Valutazione del rischio nel Paesaggio terrazzato ligure.

Risk assessment of terraced landscape in Liguria - The territory of Liguria is of a complex morphology characterized by the presence of steep slopes and erosive torrents. Terracing here dates back centuries and as previously pointed out (see the description of the region’s geomorphology) played an important role in restricting soil erosion. However, that role was severely undermined when, as a result of industrial growth within the region, people moved away from the mountain areas and the system of terracing was left to go into slow and inexorable decline. This process of decay can without any doubt be defined as a key cause of the hydro-geological instability of the slopes of Liguria, and as such is to be studied in order to develop the instruments best suited to remedy it. Working on behalf of the Regional Government of Liguria, the team from the University of Genoa that participated in the alpter project focussed on the development of a method for assessing the risks arising from the gradual abandonment of terraced hillsides. As is observed in Shresta et al. (2004), terraced hillsides cannot be studied using traditional methods as they constitute a man-made environment, where the dynamics in the behaviour of terrain have been modified. The evaluation of the risks faced/posed by terracing requires a method that makes it possible to identify as quickly as possible the places where the situation has become critical. It is impossible to adopt a “blanket approach” in intervention to protect terraced hillsides, so what is needed is a tool that enables one to establish which areas should get priority. Given the scale of the problem when seen in the region as a whole, the first step was to identify the main characteristics of individual areas of terracing. This was done by the compilation of a specific information sheet that covered such features as surface area, state of maintenance, land use and the evidence of instability (the three categories of “wall buckling”, “rupture” and “total collapse”). The survey information – and the subsequent analysis of the data gathered – was carried out hillside by hillside, following the method illustrated by Carrara et al. (1991,1995). The validity of this “hillside scale” is borne out by the fact that terracing forms a mutually interdependent system of support: if there is a decline in the maintenance of just one part thereof, the entire system suffers as the result of a sort of “domino effect”. If nothing is done about the collapse of a single stretch of terracing, the entire hillside suffers eventually. The data collected in the field was then processed using gis and examined in relation to the geomorphology of the territory concerned: gradient, degree of exposure, lithological characteristics. The cross-referencing of the data then made it possible to identify the factors that have a critical influence on the stability of terraced hillsides. the sample areas: the hydrographical basins of the bisagno and argentina torrents The sample area within the Cinque Terre National park was not included in the overall processing of data: given its small size, it could not be considered as representative and therefore did not serve the primary aims of the project. The survey of the Bisagno torrent resulted in the development of certain morphological-statistical parameters that brought out the relations between the structures of the terracing and the type of hillsides where they were to be found. These parameters then served to develop a model that might be used in identifying the areas of terraced terrain where there were risks of instability (brancucci, paliaga, 2006). The validity of this model was then tried out in a survey of the Valle Argentina. The water basin of the Bisagno torrent covers an area of about 96 square kilometres. The substratum is made up of marly limestone, argillites, slates and schist. The terrain is steep-sloped and, at a short distance from the coast, reaches up to a maximum of 1,000 metres above sea-level. The climate is typically Mediterranean along the coast, whilst further inland the winters can be harsh; the average rainfall varies between 1,600 and 1,800 millimetres (Brancucci, 1994). Within the basin the main forms of terrain instability are the result of debris flow, surface landslides and some sizeable paleolandslides (these latter themselves now the site of large areas of terracing). In the lower part of the basin there is a high risk of flooding, a phenomenon which in the last 40 years has caused considerable damage and some fatalities. More than 20% of the basin is occupied by terraced terrain, most of which is now in a state of total or semitotal neglect. The basin of the Argentina torrent extends over more than 180 square kilometres and has similar morphological and lithological characteristics, with steep slopes and even higher maximum altitudes (Monte Saccarello reaches up to more than 2,000 metres above seal-level). In both torrent basins, most of the terrain instability is to be found on the north- and south-facing slopes, which are of steeper gradient. It is, however, distributed equally between cultivated and abandoned areas (in the former occurring independently of the type of land use). Finally, the instability seems to be concentrated on the smaller rather than the wider slopes. The lithology of the substratum, which in part determines the drainage qualities of the terrain, does not seem to be a decisive factor in either zone; however, this observation is yet to be proved conclusively. Such a result, which might appear to contradict the usual models of erosion, might be explained by the greater importance to be attributed to other parameters affecting stability: gradient, exposed position and the characteristics of the techniques used in building the retaining walls. The first factor is related to the intensity of the processes of erosion, whilst the second plays a role in determining weather conditions (in both areas, these included heavy rainfall, often associated with strong southern winds). The third factor (construction techniques) plays a role in determining the efficiency with which the terrace walls withstand the thrust exerted by terrain. The north-facing slopes revealed a greater incidence of instability, which can perhaps be explained as due to weather conditions: in fact, these slopes are generally colder and wetter and, at higher latitudes, are more subject to the freezing and frosts that accelerate the crumbling of the dry-stone terrace walls. This observation of the link between exposure and terrain instability – together with analyses to-date of the available data – suggests that gradients of 30° and above are those at risk. There is yet to be in-depth research into the apparent lack of relation between human settlement/use of terraced terrain and its susceptibly to landslides. One first, rough-and-ready, explanation of this might be that the techniques used in restoring the walls are inadequate or unsuitable, particularly with regard to the permeability of such structures. This would mean that such “restored” walls are just as dangerous and harmful as those which have been left to decay and crumble.