Representative rainfall thresholds for landslides in the Nepal Himalaya

Measuring some 2400 km in length, the Himalaya accommodate millions of people in northern India and Pakistan, Nepal, Bhutan, and parts of other Asian nations. Every year, especially during monsoon rains, landslides and related natural events in these mountains cause tremendous damage to lives, property, infrastructure, and environment. In the context of the Himalaya, however, the rainfall thresholds for landslide initiation are not well understood. This paper describes regional aspects of rainfall thresholds for landslides in the Himalaya. Some 677 landslides occurring from 1951 to 2006 were studied to analyze rainfall thresholds. Out of the 677 landslides, however, only 193 associated with rainfall data were analyzed to yield a threshold relationship between rainfall intensity, rainfall duration, and landslide initiation. The threshold relationship fitted to the lower boundary of the field defined by landslide-triggering rainfall events is I = 73.90D^− 0.79 (I = rainfall intensity in mm h^− 1 and D = duration in hours), revealing that when the daily precipitation exceeds 144 mm, the risk of landslides on Himalayan mountain slopes is high. Normalized rainfall intensity–duration relationships and landslide initiation thresholds were established from the data after normalizing rainfall-intensity data with respect to mean annual precipitation (MAP) as an index in which N_I = 1.10D^− 0.59 (N_I = normalized intensity in h^− 1). Finally, the role of antecedent rainfall in causing landslides was also investigated by considering daily rainfall during failure and the cumulative rainfall to discover at what point antecedent rainfall plays an important role in Himalayan landslide processes. Rainfall thresholds presented in this paper are generalized so they can be used in landslide warning systems in the Nepal Himalaya.     

Analysis of rainstorm-induced slide-debris flows on natural terrain of Lantau Island, Hong Kong

Lantau Island, the largest outlying island of the territory of Hong Kong, experienced a severe rainstorm on 4–5 November 1993, which induced >800 slope failures on natural terrain there. Detailed field investigations were carried out to study the failure modes, in relation with various influencing factors. It was found that the occurrence of slide-debris flows has a close relationship with bedrock geology, slope gradient, vegetation cover and micro landform. The failure modes of slide-debris flows may be classified into translational slides and rotational slides, and the former are predominant. Analysis of the hydrological response of colluvial slopes during the rainstorm indicated that the majority of the failures were caused by the development of a perched water table in the thin surface layer of colluvium of volcanic origin due to infiltration during the heavy rain. Undisturbed soil samples from south Lantau have been subjected to anisotropically consolidated undrained compression tests at comparatively low stress levels. Constant deviatoric stress path tests (CQD) simulating the stress path in the field at in situ stress levels have been performed to investigate soil behavior. The CQD test results indicate that the material of slopes at undisturbed state is brought to dilation because of the increase in pore water pressure caused by infiltration of rain water. For a translational slide, the displacement, resulting from dilation, may destroy cohesion along the failure surface and locally within the interior of the slide. The surplus water during the intense rainstorm was able to equilibrate the reduction in pore pressure caused by dilation, and the dilation and displacement may be further increased. The strain-softening after significant strains triggered debris flow mobilization. However, for a rotational slide, the increase in pore water pressure caused by surplus water infiltration during the intense rainstorm could not equilibrate the reduction in pore pressure caused by dilation, much or even all of the sliding block could not mobilize into a debris flow.     

Boundary effects of rainfall-induced landslides

In the study of landslides, it is generally assumed that an impermeable boundary exists at a certain depth and failure occurs at this boundary. In reality this is not always the case and failures can occur at any depth. This paper aims to study the effect of boundary conditions on landslides, using a series of seepage and stability analyses performed over a range of rainfall intensities, and for different failure mechanisms, by studying the failure time and depths corresponding to fully drained, partially drained, and impermeable boundaries. It is shown that these conditions can significantly affect the occurrence and depth of rainfall-induced landslides.     

关于滑坡预测预报方法的思考

滑坡是世界上十大自然灾害之一 ,容易造成极大的经济损失和人员伤亡 ,对社会危害很大。目前有关它的预测预报的效果还不甚理想。滑坡灾害的诱发因素包括自然因素和人为因素 ,而人为因素主要与人类工程活动有关 ;在一定的地质地形条件下 ,自然诱发因素则主要是降雨 ,特别是暴雨。通过对滑坡预测预报方法的哲学思考 ,作者认为今后应该加强由天预报地 ,即以降雨预报结果来推测区域滑坡发生的概率方法的研究。将该方法与单点滑坡的监测预报法结合起来 ,可望进一步提高滑坡预测预报的水平     

Comparison between the two triggered landslides in Mid-Niigata, Japan by July 13 heavy rainfall and October 23 intensive earthquakes in 2004

On July 13, 2004, heavy rainfalls because of the intensive activities of the rain front occurred in the Mid-Niigata Region, Japan. They were as much as 400 mm in 24 h, bringing about serious flooding by breaking the river banks. The heavy rainfalls also triggered more than 3,500 landslides. Three months later, the southern region of Mid-Niigata was attacked by an earthquake of magnitude 6.8 on the Richter scale on October 23, 2004. The main earthquakes were followed by intensive aftershocks, which continued until December 2004. By these earthquakes, variable landslides of more than 4,400 also occurred in the hilly and mountainous areas. Namely, different triggers brought about the variable landslides in the hilly mountains whose features are very similar in geological and geomorphological points of view. Therefore, these two events are very useful for clearing the difference in features of the landslides between the two. We have been researching on both landslides in the field just after both occurred and later analyzing air photographs using the geographic information system (GIS). In this paper, we describe the comparison in the distribution features using GIS analytical data between the heavy rainfall-induced and the intensive earthquake-induced landslides.     

A method for producing digital probabilistic seismic landslide hazard maps

The 1994 Northridge, California, earthquake is the first earthquake for which we have all of the data sets needed to conduct a rigorous regional analysis of seismic slope instability. These data sets include: (1) a comprehensive inventory of triggered landslides, (2) about 200 strong-motion records of the mainshock, (3) 1:24 000-scale geologic mapping of the region, (4) extensive data on engineering properties of geologic units, and (5) high-resolution digital elevation models of the topography. All of these data sets have been digitized and rasterized at 10 m grid spacing using ARC/INFO GIS software on a UNIX computer. Combining these data sets in a dynamic model based on Newmark's permanent-deformation (sliding-block) analysis yields estimates of coseismic landslide displacement in each grid cell from the Northridge earthquake. The modeled displacements are then compared with the digital inventory of landslides triggered by the Northridge earthquake to construct a probability curve relating predicted displacement to probability of failure. This probability function can be applied to predict and map the spatial variability in failure probability in any ground-shaking conditions of interest. We anticipate that this mapping procedure will be used to construct seismic landslide hazard maps that will assist in emergency preparedness planning and in making rational decisions regarding development and construction in areas susceptible to seismic slope failure.     

The slope movements within the Mondorès graben (Drôme, France); the interaction between geology, hydrology and typology

The Mondorès graben in the south-eastern French Alps is an uncommon structural feature, which originates in a complex polyphasic tectonic evolution. In contrast with its immediate surroundings, with hardly any huge landslides, the Mondorès graben is characterised by various types of landslides. A huge sagging caused part of the limestone cliff to subside some 50 m within 50 years. Two recent mud flows that occurred were considered a potential threat to some inhabited places downstream. The hydrologic aspects of the Boulc-Mondorès landslide resemble the geology: infiltration (and karst input?) in the rock-sliding zone and exfiltration in the marls resulting in slumps and mud flows. The mass movement interactions could be explained by a structural geology analysis with geodetic monitoring using different techniques. It is also shown that hydrochemistry as well as geophysical surveys are of importance in unravelling the hydrologic systems and the geological subsurface structures. The present paper aims at explaining the geological control of the different slides in view of estimating their potential danger. Understanding the geological structure and its evolution therefore is a necessary prerequisite.     

The determination of statistical and deterministic hydrological landslide-triggering thresholds

 Hydrological landslide-triggering thresholds separate combinations of daily and antecedent rainfall or of rainfall intensity and duration that triggered landslides from those that failed to trigger landslides. They are required for the development of landslide early warning systems. When a large data set on rainfall and landslide occurrence is available, hydrological triggering thresholds are determined in a statistical way. When the data on landslide occurrence is limited, deterministic models have to be used. For shallow landslides directly triggered by percolating rainfall, triggering thresholds can be established by means of one-dimensional hydrological models linked to the infinite slope model. In the case of relatively deep landslides located in topographic hollows and triggered by a slow accumulation of water at the soil-bedrock contact, simple correlations between landslide occurrence and rainfall can no longer be established. Therefore real-time failure probabilities have to be determined using hydrological catchment models in combination with the infinite slope model. Received: 15 October 1997 · Accepted: 25 June 1997     

Stream response to subsidence from underground coal mining in central Utah

 Short-term geomorphic and hydrologic effects of subsidence induced by longwall mining under Burnout Creek, Utah were evaluated. During the year after longwall mining, 0.3–1.5 m of subsidence was measured near impacted reaches of the mountain stream channel. The major channel changes that occurred in a 700-m reach of Burnout Creek that was subsided from 1992 to 1993 were: (1) increase in lengths of cascades and to a lesser extent glides; (2) increases in pool length, numbers and volumes; (3) increase in median particle diameter of bed sediment in pools; and (4) some constriction in channel geometry. Most of the changes appeared short-lived, with channel recovery approaching pre-mining conditions by 1994. In a 300-m reach of the South Fork that was subsided from 1993 to 1994, only channel constriction was observed, although any impacts on pool morphology may have been confounded by heavy grazing in the riparian reaches during the dry summer of 1994. Similar near-channel sedimentation and loss of pool volume between 1993 and 1994 were noted throughout Burnout Creek and in adjacent, unmined James Creek. Subsidence during the 3-year period had no effect on baseflows or near-channel landslides. Received: 10 April 1998 · Accepted: 21 September 1998     

Application of multi-temporal differential interferometry to slope instability detection in urban/peri-urban areas

We present two case studies regarding the application of Synthetic Aperture Radar (SAR) Persistent Scatterers Interferometry (PSI) techniques to landslide-prone slopes situated in the municipal territories of Caramanico Terme and Volturino (Italy). The analysis of satellite SAR data with PSI techniques poses often problems on sites where, due to the scarcity of human artefacts and the presence of vegetation cover, density of coherent points (PS) is low (< 10 per km²). Moreover, the steep and rough topography typical of landslide-prone areas hamper the interferometric pre-processing, making more difficult the joint estimation of displacements and of DEM errors. Under these conditions the significance of temporal interferometric phase trends can be uncertain and conservative assumptions, necessary to ensure low false detection probabilities, need to be coupled with innovative processing strategies to increase the detection efficiency of PS objects. Here, the SPINUA (Stable Point Interferometry over Un-urbanised Areas) processing technique is applied together with an alternative PS Candidate (PSC) selection procedure based on the use of pixels classified as urban.

The cases of Caramanico and Volturino are representative, respectively, of harsh and favourable conditions for PSI applications. The results from Caramanico show clusters of PS exhibiting similar line-of-sight (LOS) deformation behaviour in the period 1995–2000. The locations of moving PS often coincide with distressed buildings and appear consistent with the areal distribution of recent and past landslide activity. The temporal displacement trends, however, are characterised by very low annual average velocities (from 3 to 7 mm/y) and it is uncertain to what extent the PS data reflect true slope movements, local deformations (e.g. settlement of engineering structures) or both.

Thanks to the more favourable conditions, the application of the standard SPINUA approach in the Volturino area was sufficient to obtain suitable densities of PS, as well as spatially and temporally consistent displacement results for a period 1992–2000. In particular, a group of moving PS was identified in a peri-urban area, known for the past and recent slope stability problems. The slowly moving PS (from 3 to 5 mm/y) fall in a location that, unlike the remaining part of the town, is characterised by the presence of many distressed buildings and structures. Although the site information confirms the reliability of PS data, in the absence of ground monitoring and detailed records of landslide movements, it is difficult to identify the main mechanism of the detected deformations. In general, in geologically and topographically complex urban/peri-urban settings, the significance of very low-velocity PSI surface displacements should always be considered together with in situ geotechnical controls and ground monitoring data.