Assessing the earthen levees’ vulnerability through practical procedure and different monitoring techniques: the experimental site along the Tatarena stream, central Italy.

Earthen levees represent one of the most common structural measures used to reduce the severe effects of flood events. The hydraulic risk assessment on the flood-prone areas is typically achieved by assuming the levee system undamaged during floods, but the levees can fail due to different collapse mechanisms, among which overtopping and seepage/piping, due to infiltration process through the levee body and foundation, are the most frequent.
The monitoring of the levee status through techniques such as geophysical methods is fundamental to gather data for levee vulnerability analysis and to evaluate in real-time the possible happening of a breach formation.
In this context, this study describes a practical procedure for assessing levee vulnerability to seepage and an experimental monitoring system to be developed for an earthen levee along the Tatarena stream, located in the Umbria Region, central Italy. The selected area was damaged in 2020 by a levee failure due to the presence of animal burrows.
The monitoring system is based on combined geophysical methods: Electrical Resistivity Tomography (ERT), Ground Penetrating Radar (GPR) and Frequency Domain Electromagnetic Methods (FDEM). These methods collect different geophysical parameters (i.e. electrical conductivity and permittivity) that are correlated to the main hydraulic characteristics of the investigated subsoil, such as porosity, water content, permeability. Therefore, the geophysical methods could provide useful information on water infiltration process, making it possible to check the hydraulic status of the levees that is fundamental to identify possible critical conditions.
The work was carried out in two steps: 1) levee characterization; 2) monitoring system design and implementation.
First, the levee (approximately 570 m long) was analyzed using GPR, FDEM and ERT methods. The GPR (Dual-frequency antenna 300-800MhZ) was acquired by running profiles approximately 50 m long, defining a high-resolution image up to 2.0 m deep, while the electromagnetic method (Profiler EMP-400) through a continuous profile. The ERT is long 320m and a multichannel system with 72 electrodes (Syscal Pro instrument) and an electrode spacing of 1m was used. Therefore, an optimized roll – along protocol allowed to acquire a series of profiles (n.10) with 24 shift electrodes approach. All acquired data were elaborated and inverted (ZondRes2D software) to obtain the final electrical resistivity image. All the geophysical results highlighted an integrated interpretation allowing to define the characteristics of the levee and to optimally design the monitoring system.
Second, a monitoring system has been designed. It comprises a longitudinal ERT, consisting of 96 electrodes spaced 0.5 m (total length of 47.5 m). Cross-sectional ERTs are planned to be placed along two sections, located 19 m and 28.5 m from the beginning of the longitudinal ERT, consisting of 24 electrodes spaced 0.5 m (total length of 11.5 m).
In addition, a continuous self-potential measurement system is foreseen based on 40 electrodes located at a mean distance of 1.5 m (total system length 60 m).
The data recorded during the monitoring period will be used to assess the reliability of the estimate derived through the practical procedure.