Academic literature on the topic 'Debris-flow rainfall thresholds'

Abstract. Debris flows are natural disasters that frequently occur in mountainous areas, usually accompanied by serious loss of lives and properties. One of the most commonly used approaches to mitigate the risk associated with debris flows is the implementation of early warning systems based on well-calibrated rainfall thresholds. However, many mountainous areas have little data regarding rainfall and hazards, especially in debris-flow-forming regions. Therefore, the traditional statistical analysis method that determines the empirical relationship between rainstorms and debris flow events cannot be effectively used to calculate reliable rainfall thresholds in these areas. After the severe Wenchuan earthquake, there were plenty of deposits deposited in the gullies, which resulted in several debris flow events. The triggering rainfall threshold has decreased obviously. To get a reliable and accurate rainfall threshold and improve the accuracy of debris flow early warning, this paper developed a quantitative method, which is suitable for debris flow triggering mechanisms in meizoseismal areas, to identify rainfall threshold for debris flow early warning in areas with a scarcity of data based on the initiation mechanism of hydraulic-driven debris flow. First, we studied the characteristics of the study area, including meteorology, hydrology, topography and physical characteristics of the loose solid materials. Then, the rainfall threshold was calculated by the initiation mechanism of the hydraulic debris flow. The comparison with other models and with alternate configurations demonstrates that the proposed rainfall threshold curve is a function of the antecedent precipitation index (API) and 1 h rainfall. To test the proposed method, we selected the Guojuanyan gully, a typical debris flow valley that during the 2008–2013 period experienced several debris flow events, located in the meizoseismal areas of the Wenchuan earthquake, as a case study. The comparison with other threshold models and configurations shows that the selected approach is the most promising starting point for further studies on debris flow early warning systems in areas with a scarcity of data.

Climate change has increased the amount and intensity of rainfall, increasing debris flow risk. Debris flow damage has become a social issue in Korea, and the need for research related to rainfall thresholds for debris flow has increased. In this study, an automatic calculation algorithm was developed to derive criteria suitable for the Korean region for the rainfall thresholds of debris flow. The study area selected was Gangwon-do, where a significant amount of debris flow occurred recently, and information on rainfall and the occurrence of debris flow were collected. As a result of applying an algorithm based on the R programming language, an equation for the relationship between the rainfall intensity and duration in Gangwon-do was automatically derived using debris flow occurrence point information and meteorological rainfall information.

Abstract. Debris-flows occurring in the area of Cancia (Dolomites, Northeastern Italy) in recent years have exposed the population to serious risk. In response to the recurring hazard, an alarm and monitoring system was installed to provide a sufficient level of safeguard for inhabitants and infrastructures. The data recorded at three rain gauges during debris-flow events has been analysed, taking into consideration the different elevation of the gauges to delineate the storm rainfall distributions. Rainfall data is compared with the occurrence of debris-flows to examine relations between debris-flow initiation and rainfall. In addition, the data is compared with that recorded during debris-flows which occurred under similar or different geological settings in the Eastern Italian Alps, in order to define triggering thresholds. A threshold for debris-flow activity in terms of mean intensity, duration and mean annual precipitation (M.A.P.) is defined for the study area The normalised rainfall and the normalised intensity are expressed as a per cent with respect to M.A.P. This threshold is compared with thresholds proposed by other authors, and the comparison shows that a lower value is obtained, indicating the debris-flow susceptibility of the area. The threshold equations are: R/M.A.P. = - 1.36 · ln(I) + 3.93 where I > 2 mm/h I /M.A.P. = 0.74 · D-0.56. The determination of a debris-flow threshold is linked to the necessity of a fast decisional phase in a warning system for debris-flow protection. This threshold cannot be used as a predictive tool, but rather as a warning signal for technicians who manage the monitoring/warning system.

Abstract. The prediction of debris flows is relevant because this type of natural hazard can pose a threat to humans and infrastructure. Debris-flow (and landslide) early warning systems often rely on rainfall intensity–duration (ID) thresholds. Multiple competing methods exist for the determination of such ID thresholds but have not been objectively and thoroughly compared at multiple scales, and a validation and uncertainty assessment is often missing in their formulation. As a consequence, updating, interpreting, generalizing and comparing rainfall thresholds is challenging. Using a 17-year record of rainfall and 67 debris flows in a Swiss Alpine catchment (Illgraben), we determined ID thresholds and associated uncertainties as a function of record duration. Furthermore, we compared two methods for rainfall definition based on linear regression and/or true-skill-statistic maximization. The main difference between these approaches and the well-known frequentist method is that non-triggering rainfall events were also considered for obtaining ID-threshold parameters. Depending on the method applied, the ID-threshold parameters and their uncertainties differed significantly. We found that 25 debris flows are sufficient to constrain uncertainties in ID-threshold parameters to ±30 % for our study site. We further demonstrated the change in predictive performance of the two methods if a regional landslide data set with a regional rainfall product was used instead of a local one with local rainfall measurements. Hence, an important finding is that the ideal method for ID-threshold determination depends on the available landslide and rainfall data sets. Furthermore, for the local data set we tested if the ID-threshold performance can be increased by considering other rainfall properties (e.g. antecedent rainfall, maximum intensity) in a multivariate statistical learning algorithm based on decision trees (random forest). The highest predictive power was reached when the peak 30 min rainfall intensity was added to the ID variables, while no improvement was achieved by considering antecedent rainfall for debris-flow predictions in Illgraben. Although the increase in predictive performance with the random forest model over the classical ID threshold was small, such a framework could be valuable for future studies if more predictors are available from measured or modelled data.

Abstract. The systematic underestimation observed in debris flow early warning thresholds has been associated with the use of sparse rain gauge networks to represent highly non-stationary rainfall fields. Remote sensing products permit concurrent estimates of debris-flow-triggering rainfall for areas poorly covered by rain gauges, but the impact of using coarse spatial resolutions to represent such rainfall fields is still to be assessed. This study uses fine-resolution radar data for ∼ 100 debris flows in the eastern Italian Alps to (i) quantify the effect of spatial aggregation (1–20 km grid size) on the estimation of debris-flow-triggering rainfall and on the identification of early warning thresholds and (ii) compare thresholds derived from aggregated estimates and rain gauge networks of different densities. The impact of spatial aggregation is influenced by the spatial organization of rainfall and by its dependence on the severity of the triggering rainfall. Thresholds from aggregated estimates show 8–21 % variation in the parameters whereas 10–25 % systematic variation results from the use of rain gauge networks, even for densities as high as 1∕10 km−2.

Abstract. In this paper, we examine variations in climate characteristics near the area of Cortina d'Ampezzo (Dolomites, Eastern Italian Alps), with particular reference to the possible implications for debris-flow occurrence. The study area is prone to debris-flow release in response to summer high-intensity short-duration rainfalls and, therefore, it is of the utmost importance to investigate the potential increase in debris-flow triggering rainfall events. The critical rainfall threshold is agreed to be a crucial triggering factor for debris-flows. Data from a monitoring system, placed in a catchment near Cortina (Acquabona), show that debris-flows were triggered by rainfalls with peak rainfall intensities ranging from 4.9 to 17.4 mm/10 min. The analyses of meteorological data, collected from 1921 to 1994 at several stations in the study area, show a negative trend of annual rainfall, a considerable variation in the monthly rainfall distribution, and an increase in the temperature range, possibly related to global climate changes. Moreover, high-intensity and short-duration rainfall events, derived from data collected from 1990 and 2008, show an increase in exceptional rainfall events. The results obtained in a peak-over-threshold framework, applied to the rainfall data measured at the Faloria rain gauge station from 1990 to 2008, clearly show that the interarrival time of over-threshold events computed for different threshold values decreased in the last decade. This suggests that local climatic changes might produce an increase in the frequency of rainfall events, potentially triggering debris flows in the study area.

ABSTRACT The extreme heat from wildfire alters soil properties and incinerates vegetation, leading to changes in infiltration capacity, ground cover, soil erodibility, and rainfall interception. These changes promote elevated rates of runoff and sediment transport that increase the likelihood of runoff-generated debris flows. Debris flows are most common in the year immediately following wildfire, but temporal changes in the likelihood and magnitude of debris flows following wildfire are not well constrained. In this study, we combine measurements of soil-hydraulic properties with vegetation survey data and numerical modeling to understand how debris-flow threats are likely to change in steep, burned watersheds during the first 3 years of recovery. We focus on documenting recovery following the 2016 Fish Fire in the San Gabriel Mountains, California, and demonstrate how a numerical model can be used to predict temporal changes in debris-flow properties and initiation thresholds. Numerical modeling suggests that the 15-minute intensity-duration (ID) threshold for debris flows in post-fire year 1 can vary from 15 to 30 mm/hr, depending on how rainfall is temporally distributed within a storm. Simulations further demonstrate that expected debris-flow volumes would be reduced by more than a factor of three following 1 year of recovery and that the 15-minute rainfall ID threshold would increase from 15 to 30 mm/hr to greater than 60 mm/hr by post-fire year 3. These results provide constraints on debris-flow thresholds within the San Gabriel Mountains and highlight the importance of considering local rainfall characteristics when using numerical models to assess debris-flow and flood potential.

Rainfall is the main factor that induces debris flow. Satellite rainfall products provide a new source of data in terms of debris flow-triggering conditions to overcome the lack of rainfall data coverage from ground-based rainfall gauges in large-scale mountainous regions. In this study, the applicability of four satellite rainfall products (CMORPH, GPM, MSWEP, and PERSIANN) in the Hengduan Mountain region (HMR) was evaluated with reference to ground observation data from 2000 to 2020. The critical rainfall and rainfall thresholds under different rainfall patterns and warning levels that trigger debris flows were analyzed according to the empirical cumulative distribution function (ECDF) and cumulative probability. The results showed that CMORPH (comprehensive indicator score (CI = 0.72) and GPM (CI = 0.70) performed better in the simulation of daily rainfall sequence consistency and extreme rainfall conditions in the study area. CMORPH also had the highest reconstruction rate for correctly capturing rainfall events that triggered debris flows, with a value of 89%. Approximately half of the rainfall patterns that cause debris flows are antecedent-effective-rainfall-dominated. Both intraday-rainfall-dominated and intraday-antecedent-rainfall-balanced patterns were below 30%. There were evident differences in the critical rainfall for different rainfall patterns under the same warning level. By comparing the results of previous studies on rainfall thresholds, it is believed that the results of this study confirm the application of satellite rainfall products; in addition, the calculated rainfall thresholds can provide a reference for the early warning of debris flows in the HMR. In general, this work is of great significance to the prediction and early warning of debris flow hazards.

Abstract Rainfall thresholds are often used in early warning systems to identify rainfall conditions that, when reached or exceeded, are likely to result in debris flows. Rain gauges are typically used for the definition of these thresholds. However, in mountainous areas in situ observations are often sparse or nonexistent. Satellite-based rainfall estimates offer a solution to overcome the coverage problem at the global scale but are associated with significant estimation uncertainty. Evaluating satellite-based rainfall thresholds is thus necessary to understand their potential and limitations. In this work, an intercomparison among satellite-based precipitation products is presented in the context of estimating rainfall thresholds for debris flow prediction. The study is performed for the upper Adige River basin in the eastern Italian Alps during 2000–10. Large differences are observed between event-based characteristics (event duration and magnitude) derived from rain gauge and satellite-based estimates, revealing considerable interproduct variability in the debris flow–triggering rainfall characteristics. The parameters of the satellite-based thresholds differ less than 30% from the corresponding rain gauge–based parameters. Results further suggest that the adjustment of satellite-based estimates (either gauge based or by applying an error model) together with spatial resolution has an important impact on the estimation of the accumulation–duration thresholds.

Abstract. Debris flows, triggered by extreme precipitation events and rapid snow melt, cause considerable damage to the Norwegian infrastructure every year. To define intensity-duration (ID) thresholds for debris flow initiation critical water supply conditions arising from intensive rainfall or snow melt were assessed on the basis of daily hydro-meteorological information for 502 documented debris flow events. Two threshold types were computed: one based on absolute ID relationships and one using ID relationships normalized by the local precipitation day normal (PDN). For each threshold type, minimum, medium and maximum threshold values were defined by fitting power law curves along the 10th, 50th and 90th percentiles of the data population. Depending on the duration of the event, the absolute threshold intensities needed for debris flow initiation vary between 15 and 107 mm day−1. Since the PDN changes locally, the normalized thresholds show spatial variations. Depending on location, duration and threshold level, the normalized threshold intensities vary between 6 and 250 mm day−1. The thresholds obtained were used for a frequency analysis of over-threshold events giving an estimation of the exceedance probability and thus potential for debris flow events in different parts of Norway. The absolute thresholds are most often exceeded along the west coast, while the normalized thresholds are most frequently exceeded on the west-facing slopes of the Norwegian mountain ranges. The minimum thresholds derived in this study are in the range of other thresholds obtained for regions with a climate comparable to Norway. Statistics reveal that the normalized threshold is more reliable than the absolute threshold as the former shows no spatial clustering of debris flows related to water supply events captured by the threshold.

Rainfall triggered shallow slides and debris flows constitute a significant hazard that causes substantial economic losses and fatalities worldwide. Regional-scale risk mitigation for these processes is challenging. Therefore, landslide early warning systems (LEWS) are a helpful tool to depict the time and location of possible landslide events so that the hazardous situation can be managed more effectively. The main objective of this thesis is to set up a regional-scale LEWS that works in real-time over Catalonia (NE Spain). The developed warning system combines in real-time susceptibility information and rainfall observations to issue qualitative warnings over the region. Susceptibility has been derived combining slope angle and land use and land cover information with a simple fuzzy logic approach. The LEWS input rainfall information consists of high-resolution radar quantitative precipitation estimates (QPEs). To assess if a rainfall situation has the potential to trigger landslides, the LEWS applies a set of intensity duration thresholds. Finally, a warning matrix combines susceptibility and rainfall hazard to obtain a qualitative warning map that classifies the terrain into four warning classes. The evaluation of the LEWS performance has been challenging because of the lack of a systematic inventory, including the time and location of recent landslides events. Within the context of this thesis, a citizen-science initiative has been set up to gather landslide data from reports in social networks. However, some of the reports have significant spatial and temporal uncertainties. With the aim of finding the most suitable mapping unit for real-time warning purposes, the LEWS has been set-up to work using susceptibility maps based on grid-cells of different resolutions and subbasins. 30 m grid-cells have been chosen to compute the warnings as they offer a compromise between performance, interpretability of the results and computational costs. However, from an end users’ perspective visualising 30 m resolution warnings at a regional scale might be difficult. Therefore, subbasins have been proposed as a good option to summarise the warning outputs. A fuzzy verification method has been applied to evaluate the LEWS performance. Generally, the LEWS has been able to issue warnings in the areas where landslides were reported. The results of the fuzzy verification suggest that the LEWS effective resolution is around 1 km. The initial version of the LEWS has been improved by including soil moisture information in the characterisation of the rainfall situation. The outputs of this new approach have been compared with the outputs of LEWS using intensity-duration thresholds. With the new rainfall-soil moisture hydrometeorological thresholds, fewer false alarms were issued in high susceptibility areas where landslides had been observed. Therefore, hydrometeorological thresholds may be useful to improve the LEWS performance. This study provided a significant contribution to regional-scale landslide emergency management and risk mitigation in Catalonia. In addition, the modularity of the proposed LEWS makes it easy to apply in other regions.
Els lliscaments superficials i els corrents d’arrossegalls són un fenomen perillós que causa significants perdudes econòmiques i humanes arreu del món. La seva principal causa desencadenant és la pluja. La mitigació del risc degut a aquets processos a escala regional no es senzilla. Ena quest context, els sistemes d’alerta són una eina útil per tal de predir el lloc i el moment en que es poden desencadenar possibles esllavissades en el futur, i poder fer una gestió del risc més eficient. L’objectiu principal d’aquesta tesi és el desenvolupament d’un sistema d’alerta per esllavissades a escala regional, que treballi en temps real a Catalunya. El Sistema d’alerta que s’ha desenvolupat combina informació sobre la susceptibilitat del terreny i estimacions de la pluja d’alta resolució per donar unes alertes qualitatives arreu del territori. La susceptibilitat s’ha obtingut a partir de la combinació d’informació del pendent del terreny, i els usos i les cobertes del sòl utilitzant un mètode de lògica difusa. Les dades de pluja són observacions del radar meteorològic. Per tal d’analitzar si un determinat episodi de pluja te el potencial per desencadenar esllavissades, el sistema d’alerta utilitza un joc de llindars intensitat-durada. Posteriorment, una matriu d’alertes combina la susceptibilitat i la magnitud del episodi de pluja. El resultat, és un mapa d’alertes que classifica el terreny en quatre nivells d’alerta. Amb l’objectiu de definir quina unitat del terreny és la més adient pel càlcul de les alertes en temps real, el sistema d’alerta s’ha configurat per treballar utilitzant mapes de susceptibilitat basats en píxels de diverses resolucions, i en subconques. Finalment, l’opció més convenient és utilitzar píxels de 30 m, ja que ofereixen un compromís entre el funcionament, la facilitat d’interpretació dels resultats i el cost computacional. Tot i això, la visualització de les alertes a escala regional emprant píxels de 30 m pot ser difícil. Per això s’ha proposat utilitzar subconques per oferir un sumari de les alertes. Degut a la manca d’un inventari d’esllavissades sistemàtic, que contingui informació sobre el lloc i el moment en que les esllavissades es van desencadenar, l’avaluació del funcionament del sistema d’alerta ha sigut un repte. En el context d’aquesta tesi, s’ha creat una iniciativa per tal de recol·lectar dades d’esllavissades a partir de posts en xarxes socials. Malauradament, algunes d’aquestes dades estan afectades per incerteses espacials i temporals força importants. Per a l’avaluació el funcionament del sistema d’alerta, s’ha aplicat un mètode de verificació difusa. Generalment, els sistema d’alerta ha estat capaç de generar alertes a les zones on s’havien reportat esllavissades. Els resultats de la verificació difusa suggereixen que la resolució efectiva del sistema d’alerta età al voltant d’1 km. Finalment, la versió inicial del sistema d’alerta s’ha millorat per tal poder incloure informació sobre l’estat d’humitat del terreny en la caracterització de la magnitud del episodi de pluja. Els resultats del sistema d’alerta utilitzant aquest nou enfoc s’han comparat amb els resultats que s’obtenen al córrer el sistema d’alerta utilitzant els llindars intensitat-durada. Mitjançant els nous llindars hidrometeorològics, el sistema emet menys falses alarmes als llocs on s’han desencadenat esllavissades. Per tant, utilitzar llindars hidrometeorològics podria ser útil per millorar el funcionament del sistema d’alerta dissenyat. L’estudi dut a terme en aquesta tesi suposa una important contribució que pot ajudar en la gestió de les emergències degudes a esllavissades a escala regional a Catalunya. A més a més, el fet de que el sistema sigui modular permet la seva fàcil aplicació en d’altres regions en un futur.
Enginyeria del terreny

Hydrogeological hazards are quite diffuse rainfall-induced phenomena that affect mountain regions and can severely impact these territories, producing damages and sometimes casualties. For this reason, hydrogeological risk reduction is crucial. Mitigation strategies aim to reduce hydrogeological risk to an acceptable level and can be classified into structural and non-structural measures. This work focuses on enhancing some non-structural risk mitigation measures for mountain areas: debris-flow rainfall thresholds, as a part of an Early Warning System (EWS), multivariate rainfall scenarios with multi-hazard mapping purpose and public awareness. Regarding debris-flow rainfall thresholds, an innovative calibration method, a suitable uncertainty analysis and a proper validation process are developed. The Backward Dynamical Approach (BDA), a physical-based calibration method, is introduced and a threshold is obtained for a study area. The BDA robustness is then tested by assessing the uncertainty in the threshold estimate. Finally, the calibrated threshold's reliability and its possible forecast use are assessed using a proper validation process. The findings set the stage for using the BDA approach to calibrate debris-flow rainfall thresholds usable in operational EWS. Regarding hazard mapping, a multivariate statistical model is developed to construct multivariate rainfall scenarios with a multi-hazards mapping purpose. A confluence between a debris-flow-prone creek and a flood-prone river is considered. The multivariate statistical model is built by combining the Simplified Metastatistical Extreme Value approach and a copula approach. The obtained rainfall scenarios are promising to be used to build multi-hazard maps. Finally, the public awareness within the LIFE FRANCA (Flood Risk ANticipation and Communication in the Alps) European project is briefly considered. The project action considered in this work focuses on training and communication activities aimed at providing a multidisciplinary view of hydrogeological risk through the holding of courses and seminars.

The present work analyzes the hydrologic conditions leading to the triggering of debris flows in an alpine region. The overall analysis has been split in two parts: the first part of the analysis has been carried out at a regional and decadal scale to improve our knowledge of rainfall thresholds for debris-flow occurrences, of the uncertainty related to rainfall estimation at debris-flow initiation sites, and of the main morphometric characteristics of debris-flow triggering locations; in the second part a focus has been devoted to the analysis of the hydrologic response of some watersheds for a selection of events with the help of detailed input information, both topographic and hydrological. The study area is upper Adige River basin (Northern Italy), which occupies a distinctive hydrometeorological niche, characterized by high frequency of orographic thunderstorms. The first part of the thesis analyzes the debris-flow triggering issue at regional and decadal-scale (2000-2010). A morphometric characterization of debris-flow triggering locations has been carried out in terms of slope-area analysis. The characterization was carried out dividing considered debris flows in classes of rainfall duration, rainfall intensity and considering also the potential relation with the different geologic settings. Another issue under investigation is related to rainfall threshold estimation (from raingauges network) for debris-flow occurrence and the uncertainty related to this estimation. First, a set of rainfall thresholds has been derived adopting a frequentist approach. The rainfall estimation procedure used for threshold identification was based on two major sources: records from the nearest gauge and an Inverse Distance interpolation of all the available records at regional and decadal scale. Thresholds have been analyzed considering all the available points and also trying to group debris-flow occurrences according to seasonal occurrence, different storm characteristics and homogeneous geologic setting. The level of uncertainty related to rainfall threshold estimation has been analyzed in detail The estimation problem is especially severe for the location of the debris flows, with raingauges that are commonly located at low elevation (e.g., in the valley floors) and debris flows that originate at high elevations, in the head part of the mountain catchments. The rainfall estimation procedure used for the uncertainty assessment, is the one based on the nearest raingauge, which is used as a proxy to estimate the rainfall that has resulted in debris flows. The objective is to investigate the impact of the uncertainty inherent in the estimation of rainfall on the definition of a threshold for possible debris-flow occurrence, and on the operational use of the threshold for prediction purposes. These effects are likely to depend on two factors: the space-time rainfall variability and the distances between the debris-flow locations and the raingauges on both the horizontal and the vertical planes. Three main problems are considered: (i) the effect of the rainfall sampling problem on the estimation of the parameters of the threshold model, (ii) the effect of applying a number of procedures to filter the rainfall information on the threshold model estimation, and (iii) the examination of the impact of the precipitation sampling problem on the performance of the threshold as a predictor of debris-flow occurrence. These aspects are examined here based on a simulation experiment. The methodology examines the intensity-duration thresholds derived from a set of raingauge locations that is assumed to represent debris flow/landslide points (DFR) and an equivalent set of raingauges assumed to have the role of closest available measurement (MR). A set of reference rainfall thresholds is used to identify the rainfall events at DFR that “triggered debris flows (i.e. exceed the threshold). For these same events, the corresponding rainfall thresholds are derived from MR observations. Comparison between the rainfall thresholds derived from DFR and MR, revealed that uncertainty in rainfall estimation has a major impact on estimated intensity-duration thresholds. Specifically, results showed that thresholds estimated from MR observations are consistently underestimated. Evaluation of the estimated thresholds for warning procedures, showed that while detection is high, the main issue is the high false alarm ratio, which limits the overall accuracy of the procedure. Overall performance on debris-flow prediction was shown to be good for low rainfall thresholds and poor for high rainfall thresholds examined. Finally different interpolation techniques have been applied to a set of gauges close to the DFR to assess the eventual improvement brought by the interpolation procedure. Results on this showed that interpolation can improve estimates specifically in the case of poor DFR-MR correlation, while in the other cases, especially considering intense and/or localized storms, the benefits of considering interpolated value in respect to only one near gauge is not so significant. In the second part of the thesis the hydrometeorological and hydrological controls of these events are examined through analyses of three storm systems occurred on October 3-4, 2006, June 20-21 2007 and September 3-4, 2009. The first storm system generated a moderate flash flood along the main streams with almost no debris flows in the tributaries, the second triggered a large number of debris flows and was characterized by a minor runoff response for the major streams, and the third resulted in both a relevant flash flood response and widespread debris flows. These events have been examined by using per-event calibrated radar rainfall data, providing an important input for the evaluation of a distributed hydrological model, and by using a database reporting location and information related to the events. Raingauge and streamflow data, debris flow spatial information and observations are used along with the distributed model to analyze the hydrological and geomorphic responses to these rainstorms. The striking contrast in flood and geomorphic responses between the three floods is related to contrasts in the antecedent moisture conditions and in the space-time structure of the triggering storm. Rainfall-runoff modeling has permitted estimating discharge and runoff volumes in a number of small ungauged catchments in which debris-flow volumes had been assessed by means of post-event surveys. The computation of sediment concentration by volume using model-estimated water runoff has resulted in realistic values. The relations between water peak discharge and debris-flow volumes show higher correlation coefficients than those involving morphometric variables and rainfall amounts.
Il presente lavoro ha analizzato le condizioni idrologiche associate all’innesco di colate detritiche in una regione alpina. L’analisi è stata suddivisa in due rami principali: la prima parte è stata effettuata a scala regionale e decennale (2000-2010) per approfondire le tematica delle soglie pluviometriche per l’innesco di colate e delle incertezze legate alla determinazione della pioggia nelle zone di innesco delle colate detritiche. Si è cercato, inoltre, di caratterizzare, dal punto di vista morfometrico, le zone di innesco delle colate stesse. Uno studio approfondito è stato dedicato successivamente all’analisi della risposta idrologica di alcuni bacini, per tre eventi di piena avvenuti fra il 2006 ed il 2009. L’area di studio è l’intera Provincia Autonoma di Bolzano (Alto Adige, Nord Italia). Tale area occupa una nicchia idrometereologica peculiare, caratterizzata da un’elevata frequenza di fenomeni temporaleschi con forzante orografica, che, specialmente nel periodo estivo, possono attivare colate detritiche e piene improvvise. La caratterizzazione morfometrica dei siti di innesco è stata effettuata esaminando l’area contribuente e la pendenza locale dei punti stessi. La caratterizzazione è stata effettuata dividendo il dataset in classi di durata di precipitazione, introducendo ulteriori filtri sull’intensità di pioggia e considerando anche la relazione con la litologia. Un’altra questione in esame è legata alla stima di soglie pluviometriche per l’innesco di colate detritiche unitamente all’incertezza legata a questa stima. In primo luogo, un insieme di soglie pluviometriche è stato derivato mediante un approccio frequentista. La procedura di stima di precipitazione per l’identificazione della soglia è basata su due fattori principali: dato del pluviometro più vicino e una interpolazione (Inverse Distance) di tutti i dati orari disponibili a scala regionale e decennale. Le soglie sono state analizzate prendendo in considerazione tutti i punti disponibili ed anche cercando di raggrup-pare i fenomeni di colata in base alla stagionalità, alle diverse caratteristiche dell’evento meteorico ed in base a zone geologicamente omogenee. Il livello di incertezza relativo alla stima di soglie di pioggia è stato analizzato in dettaglio. Il problema della stima di risulta essere particolarmente importante proprio a causa della posizione delle zone di innesco di colate detritiche, con pluviometri che sono comunemente situati a bassa quota (e.g., nei fondovalle) e colate detritiche che hanno origine ad altitudini elevate, nella parte di testata di bacini montani. La procedura di stima di precipitazioni adottata per la valutazione dell’incertezza, è quella basata sull’utilizzo del pluviometro più vicino, che viene considerato come proxy per stimare la pioggia innescante sul sito di colata. L’obiettivo è quello di studiare l’impatto dell’incertezza insita nella stima delle precipitazioni sulla definizione di una soglia per l’innesco di colate detritiche, e l’uso operativo della soglia per scopi di previsione. Due sono le condizioni che maggiormente influenzano l’icertezza di stima: la variabilità spazio-temporale delle precipitazioni e le distanze tra le zone d’innesco ed i relativi pluviometri più prossimi sia sul piano orizzontale che sul piano verticale. Tre sono gli effetti principali considerati: (i) l’effetto del campionamento delle precipitazioni sulla stima dei parametri del modello di soglia, (ii) l’effetto dell’applicazione di una serie di procedure per filtrare le informazioni di precipitazione sul modello di stima della soglia, e (iii) l’incidenza del campionamento della precipitazione sulle performance della soglia come predittore di accadimento di colate. Questi aspetti vengono esaminati mediante una simulazione condotta a scala regionale. La metodologia adottata esamina le soglie di intensità-durata mediante la selezione di una serie di pluviometri che si assumono rappresentare un sito di innesco di colata, denominati DFR, ed una serie di pluviometri prossimi a questi (in rapporto 1:1), denominati MR, che vengono usati per stimare la precipitazione sul sito DFR. Una serie di soglie pluviometriche di riferimento viene utilizzate per identificare gli eventi di precipitazione che “innescano” colate de- tritiche su DFR (cio` eventi che superano la soglia). Per questi stessi eventi, le corrispondenti soglie pluviometriche sono derivate da osservazioni MR. Il confronto tra le soglie pluviometriche derivate da DFR e MR, ha rivelato che l’incertezza nella stima di precipitazione ha un impatto importante sulle soglie di intensità-durata. In particolare, i risultati hanno mostrato che le soglie stimate dalle osservazioni MR presentano stime in difetto. La valutazione delle soglie stimate per procedure di allarme, ha mostrato che, mentre la probabilità di rilevamento è alta, il problema principale è l’elevato rapporto di falsi allarmi, che limita la precisione complessiva del procedimento. Le performance generali sulla previsione di colate detritiche si sono dimostrate buone per soglie pluviometriche moderate e scarse per soglie elevate. Infine, diverse tecniche di interpolazione sono state applicate ad una selezione di pluviometri prossimi al DFR per valutare l’eventuale miglioramento portato dagli interpolatori. I risultati hanno dimostrato che l’interpolazione può migliorare le stime, in particolare i miglioramenti più significativi si sono notati nei casi di correlazione debole tra DFR e MR, mentre per gli altri casi ed in particolare considerando eventi intensi e/o localizzati, i benefici derivanti dal considerare i valori interpolati rispetto al valore del pluviometro più prossimo non sono così significativi. Nella seconda parte della tesi viene esaminata la forzante idrometeorologica ed idrologica relativa all’innesco di colate, attraverso l’analisi dei tre eventi meteorici avvenuti il 3-4 ottobre 2006, 20-21 giugno 2007 e 3-4 Settembre 2009 per capire come la variabilità meteorologica si rifletta nelle diverse risposte dei bacini. Il primo evento ha generato infatti piene improvvise di modesta intensità lungo le aste torrentizie principali con quasi nessuna colata registrata nei corsi d’acqua secondari, il secondo ha innescato un gran numero di colate in bacini secondari ed è stato caratterizzato da una risposta idrica minore nei collettori principali, il terzo ha visto sia piene improvvise importanti che colate detritiche diffuse. Tali eventi sono stati studiati utilizzando dati di pioggia calibrati da radar sul singolo evento, fornendo un input importante per la calibrazione di un modello idrologico distribuito, ed utilizzando inoltre informazioni derivanti dall’analisi di un ampio database di fenomeni di instabilità (incluse le colate detritiche) che copre l’intero territorio provinciale e che viene mantenuto costantemente aggiornato. L’integrazione di dati radar e da rete pluviometrica, dati di portata, informazioni spaziali e volumetriche sugli eventi di colata assieme ad osservazioni sul campo sono usati, unitamente al modello idrologico distribuito, per analizzare le risposte idrologiche e geomorfologiche agli eventi innescanti. Il forte contrasto in termini di risposta idrica ed effetti morfologici fra le tre piene risulta legato a differenze nelle condizioni di umidità antecedenti e nella struttura spazio-temporale degli eventi innescanti. La modellazione afflussi-deflussi ha permesso la stima e di portate al picco e di volumi di deflusso in una serie di piccoli bacini non strumentati in cui i volumi depositati dalle colate sono stati stimati mediante indagini post-evento. Il calcolo della concentrazione volumetrica di sedimento eseguito utilizzando i risultati dell’approccio di modellazione scalato ai bacini da colata ha portato a valori realistici. Le relazioni tra portate al picco e volumi mobilizzati dalle colate mostrano coefficienti di correlazione più elevati di quelli che considerano le variabili morfometriche ed i volumi di precipitazione.

The main subject of the present thesis is the triggering of debris flows, which has been studied from a geomorphological and from a hydrological point of view. The thesis is a compilation of three papers, each one focused on a specific part of the triggering mechanism. In fact, debris flows are dangerous events, typical of mountain territories all over the world and they necessitate the concurrence of many variables to be triggered. In the last decades, different studies stated that the main variables involved in the initiation of a debris flow are: terrain slope, water input and sediment availability. If these factors exceed some specific critical thresholds, the probability of a debris flow triggering can be very high. However, the fact that so many variables are involved makes these events difficult to forecast. In the present study, we analysed different aspects related to the triggering probability. For this reason, the thesis is composed of three different papers: (1) Chapter 2, titled “On the criteria to create a susceptibility map to debris flow at a regional scale”; (2) Chapter 3, titled “Correlation between the rainfall, sediment recharge and triggering of torrential flows in the Rebaixader catchment (Pyrenees, Spain); (3) Chapter 4, titled “Rainfall durations and corresponding dominant mechanism for the initiation of debris flows in three basins characterized by different geomorphological settings”. In Chapter 2, we analysed the geomorphological variables of the catchments that have the most important role in the triggering of debris flows. We used a model named Flow-R that works at a regional scale (allowing to analyse an entire valley) to study the potential triggering areas, neglecting the hydrological part of the mechanism. In fact, starting from a real case study (event of 4th August 2012), we used the collected data regarding the triggering and deposition areas to model the debris flow event. We analysed the morphological parameters that had better discriminate the potential triggering areas from the zones in which the erosion and slope failure are highly improbable. In Chapter 3, we studied the two triggering variables related to water input and sediment recharge. In this case, we focused on: (1) rainfall, investigating its influence on the triggering of debris flows and on the accumulation and mobilization of sediments; (2) sediment availability, considering the registered debris flow volumes as the previously available sediment quantity inside the triggering area, in the period before the triggering of the event. The analysed catchment is in the Spanish Pyrenees and it is a good case study because since summer 2009 it has been equipped with a monitoring station (rain gauges, piezometers, video cameras, geo-phones). Therefore, the rainfall datasets registered with a time interval of 5 minutes inside the catchment are relatively long and allows to study potential correlations between the water input and the sediment mobilization. We searched if there are any correlations between the volume of the triggered event and the total precipitation of the recharge period (the period between the considered debris flow and the previous one). We then analysed the correlations between the maximum rainfall intensities of the triggering rainfall events and the volume of the debris flows. Finally, we used a parameter called “rainfall erosivity” that, calculated for each rainfall event, considers at the same time the total precipitation, the maximum intensity and the kinetic energy of the rain. This parameter has been calculated using two different time scales: (1) for the single triggering rainfall event; (2) as the sum of all the rainfall events happened during every recharge period. The results are interesting because it is clear that rainfall scarcely influences the quantity of the mobilized sediment, evidencing that there are other important variables involved in this mechanism. Whereas, the triggering or non-triggering of a debris flow is strongly dependent on the rainfall erosivity of the triggering rainfall event. After these two analyses, made at the regional scale and at the basin scale, in Chapter 4 we went more in the detail, focusing on the headwater basins (the upper parts of debris flow catchments). We analysed three study areas: (1) the headwater basin of Rio Rudan, located in the south side of Mount Antelao (Italian Dolomites); (2) the headwater basin of Rio Chiesa, located in the south side of Col di Lana (that is also located inside the dolomitic region, but it is characterized by a different geology, mainly composed of volcanic and sedimentary rocks); (3) the headwater basin of Rio Rebaixader, located in the Spanish Pyrenees (mainly composed of metamorphic rocks). In each headwater basin, we extracted three control cross sections along the channel network. For every cross section we calculated the critical triggering discharge for the debris flow initiation using the formulas of Gregoretti and Dalla Fontana (2008) and of Whittaker and Jaggi (1986). Then, we made some hydrological simulations using the software FLO-2D, using as water input different hyetographs created using the Intensity-Duration equations of Gregoretti and Dalla Fontana (2007) and of Cannon and Ellen (1985). These simulations allowed us to verify which is the minimum rainfall duration (related to the corresponding rainfall intensity) needed to reach the critical discharge in the control cross sections. To test also the possible shallow slope failure triggering mechanism, we also made a slope stability analysis in the three initiation areas, using the geotechnical parameters derived from the analysis of terrain samples collected inside the headwater basins. These two different analysis gave an overall view on the mechanism of mobilization of sediments in the analysed areas. The results show that the critical discharges in the three basins are comparable, whereas the slope stability analysis evidences some differences between the basins. In fact, Rio Rudan resulted generally more stable than the two other basins, even with high saturation conditions of the terrain. This can mean that in this basin the principal triggering mechanism is the “channel-bed failure”, whereas in the two other basins mechanisms of “shallow slope failure” are also probable.
La seguente tesi è stata sviluppata in forma compilativa, come raccolta di articoli. Il filo conduttore di tutto il manoscritto è l’analisi del fenomeno di innesco di colate detritiche. Questo tipo di eventi, tipico di zone montane di tutto il mondo, necessita della concomitanza di particolari fattori per poter accadere. Negli ultimi decenni, differenti studi a riguardo, hanno dimostrato che tra le principali variabili in gioco nel determinare l’innesco di una colata, ci sono: la pendenza del terreno, una sufficiente quantità d’acqua e una certa disponibilità di sedimento nell’area sorgente che possa venire mobilizzata. Questi fattori, quando concomitanti sopra ad una certa soglia limite, determinano una elevata probabilità di innesco di un fenomeno di colata detritica. Il fatto però che ci siano in gioco molte differenti variabili, rende questi meccanismi molto difficili da comprendere e predire con estrema esattezza. In questo studio, si è cercato di analizzare tutti gli aspetti legati alla probabilità di innesco per dare un quadro complessivo del fenomeno, prendendo in considerazione differenti variabili in differenti aree di studio. Per questo motivo la tesi è strutturata in tre parti distinte: ad un primo capitolo introduttivo in cui viene presentato il fenomeno di colata detritica nella sua interezza, segue il Capitolo 2, intitolato “On the criteria to create a susceptibility map to debris flow at a regional scale”. In questa parte della tesi, vengono analizzate le variabili geomorfologiche del terreno che incidono maggiormente nel possibile innesco di una colata detritica. Utilizzando un modello chiamato Flow-R che lavora a scala regionale (permettendo di analizzare un’intera vallata e non solamente singoli bacini), si è trascurata la parte idrologica del fenomeno concentrandosi sulla morfologia del terreno. Partendo infatti dai dati reali (dati di pioggia, volumi di colate, mappatura delle aree di innesco e delle aree di deposito) misurati durante e successivamente l’evento del 4 agosto 2012 che ha interessato l’intera Val di Vizze (Provincia di Bolzano) si è cercato di ricostruire nel modo più verosimile l’innesco e la propagazione di colate nel territorio analizzato, cercando di trovare i parametri morfologici che permettessero di discriminare accuratamente le possibili aree sorgenti dalle zone in cui invece l’erosione e l’innesco sono altamente improbabili. Proseguendo con il Capitolo 3 della tesi, denominato “Correlation between the rainfall, sediment recharge and triggering of torrential flows in the Rebaixader catchment (Pyrenees, Spain)” si è invece passati ad analizzare le due variabili pioggia e quantità di sedimento, legate all’innesco di colata detritica. In questo caso, a differenza della precedente analisi, ci si è concentrati: (1) sulla pioggia, analizzando se questa influisca non solo nel determinare lo switch innesco si/innesco no, ma provochi degli effetti anche sull’accumulo e/o mobilizzazione dei sedimenti nelle aree di innesco; (2) sulla quantità di sedimento disponibile per un eventuale innesco di colata, considerando i volumi detritici registrati come il sedimento disponibile, nell’area di innesco, durante il periodo pre-colata. Il bacino analizzato in questa seconda parte della tesi, si trova nei Pirenei spagnoli ed è un ottimo caso studio in quanto fin dall’estate 2009 è stato equipaggiato con una stazione di monitoraggio che comprende pluviometri, geofoni, piezometri e videocamere. La serie storica dei dati di pioggia, che viene registrata con un intervallo temporale di 5 minuti, è quindi relativamente ampia. Inoltre, essendo questo un bacino caratterizzato da un’elevata frequenza di fenomeni di colata detritica e di correnti iperconcentrate, si è avuta a disposizione una serie di una ventina di eventi (con relativo volume di detriti) registrati sempre a partire dall’estate 2009. Una serie di dati di questo tipo, permette quindi di effettuare analisi molto più approfondite rispetto a quelle che si possono fare in singoli bacini non costantemente monitorati in cui ci si limita a prendere in considerazione le giornate caratterizzate da eventi di colata. Si è quindi studiato se ci fosse correlazione tra il volume dell’evento innescato e la quantità di pioggia caduta nel periodo trascorso tra l’evento di colata stesso e il precedente (questo viene denominato in letteratura “periodo di ricarica”). Si sono successivamente verificate le eventuali correlazioni tra l’intensità massima degli eventi di pioggia del periodo di ricarica e il volume del successivo evento innescato. Per fare uno studio più complesso si è deciso di utilizzare una variabile denominata “rainfall erosivity”, questo parametro calcolato per ogni evento di pioggia registrato, mette insieme la quantità totale di precipitazione misurata con l’energia cinetica della pioggia stessa, calcolata utilizzando la massima intensità media nella mezz’ora. Con questo parametro si è differenziato tra la pioggia totale caduta durante il periodo di ricarica e la pioggia del singolo evento innescante. I risultati ottenuti sono molto interessanti, infatti risulta chiaro come la pioggia abbia un’influenza relativamente scarsa sull’accumulo di sedimenti e sulle quantità mobilizzate, dimostrando come queste quantità siano influenzate da altre variabili in gioco, mentre l’innesco o il non innesco di una colata è fortemente dipendente dall’energia dell’evento di pioggia che si verifica sul bacino. Dopo aver svolto una prima analisi a scala regionale e una seconda a scala di singolo bacino, nel Capitolo 4 si è entrati ancora più nel dettaglio, prendendo in considerazione solamente il sottobacino di testata. In questo capitolo, intitolato “Rainfall durations and corresponding dominant mechanism for the initiation of debris flows in three basins characterized by different geomorphological settings”, sono state analizzate tre differenti aree studio: (1) il sottobacino del Rio Rudan, che si trova nel versante meridionale del Monte Antelao, in pieno territorio dolomitico; (2) il sottobacino del Rio Chiesa, posto sul versante meridionale del Col di Lana, caratterizzato da una geologia differente rispetto al primo, composta da una mescolanza di rocce vulcaniche e sedimentarie; (3) il sottobacino del Rio Rebaixader, che si trova nei Pirenei spagnoli ed è composto principalmente da rocce metamorfiche. In ognuno dei tre sottobacini, sono state estratte tre sezioni di controllo, lungo la rete idrografica, e per ognuna di esse sono state calcolate le relative portate critiche di innesco di colata detritica utilizzando le due formule di Gregoretti e Dalla Fontana (2008) e Whittaker e Jaggi (1986). Successivamente in ciascuno dei sottobacini, utilizzando il software di modellazione FLO-2D, sono state effettuate diverse modellazioni idrologiche utilizzando come input di pioggia, differenti pluviogrammi creati utilizzando le equazioni di Intensità-Durata sviluppate da Gregoretti e Dalla Fontana (2007) e da Cannon e Ellen (1985). Queste indagini hanno permesso di verificare quale sia la durata minima di pioggia (legata alla corrispondente Intensità soglia) necessaria per raggiungere la portata critica di innesco nelle sezioni di controllo analizzate. Per completare lo studio sul meccanismo d’innesco nei tre sottobacini analizzati, è stata fatta anche un’analisi di stabilità di versante, utilizzando i parametri geotecnici derivanti da campioni di suolo prelevati nelle aree di innesco. Queste due analisi danno insieme una visione complessiva del modo in cui le colate detritiche si sviluppino nelle aree analizzate. I risultati mostrano infatti come, in tutti e tre i sottobacini, le portate critiche di innesco siano comparabili come grandezza, mentre le analisi si stabilità di versante evidenziano come il bacino del Rio Rudan sia mediamente più stabile rispetto alle altre due aree, anche in condizioni di elevata saturazione del suolo. Questo fa pensare che in questo bacino il meccanismo di innesco più probabile sia il cosiddetto “channel bed failure”, mentre negli altri due bacini ci sono sicuramente anche fenomeni di “shallow slope failure” che avvengono nelle aree dissestate di versante portando grandi quantità di detriti all’interno del reticolo idrologico.

Les crues – telles que les laves torrentielles – engendrées dans les torrents lors de fortes précipitations peuvent mobiliser de grande quantité de sédiments. Lorsqu'elles atteignent les zones urbanisées, elles peuvent mettre en dangers à la fois les personnes et les biens. Les approches visant à réduire le risque torrentiel se basent largement sur des seuils intensité-durée de pluie qui déterminent les conditions minimum de déclenchement d’une lave torrentielle. Pourtant, ces seuils sont sujets à une forte variabilité liée, non seulement aux différences inter-sites, mais aussi à la méthode appliquée lors de leur établissement. De plus, ils peuvent entraîner des fausses prédictions, l’intensité et la durée de l’épisode de pluie n’étant pas les seules variables explicatives. Ce travail de thèse vise (i) à fournir un cadre méthodologique rigoureux pour l’établissement des seuils de pluie afin de limiter les sources de variabilité, et (ii) à améliorer leurs performances en considérant à la fois les facteurs de déclenchement et de prédisposition. Il s’appuie sur les données d’un observatoire des crues torrentielles, mis en place dans les Alpes françaises en 2011 sur les torrents très actifs du Manival et du Réal. Dans un premier temps, les images et mesures hautes-fréquences collectées entre 2011 et 2016 ont été analysées afin de détecter et de caractériser les crues torrentielles. Pour appréhender la diversité des écoulements observés, une classification phénoménologique a été proposée. Dans un second temps, la condition minimum intensité-durée de pluie requise pour déclencher une lave torrentielle a été établie. La sensibilité du seuil à la définition d’un épisode de pluie a été évaluée. Dans un troisième temps, un modèle de régression logistique a été implémenté pour discriminer les épisodes de pluies critiques qui n’ont pas engendré de lave torrentielle. Il a permis de sélectionner les variables explicatives les plus pertinentes. Finalement, des pistes de travail ont été avancées pour (i) passer de conditions critiques établies à une échelle locale vers une échelle régionale, en perspective d’une application au sein d’un système d’alerte dédié aux risques hydrométéorologiques, et (ii) passer des conditions de déclenchement d’une lave torrentielle dans la zone de production sédimentaire aux conditions de propagation jusqu'aux zones à enjeux
Flows – such as debris flows – caused by heavy rainfalls in torrents can mobilise a huge amount of sediments. When they reach the urbanised areas, they may endanger the people’s safety or cause damages. Approaches aimed at mitigating torrential risk widely rely on rainfall intensity-duration thresholds which determine the minimum debris-flow triggering conditions. However, these thresholds suffer from a high variability related not only to inter-site differences but also to the method applied to design them. In addition, they are likely to cause false prediction because the intensity and the duration of the rainfall event are not the only explanatory variables. This PhD research work aim (i) to provide a rigorous methodological framework for designing rainfall threshold in order to limit the variability sources, and (ii) to improve their performances by including both the triggering and the predisposing factors. It is supported by field observations stemming from high-frequency monitoring stations installed since 2011 on two very active debris flow-prone torrents in the French Alps: the Manival and the Réal. First, the images and data gathered between 2011 and 2016 were analysed in order to detect and characterise the sediment laden-flows. To deal with the variety of recorded flows, a phenomenological classification was performed. Second, the minimum intensity-duration threshold for debris-flow triggering was assessed. The threshold sensitivity to the rainfall event definition was estimated. Third, a logistic regression model was used to discriminate the critical rainfall events which do not lead to a debris flow. It makes it possible to select the most relevant explanatory variables. At last, several avenues of work were proposed (i) to move the knowledge of debris-flow initiation conditions from a local to a regional level, with a view to application in a warning system dedicated to hydrometeorological risks, and (ii) to improve the ability to predict, not the debris-flow triggering in the production zone, but the debris-flow propagation up to the area concerned

Debris flows and shallow landslides, due to the high velocity, in recent decades have caused the most damage in Italy both in terms of casualties that economic losses. The triggering is in almost all cases related to intense rainfall events. For these types of landslides one of the main risk mitigation measures is the adoption of early warning systems based on rainfall thresholds that identify the critical amount of precipitation for landslide initiation. The aim of this research is the development of objective, repeatable and exportable methodologies for the identification, analysis and characterization of rainfall events responsible for the triggering of shallow landslides and debris flows and the definition of empirical rainfall thresholds. The study area is the province of Trento (6,208 km2), located in the north-eastern Alps, and characterized by complex orography, with 70% of the area at an altitude above 1,000 m. 260 debris flows and shallow landslides with known date of occurrence in the study area have been extracted from the Italian Landslide Inventory (Progetto IFFI) and descriptive statistical analysis related to the main landslide controlling factors have been performed in order to assess the representativeness of the sample with known date of activation respect to the total debris flows and shallow landslides of Trento province. An objective and rigorous statistical methodology has been defined for the identification of the beginning of the triggering event based on the critical duration, that is the minimum dry period duration separating two stochastically independent rainy periods. The critical duration has been calculated for each rain gauge of the study area and its variability during the months of the year has been analysed. An analysis of the rainfall spatial variability in a neighbourhood of the landslide detachment zone has been then carried out through the examination of the Monte Macaion radar maps during some summer convective events, the comparison of rainfall records of rain gauges located in a 10 km buffer around the landslide, and the calculation of the Pearson's correlation coefficient between pairs of neighbouring rain gauges. An automatic procedure for the identification and characterization of the triggering rainfall event has been developed, which provides in output the event duration, cumulated rainfall and average intensity, the maximum rainfall height for fixed durations (from 5 minutes to 96 hours), the cumulated antecedent rainfall (from 1 to 30 antecedent days), the maximum return period of the event, the duration, rainfall amount and intensity associated with the maximum return period. The following rainfall thresholds have been then calibrated with the frequentist approach: cumulated event rainfall-duration (E-D) and average intensity-duration (I-D), which represent the rainfall event in its entirety, and rainfall amount-duration and intensity-duration associated with the event maximum return period (ETR-DTR and ITR-DTR), which consider the most critical portion of the event therefore responsible for the landslide triggering. In the absence of information about the landslide time of activation, the end of the triggering event has been identified using two criteria: the rainfall peak intensity and the last registration of the day. The relationship between rainfall thresholds and some environmental landslide controlling factors (i.e. lithology, land use and elevation) has been analysed and finally the contribution of antecedent rainfall has been evaluated. The main outcomes of the research are: the good applicability of the methodology adopted for the objective identification of the beginning of the triggering event, the low representativeness during convective summer events of the rainfall information recorded at the nearest rain gauge with respect to the precipitation over the landslide source area, the influence of the two criteria for the identification of the end of the event on the thresholds coefficients.

碩士
國立臺灣大學
生物環境系統工程學研究所
100
The main purpose of this study is to investigate the effects of landslides and other physiographic factors on the rainfall threshold value for debris flow warning and then to evaluate it. In Taiwan, the rainfall threshold value for debris flow warning has been used for years as a basis to announce debris flow warning and it is amended annually or after the event of great mass movement disasters. The site of this study was selected at Chenyoulan stream watershed. The statistics examines to landslide ratio, the average riverbed slope steepness , effective watershed area , form factor of watersheds and the debris flow take place higher relativity of physiographic factor, and among them with landslide ratio and debris flow to establish relationship most for notable. Then, with method of logistic regression was used for analysis, building a model to assess rainfall threshold value for debris flow warning with landslides and other physiography factors. In the meantime, compare the effect of two different rainfall-field division ,and the influence of physiographic factor conversion of membership function. From the result, Fan et al.(2003) the rain-field division method and the mode of has been convert to membership function physiography factor the best, the accuracy rate of debris flow occurrence reaches to 80% above. Therefore, consider the debris flow warning usually more conservative standard to publish a warning notice, this results to build a debris flow warning mode. Deduce of the physical mechanism of mode and debris flow occurrence mutually matches. While landslide ratio (DN), average riverbed slope steepness (SN) and effective watershed area (AN) and form factor of watersheds (FN) increment, it takes place a probability to also immediately increase. If the debris flow takes place a probability(p) homology, the rainfall threshold value for debris flow warning to reduce while landslide ratio(DN), average riverbed slope steepness (SN) and effective watershed area (AN) and form factor of watersheds(FN) increment. This debris flow warnning mode, after to provide major disaster (as earthquake, typhoon pouring rain) ,through examine the quantity of four kinds of physiographic factors, is rapid to revise the rainfall threshold value for debris flow warning ,is well timed to reflect now condition.

碩士
國立臺灣大學
生物環境系統工程學研究所
99
The main purpose of this study is to investigate the effects of landslides and other physiographic factors on the rainfall threshold value for debris flow warning and then to evaluate it. In Taiwan, the rainfall threshold value for debris flow warning has been used for years as a basis to announce debris flow warning and it is amended annually or after the event of great mass movement disasters. The site of this study was selected at Shen-mu, Nan-tou. The debris flow events occurred in the site were collected and the method of logistic regression was used for analysis. Statistical analysis was made on the physiographic factors which were mostly commonly used in the previous studies to investigate the occurrence of debris flow. In this study, average riverbed slope steepness, landslide area and effective watershed area were used to estimate the rainfall threshold value for debris flow warning. From the results, it was found that when the ratio of the data in proportion to the occurrence of debris flow and non occurrence of debris flow was 1 to 1,the sensitivity was higher and up to 87%. This indicated that when the debris flow events of occurrence and non occurrence were similar, there would be a better prediction. In addition, the equation for evaluating rainfall threshold value for debris flow using the logistic regression analysis was obtained. The parameters used for evaluation were average riverbed slope steepness, landslide area, effective watershed area and effective cumulative rainfall. The event occurred during Herb typhoon was used as an example for validation. If the physiographic factors are similar to that during Herb typhoon, debris flow may occur while the effective cumulative rainfall reach 713.5mm .