Case study of the Entella river floodplain (Liguria, Italy): engineering geological mapping for the development of urban areas

The advancement of the engineering geological mapping for a correct management of the Entella River floodplain is of primary importance for a general town planning and a sustainable development of the nearby areas. This paper presents the case-study of the Entella Stream floodplain including the final catchment area with a total surface of 376 km2: the presence of the towns of Lavagna, Chiavari and Cogorno which sum up over 50,000 inhabitants makes this area extremely vulnerable and therefore susceptible to high risk values. The final sector of the catchment is divided in three zones of different lithology: the first one consists of shales and presents gentle slopes, the second part shows steeper sides made by slates and sandstones, while the last one corresponds to the plain area and it’s made up of fluvial and marine deposits. The Entella Stream floodplain is 4,5 km long and its width ranges between 150 m and 400 m; the stream flow rate during a flooding event is about 3000 m3/s and since the 17th century more than 50 critical events have been recorded making it possible to determine a mean recurrence time for overflowing in the lower portion of the Entella basin of about 10 years. Recent deposits can be identified in the valley floor area corresponding to the actual riverbed of the Entella Stream; present-day terraced alluvial deposits actually give origin to the flat areas along the stream becoming wider in the middle-lower sector; approaching the delta, ancient fluvial terraces represented by small residual lenses on a higher level compared to the actual valley floor can be also identified. Alluvial fans on the orographic left along the central and southern sectors show a grade range between 2% and 10% while the ones located on the orographic right are less extended and steeper than the previous ones (more than 10%). From the analysis of the collected data an engineering geomorphological map (scale 1:10,000) has been carried out as well as a lithotechnical sketch of the soils and a hydrogeological sketch of the plain sector in which the soils have been distinguished according to their geotechnical features and their permeability degree. The whole study area has been involved in a strong urbanization process during the last one hundred years which considerably reduced the hydraulic efficiency of the river. Slopes have also been modified by the ancient practice of building terraces made up of dry-stone walls while the coastal sector has been altered by embankments and artificial cliffs. Further infrastructural projects such as the extension of existing roads, the enlargement of the Chiavari City stadium and a purification plant close to the Entella stream delta will soon concern this area: an evaluation of the stream catchment main features is therefore needed also to evaluate the present and future anthropic impact.