Rainfall thresholds for prediction of shallow landslides around Chamoli-Joshimath region,Garhwal Himalayas,India

Majority of landslides in the Indian sub-continent are triggered by rainfall. Several attempts in the global scenario have been made to establish rainfall thresholds in terms of intensity-duration and antecedent rainfall models on global, regional and local scales for the occurrence of landslides. However, in the context of the Indian Himalayas, the rainfall thresholds for landslide occurrences are not yet understood fully. Neither on regional scale nor on local scale, establishing such rainfall thresholds for landslide occurrences in Indian Himalayas has yet been attempted. This paper presents an attempt towards deriving local rainfall thresholds for landslides based on daily rainfall data in and around Chamoli-Joshimath region of the Garhwal Himalayas, India. Around 128 landslides taken place in last 4 years from 2009 to 2012 have been studied to derive rainfall thresholds. Out of 128 landslides, however, rainfall events pertaining to 81 landslides were analysed to yield an empirical intensity–duration threshold for landslide occurrences. The rainfall threshold relationship fitted to the lower boundary of the landslide triggering rainfall events is I?=?1.82 D ^?0.23 (I?=?rainfall intensity in millimeters per hour and D?=?duration in hours). It is revealed that for rainfall events of shorter duration (≤24 h) with a rainfall intensity of 0.87 mm/h, the risk of landslide occurrence in this part of the terrain is expected to be high. Also, the role of antecedent rainfall in causing landslides was analysed by considering daily rainfall at failure and different period cumulative rainfall prior to failure considering all 128 landslides. It is observed that a minimum 10-day antecedent rainfall of 55 mm and a 20-day antecedent rainfall of 185 mm are required for the initiation of landslides in this area. These rainfall thresholds presented in this paper may be improved with the hourly rainfall data vis-à-vis landslide occurrences and also data of later years. However, these thresholds may be used in landslide warning systems for this particular region of the Garhwal Himalayas to guide the traffic and provide safety to the tourists travelling along this pilgrim route during monsoon seasons.     

Rainfall thresholds for rainfall-induced landslides in Slovenia

In Slovenia, rainfall-induced landslides lead to considerable damages, even causing human losses. In order to reduce the impact of this kind of landslide, several researchers analyzed rainfall-induced landslides in this country, but to date, no rainfall thresholds have been developed for a landslide warning system at national scale. In this paper, the definition of rainfall thresholds for rainfall-induced landslides in Slovenia is presented. The thresholds have been calculated by collecting approximately 900 landslide data and the relative rainfall amounts, which have been collected from 41 rain gauges. To define the thresholds, an existing procedure characterized by a high degree of objectiveness has been used. This approach is based on a software that was developed for a test site with very different characteristics (Tuscany, central Italy). At first, a single national threshold has been defined; subsequently, the country was divided into four zones, on the basis of the major river basins. The effectiveness of the thresholds has been verified by the use of several statistical parameters and it resulted in quite good performances, even if with some uncertainties, probably due to the quality of the available data. Besides the setting of a threshold system, usable for civil protection purposes at national scale, an additional outcome of this work was the possibility of applying, with good results, a methodology defined for another region, therefore testing its degree of exportability in different settings.     

Rainfall induced debris flows in pyroclastic deposits, Campania (southern Italy)

A combination of empirical and physically based hydrological models has been used to analyze historical data on rainfall and debris-flow occurrence in western Campania, to examine the correlation between rainfall and debris-flow events.

Rainfall data from major storms recorded in recent decades in western Campania were compiled, including daily series from several rain gauges located inside landslide areas, supplemented by hourly rainfall data from some of the principal storms.

A two-phase approach is proposed. During phase 1, soil moisture levels have been modelled as the hydrological balance between precipitation and evapotranspiration, on a daily scale, using the method of Thornthwaite [Geograph. Rev. 38 (1948) 55].

Phase 2 is related to the accumulation of surplus moisture from intense rainfall, leading to the development of positive pore pressures. These interactions take place on an hourly time scale by the “leaky barrel” (LB) model described by Wilson and Wiezoreck [Env. Eng. Geoscience, 1 (1995) 11]. In combination with hourly rainfall records, the LB model has been used to compare hydrological effects of different storms. The critical level of retained rain water has been fixed by the timing of debris-flow activity, related to recorded storm events.

New rainfall intensity–duration thresholds for debris-flow initiation in western Campania are proposed. These thresholds are related to individual rain gauge and assume a previously satisfied field capacity condition. The new thresholds are somewhat higher than those plotted by previous authors, but are thought to be more accurate and thus need less conservatism.     

Impact of climate change on landslides frequency: the Esino river basin case study (Central Italy)

Researchers have long attempted to determine the amount of rainfall needed to trigger slope failures, yet relatively little progress has been reported on the effects of climate change on landslide initiation. Indeed, some relationships between landslides and climate change have been highlighted, but sign and magnitude of this correlation remain uncertain and influenced by the spatial and temporal horizon considered. This work makes use of statistically adjusted high-resolution regional climate model simulations, to study the expected changes of landslides frequency in the eastern Esino river basin (Central Italy). Simulated rainfall was used in comparison with rainfall thresholds for landslide occurrence derived by two observation-based statistical models (1) the cumulative event rainfall–rainfall duration model, and (2) the Bayesian probabilistic model. Results show an overall increase in projected landslide occurrence over the twenty-first century. This is especially confirmed in the high-emission scenario representative concentration pathway 8.5, where according to the first model, the events above rainfall thresholds frequency shift from ~?0.025 to ~?0.05 in the mountainous sector of the study area. Moreover, Bayesian analysis revealed the possible occurrence of landslide-triggering rainfall with a magnitude never occurred over the historical period. Landslides frequency change signal presents also considerable seasonal patterns, with summer displaying the steepest positive trend coupled to the highest inter-model spread. The methodological chain here proposed aims at representing a flexible tool for future landslide-hazard assessment, applicable over different areas and time horizons (e.g., short-term climate projections or seasonal forecasts).     

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     

The new landslide inventory of Tuscany (Italy) updated with PS-InSAR: geomorphological features and landslide distribution

In this paper, the updating of the landslide inventory of Tuscany region is presented. To achieve this goal, satellite SAR data processed with persistent scatter interferometry (PSI) technique have been used. The updating leads to a consistent reduction of unclassified landslides and to an increasing of active landslides. After the updating, we explored the characteristics of the new inventory, analysing landslide distribution and geomorphological features. Several maps have been elaborated, as sliding index or landslide density map; we also propose a density-area map to highlight areas with different landslide densities and sizes. A frequency-area analysis has been performed, highlighting a classical negative power-law distribution. We also explored landslide frequency for lithology, soil use and several morphological attributes (elevation, slope gradient, slope curvature), considering both all landslides and classified landslide types (flows, falls and slides).     

The influence of intense rainfall on the activity of large-scale crystalline schist landslides in Shikoku Island, Japan

Many large landslides in the crystalline schist region of Shikoku Island, Japan, are susceptible to intense rainfall. Through the use of on-site monitoring systems, the activity of landslides and their meteorological triggers can be assessed. Continuous high-intensity rainfall was found to play a key role in provoking landslide movement. This paper investigates the influence of intense rainfall on the activity of crystalline schist landslides by examining rainfall and displacement of four typical landslides. By defining and calculating the effective rainfall and the relative landslide displacement, the relationship between intense rainfall and rainfall-induced landslide movement was analysed. Results indicate that the intense rainfall-induced landslide movement can be correlated with the effective rainfall. From these results, two rainfall thresholds were identified for the landslide risk management of Shikoku Island.     

Rainfall intensity-duration thresholds for landslide prediction in South Korea by considering the effects of antecedent rainfall

This study was conducted to determine the rainfall intensity-duration thresholds (ID curves) for landslide prediction by considering the effects of antecedent rainfall. Data for the time and location of landslides that occurred in South Korea from 1999 to 2016 were collected. Overall, 231 landslide histories from 1999 to 2013 were used to determine the rainfall thresholds, and 12 landslide histories from 2014 to 2016 were used to verify the proposed rainfall thresholds. Probabilistic ID curves were proposed to reflect the influence of other factors except rainfall, and ID curves for various inter-event time definitions (IETDs) were suggested to analyze the variation in the ID curves with the effects of antecedent rainfall. The results suggest that the IETD indicates the duration for which the antecedent rainfall affects the ground condition. It was also found that the ID curve for 12 h of the IETD was the most reliable of the verification procedures using the receiver operating characteristic (ROC) plot and threat score (TS).     

基于位移比模型的三峡库区云阳县域内蠕变型滑坡降雨预警

建立高效合理的区域滑坡灾害降雨预警模型对滑坡防治具有重要意义.然而以往的研究多侧重于临滑预警,对蠕变型滑坡在强降雨工况下的短暂加速变形的预警研究还有待深入.以三峡库区云阳县域内滑坡为例,首先根据滑坡地表位移监测数据的特点对统计样本进行合理筛选.再通过降雨因子与滑坡发生的相关性分析以及对滑坡在降雨条件下位移变化情况的数值模拟,确定了适用于不同时间阶段的降雨统计变量.然后将考虑了滑坡规模特征的滑坡位移比（累计位移与滑坡纵长之比）作为变形指标,分时段统计滑坡地表位移监测数据与历史降雨信息,建立了日降雨数据与月位移数据的对应关系,得到了可用于确定降雨量阈值的位移比模型,并获得了云阳县蠕变型滑坡的五级预警分区.最后分别选用研究区滑坡险情实例、长年位移监测数据及极端降雨事件对模型预警效果进行检验.结果显示基于专业监测数据的位移比模型的滑坡降雨预警结果与实际情况相符,可为蠕变型滑坡的预警预报提供依据.      

Understanding rainfall-landslide relationships in man-modified environments: a case-history from Caramanico Terme, Italy

 The expansion of Caramanico Terme in this century has led to the urbanization of marginally stable valley slopes, and this has coincided with the apparent acceleration of landslide processes. Recent landslides on man-modified slopes were caused, but not necessarily triggered, by heavy precipitation (antecedent moisture was a more critical factor than the amount of storm rainfall). Because no important landslides on natural slopes in the same period were reported in the Caramanico area, a clear distinction must be made between natural settings and those modified by man when determining rainfall thresholds for predictive purposes. In recently urbanized mountainous environments, the thresholds used to assess landslide hazards should not be weighted too heavily on old historical records of precipitation and associated mass movements. Instead, more weight ought to be given to the period following the occurrence of any major anthropogenic and natural (e.g. high-magnitude earthquake) modification of slope setting. Received: 19 October 1996 · Accepted: 25 June 1997     

Rainfall thresholds for the initiation of landslide at Lanta Khola in north Sikkim,India

In the Indian Himalaya, a 15 km stretch of the North Sikkim Highway that is exceptionally susceptible to landsliding is characterized by fine-grained, low permeability debris material. Lanta Khola is one of the major debris slides in this stretch and is active every year during the monsoons. Although the relationship between rainfall and landsliding in the area is obvious, there is no previous study of precipitation thresholds for landslide initiation. Review of available rainfall and landslide activity data for the area between 1998 and 2006 suggests that sliding cannot be modeled by typical exponential relationships between cumulative rainfall (E) and rainfall duration (D). An alternative rainfall threshold has been proposed that predicts sliding if normalized cumulative rainfall for more than 15 days exceeds 250 mm. It is suggested that when this cumulative rainfall threshold is exceeded, the debris zone in the affected stretch becomes saturated and fails, causing landsliding.     

Utilizing crowdsourcing to enhance the mitigation and management of landslides

Landslides are mainly triggered by earthquakes and rainfall and have poor temporal predictability. Landslides pose significant threats to settlements and infrastructure in mountainous regions around the world. To mitigate this natural hazard, a new paradigm of landslide mitigation and management is required. Increasing smartphone ownership around the world, especially in developing countries, offers scientists an opportunity to embrace crowdsourcing so as to improve landslide research. This paper presents a new landslide information system (LIS) comprising a smartphone app and an administrative interface and database. The mobile app has been published for both iPhone and Android platforms. The interface of the smartphone app is powered by the highly-customizable Google Maps platform, which is overlaid with real-time landslide data. Users can choose between visualizing “known sites” and “contribution” of landslide data. The visualization option shows published landslides and areas that are susceptible. Users can contribute their GPS coordinates and multimedia to enhance landslide reports. A comparison with similar systems, potential applications, and challenges of using smartphone technology for mitigating landslides are also discussed.     

Exploiting historical rainfall and landslide data in a spatial database for the derivation of critical rainfall thresholds

Critical rainfall thresholds for landslides are powerful tools for preventing landslide hazard. The thresholds are commonly estimated empirically starting from rainfall events that triggered landslides in the past. The creation of the appropriate rainfall–landslide database is one of the main efforts in this approach. In fact, an accurate agreement between the landslide and rainfall information, in terms of location and timing, is essential in order to correctly estimate the rainfall–landslide relationships. A further issue is taking into account the average moisture conditions prior the triggering event, which reasonably may be crucial in determining the sufficient amount of precipitation. In this context, the aim of this paper is exploiting historical landslide and rainfall data in a spatial database for the derivation of critical rainfall thresholds for landslide occurrence in Sicily, southern Italy. The hourly rainfall events that caused landslides occurred in the twentieth century were specifically identified and reconstructed. A procedure was proposed to automatically convert rain guages charts recorded on paper tape into digital format and then to provide the cumulative rainfall hyetograph in digital format. This procedure is based on a segmentation followed by signal recognition techniques which allow to digitalize and to recognize the hyetograph automatically. The role of rainfall prior to the landslide events was taken into account by including in the analysis the rainfall occurred 5, 15 and 30 days before each landslide. Finally, cumulated rainfall duration thresholds for different exceedance probability levels were determined. The obtained thresholds resulted in agreement with the regional curves proposed by other authors for the same area; antecedent rainfall turned out to be particularly important in triggering landslides.     

降雨诱发滑坡阶跃型变形的预测分析及应用

滑坡进入蠕动变形阶段之后,往往难以及时开展勘察治理工作,合理的临灾预警是有效减少滑坡灾害损失的重要手段。首先确定降雨诱发“阶跃型”滑坡的预警关键判据为前期降雨、当次降雨、位移速率,并引入“一个降雨过程”定义滑坡监测的降雨区间,将预警过程分为当次降雨和前期降雨+当次降雨两种模式。然后运用最小二乘法确定滑坡“阶跃”变形曲线上的“破坏拐点”和“稳定拐点”用以确定变形加速区间,以此求解前期降雨、当次降雨以及移速率阈值。最后以王家坡滑坡为例,设计了两种模式下的5级递进式分级预警模型。研究表明：（1）前期降雨与当次降雨组成“一个降雨过程”的时间间隔为7 d;（2）王家坡滑坡的位移速率阈值为20 mm/d;（3）前期降雨+当次降雨模式下王家坡滑坡的前期降雨、当次降雨阈值分别为10、15 mm,当次降雨模式下王家坡滑坡的降雨阈值为25 mm。     

Analysis of the relationship of landslide occurrence with rainfall: a case study of Wudu County,China

Landslides are a significant hazard in many parts of the world and represent an important geohazard in China. Rainfall is the primary triggering agent for landslides and often used for prediction slope failures. However, the relationship between rainfall and landslide occurrences is very complex. Great efforts have been made on the study of regional rainfall-induced landslide forecasting models in recent years; still, there is no commonly accepted method for rainfall-induced landslide prediction. In this paper, the quantitative antecedent soil water status (ASWS) model is applied to investigate the influence of daily and antecedent rainfall on the triggering of landslides and debris flows. The study area is Wudu County in Gansu Province, an area which exhibits frequent landslide occurrences. The results demonstrate a significant influence of high intensity rainfall events on landslide triggering. Still, antecedent rainfall conditions are very important and once a threshold of approximately 20 mm is exceeded, landslides and debris flows can occur even without additional rainfall. The study presented could also facilitate the implementation of a regional forecasting scheme once additional validation has been carried out.     

The rainfall intensity–duration control of shallow landslides and debris flows: an update

A global database of 2,626 rainfall events that have resulted in shallow landslides and debris flows was compiled through a thorough literature search. The rainfall and landslide information was used to update the dependency of the minimum level of rainfall duration and intensity likely to result in shallow landslides and debris flows established by Nel Caine in 1980. The rainfall intensity–duration (ID) values were plotted in logarithmic coordinates, and it was established that with increased rainfall duration, the minimum average intensity likely to trigger shallow slope failures decreases linearly, in the range of durations from 10 min to 35 days. The minimum ID for the possible initiation of shallow landslides and debris flows was determined. The threshold curve was obtained from the rainfall data using an objective statistical technique. To cope with differences in the intensity and duration of rainfall likely to result in shallow slope failures in different climatic regions, the rainfall information was normalized to the mean annual precipitation and the rainy-day normal. Climate information was obtained from the global climate dataset compiled by the Climate Research Unit of the East Anglia University. The obtained global ID thresholds are significantly lower than the threshold proposed by Caine (Geogr Ann A 62:23–27, 1980), and lower than other global thresholds proposed in the literature. The new global ID thresholds can be used in a worldwide operational landslide warning system based on global precipitation measurements where local and regional thresholds are not available..     

Comparison of statistical methods and multi-time validation for the determination of the shallow landslide rainfall thresholds

Shallow landslides are unforeseeable phenomena often resulting in critical conditions in terms of people’s safety and damage. The main purpose of this paper is the comparison of different statistical methods used to determine the rainfall thresholds for the shallow landslide occurrence. Rainfall data over a 46-year period were collected for one rain gauge located in a test area of northwest Italy (Riviera Spezzina; RS). In the RS, intense rainfalls often induce shallow landslides causing damage and sometimes casualties. The rainfall events occurred in the 1967–2006 period were classified as events inducing shallow landslides (SLEs_1967–2006) and events that did not trigger shallow landslides (NSLEs_1967–2006). Thresholds for various percentiles of SLEs_1967–2006 were computed by identifying the lower limit above which shallow landslides occurred. Another set of thresholds, corresponding to different probabilities of occurrence, was determined using SLEs_1967–2006 and NSLEs_1967–2006. The least-squares linear fit (LSF) and the quantile regression (QR) techniques were employed in the former approach, while the logistic regression (LR) was applied in the latter. The thresholds were validated with the same data used for their definition and with the data recorded in the 2008–2014 period. Contingency tables were created and contingencies and skill scores were computed. The 10% probability threshold obtained using the LR method is characterized by the best values of at least two skill scores for both periods considered; therefore, it may be considered the “best” threshold for the RS. The results of this work can help the choice of the best statistical method to determine the shallow landslide rainfall thresholds.     

Assessment of rainfall thresholds and soil moisture modeling for operational hydrogeological risk prevention in the Umbria region (central Italy)

Rainfall thresholds represent the main tool for the Italian Civil Protection System for early warning of the threat of landslides. However, it is well-known that soil moisture conditions at the onset of a storm event also play a critical role in triggering slope failures, especially in the case of shallow landslides. This study attempts to define soil moisture (estimated by using a soil water balance model) and rainfall thresholds that can be employed for hydrogeological risk prevention by the Civil Protection Decentrate Functional Centre (CFD) located in the Umbria Region (central Italy). Two different analyses were carried out by determining rainfall and soil moisture conditions prior to widespread landslide events that occurred in the Umbria Region and that are reported in the AVI (Italian Vulnerable Areas) inventory for the period 1991?C2001. Specifically, a ??local?? analysis that considered the major landslide events of the AVI inventory and an ??areal?? analysis subdividing the Umbria Region in ten sub-areas were carried out. Comparison with rainfall thresholds used by the Umbria Region CFD was also carried out to evaluate the reliability of the current procedures employed for landslide warning. The main result of the analysis is the quantification of the decreasing linear trend between the maximum cumulated rainfall values over 24, 36 and 48?h and the soil moisture conditions prior to landslide events. This trend provides a guideline to dynamically adjust the operational rainfall thresholds used for warning. Moreover, the areal analysis, which was aimed to test the operational use of the combined soil moisture?Crainfall thresholds showed, particularly for low values of rainfall, the key role of soil moisture conditions for the triggering of landslides. On the basis of these results, the Umbria Region CFD is implementing a procedure aimed to the near real-time estimation of soil moisture conditions based on the soil water balance model developed ad hoc for the region. In fact, it was evident that a better assessment of the initial soil moisture conditions would support and improve the hydrogeological risk assessment.     

Spatially distributed rainfall information and its potential for regional landslide early warning systems

Crucial to most landslide early warning system (EWS) is the precise prediction of rainfall in space and time. Researchers are aware of the importance of the spatial variability of rainfall in landslide studies. Commonly, however, it is neglected by implementing simplified approaches (e.g. representative rain gauges for an entire area). With spatially differentiated rainfall information, real-time comparison with rainfall thresholds or the implementation in process-based approaches might form the basis for improved landslide warnings. This study suggests an automated workflow from the hourly, web-based collection of rain gauge data to the generation of spatially differentiated rainfall predictions based on deterministic and geostatistical methods. With kriging usually being a labour-intensive, manual task, a simplified variogram modelling routine was applied for the automated processing of up-to-date point information data. Validation showed quite satisfactory results, yet it also revealed the drawbacks that are associated with univariate geostatistical interpolation techniques which solely rely on rain gauges (e.g. smoothing of data, difficulties in resolving small-scale, highly intermittent rainfall). In the perspective, the potential use of citizen scientific data is highlighted for the improvement of studies on landslide EWS.     

滑坡监测预警国内外研究现状及评述

本文从降雨临界值研究、监测技术方法、区域性监测预警系统三个方面对滑坡监测预警的国内外研究现状进行了回顾和总结。首先归纳了国内外28个国家或地区的滑坡降雨临界值及统计方法,三个模型——日降雨量模型、前期降雨量模型和前期土体含水状态模型,基本概括了当前降雨诱发滑坡临界值的确定方法;但由于降雨入渗触发滑坡的复杂性,不同机理的滑坡"需要"不同的降雨临界值;目前的研究趋势是对雨量雨强雨时—土体渗流场动态变化—土体抗剪强度变化的耦合关系进行研究。按监测对象的不同,滑坡监测可分为四大类,即位移监测、物理场监测、地下水监测和外部诱发因素监测;按监测手段的不同,则可分为人工监测、简易监测、专业监测三大类;目前国内外在滑坡监测技术、方法、手段上并无太大差距,专业仪器已成为常规设备,只是由于价格因素得不到普及;一些新技术如InSAR、三维激光扫描等能很快应用到滑坡监测领域;监测数据的采集和传输也都实现了自动化和远程化;监测和预警系统有向Web—GIS发展的趋势。利用一个地区的滑坡易发区划或危险区划,结合降雨临界值,可以设定不同的预警级别,在区内布设一定数量的雨量站,监测雨量加上预报雨量,就可进行滑坡预警预报,国内外的区域性降雨型滑坡监测预警大体都是这个思路和做法,该方法在对公众进行警示方面起到了良好效果,但由于预警的范围太大,在具体的单点防治上,难以做到有效。我国在近10年开展了大量的监测预警工作,并取得了丰硕的成果,但根据统计数据,其成功预警率却并不理想,这一方面表现在成功预警实例中专业预警所占比例过低,另一方面同时表现在发生的大量的地质灾害在已有的预警点之外。制约目前工作有效性的主要问题是滑坡隐患点的排查和识别问题,因为只有识别出了隐患点才能进行下一步的监测和预警,它是一切工作的基础。而解决这一问题的重要途径是分析区域上的滑坡发育规律,找到有效的隐患点识别技术方法,以及引进风险管理的概念,进行监测资源的合理分配和有效预警。