Potentiality and limitation of distributed optical fiber sensing system to measure seepage flows under levee embankments

The development of innovative monitoring systems to mitigate the risk associated to river levee failure is a recent challenge involving synergic efforts of public institutions, specialized companies and academics. To this aim, this paper presents the main results of two experiences carried out on the levee of Adige River in the Province of Bolzano (Italy), that in the recent past was interested by some moderate piping phenomena, probably connected to the presence of ancient riverbeds or water-meadows which form preferential seepage paths under the levee. Two test sites were selected and deeply investigated through geotechnical surveys. There it was adopted two different installa- tions of distributed optical fiber sensing systems to measure temperature evolution in the soil potentially subjected to subsurface se- epage flows: in the first one, a fiber cable was buried inside a 350 m shallow trench at the landside foot of the levee; in the second one the fiber was set up in 5 vertical boreholes, driven down to 25 m at the center and the corners of a 400 m2 square area. In the first case, in October 2018 the system was thermally stressed by an exceptional storm, that generated in the river a relevant double-peak flooding event. In the second site, installed in 2021, the seepage was artificially induced by inflating water in the central borehole and measuring temperature evolution in the wells. The paper presents the main results of these installations and discusses the potentialities and the limitations of this relatively new monitoring system to provide insight into the seepage process and piping phenomena undermining the levee stability.