Long-term geotechnical response of Venice coastal defences detected by Persistent Scatterer Interferometry

With a surface area of about 550 km2 the Venice Lagoon is the largest Italian wetland, open in the Upper Adriatic Sea to the highest tides of the Mediterranean Sea. The lagoon is connected to the sea through three inlets, which divide the narrow littoral strip separating the inner water body from the Adriatic. Several nearshore and offshore structures have been constructed over the decades to protect such a unique city and its coastal environment from sea storms and high waters, whose frequency and level are increasing due to relative sea level rise. Long jetties have been built at the inlets between the end of the 18th and beginning of the 19th centuries and then reinforced between 1994 and 1997. Since 2003, in the framework of the MOSE construction (i.e., the project of mobile barriers for the temporarily closure of the lagoon to the sea), the jetties have been extended, new breakwaters have been constructed in front of the inlets, and a small island has been realized within the Lido inlet to support the MOSE gates. An accurate quantification of the movements of these coastal defense structures due to long-term consolidation has been carried out by Persistent Scatterer Interferometry (PSI) using ENVISAT ASAR and TerraSAR-X images acquired from April 2003 to December 2009 and from March 2008 to January 2009, respectively. The displacements range between few mm/yr for the structures older than 10 years up to 50-70 mm/yr for those realized few years ago. The PSI measurements have been combined with the outcome of a detailed geomechanical characterization of the lagoon subsoil down to -50 m depth below msl. The geotechnical dataset has been collected at a test site located on the northern littoral where an instrumented 20 m radius, 6.7 m high vertically-walled reinforced sand embankment was built at the end of 2002 and monitored till to 2007. The use of the derived stress-strain properties together with the actual lithostratigraphy below the structures, which is available from several piezocone profiles and boreholes, allow for the computation of secondary compression (consolidation) rates that match very well the PSI-derived movements. The results provide important information on the expected time-dependent settlement of these structures and prove the potentiality of PSI in controlling the geomechanical response of large coastal structures.