Landslides are widespread in the Dolomites and mainly consist of mass movements occurring from the Lateglacial to date. All the the different types of landslides described by Cruden and Varnes (1996) can be found in the Dolomites (Plate MP-3). The frequency and magnitude of gravitational phenomena is proved to be very high in the last Post-glacial period when slopes no longer sustained by ice masses, were affected by many large scale landslides. Panizza (1973) showed how these landslides seem to be concentrated downstream of the confluence of glaciated valleys where “glaciopressure” might have been more intense. These stresses were particularly strong at the convergence point of glacier tongues in a narrow valley during the advancement of the LGM glaciers. Deformations seem to have taken place in correspondence with surfaces of structural discontinuity such as stratification, faults and joint planes, giving rise to or accentuating the process of disjointing the adjacent rock masses and therefore creating surfaces of potential displacement. Among these mass movements due to glaciopressure, the following can be quoted: cima Rosetta in Val Cismon, Mt. Faloria and Mt. Antelao in Boite valley, Mt. Ponsin in Duron valley. The analysis of the dated landslides has allowed correlations to be outlined between increases in landslide activity and climatic changes in the study areas (Soldati et al., 2004). The first phase of marked slope instability is observed in the Preboreal and Boreal (about 11,500 to 8500 cal BP) and includes both large translational rock slides, which affected the dolomite slopes following the withdrawal of the LGM glaciers, and complex movements (rotational slides and flows) which affected the underlying pelitic formations probably favoured by high groundwater levels due to an increase of precipitation and/or permafrost melting. A second concentration of landslide events occurred during the Sub-boreal (about 5800 to 2000 cal BP), when slope processes mainly ascribable to rotational slides and/or flows took place in both the study areas. In the light of collected data, the events dated may be considered reactivations of older events linked to the phase of precipitation increase which has been documented in several European regions during this mid-Holocene period. The recurrence in time of this landslide activity was certainly influenced also by other non-climatic factors, in particular geological-structural ones. First of all the spatial distribution of geological formations with different mechanical characteristics must be taken into account. In particular, the occurrence of landslides is high where rigid and resistant rocks (dolomites, limestones etc.) showing brittle behaviour, overlie weake rocks characterised by ductile behaviour (marls, clays etc.). Furthermore, also where the effects of tectonics were most intense, in correspondence with faults or overthrusts, mass movements have been favoured. Gravitational landforms are also connected with the existence of deep-seated gravitational slope deformations, recently recognised in the Dolomites and particularly in the area of Cortina d’Ampezzo. With respect to morphological evidence, they are generally characterised by the presence of trenches, gulls and uphill-facing scarps in the upper parts of the slopes and bulges in the lower parts (e.g. Tofàne, Lastoni di Formin and Faloria groups). It must be emphasised that deep-seated deformations may be the initial stage of large-scale mass movements whenever the deformation belt develops into a sliding plane. However it has been observed that the presence of these processes favours or induces “collateral movements” (rock falls, slides and flows) in the surrounding areas. With reference to rock falls that have recently occurred from some Dolomite peaks (Cinque Torri, mount Cir, Cima Una ecc.), it can be stated that these are quite normal events of the genetic evolution of these as well as others mountains of the Alpine chaine.
|Landslide type /
Prà e Lagunaz
System : Pelmo – Nuvolau (1)
Material involved: Dolomia Principale
System : Puez/Odle (6)
Material involved: La Valle and
System : Dolomiti settentrionali (5)
Material involved: Dolomia Principale
and Travenanzes Formation
System : Dolomiti settentrionali (5)
Material involved: scree slopes
System: Dolomiti settentrionali (5)
|Spread||Lastoni di Formin
System : Pelmo – Nuvolau (1)
Material involved: Cassian Dolomite,
Heiligkreuz Formation and San Cassiano
Plate B. Examples of landslides in the Dolomites, according to the scheme of Cruden and Varnes (1996). By M. Soldati, unpublished.
DETAILS ON THE LANDSLIDES QUOTED IN PLATE B
Prà and Lagunaz
The rock fall occurred on 3 December 1908 on the southern slope of the Pale di San Lucano. The villages of Prà and Lagunaz, located in the San Lucano valley, were almost destroyed and 28 peoples were killed. The volume of material detached from the vertical slopes of the mountain massif, consisting of Dolomia Cassiana, has been estimated in 250,000 m3.
The rock fall occurred on 20 July 2006 at Piz Sompluf in the Community of S. Martino in Badia. It is an emblematic example of recent rock falls probably favoured by permafrost degradation and frost shattering processes. A volume of 40,000 m3 detached from a vertical cliff made up of Dolomia Cassiana, at a height of 2400 m. The fallen material travelled for a distance of 800 m on the debris cone at the foot of the slope.
The Mt. Salta landslide is located on the opposite slope of the more famous Vajont landslide. The landslide consists of rock fall deposits mainly accumulated due to an event dating back to 1674. The landslide has a volume of ca. 1,000,000 m3 and covers an area of about 160,000 m2: It is prevalently made up of a chaotic accumulation of blocks of the Vajont Limestones, partially covered by scree slopes, some of which reaching more than 800 m3. Isolated blocks belonging can be also found below the village of Casso.
The Sassere landside is located valleyward of Malga Flavona, near the boundary of the system. It is a huge Holocene rock fall accumulation with an estimated volume of 55 million m3. The accumulation is chaotic and characterized by the presence of large blocks of limestones of the Calcari Grigi formation. The landslide is characterized by a clear correspondence between the source and accumulation areas, which is not the case for most large scale landslides observable in the system.
An evident topple is observable at the Cinque Torri, isolated rock monoliths made up of Dolomia Principale in the vicinity of Cortina d’Ampezzo. The Torre Grande, the highest monolith (2361 m), characterized by almost vertical walls reaching the height of 150 m, is affected by open fractures which extend as far as the substratum; this leads to the subdivision of Torre Grande in some rock blocks and has favoured a rock topple.
The upper part of the Passo Gardena landslide is a significant example of rock slide affecting the Dolomia Cassiana. In the Upper Badia Valley. It evolves into a rotational slide affecting weak clayey rocks of the S. Cassiano and La Valle formations, and then becomes an earth slide – earth flow of some million m3 of clayey material (cosmogenic 36Cl AMS dating: from about 11,800 to 8500 yr BP).
A spectacular roto-translational rock slide is clearly visible on the west side of the Settsass mountain group between 2200 and 2000 m of elevation. The slide appears as a chaotic accumulation of large blocks of Dolomia Principale mobilised because of displacements that affected the underlying Raibl Formation.
The Corvara landslide affects an area of more than 2.5 km2 located immediately uphill of the village of Corvara in Badia (Pralongià plateau), at the SE boundary of System 6. It can be classified as a rotational earth slide, which locally shows evidence of earth flow, giving to the landslide a complex style of activity. It affects slopes made up of the La Valle and San Cassiano formations. The estimated overall volume is of more than 300 million m3. It is known from several radiocarbon dates that the landslide has moved since at least about 10,000 cal BP, and that it underwent a second major phase of morphological development from about 5000 to 2500 cal BP. The landslide is active at present, with movement rates ranging from about 0.01 to 2 m/year.
Nomination of the Dolomites for inscription on the World Natural Heritage List UNESCO
A significant example of rock flow (sackung) have been recognised at Mt. Faloria, which is located the eastern slopes of the Cortina d’Ampezzo area. Here steep cliffs of Dolomia Principale overlie marls and clays of the Raibl Formation. The Dolomia Principale shows a dense net of joints, which heavily modified the mechanic behaviour of the formation, thus weakening the rock strength. Sackung-type deformations occur within its SW slope. In the highest parts numerous trenches, partially filled by large quantities of debris, are evident indicating a recent activity. Actually, a sector of the slope eventually encountered a sudden evolution, represented by a rock avalanche.
A debris flow occurred on 4 September 1997 in the area of Cortina d’Ampezzo where it caused a significant threat owing to the intense urban development*. The event, which affected the talus fans at the foot of Mt. Pomagagnon near the village of Fiames, blocked the state road no. 51 “Alemagna” and, after sparing some houses, barred the course of the Torrent Boite and formed an impoundment. This debris flow aroused great concern among local authorities; therefore the construction of embankments for protecting the buildings threatened by the landslide was carried out. Other debris flows are along the Boite Valley (Acquabona, Cancia etc.), Parola Valley and Upper Gardena Valley.
One of the oldest dated earth flow in the Dolomites (a wood sample find in a borehole at a depth of
42.3 metres dated 10,035 ± 110 yr B.P.) is located at the boundary of System 5, on the western slopes of the Cortina d’Ampezzo hollow. Movements have repeatedly involved the terrains of the San Cassiano Formation during the Holocene and the landslide is still partially active. The thickness of the deposit, which has been estimated at 60 m through the evidence of a borehole, witnesses a prolonged sliding activity, particularly intense between 10,000 and 9000 yr B.P. The rate of movement has lately reached the value of 2 m/yr.
The landslide affects the Cherz plateau (ca. 2000 m a.s.l.) that is located at the foot of the Settsass mountain group ), at the SW boundary of System 5. The landslide shows a complex style of activity, but the main type of movement is that of coalescent earth flows which convey material of the La Valle and San Cassiano formations from the upper part of the slopes, where rotational slides occur, to the Cordevole valley (near the villages of Cherz and Contrin). The accumulation collects also debris flowing down from the foot of the southern sector of the Settsass mountain group. The landslide is clearly active and the movement is favoured by the presence of several ponds on the accumulation.
Lastoni di Formin
One of the most peculiar cases of rock spreading in the Dolomites is that of the Lastoni di Formin which are located SW of Cortina d’Ampezzo on the right side of the Rio Falzarego. The Lastoni di Formin can be described as a thick jointed plate of Dürrenstein Formation and Dolomia Cassiana overlying the marls, arenites and clays of the S. Cassiano Formation. The peculiarity of this phenomenon consists in the presence of various stages of evolution at the same time. Lateral spread phenomena are prevalent in the upper part accompanied by progressive displacements of the blocks downslope, which gives the slope a step-like morphology.
The analysis of the dated landslides has allowed correlations to be outlined between increases in landslide activity and climatic changes in the study areas (Soldati et al., 2004). The first phase of marked slope instability is observed in the Preboreal and Boreal (about 11,500 to 8500 cal BP) and includes both large translational rock slides, which affected the dolomite slopes following the withdrawal of the LGM glaciers, and complex movements (rotational slides and flows) which affected the underlying pelitic formations probably favoured by high groundwater levels due to an increase of precipitation and/or permafrost melt