GNGTS 2019 - Atti del 38° Convegno Nazionale

718 GNGTS 2019 S essione 3.2 LONG-TERM MONITORING STRATEGY TO ASSESS THE INNER CONDITIONS OF RIVER LEVEES. CASE STUDY: COLORNO, ITALY G. Tresoldi 1 , A. Hojat 1,2 , L. Zanzi 1 1 Dipartimento di Ingegneria Civile e Ambientale, Politecnico di Milano, Milano, Italy 2 Department of Mining Engineering, Shahid Bahonar University of Kerman, Kerman, Iran Introduction. The frequency of extreme rainfall events has dramatically increased in the last decade, causing unexpected floods which threaten human lives and properties. Long- term monitoring and proper maintenance of river embankments are key factors to reduce hydrogeological risks. In most cases, assessment of earthen embankments relies only on periodic visual inspections, and the inner conditions of the structure usually remain unknown. Earthen embankments are vulnerable to inhomogeneous seepages, long-lasting high levels of water in the river, and fast wet-dry cycles. Geophysical techniques have shown to be successful in investigating the inner conditions of embankments in a non-destructive manner. Geo-electrical methods have specifically proved to be efficient in monitoring concentrated seepages and critical saturation zones within earthen slopes, dams and dikes (Dahlin et al. , 2008; Kuras et al. , 2009; Jomard et al. , 2010; Supper et al. , 2012; Perri et al. , 2014; Weller et al. 2014; Loperte et al. , 2016; Hojat et al. , 2019a). After careful evaluation of critical sites in the territory of three managing authorities for rivers and canals in northern Italy, and numerous preliminary electrical resistivity tomography (ERT) measurements, a long-term geoelectrical monitoring system was installed on a river levee in Colorno, Parma, in November 2018 (Hojat et al. , 2019b). This is the second installation of a prototype ERTmonitoring system. The first customized ERTmonitoring system was installed along the embankment of an irrigation channel in San Giacomo delle Segnate in 2016 (Arosio et al. 2017; Tresoldi et al. 2018; Tresoldi et al. 2019). Thanks to these monitoring systems, the inner conditions of the study sites are assessed remotely in real time. The study site. In this paper, we present the results from the G.Re.T.A. monitoring system installed in Colorno in a site named La Penza. This site was selected for long-term monitoring after receiving suggestions from three managing authorities for canals and rivers in northern Italy (Consorzio di Bonifica Terre dei Gonzaga in Destra Po, Consorzio di Bonifica Est Ticino Villoresi and AIPO). The decision was based on the results of reconnaissance ERT measurements along 1448m of candidate segments of Parma river embankments (Hojat et al. , 2019b). In addition, the high exposure of the zone and the past instability during Colorno flood in December 2017 were considered: during the event, a concentrated seepage caused the re- building of the levee segment (Fig. 1a and Fig. 1b). The prototype ERT system is composed of 48 electrodes and was installed along 94m of the embankment. Wenner array with a=2m was used for monitoring this site. Such a configuration ensures adequate lateral and vertical coverage of the embankment. The two 24-electrode cables are protected with anti-rodent plastic cases and were put in a 0.5m-deep trench along the embankment. To ensure permanent good contact of electrodes with the soil, plate stainless steel electrodes of 0.2m x 0.2m were used. The G.Re.T.A. geo-resistivity meter, designed by LSI Lastem with scientific support of Politecnico di Milano, was placed in the centre of the critical levee segment and is accompanied with a meteorological station to register rainfall, air humidity and temperature (Fig. 1c). The whole system is powered by a solar panel, it is programmable and can send data through internet connection. The frequency of measurements is set by the operator through a cloud platform where data are saved and can be processed. Long-term monitoring results. Resistivity pseudosections were initially measured twice a day to control the capabilities of the system. The frequency of measurements was set to once a day from February 2019. Measured data are corrected to remove the influence of burial of electrodes. Moreover, since the ERT measurements are 2D, the 3D geometry effect of the levee and of the boundary conditions is also studied. Forward modelling calculations were performed in RES2DMOD (Loke 2016) and RES3DMODx64 (Loke 2014) to estimate the influence of the