POST-GLACIAL ENVIRONMENTAL EVOLUTION AND GROUND SUBSIDENCE IN THE COMO URBAN AREA (NORTHERN ITALY)

We conducted investigations on the latest Pleistocene to Holocene geomorphology and stratigraphy of Lake Como region (Lombardia, Italy), based on field survey, interpretation of airphoto coverages, literature stratigraphic, archaeological and topographic data, and the compilation of available subsurface data for the urban area of Como. In this town, located at the S edge of the W branch of the lacustrine basin, remarkable phenomena of ground subsidence have been recorded in the past decades.

Using the reconstruction of the (late) post-glacial landscape evolution of the Como area, we expect to quantify the role played by natural processes and human activity in controlling the lowering of the ground surface. This study is part of the “Lario Project”, a multidisciplinary limnogeological research aimed at assessing the vulnerability of the physical environment for land use planning and natural risk mitigation. Lake Como (“Lario”) lies at 198 m a.s.l. in a 60 km long, downstream-bifurcated glacial trough sited across the Central Alps, which was intensely modelled by an ice tongue up to ca. 2 km thick during the Last Glacial Maximum.

The urban area of Como, built on a Holocene marshy basin on the lakeshore, in 1955 to 1975 experienced a crisis of subsidence; maximum values of 20 - 40 cm up to ca. 1 m of ground lowering have been measured in the districts close to the lakeshore. In the last ca. 30 years subsidence rates in the same districts decelerated to mean values of 3 to 4 mm/yr, following the ban of extraction of deep ground water, indiscriminately exploited immediately after the II world war. We wanted to know if these values represent the long-term “natural” trend, and therefore have to be expected for the near future. The preliminary results of our study show that ca. 4 mm/yr is the subsidence value over the last ca. 12.000 years in the central part of the town. The anthropic component seems therefore to have been relevant only during the named 1955 - 75 critical phase of ground subsidence.

A sequence of shallow boreholes will be drilled in April 2003 in order to check the proposed model of recent landscape evolution in the Como urban area, and the resulting assessment of Holocene ground subsidence rates. Details of these stratigraphic investigations will be presented during the INQUA Congress.