ORI GIN AL PA PER

A geotechnical investigation of the retrogressiveYaka Landslide and the debris flow threateningthe town of Yaka (Isparta, SW Turkey)

Adnan Ozdemir Æ Mehmet Delikanli

Received: 15 April 2008 / Accepted: 6 August 2008 / Published online: 27 August 2008� Springer Science+Business Media B.V. 2008

Abstract In this study the factors affecting the retrogressive Yaka Landslide, its mech-

anism and the hazard of debris flow on the town of Yaka are investigated. In the landslide

area, the first landslide was small and occurred in March 2006 on the lower part of the

Alaardıc Slope near the Gelendost District town of Yaka (Isparta, SW Turkey). The

second, the Yaka Landslide, was large and occurred on 19 February 2007 in the soil-like

marl on the central part of Alaardıc Slope. The geometry of the failure surface was circular

and the depth of the failure surface was about 3 m. Following the landslide, a 85,800 m3 of

displaced material transformed to a debris flow. Then, the debris flow moved down the

Eglence Valley, traveling a total distance of about 750 m. The town of Yaka is located

1,600 m downstream of Eglence Creek and hence poses a considerable risk of debris flow,

should the creek be temporarily dammed as a result of further mass movement. Material

from the debris accumulation has been deposited on the base of Eglence Valley and has

formed a debris-dam lake behind a debris dam. Trees, agricultural areas, and weirs in the

Eglence Creek have seen serious damage resulting from the debris flow. The slope angle,

slope aspect and elevation of the area in this study were generated using a GIS-based

digital elevation model (DEM). The stability of the Alaardıc Slope was assessed using limit

equilibrium analysis with undrained peak and residual shear strength parameters. In the

stability analyses, laboratory test results performed on the soil-like marls were used. It was

determined that the Alaardıc Slope is found to be stable under dry conditions and unstable

under completely saturated conditions. The Alaardıc Slope and its vicinity is a paleo-

landslide area, and there the factor of safety for sliding was found to be about 1.0 under

saturated conditions. The Alaardıc Slope and the deposited earthen materials in Eglence

Creek could easily be triggered into movement by any factors or combination of factors,

such as prolonged or heavy rainfall, snowmelt or an earthquake. It was established that the

depth of the debris flow initiated on the Yaka Landslide reached up to 8 m in Eglence

Creek at the point it is 20 m wide. If this deposited material in Eglence Creek is set into

motion, the canal that passes through Yaka, with its respective width and depth of 7 and

1.45 m, could not possibly discharge the flow. The destruction or spillover of this canal in

A. Ozdemir (&) � M. DelikanliGeological Engineering Department, Selcuk University, Konya, Turkeye-mail: aozdemirkonya@hotmail.com

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Nat Hazards (2009) 49:113–136DOI 10.1007/s11069-008-9282-y

Yaka could bring catastrophic loss to residents which are located within 3–5 m of the bank

of the canal. Furthermore, if material present in the landslide source area slides and this

displaced material puts pressure on the unstable deposited material in Eglence Creek, even

more catastrophic loss would occur to the town of Yaka. In this study, it was determined

that debris flows are still a major hazard to Yaka and its population of 3,000. The results

provided in this study could help citizens, planners, and engineers to reduce losses caused

by existing and future landslides and debris flow in rainfall and snowmelt conditions by

means of prevention and mitigation.

Keywords Debris flow � Landslide � Retrogressive landslide � Yaka Landslide �Turkey

1 Introduction

Landslides are natural geologic processes that cause different types of damage, ranging

from affecting people, organizations, and industries, to the environment (Glade 1998).

Globally, landslides cause billions of dollars in damage and thousands of deaths and

injuries each year. Developing countries, where 95% of landslide disasters have been

recorded, suffer the most, up to 0.5% of the gross national product has been lost to

landslides (Chung et al. 1995). Natural disasters, such as earthquakes, floods,and landslides

are major, yet common, natural hazards in Turkey. Every year landslides cause damage to

property and infrastructure, and the loss of life in Turkey. In the period 1959–1994,

landslides damaged 76,995 buildings throughout Turkey (Ildir 1995) in addition to causing

death and destroying farmland, etc. (Duman et al. 2005). Landslides are influenced by a

variety of control factors, such as geology and topography, and trigger factors, such as

earthquakes and prolonged and/or heavy rains. Sometimes a combination of various factors

is responsible for landsliding. Many researchers have investigated the landslides triggered

by rainfall (Ocakoglu et al. 2002; Petrucci and Polemio 2003; Fiorillo and Wilson 2004;

Mikos et al. 2004; Wen and Aydin 2005; Mikos et al. 2006; Sivrikaya et al. 2008). It is

generally recognized that the rainfall-induced landslides are caused by excess pore pres-

sures and seepage forces during periods of intense rainfall. It is the excess pore water

pressure that decreases the effective stress in the soil and thus, reduces the soil shear

strength, consequently resulting in slope failure (Anderson and Sitar 1995). In recent years,

many studies have been published on shallow landslides, debris flow and rainfall-induced

landslides (Chowdhury and Flentje 2002; Delmonaco et al. 2003; Teoman et al. 2004;

Benac et al. 2005; Dunning et al. 2006; Yilmaz and Yildirim 2006; Wang et al. 2006;

Ulusay et al. 2007).

In the Yaka Landslide area, the first, but small, landslide, which displaced approxi-

mately 12,000 m3 of material occurred on March 2006 in the lower part of the AlaardıcSlope. The second and large landslide, called the Yaka Landslide, which displaced

85,800 m3 of material occurred on 19 February 2007 in the central part of the AlaardıcSlope, near the Gelendost town of Yaka (Isparta, SW Turkey). The Yaka Landslide had a

retrogressive movement, progressing uphill.

Following a period of rapid snowmelt and heavy rainfall, the Yaka Landslide occurred.

The Yaka Landslide initiated within highly weathered soil-like marl in a sliding mode, then

transformed to a debris flow and finally moved down Eglence Creek toward the town of

Yaka. Yaka is located 1,600 m downstream of the landslide source area and hence poses a

considerable risk of debris flow and the creek must be temporarily dammed as a result of

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further mass movement. The debris flow traveled about 750 m toward Yaka. This

85,800 m3 mass of displaced earthen material was deposited in Eglence Creek and still

remains there. In addition, destabilized, but not completely failed, a huge potentially

unstable mass with a volume of 185,391 m3, still remains in the source area. These two

masses could easily be triggered into movement by any factors, such as prolonged or heavy

rainfall or snowmelt. Debris flow from landslide remains a major hazard to Yaka.

The geological, morphological, and meteorological specifications of the Senirkent

District Center and the village of Bahiyar in the Yalvac District that are in close proximity

to the study area especially present many similarities to the area covered by this study. On

July 13, 1995 in Senirkent 74 lives were lost, numerous houses were destroyed, and there

was much property damage resultant of a debris flood caused by prolonged and heavy

rainfall (Nurlu et al. 1997). On June 17, 2003 in the village of Bahtiyar 2 lives were lost in

a similar incident of a debris flood. The town of Yaka, situated in the same region and with

its population of 3,000, the safety of approximately 20% of the town’s lives and property

are under threat of an imminent and unavoidable debris flood.

The main purposes of this study are to describe the affecting factors of the retrogressive

Yaka Landslide; to explain the mechanism of this landslide and to describe the hazard of

debris flow to Yaka.

2 General characteristics of the study area

The study area is located in the Central Mediterranean Region of Turkey, east of the

province of Isparta, and about 3 km to the south of the center of the Gelendost District. To

otherwise state this, the study area is in vicinity of the town of Yaka. In the year 2000, the

population of Yaka was about 3,000. The study area has a total area of about 20 km2 and

extends from 4215,000 to 4220,000 m North latitude and from 327,000 to 331,000 m East

longitude. The location map of the study area is given in Fig. 1.

The geological, geomorphological, and hydrogeological environment of the study area

is favorable to landslide activity. Several landslides have occurred in the past; there are

also numerous evidences of active landslides in the study area. Landslides in study area

terrain have varying controlling factors, such as lithology and slope that seem to be the

main controlling factors in causing slope instability. The main parameters that are influ-

ential to the slope instability and which were considered in the present study are described

below.

2.1 Preparation of the digital elevation model

A digital elevation model (DEM) was prepared by digitizing the detailed topographic map

of the area with a scale of 1:25000 and a contour interval of 10 m. They are converted into

raster format with a grid size of 10 9 10 m. The grid has 501 rows and 401 columns which

cover the study area. Maps of slope, aspect and elevation, and other maps of the study area,

have been created using the DEM.

2.2 Geomorphology

The study area has a highly uneven topography. The topography of the landslide area is

considerably steep and highly elevated. There are distinct geomorphic formations resulting

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from differences in the lithology of the study area, where faulting processes have formed

the main and predominant topography. The study area has a dendritic drainage pattern.

This area is characterized by several orders of terraces and remnants of ancient erosional

surfaces formed by the first phases of river down-cutting. Another type of area, very

different from that of the terrigenous materials, is characterized by the carbonatic domains

in the southern part of the area shown in Fig. 2. There, high local relief, steep and sub-

vertical slopes, and a structurally controlled hydrographic network are the most typical

geomorphic features. Many landslides and erosional processes have modified the escarp-

ments. Some of the geomorphological features associated with gravitational movements

include large irregular slides of earthen material and relatively deep-seated gravitational

deformation steps.

The well known faults of the study area, named the Yaka Fault and Balcı Fault, run

approximately in an E–W direction at the north–south of the study area. Other geomorphic

features are associated with fluvial and slope processes. Weathered and loose unconsoli-

dated materials have been eroded, transported, and deposited in low elevation areas by

such processes as heavy rain, sheet wash, and gullying. The loose materials transported

through the valleys have been deposited as alluvial and colluvial fans. Eglence Creek has

formed deep gullies that start from the elevated Alaardıc and Kecikırı Ridges. The Eglence

Valley, with an elevation of about 1,300 m, is a constructed V-shaped valley with a high

run-off and steep gradient, indicating the youthful geomorphological nature of the study

area. In the study area, the general flow direction of Eglence Creek is to northwest, and it

discharges to the Gelendost Plain. There are also many lower order streams which flow

only after rainy periods and generally in a northwest direction. The general physiographic

Fig. 1 Location map of the study area

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trend of the area is NE–SW, as can be seen in Fig. 2. The elevation values vary between

948 and 1,869 m above sea level, while dominant altitude ranges from 1,300 to 1,500 m

(Fig. 2a). A darker color on the map indicates a higher elevation. The slope steepness,

aspect, and flow direction maps of the study area are shown in Fig. 2b, c and d. The slopes’

aspect were divided into 8 sections depending on the direction of the faces of the slopes as

either N, NE, E, SE, S, SW, W, or NW. Therefore, including the flat slope category, nine

categories of slope aspects are assumed. In the landslide area, the slope, aspect, and

elevation vary from about 20 to 40�, 247 to 337�and 1300 to 1400 m, respectively (Fig. 2).

2.3 Geology

In the study area, there is an outcropping of sedimentary formations (Fig. 3). In strati-

graphic succession, the Anamas Formation (Soyarslan 2004) that can be observed in the

central and southern section of the study area is Cretecaous–Triassic aged and the oldest in

the study area. This formation forms the base rock and consists of karstic limestone shaped

into high steep slopes. The Goksogut formation (Demirkol 1986) is generally composed of

white-gray marl, mudstone and gray-white, clayey, and micritic limestone. Lithologies of

the Goksogut formation are characterized by heterogeneity with frequent vertical and

lateral alternations of different lithological sequences (Yagmurlu 1991). This formation

Fig. 2 Elevation, slope, aspect, and flow direction maps of the study area derived from the digital elevationmodel (DEM)

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can be observed in the eastern, central-southern parts of the research site. The age of the

Goksogut formation is Miocene–Pliocene (Ozturk M et al. 1981). The Goksogut formation

lies unconformably over the Anamas formation. Bedding of clayey limestone and marl has

a dip of 20–30� and a strike appears to the northeast. The thickness of the Goksogut

formation varies from 100 to 130 m (Soyarslan 2004). Unconsolidated recent sediments,

such as alluvium and talus deposits, unconformably cover the oldest rock units. These units

can be observed in the northern part of the study site. More recent sedimentary units of

Holocene aged are represented by alluvium of the streams, river valleys, and talus deposits.

Alluvial deposits are restricted to low-lying areas close to creek courses and deeply incised

gullies. They consist of cobbles, gravels, sands, silts, and clays. Talus deposits are com-

posed of block, gravel, sand, silt, and clay-sized particles. The talus material often contains

big blocks of limestone and clayey limestone. The thicknesses of the alluvium and talus

deposits vary in range from 0.5–20 m, and 5–40 m, respectively.

Fig. 3 Geological map of thestudy area and its vicinity;modified from Soyarslan (2004)and Topcam et al. (1977)

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2.4 Seismotectonics of Yaka and its vicinity

There are many major active faults in the region. The general directions of these faults are

northeast to southwest and east to west (Fig. 4). The east–west trend fault, named the BalcıFault, is located in the southern part of the study area. This fault can primarily be observed at

the junction of the Anamas and Goksogut formations. The other fault, named the Yaka fault,

is located in the northern part of the study area. The Yaka fault can also be observed at the

northern boundary of the Anamas Formation. The region where the study area is situated is

in the West Anatolian Earthquake Zone (WAEZ). The seismicity of Yaka and its vicinity is

mainly controlled by many faults which are in the West Anatolian Fault Zone (WAFZ).

The Yaka Landslide is 2 km distant from the active Yaka fault. The distribution of the

epicenters of major earthquakes with magnitudes [4 that have occurred in the last

107 years is shown in Fig. 4 (KOERI-Kandilli Observatory and Earthquake Research

Institute). About 223 seismic activities have occurred in the study area and at its near

vicinity in the period 1900—the last 107 years. All these seismic events have been shallow

earthquakes. Figure 4 indicates that the main sources of earthquake for the study area are

the Yaka, Gelendost, Beysehir, and Anamas Faults, although the other faults at the north,

southeast, and southwest of the study area also constitute potential sources of earthquake.

Kocyigit (1983), Kocyigit et al. (2000) and Boray et al. (1985) give a detailed account of

the tectonics of the study area and its vicinity. The study area is located in a first-degree

earthquake zone according to the Turkish Earthquake Zoning Map, which was prepared by

the Earthquake Research Department of the Turkish General Directorate of Disaster

Affairs (ERD 2003). Peak Horizontal Ground Accelerations (PHGAs) for the study area

are 0.353 g for 100, 0.43 g for 225, 0.507 g for 475, and 0.586 g per 1000 year return

Fig. 4 Seismotectonics map ofthe Yaka and its vicinity(earthquake epicenters withmagnitude [4.0 in the past100 years and major fault zonesaround Yaka; modified fromKocyigit (1984)

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period (Gulkan et al. 1993). There was no occurrence of an earthquake in the very close

vicinity of the study area before or during the main landslide event.

2.5 Hydrometeorological conditions

In the study area and its vicinity a Mediterranean climate prevails . Generally, in the

summer season, the weather is hot and dry, while the weather in the winter season is

slightly cold and rainy (Topcam et al. 1977). Presence of lakes, in the region, affects the

weather conditions. Rainfall data from the Egirdir, Gelendost, and Yalvac Meteorological

Stations located in the vicinity of the study area was assessed. The Gelendost and Yalvac

Meteorological Stations are those that are in closest vicinity to the study area. Because the

Gelendost Meteorological Station was closed in 1987, no further data can be found after

that year. The use of the data from the Yalvac Meteorological Station, located in closer

proximity to the study area, was preferred for use in the article because its altitude is closer

to that of the average altitude of the study area. To determine the influence of rainfall in the

occurrence of a landslide, precipitation data for the 9-year period from 1962 to 1971 from

the Gelendost Meteorological Station and for a 40-year period from 1962 to 2002 from the

Yalvac Meteorological Station were considered. The average recorded annual precipita-

tions by the Gelendost and Yalvac Meteorology Stations were 577 mm and 509 mm,

respectively (Fig. 5). The long-term average of the Gelendost Meteorology Station’s

monthly average rainfall of the study area, varied from 6 to 102 mm. Based on the

histograms of annual rainfall of the Gelendost Meteorology Station, which were charted

from starting in September and running through August, the highest rainfall occurred in

December and there was a decreasing trend in rainfall from December to March (Fig. 6).

The landslide area is frequently covered by snow in the winter season. The average

thickness of the snow cover and monthly rainfall in the landslide source area before the

Yaka Landslide event were approximately 65 cm and 75 mm, respectively. In the summer

season, there is very little rainfall in the area where this study was conducted. Also tied to

this, the ground water content falls to its lowest level in this season. According to the data

reported by the Yalvac Meteorological Station, 130 mm of precipitation was observed in

October of 2006. As this precipitation occurred during a period when the weather was hot,

Fig. 5 Precipitation records for the study area and its near vicinity from Gelendost and YalvacMeteorological Stations

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it fell in the form of rain. Owing to reasons like a low ground water content, precipitation

falling in the form of rain, and a considerable portion of the rainfall immediately running

off in the medium of surface or shallow run off, this excessive precipitation, observed in

October of 2006, did not cause a landslide. However, the increase of the ground water

content by the precipitation of October and its following months, the occurrence of the

precipitation in a period of cold weather causing it to fall as snow and the accumulation of

the snow and its subsequent melting were all factors related to the occurrence of the

landslide. Using data from the Yalvac meteorological station, the cumulative monthly

rainfall relating to dry and wet years and between the years 1960 to 2002 and 2006 to 2007

Fig. 6 Monthly precipitation histograms based on the records from Gelendost and Yalvac Meteorologicalstations

Fig. 7 Monthly cumulative rainfall plots for the periods of 1960–2002 and 2006–2007 based on the recordsfrom Yalvac Meteorological Station

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was charted in graphic form and has been presented in Fig. 7. As can be observed from

Fig. 7, in the year 2007, the year in which the Yaka Landslide occurred, the cumulative

February rainfall was 337 mm, whereas the average cumulative February rainfall for the

years 1960 to 2002 was 294 mm. The cumulative precipitation of February 2007, when the

landslide occurred, was 15% more than the long-term average and approached 90% of the

amount in 1998, the year in which maximum precipitation was observed. Adding other

contributing factors such as snowmelt to these conditions, the landslide occurred.

The period between February and April generally coincides with the snowmelt period.

The Yaka Landslide occurred during this time. It can be concluded that the study area

possesses the landslide triggers of snowmelt and prolonged and/or heavy precipitation. It

can also be concluded that snowmelt and prolonged and/or heavy rainfall seems to be more

important factors than others, such as earthquake, in the landslide area.

Some hydrological and hydraulic values relating to the investigation site and Eglence

Creek, which were obtained or calculated from the Yalvac Meteorology Station data, is

presented in Table 1. The canal which passes through Yaka and projected by the engineers

of the State Hydraulic Works of Turkey in 1991, has a width of 7 m, a height of 1.45 m, a

slope of 0.01, and roughness coefficient of 0.018 s/m1/3. The values presented in Table 1

were employed in conjunction with the Rational Method by the engineers of the State

Hydraulic Works of Turkey to calculate a 500-year return period (Q500) value of 56.29 m3/

s. It has also been proposed that the peak flow velocity of debris flows may be estimated

using the Manning–Strickler Equation (Rickenmann 1999) with Manning coefficient (n)

&0.1 s/m1/3 (Pierson 1986; Rickenmann and Zimmermann 1993). It was determined that

the debris peak discharge to clear water peak discharge ratio is 0.18 by application of the

Manning–Strickler Equation using an average n = 0.1 s/m1/3 for debris flows and a mean

value n = 0.018 s/m1/3 for clear water flows. A canal designed for clear water will only

provide for 18% of the projected volume to be discharged in the case of debris flow.

Details of debris flows and/or hydrologic models of catchment are not included in the

scope of this study.

3 Geotechnical properties of the landslide material

Landslide material properties were determined on samples (D1, D2, D3, and UD1) taken

from the landslide area (Fig. 8). Laboratory tests in accordance with the American

Table 1 Some hydrologic and hydraulic values of Eglence Creek

Return period—year 5 10 25 50 100 500

Catchment area of Eglence creek (A)—km2 12.41

Length of the main canal (L)—m 7.95

Length of centered of drainage areafrom the outlet along the river (Lc)—km

4.35

Difference of altitude for maincanal (H)—m

1970–988 = 982

Time of concentration of water (tc)—min 67

Runoff coefficient (Ct) 0.27 0.3 0.35 0.4 0.44

Average rainfall intensity (It)—mm/h 17.1 19.8 23.8 25.1 27.5

Peak discharge runoff (Qt)—m3/s 15.94 20.48 27.71 34.61 41.71 56.29

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Standards Testing of Materials Standards (Bowles 1992) were employed. An undisturbed

block sample from the stable units in highly weathered soil-like marl and three disturbed

samples from the toe of the accumulation zone of the Yaka Landslide material were

extracted. Undisturbed soil samples using specimen cutters were also collected from a

trench excavated near the scarp, very close to the shallowest part of the sliding face. These

samples, which were derived from the zone near the sliding face, were used for laboratory

shear strength determinations.

Grain size analysis and Atterberg Limits were conducted on all samples. Unit weights

were determined and direct shear tests were carried out both on dry, undisturbed samples

and on saturated, undisturbed samples. Conventional coarse sieve, fine sieve, and

hydrometer methods were utilized in grain size analysis. The range of grain size of the

Fig. 8 (a) Generalized map of Yaka Landslide, (b) longitudinal profile of the Alaardıc slope and YakaLandslide, and (c) old and new topographic profiles of Alaardıc slope

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samples is given in Fig. 9. Samples of the landslide material (D1, D2, and D3) are com-

posed of 0–32% gravel, 13–24% sand, 25–51% silt, and about 25–44% clay-sized material

(Fig. 9; Table 2). Grain size analyses of the samples indicate predominantly low plasticity

silt and clay. The liquid limit (LL) and plastic limit (PL) of the landslide materials range,

respectively, between 33–46% and 17–30%. According to the Unified Soil Classification

System, these soils are classified as low plasticity inorganic silt (ML) and low plasticity

inorganic clay (CL). The debris soil exhibits low to medium swelling potentials according

to the Williams and Donaldson’s (1980) swelling potential chart modified from Van Der

Merwe (1964). The bulk unit weight of natural undisturbed samples ranges between 19.13

and 19.799 kN/m3 and saturated samples range between 19.5 and 20.45 kN/m3, respec-

tively with a mean value of 20.05 kN/m3. All of the tests were performed on samples

according to ASTM standards (Bowles 1992). Direct shear strength tests were performed

on undisturbed samples (UD) extracted from the block of weathered soil-like marl

according to ASTM standards (Bowles 1992) following an unconsolidated, undrained test

method. Tests were run on 60 mm2 samples. Both peak and residual strength values were

Fig. 9 Grain size curves of samples and landslide materials

Table 2 Some properties and index values of samples

Soil properties/sample type and no. D1 D2 D3 UD4

Moisture content (%) 6 5 2 17

Unit weight (kN/m3) 19.79 19.02 19.13 19.46

Specific gravity (Gs) 2.61 2.62 2.59 2.6

Liquid limit (%) 46 46 33 33

Plastic limit (%) 27 28 30 17

Plasticity index (%) 19 18 3 16

Clay (%) 34 44 25 37

Silt (%) 51 33 30 25

Sand (%) 15 14 13 24

Gravel (%) 0 9 32 14

Class of soil ML CL ML CL

Swelling potential Medium Low Low Low

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determined (Figs. 10, 11). The summary of the laboratory test results and the determination

of some soil values are given in Tables 2 and 3.

4 A description and the mechanism of the Yaka Landslide and debris flows

The first, but small, landslide on the Alaardıc Slope occurred on March 2006. There is no

further information about this small landslide other than it occurred in the lower part of the

Alaardıc Slope. On this same slope, the most recent and significant Yaka Landslide event,

occurred on 19 February 2007 at 13:15 local time (EET) following a period of heavy

rainfall and resultant rapid snowmelt. As mentioned before, the Yaka Landslide initiated

within highly weathered soil-like marl already in a sliding mode, transformed to a debris

flow and then moved through Eglence Creek to Yaka (Fig. 8). A large debris flow of the

unstable materials in the valley passed down the gorge and into the channel of Eglence

Creek, forming a dam. The dam is approximately 4 m height and forms a debris dam in the

creek. The occurring debris flow from Yaka Landslide, traveled down the Eglence Creek a

Fig. 10 (a) Shear stress versus shear displacement and (b) failure envelope for natural soil samples plots

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distance of about 750 m toward the town of Yaka (in Fig. 8; D point). A site view and

longitudinal profile of the Yaka Landslide showing flow path and debris accumulation in

Eglence Creek is presented in Fig. 12. The landslide area and potential unstable mass

locations, as determined by field surveys, such as A—unstable mass zone, B—unstable

mass zone, and C—unstable mass zone are shown in Figs. 8a and 12. Destabilized, but not

completely failed, a huge potentially unstable mass remains in the source area. The surface

area of unstable masses A, B, and C are approximately 61,797, 136,000, and 154,398 m2,

Fig. 11 (a) Shear stress versusshear displacement and (b)failure envelope for saturated soilsamples plots

Table 3 Some geomechanics values of samples

Condition ofsample

Peak orresidual

Cohesion(kN/m2)

Internal frictionangle (�)

Unit weight(kN/m3)

Natural water content Peak 65 41 19.5

Residual 45 31 19.5

Saturated Peak 45 22 20.05

Residual 40 18 20.05

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respectively. If a 3 m depth of the failure surfaces is accepted, volume of the unstable

masses can be calculated as 185,391, 408,000, and 461,194 m3, respectively for A, B, and

C. Additionally, previously mobilized earthen flow material still remains in Eglence Creek.

These masses could easily be triggered by any factor, such as prolonged or heavy rain,

snowmelt or an earthquake. The town of Yaka is located 850 m downstream the Eglence

Creek debris dam and hence poses a considerable risk of being flooded should the creek be

temporarily dammed as a result of further mass movement. Debris flow from landslide is

still a major hazard to Yaka.

Fig. 12 (a and b) Views of Yaka Landslide and its vicinity from downstream, (c) view of Yaka Landslidefrom upstream

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The thickness of the displaced material and the position of failure surfaces were also

determined based on the field surveys. The geometry of the Yaka Landslide is described

below, following the WP/WLI suggested nomenclature for landslides (IAEG Commission

on landslides 1990):

Total length L = 220 m

Length of the displaced mass Ld = 220 m

Length of the rupture surface Lr = 230 m

Width of the displaced mass Wd = 130 m

Width of the rupture mass Wr = 140 m

Depth of the displaced mass Dd = 3 m

Depth of the rupture surface Dr = 3 m

Total height (the height from the crown to the tip of toe) DH = 55 m

The thickness of the displaced mass was determined to be 12 m in the central region of

the pre-failure slope topography, but it was found to thin considerably at the down-slope

and up-slope edges of the landslide area. The mean depth of the failure surface is

approximately 3 m and the surface area of the landslide is 28,600 m2. The volume of the

destabilize landslide material may then be calculated at approximately 85,800 m3. This

debris material is deposited on a high gradient at the base of the Eglence Valley. The

thickness of the deposit varies between 3 and 5 m at the base of the valley. During the

debris flow, this thickness reached up to 10 m in the Eglence Valley (Fig. 13).

It was also determined that the debris dam in Eglence Creek has not yet been stabilized,

while the water stored behind the landslide dam (Fig. 13g, f) has infiltrated the deposited

material. Therefore it should be noted that the debris dam still poses a significant threat to

those downstream. Such landslide dams have appeared stable but later have failed bringing

catastrophic results following significant influxes of water to the impoundment. The sea-

sonal washes have a noteworthy potential for bringing significant runoff and in the form of

large influxes of water behind this debris dam. This condition could result in dam failure.

The plasticity of the soil and the angle of internal friction are conducive to failure when

saturated. Research also found that the failure is more likely to occur on A, B, and C

unstable slopes (Figs. 8a, 12b, c). A—not-completely-failed, huge, and potentially unstable

mass (in Fig. 8, A—unstable mass and B—unstable mass) still remains in the source area.

Primarily the A-unstable and secondarily the B-unstable masses could easily be triggered

by any factor, such as prolonged or heavy rainfall, snowmelt, or earthquake. Debris flow is

still a major and serious potential hazard to the Yaka settlement area.

5 Stability analyses on the Alaardıc slope

To evaluate the stability of a slope utilizing limit equilibrium analyses methods, it is

necessary to perform calculations of a considerable number of possible slip surfaces to

determine the location of the most critical slip surface, and the corresponding minimum

value of factor of safety (FS). A limit equilibrium analysis of both the original and the

failed slopes was performed using the Bishop’s Simplified Method (Bishop 1955) and the

Janbu Method (Janbu 1968) with peak and residual undrained unconsolidated shear

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strength parameters under natural moisture content and completely saturated conditions

(Table 3). The cross section showed in Fig. 8b and c presents profiles of the slope both

before and after the failure. The location of this cross section is shown in Fig. 8a as line

EF. Figure 14 illustrates the limit-equilibrium analyses for the first and second failed

slopes, respectively. Sliding and retrogressive development had took place in the Yaka

Landslide area (Fig. 12b). The first failure initiated on the S1–S2–S3 section of slope in

March 2006 (Figs. 12b, 14a), and then the main failure occurred on the S4–S5–S6 section

of slope on 19 February 2007 (Figs. 12b, 14b). In the near future, failures are likely to

occur on the S7–S8–S9 and/or on the S10–S11–S12 surfaces (Figs. 12b, 14a, 15b).

Fig. 13 Some views of debris deposits (a, b, c) and/or deposited Eglence Creek (c) and grooves in valleydeposits (c, d, e) and formed debris-dam lake (f, g)

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The limit equilibrium analyses were conducted utilizing software named SLOPE/W

(Geo-Slope 2000). Slope stability analyses were performed by considering the laboratory

test results. A circular slip surface was assumed in the analyses. The analyses are presented

in Table 4 noting parameters and calculated safety factors.

A number of different computer runs were conducted to calculate the factor of safety,

both under natural moisture content and saturated conditions to determine peak and

residual shear strength values. In the analyses on the S1–S2–S3 surface, the safety factors

using Bishop’s Method were found as 2.21 and 1.53 for peak and residual shear strength

Fig. 14 Limit equilibrium analyses of the Alaardıc Slope: (a) initial failure in 2006 and (b) main failure(Yaka Landside) on February 19, 2007

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Fig. 15 Limit equilibrium analyses of Alaradıc Slope after Yaka Landslide (possible future failuresurfaces)

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parameters of natural moisture content conditions, respectively. On other hand, the factors

of safety were determined to be 1.18 and 0.99, respectively, for peak and residual shear

strength parameters under completely saturated conditions. The same values for the S4–

S5–S6 surface were found to be 2.64, 1.83, 1.33, and 1.09 for natural water content-peak,

natural water content-residual, saturated-peak, and saturated-residual shear strength values,

respectively. Under natural moisture conditions, the safety factors for the residual shear

strength parameters were calculated at 1.65 and 1.59. At the same time, under saturated

conditions, the slopes had safety factors of only 1.00 and 0.96 for the S7–S8–S9 and the

S10–S11–S12 surfaces (Fig. 15a, b), respectively. This supports the contention that in its

natural moisture content state, the slope is stable; however, the slope becomes unstable in

saturated conditions. The first failure occurred on the S1–S2–S3 surface of the AlaaardıcSlope (Fig. 14). Following this first failure, one year later, the second failure occurred on

the S4–S5–S6 surface of the slope. In the future and upon the next occurrence, retro-

gressive failure could take place on the S7–S8–S9 and the S10–S11–S12 surfaces of

Alaardıc slope (Fig. 16). The factors of safety values have been determined to be 1.0 and

0.96 (employing Bishop’s Method for saturated residual shear strength parameters) for the

S7–S8–S9 and the S10–S11–S12 surfaces (Fig. 16), respectively.

6 Discussion and conclusions

On February 19, 2007, the large Yaka Landslide was triggered by heavy precipitations and

snowmelt in the soil-like marl and in low plasticity inorganic clay units in the paleo-

landslide area. Site investigation and stability analyses indicate that the sliding surface was

rotational. Following the rotational failure, owing to the steep topography and high water

saturation, the landslide material moved into the V-shaped Eglence Creek, flowing in it

Table 4 Used soil parameters in the analyses and calculated factor of safety values

Profile Watercondition

Peak orresidual

Cohesion(kN/m2)

Internal frictionangle (�)

Unit weight(kN/m3)

Factor of safety

FBishop FJanbu

S1–S2–S3 Natural Peak 65 41 19.5 2.21 2.12

Residual 45 31 19.5 1.53 1.46

Saturated Peak 45 22 20.05 1.18 1.14

Residual 40 18 20.05 0.99 0.96

S4–S5–S6 Natural Peak 65 41 19.5 2.64 2.54

Residual 45 31 19.5 1.83 1.76

Saturated Peak 45 22 20.05 1.33 1.28

Residual 40 18 20.05 1.09 1.06

S7–S8–S9 Natural Peak 65 41 19.5 2.38 2.33

Residual 45 31 19.5 1.65 1.61

Saturated Peak 45 22 20.05 1.22 1.19

Residual 40 18 20.05 1.00 0.99

S10–S11–S12 Natural Peak 65 41 19.5 2.28 2.23

Residual 45 31 19.5 1.59 1.54

Saturated Peak 45 22 20.05 1.18 1.15

Residual 40 18 20.05 0.96 0.96

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until coming to a halt at a considerably lower elevation. Possible causes of this landslide,

its mechanism, debris flows to Eglence Creek, and the immobilized unstable mass still

remaining at the source area as a possible future threat have all been discussed in this

study.

During the field surveys, it was observed that landslides are common in this area

dominated by marl, mudstone, and clayey limestone. The lithologies of the Goksogut

formation, such as marl, mudstone, and clayey limestone, are highly susceptible to

weathering. It is a known fact that the type of rock and its morphology govern the stability

of a slope. Discontinuities, like bedding planes, faults, and fracture reduce the strength of

rock masses. In the case of the lithologies of this study area, the presence of the weak

surfaces in marl, mudstone, and clayey limestone weaken the rocks, thereby creating slope

instability. Areas of marl, a great amount of weathering and the response of the clay-

enriched soils seem to be responsible for making the area’s slopes unstable. Topographic

parameters like slope and slope aspect play a crucial role in governing the stability of the

terrain in the study area. As the slope increases, the chance of failure also increases.

Important factors to slope instability are poised by steep slopes composed of these soil-like

marls.

In general, the investigation site and particularly the landslide area including its near

vicinity presents a typically paleolandslide morphology. The Yaka Landslide began as a

shallow circular landslide on the western and northern part of the Alaardıc Slope after a

period of heavy rainfall and snowmelt which weakened the material through increasing

saturation and subsequent weight. The sliding mode then transformed to a debris flow and

then the displaced material moved down Eglence Creek toward the town of Yaka (Figs. 12

and 13). While many landslides have already occurred in the study area, there is also a

great potential of future slides and subsequent debris flow taking place during rains and

snowmelts. The Yaka Landslide was triggered only by heavy rains and snowmelt. There

was no nearby earthquake at or before the time of the Yaka Landslide. The landslides that

have occurred in the study area have been retrogressive, circular failures. The resultant

landslide material flowed to the base of the Eglence Valley and was deposited there. These

deposited material formed a landslide dam. As a result, a debris-dam lake has been formed

Fig. 16 Retrogressive failure model of Alaardıc slope (failure occurred on the S1–S2–S3 and S4–S5–S6surfaces, possible failure that will occur at S7–S8–S9 and S10–S11–S12 surfaces)

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behind this dam. The distance from the toe of the failure surface to the toe of the debris

flow was about 750 m. Volume of displaced earthen material is about 85,800 m3.

Eglence Creek passes through Yaka. The width at the base of this creek’s valley varies

between 10 and 35 m near the Alaardıc Slope. Flow obstructing weirs of 2 m in height had

been constructed in Eglence Creek in order to prevent the flow which might occur during a

heavy rainfall and/or snowmelt in the area. All of these weirs have been since destroyed by

past floods. In addition, the downstream segment of Eglence Creek where it flows through

Yaka has been channelled into a canal to prevent flooding by Eglence Creek floods which

might occur during a prolonged or heavy rainfall or period of snowmelt. The width and

height of this canal are 7 and 1.45 m, respectively.

The distance from the source area of the Yaka Landslide to the settlement area of Yaka

is about 1,600 m. The distance between Yaka and the tip of the toe of the debris in Eglence

Creek is 850 m. Proceeding the flow of earthen material, the depth of Eglence Creek’s

thalweg was 6–10 m and its base width was approximately 20 m at that point. Presently,

the earthen material accumulated in Eglence Creek has a width of 10–35 m and a mean

depth of 3–4 m, respectively. The subsequent accumulation of water behind the debris dam

has infiltrated the deposited landslide material. In addition, it has been determined that the

width, depth, and persistency of tension cracks are up to 150 cm, 200 cm, and 20 m,

respectively, on the back of the scarp at the top of the slope. The opening of new fractures

at the top of slope has enabled the infiltration of rain waters into the slide mass which has

resulted in the softening of the soil-like marl, subsequently decreasing its shear strength.

Furthermore, the investigation of slides which have occurred, the morphology of the rock

and the applied stability analyses of the Alaardıc Slope have shown the area’s failure mode

to be retrogressive. All of these data point to future retrogressive failures on the AlaardıcSlope. The stability of the Alaardıc Slope is dependent on saturation conditions of the mass

and the steepness of the slope. Under completely unsaturated conditions, the majority of

the area is found to be stable, while under completely saturated conditions, the majority of

the area is found to be unstable.

The accumulation of debris and sediments could lead to a debris flow after a significant

amount of precipitation and/or snowmelt. In this study, the field surveys and the stability

analyses that were performed pointed out that if any failure should occur, the displaced

material volume, which could be set into movement, would be more than 85,800 m3. In

addition, unstable material that has been deposited in Eglence Creek may be moved by

affects of prolonged or heavy rainfall, snowmelt or further displaced landslide material. It

has been established that the thickness of the mudflow in Eglence Creek reaches a depth of

8 m and a width of 20 m. If deposited material in Eglence Creek is set to motion, the canal

that passes through the town of Yaka, with its width of 7 m and depth of 1.45 m, could not

likely discharge the flood. The destruction or spillover of this canal in Yaka could bring

catastrophic damage to the residents located at 3–5 m from the bank of the canal. Fur-

thermore, if material present in the landslide source area slides and this displaced material

puts pressure on the unstable deposited material in Eglence Creek even more catastrophic

damage are likely to occur to Yaka. Yaka is situated in a first-degree earthquake zone, thus

additional precautions may also be needed in order to prevent further damage that might be

caused by a possible earthquake.

Significant damage to the masonry retaining walls and trees in Eglence Creek has

resulted from the flows. Turkish governmental research institutions, and various other

research institutions world-wide have attempted for years to assess landslides, debris flow

hazards and risks, and to show their spatial distribution (Yumuang 2006). Studies point out

that landslides and debris flow is among the most hazardous of natural disasters. Debris

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flows pose a hazard, different than other types of landslides and flow due to their rapid

movement and destructive power. In addition to the threatening of lives, debris flow can

damage buildings, and, infrastructure by means of sediment burial, erosion, direct impact,

and associated flooding (Yumuang 2006).

In this study, it was determined that debris flow represents a serious threat to Yaka and

there is a urgent need to undertake immediate remedial action. Furthermore, it is estab-

lished that the destructive significance of landslides and debris dams, whose triggers may

derive from seismic or meteorological factors, may increase upon the occasion of more

rapid phases of movement. Natural slope instability in the terrain of the Alaardıc Slope,

where failures might cause catastrophic distribution on the surrounding area, is a major

concern.

Mainly this study was primarily conducted for the purposes of the determination of

affecting factors of this potential landslide and debris flow, the subsequent effects upon

Yaka of such a landslide’s unavoidable debris flow and flood and to warn administrators,

planners, and engineers with some necessary information to aid in the matter of prevention

of loss to life and property.

References

Anderson SA, Sitar N (1995) Analysis of rainfall-induced debris flows. J Geotech Eng 121:544–552. doi:10.1061/(ASCE)0733-9410(1995)121:7(544)

Benac C, Arbanas I, Jurak V, Ostric M, Ozanic N (2005) Complex landslide in the Rjecina valley (Croatia):origin and sliding mechanism. Bull Eng Environ 64:361–371. doi:10.1007/s10064-005-0002-5

Bishop AW (1955) The use of the slip circle in the stability analysis of slopes. Geotechnique 5:7–17Boray A, Saroglu F, Emre O (1985) Isparta buklumunun kuzey kesiminde Dogu-Batı daralma icin bazı

veriler. Jeoloji Muhendisligi 23:9–20 (in Turkish)Bowles JE (1992) Engineering properties of soils and their measurement. McGraw-Hill, Inc., New York,

pp 241Chowdhury R, Flentje P (2002) Uncertainties in rainfall-induced landslide hazard. Q J Eng Geol Hydrogeol

35:61–69Chung CF, Fabbri AG, van Westen CJ (1995) Multivariate regression analysis for landslide hazard zonation.

In: Carrara A, Guzetti F (eds) Geographical ınformation systems in assessing natural hazards. KluwerAcademic Publishers, The Netherlands, pp 107–133

Delmonaco G, Leoni G, Margottini C, Puglisi C, Spizzichino D (2003) Large scale debris-flow hazardassessment: a geotechnical approach and GIS modeling. Nat Hazards Earth Syst Sci 3:443–455

Demirkol C (1986) Sultandag ve dolayının tektonigi. MTA Derg 107:111–118 (in Turkish)Duman TY, Can T, Gokceoglu C, Nefeslioglu HA (2005) Landslide susceptibility mapping of Cekmece area

(Istanbul, Turkey) by conditional probability. Hydrol Earth Syst Sci Discuss 2:155–208.http://www.hydrol-earth-syst-sci-discuss.net/2/155/2005/hessd-2-155-2005.html

Dunning SA, Rosser NJ, Massey CR (2006) Formation and failure of the Tsatichhu landslide dam, Bhutan.Landslides 3:107–113. doi:10.1007/s10346-005-0032-x

ERD (Earthquake Research Department) (2003) http://www.deprem.gov.tr/depyon/DBYBHY-2007.pdfFiorillo F, Wilson RC (2004) Rainfall induced debris flows in pyroclastic deposits, Campania (southern

Italy). Eng Geol 75:263–289. doi:10.1016/j.enggeo.2004.06.014Geo-Slope (2000) Geo-Slope office. Geo-slope International Ltd., Calgary, CanadaGlade T (1998) Establishing the frequency and magnitude of landslide triggering rainstorm events in New

Zealand. Environ Geol 35:160–174. doi:10.1007/s002540050302Gulkan P, Yucemen S, Kocyigit A, Doyuran V, Basgoz N (1993) En son verilere gore hazırlanan Turkiye

deprem bolgeleri haritası. ODTU Deprem Muhendisligi Arastırma Merkezi, Rapor No. 93-01(in Turkish)

IAEG Commission on Landslides (1990) Suggested nomenclature for landslides. Bull Eng Geol Environ41:13–16. doi:10.1007/BF02590202

Ildir B (1995) Turkiyede heyelanların dagılımı ve afetler yasası ile ilgili uygulamalar. In: Onalp A (ed)Proceedings of 2nd national landslide symposium. Sakarya University, Turkey, pp 1–9 (in Turkish)

Nat Hazards (2009) 49:113–136 135

123

Janbu N (1968) Slope stability computations. Institutt for Geotknikk og Fundamenteringslære, NorgesTekniske Høgskole, Soils Mechanics and Foundation Engineering, the Technical University ofNorway, Trondheim, Norway

KOERI (Bogazici University Kandilli Observatory Earthquake Research Institute). Earthquake database,http://www.koeri.boun.edu.tr

Kocyigit A (1983) Hoyran Golu (Isparta Buklumu) dolayının tektonigi. Geol Soc Turk Bull 26:1–10(in Turkish)

Kocyigit A (1984) Intra-plate neotectonic development in southwestern Turkey and adjacent areas. Geol SocTurk Bull 27:1–16

Kocyigit A, Unay E, Sarac G (2000) Episodic graben formation and extensional neotectonic regime in westCentral Anatolia and Isparta Angle: a case study in the Aksehir-Afyon Graben, Turkey. Tectonics andmagmatism in Turkey and the surrounding area, vol 173. Geological Society, London, Special Pub-lications, pp 405–421

Mikos M, Cetina M, Brilly M (2004) Hydrologic conditions responsible for triggering the Stoze landslide,Slovenia. Eng Geol 73:193–213. doi:10.1016/j.enggeo.2004.01.011

Mikos M, Brilly M, Fazarnic R, Ribicic M (2006) Strug landslide in W Slovenia: a complex multi-processphenomenon. Eng Geol 83:22–35. doi:10.1016/j.enggeo.2005.06.037

Nurlu M, Ozsarac V, Ozmen BA (1997) Case of study using remote sensing and GIS techniques after Dinar(SW Turkey). Earthquake international symposium on geology and environment, September 1–5,_Istanbul

Ocakoglu F, Gokceoglu C, Ercanoglu M (2002) Dynamics of a complex mass movement triggered by heavyrainfall: a case study from NW Turkey. Geomorphology 42:329–341. doi:10.1016/S0169-555X(01)00094-0

Ozturk ME, Ozturk Z, Acar S, Ayaroglu A (1981) Sarkikaagac (Isparta) ve dolayının jeolojisi. MTA, JeolojiDairesi, Rapor No: 7045, pp 190 (in Turkish)

Petrucci O, Polemio M (2003) The use of historical data for the characterisation of multiple damaginghydrogeological events. Nat Hazards Earth Syst 3:17–30

Pierson TC (1986) Flow behavior of channelized debris flows, Mt. St. Helens, Washington. In: Abraham AD(ed) Hillslope processes. Allen & Unwin, Boston, pp 269–296

Rickenmann D (1999) Empirical relationships for debris flows. Nat Hazards 19:47–77. doi:10.1023/A:1008064220727

Rickenmann D, Zimmermann M (1993) The 1987 debris flows in Switzerland: documentation and analysis.Geomorphology 8:175–189. doi:10.1016/0169-555X(93)90036-2

Sivrikaya O, Kilic AM, Yalcin MG, Aykamis AS, Sonmez M (2008) The 2001 Adana landslide and itsdestructive effects, Turkey. Environ Geol 54:1489–1500. doi:10.1007/s00254-007-0930-4

Soyarslan _I_I (2004) Egirdir Golu dogusunun hidrojeoloji incelemesi ve yeraltı suyu modellemesi. SuleymanDemirel Universitesi, Fen Bilimleri Enstitusu doktora tezi, pp 210, Isparta (in Turkish)

Teoman MB, Topal T, Isık NS (2004) Assessment of slope stability in Ankara clay: a case study along E90highway. Environ Geol 45:963–977. doi:10.1007/s00254-003-0954-3

Topcam A, Atıgan A, Alkan N, Ozpınar B, Agack O (1977) Hoyran-Gelendost ve Yalvac Ovaları hid-rojeolojik etud raporu. Devlet Su _Isleri Genel Mudurlugu, Jeoteknik Hizmetler ve Yeraltısuları DairesiBaskanlıgı, DS_I Basım ve Foto-film _Isletme Mudurlugu Matbaası, 56 pp, Ankara (in Turkish)

Ulusay R, Aydan O, Kılıc R (2007) Geotechnical assessment of the 2005 Kuzulu landslide (Turkey). EngGeol 89:112–128. doi:10.1016/j.enggeo.2006.09.020

Van Der Merwe DH (1964) The prediction of heave from the plasticity index and percentage clay fraction ofsoils. Civ Eng S Afr 6(6):103–106

Wang C, Esaki T, Xie M, Qiu C (2006) Landslide and debris-flow hazard analysis and prediction using GISin Minamata-Hougawachi area, Japan. Environ Geol 51:91–102. doi:10.1007/s00254-006-0307-0

Wen BP, Aydin A (2005) Mechanism of a rainfall-induced slide-debris flow: constraints from micro-structure of its slip zone. Eng Geol 78:69–88. doi:10.1016/j.enggeo.2004.10.007

Williams AAB, Donaldson G (1980) Building on expansive soils in South Africa. In: Proceedings of the 4thinternational conference, vol 2, Expansive soils, Denver, pp 234–238

Yagmurlu F (1991) Yalvac-Yarıkkaya Neojen havzasının stratigrafisi ve depolama ortamları. Turkiye JeolojiBulteni 34:9–19 (in Turkish)

Yilmaz I, Yildirim M (2006) Structural and geomorphological aspects of the Kat landslides (Tokat-Turkey)and susceptibility mapping by means of GIS. Environ Geol 50:461–472. doi:10.1007/s00254-005-0107-y

Yumuang S (2006) 2001 debris flow and debris flood in Nam Ko area, Phetchabun province, centralThailand. Environ Geol 51:545–564. doi:10.1007/s00254-006-0351-9

136 Nat Hazards (2009) 49:113–136

123