Academic literature on the topic 'Rainfall-triggered shallow landslides'

Abstract. Hazard assessment of shallow landslides represents an important aspect of land management in mountainous areas. Among all the methods proposed in the literature, physically based methods are the only ones that explicitly includes the dynamic factors that control landslide triggering (rainfall pattern, land-use). For this reason, they allow forecasting both the temporal and the spatial distribution of shallow landslides. Physically based methods for shallow landslides are based on the coupling of the infinite slope stability analysis with hydrological models. Three different grid-based distributed hydrological models are presented in this paper: a steady state model, a transient "piston-flow" wetting front model, and a transient diffusive model. A comparative test of these models was performed to simulate landslide occurred during a rainfall event (27–28 June 1997) that triggered hundreds of shallow landslides within Lecco province (central Southern Alps, Italy). In order to test the potential for a completely distributed model for rainfall-triggered landslides, radar detected rainfall intensity has been used. A new procedure for quantitative evaluation of distributed model performance is presented and used in this paper. The diffusive model results in the best model for the simulation of shallow landslide triggering after a rainfall event like the one that we have analysed. Finally, radar data available for the June 1997 event permitted greatly improving the simulation. In particular, radar data allowed to explain the non-uniform distribution of landslides within the study area.

Earthquakes, rainfall, or a combination of both can trigger landslides, which can be classified into shallow and deep-seated types according to scale. Landslide risk potential can be charted according to the spatiotemporal characteristics of a combination of triggering factors that can be collated for similar historical events by various methods. The geographic information system (GIS) and the instability index method are two approaches commonly used to perform such a task; however, the nature of the event and the quality of imported data affect the degree of bias of model predictions against real-time values. To identify the differences between shallow and deep-seated landslides, 324 cases of landslides triggered by single rainfall events in Taiwan are analyzed in this study. It is determined that the principal factor governing shallow failure for rainfall-induced landslides is slope and that deep-seated failure is controlled by the amount of accumulated rainfall. By arranging the weighting, these factors could predict 93% and 75% of the occurrences of shallow and deep-seated landslides, respectively, based on a pre-event digital terrain model.

Abstract. In recent decades, the Entella River basin, in the Liguria Apennines, northern Italy, was hit by numerous intense rainfall events that triggered shallow landslides and earth flows, causing casualties and extensive damage. We analyzed landslide information obtained from different sources and rainfall data recorded in the period 2002–2016 by rain gauges scattered throughout the catchment, to identify the event rainfall duration, D (in h), and rainfall intensity, I (in mm h−1), that presumably caused the landslide events. Rainfall-induced landslides affected the whole catchment area, but were most frequent and abundant in the central part, where the three most severe events hit on 23–24 November 2002, 21–22 October 2013 and 10–11 November 2014. Examining the timing and location of the slope failures, we found that the rainfall-induced landslides occurred primarily at the same time or within 6 h from the maximum peak rainfall intensity, and at or near the geographical location where the rainfall intensity was largest. Failures involved mainly forested and natural surfaces, and secondarily cultivated and terraced slopes, with different levels of maintenance. Man-made structures frequently characterize the landslide source areas. Adopting a frequentist approach, we define the event rainfall intensity–event duration (ID) threshold for the possible initiation of shallow landslides and hyper-concentrated flows in the Entella River basin. The threshold is lower than most of the curves proposed in the literature for similar mountain catchments, local areas and single regions in Italy. The result suggests a high susceptibility to rainfall-induced shallow landslides of the Entella catchment due to its high-relief topography, geological and geomorphological settings, meteorological and rainfall conditions, and human interference. Analysis of the antecedent rainfall conditions for different periods, from 3 to 15 days, revealed that the antecedent rainfall did not play a significant role in the initiation of landslides in the Entella catchment. We expect that our findings will be useful in regional to local landslides early warning systems, and for land planning aimed at reducing landslide risk in the study area.

The Niigata Ken Chuetsu earthquake triggered a vast number of landslides in the epicentral region. Landslide concentrations were among the highest ever measured after an earthquake, and most of the triggered landslides were relatively shallow failures parallel to the steep slope faces. The dense concentration of landslides can be attributed to steep local topography in relatively weak geologic units, adverse hydrologic conditions caused by significant antecedent rainfall, and very strong shaking. Many of the landslides could be discerned from high-resolution satellite imagery acquired immediately after the earthquake.

Abstract. Over the last 40 years, many contributions have identified empirical rainfall thresholds (e.g. rainfall intensity (I) vs. rainfall duration (D), cumulated rainfall vs. rainfall duration (ED), cumulated rainfall vs. rainfall intensity (EI)) for the possible initiation of shallow landslides, based on local and global inventories. Although different methods to trace the threshold curves have been proposed and discussed in literature, a systematic study to develop an automated procedure to select the rainfall event responsible for the landslide occurrence has only rarely been addressed. Objective criteria for estimating the rainfall responsible for the landslide occurrence play a prominent role on the threshold values. In this paper, two criteria for the identification of the effective rainfall events are presented. The first criterion is based on the analysis of the time series of rainfall mean intensity values over 1 month preceding the landslide occurrence. The second criterion is based on the analysis of the trend in the time function of the cumulated mean intensity series calculated from the rainfall records measured through rain gauges. The two criteria have been implemented in an automated procedure that is written in the R language. A sample of 100 shallow landslides collected in Italy from 2002 to 2012 was used to calibrate the procedure. The cumulated event rainfall (E) and duration (D) of rainfall events that triggered the documented landslides are calculated through the new procedure and are fitted with power law in the D, E diagram. The results are discussed by comparing the D, E pairs calculated by the automated procedure and the ones by the expert method.

A new methodology for shallow landslide forecasting in wildfire burned areas is proposed by estimating the annual probability of rainfall threshold exceedance. For this purpose, extensive geological fieldwork was carried out in 122 landslides, which have been periodically activated in Western Greece, after the devastating wildfires that occurred in August 2007 and burned large areas in several parts of Western Greece. In addition, daily rainfall data covering more than 40 years has been collected and statistically processed to estimate the exceedance probability of the rainfall threshold above which these landslides are activated. The objectives of this study are to quantify the magnitude and duration of rainfall above which landslides in burned areas are activated, as well as to introduce a novel methodology on rainfall-induced landslide forecasting. It has been concluded that rainfall-induced landslide annual exceedance probability in the burned areas is higher when cumulative rainfall duration ranges from 6 to 9 days with local differences due to the prevailing geological conditions and landscape characteristics. The proposed methodology can be used as a basis for landslide forecasting in wildfire-affected areas, especially when triggered by rainfall, and can be further developed as a tool for preliminary landslide hazard assessment.

Rainfall-triggered shallow landslides represent a major threat to people and infrastructure worldwide. Predicting the possibility of a landslide occurrence accurately means understanding the trigger mechanisms adequately. Rainfall is the main cause of slope failures in Slovenia, and rainfall thresholds are among the most-used tools to predict the possible occurrence of rainfall-triggered landslides. The recent validation of the prototype landslide early system in Slovenia highlighted the need to define new reliable rainfall thresholds. In this study, several empirical thresholds are determined using an automatic tool. The thresholds are represented by a power law curve that links the cumulated event rainfall (E, in mm) with the duration of the rainfall event (D, in h). By eliminating all subjective criteria thanks to the automated calculation, thresholds at diverse non-exceedance probabilities are defined and validated, and the uncertainties associated with their parameters are estimated. Additional thresholds are also calculated for two different environmental classifications. The first classification is based on mean annual rainfall (MAR) with the national territory divided into three classes. The area with the highest MAR has the highest thresholds, which indicates a likely adaptation of the landscape to higher amounts of rainfall. The second classification is based on four lithological units. Two-thirds of the considered landslides occur in the unit of any type of clastic sedimentary rocks, which proves an influence of the lithology on the occurrence of shallow landslides. Sedimentary rocks that are prone to weathering have the lowest thresholds, while magmatic and metamorphic rocks have the highest thresholds. Thresholds obtained for both classifications are far less reliable due to the low number of empirical points and can only be used as indicators of rainfall conditions for each of the classes. Finally, the new national thresholds for Slovenia are also compared with other regional, national, and global thresholds. The thresholds can be used to define probabilistic schemes aiming at the operative prediction of rainfall-induced shallow landslides in Slovenia, in the framework of the Slovenian prototype early warning system.

Abstract. Over the last 40 years, many contributions have been devoted to identifying the empirical rainfall thresholds (e.g. intensity vs. duration ID, cumulated rainfall vs. duration ED, cumulated rainfall vs. intensity EI) for the initiation of shallow landslides, based on local as well as worldwide inventories. Although different methods to trace the threshold curves have been proposed and discussed in literature, a systematic study to develop an automated procedure to select the rainfall event responsible for the landslide occurrence has rarely been addressed. Nonetheless, objective criteria for estimating the rainfall responsible for the landslide occurrence (effective rainfall) play a prominent role on the threshold values. In this paper, two criteria for the identification of the effective rainfall events are presented: (1) the first is based on the analysis of the time series of rainfall mean intensity values over one month preceding the landslide occurrence, and (2) the second on the analysis of the trend in the time function of the cumulated mean intensity series calculated from the rainfall records measured through rain gauges. The two criteria have been implemented in an automated procedure written in R language. A sample of 100 shallow landslides collected in Italy by the CNR-IRPI research group from 2002 to 2012 has been used to calibrate the proposed procedure. The cumulated rainfall E and duration D of rainfall events that triggered the documented landslides are calculated through the new procedure and are fitted with power law in the (D,E) diagram. The results are discussed by comparing the (D,E) pairs calculated by the automated procedure and the ones by the expert method.

Abstract. The Apuan Alps region is one of the rainiest areas in Italy (more than 3000 mm/year), in which frequently heavy and concentrated rainfall occurs. This is particularly due to its geographical position and conformation: the Apuan chain is located along the northern Tuscan coast, close to the Ligurian Sea, and the main peaks reach almost 2000 m. In several cases, the storms that hit the area have triggered many shallow landslides (soil slip-debris flows), which exposed the population to serious risks (during the 19 June 1996 rainstorm about 1000 landslides were triggered and 14 people died). The assessment of the rainfall thresholds is very important in order to prepare efficient alarm systems in a region particularly dedicated to tourism and marble activities. With the aim of contributing to the landslide hazard evaluation of the southern Apuan Alps territory (upper Versilia area), a detailed analysis of the main pluviometric events was carried out. The data recorded at the main rain gauge of the area from 1975 to 2002 were analysed and compared with the occurrence of soil slips, in order to examine the relationship between soil slip initiation and rainfall. The most important rainstorms which triggered shallow landslides occurred in 1984, 1992, 1994, 1996, 1998 and 2000. Many attempts were made to obtain a possible correlation between rainfall parameters and the occurrence of soil slip phenomena and to identify the local rainfall threshold for triggering shallow landslides. A threshold for soil slip activity in terms of mean intensity, duration and mean annual precipitation (MAP) was defined for the study area. The thresholds obtained for the southern Apuan Alps were also compared with those proposed by other authors for several regions in the world. This emphasized the high value of the rain threshold for shallow landslide activity in the Apuan area. The high threshold is probably also linked to the high mean annual precipitation and to the high frequency of rainstorms.

The hydrographic basin of Ribeira Grande (S. Miguel Island, Azores) has a set of characteristics that enhance the occurrence of shallow slides that have been triggered by rainfall and earthquakes. Two landslide inventories were built according to the landslide triggers: Landslide Inventory 2 (LI 2), which includes 174 earthquake-triggered shallow slides occurred in 2005; and Landslide Inventory 1 (LI 1), which includes 442 shallow slides triggered by rainfall in several periods from 2005 to 2016. Both landslide inventories were characterized and compared from the morphometric point of view and were used individually to produce susceptibility models to failure using a simple bivariate state-of-the-art statistical method (the Information Value). The landslide susceptibility Models were validated using success rates, prediction rates, and Kappa statistics. The results show that shallow slides triggered by rainfall and earthquakes in the study area have different morphometric characteristics. It was verified that models produced with LI 1 are very effective in predicting the spatial location of LI 2, but the same does not happen in the inverse situation. Finally, landslide susceptibility models developed with LI 1 and LI 2 for the upper sector of the hydrographic basin (where most landslides occurred), and latter applied to the complete watershed, present more modest predictive results but are more reliable to characterize the landslide susceptibility in the study area.

Shallow landslides studies are usually extended over landscape scale, where the investigations about geotechnical and hydrological properties of the soil are limited to some local points and not sufficient to assure an in-depth explanation of failure trigger. The physics of the phenomenon is thus minimized, and resolution in space and time is maximized. Such as approach can be useful to predict landslide occurrences for emergency purposes, but it is not effective to interpret the real triggering landslide mechanism. A local scale analysis become needed to achieve an understanding of the processes leading to the failure. Specifically, a full comprehension requires to provide experimental data from a carefully monitored and controlled landslide field site. The present study focuses on a large-scale device aimed at simulating shallow landslides triggered by heavy intensity rainfall. The physical model consists of an artificial hillslope built with a reinforced concrete box: the maximum height is 3.5 m, with length of 6 m and width of 2 m, so that a 2:3 slope can be built. On each lateral side of the box, 50 openings closed with screw caps allow the insertion on properly chosen positions of the control instrumentation (6 tensiometers and 6 Water Content Reflectometer sensors). The monitoring network, connected to an automatic acquisition system, was completed by two piezometers, and two stream gages able to evaluate both the surface runoff and subsurface contributions to the total outflow. The work developed in this study concerns the design and the performance analysis of the main features characterizing the large-scale hillslope model, up to the performance of two landslide experiments on a 60 cm thick sandy soil layer. A rainfall simulator was designed and built to reproduce an intensive precipitation causing the soil collapse. It was realized with a one-loop network equipped with spray nozzles appropriately chosen to minimize the surface splash erosion. In such a way the effects induced by the simulator concern infiltration dynamics without generating top erosion, which could introduce further factors of more difficult understanding. The nozzle configurations on the network were chosen to reproduce i) the desired range of the rainfall intensity, varying from 50 to 150 mm/h, and ii) the spatial uniformity of the produced rain. A careful analysis of the rain sprayed by a single nozzle was developed on a prototype, in order to recognize the main variables affecting the nozzle functioning and performance. Further investigations were then carried out to test the performance of the final full-scale version of the rainfall simulator, highlighting its flexibility for the regulation and the control of the generated rain intensity. Depending on the desired rainfall range, four different configurations of nozzles, distinguished by the number of active nozzles and their location, were chosen to cover the required intensity interval. A careful analysis about the drop diameters was conducted by recurring to an oil mixture poured in Petri dishes that were exposed to the rain. The drop size distribution thus collected characterizes the induced rainfall and was used for a numerical simulation aimed at estimating the impact energy of the drops falling on the soil. The proposed model calculates the trajectories of the particles injected by the nozzle using a constitutive law of sphere aerodynamics in a 3D space. As a result, the rainfall potential erosion and its spatial distribution were assessed, highlighting the limited surface erosion generated by the proposed rainfall simulator. In a second step, a suitable device was realized to calibrate the WCR (Water Content Reflectometer) sensors. It consists of a 0.6 x 0.5 x 0.6 cubic meters Plexiglas box containing the soil with the top exposed to rainfall and the bottom sustained by a perforated base. The calibration of the WCR sensors pointed to obtain an effective law for an accurate assessment of the water infiltration evolution in the soil during the landslide experiments. Several tests were performed with varying porosity values of the sand sample placed into the Plexiglas box, where three tensiometers and as many as WCR probes were arranged. The final results suggest a calibration relationship linearly depending on the WCR output signal and porosity. Two experiments on the artificial slope were then performed by applying two different porosities of the soil during the placement. The chosen soil consists of a fine sand with high particle size uniformity. The first porosity was obtained by dumping the sand without applying compacting action, such that the sand was in loose conditions. In a second case, the sand was compacted to yield a dense sand. The two experiments were carried out by applying the rainfall at until the sand collapse. The observation of the experiments and the analysis of the recorded data allow to examine the hydrological dynamics leading to the landslide and the triggering factors. With loose sand, the failure occurred suddenly without warning signs; at the failure, the soil appeared like a viscous fluid and the tensiometers recorded an instantaneous peak of the water pressure head. In the case with dense sand, the failure occurred really slowly, and some local detachments of top layer preceded the advance of the whole sand volume. A numerical model solving Richards equation was used to reproduce the hydrological processes leading to failure in the two experiments. A numerical inverse method was adopted to improve the reliability of the numerical solution with respect to the data recorded from the experiments. The comparison reveals a good agreement between the experimental and numerical results for the loose sand experiment. In the case regarding dense sand, the limits of Richards solution does not allow to reach an acceptable agreement with experimental recorded. The causes might be linked with the affection of the air phase in sand pores and the incipient deformation of the soil matrix at micro-scale.
Le valutazioni di pericolosità sulle frane superficiali sono in genere estese su scala di versante, le cui limitate indagini geotecniche non consentono una caratterizzazione fedele che miri ad una comprensione dettagliata dei fenomeni di innesco. Per tale motivo, i processi fisici considerati nella spiegazione del fenomeno sono spesso riduttivi e tali da consentire una modesta risoluzione sia spaziale che temporale. Tale approccio risulta utile per valutazioni legate alla previsione di innesco, e quindi, correttamente, non necessitano di un’elevata affidabilità nell’interpretazione del fenomeno idromeccanico, ma risultano compatibili con un’analisi sommaria e generalizzata. Tuttavia, un’analisi a scala locale diventa necessaria quando sia richiesta una comprensione dettagliata del fenomeno di innesco che permetta di riconoscere gli elementi, e la loro intensità, nell’innesco delle frane superficiali. Tale esigenza può essere soddisfatta solo riproducendo dati sperimentali raccolti su siti a monitoraggio elevato. Lo studio presente si occupa di un pendio artificiale per la simulazione dei fenomeni di innesco delle frane superficiali determinate da precipitazioni intense. Il modello fisico consiste di un muro di contenimento in calcestruzzo armato: l’altezza massima è di 3.5 m, la lunghezza alla base è di 6 m e la larghezza di 2 m, così da realizzare una pendenza superficiale di 2:3. Su ogni muro laterale, sono applicate 50 forature realizzate mediante tronchetti flangiati che consentono l’inserimento di 6 tensiometri e 6 sonde WCR (Water Content Reflectometer) nelle posizioni desiderate. La strumentazione è completata con 2 piezometri e due pozzetti per la misura delle portate di ruscellamento e sub-superficiale in uscita. Ogni misura viene acquisita e registrata mediante un sistema di acquisizione. Il lavoro sviluppato riguarda la progettazione e l’analisi delle prestazioni dei principali dispositivi impiegati nel modello fisico, fino alla realizzazione di due prove sperimentali su uno strato di sabbia con spessore verticale di 60 cm. Un simulatore di pioggia è stato progettato e realizzato per generare una precipitazione intensa tale da provocare l’instabilità del materiale posato nel modello fisico. Il simulatore consiste in una rete ad anello chiuso sulla quale sono distribuiti degli ugelli appositamente scelti per evitare erosione sulla superficie del terreno dovuta all’impatto delle gocce erogate. In questo modo gli effetti indotti dalla precipitazione si riconoscono nei soli fenomeni di infiltrazione, senza innesco di processi erosivi che potrebbero aggiungere fenomeni di difficile comprensione. Le configurazioni degli ugelli distribuiti sul simulatore vengono scelte per coprire i) il campo desiderato delle intensità di pioggia, variante tra 50 e 150 mm/h, e per assicurare ii) un’elevata uniformità spaziale della precipitazione prodotta. Un’attenta analisi è stata svolta per caratterizzare un singolo ugello mediante un apposito dispositivo, così da individuarne le principali variabili caratterizzanti il funzionamento e le prestazioni. Successivamente, l’indagine sperimentale è stata concentrata sulla versione finale del simulatore di pioggia, al fine di definire le modalità di gestione e di regolazione della precipitazione riprodotta. A seconda del campo di intensità desiderato, quattro differenti configurazioni di ugelli, distinte per il numero e la posizione degli ugelli attivi, sono state individuate per ricoprire l’intervallo totale di intensità da 50 a 150 mm/h. Inoltre, è stata eseguita un’analisi per valutare i diametri delle gocce erogate mediante olio ad alta viscosità versato in dischetti Petri esposti alla pioggia artificiale. La distribuzione dei diametri così ottenuta è stata successivamente impiegata in un modello numerico per stimare la distribuzione dell’energia di impatto delle gocce sul suolo. Il modello numerico proposto calcola la traiettoria delle particelle erogate dall’ugello mediante una legge costitutiva basata sull’aerodinamica di sfere nello spazio 3D. I risultati hanno posto in evidenza la limitata erosione superficiale determinata dalla precipitazione erogata. È stata poi realizzato un dispositivo per la calibrazione delle sonde WCR (Water Content Reflectometer), impiegate per la stima del contenuto volumetrico d’acqua del terreno. Il dispositivo consiste in un contenitore in Plexiglas di dimensione interne pari a 0.6 x 0.5 x 0.6 m3, contenente il suolo che risulta libero nella parte superiore e, alla base, trattenuto da una piastra forata. La procedura di calibrazione delle sonde WCR ha mirato a definire una legge per una stima accurata dei processi di infiltrazione nel suolo durante gli esperimenti di frana. Numerose prove sono state quindi condotte variando, rispettivamente, la porosità del provino di materiale posato nel contenitore; le caratteristiche del suolo erano costantemente monitorate da 3 tensiometri infissi e da altrettante sonde WCR. Il risultato finale ha restituito una legge di calibrazione dello strumento linearmente dipendente dal segnale di uscita della sonda WCR e anche dalla porosità del terreno. Due esperimenti sul modello fisico di frana sono stati quindi realizzati su uno strato di materiale con due rispettive porosità. Il materiale impiegato consiste in una sabbia fine con distribuzione granulometrica molto uniforme. In un primo caso la sabbia è stata posata senza introdurre azioni di compattazione, a meno di una leggera battitura sulla superficie per evitare eccessive deformazioni successive alla precipitazione. In un secondo esperimento, la sabbia è stata invece posata e compattata per strati successivi, così da raggiungere uno stato addensato. I due esperimenti sono stati rispettivamente eseguiti applicando un’intensità di precipitazione pari a 150 mm/h fino a collasso avvenuto. Le modalità di innesco osservate e l’analisi dei dati raccolti permettono di individuare gli elementi idrologici che determinano il collasso in entrambi i casi, mettendo anche in rilievo le diversità. Con sabbia sciolta, il collasso si verifica istantaneamente, senza segni premonitori che avvertano dell’imminente frana. All’innesco, il volume di terreno ha assunto le sembianze di un fluido ad alta viscosità, e i tensiometri installati registrano un picco istantaneo di pressione idraulica. Nel caso di sabbia addensata, il collasso avviene molto lentamente ed è preceduto da distacchi localizzati di strati sottili di terreno. Un modello per la risoluzione dell’equazione di Richards è stato impiegato per riprodurre le dinamiche idrologiche che determinano l’innesco delle frane nei due casi distinti. Si è ricorso, inoltre, ad una procedura inversa per migliorare l’affidabilità della soluzione numerica rispetto ai dati sperimentali registrati durante le prove di frana. Il confronto esprime un’elevata corrispondenza tra dati numerici e sperimentali nel caso di sabbia sciolta. Nel secondo caso con sabbia addensata, le ipotesi del modello di Richards non sono sufficienti per raggiungere una corrispondenza accettabile con i dati sperimentali. Le cause possono ritrovarsi nell’influenza che la fase gassosa contenuta nei pori può determinare, nonché le deformazioni incipienti a micro-scala che si manifestano durante l’esperimento.

Landslides cause significant damage to property and human lives throughout the world. Rainfall is the most common triggering factor for the occurrence of landslides. This dissertation presents two novel methodologies for assessment of rainfall-triggered shallow landslide hazard. The first method focuses on using remotely sensed soil moisture and soil surface properties in developing a framework for real-time regional scale landslide hazard assessment while the second method is a deterministic approach to landslide hazard assessment of the specific sites identified during first assessment. In the latter approach, landslide inducing transient seepage in soil during rainfall and its effect on slope stability are modeled using numerical analysis. Traditionally, the prediction of rainfall-triggered landslides has been performed using pre-determined rainfall intensity-duration thresholds. However, it is the infiltration of rainwater into soil slopes which leads to an increase of porewater pressure and destruction of matric suction that causes a reduction in soil shear strength and slope instability. Hence, soil moisture, pore pressure and infiltration properties of soil must be direct inputs to reliable landslide hazard assessment methods. In-situ measurement of pore pressure for real-time landslide hazard assessment is an expensive endeavor and thus, the use of more practical remote sensing of soil moisture is constantly sought. In past studies, a statistical framework for regional scale landslide hazard assessment using remotely sensed soil moisture has not been developed. Thus, the first major objective of this study is to develop a framework for using downscaled remotely sensed soil moisture available on a daily basis to monitor locations that are highly susceptible to rainfall- triggered shallow landslides, using a well-structured assessment procedure. Downscaled soil moisture, the relevant geotechnical properties of saturated hydraulic conductivity and soil type, and the conditioning factors of elevation, slope, and distance to roads are used to develop an improved logistic regression model to predict the soil slide hazard of soil slopes using data from two geographically different regions. A soil moisture downscaling model with a proven superior prediction accuracy than the downscaling models that have been used in previous landslide studies is employed in this study. Furthermore, this model provides satisfactory classification accuracy and performs better than the alternative water drainage-based indices that are conventionally used to quantify the effect that elevated soil moisture has upon the soil sliding. Furthermore, the downscaling of soil moisture content is shown to improve the prediction accuracy. Finally, a technique that can determine the threshold probability for identifying locations with a high soil slide hazard is proposed. On the other hand, many deterministic methods based on analytical and numerical methodologies have been developed in the past to model the effects of infiltration and subsequent transient seepage during rainfall on the stability of natural and manmade slopes. However, the effects of continuous interplay between surface and subsurface water flows on slope stability is seldom considered in the above-mentioned numerical and analytical models. Furthermore, the existing seepage models are based on the Richards equation, which is derived using Darcy’s law, under a pseudo-steady state assumption. Thus, the inertial components of flow have not been incorporated typically in modeling the flow of water through the subsurface. Hence, the second objective of this study is to develop a numerical model which has the capability to model surface, subsurface and infiltration water flows based on a unified approach, employing fundamental fluid dynamics, to assess slope stability during rainfall-induced transient seepage conditions. The developed model is based on the Navier-Stokes equations, which possess the capability to model surface, subsurface and infiltration water flows in a unified manner. The extended Mohr-Coulomb criterion is used in evaluating the shear strength reduction due to infiltration. Finally, the effect of soil hydraulic conductivity on slope stability is examined. The interplay between surface and subsurface water flows is observed to have a significant impact on slope stability, especially at low hydraulic conductivity values. The developed numerical model facilitates site-specific calibration with respect to saturated hydraulic conductivity, remotely sensed soil moisture content and rainfall intensity to predict landslide inducing subsurface pore pressure variations in real time.

碩士
國立成功大學
資源工程學系碩博士班
97
There are a lot of landslides happened in Taiwan during rainy seasons. A correlation between rainfall and slope stability has been studied. There has been much research conducted on the effects of rainfall on unsaturated soil slopes. The one of mechanisms that leads to slope failures is the pore water pressures starting to increase when water starts to infiltrate the unsaturated soil zone. The failures have been attributed to the advancement of a wetting front into slopes until it reaches a depth where it triggers failure. Conventional methods for slopes stability analysis, assuming fully saturated (or dry) behaviors on slope, cannot be described successfully under unsaturated conditions. In this study, the finite element computer program SEEP/W is used to simulate the changes in pore water pressure when the slope is subjected to different rainfall events. Finally, the pore water pressure distribution computed in the program SEEP/W is imported into SLOPE/W for the slope stability analysis. The time-dependent pore water pressure distribution could therefore be used to compute the factor of safety with time directly. This thesis is divided into two parts, the first part is to process sensitivity analysis of controlling parameters, including general geometry, soil property, rainfall intensity and rainfall pattern. This results indicate soil property and rainfall intensity are the most important factors to influence slope stability. In the second part, the Songmao region in Taichung, Taiwan which has been examined by modeling local rainfall events and coupled with the monitored data from field to determine the slope stability mechanism. By the rainfall events, the simulation indicated safety factor of slope decreased by less than 1.0 when the rainfall infiltrates at 2.0m depth and the monitored data also revealed displacement in the evidence. In conclusion, the slope failures have been attributed to the increase of pore water pressure and decrease of shear strength when rainfall triggers failure. Therefore, the main reason to make Songmao region slope instability is rainfall infiltration.

碩士
國立嘉義大學
土木與水資源工程學系研究所
98
Typhoon Morakot on August 7, 2009 hit central and southern Taiwan and lead to many slope disasters which destroyed villages and killed hundreds of people. The objective of this study is to assess the possibility of rainfall-induced landslides in Jiashian area located in southern Taiwan. The results show that the landslide occurrence is significantly affected by the rainfall condition. For a 48hr rainfall event, the landslide tendencies of the four rainfall patterns, including the uniform, advanced, intermediated, and delayed rainfalls, seem to be the same when the rainfall amount is less than 250mm. The positions of landslide occurrence probability larger than 50% significantly increase while the 48hr rainfall is between 250mm and 500mm. Among the four rainfall patterns, the advanced rainfall has the largest rate of increase, whereas the delayed rainfall provides the smallest rate, and the other two almost have the same rate. When the rainfall is greater than 500mm, the increase rate of the advanced rainfall in the position of landslide occurrence probability larger than 50% dramatically decreases, but the delayed rainfall has the contrary outcome, and the increase rates of the other two rainfall patterns are between those of the advanced and delayed rainfalls. However, like the rainfall amount less than 250mm, the four rainfall patterns with large rainfall amount seem to again have the same landslide tendency.

The shallow landslides are hazardous mass movements commonly triggered by intense rainfall. The hazardousness of these events is mainly due to their common evolution in rapid mass movements as debris avalanches and flows and to the frequently occurring in the form of clusters of events. Because of their characteristics, the forecasting is a particularly valuable tool to protect people and infrastructures from this kind of landslide events. The presence of vegetation on hillslopes significantly reduces the slopes susceptibility to the shallow landslides, and the stabilising action is mainly due to the reinforcement of the soil by the roots. The spatial variation of the root reinforcement should be therefore considered in distributed slope stability analyses. However, the natural variability of the parameter makes it challenging to insert the root reinforcement into the models. Many approaches to the problem were tested, but nowadays there are still lacking a distributed slope stability model capable of very quick processing in which the root reinforcement is considered and an approach to estimate the root cohesion at the regional scale that it has been tested in very wide areas and for long period-simulations. In this study, we present the effect of the root cohesion on slope stability simulations at the regional scale obtained using a physically-based distributed slope stability model, the HIRESSS (HIgh REsolution Slope Stability Simulator). The HIRESSS model was selected for the purposes, being capable of rapid processing even in wide areas thanks to the parallel structure of its code. The simulator was modified to insert the root reinforcement among the geotechnical parameters considered to computing the factor of safety in probabilistic terms, and for this purpose a commonly adopted model for the root cohesion was chosen. To build a map of the root cohesion for the study areas, the distribution of plant species in the area was obtained from vegetation distribution map and in situ surveys, then a value of root cohesion and a range of variation was defined for each plant species based on the most recent literature in this field, finally, to reproduce the natural variability, the root reinforcement was treated as variable in Monte Carlo simulations, as well as the other geotechnical parameters. The results of the simulations for the study areas were processed and analysed in order to evaluate the effect of the root cohesion on the failure probabilities and the adopted approach to estimate the root cohesion at the regional scale. The comparative analyses carried out on the results of the simulations performed inserting or not the root reinforcement brought out little differences between the two from the point of view the failure probabilities, particularly when the saturated conditions of the soil are reached. 10 Based on the findings of this research, it is considered that a root cohesion model different to the one adopted is preferable in the context of the shallow landslides, in applications in which working with failure probabilities (instead of factor of safety values) is desirable, and in areas similar to the ones of the study.

碩士
國立交通大學
土木工程系所
93
In this study, based on Richards equation and infinite slope stability theory, a rainfall-triggered shallow landslide model is developed and examined. After the developed model is verified, the type and mechanism of failure in steep and mild slopes are investigated first. The influences of rainfall characters including intensity, duration, and pattern on shallow landslide are then examined. The results show that the failure of steep slope could happen when the soils are unsaturated or saturated. However, the failure of mild slope seems to occur in saturated soils only. Both the decrease of soil suction and the rise of groundwater table caused by rainfall could trigger landslide. The intensity, duration, and pattern of rainfall have significantly influence not only on the occurrence of shallow landslide but also on the depth and time of soil failure. In addition, the sufficient condition of landslide caused by the rise of groundwater table is derived and it could be used in engineering practices.

碩士
國立交通大學
土木工程系所
96
Rainfall-triggered shallow landslide is one of the major natural hazards. Recently, the infinite slope theory combining reliability analysis was widely applied to assess the grid-based regional slope stability (Baum et al. 2002). The advantages of above method are based on sound physical mechanic and accounting the uncertainties of hydrogeological parameters, simultaneously. However, for a specified region consisting of several grid points, most researches accounted for the slope reliability of each individual point, and ignored the influence of its neighboring points, thus, the overall landslide potential for whole region cannot be quantified. In this study, a framework to evaluate the regional reliability during rainstorm event is presented which explicitly incorporating the spatial correlation between each grid points. According to Su (2007), the cohesion, friction angle, unit weight of saturated soil, and saturated hydraulic conductivity are considered as the random hydrogeological parameters in this study. Based on the assumption that the spatial variability of random hydrogeological parameters are second-order stationary with exponential covariance function, the spatial correlation of uncertain parameters between each grid points inside the pre-specified region are accounted firstly. From the “Rainfall-Triggered Shallow Landslide Model” developed by Tsai and Yang (2006) along with the first-order second-moment method (FOSM), the statistical properties of safety factor (FS), including expectation, standard deviation, and correlation coefficients, are quantified. Furthermore, based on the assumption that the joint probability function for safety factors is multivariate normal distribution, the concept of “series system” is adopted to obtain the regional reliability (i.e. the reliability that all the grid points do not failure during rainstorm event). To examine the accuracy of proposed framework, a hypothetical example is utilized. The examination is conducted through the comparison of the regional reliability calculated by the proposed framework and Monte Carlo simulation (MCS). The results indicate that the multivariate normal distribution assumption of safety factors and the FOSM are applicable for risk assessment of landslide, regardless of the uncertainties degrees of hydrogeologic parameters. After the proposed framework has been examined, it is applied to the Shihmen reservoir watershed. From the application results, comparing with the traditional methods which determine the reliabilities for each individual grid points, the regional reliability is more suitable to assess the overall landslide potential for whole region because it incorporate the spatial variability of hydrogeological parameters and the spread of reliabilities among all the grid points simultaneously. Thus, the proposed framework could assist the engineers outline the management priorities for different regions according to various degrees of regional reliabilities.

碩士
國立交通大學
土木工程學系
100
Rainfall-triggered shallow landslide is one of the major natural disasters over the world which may immediately cause large numbers of casualties and huge economic losses. To mitigate the landslide disasters, the numerical model simulations based on the infinite slope theory have been widely applied to predict the slope stability during a rainfall event. However, due to the inherent heterogeneity and lack of complete information about model input variables, uncertainties exist in specifying the values of these variables in the numerical model rendering potential failure to obtain the authentic model output (safety factor, FS) for the slope under consideration. In this study, three approximated uncertainty analysis methods, including the first-order second-moment (FOSM), Rosenblueth’s point estimation (R-PE), and Li’s point estimation (LI-PE) were utilized with the developed rainfall triggered shallow landslide model (Tsai and Yang, 2006) to obtain the statistical properties of FS and the landslide probability (Pf) at the Salunzai slope during typhoon Aere. Besides, the relative errors of computed Pf with respect to Monte Carlo simulation (MCS) result were compared. Six stochastic model input variables, including the saturated hydraulic conductivity (Ksat), friction angle (??, cohesion (c), initial groundwater depth (dZ), soil thickness (dLZ), and slope (α) were considered. Moreover, six cases involving different uncertainty levels of slope angle and soil thickness were considered. The results showed that each of the three approximated methods has its own advantages and drawbacks for the uncertainty analysis of rainfall triggered shallow landslide model. The performances of the approximated uncertainty analysis methods were evaluated through three criteria including: (1) accuracy; (2) efficiency; and (3) prior information requirements. For the accuracy of the approximated methods, the differences in obtained landslide probability between the point estimation (Rosenblueth’s and Li’s) and MCS are minimal in all of the six cases. However, the FOSM method tends to significantly overestimate the landslide probability especially in the case with higher model inputs uncertainties. In contrast to accuracy, the FOSM method has highest computational efficiency because the required number of numerical model evaluation in one simulation grid is 9, while the Rosenblueth’s and Li’s point estimation methods require 16 and 15 model evaluations, respectively. In view of the prior information requirement, the FOSM method only requires the first two moments of the stochastic model input variables while the Rosenblueth’s and Li’s point estimation methods require the first three and four moments of model inputs, respectively. In summary, the applicability of the three approximated uncertainty analysis methods for rainfall-triggered shallow landslide model depend on the space scale of the application. For the estimation of the distributions of the landslide probabilities within a watershed, the third and fourth moments of stochastic model inputs might not be reliably obtained, besides, the amounts of simulation grids might be huge, thus the FOSM method is more applicable than the Rosenblueth’s and Li’s point estimation methods. In contrast, for the estimation of landslide probability at a specified slope, the third and fourth moments of stochastic model inputs might be reliably through a more comprehensive field investigation, thus the Rosenblueth’s and Li’s point estimation methods are more applicable due to the higher accuracy.

AbstractThis paper presents and discusses the mechanisms of rainfall-induced shallow landslides that commonly occur in South East Queensland (SEQ) and northern New South Wales (NSW), Australia. The major factors causing the formation of landslide mass such as geology, weathering, and rainfall patterns were discussed. Results from field surveys and laboratory testing of rock/soil material from landslide masses were presented, and relationships between the material strength and landslide occurrence were drawn. It was found that most of shallow slides were related to sandstone deposits. Those failures occurred on natural slopes and road cuts with the inclination of the failure plane being in the range of 35–45°. For natural slopes where the landslide mass mostly consisted of coarse-grained soil, the relationship between the soil strength and water content was established. In addition, the relationship between rainfall patterns such as intensity and duration, and the landslide occurrence was presented. Based on the data from field work and laboratory results including a series of flume tests, the mechanism of shallow landslides triggered by rainfall events was identified and discussed.

Rainfall-triggered shallow landslide events have caused losses of human lives and millions of euros in damage to property in all parts of the world. The need to prevent such hazards combined with the difficulty of describing the geomorphological processes over regional scales led to the adoption of empirical rainfall thresholds derived from records of rainfall events triggering landslides. These rainfall intensity thresholds are generally computed, assuming that all events are not influenced by antecedent soil moisture conditions. Nevertheless, it is expected that antecedent soil moisture conditions may provide critical support for the correct definition of the triggering conditions. Therefore, we explored the role of antecedent soil moisture on critical rainfall intensity-duration thresholds to evaluate the possibility of modifying or improving traditional approaches. The study was carried out using 326 landslide events that occurred in the last 18 years in the Basilicata region (southern Italy). Besides the ordinary data (i.e., rainstorm intensity and duration), we also derived the antecedent soil moisture conditions using a parsimonious hydrological model. These data have been used to derive the rainfall intensity thresholds conditional on the antecedent saturation of soil quantifying the impact of such parameters on rainfall thresholds.