram that reveals the presence of a huge volume of very-low-quality rock with ultralow to very low P-wave velocities of [Formula: see text]. These values are astonishingly low compared to the average horizontal P-wave velocity of [Formula: see text] determined from laboratory analyses of intact rocks collected at the investigation site. The extremely low field velocities likely result from the ubiquitous presence of dry cracks, fracture zones, and faults on a wide variety of scales. They extend to more than [Formula: see text] depth over a [Formula: see text] area that encompasses the mobile segment of the mountain slope, which is transected by a number of actively opening fracture zones and faults, and a large part of the adjacent stationary slope. Although hazards related to the mobile segment have been recognized since the last major rockslides affected the mountain in 1991, those related to the adjacent low-quality stationary rock mass have not.

In situ characterization of flows in a fractured unstable slope

In the case of large unstable mountain slopes, the spatial repartition of the saturated zones influence the pressure repartitions in the massif and the slope stability. This contribution proposes an evaluation of water origin and pressure transit time of groundwater flows from hydrochemical and isotopic measurements. The objective is to propose a conceptual model of the groundwater flows and a location of the saturated zones in the slope. The interpretation of time measurements of water pressure in the collapsed zone highlights (i) an evolution of flows with deformation that induces a damaging of the rock properties and (ii) the necessity to connect hydrogeological conditions and geological background in a dynamical system to analyse the pore pressure in the fractured collapsed zone.

Characterization of an unstable rock mass based on borehole logs and diverse borehole radar data

Unstable rocky slopes are major hazards to the growing number of people that live and travel though mountainous regions. To construct effective barriers to falling rock, it is necessary to know the positions, dimensions and shapes of structures along which failure may occur. To investigate an unstable mountain slope distinguished by numerous open fracture zones, we have taken advantage of three moderately deep (51.0–120.8 m) boreholes to acquire geophysical logs and record single-hole radar, vertical radar profiling (VRP) and crosshole radar data. We observed spallation zones, displacements and borehole radar velocity and amplitude anomalies at 16 of the 46 discontinuities identified in the borehole optical televiewer images. The results of the VRP and crosshole experiments were disappointing at our study site; the source of only one VRP reflection was determined and the crosshole velocity and amplitude tomograms were remarkably featureless. In contrast, much useful structural information was provided by the single-hole radar experiments. Radar reflections were recorded from many surface and borehole fracture zones, demonstrating that the strong electrical property contrasts of these features extended some distance into the adjacent rock mass. The single-hole radar data suggested possible connections between 6 surface and 4 borehole fractures and led to the discovery of 5 additional near-surface fracture zones. Of particular importance, they supplied key details on the subsurface geometries and minimum subsurface lengths of 8 of the 10 previously known surface fracture zones and all of the newly discovered ones. The vast majority of surface fracture zones extended at least 40–60 m into the subsurface, demonstrating that their depth and surface dimensions are comparable.

Semblance‐based topographic migration (SBTM): a method for identifying fracture zones in 3D georadar data

ABSTRACTSteep‐dipping fracture zones are generally difficult to delineate using traditional ground‐penetrating radar (georadar) techniques. Evidence for their presence in standard georadar images may be either completely absent or limited to diffractions and/or chaotic reflection patterns. To address this issue, we present a novel three‐dimensional (3D) migration scheme based on computations of semblance. This new approach, which accounts for undulating surface topography, emphasizes diffractors while markedly reducing the effects of specular reflectors. After demonstrating the efficiency of the technique on 3D synthetic data, we apply it to a 3D georadar data set acquired across an unstable mountain slope in the Swiss Alps. This region is characterized by rugged topography and numerous shallow‐ to steep‐dipping fracture zones and faults. Only the shallow‐ to moderate‐dipping structures are imaged as reflectors in conventionally migrated versions of the georadar data. Our semblance‐based topographic migration (SBTM) scheme produces a 3D volume containing clouds of high‐semblance values. Application of morphology image processing to these clouds reveals the presence of geologically meaningful structures, most of which are very steeply dipping (>75°). Several of the steep‐dipping structures project to open fracture zones and associated lineaments at the surface, thus demonstrating the capability of the combined SBTM and morphology procedure for mapping near‐vertical fracture zones.

Hydrometeorological and landuse attributes of debris flows and debris floods during typhoon Toraji, July 29–30, 2001 in central Taiwan

The 921 Chi-Chi earthquake, a 7.5 M earthquake with center located near Chi-Chi of Nan Tou County occurred on September 21, 1999 and caused more than 2400 fatalities and totaling billions of dollars losses due to destruction and damages to private and public buildings, roads, bridges and hydraulic structures in central Taiwan. This 921 Chi-Chi earthquake also triggered close to 26,000 landslides with a total area of 15,977 ha and severely disturbed mountain slopes, thus setting the conditions for occurrence of more landslides and other related disasters. Post-earthquake watershed restoration works progressed without disruptions by major typhoon rainstorms until July 29–30, 2001 when typhoon Toraji hit eastern and central Taiwan with heavy rainfall having peak hourly intensity exceeding 70 mm/h. This rainstorm caused several major debris flows and debris floods that destroyed or severely damaged dams, roads, bridges, dikes and houses, in addition to causing 103 casualties, 111 missing (presumed dead), 189 injured. This study (1) explores the characteristics and impacts of debris flows/floods and (2) investigates hydrometeorological and land use attributes of debris flows and debris floods associated with this typhoon Toraji in central Taiwan. Debris floods, different from more familiar debris flows, are extreme peak streamflows containing large quantities of muds, rocks and debris normally occurring in third and fourth order watersheds. Data analysis and field investigations revealed that the disastrous debris flows and debris floods during typhoon Toraji were essentially caused by very heavy rainfalls on inherently unstable mountain slopes that were loosened further earlier by the 921 Chi-Chi earthquake. The extremely intense rainfall of typhoon Toraji provided not only additional materials by creating new and reactivating or enlarging old landslides but also high peak stream flows required for occurrence of debris flows and debris floods. The destruction and damaging power of debris flows and debris floods witnessed during typhoon Toraji suggests that the effectiveness of engineering structures for disaster prevention is limited. For disaster mitigation, it is required to have more comprehensive strategies such as (1) land use zoning that regulates and restricts human encroachment on hazardous areas and (2) implementation of evacuation drills and relocation plan for people currently living in the hazardous zones.

Irrigation system design for management in mountainous areas

The paper reports research on engineering designs of irrigation systems in mountainous areas and the resulting implications for management. The engineering designs are important because they may or may not provide the social basis for local people to participate in system development and management. This hypothesis is analyzed by using field data from an irrigation development program in the Himachal Pradesh State in India. The development program will construct about 150 small-scale, gravity flow and lift irrigation schemes in a seven year period. The results of the analysis support the argument that the engineering designs critically influence local people's capability and willingness to manage irrigation facilities. In gravity flow irrigation schemes, water capture by permanent flow diversion structuress is extremely expensive and yet unreliable because of large variation in stream flow and unstable mountain slopes. A more practical alternative is to utilize diversion structures of temporary nature which are made with local materials. Construction and management of such structures will encourage local participation because of high maintenance requirements. In lift irrigation schemes a conventional design is to lift all water requirements to a single highest point in the command area. This practice results in unnecessarily high energy costs and in a water distribution system which is complex to manage. A better alternative is to design the pumps so that water can be delivered at variable elevations in the scheme area. This will allow disaggregation of the total area into smaller rotational units and, therefore, management of water distribution can be potentially easier and equitable.