Effetti dell'attività della fauna sulla stabilità degli argini: Valutazione Ecoidraulica dei meccanismi di rottura arginale e tecniche di ripristino

The greatest civilizations in human history have always developed near water and, in order to increase arable land, have built serval types of river structures to control the flow. Levees (also known as dikes), for instance, are simply man-made embankments built to keep a river from overflowing its banks. However, river flooding is a natural phenomenon so that it is not rare that rivers overflow their banks and flood the surrounding landscape causing considerable damages. Researchers, technicians, land managers, and other stakeholders have attempted to overcome the problems of river flooding. However, the flood protection systems are complex, and their failure mechanisms are not yet completely understood. Additionally, contrary to past practices, it is not possible to investigate levee failure mechanisms only from the hydraulic and geotechnical perspectives. Levees are inserted in fluvial ecosystems where animals and vegetation play key roles. Indeed, levees are a habitat for animals, which often dig tunnels and holes inside these soil structures, altering their performance and possibly triggering failure phenomena. The present dissertation operated in this framework of the failure levee induced by the presence of animals. This thesis aims to contribute to the flood risk mitigation in river ecosystem through the understanding of levee failure mechanisms induced by burrowing animal activities. Specifically, the main aspect of this work is to overcome the traditional approach used to study levee failures, to develop an eco-hydraulic approach, and to define a suitable remediation technique. To achieve these goals, after building a consistent view of the state of art, the research is divided into three main components. The first part studies a flood that occurred in Sicily in 2012. In particular, the performance of the levees during this event and the probable levee failure mechanisms are investigated. Then levee structures are investigated in order to describe the main geometric characteristics of animal burrows. Specifically, after a visual inspection of the activity of burrowing animals, and after measuring the dimensions of the hole entrances, a Ground Penetrating Radar (GPR) survey was carried out to investigate depths and paths of these dens. The second part studies the effects derived from the presence of holes on the levee stability. In collaboration with the University of Hull, an eco-hydraulic analogue model and an image analysis procedure were designed. To look the effect that some geometrical paraments of holes have on the levee performance, the density, the position, and the length of the burrows are investigated. The results from comparing erosion in undisturbed levee with the case of weakened levees (i.e. with burrows) indicate that the erosion mechanism changes as a function of the vertical position of the burrows: the burrows above water level increase the total erosion, while the burrows located under the water level seem to reduce it. Moreover, the not orthogonal burrows related to the flow direction produce a greater surface erosion. The third part is focused on industrial applications. To reduce soil permeability and to increase levee stability, in collaboration with Trevi S.p.A. engineering society, some remediation techniques were studied and selected. The research showed that a geotextile is the most suitable technique in order to reduce the levee instability. Indeed, this material is proper to different kind of morphological and geotechnical characteristics of soil conditions, and its application and maintenance are easy. Furthermore, it could be covered by natural vegetation to achieve a rapid regeneration of river flora on the levees.