Levee body vulnerability to seepage and impact of animal burrows - The Tanaro River case study

Floodplains are naturally subjected to inundations, that pose a threat to the populations and facilities located along the watercourses. To reduce the hydraulic risk in flood-prone areas, various protection measures have been implemented over the centuries, including the realization of earthen levees. However, flooding is frequently related to the failure of levees, whose performances can thus induce some level of risk. Chapter 1 provides a brief description of the concept of Residual Risk in protected areas and introduces the issues of seepage-induced failures and impact of animal burrows in the seepage vulnerability of earthen embankments. In Chapter 2 the most common causes of failure of earthen levees are described and some statistics concerning the diffusion of levee failure types are presented, with the main purpose to highlight the large diffusion of seepage-induced failures. The seepage process is triggered by the hydraulic gradients established between the outboard and inboard faces of the levee, able to induce a seepage flow and the consequent removal of soil particles from the levee body. The governing equations of seepage are presented in Chapter 3. Seepage is one of the most dangerous causes of levee failure. Indeed, if the erosion process triggered by seepage remains undetected, the collapse of the embankment is barely predictable and, consequently, unexpected and catastrophic. Several research studies have dealt with the evaluation of the seepage vulnerability of earthen embankments. Some of these studies, subdivided into hydro-geotechnical modelling and reliability-based approaches, are illustrated in Chapter 4, focusing on their advantages and gaps. Currently it is widely acknowledged that the presence of discontinuities, such as animal burrows, within the embankments facilitates the onset of seepage. The possible impact of burrows in the seepage vulnerability of earthen levees is analysed in Chapter 5: after a literature review addressed to highlight the relevance of the issue and some possible mitigation measures, a new methodology for the assessment of the effect of burrows on earthen levee vulnerability to seepage, specially developed in this thesis work, is presented. The procedure couples an existing methodology for the estimation of the seepage probability of undamaged embankments, developed by the IRPI-CNR, with the use of a finite element analysis software, SEEP/W (Geostudio ®, 2008). In Chapter 6 the procedure is performed on 21 levees selected along the Tanaro River (northern Italy) and the results are presented and discussed. They put into evidence that the IRPI-CNR procedure is quite reliable in the identification of the most vulnerable stretches in the flood defence system, when the undamaged structures are investigated. Furthermore, the new procedure allows to catch the worsening of the stability conditions of the levees when the presence of burrows is simulated: they accelerate the saturation of the levees and reduce the required time for the saturation line to reach the landside toe, a configuration which marks the onset of the seepage process. The cavities cause a general increase in the seepage vulnerability of the embankments, which is higher as the length of the burrows increases. The collected data concerning the levee system and the results deriving from the application of the procedure have joined the Italian earthen levee Database (Database nazionale ArgiNature in TErra, DANTE), developed by IRPI-CNR and addressed to civil protection purposes. The database constitutes by itself a precious base to support flood risk management activities in the study area. The most important novelty introduced by the developed methodology consists in the capability to predict the performances of existing levees in the hypothesis they get damaged, due to the presence of burrowing animals or other causes. The main advantages of the procedure consist in: 1) the capability to provide a quantitative estimation of the impact induced by burrows, through e relatively simple and expeditious procedure; 2) the use of a probabilistic approach, which allows the estimation of the seepage probability regardless of the knowledge of the hydraulic conductivity, a common unknown of the problem; 3) the definition of seepage vulnerability classes, useful to identify possible weak points and to establish the priority of intervention within extended levee systems Chapter 7 presents a critical analysis of the advantages and limitations of the purposed procedure, and suggests some possible further developments

REFINEMENT OF SEEPAGE VULNERABILITY ESTIMATE IN NATIONAL LEVEE DATABASE OF ITALY

Piping induced by seepage is a frequent cause of failure for earthen levees. The probability that the seepage line in the levee body intercepts the landside slope provides an important indication of the levee vulnerability to seepage. Nevertheless, the definition of the seepage paths associated to assigned water levels is affected by the difficulty in estimating the hydraulic parameters that control the filtration process, chiefly the soil hydraulic conductivity. The ideation of expeditious and operational methods, able to analyse extended levee systems quickly and to identify the most vulnerable portions of the systems, is therefore of paramount importance. To this purpose, an existing practical procedure for the evaluation of levee vulnerability to seepage, based on the definition of a vulnerability index (Camici et al., 2015), is here enhanced and it is used to produce 'synthetic diagrams', easily applicable to derive the seepage probability of the dykes whose hydraulic conductivity is unknown. The procedure is applied for the Tiber River (central Italy) and the Tanaro River (northern Italy). The Italian levee database (DANTE) is briefly presented in the final part of the paper: it is conceived as a dynamic geospatial tool, addressed to collect all the available information on levee systems and to usefully support authorities involved in hydraulic risk mitigation

Levee body seepage: a refinement of an expeditious procedure for fragility curves and vulnerability diagrams' assessment

Extensive flooding can be the result of levee system failures most frequently caused by the piping process due to seepage. The proper description of the seepage line is affected by the difficulty of estimating the hydraulic parameters, mainly the soil hydraulic conductivity. Therefore, the development of simple methods for a quick analysis of extended levee systems is fundamental to identify critical points. In this context, a practical procedure, recently proposed, based on a simple vulnerability index is here enhanced and used to derive diagrams easily applicable for seepage vulnerability estimate, taking the hydraulic parameters' uncertainty into account. The procedure is applied for the Tiber River, in central Italy, and the Tanaro River, in northern Italy, by analyzing 67 and 6 levees, respectively. The results show that the method provides the highest seepage probabilities for levees affected by failures in the past. Therefore, the procedure seems to be able to identify the levees that require detailed investigations. Finally, the Italian levee database (DANTE) is presented as a dynamic geospatial tool for collecting all the available data/information on levee systems to usefully support authorities with the charge of hydraulic risk mitigation for identifying the most vulnerable levees

Rainfall thresholds for the possible initiation of shallow landslides in the Italian Alps

Abstract Rainfall-induced shallow landslides are frequent in the Italian Alps, where they cause severe economic damages and loss of life. The prediction of rainfall-induced slope failures is of utmost importance for civil protection purposes and relies upon the definition of physically based or empirical rainfall thresholds. Reliable empirical rainfall thresholds require a large amount of information on the geographical and temporal location of past rainfall events that caused the observed mass movements. We have compiled a catalogue listing 453 rainfall events that have triggered landslides in the Italian Alps in the 13-year period 2000-2012. For the purpose, we searched national and local newspapers, blogs, technical reports, historical databases, and scientific journals. In the catalogue, for each rainfall event that triggered one or more failures, the information includes: (i) landslide geographical position, (ii) date of the landslide occurrence, (iii) landslide type (if available from the source of information), and (iv) rainfall information. Using the available information, we calculated the cumulated amount (E) and the duration (D) of the rainfall that likely caused the documented slope failures. We exploited the catalogue to calculate new ED threshold curves and their associated uncertainties for the Italian Alps adopting a frequentist approach. To define seasonal rainfall thresholds, we also investigated the monthly distribution of the landslides. The new thresholds are compared with similar curves in the same general area. We expect the results of our study to improve the ability to forecast shallow landslides in the Italian Alps and, more generally, in the wider Alpine region

Rainfall thresholds for the possible initiation of shallow landslides in the Italian Alps

Abstract Rainfall-induced shallow landslides are frequent in the Italian Alps, where they cause severe economic damages and loss of life. The prediction of rainfall-induced slope failures is of utmost importance for civil protection purposes and relies upon the definition of physically based or empirical rainfall thresholds. Reliable empirical rainfall thresholds require a large amount of information on the geographical and temporal location of past rainfall events that caused the observed mass movements. We have compiled a catalogue listing 453 rainfall events that have triggered landslides in the Italian Alps in the 13-year period 2000-2012. For the purpose, we searched national and local newspapers, blogs, technical reports, historical databases, and scientific journals. In the catalogue, for each rainfall event that triggered one or more failures, the information includes: (i) landslide geographical position, (ii) date of the landslide occurrence, (iii) landslide type (if available from the source of information), and (iv) rainfall information. Using the available information, we calculated the cumulated amount (E) and the duration (D) of the rainfall that likely caused the documented slope failures. We exploited the catalogue to calculate new ED threshold curves and their associated uncertainties for the Italian Alps adopting a frequentist approach. To define seasonal rainfall thresholds, we also investigated the monthly distribution of the landslides. The new thresholds are compared with similar curves in the same general area. We expect the results of our study to improve the ability to forecast shallow landslides in the Italian Alps and, more generally, in the wider Alpine region

Influence of geological, morphological and climatic factors in the initiation of shallow landslides in North Western Italy

Landslides are recurrent geomorphic processes in Italy and represent a serious economic and societal problem. Historical catalogues report thousands of landslide casualties since the 9th Century. The importance to predict landslide occurrence is particularly evident to prevent the effects of rapid mass movements. The mitigation of landslide-related risk needs the improvement of prediction skills, obtained with a better understanding of triggering mechanisms, which are peculiar of the specific geological and environmental settings. The prominent role of rainfall in mass movement activations is widely recognised in the scientific community and several authors all over the world have long attempted to define the critical values responsible for landslide initiation. In Italy, the National Department for Civil Protection (DPC) is funding a CNR-IRPI (National Research Council-Research Institute for Hydro-geological Protection) research, aiming at the definition of regional rainfall thresholds for shallow landslides for the entire national territory. In this study, we investigated the role of geological, morphological and climatic factors on the initiation of rainfall-induced slope failures in the Piedmont, the Aosta Valley and the Lombardy regions (NW Italy

Rainfall thresholds for the activation of shallow landslides in the Italian Alps: the role of environmental conditioning factors

The aim of the present work is to investigate the role exerted by selected environmental factors in the activation of rainfall-triggered shallow landslides and to identify site-specific rainfall thresholds. The study concerns the Italian Alps. The region is exposed to widespread slope instability phenomena due to its geological, morphological and climatic features. Furthermore, the high level of anthropization that characterizes wide portions of the territory increases the associated risk. Hence, the analysis of potential predisposing factors influencing landslides triggering is worthwhile to improve the current prediction skills and to enhance the preparedness and the response to these natural hazards. During the last years, the Italian National Research Council’s Research Institute for Hydro-geological Protection (CNR-IRPI) has contributed to the analysis of triggering conditions for rainfall-induced landslides in the framework of a national project. The project, funded by the National Department for Civil Protection (DPC), focuses on the identification of the empirical rainfall thresholds for the activation of shallow landslides in Italy. The first outcomes of the project reveal a certain variability of the pluviometric conditions responsible for the mass movements activation, when different environmental settings are compared. This variability is probably related to the action of local environmental factors, such as lithology, climatic regime or soil characteristics. Based on this hypothesis, the present study aims to identify separated domains within the Italian Alps, where different triggering conditions exist and different countermeasures are needed for risk prevention. For this purpose, we collected information concerning 511 landslides activated in the period 2000-2012 and reconstructed 453 rainfall events supposed to be responsible for the activations. Then, we selected a set of thematic maps to represent the hypothesised landslide conditioning factors and to identify the supposed homogeneous domains within the study area. We employed an existing statistical method for the definition of the cumulated event rainfall vs. rainfall duration (ED) thresholds, for both the entire catalogue of rainfall events and for the events falling in the separated domains. The obtained results contribute to a better understanding of the role exerted by geological, pedological and climatic factors in landslides activation and help identifying separated domains where different risk managing strategies should be adopted. The proposed methodology can be a valid support for risk reduction strategies planning at regional scale

Populating a catalogue of rainfall events that triggered shallow landslides in Italy

In Italy, rainfall induced landslides - including soil slips and debris flows - occur every year, claiming lives and causing severe economic damages. In the 61-year period 1950-2010, such phenomena have caused more than 6400 casualties (SALVATI et alii, 2010). During 2011, 25 people have been either killed or wounded by landslides. As a result, the prediction of slope failures triggered by rainfall is of primary importance for decision makers and civil protection authorities. The predictive ability for rainfall induced landslides is still limited due to the complexity of the problem, to the number of the involved variables, and to the methodological approaches that are not always rigorous. As regards shallow landslides, a team of researchers working at CNR-IRPI (Italian National Research Council, Institute of Research for Geo-Hydrologic Protection) is carrying out a research project funded by the Italian national Department for Civil Protection (DPCN), aimed at defining regional and subregional rainfall thresholds

Rainfall events able to trigger shallow landslides in Calabria (Southern Italy)

The key role of rainfall in landslides activation is highlighted by the increasing number of studies on rainfall triggering thresholds for shallow landslides over the last decades. Such thresholds are generally defined in terms of cumulated rainfalls in a period preceding the mobilization itself (TERRANOVA et al., 2007). Many areas of the Mediterranean basin are recurrently subject to catastrophic geo-hydrological events with high economic and social impact. In Calabria, an active tectonic region characterised by small basins with steep slopes, rainfall-induced slope movements frequently cause dramatic consequences to the environment and to population. Proper understanding and forecasting of rainfall events is a prerequisite for the adoption of appropriate mitigation measures and for reducing the risk. Aiming at characterising the rainfall events able to trigger shallow landslides in Calabria, the temporal structure of 152,575 rainfall events characterised by different durations has been analysed by applying the approach proposed by TERRANOVA & IAQUINTA (2011). Considered events were recorded between 1989 and 2008 at 155 rain gauges, at time steps of five minutes

Catalogue of rainfall events with shallow landslides and new rainfall thresholds in Italy

In Italy, rainfall-induced shallow landslides are frequent and harmful phenomena. The prediction of their occurrence is of social significance for civil protection purposes. For the operational prediction of rainfall-induced shallow landslides empirical rainfall thresholds based on the statistical analysis of past rainfall conditions that triggered slope failures are commonly used. The paper describes a catalogue of 1981 rainfall events, which caused 2408 shallow landslides in Italy in the period 1996-2012. Information on rainfall-induced landslides was collected searching chiefly online newspaper archives, blogs, and fire brigade reports. For each documented failure, we reconstructed the triggering rainfall conditions (rainfall duration D and cumulated rainfall E) using national and regional rain gauge networks. We analysed the rainfall conditions to determine new ED rainfall thresholds for Italy. The calculated thresholds can be implemented in a landslide forecasting system to mitigate landslide hazard and ris