Critical Rainfall Conditions Research Articles

SLEM (Shallow Landslide Express Model): A Simplified Geo-Hydrological Model for Powerlines Geo-Hazard Assessment

Powerlines are strategic infrastructures for the Italian electro-energetic network, and natural threats represent a potential risk that may influence their operativity and functionality. Geo-hydrological hazards triggered by heavy rainfall, such as shallow landslides, have historically affected electrical infrastructure networks, causing pylon failures and extensive blackouts. In this work, an application of the reworked version of the model proposed by Borga et al. and Tarolli et al. for rainfall-induced shallow landslide hazard assessment is presented. The revised model is called SLEM (Shallow Landslide Express Model) and is designed to merge in a closed-from equation the infinite slope stability with a simplified hydrogeological model. SLEM was written in Python language to automatise the parameter calculations, and a new strategy for evaluating the Dynamic Contributing Area (DCA) and its dependence on the initial soil moisture condition was included. The model was tested for the case study basin of Trebbia River, in the Emilia-Romagna region (Italy) which in the recent past experienced severe episodes of geo-hydrological hazards. The critical rainfall ratio (rcrit) able to trigger slope instability prediction was validated against the available local rainfall threshold curves, showing good performance skills. The rainfall return time (TR) was calculated from rcrit identifying the most hazardous area across the Trebbia basin with respect to the position of powerlines. TR was interpreted as an index of the magnitude of the geo-hydrological events considering the hypothesis of iso-frequency with precipitation. Thanks to its fast computing, the critical rainfall conditions, the temporal recurrence and the location of the most vulnerable powerlines are identified by the model. SLEM is designed to carry out risk analysis useful for defining infrastructure resilience plans and for implementing mitigation strategies against geo-hazards.

Antecedent rainfall as a critical factor for the triggering of debris flows in arid regions

Abstract. In arid regions, debris flows (DFs) are occasionally observed when heavy rainfall hits steep slopes with unconsolidated materials. Despite the related hazards, not much is yet known about the critical rainfall conditions for DF initiation in dryland areas mainly due to a lack of observations and data. In this study, we use high-resolution digital surface models and orthophotos to detect DFs on the arid slopes of the Dead Sea's northwestern margins, as well as weather radar rainfall estimates for the characterization of the triggering and non-triggering storms. We identify deposits from over 40 relatively small DFs that occurred between 2013 and 2019. We divide them into four groups based on their spatial distribution and triggering period, and we identify the most likely triggering storms based on weather radar data and additional information. We show that the DFs were likely triggered by an intense convective cell (lasting less than 45 min) which was preceded by non-negligible rainfall amounts (about 10 mm) delivered during the storm. Comparing triggering and non-triggering storms, we observed that rain intensity alone is insufficient to explain the phenomena and discuss the possibility that antecedent rainfall could represent a critical factor for the triggering of DFs on steep slopes of arid environments.

Application of Rainfall Threshold for Sediment-Related Disasters in Malaysia: Status, Issues and Challenges

Sediment-related disaster is one of the most significant natural disasters, from the perspective of magnitude, damage and loss to human life and infrastructure, and disruption to socio-economic activities. Debris, mud flood, landslide and cliff failure are the major catastrophic problems commonly experienced in most developing countries, including Malaysia. As rainfall is the main culprit to sediment-related disaster occurrences, rainfall data are crucial in the correlation of the occurred events. Several studies have been undertaken worldwide to estimate the critical rainfall conditions and draw the benchmark to predict landslide occurrences, specifically for debris and mudflows (DMF), and shallow landslides. Therefore, this paper presents an up-to-date picture on the development of the rainfall threshold from Malaysia’s perspective. Additionally, the open issues and challenges of deriving the rain threshold are also discussed in three aspects: collection of the dataset features, identification of the threshold and validation of the threshold. The outcomes of this review could serve as references for future studies in Malaysia and other developing countries in managing sediment-related disasters.

AN APPLICATION OF RAINFALL THRESHOLD FOR SEDIMENT RELATED DISASTER IN MALAYSIA, ISSUES AND CHALLENGES

Sediment related disaster is the most significant natural disaster in Malaysia and many countries in this world from the perspective of magnitude, damage and loss to human life and infrastructure as well as disruption to socio-economic activities. Debris, mud flood, landslide and cliff failure are some of the major catastrophic problems and became a history for the country especially Cameron Highlands in a state of Pahang. As rainfall is the main culprit to sediment-related disaster occurrences, therefore the rainfall data is very crucial to be used in the correlation of the occurred events. Due to that fact, several studies worldwide have been made to estimate critical rainfall conditions and this being useful to draw the benchmark to predict the occurrences of the landslide specifically for DMF and shallow landslides. This paper discussed the development of the rainfall threshold in Malaysia by compiling the framework of the threshold to determine the lesson learned as well as the way forward. As Malaysia needs to move at a faster pace towards embracing the whole aspects in determining the threshold as well to implement it into the operational threshold, therefore the first step is very important to initiate the momentum while the collaboration or networking among government agencies in National Disaster Risk Reduction (DDR) should be enhanced and strengthened.

Monitoring of Rainfall and Soil Moisture at the Rebaixader Catchment (Central Pyrenees)

ABSTRACT The instrumental monitoring of torrential catchments is a fundamental research task that provides necessary information to improve our understanding of the mechanisms of debris flows. While most monitoring sites include meteorological sensors and analyze the critical rainfall conditions, very few contain soil moisture measurements. In our monitoring site, the Rebaixader catchment, 11 debris flows and 24 debris floods were detected during the last 9 years. Herein, the initiation mechanisms of these torrential flows were analyzed, focusing on the critical rainfall conditions and the soil water dynamics. Comparing the temporal distribution of both rainfall episodes and torrential flows, the Kernel density plots showed maximum values for rainfalls at the beginning of June, while the peak for torrential flows is on July 20. Thus, the antecedent rainfall, and especially the soil moisture conditions, may influence the triggering of torrential flows. In a second step, a new updated rainfall threshold was proposed that included total rainfall duration and mean intensity. The analysis of soil moisture data was more complicated, and no clear trends were observed in the data set. Therefore, additional data have to be recorded in order to quantitatively analyze the role of soil moisture on the triggering of torrential flows and for the definition of thresholds. Some preliminary results show that the soil moisture at the beginning of a rainfall event affects the maximum increase of soil moisture, while a slight trend was visible comparing the initial soil moisture with the necessary rainfall amount to trigger a torrential flow.

Variation of debris flow/flood formation conditions at the watershed scale in the Wenchuan Earthquake area

A channelized debris flow/flood generally originates from initial gully erosion by superficial runoff that evolves rapidly into massive erosion of the channel bed. Knowledge of the formation conditions of such events is crucial for accurate forecasting, and determination of rainfall and runoff thresholds for such hazards is a primary concern following a strong earthquake. This work proposed a framework for debris flow/flood formation at the watershed scale in two watersheds (area: 2.4 and 32.4 km2) in the Wenchuan Earthquake area (China). The critical runoff and rainfall conditions required for debris flow/flood formation were simulated and their annual variations investigated. Ultimately, the runoff conditions required for debris flow/flood formation in the two studied watersheds were calculated on an annual basis and found to increase in time. Similarly, following consideration of three different rainfall types, critical rainfall conditions were proposed that also showed an increasing tendency. The increase of rainfall and runoff conditions for debris flow/flood formation is attributable to both the recovery of vegetation and the reduction of source materials. In comparison with actual monitored flow behaviors and previously proposed rainfall thresholds, the results showed strong consistency and high forecasting efficiency.

Rainfall analysis of debris flows in the Obří důl Valley in the Krkonoše Mts., Czechia

Rainfall is the most important triggering factor responsible for the occurrence of debris flows in the Obři důl Valley in the Krkonose Mts. The critical rainfall conditions for slope failures are not the same for different debris flows, and may be strongly influenced by regional geological and geomorphological conditions. Nevertheless, analysis of hourly intensities, daily rainfall, cumulative data and the antecedent precipitation index (API) revealed that several of the above-mentioned factors are necessary to trigger the debris flow. On the other hand, a significant amount of daily rainfall (e.g. 225 mm) could trigger a debris flow without the support of any other rainfall characteristics in the monitored area and period under review. We used several rain gauges from the study area but the local differences in rainfall were so high that data from more remote stations was difficult to include in the Obři důl Valley. This is why only a limited amount of precise data is available for some years.

Evaluating critical rainfall conditions for large-scale landslides by detecting event times from seismic records

Abstract. One purpose of landslide research is to establish early warning thresholds for rainfall-induced landslides. Insufficient observations of past events have inhibited the analysis of critical rainfall conditions triggering landslides. This difficulty may be resolved by extracting the timing of landslide occurrences through analysis of seismic signals. In this study, seismic records of the Broadband Array in Taiwan for Seismology were examined to identify ground motion triggered by large landslides that occurred in the years 2005 to 2014. A total of 62 landslide-induced seismic signals were identified. The seismic signals were analyzed to determine the timing of landslide occurrences, and the rainfall conditions at those times – including rainfall intensity (I), duration (D), and effective rainfall (Rt) – were assessed. Three common rainfall threshold models (I–D, I–Rt, and Rt–D) were compared, and the crucial factors of a forecast warning model were found to be duration and effective rainfall. In addition, rainfall information related to the 62 landslides was analyzed to establish a critical height of water model, (I-1.5)⋅D=430.2. The critical height of water model was applied to data from Typhoon Soudelor of 2015, and the model issued a large landslide warning for southern Taiwan.

Method to Develop Critical Rainfall Conditions for Occurrences of Sediment-Induced Disasters and to Identify Areas Prone to Landslides

The present study demonstrates a method to specify critical rainfall conditions for the occurrence of a sediment disaster and identify areas prone to landslides using a simulator proposed by the current authors for sediment hazards. The simulator predicts spatial and temporal distributions for surface and subsurface flows, landslides, and debris flow resulting from rainfall events. The method to develop a critical curve for the occurrence of a disaster is proposed using simulated landslide data derived from artificially specified rainfall conditions, past rainfall data, and disaster records. Usually, a rainfall event also constitutes a period without rain, and this method can be used to evaluate the influence of the no-rain period. In addition, we propose a method to classify slopes according to the probability of landslide occurrences on a domain defined by slope gradient versus catchment area, using data on landslides resulting from a specified rainfall amount and intensity. Areas identified as having a high probability of landslide occurrences correspond to the runout mark of landslides and debris flow in August 2014.

Critical rainfall conditions for the initiation of torrential flows. Results from the Rebaixader catchment (Central Pyrenees)

Summary Torrential flows like debris flows or debris floods are fast movements formed by a mix of water and different amounts of unsorted solid material. They generally occur in steep torrents and pose high risk in mountainous areas. Rainfall is their most common triggering factor and the analysis of the critical rainfall conditions is a fundamental research task. Due to their wide use in warning systems, rainfall thresholds for the triggering of torrential flows are an important outcome of such analysis and are empirically derived using data from past events. In 2009, a monitoring system was installed in the Rebaixader catchment, Central Pyrenees (Spain). Since then, rainfall data of 25 torrential flows (“TRIG rainfalls”) were recorded, with a 5-min sampling frequency. Other 142 rainfalls that did not trigger torrential flows (“NonTRIG rainfalls”) were also collected and analyzed. The goal of this work was threefold: (i) characterize rainfall episodes in the Rebaixader catchment and compare rainfall data that triggered torrential flows and others that did not; (ii) define and test Intensity–Duration (ID) thresholds using rainfall data measured inside the catchment by with different techniques; (iii) analyze how the criterion used for defining the rainfall duration and the spatial variability of rainfall influences the value obtained for the thresholds. The statistical analysis of the rainfall characteristics showed that the parameters that discriminate better the TRIG and NonTRIG rainfalls are the rainfall intensities, the mean rainfall and the total rainfall amount. The antecedent rainfall was not significantly different between TRIG and NonTRIG rainfalls, as it can be expected when the source material is very pervious (a sandy glacial soil in the study site). Thresholds were derived from data collected at one rain gauge located inside the catchment. Two different methods were applied to calculate the duration and intensity of rainfall: (i) using total duration, Dtot, and mean intensity, Imean, of the rainfall event, and (ii) using floating durations, D, and intensities, Ifl, based on the maximum values over floating periods of different duration. The resulting thresholds are considerably different (Imean = 6.20 Dtot−0.36 and Ifl_90% = 5.49 D−0.75, respectively) showing a strong dependence on the applied methodology. On the other hand, the definition of the thresholds is affected by several types of uncertainties. Data from both rain gauges and weather radar were used to analyze the uncertainty associated with the spatial variability of the triggering rainfalls. The analysis indicates that the precipitation recorded by the nearby rain gauges can introduce major uncertainties, especially for convective summer storms. Thus, incorporating radar rainfall can significantly improve the accuracy of the measured triggering rainfall. Finally, thresholds were also derived according to three different criteria for the definition of the duration of the triggering rainfall: (i) the duration until the peak intensity, (ii) the duration until the end of the rainfall; and, (iii) the duration until the trigger of the torrential flow. An important contribution of this work is the assessment of the threshold relationships obtained using the third definition of duration. Moreover, important differences are observed in the obtained thresholds, showing that ID relationships are significantly dependent on the applied methodology.

Rainfall thresholds for landslide activity in Portugal: a state of the art

Rainfall is the most important physical process responsible for the landslide triggering in Portugal. Results obtained worldwide have shown that control of rainfall on landslides differs substantially depending upon landslide depth and kinematics and the affected material. Therefore, the critical rainfall conditions for failure are not the same for different types of landslides, and may be strongly influenced by regional geologic and geomorphologic conditions. Rapid debris flows are typically triggered by very intense showers concentrated in just a few hours, and shallow translational soil slips are usually triggered by intense precipitation falling within the 1–15 days long range. On the contrary, activity of deep-seated landslides of rotational, translational and complex types is related to periods of nearly constant rainfall, lasting from several weeks to several months. The different rainfall intensity–duration conditions are associated with different hydrologic mechanisms for slope failure. The generation of surface run-off and high peak discharges in first-order mountain catchments is a critical triggering mechanism for debris flows. The intense rainfall allows the rapid growth of pore water pressure and the drop of capillarity forces that sustain the apparent cohesion of thin soils. As a consequence, shallow soil slips occur within the soil material or at the contact with the underlying less permeable bedrock. Long lasting rainfall episodes enable the steady rise of the groundwater table and the development of positive pore water pressures into the soil. Consequently, deep-seated failures occur in relation to the reduction of shear strength of affected materials. In this work, we present the state of the art concerning the proposition of empirical rainfall thresholds in Portugal for different types of landslides observed in different zones of the country: the Lisbon region, the Douro Valley and the NW Mountains, and the Povoacao Municipality in Sao Miguel Island (Azores). The empirical thresholds applied in Portugal are based on the identification of past landslide events and include (i) the computation of antecedent rainfall threshold defined by linear regression, (ii) the normalization of rainfall by the mean annual precipitation, (iii) the definition of lower limit and upper limit rainfall thresholds and (iv) the definition of combined rainfall thresholds, which integrates the rainfall event and the antecedent rainfall for different time periods.

Effects of Long-term Organic Material Amendments on Soil Properties and Corn Yield in Rainfed Area of Thailand

A long-term field experiment on organic material application has been conducted since 1980 at Phraphuttabat Technical Service Center, Lop Buri Province, Thailand, to clarify the effectiveness of organic materials on chemical and physical properties of soil and yield of corn. Mimosa was the most effective crop for increasing organic matter content of soil. Total and Bray II extractable phosphorus content markedly increased in city compost plots. Rice straw showed significant effect on increasing total and exchangeable potassium in soils. Total Ca, Mg, Cu, Zn, extractable Cu, Zn and exchangeable Ca contents distinctively increased in city compost plots. Extractable Fe content significantly increased in the mimosa plot. Average yield of corn with and without fertilizer from 2000 to 2005 showed that the use of crotalaria, mimosa, rice bean and city compost was beneficial to increase the production of corn yield for one, two, four and three years out of a six year experimental duration, respectively. The 5-year moving average of corn yield for 26 years showed the following trends. Corn yield in the mimosa plot without fertilizer increased but the increase was relatively low. The corn yields in the rice bean plot were relatively high during 1982-1986 and 1999-2003 in both fertilizer plots. The yield of corn in crotalaria plots with and without fertilizer was at a relatively low level from 1980 to 1988, and increased from 1989 to 2005. This long-term experiment suggests that application of green manure improved soil properties. Competition of green manure with corn resulted in a relatively low yield of corn and on the contrary, without the competition of green manure corn yields were relatively high. Rice straw mulch with fertilizer was effective in maintaining corn yield under critical rainfall years, while rice straw mulch without fertilizer did not increase the corn yield like that of the fertilizer plot under critical rainfall conditions. Although city compost application was effective in increasing corn yield, precautions against heavy metal contamination should be taken into consideration.

Analysing the Relationship Between Typhoon‐Triggered Landslides and Critical Rainfall Conditions

AbstractMany landslides are triggered by rainfall. Previous studies of the relationship between landslides and rainfall have concentrated on deriving minimum rainfall thresholds that are likely to trigger landslides. Though useful, these minimum thresholds derived from a log–log plot do not offer any measure of confidence in a landslide monitoring or warning system. This study presents a new and innovative method for incorporating rainfall into landslide modelling and prediction. The method involves three steps: compiling radar reflectivity data in a QPESUMS (quantitative precipitation estimation and segregation using multiple sensors) system during a typhoon (tropical hurricane) event, estimating rainfall from radar data and using rainfall intensity and rainfall duration as explanatory variables to develop a landslide logit model. Given the logit model, this paper discusses ways in which the model can be used for computing probabilities of landslide occurrence for a real‐time monitoring system or a warning system, and for delineating and mapping landslides. Copyright © 2007 John Wiley & Sons, Ltd.

Modelling of the initiation of rainfall-induced debris flows in the Cardoso basin (Apuan Alps, Italy)

A case study is being carried out in relation to the event of June 19, 1996, when an intense rainstorm hit the Cardoso area in the Apuan Alps, Italy. The catastrophe triggered several gravitational movements, mainly characterized by soil slip-debris flows. A dataset coming from field and laboratory surveys allowed a complete parameterization of the event, and consequently the realization of a physically based dynamic model related to the initiation of the rainfall-induced landslides. The aim of the study is to analyze the processes involved in the light of the modelling results and to assess the spatio-temporal activity of the landslides, both in terms of location of the triggering sites and of determination of the critical rainfall conditions. The critical rainfall input thresholds, obtained with a deterministic approach, are then compared with the thresholds obtained by an empirical analysis of the main rainfall events occurred in the southern Apuan area in the period from 1975 to 2002.