A model of Como (N italy) urban subsurface: a multidisciplinary tool for hydraulic, hydrogeologic and subsidence risk management

The city of Como lies on the shores of Lake Como and is built on a sedimentary basin made of at least 155 m of post-LGM (Last Glacial Maximum) lacustrine, palustrine and alluvial deposits. The area is threatened by different kinds of geological hazards, mainly related to lake flooding and subsidence; the city is also particularly vulnerable in case of possible strong ground shaking, because of the local high liquefaction potential and the likely occurrence of slope instabilities and amplification phenomena. We applied a multidisciplinary approach aimed at reconstructing the late Quaternary and Holocene evolution of the area, based on field surveys and the analyses of stratigraphic and geotechnical data, hydrogeological and subsidence monitoring. Our model has been tested during the design of the new defense system for the mitigation of the flood hazard. We anticipated that the worst geological problems for this facility would have occurred in the area where, according to our model, the Roman lake harbour was located. We realized at this site a specific campaign consisting in the drilling of 7 new boreholes, a number of14C dating, and geotechnical and seismic surveys. A previously unrecognized organic silty unit, rich in archaeological remains and consistent with our hypothesis, has been found. This unit is the most critical for engineering planning due to its very bad mechanical properties. These results demonstrate that the model can be used as a predictive tool for hazard management and for a more efficient urban planning.