I sistemi terrazzati: un patrimonio, un rischio.

Terraced systems: heritage and risk - Artificial terracing is a “complex system of transforming steep slopes to create cultivatable areas through the conservation of land resources and the use and optimal management of water resources”. The results of this transformation can be seen in the extremely varied and complex landscapes qualified as World Heritage sites (Cinque Terre): probably the only case of a natural landscape achieving this status. In actual fact, terracing is much more than just a scenic resource regarding which a great number of papers and essays have been written and its value is indisputable: despite being anthropogenic, terracing is, above all, an integral element of the morphogenetic system, namely of the series of processes that generate the shapes of the earth’s surface. As soon as a rock comes into contact with the atmosphere, it is subjected to a series of physical and chemical processes and agents that modify its status, transforming its original properties. Firstly, the rock is weathered and debris is gradually generated: this brought to the formation of broken material formed by small pieces of varying size. This debris can then be transported for small or great distance depending on its position in space. If the debris is in an area where the morphology is gentle, featuring low gradients, it will most probably not move a great distance from its original position (autochthonous debris). On the other hand, if the mass of debris sits on a steep slope, it can travel for long distances (allochthonous debris), depending on the factors described below. So let’s take into consideration a natural slope, i.e. that portion of the earth’s surface that runs between a watershed and a valley floor. It is characterized by geometric parameters, such as: exposure, gradient and length of the slope. By exposure we mean how the “face” of the slope is oriented with respect to the path of the sun; the gradient is the angle formed between the slope and the horizontal plane; and the length is the distance from the watershed to the valley floor. These three parameters, when inserted in the context of erosion processes, determine how mountain ranges evolve. At latitudes where water is the main modelling agent, its erosive power usually increases as the gradient and length of the slope increase. The increase in the erosive power of unchannellized water means that the material produced as a result of rocks weathering is unable to stay where it is and is instead carried away by the water (and by the ever-present force of gravity). There is another piece that has to be added to the puzzle: if a certain amount of organic material – whether of animal or plant origin – is added to the mass of debris, we get a product that goes by the name of soil, which is the raw material required for farming. The soil-forming process is called pedogenesis (pedogenetic process). The more organic material is added and the greater the mass of debris, with the addition of water and a favourable climate, the richer (more fertile) the resulting soil will be. In order to develop, the pedogenetic process nonetheless needs a stable geomorphologic environment, which implies a low gradient of the slopes. Indeed, it is the gradient that increase the velocity of surface water , and hence the strength required to move the debris. Consequently, steep slopes prevent the pedogenetic process from being completed with the result that no or a very poor and not fertile soil develops, making it inhospitable for agricultural purposes. This, in brief, is the morphological situation typical of the areas involved in the alpter project: hilly or mountainous areas, hence featuring quite steep gradients, with few flat areas and little soil avalaible for agricultural practices. The need for slopes that are compatible with soil formation led the old farmers to “build” their own flat ground. Thus they resorted to terracing, which, as we mentioned earlier, is a “complex system of transforming steep slopes to create cultivatable areas through the conservation of land resources and the use and optimal management of water resources”. Terraced hillsides, though, interact with the morphogenetic system. This interaction mainly lies in the forced immobilizing of large amounts of material by means of dry stone walls or terrace steps (calculate approx. 1 cu m of earth for every linear metre of dry stone wall). Looking at the interaction of terracing in terms of interaction with the morphogenetic system, we see that it can have feedback – like any anthropogenic action on the land – leading to positive and/or negative effects. Terracing has both effects: positive in that it limits the erosive action of water washing away soil and debris, encourages pedogenesis, contributes to the overall stabilization of evolution in sloping areas and helps optimize the regimes of rivers and streams; and negative because it actually robs the erosion cycle of material, limiting the amount of debris that could contribute to the formation of alluvial plains or, in the case of coastal areas, possibly compounding the shrinking beach problem as less material is carried to the sea. However, we should not forget that terracing is an anthropogenic process, i.e. it is the work of man. Ever since man appeared on the Earth, he has interacted increasingly with the modelling processes shaping the earth’s surface. This increase has been and continues to be directly proportional to the technical skill and technology that man manages to bring into play. Human intervention of any kind must be kept efficient and this is particularly true in the case of work like terraces, which – despite being brilliant in concept – are actually fragile unless their structure is kept intact. As a matter of fact, the terraced system’s constituent parts, retaining works and, above all, water control elements need constant attention. Indeed, in those areas where changed social conditions have led to the migration from countryside to urban areas, risk factors linked to the increase of the erosion rate become an actual problem, with the result that hill- and mountainsides tend to revert to the original conditions of slopes described earlier. Water washing away debris, once its flow is no longer regulated, and retaining works that are left unattended have a part to play in causing areas of instability. The more extensive the portion of abandoned land and the longer it has been abandoned, the more important are the instability processes affecting areas. The balance, which has been skilfully maintained for so many years, quickly reaches a breaking point. To make matters worse, on the one hand the expertise required to maintain the terraced hillsides efficiently is practically extinct in today’s society and, on the other, with regard to the type of agriculture (appropriately called “heroic”), there is no economic basis to encourage wide-ranging repair work. In a best case scenario, repairs are limited to restoring very small portions of land that are mainly devited for often high-quality niche production. This neglect of the terracing is seen, as will be illustrated in the following sections, even where reconstruction measures have been attempted but prove incompatible with the structure of the area both in terms of the scenery and, from a functional point of view. As we have mentioned, terracing is found mainly on steeper slopes. Nonetheless, it is not unusual to see terracing in privileged locations on hillsides made up of loose deposits driven by gravity, bedrock debris, ancient stabilized landslide deposits etc. that have built up to a certain thickness. In most cases, the grain size features of these deposits are ideal for the ground to be exploited for agricultural purposes since they mostly feature: – a matrix that is not too fine and incorporates stone fragments in a diversity of sizes; – chips that are tens of centimetres in diameter; – blocks that can be used to build retaining walls or rural buildings associated with the farming practices; – erratic boulders, on occasion, which can measure several cubic metres. In the areas investigated by the project, moreover, it is not unusual to find terracing built on stabilized landslide deposits (Fig. 2), namely based on landslides that occurred in the ancient past where the gradient - which is usually less steep than in the surrounding areas - allows a kind of terrace to be built that is wider and not as high. The extent of the change on the landscape, both from an aesthetic point of view and in terms of the whole hillside’s drainage system, is radical. The drainage element is essential to maintain these structures since the moment they are neglected/no longer used, the environment tends to revert to its original form, progressively hiding the “irregularities” constituted by the terracing (blake et al., 2003; gisotti, 2003). When farming is discontinued, it leads to the loss of land qualities due to the humic/clay complex, which is rich in organic substances. As a result, the particles of loose earth with a sandy matrix tend to slip easily through the gaps between the elements of the wall holding up the embankment (brancucci et al., 2001). The surface water that seeps into the subsurface flows toward a medium whose permeability has been reduced, starting a failure process that affects the wall and, consequently, the terrace (masetti et al., 2005). The eventual failure of part of the terrace can develop a preferential path for runoff, which becomes less and less controllable and increasingly erosive. Consequently, a chain reaction can ensue undermining the whole hillside system, to the point where its stability is compromised (Fig. 3). From a geomorphologic point of view, the most interesting aspect of the problem arising from the abandoned use of terraced slopes is precisely this one, implying, the ways in which the landscape reverts to its original situation, preceded by the structures’ various stages of deterioration. Based on observations, the deterioration phenomena can be classified as: a) internal phenomena, depending on the walls’ construction features: – phenomena depending on defects in the wall’s construction, such as incorrect sizing of the wall or incorrect arrangement of the stones the actual wall is built with; – phenomena depending on the walls’ “natural” deterioration processes. b) external phenomena, not depending on the walls’ construction features (Fig. 4). – of natural origin; – induced by human activity. As far as deterioration generated by natural factors that do not depend on the walls’ structure is concerned, the following phenomena have been pointed out: – failure of elements at the top of the wall (Fig. 5) due to surface water runoff (when the top of the wall is built with small-sized elements); – partial failure of the wall due to loss of stability as a result of a progressive increase of strain induced in the median part of the wall by backfill pressure. – base of the wall shifts, probably due to the force exerted by the backfill (this phenomenon can be accentuated by incorrect building of wall foundations as well as by the action of animals). As far as deterioration induced by human activity is concerned, we basically refer to the practice of abandoning farming and, consequently, to the neglect of terrace maintenance, without which terraces easily loss their benefic effects on slope stability. The maintenance consists of a series of small, neverending operations such as pulling out weeds; clearing stones from the cultivated land; tidying up and repairing dry stone walls; and cleaning drainage channels. The absence of these operations triggers the collapse of the whole hydrogeological control system constituted by the terracing. At the beginning the irrigation ditches and water collection channels become clogged (Fig. 6): grass, stones and earth prevent rain from flowing into the proper channels, meaning that water runs over the whole surface of the terrace, which is made less permeable due to the overgrown grass that is left unmown and the weeds that suffocate the crops. The hillside, which is divided into a succession of terraces, ends up interrupting the water’s flow with drops and obstacles, causing it to become turbulent and even more erosive than a laminar flow of water over even slopes (see cemagref, 1988). In any case this subdivision of the causes of deterioration into natural and anthropogenic causes must be considered as a theoretical schematization only, which we can gather from the very meaning of terracing, i.e. the artificial shaping (therefore, decided by man) of an area that would otherwise not be farming friendly. Consequently, man is the main engine behind the “birth”, “survival” and “death” of his creation. 1. types of dry stone wall deterioration In a terraced environment that is being neglected, various kinds of failure can arise: the erosive action of water can result in the undermining of the foot of the wall retaining the terraced strips, which can then topple. Water is free to seep into the terraces in an uncontrolled and ever more violent way, generating high pore-water pressure on the walls, which start to bulge and then leading to actual landslides. Moreover, landslides and bulging walls can be caused by uncontrolled groundwater, or by a lack of maintenance on walls. The structure of the walls is weakened by frost or by the thrust of tree roots allowed to grow unchecked, thus triggering the failure of parts of the walls, starting with the top stones, which creates openings for water to flow through. It is common to see parts of wall that have collapsed, leaving a clear small scarp in the part of the backfill previously retained by the wall, and a downhil area where the rest of the wall have been accumulated mixed with the and slipped soil. When a landslide occurs, an opening is created that causes an increase in water flow rate when it rains. As a result, the next wall down is in greater danger of succumbing to deterioration itself and this eventually leads to trails of wreckage that can be clearly seen on hillsides that are being neglected1. The wall-less earth banks, on the other hand, begin to fail when the drainage channel system is abandoned: water runs unchannelized along the banks where the grass is no longer mown, potentially causing small landslides as a result of erosion induced by the surface runoff. Terraces failure triggers a domino effect: from the top down, with the slippage of a terraced strip high up the slope eventually affecting all the strips downhill; or from the bottom up, with the collapse of a strip of land inducing instability in the wall above it, which no longer has foot protection. The urbanization of valley floors is a trait common to many terraced valleys: the morphologic situation has forced almost the entire population to cram into the only existing flat or almost flat areas, meaning the danger associated with the deterioration of the terraces lying above densely-populated areas is considerable. The abandonment of crops is generally followed by the hillside’s renaturalization as spontaneous vegetation (often weeds) takes over. This phase, which can vary in length depending on the location’s characteristics, presages the appearance of various shrubby pioneer species, which will develop and give rise to complex associations, until the wood is fully reinstated. Nonetheless, we should bear in mind that the transition from cultivated to revegetated goes through a period of extreme dangerousness, especially considering the fire risk, which is typical of the stage during which terraces are abandoned and the terraced structures deteriorate. Analysing how these renaturalization phenomena occur and how the growth of suitable species to consolidate the earth might be fostered is a rather interesting, albeit extremely complex, matter. Nonetheless, even supposing that wood growth restores stability to the hillside, this entails losing the agricultural land and cultural heritage offered by the terraced systems, which is not to be underestimated. 2. abandonment, visual conflict and disarray on the landscape So-called semi-abandonment seems to be almost as frequent in terraced areas as the abandonment problem. We frequently come across non-traditional farming techniques that fail to use the terrace structure correctly, from the point of view of function. Rubbish, such as bed frames by way of fencing and bathtubs for collecting rainwater, is often used in the farming of terraced strips, thus employing fewer financial resources and less energy. The visual conflict caused by these forms of “sub-farming” on the landscape is no less serious than the risk of hillside instability resulting from lack of maintenance, which the terraces really needed. Greenhouse farming deserves its own special mention. If built without the necessary precautions, greenhouses can cause serious damages. Even when the hillside is not completely transformed by a series of concretereinforced terraces, during heavy rains, the waterproof surface of the greenhouses still causes a surficial runoff dangerous for the terracing itself as well as the people living below. Moreover, the large amounts of chemical fertilizers, weed killers and insecticides used must be suitably removed from the terraced land so as not to pollute the area downhill or contaminate the aquifers. From the point of view of landscape ecology, greenhouse farming can be seen as an industry for all intents and purposes2. What’s more, when it comes to how the landscape is perceived, greenhouses stand out as rigid, messy volumes that are certainly not a joy to behold, unlike a terraced hillside planted with vines or olive trees. The loss of knowledge of the terracing culture also has a considerable effect on what consolidation systems are chosen: replacing dry stone walls with reinforced concrete walls with no outlet for water, without allowing for drainage, can lead to even more serious failures.