Critical rainfall thresholds for triggering shallow landslides in the Serchio River Valley (Tuscany, Italy)

Abstract. The Serchio River Valley, in north-western Tuscany, is a well-known tourism area between the Apuan Alps and the Apennines. This area is frequently hit by heavy rainfall, which often triggers shallow landslides, debris flows and debris torrents, sometimes causing damage and death. The assessment of the rainfall thresholds for the initiation of shallow landslides is very important in order to improve forecasting and to arrange efficient alarm systems. With the aim of defining the critical rainfall thresholds for the Middle Serchio River Valley, a detailed analysis of the main rainstorm events was carried out. The hourly rainfall recorded by three rain gauges in the 1935–2010 interval was analysed and compared with the occurrence of shallow landslides. The rainfall thresholds were defined in terms of mean intensity I, rainfall duration D, and normalized using the mean annual precipitation. Some attempts were also carried out to analyze the role of rainfall prior to the damaging events. Finally, the rainfall threshold curves obtained for the study area were compared with the local, regional and global curves proposed by various authors. The results of this analysis suggest that in the study area landslide activity initiation requires a higher amount of rainfall and greater intensity than elsewhere

Growth of a sinkhole in a seismic zone of the northern Apennines (Italy)

Sinkhole collapse is a major hazard causing substantial social and economic losses. However, the surface deformations and sinkhole evolution are rarely recorded, as these sites are known mainly after a collapse, making the assessment of sinkhole-related hazard challenging. Furthermore, more than 40% of the sinkholes of Italy are in seismically hazardous zones; it remains unclear whether seismicity may trigger sinkhole collapse. Here we use a multidisciplinary data set of InSAR, surface mapping and historical records of sinkhole activity to show that the Prà di Lama lake is a long-lived sinkhole that was formed in an active fault zone and grew through several events of unrest characterized by episodic subsidence and lake-level changes. Moreover, InSAR shows that continuous aseismic subsidence at rates of up to 7.1mmyr-1occurred during 2003-2008, between events of unrest. Earthquakes on the major faults near the sinkhole do not trigger sinkhole activity but low-magnitude earthquakes at 4-12 km depth occurred during sinkhole unrest in 1996 and 2016. We interpret our observations as evidence of seismic creep at depth causing fracturing and ultimately leading to the formation and growth of the Prà di Lama sinkhole

Landslides triggered by the intense rainstorm of June 19, 1996 in southern Apuan Alps (Tuscany, Italy)

On June 19, 1996 an exceptionally heavy rainstorm (max. rain intensity 157.6 mm/h, total amount 477.6 mm/24 h) triggered hundreds of landslides in the southern Apuan Alps, particularly in the Cardoso Torrent basin. Here, the results of a preliminary study on the landslides triggered by the rainstorm are presented. Most of the landslides were probably first time landslides and mainly involved the regolith cover of the slopes. Very rapid or extremely rapid, debris slide-debris flow was the most frequent type of movement. Because of their high fluidity, sliding masses and debris flows often did not accumulate at slope toes, but flowed into the riverbeds together with the runoff. Thus, some villages in the valley floors were nearly destroyed and there were many fatalities. Landslides were probably responsible for most damage and victims: they abnormally overloaded streams with sediments and uprooted trees, which increased their destructive capacity. The aggradation of riverbeds reduced the supportable discharge of streams and favoured their flooding. The clogging of bridge spans caused local flooding and the collapse of landslide or sediment dams produced flood waves. The catastrophe demonstrated the extreme vulnerability of the area and the need for risk scenarios and prevention measures, which are dealt with in conclusion

Quarrying activities and geosites of the Apuan Alps (north-western Tuscany, Italy): coexistence possibilities and protection criteria

The Apuan Alps are a mainly calcareous-metamorphic range in Northern Tuscany that attains the height of ca. 2.000 m above sea level; they imposingly rise on the coastal plain of Versilia, along the Ligurian-Tirrenian Sea. On account of some peculiarities, the whole range, where a Regional Park was established in 1985, may be regarded as a geological heritage, or geosite, of particular environmental value; we refer, for example, to its “alpine” type morphology, characterized by sharp peaks, steep slopes and deeply cut valleys, that make the Apuan Alps very different from the adjacent main chain of the Apennines. Also relevant - and of still greater ambient importance - is the landscape which in a short distance grades from sharp white marble peaks to the coastal plain. There are also specific geosites, related to particular geological, geomorphological or environmental aspects. Among them, the following must be mentioned: surface and underground karst features (Antro del Corchia is among the widest cave system in Europe), such erosive phenomena as pot-holes and ancient glacial evidences, mainly cirques and moraines. By contrast, a marble quarrying activity has been in operation in the Apuan Alps for more than twenty centuries; it often interfered with some of the geosites. This interference resulted either in the destruction of the geosites, or in an important landscape modification due to the quarry excavation itself and the accumulation of marble fragments in quarry dumps (locally called “ravaneti ”). Moreover, the pollution of water supplies contributes to the alteration of natural conditions. With regard to ravaneti, their role is particularly complex; while changing the physical reality, in particular cases their intimate connection with the landscape raises ravaneti to the rank of geosites. So they, or at least the most important of them, become elements to be defended from reworking, through excavation of inert materials, a possibility frequently proposed today. Since the present legislation does not guarantee effective environmental protection, safeguard criteria must be sought and established, which may permit the coexistence of a productive industry, like quarrying, with the safeguard of geosites. The imminent drafting of the “Park Plan”, which should specify the areas to be used as a “total reservation”, appears to be the operational moment. This phase of territorial planning will start with a census of existing geosites, to be followed by the sanctioning of a set of obligations, aimed at protecting the identified geosites. In fact, only a global safeguard policy will contribute to the preservation of “landscape” and “environment” resources, as vital for the Apuan area as the economically important marble industry

Eventi alluvionali e fenomeni franosi nelle Alpi Apuane (Toscana): primi risultati di un’indagine retrospettiva nel bacino del Fiume Versilia

The most significant results of an archive investigation on the floods and landslides occurred in the Versilia River basin in the last centuries are presented. The main purposes are to contribute to in the assessment of the hydrogeological hazard in the Apuan-Versilian area as well as to collect useful data to catalogue disastrous events. Information on many past events was analysed. The research confirmed the vulnerability of the studied territory, which was emphasized by the tragic June 19th, 1996 hydrogeological catastrophe. Other high severity events were identified in 1636, 1774, 1846, 1885 and 1902, together with a lot of less in- tense events, causing however sensible effects. The events showed a tendency to recur in the same areas with a pronounced rise in frequency during the last centuries, likely depending on concurring factors: increased number and reliability of the information sources; increased attention to the damaging phenomena; springing up of the elements at risk; climate changes. In the Apuan-Versilian territory, the average frequency resulted of 1 sensible event every 4.2 years. Moreover, the collected data allowed a preliminary, but significant classification of the identified events; this classification was based on the event severity, deduced or estimated from the collected information. Finally, to better understand the peculiarities, a few of the most important events (years 1774, 1885 e 1902) were described in detail

Valenze ambientali ed economiche dei ravaneti delle Alpi Apuane (Toscana, Italia)

The Apuan Alps, in Northern Tuscany, are a mainly calcareous-metamorphic range where quarrying is particularly extensive, due to the abundance, quality and beauty of the marble (the Carrara marble is of worldwide renown). In this mountain range geological heritages, or geosites, are widely represented. These include surface and underground karst features, features of Quaternary glaciations and also the particular alpine-type morphology (sharp peaks, steep slopes, deeply cut valleys). Signs of the age-old quarrying activities are also evident. Among these, the accumulation of rock fragments in quarry dumps - locally named ravaneti - is typical of the Apuan Alps and is one of its most important characteristics. The ravaneti result from marble excavation and cover a large surface area of quarried zones. They are the result of an extensive quarrying method, with extensive use of mines, now obsolete and replaced by more up-to-date techniques. At present the ravaneti are taken into consideration from two opposing points of view, that is their environmental and economic-productive relevance. However, both viewpoints must find an accordance with each other. Indeed, over the last few years, these quarry dumps have been increasingly used as a source of inert materials and calcium carbonate, without any planning or regulation. By contrast, the environmental value of the ravaneti is scarcely taken into consideration, even though they have become an integral part of the Apuan landscape: their whiteness seen in the distance makes the mountainous massif even more particular. Moreover, the ravaneti, which resulted from ancient quarrying techniques, are an important indication of past extractive activity. They sometimes still show traces of quarrying work (e.g. the vie di lizza - rocky chutes once used by quarry workers for sliding marble blocks downslope -, the fragment retaining constructions and the ramps going up to the quarries). They are therefore important features of local industrial archaeology. Finally, the quarry dumps are very large and very pervious. They therefore slow down the rainwater concentration in the watercourses, thereby helping to protect inhabited centres against floods and contributing to aquifer replenishment. This is an important fact in favour of maintenance of the ravaneti . The present market trends lead to the fear that the intensive re-utilization of the ravaneti may become uncontrolled. This has certainly been the case of the indiscriminate quarrying activity in the Apuan Alps over the centuries. We believe it is essential to promote as soon as possible a series of studies, to determine which ravaneti deserve protection from indiscriminate re-utilization. Any selection criteria, based on thematic maps and project files, should consider classification parameters, including: scenic significance, morphology, characteristics of plant adaptation, historical and anthropic value. Even if the introduction of regulatory measures restrict the exploitation of a certain number of ravaneti, millions and millions tons of rock fragments will remain for market use. Moreover, such protective action will raise the tourist value of the area. In conclusion, no damage to the economy of the Apuan district is expected from the adoption of a rational policy for safeguarding the environment

The contribution of the historical data in evaluation the flood and landslide hazard in mountain areas: the case of the Versilia River basin (Italy)

The Versilia (NW Tuscany) is an important tourist region of Italy, enclosed between the Ligurian Sea and the Apuan Alps. Because of this particular geographic position, Versilia is often interested by intense rainfalls that sometimes cause severe damage and destruction. In this context, an historical archive investigation on the floods and landslides occurred in the Versilia River basin in the last centuries was carried out. The research aimed at contributing to the hydrogeological hazard assessment in the Apuan-Versilian area as well as to collect useful data to realize a catalogue of disastrous events. Thus, much information on many past events was analized. The results confirmed the hazard and vulnerability of the studied territory, which was emphasized by the June 19, 1996 hydrogeological catastrophe (14 deaths and about 500 millions of Euro). The studied area was hit by severe events also in 1636, 1774, 1846, 1885 and 1902, together with a lot of less intense events, causing however sensible geomorphologic effects and damage. Moreover, the considered events showed a tendency to recur in the same areas and a pronounced rise in frequency during the last centuries. This likely depends on many concurring factors, among which there are: increased number and reliability of the information sources; increased attention to the damaging phenomena; springing up of the elements at risk in hazardous areas; possible climate changes. In particular, in the Apuan-Versilian territory, the average frequency resulted of 1 sensible event every 4.2 years. Furthermore, a preliminary, but significant classification of the identified events was elaborated; such a classification was based on the event severity, deduced or estimated from the collected data. The main results showed that an event characterized by intensity comparable with that of the June 19, 1996 event could have a recurrence time of about 100 years. On the contrary, other hydrologic/pluviometric studies estimated the recurrence time of such an event at a few hundreds of years. It could be inferred that in some particular situation the hydrological studies underestimate the hazard. From this point of view, an accurate historical research could provide a significant contribution in evaluating the hazard of a territory

Deep - seated gravitational slope deformations in north - western Tuscany (Italy): remarks on typology, distribution and tectonic connections

Some features make north-western Tuscany prone to Deep-seated Gravitational Slope Deformations (DGSD), such as: high relief energy, high rainfall, intensely fractured and deformed rocks, active or recently active tectonics, strong seismicity. The territory investigated shows many examples of such phenomena, which differ owing to their geological and structural conditions, typology and activity. The case history shows that both tectonics and lithostratigraphic structures have greatly influenced typologies and kinematic mechanisms, whilst neotectonic evolution and climatic conditions have had their influence on all phenomena studied. Among the cases where the tectonic structure is a prevailing factor, deep-seated gravitational slope deformations located on normal faults could be quoted, as those outlining the tectonic depressions of the Serchio and Magra valleys or other regional normal fault systems. These faults release and subdivide the bedrock into very large blocks, making them subject to gravitational adjustment. In addition, a preferential orientation of geomorphic features (scarps, trenches, reverse slopes, etc.) can be verified in accordance with the trends of tectonic displacements and brittle deformation systems. Some examples include the gravitational processes near Canossa, Bagnone and Chioso in Magra Valley, near San Romano in Garfagnana in the Serchio Valley and near the Abetone Pass. Block slide and rock flow are generally common types of movement, often structurally controlled by fault planes. The lithostratigraphic structure may be frequently regarded as the main control feature in the development of deep-seated gravitational slope deformations; in particular, a structure of thick rigid rocks overlying ductile rock types is instrumental. In this case, block slides and lateral spreads are the most common kinds of movement. Some examples are found at Mt. Castri in the Serchio Valley (sandstone over shale) and in the Magra Valley near Bagnone and Chioso (marly limestone overlying shale). The underlying ductile rocks are remarkably deformed by tectonics and sometimes show deformations resulting from the load of overlying brittle rocks, such as bulges and reverse slopes. Along the deep valleys which transversally cut the main tectonic structures, the topographic stress is sometimes considerable, owing to the Pleistocene - Holocene uplift; brittle/ductile deformations may then occur, due to gravitational slope tension rather than tectonic stress. Finally, a considerable amount of the DGSD here studied are believed to be dormant, because of debris filling up the trenches and no movement evidence in the past decades; in some cases, evidences of activity were recognised

A swedge stability analysis in critical stability conditions and definition of a possible remediation

A large block of limestone in rather critical stability conditions is placed above the road that leads to the Wind Cave (Vergemoli, LU). The stability of this part of the slope has been investigated since it is sited on the same road and especially over a public car park. It was identified the potential remediation to obtain an adequate safety level. A geomechanical analysis and several laboratory tests for rock mass characterization were conducted to define the block stability

Carta geologica d'Italia a scala 1:50.000, Foglio 234 Fivizzano

The geological sheet n. 234 Fivizzano is part of the new Geological Map of Italy, at 1:50,000 scale (CARG Project). The results are described in the explanatory notes, the following main results should be emphasized: lithostratigraphic review and dating of several formations of the tectonic units; mesostructural analysis of mapped tectonic units; identification and mapping of many landslides which were unknown