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.