Relationships Between Slope Instabilities, Active Tectonics and ...

Relationships Between Slope Instabilities, Active Tectonics and Drainage Systems: The Du´dar Landslide Case (Granada, Southern Spain) Martıń Jesu´s Rodrı´guez-Peces, Jose´ Vicente Pe´rez-Penã, Jose´ Miguel Azanõń, and Alicia Jimeńez-Gutierrez

Abstract

A geomorphologic description of the Du´dar landslide (Granada, S Spain) has been carried out using a high-resolution digital elevation model derived from LIDAR (Light Detection and Ranging) data. We have analysed the significant changes that the landslide caused in the drainage system of the Aguas Blancas and Darro rivers, which in turn are the consequence of the tectonic activity of the north-eastern border of the Granada Basin. These modifications comprise river diversions and active incision within the body of the landslide, making it susceptible to future reactivations. A stability back-analysis of the landslide has been performed to identify the mechanism of failure and the most-likely triggering factors. This analysis shows that a low-to-moderate magnitude earthquake (Mw 5.0–6.5) related to the active faults in the Granada Basin seems to be the main triggering factor of the Du´dar landslide. Keywords

Active tectonic Seismicity

Betic Cordillera

Introduction Large and giant landslides (>5 hm3 according to Fell 1994) develop typically in active mountain belts, in areas with high local relief (Weidinger 2006; Korup et al. 2007; Prager et al. 2008). These slope movements act as active geomorphic agents, modifying the landscape and disturbing drainage

M.J. Rodrı´guez-Peces (*) Department of Geodynamics, University Complutense of Madrid, Ciudad Universitaria s/n, Madrid 28040, Spain e-mail: [email protected] J.V. Pe´rez-Penã A. Jimeńez-Gutierrez Department of Geodynamics, University of Granada, Campus de Fuentenueva s/n, 18071 Granada, Spain J.M. Azanõń Department of Geodynamics, University of Granada, Campus de Fuentenueva s/n, 18071 Granada, Spain Instituto Andaluz de Ciencias de la Tierra (UGR-CSIC), Granada, Spain

Drainage system

Du´dar

Landslide

Newmark

networks (Turnbull and Davies 2006; Korup et al. 2007). Nevertheless, there are examples of extremely large landslides developed in areas with medium to low landscape gradients (Philip and Ritz 1999; Strasser and Schlunegger 2005; Davies et al. 2006; Van Den Eeckhaut et al. 2007; Pańek et al. 2007, 2010). In such areas, those giant slope instabilities can be considered as rare, and normally they are controlled by specific geomorphologic and tectonic settings. In this paper, we have carried out a geomorphologic description of the Du´dar landslide. This large fossil landslide was developed in an area with medium landscape gradient as is the Granada Basin (Betic Cordillera, SE Spain). In the NE border of this basin, the landscape evolution during the Quaternary is the consequence of its active tectonic configuration (Rodrı´guez-Fernańdez and Sanz de Galdeano 2006). We analyzed the landslide geomorphic features with the aid of a high-resolution digital elevation model (DEM) derived from LIDAR (Light Detection and Ranging) data. We studied the significant changes that the landslide caused in the drainage system of the Aguas Blancas and Darro rivers.

C. Margottini et al. (eds.), Landslide Science and Practice, Vol. 5, DOI 10.1007/978-3-642-31427-8_43, # Springer-Verlag Berlin Heidelberg 2013

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Finally, a stability back-analysis of the Du´dar landslide has been performed to identify the mechanism of failure and the most-likely triggering factors. By means of this analysis we realized if the presence of water could be the main controlling factor required to trigger the landslide, and the seismic activity related to the active faults located in the Granada Basin could be regarded as the main cause of the occurrence of this landslide.

The most-likely triggering factor for the Du´dar landslide seems to be a great earthquake, as for other large landslides located in the Granada Basin such as the Guëve´jar landslide (Rodrı´guez-Peces et al. 2011). Nevertheless, there are not historical data for such event but probably occurred prior to 1400, which is the date of the first historical records of the Du´dar village, located in one side of the landslide. In addition, the active normal faults of this part of the Granada Basin can potentially generate earthquakes with magnitudes greater than Mw 6.0 (Sanz de Galdeano et al. 2003).

Geological and Seismotectonic Context of the Du´dar Landslide Geotechnical Investigations The Du´dar landslide is located in the eastern border of the Granada Basin (South Spain), which is one of the largest Neogene basins of the central Betic Cordillera (Fig. 1). The Du´dar landslide, with an extension of around 380 ha, is one of the largest landslides developed in this part of the Cordillera. The landslide mass comprises deposits from the sedimentary infilling of the Granada Basin. The sedimentary sequence within the landslide area comprises from bottom to top: (a) calcarenites, sandstones and marls (Lower Tortonian); (b) grey conglomerates, sandstones and marls (Upper Tortonian); (c) reddish conglomerates (Uppermost Tortonian); (d) reddish clays, sands and conglomerates (Lower Pleistocene to Upper Pliocene). The sedimentary units (a) and (b) were deposited in a marine environment related to a shallow coastal platform and fan deltas (Garcıá-Garcıá et al. 1999). The reddish conglomerates are related with alluvial fans, marking the change to continental conditions in the Granada Basin. The reddish clays, sand and conglomerates are related with a glacis residual surface developed at the end of the sedimentary record (Fernańdez et al. 1996). The contacts between these sedimentary units, in spite of being discordant, are mainly sub-horizontals. One of the most important determinant factors for the Du´dar landslide development was the altitude difference between the Aguas Blancas and Arroyo de Belen Creek (tributary of the Darro River, Fig. 2). These two rivers, in despite of running parallel and separated by less than 2 km of distance, have an average altitude difference of around 200 m. This geomorphic configuration is the consequence of the tectonic activity in the NE border of the Granada Basin. This tectonic activity comprises NW-SE striking normal faults and roughly N70E directed-folds. Moreover, the Aguas Blancas River flows in this area through a synclinal hinge and the Darro drainage basin is located in the SE dipping flank of the fold. As a consequence of this tectonic configuration, the northwest margin of the Aguas Blancas River is highly unstable and presents a number of small landslides.

The main failure surface of the Du´dar landslide was estimated based on the field observations. This critical surface can be related to a translational movement developed successively in the grey conglomerates, sandstones and marls. The main shear strength parameters corresponding to the sedimentary units involved in the landslide have been derived from some geotechnical tests developed in prior studies in similar Neogene marly and silty soils in the Granada Basin (El Amrani Paaza et al. 1998, 2000; Oteo 2001; Azanõń et al. 2006, 2010, 2011; Rodrı´guez-Peces 2010; Rodrı´guez-Peces et al. 2011). The average values of these parameters (Table 1) have been used in the backanalysis of the Du´dar landslide, reported later.

Geomorphological Features of the Landslide The Du´dar landslide covers an area of about 343 ha, with a maximum length of 2.9 km and maximum width of 1.9 km (Fig. 2). The complete geometry of the Du´dar landslide is described in Table 2, following the nomenclature for landslides suggested by the IAEG Commission on landslides (1990). Assuming an average depth of the failure surface of approximately 111 m (66–156 m), the mean volume of the landslide may then be roughly estimated at 379 hm3 (226–533 hm3). The volume of landslides can also be estimated by means of empirical equations relating the landslide volume to geometrical features, mainly the landslide area. Recently, Guzzetti et al. (2009) have developed an area versus volume empirical relationship from a worldwide catalogue of landslides. These authors consider that the relationship is largely geometric, and not significantly influenced by geomorphological or mechanical properties of the failed soils or rocks, or the type of landslide. For this reason, this area-volume relationship has been used to obtain an additional estimation of the Du´dar landslide volume. The obtained mean volume is 286 hm3

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Fig. 1 (a) Simplified geological sketch of the central part of the Betic Cordillera (South Spain). The location of the Du´dar landslide area is marked with a black rectangle. (b) Digital elevation model (DEM) showing the location of the Darro and Aguas Blancas rivers. The location of the Du´dar landslide and the main tectonic structures are depicted

(251–326 hm3). This mean value is slightly smaller than the one estimated above, but the minimum volumes estimated using this two different methods are of the same order of magnitude. From field observations, the landslide body can be divided into three different parts: the head, the intermediate and the toe zones (Fig. 3). The head area

corresponds to a large hill of about 110 m thick with an average slope gradient of 55 . These values are related to the main scarp of the landslide. The intermediate part of the landslide is related to the erosion of the landslide body by a river incision. The incision is particularly stronger in the landslide body since the sediments have been weakened as a consequence of the landslide


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Table 2 Summary of the main geomorphologic parameters of the Du´dar landslide following the nomenclature suggested by the IAEG Commission on landslides (1990) Total length Length of the displaced mass Length of the rupture surface Width of the displaced mass Width of the rupture surface Depth of the displaced mass (maximum) Depth of the rupture surface (maximum) Total height (height from the crown to the tip of toe) Perimeter Total area Fig. 2 High-resolution digital elevation model (DEM) of the Du´dar landslide derived from LIDAR (light detection and ranging) data Table 1 Summary of the main geotechnical properties of the lithological units found in the Du´dar landslide. g, unsaturated unit weight; gsat, saturated unit weight; cr, residual cohesion; Fr, residual friction angle Lithological unit Reddish conglomerates Grey conglomerates, sandstones and marls Calcarenites, sandstones and marls

g (kN/m3) 21.53 21.90

gsat (kN/m3) 23.16 15.40

cr (kPa) 81.90 10.70

Fr ( ) 38 20

23.76

24.53

30.41

29

movement. At the toe sector the thickness of the landslide mass is greater, but it was eroded by the Aguas Blancas River at the most distal sector, between the Du´dar and Queńtar villages.

Drainage System Changes The Du´dar landslide caused significant changes in the drainage system of the Aguas Blancas and Darro rivers (Fig. 4). These modifications comprise river diversions and active incision within the body of the landslide, making it susceptible to future reactivations The Arroyo de Belen Creek (tributary of the Darro River) ends abruptly when it reaches the landslide, and the Darro main watershed makes an anomalous turn in that point (Fig. 4). The creek that flows at present-day to the Aguas Blancas River matches perfectly in shape with the Arroyo de Belen Creek, indicating that it should be part of the same watershed. The altitude difference between the Arroyo de Belen Creek and the Aguas Blancas is around 200 m. The Du´dar landslide altered the drainage, capturing part of the Darro Basin (i.e. the upper part of the Arroyo de Belen watershed). This capture had as a consequence a lowering in the base level and a prominent incision within the landslide mass (Fig. 3). Even though the landslide

L ¼ 2,893 m Ld ¼ 2,758 m Lr ¼ 2,343 m Wd ¼ 1,880 m Wr ¼ 1,493 m Dd ¼ 202 m Dr ¼ 202 m DH ¼ 358 m P ¼ 8,319 m A ¼ 3,427,062 m2

is now inactive, the river incision caused by this base-level lowering could reactivate the landslide in some points, making it a natural hazard to take into account.

Stability Back-Analysis of the Du´dar Landslide The first step in performing the stability back-analysis was the reconstruction of the topography of the slope before the sliding based on a high-resolution digital elevation model (DEM) of the Du´dar landslide area. This DEM was derived by means of a LIDAR survey resulting in a model with a grid size of 3 3 m. Pre-sliding topography was reconstructed subtracting the contour lines of the total landslide area, interpolating a new DEM by means of a geographic information system (GIS) and moving the displaced mass to its original location. The back-analyses of the Du´dar landslide have been made using Slide (Rocscience Inc 2003), a 2D slope stability software which calculates safety factors (SF) for circular and non-circular slope failure surfaces based on a number of widely used limit equilibrium methods. The MorgensternPrice method was used as it is considered the most appropriate for slope ruptures developed in soils and is valid for circular and non-circular failure surfaces. In general, to evaluate the stability of a slope the Slide program calculates a significant number of possible circular slip surfaces in order to find the location of the most critical one with the minimum safety factor value. The slope profile was derived from the high-resolution DEM (3 3 m) obtained of the LIDAR survey cited above. This cross-section represents the observed main path of the landslide. The geometry and location of the potential failure surfaces were particularly fixed by means of different control points and analysing the slope surface geometry (Fig. 5). The main scarp and toe location have been set based on field observations. Hence, the possible slip surfaces were obtained fitting their location to these control points.

Relationships Between Slope Instabilities, Active Tectonics and Drainage Systems:. . .

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ac ¼ ðSF 1Þg sin a

(1)

where ac is the critical seismic acceleration (in gravity units, 1 g ¼ 9.81 m/s2), g is the gravity acceleration, SF is the static safety factor and a is the thrust angle. For rotational

2750 2500 2250 2000 1750 1500 1250

W

1000

We have considered two main potential triggering factors in the back-analysis: water saturation and seismicity. The presence of water is a major factor controlling the triggering of landslides. This condition is very common in the south of Spain as a consequence of the typical Mediterranean heavy rainfall regime. To model this situation, we have considered a complete saturation of the sedimentary materials involved in the landslide. Furthermore, the seismic activity related to the active faults located in the Granada Basin can be regarded as an additional cause of the occurrence of landslides. In this case, we have considered the horizontal peak ground acceleration (PGA) as a representative seismic parameter to model the seismic motion in the landslide. The minimum seismic acceleration required for overcoming the shear resistance and initiating the displacement of the landslide (i.e. the critical acceleration) was evaluated fitting the PGA value by iteration until the safety factor obtained was equal to one (stability condition). However, the critical acceleration related to a circular approximation of the failure surface was also obtained by means of the equation (Newmark 1965):

750 m

Fig. 3 Geomorphologic map of the Du´dar landslide showing the location of the main scarp, active gullies, incised channels, and the main displaced mass of the landslide

3000

3250

Fig. 4 Significant changes caused by the Du´dar landslide in the drainage system of the Aguas Blancas and Darro rivers

0m

250

500

750

1000

1250

1500

1750

2000

2250

2500

2750

3000

Fig. 5 Longitudinal cross-section of the Du´dar area prior to the landslide considering a shallow water table (blue line) and residual shear strength parameters. Failure surface is shown by a red line

movement, Newmark (1965) showed that the thrust angle is the angle between the vertical and a line segment connecting the centre of gravity of the landslide mass and the centre of the slip circle. Then, the static safety factor prior to the earthquake has been obtained removing the seismic acceleration value. Finally, we have estimated the most likely magnitudedistance pairs of the potential earthquake which PGA could exceed the critical acceleration and, thus, triggered the


340 Table 3 Most likely magnitude-distance pairs of potential earthquakes which might exceed the critical acceleration in the Du´dar landslide. Mw: moment magnitude; Rep: epicentral distance to landslide (km) Mw Rep

5.0 1

5.5 7

6.0 11

6.5 16

Du´dar landslide. The magnitude and epicentral values have been obtained using a number of ground motion prediction equations (GMPEs) selected from the literature (Skarlatoudis et al. 2003; Ambraseys et al. 2005; Akkar and Bommer 2007; Bindi et al. 2010).

The Du´dar Landslide Considering the Presence of Water as the Main Triggering Factor The slope stability analysis indicated the Du´dar slope was stable before the occurrence of the landslide. We have obtained a high safety factor (SF ¼ 2.51), assuming a relative shallow water table. From the analysis, the slope would remain stable even after considering a complete saturation of the slope. In this case the minimum safety factor is still high (SF ¼ 1.71).

The Du´dar Landslide Considering the Seismicity as the Main Triggering Factor The critical acceleration estimated for the Du´dar landslide is 0.19 g. In addition, a thrust angle of 8 and a critical acceleration of 0.21 g have been estimated assuming a circular approximation of the failure surface by means of (1). This critical acceleration is slightly greater than the value obtained using the Slide software (0.19 g), which is a more accurate estimation. The most likely magnitude-distance pairs of potential earthquakes which have been able to overcome this critical acceleration value have been obtained using the different GMPEs selected (Table 3). This analysis indicates the Du´dar landslide could be triggered by an earthquake of Mw ¼ 5.0–6.5, provided that it takes place within < 16 km of the landslide. The magnitude values must be considered as the minimum ones, so the occurrence of an earthquake with a larger magnitude than the estimated ones for each magnitudedistance pair could trigger also the Du´dar landslide. Besides, all the estimated earthquakes would very likely be associated with the rupture of one of the active faults present in the Granada Basin, which can potentially generate earthquakes with magnitudes greater than Mw ¼ 6.0 (Sanz de Galdeano et al. 2003) and are located very close to the Du´dar landslide.

Discussion and Conclusions The Du´dar landslide is one of the largest landslides of the Betic Cordillera. Despite of being located in an area of medium-low relief, can be considered as a giant landslide on the basis of its dimensions. Giant landslides are rare in such areas with low-tomedium relief gradients, and normally they respond to specific tectonic or lithologic configurations. In the case of the NE border of the Granada Basin, the active tectonic played an important role in the genesis of the Du´dar landslide. The altitude difference of the Arroyo de Belen Creek (tributary of the Darro River) and the Aguas Blancas River is one of the most important determinant factors for this landslide. This elevation difference is, in part, due to the location of both sub-basins. The Aguas Blancas River flows through an N70E synclinal axe, whereas the Belen Creek is located in the south-eastern dipping flank. The Du´dar landslide acted as a geomorphological agent, changing the landscape and diverting the head of the Arroyo de Belen to the Aguas Blancas River. This river diversion caused, in turn, a lowering in the base level, resulting in a increase of the river incision. Despite the Du´dar landslide can be considered as a fossil landslide, the river incision in the landslide mass could be a risk due to future partial reactivations. The paper also reports a stability back-analysis of the Du´dar landslide, in particular looking at the role played by the main potential triggering factors: water saturation and seismicity. We have found that the presence of water can be not regarded as the triggering factor of the Du´dar landslide. By the contrary, our calculations indicate the Du´dar landslide could be triggered by a low-to-moderate magnitude earthquake (Mw 5.0–6.5), which could very likely be associated with the rupture of one of the active faults present in the Granada Basin, located very close to the landslide. Acknowledgments This study was supported by research projects CGL2008-03249/BTE, TOPOIBERIA CONSOLIDER-INGENIO2010 CSD2006-00041.

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