Metodi di analisi del dissesto e del rischio in zone terrazzate nell’ambito del Progetto ALPTER.

Instability of terraced landscapes: methods of risk analysis. - Within the alpter projectWithin the studies that form part of the alpter project particular attention was focused upon the evaluation of the risks facing man-made terraced hillsides. In Italy it was the Universities of Genoa, Padua and Milan, in France the University of Nice, which experimented various methods of enquiry aimed at developing scientific procedures. A range of conceptual approaches was adopted in order to develop a wide range of measures that might applied be on a wide scale in detailed studies of the structural strengths and weaknesses in terracing. Other chapters in this work highlight the fact that terraced hillsides pose a problem that can no longer be ignored; like all man-made structures, they require constant maintenance and particular care in the way they are used. If these requirements are not met, any structure will go into decline. This is particularly true of terracing, where neglect and decline can have serious – and sometimes disastrous – environmental effects on lower valley areas. The University of Genoa developed a large-scale onsite methodology. The experiments involved two sample valleys, the Val Bisagno (96 square kilometres) near the city of Genoa and the Valle Argentina (180 square kilometres) at the western end of the Ligurian coast, plus a small area of a few square kilometres around San Bernardino in the Cinque Terre National Park. The research looked at single terraced hillsides in order to identify how different forms of instability might be related to the structural characteristics of the type of retaining walls present. Apart from the objective problems posed by difficulty of access and the fact that a number of terraces were overgrown, this method proved valuable for two main reasons: it was relatively easy to acquire on-site data (thanks in part to the prior analysis of aerial photos and orthogonal photomaps) and to identify the areas at risk of instability. Applicable on a regional scale, the method is complemented by a geographical archive that makes it possible to cross-reference results with the other information already possessed by the bodies responsible for territorial management. The studies carried out by the Universities of Milan and Padua were more detailed in focus and more concerned with the development of instruments of enquiry. However, they both adopted different approaches. The University of Milan studied such systems in terms of the hydro-geological behaviour of terracing structures. The results highlighted that, whilst on the one hand, the flow of water within such terrain is of great benefit to agriculture, on the other it is this presence of water which makes terracing structures unstable: the degree of such instability varies sizeably according to the amount of water circulating within hillside terrain. The on-site studies made it possible to set up a very reliable model to forecast possible risks on the basis of an assessment of the relation between the circulation of water within the hillside and the thrust exerted against the retaining walls. This data is important because it allows one to identify the optimum flow of water within terraced terrain – that is, one that combine the best results for agriculture with the most acceptable levels of risk for terracing structures – and thence identify the type of crops that are most compatible with these levels of water supply. In conceptual terms, the University of Padua project was similar to that developed by the University of Genoa. However, it adopted a quantitative approach and made use of sophisticated new lidar technology (Laser Imaging Detection and Ranging), which makes it possible to acquire a remarkable quantity of data of exceptional precision (in theory, image resolution is 1- 1.5 points per square metre). As the authors themselves point out, the technique does have some drawbacks: not only is data expensive to collect and process, but its acquisition is substantially hindered by the fact that terraces are overgrown (a problem also faced by the methods adopted in Liguria). What this means is that the Padua method is applicable only in certain situations: on wide open hillsides that have little or no covering vegetation. I would also add that the method is still subject to a rather high margin of error, particularly in the presence of trees or shrubs. However, given the development of ever better data-filtering software, there is no doubt that this approach has one great advantage: it makes it possible to carry out surveys quickly and thus at regular intervals, hence providing a valid system for the constant monitoring of terrain. The French study looked into the role of terracing in preventing land erosion, focusing in particular on the protection it offers along the roads that flank the river Roja. Here various types of instability were identified, varying in relation to the use/non-use of the terraces and to the various characteristics of the retaining walls themselves (for example, incline, width and depth). These characteristics were thus seen in relation to threats faced by road structures in the area investigated. More than a method proper, this approach consisted of a survey technique employing GIS technology. Put briefly, all of the studies revealed that we still have a long way to go in our understanding of terraced areas and how they actually “work”. This may seem rather contradictory given that the terracing themselves were built using a strictly “common sense” approach inspired by great sensitivity to the characteristics of terrain. But whether the method of enquiry was on a regional scale or of more narrow focus, it brought out something which may perhaps be described as stunningly obvious: those areas of terracing which present the lowest risk of instability are those where there is still some kind of human settlement (and therefore use). Obviously, it is not these areas which should concern us (at least not as long as that human settlement lasts). What is required is the development of methods for the study and reclamation of areas where there is no longer such settlement and where there are the most evident signs of decay and decline. In effect, a range of methods has to be developed so that we can map the risks to terraced areas quickly and efficiently, to then propose strategies for mitigiting their effects. Only when we have a complete picture of these areas can we intervene with planning procedures that will improve conditions in mountain areas. This will not only result in better opportunities for those living in the mountains but might well stimulate a return to such areas – particularly to the lower valleys, where terracing represents a crucial problem.