## 3 Distributed Optical Fibre Sensing Applied to Geo-Hydrological Applications
### Soil Leves and Embankments Monitoring  
Monitoring of soil levees and embankments is aimed at preventing the collapse of the structure. The main hazard is related to occasional or exceptional changes in the watercourses stage level due to intense rainfalls. Many mechanisms can contribute to the collapse of the retaining structure, often involving either the upstream or downstream slope of the embankment as well as its foundation layer. However, one of the major driving phenomena that may lead to the collapse of the structure is the seepage regime variation inside the foundation soil and in the embankment body. Extended periods of high water levels can gradually saturate the leveee, thus reducing the soil strength. On the contrary, sudden water level decrease may originate dangerous seepage forces at the upstream slope (i.e., at riverside). Heterogeneities in the foundation soil and discontinuities across the embankment section can contribute to worsening the scenario. Non-uniform hydraulic properties can indeed lead to preferential paths for water infiltration and seepage. Flow localisation contributes to the development of high seepage forces and high flow velocities, and ultimately to the loss of resistance or the formation of pipes inside the structure with the removal of fine matrix materials, eventually leading to its collapse by internal erosion. Besides overtopping, internal erosion is the most common cause of failure in embankments, dams, and levees, with almost 50% of the total reported failures resulting from internal erosion [99]. Other causes of failure are external erosion due to overtopping, or lateral sliding of foundations due to internal water pressure and back erosion. Regarding this process, the local measurement of temperature inside the leveee is recognised as a useful tool for the identification of anomalous water flows across the leveee. The temperature in a leveee is determined by the temperature of the upstream reservoir and the air, both varying with seasons. As a consequence, a seasonal variation is also observed within the leveee due to heat conduction and advection from seepage flows. In most of the medium-size levees (up to 20 m in height), seepage flows are usually rather small, and the seasonal temperature variation downstream (i.e., at field side) is determined essentially by the heat exchange with air, at least for shallow depth. The effect from air temperature decreases with depth: therefore, at some metres depth, seasonal fluctuation of few degrees Celsius is measured. The temperature in the leveee can be confidently assumed to be only moderately affected by low seepage flows, but rather significantly by the seasonal fluctuation of the air temperature, whose effect can be minimised if measured at sufficient depth. As soon as the seepage flows increase, the seasonal temperature fluctuation will be enhanced due to advection fromthe water upstream, with the amplitude of fluctuation depending on seepage flows, inflow boundary temperature seasonal variations, and thermal stratification of the reservoir. To summarise, the basic principle of passive thermometric detection of seepage in levees is that in the absence of abnormal flows, temperature fluctuations have to be attributed entirely to heat conduction (from the crest and the foundation). When a filtration flow arises or severely varies across the embankment, a significant change in the amplitude of temperature fluctuations due to advection is expected. Thus, the detection of abnormal flows requires the temperature variations to be monitored both in time and in space: a long-term monitoring of the thermal behaviour of the embankment with sufficient spatial resolution is in fact mandatory to track any relevant change. Besides that, from a geotechnical point of view, soil water content and pore-water pressures strongly influence the shear strength of the embankment. Furthermore, the hydraulic forces acting on the soil determine localised strains, occurring before the collapse and rapidly increasing up to failure.