Technical Note

Landslides (2012) 9:417–432 Rosa María Mateos I Inmaculada García-Moreno I Jose Miguel Azañón DOI 10.1007/s10346-011-0290-8 Received: 10 December 2010 – Accepted: 1 August 2011 Freeze thaw cycles and rainfall as triggering factors Published online: 20 August 2011 © Springer-Verlag 2011 of mass movements in a warm Mediterranean region: the case of the Tramuntana Range (Majorca, Spain)

Abstract Between 2008 and 2010, the island of Majorca (Spain) minima at −6.8°C, which are anomalous values in the mild experienced the coldest and wettest winters of the last 40 years. Mediterranean climate. The result was that 34 mass movements Accumulated rainfall was twice the average and values of intense were triggered (Table 1), distributed along the Tramuntana Range rainfall up to 296 mm/24h were recorded, very similar to those (Fig. 1), namely 14 rockfalls, 1 rock avalanche and 15 earth slides– calculated for a return period of 100 years. Additionally, high earth flows (Cruden and Varnes 1996). There are also records of four precipitation coincided with anomalous, low temperatures, with karstic collapses, which shall not be analysed in this study as they abundant snowfall and freezing in the highest zones of the require a distinct analysis. Tramuntana Range, in the northwest sector of the island. As a result, The mass-movement inventory covering this period was carried 34 mass movements were recorded on the range, which seriously out with the support from the Emergency and Road Maintenance affected the road network in an area of great importance for tourism, Services, which informed us of every occurrence as such, and most of as it welcomes 8.5 million visitors each year. Fourteen rockfalls, 1 rock the movements were dated. Others were reported by private avalanche, 15 landslides and 4 karstic collapses were inventoried. The individuals, and the remaining cases were located during two geological structure, formed by a series of NW overlapping thrusts, helicopter flights over the range. As Table 1 shows, some of them determines the distribution as well as the failure pattern of the could not be dated with precision. For each of them, a detailed report movements. Thus, the northern face of the range registered 68% of was written with information regarding size, type of failure, runout, the mass movements: nine rockfalls with planar failure took place as materials involved, damage, etc. well as all the landslides recorded. Likewise, south-facing slopes have Fortunately, there were no deaths, but there were numerous been affected by longer runout rockfalls with a wedge-type failure. cases of damage to dwellings, holiday apartment blocks, barns and The thorough analysis of the meteorological data shows that most of power stations etc., and especially the road network in the range, the movements have taken place after antecedent rainfall over most significantly the numerous blockages on the Ma-10 road, which 800mm. Additionally, the rockfalls have also occurred after several caused significative economic losses in the different tourist sites, freeze–thaw cycles, being a determining and unusual factor in this valued at up to €6M. warm region. Intense rainfall >90 mm/24h also caused rockfalls as This study aims to contribute to the examination of different well as exceptional very intense rainfall >120 mm/24h caused parameters that condition the spatial and temporal distribution of the landslides. The results aim to contribute to the design of an early slope movements that have taken place. To accomplish this, the warning system coordinating emergency, infrastructure services and following specific goals were set: (1) to establish the prominent role of meteorological centres in a region of high risk. the mountain range’s geological structure in the spatial distribution of the landslides and rockfalls, as well as in the failure pattern of the Keywords Landslides . Rockfalls . Rainfall . Freeze–thaw latter; (2) to determine the influence of meteorological factors in cycles . High risk . Early warning system triggering the movements. Relationships were established between theintensityofdailyrainfallaswellasthepreviousrainfallandthe Introduction start of both kinds of slope movements recorded. To do this, data on The island of Majorca, located in the western Mediterranean, has a the daily rainfall and minimum temperature at the 36 weather variety of different geomorphological domains, most prominently the stations distributed along the entire mountain range were analysed, Tramuntana Range (1,100 km2) in the northwestern part of the island from the 1st of October 2008 until the 9th of May 2010. The purpose is (Fig. 1). The steep topography of this chain, which is linked to its to set a series of thresholds for the initiation of the movements. These geological complexity and Mediterranean climate, determines intense values have been checked against the ones obtained by other authors slope dynamics with the consequent movements of all categories in Mediterranean regions close to the Balearic Islands; and (3) to (Mateos 2002; Mateos and Azañón 2005). The main income of the determine the influence of low temperatures in the failures linked to island of Majorca comes from tourism (83% of its GDP), as it rocky massifs and to determine the influence of freeze–thaw cycles welcomes 8.5 million visitors each year. The vast urban development prior to the failure, processes which are very scantly documented in that the Tramuntana region has undergone in the past 30 years has warm Mediterranean regions. considerably increased the risk originating from mass movements. During the hydrological years 2008 to 2010, Majorca experi- Slope movements on the island of Majorca: background enced one of the coldest and wettest winters in living memory. Not Practically all the slope movements recorded on Majorca have taken only did the accumulated rainfall show twice the average recorded place in the Tramuntana Range. The variety of lithologies cropping values, this period also witnessed the highest rates of intense rainfall out in this mountain chain determines a wide range of slope (up to 296 mm/24 h) since instrumental records have been available movements. Landslides and earth flows are frequent phenomena, (1944). These rainy episodes have also coincided with cold periods in primarily affecting soft sediments from the Late Triassic (Keuper), which several days elapsed with temperatures around 0°C and made up of clays with gypsum, as well as an entire series of loamy

Landslides 9 & (2012) 417 Technical Note

Fig. 1 Location of the island of Majorca in the western Mediterranean and the Tramuntana Range on the northwest extreme of the island. The location of the 34 mass movements registered, as well as the location of the weather stations used in this study, is shown over the digital elevation model

materials from the Palaeogene and Neogene that occasionally rockfall on the Valldemossa area razed and buried a large extension outcrop along the mountain range. The most prominent movements of cropland, leaving reports in the daily news. More recently, include the Fornalutx landslide which took place on the 17th December numerous rockfalls have made the news as well, such as the one in 1924 (Mateos et al. 2009), affecting an area of around 150,000 m2. Cala de Banyalbufar (1993), which affected several fishing huts However, the most important mass movement in the Balearic (Ferrer et al. 1997) and the rockfall in Son Matge (Valldemossa) in Islands, because of both its dimensions and the damage it caused, 2005, in which one of the most important archaeological sites from was the Biniarroi landslide in spring 1721, with later local reactiva- Majorca’s prehistory was buried (Mateos and Azañón 2005). The tions in 1816, 1857 and 1943. This landslide affected an area measuring main traffic arteries in the mountain range, both road and rail, have around 300,000 m2 and totally modified the original topography in often been intercepted by slope movements (Mateos 2006), trigger- the region (Mateos and Giménez 2007;Mateosetal.2008). ing serious circulation problems as well as major economic losses. Rockfalls are the most frequent slope movements in the The historical compilation of the slope movements on the island Tramuntana Range due to the predominance of Jurassic rocky (Mateos 2002, 2006), as well as the record of those that have occurred massifs made up of limestone and dolostone. Historically, there are more recently, reveals that all processes have taken place after short records of several major rockfalls. On the 16th March 1857, a huge intense and/or continuous rainfall. Nevertheless, there is no evidence

418 Landslides 9 & (2012) Table 1 Slope movements inventory Inventory number name and date Type Damage 0. Cala Deià (29 Oct. 2008) L Power station 1. Puig Tomir (27 Nov. 2008) R Track Lluc-Pollença 2. Sa Calobra (03 Dec. 2008) L Ma-10 road 3. Es Verger (15 Dec. 2008) L Es Verger road 4. Es Verger (15 Dec. 2008) C Es Verger road 5. Cala Tuent (15 Dec. 2008) L Cala Tuent road 6. Son Cocó (19 Dec. 2008) RA Pine wood 7. Gorg Blau (31 Dec. 2008) R Ma-10 road 8. Cantera (Dec. 2008–Jan. 2010) L Drystone wall 9. Carretera Banyalbufar (Jan. 2009) L Ma-10 road 10. Biniforani (05 Jan. 2009) R Pine wood and drystone walls 11. Biniaraix (06 Jan. 2009) R Small building 12. Port de Valldemossa (07 Jan. 2009) R Port Valldemossa road 13. Crestatx (Jan. 2009) C Housing development street 14. Edificio Siesta. Calvià (08 Jan. 2009) L Building 15. Son Alberti (23 Jan. 2009–Jan. 2010) R Small building 16. Casa Puigpunyent (14 Sept. 2009) R Dwelling 17. Carretera Estellencs (15 Jan. 2010) R Ma-10 road 18. Búger (Jan. 2010) C Old road Búger-Sa Pobla 19. Costa d’en Blanes (Jan. 2010) R Dwellings 20. Carretera Estellencs (Jan. 2010) L Pine wood 21. Costa Cala Estellencs (Jan. 2010) L Coast 22. Costa Estellencs- Banyalbufar (Jan. 2010) L Coast 23. Costa Banyalbufar (Jan. 2010) L Coast 24. Valldemossa (Jan. 2010) R 25. Carretera Valldemossa (Jan. 2010) L 26. Ses Rossegueres Banyalbufar (Jan. 2010) R 27. Son Antic (17 Feb. 2010) R Barn 28. Son Bunyola (19 Feb. 2010) R Lane 29. Carretera Estellencs (08 Mar. 2010) L Ma-10 road 30. Carretera Estellencs (Mar 2010) R Ma-10 road 31. Gasolinera Estellencs (09 Apr 2010) L Ma-10 road 32. Crestatx (03 May 2010) C 33. Banyalbufar (09 May 2010) L Ma-10 road

These movements have been recorded between Oct. 2008 and May 2010 R rockfall, L landslide, RA rock avalanche, C collapse in the historical record (since the seventeenth century; Mateos 2006) Mateos et al. (2007) carried out a correlation of the known of any similar event as that presented in this work, where 34 mass dates of historical landslides since 1956 with rainfall data movements have been recorded in a period of 2 years. For this provided by the rain gauges nearest to the location of the reason, this study is of relevant interest especially given the touristic slides, revealing that most of the movements took place when importance of the area. maximum 24-h rainfall values are around 130 mm or above. In relation to the meteorological factors which triggered The triggering threshold for rockfalls was considered to be shallow failures—debris flows, debris slides and soil slips— 85 mm/day (Mateos 2006); however, there were no references

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available to take into account the effect of low temperatures (freeze– 90 km and an average width of 15 km. The geological structure thaw cycles) on this kind of failure in the range. There are no reliable of this Alpine chain leads to a southern slope that is gentler data regarding rainfall accumulated during historical episodes due to than the coastal one, which is much more rugged and abrupt, incomplete records at the different weather stations in the range and with a prevalence of very high cliffs overlooking the sea. The the differing dates of installation. peak line is more than 600 m.a.s.l., and the central sector is the highest (Lluc area, Fig. 1). There are numerous peaks with Geographic and geological context altitudes higher than 1,000 m, being the Puig Major (1,445 m) The Tramuntana mountain range is the main mountainous thehighestpeakontheisland.TheTramuntanaregion alignment of the island of Majorca, with a maximum length of encompasses 16 townships with a total population of 115,000

Fig. 2 Distribution of the rockfalls on a geological map of the mountain range and a schematic representation of rockfalls on both faces. On the northern face, the rockfalls mainly take place on the thrust fronts, with predominantly vertical movement (see Gorg Blau, 3 of Fig. 3). On the southern face, the rockfalls also affect the Jurassic limestone massifs, but they overlie the softer materials from the Rhaetian, giving rise to less steep slopes. For this reason, the movements have a longer runout as the rocks have plenty of room to move and roll (see Son Cocó photograph, 2 of Fig. 3)

420 Landslides 9 & (2012) inhabitants, with the northern face much more heavily soft, in contrast to the hard rocks that prevail in the rest of populated and urbanised. The Ma-10 road has an intense the range. vehicular traffic estimated at 7,200 vehicles on a daily average, and constitutes the main road on the northern face of the Geological structure controlling the spatial distribution of recent mass mountain range. The economy of the Tramuntana region movements revolves exclusively around tourism, which accounts for 95% The geological structure of a mountain chain tends to of its income. determine the layout of outcroppings, the distribution of The mountains correspond to the reliefs caused by the slopes and particularly the presence of faults and disconti- Miocenic structuring linked to the Alpine fold, formed by a nuities that often determine the planes and failure surfaces of series of NW overlapping thrusts (Gelabert et al. 1992). The the movements. In this way, numerous studies relate the stratigraphy of the Tramuntana Range begins with the geological structure of a region to the distribution of the deposits of siliceous sandstone from the Buntsandstein (Lower different kinds of slope movements, usually landslides and Triassic) until the more recent colluvial sediments from the rockfalls (Yong et al. 2006; Chang and Slaymaker 2002;Shang Quaternary. Carbonated lithologies clearly predominate and et al. 2005;Zhouetal.2002;Saroetal.2002). In this study, especially Jurassic limestone and dolostone, which constitute the structural layout of the materials that outcrop on the the framework of the mountains. The NE–SW mountain Tramuntana Range conditioned the distribution of the move- alignments correspond to the overlapping system, and the ments. The northern face, where 23 of the 34 movements took regional detachment level are the Later Triassic (Keuper) place(68%),ismorehazardousduetotheexistenceof sediments (Alvaro 1987), clay and gypsum with volcanic rock. steeper slopes and a higher presence of outcroppings of soft The oldest materials in the range outcrop on the coastal strip materials; both of these factors are conditioned by the NW in the southwest sector. These materials are predominantly overlapping thrusts and the regional tectonics.

Table 2 Characteristics of the rockfalls dated during the period from October 2008 to February 2010 Rockfall inventory Date Face Volume Altitude of failed Runout Type of failure Lithology no. (m3) massif (m) (m) (failure plane orientations) Puig Tomir no. 1 27/11/2008 N 16,000 620 350 Planar Liassic limestone N30°E Son Cocó no. 6 19/12/2008 S 300,000 700 650 Wedge Liassic limestone N30°E N90ºE Gorg Blau no. 7 31/12/2008 N 30,000 600 450 N120°E Liassic limestone Biniforani no. 10 5/01/2009 S 28,000 650 350 Wedge Liassic limestone N30°E N110°E Biniaraix no. 11 6/01/2009 N 3 600 100 Planar Liassic limestone N70°E Port de Valldemossa no. 12 07/01/2009 N 8 340 20 ¿? Liassic limestone Son Alberti no. 15 23/01/2009 N 10 200 5 – Liassic limestone Casa Puigpunyent 14/09/2009 N 2 360 150 Wedge Liassic limestone N120°E N30°E Estellencs 1 no. 17 15/01/2010 N 10,000 350 50 Planar Liassic limestone N110°E Son Antic no. 27 17/02/2010 S 14,000 350 250 Wedge Liassic limestone N120°E N30°E Son Bunyola no. 28 19/02/2010 N 10 250 20 Planar Liassic limestone N30°E

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Rockfalls the characteristics of the rockfalls whose precise date is known, Over the hydrological years 2008–2010, 15 rockfalls have taken and Fig. 3 shows the photographs of some of the most important. The place in the Tramuntana Range. The distribution of these largest rockfall was at Son Cocó (Nº 6 of the inventory), which movements (Fig. 2) shows that both the northern (ten events) involves 300,000 m3, and it was classified as a rock avalanche and the southern face (five events) were affected. Table 2 shows (Mateos et al. 2010). It left a tongue of enormous blocks (some

Fig. 3 Photographs of the most important dated rockfalls. 1 Puig Tomir, no. 1 (27/11/2008). 2 Son Cocó, no. 6 (19/12/2008). 3 Gorg Blau, no. 7 (31/12/2008). 4 Biniforani, no. 10 (5/01/2009). 5 Estellencs 1, no. 17 (15/01/2010). 6 Son Antic, no. 27 (17/02/2010). The Gorg Blau rockfall (photo 3) cut off the Ma-10 road for 4 months, with repairing costs up to 1.5 million Euro

422 Landslides 9 & (2012) of them >1,500 m3) with a runout of almost 650 m, razing the the layout of the materials on the slopes. Thus, on the surface of a pine wood measuring around 60,000 m2 (2 of southern face, rockfalls took place on vertical rock massifs of Fig. 3). All the rockfalls have affected practically vertical rock Liassic limestone which overlie the softer materials from the scarps made up of Liassic limestone located at altitudes Rhaetian (Upper Triassic), made up of dolomites and marls. between 250 and 700 m. On the northern face, the rockfalls The latter give rise to less rugged slopes with angles around take place through planar failures, with the predominant 30°, which allow the loose blocks plenty of room to move and direction being N30ºE, which coincides with the direction of roll. However, on the northern face, the thrusts give rise to the thrust fronts. On the southern face, the failure model in very vertical fronts that define the slope, as the blocks the rockfalls is a wedge type, with two main failure planes—N30° E accumulate at the foot with a lower horizontal component to and N110° E—that mark the fracture pattern of the mountain the movement. range on this face (Gelabert et al. 1992). The magnitude of the damage caused by this type of The runout of the rockfalls that took place on the southern movements can be clearly illustrated by the Gorg Blau rockfall face was longer than those on the northern face (Fig. 2)dueto (no. 7 of the inventory), with a volumen of 50,000 m3,which

Fig. 4 Distribution of the landslides, concentrated primarily in the Keuper outcroppings on the northern face of the mountain range and especially along the coastline on the extreme southwest. Generally speaking, they were rotational movements with associated mudflows at the foot, as illustrated in the landslide geological scheme, corresponding to Estellencs 3 landslide (no. 29)

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blocked the Ma-10 road for 3 1/2 months (3 of Fig. 3), cutting gradually drops towards the extreme SW of the range (Calviá), off the access to Sa Calobra, one of the most visited places in where average annual precipitation is no more than 300 mm Majorca. The repairing costs reached €1.5 M. (Fig. 6a). During the period spanning from October 2008 to May 2010, Landslides the island of Majorca experienced two of the coldest and rainiest Landslides are small to medium-sized movements that affect clays winters in living memory, standing out as the wettest years in the with gypsum and volcanic rocks dating to the Late Triassic past 40 years. Between the 21st of October 2008 and the 10th of (Keuper). All landslides were located on the northern face of the January 2009, the highest precipitation was registered in the Tramuntana Range, and especially on the southwestern coastline central sector of the Tramuntana Range since instrumental data (Fig. 4). These Keuper outcroppings are located at the foot of the have been available (1944). On the 15th of December 2008, a total thrusts or constitute the bottom of the largest valleys in this of 296 mm of rain fell in 24 h near Sóller, and the accumulated mountain chain (Esporles, Sóller, etc.). They are fine-grained soils rainfall during hydrological year 2008–2009 was over 2,260 mm, with low to medium plasticity, with effective cohesion values of compared to the average annual precipitation of 1,300 mm around 0.4 Kp/cm2 and very low permeability values of around (Fig. 6). Mateos et al. (2007) carried out a statistical analysis of − 1.8×10 6 cm/s (Mateos et al. 2003; Mateos et al. 2009). The intense rainfall in the Tramuntana range, using the Gumbel movements tend to be complex landslides—rotational landslides probability distribution function, which allowed to obtain the with mudflows at the bottom that affected roadside slopes with maximum 24-h rainfall values for return periods of 5, 10, 25 and steep inclines (>30°); in some cases, they are reactivations of 100 years. The values obtained on the 15th of December 2008 in previous movements (Sa Calobra no. 2 and Banyalbufar no. 33). the mountain range are very similar to those calculated for the Table 3 shows the characteristics of the landslides dated during return period of 100 years (Fig. 7), which reflects the uniqueness rainy periods, and Fig. 5 shows some photographs of the most of this extreme precipitation record. important ones. They all affected the road network in the Likewise, the period between September 2009 and May 2010 mountain range, primarily the Ma-10 road. The Estellencs 3 was exceptionally rainy, with accumulated rainfall being almost landslide (no. 29; 3 of Fig. 5) on the 8th March 2010 kept this twice the average value. As an equivalence, in the area near Lluc, motorway closed for 3 months, triggering numerous economic 235 mm of rain fell in 24 h on the 3th of May 2010, and the losses in the towns of Estellencs and Banyalbufar, and with accumulated rainfall for these 7 months was almost 1,700 mm. repairing costs reaching €2 M. Once again, the distribution of the However, the 2008–2010 period was not only exceptionally movements was determined by the structural layout of the rainy on the island but also anomalously cold, with abundant mountain range, which shows a greater presence of Keuper snowfall as well as freezing in the highest zones in the range. outcroppings on the northern face, linked to the detaching This is an unusual occurrence in the mild Mediterranean surfaces of the thrusts. climate, where the extreme temperatures are attenuated by the proximity of the sea. Additionally, the high precipitation and Triggering factors the low temperatures coincided, as the rainfall took place The island of Majorca has a typical Mediterranean climate, with mainly during the winter months, linked to cold fronts coming mild winters and warm, dry summers. The maximum precipita- in from northern Europe. During the last two winters, the tion takes place during the autumn months due to the arrival of majority of the weather stations in the range had registered the first high cold air masses, which contrast with the high temperatures oscillating around 0°C, with minimum values as temperature retained by the sea. This phenomenon is known as low as −6.8°C at altitudes of around 500 m. “gota fría” (cold rain), in which heavy storms are accompanied by intense rainfall with episodes of up to 250 mm in 24 h. The orography of the island clearly controls the distribution of Rockfalls precipitation. The central sector (Lluc) of the Tramuntana Range Figure 8 shows the graphs that relate the daily precipitation, registers average annual precipitation of 1,200 mm, which accumulated rainfall and minimum temperatures recorded to the

Table 3 Characteristics of the landslides dated during the period from October 2008 to May 2010 Movement date Volume Altitude Typology Lithology Slope (m3) (m) (Cruden and Varnes 1996)

Costa Deià (no. 0) 29 Oct 2008 1,500 80 Debris flow Colluvial deposits Quaternary 45° Sa Calobra (no. 2) 3 Dec 2008 7,000 585 Rotational landslide and earth flow Clays with gypsum from Keuper 40° Cala Tuent (no. 5) 15 Dec 2008 12,000 168 Rotational landslide Clays with gypsum from Keuper 42° Es Verger (no. 3) 15 Dec 2008 200 410 Debris flow Colluvial deposits Quaternary 50° Edificio Siesta (no. 14) 8 Jan 2009 100 158 Debris flow Marls and calcareous sandstones Oligocene 90° Estellencs 3 (no. 29) 8 Mar 2010 30,000 350 Rotational landslide and earth flow Clays with gypsum from Keuper 30° Petrol station (no. 31) 9 Mar 2010 3,000 250 Earth and debris flow Clays with gypsum from Keuper 35° Banyalbufar (no. 33) 9 May 2010 4,500 300 Rotational landslide and earth flow Clays with gypsum from Keuper 45°

424 Landslides 9 & (2012) Fig. 5 Photographs of the most important landslides recorded that affected the road network in the mountain range. 1 Sa Calobra, no. 2 (3/12/2008), 2 Cala Tuent, no. 5 (15/12/2008), 3 Estellencs 3, no. 29 (8/3/2010) and 4 Banyalbufar, no. 33 (9/5/2010). The Estellencs 3 road landslide (photo 3) cut off the Ma-10 road for 3 months and with repairing costs reaching 2 million Euro occurrence of the dated rockfalls. In order to obtain a general threshold coincides with that of 85 mm/24 h estimated by picture, the reference was the meteorological data from the B013- Mateos (2006) for historical rockfalls. Lluc weather station, located in the heart of the mountain range & Most of the rockfalls started in saturated rocky massifs (Fig. 1) at an elevation of 490 m, approximately the average (accumulated rain >800 mm) when there were several freeze– altitude of the rockfall scarps. As a general criterion, all the thaw cycles in the days prior to the failure, regardless of whether rockfalls took place after intense precipitations and very the maximum daily rainfall was overly intense (around 30 mm). clearly in the coldest periods, when the temperature hovered at around 0°C. With the aim of carrying out a thorough The case of the rock avalanche in Son Cocó (no. 6) is analysis, Table 4 was drawn up, in which the following unique, as very intense precipitation (150 mm/24 h, 4 days parameters were quantified for each rockfall, taking into prior to the failure) coincides with low temperatures and account the meteorological data from the stations closest to various (3) freeze–thaw cycles. The Casa Puigpunyent, no. 16 the failure point: (1) maximum precipitation in 24 h recorded rockfall coincided with an intense storm in the area (45 mm/ in the 10 days prior to the failure, (2) minimum temperature h) where more than 37,000 lightning strikes were recorded. recorded in the 10 days prior to the failure. To get these Tables 2 and 4 provide an overview which enables us to values, the temperature at the elevation of the rockfall scarp establish that largest rockfalls were produced on rocky scarps was estimated by applying a thermal gradient of −6.5°C for located above 600 m, showing a clear relationship to freeze– each 1,000 m (Mateos et al. 2007), (3) accumulated rainfall thaw cycles prior to the failure. Minor rockfalls were caused from the start of the rainy period until the moment of failure, and (4) on lower scarps, 250–350 m, and were more closely related to number of freeze–thaw cycles that took place prior to the rockfall. episodes of moderate–intense rainfall. The analysis of these data reveals the following:

& Some of the rockfalls (∼27%) took place after occurrences of Landslides intense rain >90 mm/24 h, regardless of the temperature. Figure 9 shows the relationships between the different meteorological These circumstances occurred in the rockfalls at Puig Tomir parameters—daily rainfall, accumulated rainfall and minimum (no. 1), Son Cocó (no. 6) and Estellencs 1 (no. 17). This temperatures—and the occurrence of dated landslides. Once

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Fig. 6 a Map of average annual precipitation in the Tramuntana Range. b Map of the distribution of precipitation during the hydrological year 2008–2009, where it can be seen that the recorded rainfall was at least twice the average

again, the B013-Lluc weather station was taken as the reference rainfall, with very high accumulated rainfall totals (>800 mm). point. This overall picture reveals that the landslides recorded Bearing in mind the same parameters as the ones used for took place after episodes of moderate to extreme intense rockfalls, Table 5 was elaborated using the information from

426 Landslides 9 & (2012) Fig. 7 Maps of maximum 24-h rainfall (millimetres) for different return periods in the Tramuntana range (Mateos et al. 2007). The values obtained on the 15th of December 2008 are very similar to those calculated for the return period of 100 years

the weather stations located the closest to the movements. documented in the Mediterranean region (D’Amato et al. 2004; Some of the landslides (five of the eight dated) were triggered Guzzetti et al., 2004; Turcotte et al., 2006; Corominas 2006; after prior medium-intensity rains, between 20 and 65 mm/ Cardinalli et al. 2006). There are also numerous studies aimed at 24 h, and with very high accumulated rainfall figure estimating the rain thresholds that trigger these movements (>1,000 mm in Cala Tuent, no. 5), while others took place (Brunetti et al. 2010; Guzzetti et al. 2007; Corominas and Moya after very intense rainfall >120 mm/24 h, with values up to 1999), whose results have served as the basis for establishing early 190 mm/24 h in the case of the Sa Calobra landslide (no. 2). warning systems. Most of the literature refers to surface land- This threshold coincides with that of 130 mm/24 h estimated by slides, as well as to mudflows triggered by brief episodes of Mateos et al. (2007) for historical landslides. However, unlike intense rain. rockfalls, freeze–thaw cycles do not seem to trigger landslides, The rockfalls recorded during the period 2008–2010 in and the altitude at which the movements took place is of no Mallorca took place under the following circumstances: (1) relevance. after occurrences of intense rain >90 mm/24 h, regardless of the temperature. These circumstances occurred in many other Discussion and results rockfalls recorded in literature (Ferrer et al. 1997;Mateos The complexity of the relationship between mass movements and 2002;MateosandAzañón2005), and (2) they started in climatic conditions makes it difficult to define “universal laws”. saturated rocky massifs (accumulated rain >800 mm) when Climate is never the sole cause (Flageollet et al. 1999; Dikau and there were several freeze–thaw cycles in the days prior to the Schrott 1999). Nevertheless, some authors consider that modifi- failure, regardless of whether the maximum daily rainfall was cations in mass movement frequency may be interpreted as overly intense (around 30 mm). The rockfalls took place changes in the hydrological conditions of slopes, which are because of the cumulative effect of the freeze–thaw cycles, directly controlled by climate. These studies enable them to which weaken the rock and propagate fissures. Some establish that slope instability processes can be considered researchers believe that the frequency of rockfalls seems to geomorphological indicators of climate change (Borgatti and be regulated by thermal fluctuations around 0°C (Frayssines Soldati 2010; Soldati et al. 2004). Some authors also reveal that and Hantz 2006; Wieczorek and Jäger 1996; Shihi et al. 2004), the frequency of these intense mass movement events is likely to as in the present study, and that the phenomenon of increase in response to global warming (Krautblatter and Moser gelifraction is intensified as the frequency of temperature 2009; Keiler et al. 2010). changes rises, while the absolute value of the thermal In the present study, we can state that the occurrence of 34 oscillation bears little influence (Orozco et al. 2002). Numerous mass movements in the Tramuntana Range recorded during the authors also established a clear relationship between rockfall period 2008–2010 is an exceptional event in the history of the intensity and wet freeze–thaw cycles frequency in different European island as there is no evidence in the historical record (since the regions, when major precipitation events are recorded (Krautblatter seventeenth century) of any similar event. These movements took and Moser 2009; Schneuwly and Stoffel 2008).Thealtitudeof place as a result of a confluence of intense rainfall and low the scarp failure is also consistently relevant. The largest temperatures in a very rainy climatic episode. rockfalls were produced on scarps located above 600 m, after The relationship between periods of intense rainfall and the several freeze–thaw cycles. Duarte and Marquinez (2002) occurrence of slope movements of different kinds is extensively established a relationship between a higher rockfall frequency

Landslides 9 & (2012) 427 Technical Note

Fig. 8 Relationship between daily rainfall, accumulated rainfall and minimum temperatures (recorded at the B013 Lluc weather station) and the occurrence of rockfalls in the periods a October 2008–February 2009 and b September 2009–March 2010

and a higher altitude of the scarp failures, in a region located al. 1998; Bolley and Oliaro 1999;Portillaetal.2010), and where the in the north of Spain, with similar weather conditions to interstitial pressure dissipates rapidly. In these cases, the preceding Majorca. rainfall is almost irrelevant, but it is the intensity of the rain which In the Mediterranean Alpine mountain ranges, several examples assumes greater importance. Corominas and Moya (1999)seta of rainy episodes which triggered surface landslides and mudflows are threshold of 180–190 mm in 24–36 h as a trigger for this kind of documented, which generally affect colluvial formations (Paronuzzi et movement in the Eastern Pyrenees. However, the behaviour is quite

428 Landslides 9 & (2012) Table 4 Values of the meteorological data related to the occurrence of rockfalls Rockfall date Max. rainfall in 24/h (mm) Minimum temp. (°C) Accumulated No. of freeze– (10 days prior to event) (10 days prior to event) rainfall (mm) thaw cycles Puig Tomir (no. 1) 27 Nov 2008 128.5 1.6 843.3 0 27 Nov 2008 26 Nov Son Cocó (no. 6) 19 Dec 2008 150.2 −1.6 874.2 3 15 Dec 2008 11 Dec Gorg Blau (no. 7) 31 Dec 2008 35.1 −3.2 1,409.2 6 26 Dec 2008 23 Dec 2008 Biniforani (no. 10) 5 Jan 2009 33.3 −3.5 931.7 8 26 Dec 2008 5 Dec 2009 Biniaraix (no. 11) 6 Jan 2009 23.5 −1.1 953.6 5 6 Jan 2009 5 Jan 2009 Port de Valldemossa (no. 12) 07 Jan 2009 24.3 −1.3 892.3 4 07 Jan 2009 7 Jan 2009 Son Alberti (no. 15) 23 Jan 2009 6.3 5.13 669.7 0 13 Jan 2009 18 Jan 2009 Casa Puigpunyent (no. 16) 14 Sept 2009 45.3 12.69 87.9 0 14 Sept 2009 12 Sept 2009 14 Sept 2009 Lightning storm Estellencs 1 (no. 17) 15 Jan 2010 93.1 2.3 789.3 0 12 Jan 2010 10 Jan 2010 Son Antic (no. 27) 17 Feb 2010 28.6 −1.5 874.4 2 15 Feb 2010 13 Feb 2010 Son Bunyola (no. 28) 19 Feb 2010 36.2 −1.5 790.4 2 12 Feb 2010 13 Feb 2010

For each case, the weather stations closest to the failure point were used

different in slopes with low permeability materials, such as regarding this issue, and this matter should be addressed in the the soft material slopes in Majorca, where presaturation is a future. necessary condition for landslides. Most of the landslides In this study, the structural layout of the materials that outcrop recorded in this study took place on roadside slopes after in the Tramuntana Range conditioned the distribution of the preceding rains of >800 mm. In roadside slopes, the critical movements. The northern face, where 23 of the 34 movements took rainfall thresholds may drop substantially compared to place, is more hazardous due to the existence of steeper slopes and a natural slopes (Corominas 2006). This argument is valid for higher presence of outcroppings of soft materials; both of these some of the landslides registered, with low–moderate rainfall factors are conditioned by the NW overlapping thrusts and the intensities of 20–62 mm/24 h prior to the failures, which are regional tectonics. This coastal slope is also the one at the highest risk completely in line with those estimated by other authors on as it is the most densely populated and urbanised. The historical roadside slopes in the mountains of Asturias and Cantabria record of movements (Mateos 2006) also reveals a higher occurrence (N Spain; Domínguez et al. 1999; Díaz de Terán et al. 1997). of movements on the northern face. However, something that had However, this does not hold true in other cases, where rain threshold notbeennotedearlieristhegreaterrunoutoftherockfallsonthe to generate the failure was higher than 120 mm/24 h, which is closer southern face due to the stratigraphic distribution of the materials to the threshold (130 mm/24 h) estimated by Mateos et al. (2007). that comprise the failed massifs. These results enable us to conclude that the main triggering factor The results of this work have allowed the first steps to be taken for landslides has been the high accumulated rainfall figures, in an towards the creation of an early warning system on the island. The exceptionally rainy period, with values being almost twice the system will be coordinated by the Emergency Service which will be average. The value of the rain intensity is less determinant. put on alert by the Meteorological Service, when rain gauges record Nevertheless, the thresholds obtained in the present study must accumulated rainfall >800 mm or when forecasts predict heavy be checked and validated. It is necessary to improve the historical rainfall >90 mm/24 h. If, in addition to this, the forecast predicts a data in order to analyse how many times the thresholds obtained cold period, the alert will be higher in scale. In these situations, the have been surpassed in the past. There is a lack of information road maintenance services will cut off the Ma-10 road at several

Landslides 9 & (2012) 429 Technical Note

Fig. 9 Relationship between the different meteorological parameters—daily rainfall, accumulated rainfall and minimum temperatures recorded—and the occurrence of landslides, taking B013-Lluc weather station as the point of reference. The landslides recorded took place after episodes of moderately to extremely intense rainfall, with high accumulated rainfall (>850 mm)

sections which have already been identified as having greater is based on tourism, the authorities are increasingly aware of the susceptibility. Taking into account the fact that the island economy need to adopt preventative measures and to put safety first.

430 Landslides 9 & (2012) Table 5 Values of the meteorological data related to the occurrence of landslides Landslide date Max. rainfall in 24 h (mm) Minimum temp. (°C) Accumulated rain (mm) No. of freeze–thaw cycles (10 days prior to event) (10 days prior to event) since the start of the season (10 days prior to event) Costa Deià 29 Oct 2008 64 10.21 179.6 0 28 Oct 2008 25 Oct 2008 Sa Calobra 3 Dec 2008 190.2 −0.1 900.3 1 27 Nov 2008 2 Dec 2008 Cala Tuent 15 Dec 2008 61.7 1.7 1,075.2 0 14 Dec 2008 13 Dec 2008 Es Verger 15 Dec 2008 178.3 −0.3 626.4 1 15 Dec 2008 1 Dec 2008 Edificio Siesta 8 Jan 2009 26.5 0.8 718.2 0 6 Jan 2009 8 Jan 2009 Estellencs 8 Mar 2010 44.5 −0.05 864.5 1 7 Mar 2008 7 Mar 2008 Petrol station 9 Apr 2010 20.2 8,9 927.6 0 4 Apr 2010 8 Apr 2010 Banyalbufar 9 May 2010 120.3 16.4 1,220.7 0 3 May 2010 8 May 2010

For each case, the weather stations closest to the failure point were used

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