11. The Importance of Geological Models in Understanding and Predicting the Life Span of Rockslide Dams: The Case of Scanno Lake, Central Italy

In order to explain the long life span of the Scanno rockslide-avalanche dammed lake (Central Italy), whose age of impoundment is older than 2,300 years, and to get indications about its present and future stability conditions, it was assumed of fundamental importance to build a representative geological model aimed at defining (i) the geometry of the boundary surface between the rockslide debris and the bedrock, (ii) the characteristics of the debris with special reference to hydraulic behaviour, (iii) the flownet within the landslide deposit taking into account the complex geological and hydraulic boundary conditions imposed by the palaeovalley morphology and by the numerous springs downstream of the dam which are fed by different aquifers. The reconstruction of the natural dam geometry (volume, heigth and width) was possible by means of integrated geological (site surveys and boreholes) and geophysical (electric tomography and seismic refraction) investigations: the so obtained data were processed in order to assess the stability conditions according to existing geomorphological indexes. As a result, good stability conditions of the analysed natural dam are highlighted on the basis of its geometric features. A campaign of pumping tests, hydrogeological measurements, chemical and isotope analyses was carried out with the aim of defining the flownet within the rockslide-avalanche deposit and its hydraulic properties. The so depicted hydrogeologic characteristics indicate that dam failures by piping processes are very unlikely, thus supporting the dam stability conditions.