Evolution of a bank failure along the River Danube at Dunaszekcső, Hungary

Geomorphology 109 (2009) 197–209

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Geomorphology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / g e o m o r p h

Evolution of a bank failure along the River Danube at Dunaszekcső, Hungary Gábor Újvári a,⁎, Gyula Mentes a, László Bányai a, János Kraft b, Attila Gyimóthy a, János Kovács c a b c

Geodetic and Geophysical Research Institute of the Hungarian Academy of Sciences, Csatkai E. u. 6–8, 9400 Sopron, Hungary Mining and Geological Bureau of Hungary, József A. u. 5, 7623 Pécs, Hungary Department of Geology, University of Pécs, Ifjúság u. 6, 7624 Pécs, Hungary

a r t i c l e

i n f o

Article history: Received 22 September 2008 Received in revised form 27 February 2009 Accepted 3 March 2009 Available online 13 March 2009 Keywords: Landslide GPS Levelling Tilt measurements Slope failure River Danube Hungary

a b s t r a c t The banks of the River Danube are one of the most susceptible areas to mass wasting in Hungary. In 2007, a large slump began to develop along the Danube at Dunaszekcső and jeopardized properties on land and navigation in the river. Several factors such as geological, hydrogeological and morphological conditions, recurrent flooding and erosion by the Danube led to a gradual development of the large rotational slide. Slope failure has been monitored using GPS, precise levelling techniques and tiltmeters since October 2007. The expected location of the maximum lateral displacement and extrusion was indicated by GPS measurements from the middle of November 2007. The main phase of the slope failure evolution, i.e. the rapid movement on 12 February 2008 was indicated by accelerated tilting of the southern moving block prior to slumping. Small rise of the relatively stable part of the slope was measured after the rapid movements, which may be explained either by the elastic rebound along the slip surface, or by the intrusion of some plastic material into the lower section of the slope. Comparison of geodetic datasets and field observations with the timing of rainfall and water level changes of the Danube suggested that hydrological properties (subsurface flow processes, soil physical properties, infiltration, and perched water table) were primarily responsible for initiation of the studied slump. A model of slope failure evolution is proposed here based on the monitoring and field observations. © 2009 Elsevier B.V. All rights reserved.

1. Introduction Several regions of Hungary in the eastern part of Central Europe are susceptible to landslides among which the bluff along the west bank of the River Danube between Budapest and Mohács is the most affected (Fig. 1a; Farkas, 1983; Kleb and Schweitzer, 2001; Szabó, 2003). Mass movements have appeared on this part of the riverbank since the Roman times (Lóczy et al., 1989; Juhász, 1999). Based on the database of the Mining and Geological Bureau of Hungary more than 20 large landslides were recorded in the area during the 20th century (Fig. 1b). Recent major movements of the landslides have been studied from geomorphological (e.g. Bulla, 1939; Pécsi, 1971; Pécsi et al., 1979), engineering geological (Egri and Párdányi, 1968; Kézdi, 1970; Karácsonyi and Scheuer 1972; Bendefy, 1972; Horváth and Scheuer, 1976; Scheuer, 1979; Pécsi and Scheuer, 1979), and hydrogeological (Domján, 1952; Galli, 1952; Schmidt Eligius, 1966) points of view. These studies mainly provided empirical descriptions of the 3D deformations. Geodetic measurements were carried out after a large slope failure at Dunaújváros in 1964 (Egri and Párdányi, 1968; Kézdi, 1970), but the results of these observations were only partially published (Kézdi, 1970). Consequently there is a lack of monitoring of landslides and their evolution in Hungary by means of both geodetic ⁎ Corresponding author. Tel.: +36 99 508 340. E-mail address: [email protected] (G. Újvári). 0169-555X/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.geomorph.2009.03.002

methods and modern tools (GPS, tiltmeters, etc.) in spite of current and potential problems in this region. In 2007, a large, about 220 m long rupture appeared parallel with the riverbank at Dunaszekcső (Figs. 1b and 2a,b). At that time the sliding mass was estimated at about 0.3 × 106 m3 and its potential energy (U = mgh) about 153.6 GJ, calculated with a centre of mass of 29 m elevation above the river (Table 1). The landslide risk affected some properties and the river navigation. Thus, a GPS network complemented with borehole tiltmeters was established at this site to monitor movements and measure deformations (Fig. 2). Observation of landslides using GPS has become a firmly established technique over the past decade (e.g. Fukuoka et al., 1995; Jackson et al., 1996; Gili et al., 2000; Malet et al., 2002). In many cases it is used together with photogrammetry (e.g. Chadwick et al., 2005; Brückl et al., 2006), geophysical surveying (Agnesi et al., 2005; Chelli et al., 2006) and deformation measurements, incorporating tiltmeters, inclinometers and extensometers (Cencetti et al., 2000; Coe et al., 2003; Corsini et al., 2005). Strategies and implementation of GPS surveys vary with the type and size of movements and some other factors, e.g. the aim of monitoring and available resources. These measurements can be continuous in time (e.g. Mora et al., 2003, Puglisi et al., 2005) or discontinuous (e.g. Moss et al., 1999; Moss, 2000; Rizzo, 2002; Squarzoni et al., 2005). The network and measurement strategy was selected and designed based on previous investigations of Bányai (2003a,b) and the above-mentioned studies. The aims of this study were to document and precisely measure the

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Fig. 1. Landslide hazard in Hungary. a) Landslide endangered areas (after Farkas, 1983; Juhász, 1999 and Szabó, 2003). 1. Zselic and Baranya–Tolna Hills; 2. Zala Hills; 3. high banks at Lake Balaton; 4. Visegrád Mountains and terraced region along the Danube; 5. high banks along the Danube; 6. North Hungarian Mountains and Hills; 7. Zemplén Mountain and high banks along the River Hernád. Black coarse lines denote the high banks along the Lake Balaton, the River Danube and the River Hernád. b) Large landslides in the 20th century along the high bank of Danube between Budapest and Mohács (after Juhász, 1999; Kleb and Schweitzer, 2001). Locations and years of the landslides are shown. Black stars indicate local outcropping blocks of Triassic limestone; sl = structural line.

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surface displacements on the moving blocks, to monitor tilting and record the evolution of the slide, to compare the geodetic and GPS deformation data with the water level fluctuations of the Danube and the precipitation data of the monitored time period. This approach

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Table 1 Morphometric and physical characteristics of the bank failure at Dunaszekcső. Characteristic

Value

Maximum altitude at top (m a.s.l.) Minimum altitude at toe (m a.s.l.) Difference of altitude Maximum length (m) Maximum width (m) Length to width ratio Surface (m2) Depth (m) Volume (m3) Weight (kg)⁎

142.27 83.3 58.97 222 30 7.4 5086 ~ 65–70 ~ 0.3 million 5.4 × 108

⁎Calculated by a mean density of loess of 1800 kg/m3 (for further details see the text).

improves the understanding of the kinematics and landslide evolution along the Danube and provides information about potential hazards and damages. 2. Geological and geomorphological settings

Fig. 2. The study area at Dunaszekcső. a) GPS network on the high bluff; and b) the network around the slide zone. Points 100–400: concrete cylinders. Points 1000–1003: benchmarks on the southern stable part at Vár Hill. Points 2000–2004: stations on the southern moving block. Points 3000–3004: benchmarks on the northern stable part at Szent János Hill. Points 4000–4006: stations on the northern moving block. Triangles (S, U) indicate two borehole tiltmeters. A and A′ denote the geological cross-section shown in Fig. 3. Note that the location of Fig. 5 can be seen in a.

The study area belongs to the Baranya Hills (Fig. 1a). The monitored bank of the Danube stretches between the Sárköz and Mohács depressions for about 15 km (Moyzes and Scheuer, 1978) (Fig. 1b). These depressions were formed due to tectonic movement along the main NE–SW and NW–SE structural lines during Quaternary, which also determined the present-day flow direction of the Danube (Moyzes and Scheuer, 1978; Síkhegyi, 2002). Geophysical measurements support the idea that recent tectonic movements play a role in the evolution of the western bank of the Danube near Dunaszekcső, as they did in the past (Erdélyi, 1967). Using seismic tomography and reflection, Hegedűs et al. (2008) found an about W–E directed low-velocity tectonic zone below the northern edge of the slope failure at Dunaszekcső. The basement formations at Dunaszekcső are Triassic–Jurassic limestones located at 200–250 m below the surface (Szederkényi, 1964; Urbancsek, 1977; Moyzes and Scheuer, 1978; Hegedűs et al., 2008). These basement rocks are covered by clayey and sandy sediments formed in the Pannonian s.l. epoch (equivalent to the Upper Miocene and the Pliocene, 12.6 to ~ 2.6–2.4 Ma; Rónai, 1985) that can be found below about 70 m depth under the southern moving block (SB) according to borehole data (Moyzes and Scheuer, 1978; Pécsi et al., 1979). The uppermost 70 m of the sediment sequence at SB are sandy and clayey loess layers with brown to red fossil soils accumulated during the Pleistocene (Fig. 3). The bluff reaches its highest point (142 m a.s.l.) at Vár Hill where the southern part of the moving blocks is located (Figs. 2 and 3). The flood plain of the Danube is very narrow or missing below SB at Vár Hill and the northern moving block (NB) at Szent János Hill. The bluff consists of a 20–30 m high vertical loess wall above the 10–20 m high slopes that consist of reworked loess from past landslides and fluvial mud, sand and gravel deposits of the Danube (Fig. 3). The slopes were intensively undercut by the river during each flood event (Moyzes and Scheuer, 1978; Kraft, 2005). The younger loess series on top is prone to collapse while the older loess below is much more compact (Moyzes and Scheuer, 1978; Scheuer, 1979). The density of the younger loess deposits is around 1.6 g cm− 3 (Papp, in press), but that of the older loess series and the intercalated paleosols is between 2.0–2.1 g cm− 3, and that of the Pannonian clays and sands reaches 2.16 g cm− 3 (Hegedűs, et al., 2008). The ground water recharged from percolated rainfall and the Lánka stream resides in the lower part of the young and more porous loess deposits (Fig. 3). Ground water flows to the SE during base flow because of the sucking effect of the Danube (Moyzes and Scheuer, 1978). Field observations show the development of tension cracks in the loess complex parallel as well as perpendicular to the channel of the

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Fig. 3. Geological cross-section of the high bank at Dunaszekcső (after Moyzes and Scheuer, 1978; Pécsi et al., 1979; Kraft, 2005). Elevation and distance were derived from the digital terrain model which was provided by geodetic measurements of 433 points. Vertical exaggeration: ×3. GWL = ground water level (measured in a well in July 2008); HW = highest water; LW = lowest water.

Danube, indicating reduced rock strength. The vertical cracks are clearly visible on the roof of the Töröklyuk cave, a unique large natural cavity on NB (Figs. 2 and 4a). Cracking was probably induced by both previous sliding events and recent slumping. Recent tectonic movements may have also influenced this process because tension cracks have also appeared in the apparently intact part of the slope 150– 200 m southwards from SB. In addition, there are also hollows of various sizes on rock walls resulting from loess corrosion and piping (Fig. 4b; Kraft, 2005). This process could be activated anytime to open major cracks, as happened during the movements of the studied landslide. Landslides in the studied hill region are concentrated in areas where relative relief is sufficiently high. This situation occurs along the Danube bank where stream undercutting has produced relatively high bluffs. One of the most important factors of landsliding is the hydrological condition of high bluffs. The Danube has a water level fluctuation in a range of nearly 10 m that influences the springs of

ground and artesian water at the foot of the bank, which is inundated during higher water stages but experiences rapid draining during lower water stages (Fábián et al., 2006). Along the steep bank of the Danube, the Upper Pannonian sediment sequence consisting of alternating permeable and impervious layers is exposed in some places below the Pleistocene or Upper Pliocene loess sequence or the Pliocene red clays. Because of previous slumping and lateral erosion by the Danube, the Upper Pannonian sediments are partly redeposited with a disturbed stratification or buried under younger deposits. The Upper Pannonian sand deposits provide confined aquifers, and their water under pressure locally moistens the overlying past slump deposits, favouring the reactivation of existing slumps and the generation of new landslides. During spring–summer floods, the river inundates the surface to the level of the springs at the base of the bluff, leading to the rise of the local groundwater table. This circumstance is noteworthy because slumps and earthslides tend to take place after prolonged high-water stages of

Fig. 4. Rock strength reducing processes on the study area. a) Closed tension cracks on the roof of the Töröklyuk cave; b) Dissolution hollows in loess at Dunaszekcső caused by corrosion. Cr = tension cracks; Hw = hollows.

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the Danube (Domján, 1952; Karácsonyi and Scheuer, 1972; Horváth and Scheuer, 1976; Pécsi et al., 1979; Fábián et al., 2006). Fig. 5 shows the recent and past mass movements along the Danube near the studied landslide at Dunaszekcső. According to the classification of Cruden and Varnes (1996), the past landslides at Dunaszekcső are historic landslide types, which developed under environmental conditions similar to today's. The numerous historic mass movements indicate the high landslide susceptibility of this area. 3. Methods 3.1. GPS measurements and precise levelling As a reference network (Fig. 2a), four “slim” reinforced concrete pillars were built (Fig. 6a). Two of the benchmarks (100 and 200) were established farther from the moving blocks, while the other two (300 and 400) were placed closer to them (Fig. 2a). Twenty-one additional smaller concrete benchmarks were built on both the stable and moving parts of the monitored area near the fracture zone (Fig. 2b). The 0.6 m deep cylinders with a diameter of 0.1 m were used during the levelling and horizontal measurements with tripods or antenna rods (Fig. 6b).

201

Seven GPS campaigns have been carried out after the establishment of the measuring network in September 2007. The zero measurement (first GPS and levelling survey) was performed on 3 October 2007 and followed by six campaigns: 18 Oct 2007, 8 Nov 2007, 13 Dec 2007, 23 Jan 2008, 26 Feb 2008 and 31 Mar 2008. Major movements on 12 February 2008 were not measured on that day, but these deformations are included in the results of the 6th campaign on 26 Feb 2008. As movements over the 1 cm level were expected a Leica 1200 series RTK GPS receiver pair was chosen for the geodetic measurements. At the first measured time period the initial coordinates of benchmark 300 were determined using the active GNSS network of Hungary (Horváth, 2005). Then six independent baselines among the four reference stations were determined for 30 min, and 21 smaller benchmarks were separately measured from the two nearer reference stations (300 and 400) for 5 min (Fig. 2a,b). The estimated baselines were rigorously adjusted by the GPS-NET program, which was developed for the identification of systematic errors (Bányai, 1991), the estimation of main phase center offsets (Bányai, 2005) and the free adjustment of a local 3D deformation network (Bányai, 2006). The GPS height determination of the simple benchmarks were improved and controlled by the precision levelling instrument Leica DNA 03.

Fig. 5. Geomorphic map of recent and past mass movements between Báta and Dunaszekcső settlements (modified after Fábián et al., 2006). 1–slope, 2–slope with gully erosion, 3–derasion valley, 4–erosion valley, 5–loess plateau, 6–scarp, 7–historic landslide, 8–creep, 9–talus, 10–loess valley, 11–loess doline, 12–spring.

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while water level data of the River Danube were recorded twice a day (7 am and 7 pm) at Mohács about 6 km to the south by the Research Institute of Environmental Protection and Water Management (VITUKI Kht) and made available to the public through an online database (www.vizugy.hu, www.vizadat.hu). 4. Results 4.1. Horizontal and vertical displacements

Fig. 6. Sketches of a) reinforced concrete pillars for the reference network and b) smaller reinforced concrete benchmarks.

The inner precision of the reference stations can be characterized by 0.3 mm in the north, 0.2 mm in the east and 0.5 mm in height component, while in the case of other measurement points 1.6 mm, 1.1 mm and 2.1 mm are the characteristic numbers. The source of the digital terrain model shown in Figs. 2 and 7 was a topographic map at the scale of 1:10,000, which was improved and specified by the measurement of 433 points on Vár and Szent János Hills using GPS, a Wild T-3000 digital thedolite and a DI 2002 high precision electronic distance meter. 3.2. Tilt measurements Two dual-axis borehole tiltmeters (Model 722A produced by Applied Geomechanics Inc., USA) were used for tilt measurements. Both tiltmeters were installed in shallow boreholes at a depth of 3 m to ensure suitable temperature stability for the instruments. The boreholes with 0.3 m diameter were cased in a PVC pipe that was fastened to the surrounding formation by filling the interspace with concrete. The tiltmeters were fixed by packed quartz sand inside the pipe to obtain a stable, strong coupling to the ground. The data logger and the batteries were placed in a thermally insulated and sealed iron chest at the end of the pipe following the method of Mentes (2003a). The tiltmeters have a dual-axis tilt sensor and a built-in sensor for temperature measurements and they operate in two measuring ranges: the resolution for the “high gain” range is 0.1 μrad while that for the “low gain” range is 1 μrad. One tiltmeter (U) was installed on the unstable section of the bank about 8 m from its edge, while the other instrument (S) was placed on the stable section about 30 m from the main crack (Fig. 2). The tiltmeters were installed so that their positive x tilt axes point to the north and their positive y axes to the east. Tilt and soil temperature data were collected hourly between 11 Nov 2007 and 31 Mar 2008 on both the stable and moving parts of SB (Fig. 2b). 3.3. Precipitation and water level data Daily precipitation data, including rain- and snowfall, were available from a station 2.4 km north of the test site at Báta Village,

At the first measured time period (between 3 and 18 Oct 2007) horizontal displacements amounted to about 0.4–0.7 cm on the northern block (NB), while the southern block (SB) moved faster (0.9–1.2 cm; Table 2). The average velocities were in the order of 0.3– 0.4 and 0.6–0.8 mm day− 1 on NB and SB, respectively. The motion vectors showed approximately ESE directions on NB, and ENE to NE directions on SB (Fig. 7a). The vertical displacements ranged between −1.4 and − 2.3 cm on NB and −0.8 and −1.7 cm on SB during this time period. Thus, NB moved more rapidly in the vertical direction than in the horizontal direction. During the second measured time period (between 18 Oct and 8 Nov 2007) the movements slightly accelerated. The horizontal displacement rates varied between 0.3 and 1.0 mm day− 1 on NB, while 0.6 and 1.2 mm day 1 on SB. Motion vectors still indicated approximately ESE directions on NB and turned to E to ENE directions on SB (Fig. 7b). The subsidence was − 3.0 to − 5.1 cm on NB and −1.8 to −3.9 cm on SB. The slide slightly slowed during the third measured time period (between 8 Nov and 13 Dec 2007). The horizontal average movement rates were about 0.2–0.5 mm day− 1 on NB and 0.3–0.7 mm day− 1 on SB and the vertical displacement rates were also lower than during the second time period, but they were in the same order of magnitude (between − 0.8 and −1.4 mm day− 1 on NB and between − 0.5 and −1.1 mm day− 1 on SB) as during the first time period. Motion vectors indicated the same ESE and ENE directions (Fig. 7c). Movements accelerated during the fourth measured time period between 13 Dec 2007 and 23 Jan 2008. The horizontal velocities significantly increased reaching 1.0–2.8 mm day− 1 on NB and 0.8– 4.2 mm day− 1 on SB. Vertical displacement rates were also increased by a factor of 3 to 4 (between −5.0 and −8.2 mm day− 1 on NB and between −1.4 and −6.4 mm day− 1 on SB). The directions of the motion vectors did not change during this time period (Fig. 7d). Only one measured location on SB (point 2002) deviated permanently from the ENE motion direction and showed ESE movement (Fig. 7, white dot). After the beginning of January 2008, movements of the sliding blocks became faster. On 12 February 2008 the unstable blocks began to move at a rate of 50–70 cm h− 1 according to field observations. On that day vertical displacements amounted to 5–6 m. During the next days the blocks reached a new apparent equilibrium state characterized by reduced motion. The sixth GPS campaign was accomplished after the above events on 26 February 2008. Some of the established benchmark points (2000, 2001, 4003 and 4005) were lost during the movements, so further information about them is missing in Table 2 and Fig. 7e,f. The measured horizontal displacements ranged between 1.9 and 2.6 m on NB and 2.8 and 4.6 m on SB during this time period (between 23 Jan and 26 Feb 2008). Vertical movements ranged between − 8.0 and −9.7 m on NB and between − 6.4 and −7.7 m on SB. Point 2002, which moved formerly in an unusual and extraordinary way, showed 2.5 cm horizontal and − 5.7 cm vertical displacement, because that part of the sliding mass hardly subsided. Motion vectors obtained in the frame of this campaign showed eastward movements on NB and the ENE direction on SB (Fig. 7e). Major mass movements slowed down during the sixth measured time period (between 26 Feb and 31 Mar 2008) and decreased to the

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Fig. 7. GPS motion vectors and development of main fractures in the study area. a) First time period (15 days between 3 and 18 Oct 2007); b) Second time period (21 days between 18 Oct and 8 Nov 2007); c) Third time period (35 days between 8 Nov and 13 Dec 2007); d) Fourth time period (41 days between 13 Dec 2007 and 23 Jan 2008); e) Fifth time period (34 days between 23 Jan and 26 Feb 2008); f) Sixth time period (34 days between 26 Feb and 31 Mar 2008). Note that movement scales are different. The white dot is point 2002.

same magnitude as it was measured during October and November 2007. The horizontal displacement rates were 0.3–1.1 mm day− 1 and 0.3–2.3 mm day− 1 on NB and SB, respectively. Slow sinking (−0.5 to

−2 cm) was observed for the majority of the remaining benchmarks. GPS motion vectors showed the ENE direction on NB and the SSW direction on SB (Fig. 7f).

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Table 2 Measured horizontal and vertical displacements (cm). Points Campaigns Oct 18, 2007 North East 1000 1001 1002 1003 2000 2001 2002 2003 2004 3000 3001 3002 3003 3004 4000 4001 4002 4003 4004 4005 4006

Nov 8, 2007 Elevation North

0 0 0.01 − 0.43 0.19 0.01 − 0.45 0.25 0.01 −0.14 0 −0.04 0.17 0.55 − 1.7 0.06 1.16 − 1.54 0.27 0.91 −0.86 0.76 0.66 −1.66 0.65 1.06 −0.99 0.2 0.14 −0.02 0.07 − 0.32 −0.01 0.22 0.06 0.02 0.21 −0.06 0 0.08 −0.09 0.01 −0.1 0.42 −1.43 −0.19 0.5 −1.55 0.04 0.56 −2.01 −0.24 0.54 − 1.85 − 0.65 0.31 − 2.12 − 0.46 0.42 − 2.29 − 0.42 0.45 − 1.92

East

Dec 13, 2007 Elevation North

− 0.04 0.1 − 0.03 − 0.05 − 0.16 − 0.03 − 0.11 0.02 0.01 −0.07 − 0.11 0.04 0.82 1.17 − 3.9 0.05 1.95 −3.64 −0.68 1.19 − 1.83 0.59 1.32 −3.92 0.15 2.53 − 2.67 − 0.17 0.08 0.04 − 0.13 0.08 0.01 − 0.14 − 0.01 − 0.01 − 0.03 0.09 − 0.02 −0.08 0.1 − 0.03 − 1.3 1.79 − 3.05 −0.94 1.74 −3.42 −0.74 1.1 −4.69 − 0.55 1.23 −4.61 − 0.44 0.99 − 4.84 − 0.32 0.64 − 5.18 − 0.43 0.87 − 4.14

0.11 0.17 0.13 0.08 0.72 0.43 −0.14 0.63 0.58 0.08 − 0.01 0.17 0.16 −0.08 − 0.8 − 1.2 − 0.52 − 0.51 − 0.51 − 0.58 − 0.74

Jan 23, 2008

Feb 26, 2008

Mar 31, 2008

East

Elevation North

East

Elevation North

East

Elevation North

East

Elevation

0.12 0.18 −0.13 0.03 1.03 1.64 0.93 1.32 2.37 − 0.05 0.07 −0.06 0.07 − 0.02 1.24 1.2 0.78 0.99 0.92 0.64 0.7

− 0.04 0 0.01 0.03 − 3.96 −3.75 − 1.89 − 3.89 − 3.35 0.05 0.02 − 0.02 −0.02 − 0.03 − 2.79 − 3.21 − 4.34 − 4.25 − 4.53 − 4.97 − 3.97

− 0.22 −0.17 − 0.31 0.1 6.64 10.24 3.42 8.27 16.35 0.01 − 0.16 0.01 0.23 0.01 10.61 9.53 8.77 8.92 7 4.37 5.46

−0.06 − 0.01 0.04 0.05 − 26.16 − 24.23 − 6.02 − 26.39 − 19.24 0.06 0.02 − 0.01 − 0.03 − 0.03 − 20.88 − 23.17 − 28.8 − 27.72 − 31.08 −33.98 − 27.49

0.51 0.28 0.04 − 0.31 – – 2.51 241.42 434.79 − 0.36 − 0.16 −0.27 − 0.82 − 0.04 244.1 237.74 253.69 – 243.53 – 198.96

− 1.76 − 0.12 0.24 − 0.2 0.04 − 0.15 0.5 0.36 – – – – −5.7 0.03 − 776.41 − 8 − 641.69 − 2.97 − 0.15 0.53 0.46 0.36 0.55 0.46 − 0.27 0.21 −0.02 0.51 − 800.06 2.01 − 815.55 2.14 − 958.24 1.44 – – −978.01 − 0.61 – – − 837.49 − 0.18

0.07 0.03 − 0.14 − 0.04 – – − 0.15 − 6.2 − 1.89 0.04 − 0.3 − 0.36 0.33 − 0.19 3.49 2.7 3.38 – 3.98 – 3.51

1.66 0 −0.08 −0.2 – – −0.77 − 2.1 0.11 0.4 0.04 0.1 0.62 0.24 − 0.86 − 1.05 − 0.63 – 0.08 – − 0.31

The horizontal displacements of benchmarks 1000–1003 and 3000– 3004, which were placed on the “stable” section of the slope within a distance of 1 and 7 m from the main rupture, ranged from 0.1 to 0.5 cm and the displacement rates were between 0.02 and 0.10 mm day− 1 during the first four measured time periods. The vertical movements were 0.1–0.5 mm. These values are within the range of the detection error limits of our methods. However, vertical displacements became slightly higher during the fifth and sixth time periods. Accordingly, subsidence (−1.76 cm) and then emergence (1.66 cm) were detected at point 1000 on SB, and emergence (1 to 5 mm) was measured at points 3000–3004 on NB. 4.2. Tilt measurements The stable part of the high bank tilted very slowly to the SW from the beginning of the measurements (S, Fig. 8). On 12 February 2008 the mobile section of SB began to slide rapidly. The intact part of the high bank tilted about 25 μrad to the SW during that day (“jump” in Fig. 8). The other instrument (U; see Fig. 2b), which was installed on the moving part of SB, was increasingly tilting approximately to the SE until the middle of January 2008. Then the y component of this tiltmeter changed to the opposite direction and the instrument began to tilt quickly to the SW. On 12 February 2008 this tiltmeter went beyond its measurement range and was not able to record movement further. However, the last recorded data showed tilting to the SE again. 4.3. Precipitation and river water level The precipitation data show three distinct wet periods between the spring of 2007 and February 2008 (Fig. 9). The largest event occurred between 23 May and 9 June 2007 with 110 mm rainfall, and followed by a 51 mm event between 10 Aug and 11 Sept 2007, and a 60 mm event between 15 Nov and 12 Dec 2007. The water level of the River Danube fluctuated between 118 and 778 cm during the analysed time period, thus the change between the minimum and the peak amounted to 660 cm. The most remarkable high stand of water occurred in mid-September 2007 exceeding 770 cm. Other secondary peaks in spring, summer and autumn reached 450–500 cm. These data and the graph in Fig. 9 show a complex hydrograph with several larger and smaller peaks and a remarkable high stand in mid-September 2007.

− 0.07 0.49 0.13 0.66 4.42 3 − 1.33 3.98 5.48 −0.09 − 0.38 0.62 − 0.5 0.08 −5.05 − 4.72 − 3.14 −2 − 2.53 − 0.32 −2.51

− 0.21 − 0.53 − 0.04 − 0.73 – – − 0.32 144.26 176.34 0.11 0.43 − 0.57 0.67 − 0.27 − 66.65 − 60.44 − 41.88 – −41.34 – −17.5

5. Discussion The bluffs along the River Danube can be divided into three main types based on their geology (Scheuer, 1979; Kleb and Schweitzer, 2001). The first type of banks only consists of Upper Pannonian deposits, the second type is made up of both Upper Pannonian and Quaternary sediments, and the third type is almost entirely composed of Quaternary deposits. Previous studies of large slope failures along the Danube underlined the fact that the slip surfaces evolved mainly in the Upper Pannonian deposits (sands, clays and sandy silts) characterized by lower shear resistance (Horváth and Scheuer, 1976; Moyzes and Scheuer, 1978; Scheuer, 1979). Therefore the first and second types are frequently affected by landslides. At the same time, the bank of the Danube is generally more stable where the Pleistocene loess complex (about 20–40 m thick) reaches below the average water level of the river (Scheuer, 1979). Nevertheless, the possibility of mass movements is enhanced by interbedded paleosols in the loess complex at the river bed level as found at SB (Fig. 3). The existence of both previous landslide scarps and recent movements indicate that, even if the studied bank is third type made up of Quaternary deposits, the local geological setting and hydrological properties could initiate slope failures along the bank of the Danube at Dunaszekcső. To investigate the effects of precipitation and water level fluctuations of the river as possible triggers for the studied mass movement, we need to pay attention to the following. The main rupture on NB developed in the midsummer of 2007 and on SB in August 2007, following a high stand of water in the Danube during a wet period (110 mm) between 23 May and 9 June 2007 (Fig. 9). The highest water levels in the middle of September 2007 most likely contributed to the further development of the slide. Rinaldi et al. (2004) emphasized that the peak river stage plays a primary role in triggering instability, but a prolonged and complex hydrograph with minor peaks preceding the main one is responsible for more unfavourable stability conditions than those of flow events with a single, distinct rising limb. The data show that accelerating tilt of the mobile section of SB started around 23–26 Dec 2007, following the high water of the river on 17 December 2007 and the wet period (60 mm) between 15 Nov and 12 Dec 2007 (Figs. 8 and 9). A significant change of the tilt direction occurred around the secondary high water stand on 26 January 2008 and was followed by the rapid movements of the slide. Previous studies in Hungary (Domján, 1952; Karácsonyi and Scheuer,

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Fig. 8. Borehole temperature and tilt records on the stable and moving part of the southern moving block at Vár Hill. ST = borehole temperature of device S; SN = x component of tiltmeter S on the stable section of Vár Hill (N–S direction); SE = y component of tiltmeter S (E–W direction); UN = x component of tiltmeter U on the moving block (N–S direction); UE = y component of tiltmeter U (E–W direction). White arrows denote the large movements on 12 February 2008. Borehole temperature record of tiltmeter U is not shown because it was almost the same as that of tiltmeter S. Trend of the tilt curves, the “jump” in the record of tiltmeter S, accelerating tilt and changing tilt directions (tiltmeter U) are totally unrelated to soil temperatures. The last sections of the curves ST, SN and SE after the major movements confirm that tiltmeter S has no detectable drift caused by temperature. This fact is further reinforced by the longer (between 11 Nov 2007 and 12 Dec 2008) tilt and temperature record of tiltmeter S which is not shown here. For further details see Mentes (2003b).

1972; Horváth and Scheuer, 1976) and worldwide (Twidale, 1964; Thorne, 1982; Springer et al., 1985; Lawler et al., 1997) emphasized the fact that bank failures are likely to occur during drawdown after a high water stage, when the bank material is still saturated and it loses the confining pressure of the river during the recession. These observations were also supported by numerical simulations in recent studies (Dapporto et al., 2003; Rinaldi et al., 2004), suggesting a strong relationship between the observed movements and hydrological conditions, involving both the magnitude and frequency of rainfall events and the periodicity of river water level fluctuations. Because ground water and pore water pressure data were not available for this study, direct conclusions about the effect of infiltrated water on the evolution of the studied landslide could not be drawn. Our geodetic and GPS measurements, geological and geomorphological observations and the relations with rainfall and water level fluctuations permitted us to build the following model to demonstrate the evolution of the studied slope failure (Fig. 10). During the first dormant stage, the slope, composed of loose materials (loess, sandy loess and fossil soils), was mainly unsaturated in ambient conditions and remained stable for many years due to matric suction (negative pore water pressures) causing an apparent cohesion (e.g. Casagli et al., 1999; Rinaldi and Casagli, 1999; Simon et al., 2000; Fig. 10a). The stability of the slopes also may have depended on the basal accumu-

lation of past slump materials, which were able to support the steep slope and could impound ground water flow (Galli, 1952). Indeed, oversaturation by this mechanism led to a similar slope failure at Dunaföldvár in the 1970s (Horváth and Scheuer, 1976). On the other hand, river erosion greatly affected the stability of the river bank by eroding basal materials, i.e. undercutting the slopes. In addition, seepage erosion significantly contributed to the erosion of the basal support material (Hagerty, 1991; Fox et al., 2007; Wilson et al., 2007). Furthermore, the observed tension cracks were able to ease and accelerate the downward infiltration of rainwater (e.g. Sidle and Ochiai, 2006) as primarily vertical, preferential flow paths (Vandekerckhove et al., 2001). This process resulted in the development of cavities, reduced shear strength due to losses of matric suction and generation of positive pore water pressures as well as an increase in unit weight (Rinaldi and Casagli, 1999; Simon et al., 2000). Recurrent floods of the River Danube caused repeated saturation through lateral seepage. This alternation between saturated and unsaturated conditions is very common in stream banks, due to the rising of the water table during flow events and its falling during the subsequent drawdown (Rinaldi and Casagli, 1999). Consequently the matric suction and shear strength were gradually reduced (Kézdi, 1970) and the first main cracks appeared on the top of the slope indicating a destabilization process.

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Fig. 9. Data series of cumulative precipitation of the study area and water level fluctuations of the River Danube. Data source: www.vizadat.hu (last accessed: 11 Mar 2008). Nr. = northern rupture; Sr. = southern rupture.

The second development stage (Fig. 10b) lasted a few months. The SB moved and tilted towards the river according to measurements by GPS and tiltmeter U (Figs. 7b,c,d and 8). The horizontal and vertical movements alternately decelerated and accelerated between September 2007 and January 2008 because of change in stability conditions. The tilt of SB steadily increased from mid-December 2007, but its SE direction did not change until mid-January 2008 (Fig. 8, UE). Then SB began to tilt backwards (towards SW) along a gently curved slip surface (Fig. 8) as the main phase of movements. About 3 weeks later (12 Feb 2008), after a drawdown of the river, when the threshold of pore water pressure was surpassed and the confining pressure of the river ceased, both moving blocks began to subside rapidly (50–70 cm h− 1). The downslope progress of the sediment blocks resulted in lateral extrusions and rising of the ground at the toe of the bluff as well as emergence of shallows on the bed of the Danube due to the extension of the landslide's toe (Fig. 10c). Since mid-November 2007 the GPS motion vectors have indicated the area where the shallows emerged subsequently. The directions of GPS vectors and the appearance of the largest extrusions in the Danube river bed (Fig. 2b), so the movements of NB and SB could be explained by the geometry of the slide and the positions of terraces, which are scraps of earlier slumps (Fig. 11). The dislocated blocks were presumably supported by the blocks of earlier slumps at their northern and southern parts, whereas there were only steep slopes in front of the moat (Fig. 11). It is also likely that the W–E tectonic line has affected the areal extent of the slide (Hegedűs et al., 2008); however, tectonics is not considered as the main triggering mechanism of the slope failure studied. The measurement points placed on the stable parts of SB and NB (1000–1003 and 3001–3002) were slightly elevated after the main motion of the slump (Fig. 2b and Table 2). The slight uplift could have happened for two reasons. The first might be the elastic rebound (Reid, 1910; Berlin, 1980) when the strain energy could have accumulated in the rock mass around the prospective and evolving

slip surface during the deformations, and then the energy was released suddenly by means of rapid movements on 12 February 2008. The rocks rebound to their original undeformed shape and that was detected as a rise on the stable part of the slope. The improved elastic rebound theory (Berlin, 1980) permits the simultaneous sliding and deformation, which could be a possible explanation for the expected, but hardly detectable (within the error limit) subsidence at points 1000–1003 and 3000–3004 before 12 Feb 2008. Perhaps the presumed rebound is apparent in the “jump” of the tilt curves during the intensive phase in Fig. 8 (SN and SE). A second reason might be the intrusion of plastic material into the lower section of the stable slope section. As the plastic material moved along the poorly defined slip surface, caused by the gravity force of the sliding block, it was pushed into the lower mass of the stable slope opposite to the formed slump toes. Ultimately, this intrusion caused a slight rise of the stable slope both on NB and SB. As mentioned above, the studied landslide posed a risk to properties and river navigation, which was a real hazard as can be seen in Fig. 11. A house on NB was collapsed and two below NB were badly damaged on 12 February 2008. The water tank at Vár Hill, which played a crucial role in the water supply for the settlement, needed to be shut off and a new one had to be built on another safe hilltop. The river navigation was stopped for some days around 12 Feb 2008 and the navigation route had to be changed because of deformation of the Danube's bed. Although the mobile blocks were at relative rest during the second half of 2008 according to subsequent measurements, there are still some warning signals indicating possible north- and southward spreading of the slump. In such a catastrophic case many family houses and public utilities would be in jeopardy. 6. Conclusions Geodetic deformation measurements can provide reliable data about the complex movements and evolution of landslides. GPS and

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Fig. 10. Local morphogenetic scheme of the bank failure at Dunaszekcső. a) Stable bank without recent movements, after landslide events; b) Destabilized high bank with slow motions, soon before the landslide initiation; c) High bank in a new equilibrium state, after rapid movements. Vertical exaggeration: ×3.

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Fig. 11. Oblique aerial view of the Dunaszekcső landslide. a) water tank; b) collapsed house; c) damaged houses. Photo was taken by László Körmendy on 17 February 2008.

precision levelling data were particularly useful to predict the spatial characteristics of the slope failure and, thus, identify the most hazardous zones in the future. This was illustrated by the motion vectors showing the area of largest prospective displacements. Furthermore, the horizontal movements of benchmark 2002 on SB were irregular compared to other adjacent points and its vertical displacements were much smaller from the beginning of the monitoring. This observation suggests that this part of the moving block was more stable and would remain approximately in its original position. GPS campaigns provided valuable information about the spatial evolution, but only restricted information about the temporal evolution of the slide, due to the fact that the GPS survey was not continuous in time. Reliable data about the assumed acceleration of movements were obtained from tilt records. A significant increase of tilt in the SW direction was shown by the record of instrument U a few days before the largest movement. These observations do not predict a landslide accurately in time, but would lead to approximate detection of the rapid movement phase of a slope failure and could forecast short-term mass movements. However, the task requires additional surveys and analyses. Rainfall and, primarily, the water level fluctuations of the Danube are found to be the main triggers of the studied landslide. At the same time, continuous monitoring of pore water pressure, ground water level changes and the magnitude of seepage erosion at the toe would provide essential details on slope failure mechanisms along the bank of the Danube. The long-term monitoring of this landslide would also provide further valuable information on mass wasting processes, redeposition of materials and their consolidation and stability. These data would help mitigate damage from future slope failures at Dunaszekcső and in areas of high risk associated with major rivers such as the Danube. Acknowledgments The present study was supported by the President Fund of the Hungarian Academy of Sciences (Reference Number: Kinnof-15/6/ 35/2007). The authors are grateful for the field assistance from Attila Horváth, Frigyes Bánfi, Tibor Molnár and Ferenc Schlaffer as well as for the technical assistance from the local government (mayor János Faller and notary Éva Pest), the Directory of Disaster Recovery at Baranya County (lieutenant colonel Tibor Oláh) and the Local Fire Service at Mohács. We also thank T. Oguchi, D. Lóczy, K.A. Brunstad, Zs.

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