Spatially distributed rainfall thresholds for the initiation of shallow landslides

The evaluation of the combined influence of rainfall patterns (in terms of mean intensity and duration) and the geomorphological and mechanical characteristics of hillslopes on their stability conditions is a major goal in the assessment of the shallow landslide triggering processes. Geographic Information Systems (GIS) represent an important tool to develop models that combine hydrological and geomechanical analyses for the evaluation of slope stability, as they allow to combine information concerning rainfall characteristics with topographic and mechanical properties of the slopes over wide areas. In this paper, a GIS-based code is developed to determine physically based intensity/duration rainfall thresholds at the local scale. Given the rainfall duration and the local geometric, hydrological and mechanical characteristics of the slopes, the code evaluates the spatial distribution of the minimum rainfall intensity that triggers shallow landslides and debris flows over a given area. The key feature of the code is the capability of evaluating the time t _p required to reach the peak pore pressure head on the failure surface and computing the corresponding critical intensity/duration thresholds based on post-event peak pore pressures. The reliability of the model is tested using a set of one-dimensional analyses, comparing the physically based thresholds obtained for three different slopes with some empirical rainfall thresholds. In a log–log scale, the thresholds provided by the model decrease linearly with increased rainfall duration and they are bracketed by the empirical thresholds considered. Finally, an example of application to a study area of the Umbria region in central Italy is presented, describing the capability of the model of providing site-specific thresholds for different rainfall scenarios.     

Estimation of rainfall thresholds triggering shallow landslides for an operational warning system implementation

The Piemonte regional warning system service, managed by the Environmental Protection Agency of Piemonte (“ARPA Piemonte” as official Italian acronym), is based on an advanced meteo-hydrological automatic monitoring system, and it is integrated with forecasting activities of severe weather-related natural hazards. At present, a meteo-hydrological chain is operated to provide flood forecasting on the main river pattern. The development of a forecasting tool for shallow landslides triggered by heavy rainfall is presented. Due to the difficulties in modelling shallow landslides triggering in a large and complex area like the Piemonte region, an empirical model is developed on the basis of the correlation between rainfall and previous landslides in historical documents. The research focuses on establishing rainfall thresholds for landslides triggering, differentiating the critical rainfall values through a geological characterisation of the different territories. The period from 1990 to 2002 is considered. A total number of 160 landslides with hourly information and time of triggering are used to calibrate the system. As a first outcome, two different zones have been identified: (1) zones in alpine environments, principally characterised by a bedrock composed of metamorphic rocks, igneous rocks, dolostones or limestones that require high values of critical rainfall and (2) zones in hilly environments, principally characterised by sedimentary bedrock that require low values of critical rainfall. Verification has been performed on a total number of 429 landslides with known date of occurrence. The results show a good agreement with the model with no missed alarms and a very low number of false alarms, thus suggesting an effective operational implementation.     

Hydrological regimes in different slope environments and implications on rainfall thresholds triggering shallow landslides

Natural Hazards - Assessing hazard of rainfall-induced shallow landslides represents a challenge for the risk management of urbanized areas for which the setting up of early warning systems, based...     

Updating EWS rainfall thresholds for the triggering of landslides

In this paper, the updating of rainfall thresholds for landslide early warning systems (EWSs) is presented. Rainfall thresholds are widely used in regional-scale landslide EWSs, but the efficiency of those systems can decrease during the time, so a periodically updating should be required to keep their functionality. The updating of 12 of the 25 thresholds used in the EWS of Tuscany region (central Italy) is presented, and a comparison between performances of new and previous thresholds has been made to highlight the need of their periodical update. The updating has been carried out by collecting ca. 1200 new landslide reports (from 2010 to March 2013) and their respective rainfall data, collected by 332 rain gauges. The comparison has been made by the use of several statistical indexes and showed a marked increasing in the performances of the new thresholds with respect to previous ones.     

A probabilistic model for evaluating the reliability of rainfall thresholds for shallow landslides based on uncertainties in rainfall characteristics and soil properties

Natural Hazards - This study aims to develop a probabilistic rainfall threshold estimation model for shallow landslides (PRTE_LS) in order to quantify its reliability while being affected by...     

Total water content thresholds for shallow landslides,Outer Western Carpathians

Shallow landslides are fairly frequent natural processes which emerge as a result of both rainfall and rapid snowmelt in the Flysch Belt of the Outer Western Carpathians. We estimated the total water content thresholds for the previously defined seven phases of increased landsliding which took place between 1939 and 2010 around the Napajedla meteorological station. The time series were reconstructed on the basis of data from surrounding stations. Rainfalls with the highest intensities (>1 mm/min) were removed from the set. Rainfall of such an intensity primarily causes overland flow and soil erosion and does not contribute to landslide threshold. The snow water equivalent was computed on the basis of the snow height, and possible errors were evaluated as interval estimations. An interval of 10 days before a landslide phase was selected for the total water content threshold. The resulting lower boundary (67.0 mm/10 days) and upper boundary (163.3 mm/10 days) thresholds of water infiltrated into soil during an event shall be part of the prepared online warning system in this area.     

Early warning system for shallow landslides using rainfall threshold and slope stability analysis

A combined cluster and regression analysis were performed for the first time to identify rainfall threshold that triggers landslide events in Amboori, Kerala, India. Amboori is a tropical area that is highly vulnerable to landslides. The 2, 3, and 5-day antecedent rainfall data versus daily rainfall was clustered to identify a cluster of critical events that could potentially trigger landslides. Further, the cluster of critical events was utilized for regression analysis to develop the threshold equations. The 5-day antecedent (x-variable) vs. daily rainfall (y-variable) provided the best fit to the data with a threshold equation of y = 80.7–0.1981x. The intercept of the equation indicates that if the 5-day antecedent rainfall is zero, the minimum daily rainfall needed to trigger the landslide in the Amboori region would be 80.7 mm. The negative coefficient of the antecedent rainfall indicates that when the cumulative antecedent rainfall increases, the amount of daily rainfall required to trigger monsoon landslide decreases. The coefficient value indicates that the contribution of the 5-day antecedent rainfall is ∼20% to the landslide trigger threshold. The slope stability analysis carried out for the area, using Probabilistic Infinite Slope Analysis Model (PISA-m), was utilized to identify the areas vulnerable to landslide in the region. The locations in the area where past landslides have occurred demonstrate lower Factors of Safety (FS) in the slope stability analysis. Thus, rainfall threshold analysis together with the FS values from slope stability can be suitable for developing a simple, cost-effective, and comprehensive early-warning system for shallow landslides in Amboori and similar regions.     

Effects of degree of saturation on shallow landslides triggered by rainfall

The empirical rainfall threshold concept and the physical-based model are two commonly used approaches for the assessment of shallow landslides triggered by rainfall. To investigate in detail the rainfall-triggered shallow landslides, many physical-based models coupling the infinite slope stability analysis with the rainfall infiltration modeling in variably saturated soil were developed. However, in those physical-based shallow landslide models, the unit weight and the unsaturated shear strength were assumed constant rather than depending on the degree of saturation. In this study, the effects of the unit weight and the unsaturated shear strength as function of degree of saturation on rainfall-triggered shallow landslides are examined. Several designed scenarios and a real case scenario are used to conduct the examinations. The results show that not only the occurrence of shallow landslides but also the failure depth and the time to failure could be misassessed if the influences of degree of saturation on the unit weight and the unsaturated shear strength are neglected.     

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.     

浅层滑坡诱发沟谷泥石流的地形和降雨条件

2011年贵州省望谟县打易镇的大范围浅层滑坡诱发的沟谷泥石流提供了研究这类泥石流地形和降雨条件的机会。在地质条件一致和小区域内的降雨条件基本一致的情况下,地形条件就是这些泥石流暴发与否的唯一决定因素。对比一些重要的地形因素与泥石流暴发的关系,得出了由流域面积、沟床纵比降和25°~45°山坡坡度面积比组成的泥石流综合地形因子T。在地形因子T的基础上,研究获得了由前期降雨量、1 h降雨强度、年平均降雨量等组成的降雨因子R。由地形因子T和降雨因子R获得的临界条件P可以判断该区域的泥石流暴发。由于研究工作部分基于泥石流的形成机理,研究成果还可用于其他区域的泥石流形成预测,为泥石流的预测预报提供了一个较好的方法。     

Susceptibility assessment of shallow landslides triggered by rainfall in tropical basins and mountainous terrains

Natural Hazards - This study presents the analyses and results of the implementation of the SHALSTAB model to study landslides caused by rainfall that occurred on September 21, 1990, in a basin of...     

台风-非台风降雨型滑坡的多时段临界雨量值预测模型

统计确定临界降雨量是滑坡早期预警常用的方法.东南沿海地区台风暴雨不同于一般降雨,常引发滑坡灾害,从而威胁沿海地区人民生命财产安全.为了建立台风和非台风降雨型滑坡临界降雨量预测模型,以浙江丽水市为例,基于2010-2020年台风暴雨、非台风降雨诱发滑坡与降雨量的统计,构建了丽水市滑坡发生概率和有效降雨量的关系,提出了多时...     

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.     

Examination of influences of rainfall patterns on shallow landslides due to dissipation of matric suction

The influences of rainfall patterns on shallow landslides due to the dissipation of matric suction are examined in this study. Four representative rainfall patterns including the uniform, advanced, intermediated, and delayed rainfalls are adopted. The results show that not only the occurrence of shallow landslides but also the failure depth and the time of failure are affected by the rainfall pattern. The different rainfall patterns seem to have the same minimum landslide-triggering rainfall amount. There is a rainfall duration threshold for landslide occurrence for a rainfall event with larger than the minimum landslide-triggering rainfall amount. For each rainfall pattern, the rainfall duration threshold for landslide occurrence decreases to constant with the increase of rainfall amount. The uniform rainfall has the least rainfall duration threshold for landslide occurrence, followed by the advanced rainfall, and then the intermediated rainfall. For each rainfall pattern, the failure depths and the times of failure from the same amount of rainfall with different durations could be largely different. In addition, the differences of the failure depths and the times of failure between various rainfall patterns with the same amount and duration of rainfall could be also significant. The failure depth and the time of failure, as compared with the occurrence of shallow landslides, are more sensitive to the rainfall condition. In other words, in comparison with the evaluation of the occurrence of shallow landslides, it needs more accurate rainfall prediction to achieve reliable estimations of the failure depth and the time of failure.     

Soil water balance model for precipitation-induced shallow landslides

Precipitation infiltration is one of the most significant triggering factors for slope failure occurrence in many places around the world. Knowledge of the mechanisms leading to precipitation-induced slope failures is of great importance to the management of landslide hazard. In this study, a soil water balance model is developed to estimate soil water flux during the process of infiltration from rainfall data, with consideration of storm periods and non-storm periods. Two important assumptions in this study are given: (1) instantaneous uniform distribution of the degree of saturation and (2) a linear relationship between evapotranspiration and the related degree of saturation. For storm periods, the Brooks and Corey model estimates both the soil water retention curve and soil water parameters. The infiltration partition is employed by an infinite-series solution of Philip in conjunction with the time compression approximation. For none-storm periods, evapotranspiration can be derived for the moisture depletion of soil water. This study presents a procedure for calculating the safety factor for an unsaturated slope suffering from precipitation infiltration. The process of infiltration into a slope due to rainfall and its effect on soil slope behavior are examined using modified Mohr–Coulomb failure criterion in conjunction with a soil water balance model. The results indicate that the matric suction, which is closely related to slope stability, is affected by the degree of saturation controlled by rainfall events.     

Initiation conditions of shallow landslides in two man-made forests and back estimation of the possible rainfall threshold

Rainfall-induced shallow landslides and debris flows are one of the most common erosion process and primary channel initiation mechanisms in many steep landscapes. Their initiation conditions are physically controlled by the soil properties and topographically influenced by the competition between area- (A) and slope-dependent (S) sediment transport process. In this work, the A-S relationship of landslides in two forests was compared with respect to the physical properties of soil and plant roots. The results reveal that landslides in the Pinus tabuleaformis forest commonly have larger surface- and contributing area, deeper failure plane and gentler slope gradient than those in the Larix Kaemphferi forest. The saturated hydraulic conductivity in the Pinus tabuleaformis forest is higher and strongly correlates to plant root biomass. The effective cohesion and inner frictional angle of soil mass in the two forests are similar. Faster saturated hydraulic conductivity may lead to the higher upslope contributing area of landslides in the the Pinus tabuleaformis forest. A combination of finite-slope model and precipitation interception model reveals that landslides in the Pinus tabuleaformis forest require higher rainfall amount that those in the Larix Kaemphferi forest. Last but not least, this work provides a clue that strong root network and high saturated hydraulic conductivity may promote the A-S condition.

     

Real-time estimation of hazard for landslides triggered by rainfall

 Landslide movements triggered by rainfall can be foreseen in real-time by modelling the relationship between rainfall amount and landslide occurrence. This paper deals with the problem of the reliability of the FLaIR (Forecasting of Landslides Induced by Rainfalls) model when applied to forecasting landslide movements in the usual condition of poor historical information availability. In this case, the identification of the admissibility field for the model parameters, instead of a point estimation, leads to an improvement of the forecasting reliability. Moreover, this approach makes the model capable of taking into account information embodied in periods of heavy rain but without movement. The concepts of informative content and foreseeability of landslide movements are introduced and their duality is analyzed. The effectiveness of the estimation procedure described has been tested by application on two landslides located in southern Italy. Received: 15 October 1996 · Accepted: 25 June 1997