The largest landslide dam in Turkey: Tortum landslide

Engineering Geology 104 (2009) 66–79

Contents lists available at ScienceDirect

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

The largest landslide dam in Turkey: Tortum landslide Tamer Y. Duman ⁎ General Directorate of Mineral Research and Exploration (MTA), Department of Geological Research, Earth Dynamics Research and Assessment Division 06520, Ankara, Turkey

a r t i c l e

i n f o

Article history: Received 31 January 2008 Received in revised form 12 August 2008 Accepted 13 August 2008 Available online 27 August 2008 Keywords: Rock slide Landslide dam Geomorphometric parameters Grain size distribution Dating

a b s t r a c t The gigantic Tortum landslide blocked the Tortum River and formed the largest landslide-dammed lake in Turkey measuring 8500 m length, 2500 m width and a surfacial area of 6.77 km2. Large and deep-seated landslides are prevalent along the steep slopes in the Tortum Valley. The Tortum landslide, located 90 km to the north of Erzurum, is one of the typical cases in the region. In this study, the characteristics, age, possible causal factors, grain size distribution and environmental impacts of the Tortum landslide, as well as the landslide dam and related lake volume, were investigated. The landslide occurred as a rapid-rock slide in the Cretaceous interbedded limestones with clastics. The surface of the rupture formed along the bedding plane. Pre-failure topography was reconstructed to estimate volumes of the Tortum landslide, dam and related dammed-lake area. Grid by number analysis is used to determine grain size distribution of the landslide dam. On the basis of the topographic reconstructions, the volumes of the displaced mass and the landslide dam were estimated as 223 million m3 and 180 million m3, respectively. The dam reached a maximum height of 270 m and impounded 1820 km2 of mountainous drainage area, forming a lake with 538 million m3 of water on the Tortum River. Geomorphometric parameters of the dam and dammed lake were compared with examined worldwide case studies. Based on the radiocarbon ages, the Tortum landslide occurred in the middle of the 17th century. The landslide-dammed lake resulted in positive environmental impacts in its vicinity, causing a change in the micro-climate of the region, providing opportunity for hydropower generation, and the development of a fishery and tourism. © 2008 Elsevier B.V. All rights reserved.

1. Introduction Landslide dams usually form in mountainous areas of high terrain (Costa and Schuster, 1988), where there are proper conditions for preparation (high hills-slope gradients and discontinuities such as bedding, faults, joints) and triggering (precipitation, snowmelt, earthquakes) factors of slope failure (Korup 2002). Most of the landslide-dammed lakes (about 90% of 390) examined worldwide relate to landslides triggered by either rainstorms/snowmelts or earthquakes (Schuster, 1993). Costa and Schuster (1991) presented a data compilation for 436 historical landslide dams and associated landslide-dammed lakes that have been recorded throughout the world. In addition, some regional works attempting to collect and classify data on the present and historical landslide dams were carried out in different parts of the world (Swanson et al., 1986; Clague and Evans, 1994; Bromhead et al., 1996; Shoaei and Ghayoumian, 1997; Hejun et al., 1998; Casagli and Ermini, 1999; Chai et al., 2000). Geomorphometric parameters of 15 large landslide dams were recently documented by King et al. (1987), Read et al. (1992), Hancox and Perrin (1994), Reneau and Dethier

⁎ MTA Gn. Mud. Jeoloji Etutleri Dai. 06520 Ankara, Turkey. E-mail address: [email protected]. 0013-7952/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.enggeo.2008.08.006

(1996), Hewitt (1998), Weidinger (1998), Hancox et al. (1999), Wayne (1999), Hanisch and Söde (2000), ICIMOD (2000). The Usoi landslide (estimated at 2–4 × 109 m3), which was triggered by the 1911 earthquake and formed Lake Sarez on the Bartang (Murgab) River in the Pamir Mountains of Tajikistan, is presently accepted to be the largest natural dam on Earth (Gasiev, 1984; Weidinger, 1998; Hanisch and Söder, 2000). Although the Usoi landslide formed a dam of some 500–700 m in height (Alford et al., 2000), the Rondu-Mendi'A' landslide blocked the Indus River in Baltistan, Pakistan constituting a dam with a height of 950 m (Hewitt, 1998). Failure of landslide dams usually resulted in catastrophic downstream flooding causing loss of life, housing and infrastructures. The Raikhot landslide dam of some 200–300 m in height which impounded a 65-km-long lake on the Indus River, Pakistan collapsed in 1841 (Mason, 1929). This phenomenon is referred to be the largest damming by landslide and resulting catastrophic flood that has been documented in the world (Shroder, 1998). Landslide dams, which are both complex and composite (Korup, 2002), are significant geomorphic forms due to their temporary existence in front of impoundment lakes. Geomorphic forms and processes related with landslide dam formation, stability and failure have been extensively explored. The grain size distribution of the debris material forming a dam influences the overall strength of a landslide dam due to the erosional processes that can cause to failures by overtopping or piping (Swanson et al., 1986; Costa and Schuster,

T.Y. Duman / Engineering Geology 104 (2009) 66–79 Table 1 Documented landslide dams in Turkey River Place (location)

Age

Dam volume Lake volume Status (Mm3) (Mm3)

Solaklı Sera

Trabzon 1929 10 Trabzon 1950 5

– 1.8

Kıratlı

Trabzon 1987 –

–

Cause

Reference

Breached Rainfall Pamir, 1930 Existing Rainfall Erguvanlı and Tarhan, 1982 Breached Rainfall Tarhan, 1991

1988; Casagli and Ermini, 1999). Casagli et al., (2003) applied different techniques to the analysis of landslide dams more than 60 cases in the Northern Apennines. After the study of Costa and Schuster (1988), some statistical conclusions of landslide dams became a matter of primary importance. Korup (2002) reviewed the literature and focused recent findings on the geomorphic and hydrologic aspects of the formation, failure and geomorphic impacts of landslide dams. Casagli and Ermini (1999), Ermini and Casagli (2002) and Korup (2004) proposed some indices for the prediction potential for landslide dam stability. They applied the indexes to the selected data from the Apennines in Italy, various worldwide sites and available New Zealand landslidedammed lakes, respectively. Weidinger (2004) suggested the Block Size Stability Diagram for classification for natural rock blockages and the life span of their dammed lakes due to work out the stability conditions of 20 landslide dams in the Indian and Nepal Himalayas as well as two in Chine. Temporary or permanent landslide dams gradually receive more attention and awareness with increasing population and land use pressure in steep terrains (Korup, 2002). Landslide dams are fairly rare in Turkey and investigation on this subject is fairly limited. All of the three listed landslide dams in Turkey are in the northeast sector of the Pontide Mountains (Table 1). A landslide, which was probably triggered by exceptional rainfall in 1929 rainstorms, formed a temporary lake on the Solaklı River in the Pontide Mountains, Turkey (Fig. 1). Breaching of this landslide dam caused the loss of 146 lives, 18 bridges and 2539 houses (Pamir, 1930), and this event is accepted as the largest catastrophic flood that is related to a landslide dam failure in Turkey. However, there is still a considerable lack of detailed case studies on the landslide dams and associated lakes throughout Turkey. In order to reveal and understand the process of landsliding in Turkey, ‘The Landslide Inventory Maps of Turkey’ project was realized between 1997 and 2007 by the General Directorate of Mineral Research and Exploration (Duman et al., 2005, 2007). Complying with the purpose of this project, landslides were mapped at 1/25,000 scale maps based on detailed aerial-photo interpretation and field studies. According to the landslide inventory

67

maps of Turkey, there are 16 important sized permanent landslidedammed lakes in Turkey (Fig. 1). These data comprise the landslide dams and associated lakes that could be depicted on the 1/25,000 scale topographic base maps. The most significant landslide-dammed lakes are concentrated at the north-eastern part of Turkey and the largest one in Tortum. The reason of this clustering is both orographic precipitation and high relief of the Pontides. The purpose of this study is to present the characteristics, age and causal factors of the Tortum landslide, to reveal the geomorphometric parameters and grain size distribution of the landslide dam, and to discuss environmental impacts of the landslide-dammed lake. In order to estimate the age of the Tortum landslide, radiocarbon dating was carried out on exhumed wood fragments from the alluvial fans at the landslide toe. The volumes of the landslide mass, the landslide dam and related lake were estimated based on the reconstruction of the original topography of the slide and lake area. 2. General characteristics of the area The study area is located in the eastern sector of the Pontide Range (Yılmaz et al., 1997) which extends along the southern margin of the Black Sea (Fig. 1). The Eastern Pontides constitute the highest peaks along the range. The region exposes a rough morphology with steep slopes and peaks. Main morphological units shaped under the control of the structural elements (main folds, faults) in the region trend to the NE–SW. The Eastern Pontides are drained by the Çoruh River, which is the most significant fluvial system in the region. Tortum River is a main tributary of the Çoruh River. Deep incision forms v-shaped valleys characterised by deep and steep slope in this drainage system. Large and deep-seated landslides are prevalent on these slopes. The relief along these slopes reaches up to 1200 m. The climate of the study area is a transition of the Black Sea and the continental east Anatolian climatic regions with warm summers (average daily July temperature 22.1 °C) and cold winters (average daily January temperature −2.8 °C). The annual average air temperature is 10.2 °C, the precipitation is 338 mm and no snowfall occurs during the year (DMI, 2005). Three formations of Late Jurassic and Cretaceous sedimentary rocks crop out in the study area (Fig. 2). These are Olurdere, Akçadağlar and Karacasu Formations (Konak et al., 2001). Stratigraphically, the Late Jurassic Olurdere formation (Jo) comprises the oldest rocks in the study area and is composed of alternating sandstone, siltstone and marl. The lower part of the Olurdere formation is abundant in sandstones and marls, and is referred to as the Altınçanak member (Joa), whereas the uppermost part, the Kivrinsintepe (Jok) is comprised of interbedded siltstone and sandstone. The Olurdere formation (Jo) is conformably overlain by the Early Cretaceous

Fig. 1. Location map of the study area with distributions of landslide dams (stars in the figure) in Turkey.

68

T.Y. Duman / Engineering Geology 104 (2009) 66–79

Fig. 2. Simplified geological map (modified from Konak et al., 2001) and landslide inventory map of the study area.

Akçadağlar formation (Ca), in which the Tortum landslide occurred, which is represented by interbedded limestones with clastics. The Akçadağlar formation (Ca) is conformably followed by sandstone, siltstone and claystone alternations of the Late Cretaceous Karacasu formation (Ck). 3. Assessment of Tortum landslide Large landslides, formed under similar geological and morphological conditions, are common in the Tortum valley (Fig. 2). They are observed to occur on the steep slopes of the valley that are constituted from bedded of clastics or clastics interbedded in a carbonatic sequence. Deep-seated translational or complex mechanisms are typical for these landslides. The Tortum landslide, located 90 km to the north of Erzurum and 60 km to the south of Artvin, is a typical case in the region (Fig. 3). However, the Tortum landslide is the only slides mass that currently dams the river in the region. Nevertheless, another landslide that failed in the Early Cretaceous Akçadağlar formation, located 8 km downstream of the Tortum landslide, reached the river and deflects it 250 m to the east (Duman et al., 2007). The Tortum landslide occurred in the Akçadağlar formation (Ca) that dip moderately downslope. Detail lithological descriptions of this

sequence are made by Konak et al. (2001) through a 867-m long section. On the basis of this type section, the lower part of this sedimentary sequence is composed of white micritic limestone levels with cherty nodules and pelecypod shells. Micritic limestones with interbedded mudstone and sandstone increase from the bottom upwards in the sequence. Towards the top, the composition of the limestone changes to semi-pelagic micrite, and its bedding changes from thick to thin layers. Sandy limestones are also observed in the upper part of the sequence. These units are disrupted, severely fractured and generally thin to moderately bedded. The cross-valley profile on the north of the Tortum landslide consists of two diverse slope segments; canyon wall and upper slope (Fig. 4). The canyon wall in the resistant limestones constitutes a cliff that is 750 m long and 600 m high at the western side of the valley. The cliff has a steep gradient of about 73–75°. The upper mountain slope of the mountain is 4 km long with a gradient of about 25°. The Tortum landslide occurred as a rock slide and the slide mass accumulated mostly on the valley floor of the Tortum River. The disintegrated mass ascended on the opposite valley side to an elevation of as much as 300 m above the valley bottom. The landslide dam reaches up to 270 m in height impounding a 1820 km2 drainage area. After filling the reservoir, the river overtopped the dam on the

T.Y. Duman / Engineering Geology 104 (2009) 66–79

69

Fig. 3. View of the Tortum landslide. (a) Facing from main scarp to toe, view to the east. Features of Tortum landslide such as major and secondary scarps, flanks, hummocky topography are apparent. The landslide mass accumulated by a majority on the valley floor of the Tortum River. (b) View from the toe to the up, toward west. Note the channelized natural diversion that formed eroding the landslide mass on the terrace in the right abutment, discharging the landslide-dammed lake. (TL) Tortum landslide-dammed lake. Dashed doted: boundary of the landslide deposit, Doted line: scarp area, arrow: direction of the movement of the landslide, dotted line arrow: diverted river.

right hand abutment by a channelized diversion founded in basement rocks. 3.1. Type of movement The Tortum landslide initially occurred as a translational slide (Varnes 1978) or stepped translational slide (Kovari and Fritz, 1984). The failure surface was controlled by bedding that dips moderately out-slope to the SE (Fig. 5). The sliding surface day-lighted close to the lower sector of the canyon. The landslide probably started as a rock slide on the failure surface but soon evolved into a rock avalanche as it accelerated down the steep slope. The slope, where the Tortum landslide formed, is a limb of a syncline (Fig. 5). The axis of the syncline is almost located in the valley floor. While the dip direction of the bedding remains nearly the same, the dip grade decreases from the upper part of the limb to the syncline axis. In addition to the bedding plane, 4 joint systems are evident in the vicinity of the landslide (Fig. 6). The bedding plane trends N33E and dips a 24–36° SE (set 1 in Fig. 6). Joint set 2 forms the acute angle with the bedding plane and trends N69W, but dips towards 54°NW. Joint set 3 trends N36W and dips 62°SW. Joint sets 4 and 5 are almost parallel to each other and trend N68–54W but dip 62 and 78°NW, respectively. The right and left flanks of the Tortum landslide were accompanied by joint set 2 and joint sets 4 and 5, respectively.

3.2. Features and geometry The landslide preserves distinct scarps and flanks (Fig. 7). The right flank has 40–45 m high and gains a concave shape towards the NE. The left flank reaches up to a height of 80–90 m. It is bifurcated into two strands in the upper part of the landslide. The height of the main scarp, which exhibits a convex geometry to the east, varies between 130 and 150 m in height. Minor scarps formed due to secondary and retrogressive slides observed both upside and downside of the landslide. The heights of the retrogressive scarps range from 20 m to 60 m. Secondary slides are observed on the landslide mass that were diverted to the downstream. These may have occurred soon after the major slide. Back-tilting on the secondary slide indicates that the mechanism was formed as type of a rotational slide. Further, the toe of the secondary slide deteriorated into a flow and travelled downstream. Three landslide lakes were formed as a result of the rotational secondary slides. From small to larger, these lakes covered 0.04, 1.34 and 2.64 ha, respectively (Fig. 7). The surface of the rupture along the bedding is covered by striated by slickensides and exposed in the upper sectors of the slide. The accumulation zone is typical with a hummocky morphology having mounds up to 20–30 m in height and depressions of 10–15 m depth and 25–300 m width. The front of the disintegrated rock mass located on the opposite eastern side of the valley, reaching to an elevation of

70

T.Y. Duman / Engineering Geology 104 (2009) 66–79

Fig. 6. Contour plot of poles to discontinuities and related great circles. Stereonets are lower hemisphere equal area nets. Numbers inside stereonets correspond to discontinuity sets described in the text.

Fig. 4. Shaded relief DEM of Tortum River produced from 10 m interval contours, showing Tortum landslide and its vicinity: (ls) Tortum landslide, (ms) mountains slope, (t) terrace, (c) cliff and (f) valley floor. Dashed line: boundary of the landslide deposit, dashed doted: scarps, arrow: direction of the movement of the landslide.

1050 m.a.s.l., a height of about 300 m above the valley floor (Fig. 7). The mass is split and diverted upstream and downstream in the valley floor. The material that diverted downstream progressed almost 2100 m from the axis of the rock avalanche. The landslide lake covered the mass diverted upstream. Beside, at the north of the Tortum landslide, the left hand side of the valley of the abandoned bed constitutes a cliff that is about 600 m high. Yıkık village located at the bottom of this cliff. At the top of the cliff, long arcuate tensions cracks (about 750 m length) have been developed (Fig. 7). 3.3. Dimensions The landslide dimensions (WP/WLI, 1990) are useful to estimate the volume of a typical landslide. However, the Tortum landslide does

not satisfy a typical landslide model. Blocking the valley by the landslide mass, diversion of the toe, and the absence of the pre-failure topographic map has caused some uncertainties when calculating the depth of the displaced mass. Therefore, a valley profile to approximate the depth of the displaced mass and the topographical reconstruction before the failure were performed to estimate the landslide volume. The dimensions of the Tortum landslide (427 ha) according to the IAEG Commission on Landslide (1990; WP/WLI, 1990) are tabulated in Table 2. The parameters of the length and the width of the landslide were obtained from the cross-sections (Fig. 5) and the geomorphological map that was acquired from the 1/25,000 scale topographic maps with a contour interval of 10 m (Fig. 7). Because the landslide mass filled the valley floor, the depth of the displaced mass could not be estimated by cross-sections. Therefore, to define the depth of the displaced mass, a longitudinal profile of 40-km in length was prepared along the valley that extends 25 km downstream and 15 km upstream from the landslide mass (Fig. 8). Along the profile, the body of the Tortum landslide dam, the landslide dammed lake, and the delta that accumulated upstream of the landslide dam was crossed. On the basis of the profile, the river has a gradient of about 1.1° and 1° downstream and upstream of the landslide, respectively. The maximum depth of

Fig. 5. Longitudinal cross-section of Tortum landslide with the dimensions. (a) Shows location of the cross-section.

T.Y. Duman / Engineering Geology 104 (2009) 66–79

71

Fig. 7. Geomorphological map of Tortum landslide showing related landslide-dammed lake, water fall, and natural diversion channel of the lake.

the displaced mass was estimated to be 270 m based on the average gradient of the valley at 1.05° (h1 in Fig. 8). Based on the cross-sections (see Fig. 5), the depth of the sliding surface is about 75–80 m at the upper part of the landslide. In contrast, the depth of the rupture surface toward the valley floor, where the main landslide mass accumulated, reached 270 m (Fig. 8). Before starting the topographic reconstruction for the estimations of volumes of the landslide and related dam lake, the present contour lines of the landslide and the lake area were clipped (Fig. 9a). Then, some cross-valley profiles were prepared to estimate the general continuation of the slope gradient toward the clipped area, and to approximate the centre of the valley in the clipped area. Following this stage, the contour lines with 10 m interval locations, which were projected from the longitudinal valley profile through the valley, were located on the approximated centreline of the valley (Fig. 9b). The contour lines were constructed manually along the valley floor considering the general gradient of the slope as a reference for the reconstruction process. Finally, topographic reconstruction was

Table 2 Dimensions of the Tortum landslide Dimensions

Value, m

Width of displaced mass, Wd Width of surface of rupture, Wr Length of displaced mass, Ld Length of surface rupture, Lr Depth of displaced mass, Dd Depth of surface of rupture, Dr Total length, L

1775 1775 1850 2950 270 70 3375

performed and pre-failure terrain geometry of both slide and lake areas were obtained (Fig. 9b). Considering the landsliding process and its consequences, three different topographical configurations such as pre-failure and pre-lake valley floor, landslide mass under water, and the actual conditions, could be estimated (Fig. 10). Applying the reconstruction procedure outlined above, the topographical contours of these configurations were evaluated. In order to calculate the volumetric variations according to these models, grid based digital elevation models having a spatial resolution of 10 × 10 m2 were constructed. The total landslide volume is estimated at 2.2 × 108 m3. 3.4. Activity State, distribution and style of activity were evaluated for the landslide according to the literature (Varnes, 1978; WLI/WLI, 1993; Cruden and Varnes, 1996). Before and during this study, instrumental measurements were not conducted to determine the current activity of the Tortum landslide. However, some observations were applied to obtain some information about the activity of the landside during 2004 and 2006 period. The surface of the rupture has being extending in the direction opposite the movement of the displaced material at the upper sector of the landside, between the elevations of 1500–2050 m.a.s.l. In this sector, a fold was observed on the surface of the rupture as a result of new sliding that occurred at the upper part of the earlier slide (Fig. 11). The fold axis gently concaves to the east. The dip of the axial surface inclines 45–48° to the west. The fold hinge dies out plunging 17–21° at the both SE and NE ends. Depending on the trend and plunge of the hinge line, this fold can be called moderately inclined horizontal and

72

T.Y. Duman / Engineering Geology 104 (2009) 66–79

Fig. 8. Longitudinal profile along the Tortum River. The profile extends toward 25 and 15 km downstream and upstream from the Tortum landslide mass, respectively. Note landslide dam, related lake and delta formed at the upstream were crossed along the profile.

Fig. 9. Stages of the reconstruction in vicinity of the Tortum landslide. (a) Recent condition of the area showing the landslide and lake area. (b) Reconstructed contours of the area with projected centreline with contour intervals from the longitudinal valley profile.

gently plunged fold. The dip of the lower limb of this fold has increased about 13–15° over the past three years. A dirt road was present in the same area and was used for the field study in 2003. However, the same road could not be used more recently because the landslide damaged it in 2004. The surface of the rupture of the Tortum landslide occurred along a bedding plane and flanks were accompanied by joint sets. Repeated movements of the same type along the bedding surface and the ensuring enlargement of the surface of rupture were observed towards the upslope directions in the landslide. Based on these observations, the state, distributions and the style of the activity of the Tortum landslide are reactivated, retrogressive and multiple, respectively. 3.5. The landslide dam and related lake The Tortum landslide mass impounds a 8500 m long, 2500 m wide lake with a surface area of 6.77 km2 and a total volume of 5.3 × 108 m3 (Fig. 12). The average March–April river discharge at the landslide site is approximately 400 m3/s. The base of the blockage varies between 160 and 330 m in width (parallel to the direction of the flow of the river). The length of the landslide dam is about 4400 m along the cross-valley, but 1350 m of this length belongs to the toe and has no significant thickness. The width of the upper sector of the landslide

Fig. 10. The essential of the volumetric calculations for three different topographical configurations considered in the study.

T.Y. Duman / Engineering Geology 104 (2009) 66–79

73

material composing the Tortum landslide dam. Photographic technique (Glinken, 1998) of this method has been employed from the characterisation of the landslide dam. For this purpose, photographs were taken from the clean and vertical windows (4 m2) of the outcrop area and a digital image processing technique was used to assess the grain size up to 4 mm (Fig. 13). Since the amount of the material finer than −4Phi (16 mm) exceeded 12%, a bulk sample was taken at the same location where the grid by number was applied and standard volumetric sieve (ASTM standards) was carried out. The diameter of the debris particles composing the landslide dam ranges in size approximately from 1.4 × 10− 4 to 2.5 × 103 mm (Fig. 14). Consequently, the blockage mass is composed principally of mixture of fine angular detritus ranging from sand, pebble and cobble size to blocks as much as 25 cm in diameter (Table 3). Some blocks those are erratically distributed in the mass are observed in size up to 10 m across. Additionally, the dam material classification, according to texture (Blikra and Nemec, 1998), is grain-supported (see Fig. 13). A delta with a length of 4 km and a width of 1 km formed upstream of the landslide dam. The delta has an average gradient of about 0.6°. The maximum thickness of the deltaic sediments is estimated roughly as 65 m by projecting the gradient to the longitudinal profile of the valley (see h2 in Fig. 8). After filling the reservoir, the lake was overtopped from a terrace, where is located at right hand abutment and the landslide mass accumulated, and a natural spillway was cut by the river. The terrace has almost the same level as the dam and divides sub-catchment of Halka Creek (see Fig. 7). The lake was overtopped throughout the natural spillway to the Halka Creek, forming a channelized diversion. The channelized diversion that is convex shaped to the east and reaches the original channel after 3 km downstream. Overtopping water created a marvellous water-fall at the tip of the channelized diversion (see Fig. 7). The water-fall is 22-m wide and 50-m high, having a discharge of 400 m3/s in the spring time. 3.6. Age of the landslide Fig. 11. A fold formed on the former sliding surface due to the retrogressive slide. (b) View to the direction opposite the movement, from east to west.

dam changes between 1380 m and 786 m. The volume of the landslide dam has been estimated as 1.8 × 108 m3. The grid by number method, which is one of the most common sampling techniques in the study of river dynamics (Church et al., 1989), has been applied to determine grain size distribution of the

A part of the landslide mass remains at the eastern part of the natural spillway, resting on the opposite eastern side of the valley. It is covered by fluvial fans derived from the east (Fig. 15a). In order to estimate the age of the Tortum landslide, radiocarbon dating was performed on exhumed wood found in and beneath the alluvial fan deposits. For this purpose, two trenches measuring 2 m deep, 4 m wide and 15–25 m long were dug in the fans on the toe of the landslide mass

Fig. 12. Tortum landslide-dammed lake and landslide dam (foreground), view to the north.

74

T.Y. Duman / Engineering Geology 104 (2009) 66–79

Fig. 13. The application of the photographic techniques for the grain size distribution estimation. The square area is 4 m2 and particles coarser than −2Phi (4 mm), which constitute 37.6%, were outlined on the image.

Fig. 14. Results of the cumulative grain size distribution of the landslide dam for the coarse and finer size that are obtained from photographic techniques (a) and standard volumetric sieve analysis (b), respectively.

(Fig. 15b). The trenches were logged in detail, showing samples locations and the exposures yielded information on the surface of the landslide mass (Fig. 15c). Sediments in the trenches consist of landslide and fan deposits and could be differentiated visually on the trench walls (Fig. 15d). The landslide deposits are composed of coarse sand and pebble material with blocks whereas the alluvial fan deposits consist of pebbly sand and silt. The coarse material of the fans decreases from apex to distal area. The thickness of the fan deposits ranges between 1 and 1.5 m at the trench sites. Beside the size of the grains, colour difference is also a distinguishing factor for the fan and landslide deposits. Yellow coloured, laminated fine sand and silt layers of the fan deposits overlie the landslide deposits. All samples that could be found in the wall of the trenches were dated. From bottom to top, five wood fragments in the fan deposits yielded radiocarbon ages ranging from BP 470 ± 40 to 150 ± 40. Calibrated Age Ranges (2σ) vary between 1784 and 1394 AD (Table 4 and Fig. 16). The age ranges provided by radiocarbon analyses are inconsistent with the stratigraphic order such that younger samples overlie older samples. Sample H5 is younger than the samples (H1, H2, and H4) collected from the same stratigraphic unit. Therefore samples H1, H2 and H4 were interpreted as having been reworked and redeposited. Based on the calibrated age range of samples H3 and H5, the Tortum landslide occurred sometime between 1641 ± 40 and 1680 ± 40 AD (48% probability).

4. Environmental impacts of Tortum landslide The landslide-dammed lake resulted in positive environmental impacts in its vicinity causing a shift in the local micro-climate and allowed development of a fishery and tourism in the region. The Tortum Waterfall is visited by approximately 7000 domestic and 5000 foreign tourists per year. The production capacity of the salmon trout, carp and stripped mullet is 84 ton/year by 13 fishery plants. The lake impounded by landsliding reaches an elevation of 1008 m.a.s.l. A triangular shaped terrace formed between the natural spillway, the abandoned river bed, and Halka Creek at the right side of the valley (see Fig. 7). There is a 150 m high fall on the slope of the Halka Creek, at the northern slope of the terrace. Diverting the river before the waterfall by a 2 km long power-canal, a hydroelectric power-plant with a

Table 3 Grain size distribution of the Tortum landslide dam Sample no

Boulder fraction (%)

Cobble fraction (%)

Pebble fraction (%)

Sand fraction (%)

Finer fraction (%)

1 2 Mean

4.2 0.8 2.5

10.4 1.7 6.05

42.3 23.9 33.1

33.1 51.2 42.15

9.7 22.1 15.9

USCS (Unified Soil Classification System) is used for the grain size classification.

T.Y. Duman / Engineering Geology 104 (2009) 66–79

75

Fig. 15. Location of the trenches, related sketch and photos. (a) Shows location of the trench site on the fans derived from left slope of the valley and deposited on the landslide mass. (b) Sketch of the south wall of the trench showing landslide and fan deposits. Sample locations marked in the fan deposits. (c) Photo facing fan deposits along the trench. (d) A close view of the wood fragment in the fan deposit.

76

T.Y. Duman / Engineering Geology 104 (2009) 66–79

Table 4 Ages reported by radiocarbon laboratory based upon the Libby half-life (5570 years) for 14C Sample no

Laboratory no

Stratigraphic unit

Sample material

13C/12C (‰)

14C age (BP)

Calibrated age range (2σ)

H3 H5 H4 H1 H2

Beta-19146 Beta-19148 Beta-19147 Beta-19145 Beta-19144

Unit Unit Unit Unit Unit

wood wood wood wood wood

−24.8 −25.3 −25.7 −24.8 −25.5

150 ± 40 230 ± 40 290 ± 50 390 ± 40 470 ± 40

AD 1666–1784 (.48) 1795–1892 (.33) 1908–1953 (.01) AD 1641–1680 (.48) 1764–1800 (.40) 1939–1951 (.01) AD 1462–1666 (.97) 1784–1795 (.02) AD 1437–1528 (.64) 1551–1634 (.35) 1545–1545 (.001) AD 1394–1475 (.97) 1327–1342 (.02)

4 silty sand 4 silty sand 3 coarse sand 3 coarse sand 3 coarse sand

The 2σ errors are presented in terms of probabilities (.97 = 97%) based on CALIB Rev. 5.1 (Stuiver and Reimer, 1993) using the calibration curves of Stuiver et al. (1998).

capacity of 100 GWh per year was established in this area. The power plant meets most of the energy need in the region. Another positive impact of the landslide-dammed lake is on the climate. The landslide lake caused a semi-mild micro-climate in its vicinity, whereas the region shows terrestrial climate characteristics. The Uzundere meteorological station is located at 8 km upstream of the Tortum landslide. Annual precipitation, temperature, and snow covered days were evaluated using the data measured from five meteorological stations (DMI, 2005) in the 100 km radius of the area around the landslide lake (Table 5). The Horasan, Hınıs, Erzurum and Tortum stations are situated 300 to 500 m higher than the Uzundere station. For this reason, data of the Uzundere, İspir and Oltu stations were compared. An obvious difference was observed on the days covered by snow. There is no record at the Uzundere station on covered snow days during the year whereas 69 and 79 days were record at the İspir and Oltu stations, respectively. Average annual rain precipitation shows a general similarity, but the type of precipitation is mostly rainfall in the vicinity of Uzundere station (1300 m.a.s.l.) whereas it is dominated by snow around the İspir (1222 m.a.s.l.) and Oltu (1321 m.a.s.l.) stations. While terrestrial climate conditions prevail in the region, vegetable and fruits productions (excepting the citrus fruits) are observed to rise owing to the semi-mild climatic condition prevailing in the vicinity of the Tortum landslide-dammed lake. In addition, there are 55 ha of greenhouses established around the area with a production capacity of about 780 ton/year in the Uzundere region (http://www.dadas.net). 5. Discussion Korup (2004) compared the landslide-dammed lake volumes and landslide dam volume from 184 worldwide examples. The Tortum landslide-dammed lake volume (estimated at 5.3 × 108 m3) and the related landslide dam (estimated at 1.8 × 108 m3) are among the top 10 and 50 of the worldwide data, respectively (Fig. 17). For the prediction potential for landslide dam stability, Casagli and Ermini (1999), Ermini and Casagli (2002) and Korup (2004) proposed six indices that are the Blockage Index, the dimensionless Blockage Index, the Impoundment Index, the Backstow Index, the Basin Index and the Relief Index (Table 6). Casagli and Ermini (1999) applied the

Blockage Index and the Impoundment Index to the cases in the Apennines, Italy. Ermini and Casagli (2002) examined 83 worldwide sites for the dimensionless Blockage Index. Korup (2004) presented results from the Blockage Index, the Impoundment Index, the Backstow Index, the Basin Index and the Relief Index based on available data from New Zealand landslide-dammed lakes. Weidinger (2004) suggested the Block Size Stability Diagram. The diagram correlates the grain, boulder and block size of landslide material and the stability of a dam, with the result that the greater the average diameter of the components, the longer the life of the dam and lake. Davies and McSaveney (2004) pointed out the implications of the presence of fragmented rock material for the stability of landslide dams. Dunning (2004) discussed the sedimentology of rock avalanche deposits and its application to dam breach-development. The Blockage Index and the Impoundment Index of the Tortum landslide dam are obtained as 6.2 and −0.4, respectively (Table 6). The Blockage Index of The Tortum landslide dam is above the threshold ratio for the unstable lakes and existing lakes. The other indices values for the Tortum landslide dam remain below the threshold ratio for the stable lake or fall into the inconclusive domains (Values of the Backstow Index between Is b −3 and Is N 0 remain inconclusive domains). On the basis of the indices, which are more useful to forecast the type of evolution of future landslide-damming events in the same region that developed the models (Casagli and Ermini, 1999; Ermini and Casagli, 2002; Korup 2004), the Tortum landslide dam remains in the unstable or inconclusive domains. At a first approximation, this state is supposed to be inconsistent with the indices, considering that the Tortum landslide dam has been present and stable for about 350 years. However, the location of the Tortum landslide dam exhibits an exceptional morphology. The lake overtopped from the landslide mass that accumulated on a terrace at the right hand abutment and discharged to a sub-catchment of the Tortum River, forming a channelized diversion on the basement rocks (see Figs. 5 and 7). No seepage is observed through the dam. This exceptional condition may result increased stability due to the nonerosional overtopping of the river. From the point of view of hazard assessment, there is almost no hazard of the failure of the landslide dam. However, there is a potential landslide hazard on the left hand side of the valley downstream. In this region tensions cracks (see Fig. 7) suggesting

Fig. 16. Probability distribution of calibrated 14C ages (Table 4) obtained from sequential radiocarbon dates (BP) using OxCal 3.10 Program (Bronk Ramsey, 1998).

T.Y. Duman / Engineering Geology 104 (2009) 66–79

77

Table 5 Mean annual meteorological data recorded at the seven stations (DMI, 2005) close to the Tortum landslide Station

Record period (year)

Elevation, (m)

Rainfall, (mm)

Snow covered day (in a year)

Temperature, (°C),

Relative moisture, (%)

Distance to Tortum lake (km)

Uzundere Oltu Tortum İspir Horasan Erzurum Hınıs

9 30 31 31 31 31 31

1300 1321 1572 1222 1540 1747 1715

338 393 841 471 406 411 599

– 76 83 69 97 113 120

10.2 9.8 8.2 10.3 6.4 5.3 6.2

53 61 61 59 61 65 65

8 30 33 53 78 82 138

that a failure process has already begun, even if it may be temporarily suspended. This area displays similarity with the geological and morphological condition of the Tortum landslide before the event. It is thus under a potential landslide threat. In the future, failure of the cliff is possible as a huge slide similar to the case of the Tortum landslide. The triggering factor of the landslide was not evaluated in the study. However, considering the date of the Tortum landslide obtained from 14C, four historical earthquakes, which were between I=VI and VII, and occurred in 1766–1794 (Ergin et al., 1967), could be interpreted as the possible triggering factor of the event. On the other hand, if it is taken into accounts that there was no landslidedammed lake and no a micro-climatic environment in the region before the landslide, possible triggering factor could strongly be deduced as a rapid snow-melting in the area. 6. Conclusion This study, for the first time, describes the Tortum landslide which forms the largest landslide-dammed lake in Turkey. The characteristics, age, causal factors and environmental impacts of the Tortum landslide, and geomorphometric parameters of the related landslide dam and its impoundment lake, were also investigated. The Tortum landslide was formed as a rock slide along the slope that forms the steep-wall of the canyon up to gentler slope of the Kemerli Mountain above an altitude of 2000 m.a.s.l. The failure occurred on the bedding plane surface dipping to the SE, toward the Tortum valley. The initial stage of the movement started as a rock slide but soon it changed into an avalanche as it accelerated down the steep slopes of the Kemerli Mountain descending toward the valley floor of the Tortum River. The landslide covers an area of about 428 ha and has total volume estimated as 2.2 × 108 m3. The lengths of the path of sliding/ avalanching and of the axis of landslide deposition along the valley

floor are both about 3.3 km. The volume of the landslide dam is calculated as 1.8 × 108 m3 with a maximum depth of about 270 m. The landslide blocked the Tortum River and resulted in the largest landslide dam in Turkey. The lake impounded by the blockage is Table 6 Geomorphometric indices distinction of discrete domains of landslide dam stability results based on available data of landslide-dammed lakes from Northern Apennine (Casagli and Ermini, 1999), New Zealand (Korup 2004) and selected worldwide (Ermini and Casagli, 2002) and indices values for the Tortum landslide dam and landslide-dammed lake Index

Landslide dam stability

Blockage Index for Northern Apennine landslide-dammed lakes data, Casagli and Ermini (1999) Blockage Index for New Zealand landslide-dammed lakes data, Korup (2004)

Ib = 6.2 Ib = 3 threshold ratio for formation lakes 4 N Ib N 3 unstable dams 5 N Ib N 4 uncertainties Ib N 5 stable and existing lakes Ib = 6.2 Ib N 2 threshold ratio for formation lakes Ib b 4 threshold ratio unstable lake Ib N 7 threshold ratio stable and existing lakes Ib' = 2.6 Ib' 2.92 lower threshold ratio for stable, Ib' 3.25 upper threshold ratio for unstable

Dimensionless Blockage Index for Selected worldwide data, Ermini and Casagli (2002) Dimensionless Blockage Index for New Zealand landslide-dammed lakes data, Korup (2004) Impoundment Index for New Zealand landslide-dammed lakes data, Korup (2004) Impoundment Index for Northern Apennine landslide-dammed lakes data, Casagli and Ermini (1999) Backstow Index for (New Zealand landslide-dammed lakes data, Korup (2004) Basin Index for New Zealand landslide-dammed lakes data, Korup (2004) Relief Index for New Zealand landslidedammed lakes data, Korup (2004)

Fig. 17. Bivariate plot of landslide-dammed lake volumes versus landslide dam volume derived from a worldwide data set (n = 184), highlighting occurrences in New Zealand, Japan, and the USA according to Korup (2004) and data from Tortum landslide dam and landslide-dammed lake.

Index values for Tortum landslide dam

Ib' 3 lower threshold ratio for stable, Ib' 5 upper threshold ratio for unstable

Ib = 2.6

Ii = 1 threshold ratio stable from unstable

Ii = −0.4

Ii = 0 threshold ratio stable from unstable

Ii = −0.4

Is b −3 upper threshold ratio for unstable Is N 0 lower threshold ratio for stable Data between these threshold remain inconclusive Ia N 3 threshold ratio stable from unstable

Is = −1.4

Ia N 3 threshold ratio stable from unstable

Ia = 0.5

Ia = 1.6

Blockage I b = log(V DA C− 1 ), Dimensionless Blockage Index I b' = log(V DA C− 1 H −D1 ), Impoundment Index Ii = log(VDV−L 1), Backstow Index Is = log(H3DV−L 1), Basin Index Ia = log (H2DAC− 1), Relief Index Ir = log(HDH−R 1) where VD is volume of landslide dam and impoundment [in m3], VL is volume of landslide dam and impoundment [in m3], AC is catchment area upstream of the blockage [in km2]. HD maximum crest height of landslide dam [in m], HR is the relief upstream of the point of blockage [in m].

78

T.Y. Duman / Engineering Geology 104 (2009) 66–79

approximately 523 × 106 m3 in volume, with a length of 8 km and a width of 0.7 km. Based on the radiocarbon age, the Tortum landslide occurred about 350 ± 40–370 ± 40 years before today. When compared with the sizes of many landslide dams that can be observed throughout the world, the Tortum landslide and the landslide-dammed lake are fairly spectacular. According to the Indices that are the criteria for a preliminary assessment of landslide dam stability evolution, the Tortum landslide dam lies in the unstable or inconclusive domains but has remained due to its exceptional morphology and because of the outflow channel is founded in rock. The landslide-dammed lake resulted in some positive environmental impacts on its vicinity, including the formation of a semi-mild micro-climate area in the generally terrestrial climatic region, the development of tourism, and the creation of the appropriate conditions for fishery and greenhouse agriculture. The lake also provides opportunity for hydropower generation. There is no historical record of the human impact or damage resulting from the Tortum landslide, and it has low risk for failure of the landslide dam. However, a settlement area located at the toe of the landslide is under a renewed threat due to a potential landslide that may occur on the left side of the valley. Acknowledgments This work was done as a part of ‘The Landslide Inventory Maps of Turkey’ supported by the General Directorate of Mineral Research and Exploration (MTA). I gratefully acknowledge MTA for the support provided. I also thank Giovanni B. Crosta, Johannes T. Weidinger and an anonymous reviewer for comments and review, which improved the paper substantially. References Alford, D., Cunha, S.F., Ives, J.D., 2000. Lake Sarez, Pamir Mountains, Tajikistan: mountain hazards and development assistance. Mountain Research and Development 20, 20–23. Blikra, L.H., Nemec, W., 1998. Postglacial colluvium in western Norway: depositional processes, facies and palaeoclimatic record. Sedimentology 45, 909–959. Bromhead, E.N., Coppola, L., Rendell, H.M., 1996. Field reconnaissance of valley blocking landslide remnants: the Cordevole and Piave catchments. Journal of the Geological Society of China 39, 373–389. Bronk Ramsey, C., 1998. Radiocarbon calibration and analysis of stratigraphy: the OxCal program. Radiocarbon 37, 425–430. Casagli, N., Ermini, L., 1999. Geomorphic analysis of landslide dams in the Northern Apennine. Transactions-Japanese Geomorphological Union 20, 219–249. Casagli, N., Ermini, L., Rosati, G., 2003. Determining grain size distribution of the material composing landslide dams in the Northern Apennines: sampling and processing methods. Engineering Geology 69, 83–97. Church, M.A., Mc Lean, D.G., Wolcott, J.F., 1989. River bed gravels: sampling and analysis. In: Thorne, C.R., Bathurst, J.C., Hey, R.D. (Eds.), Sediment Transport in Gravel-Bed Rivers. Wiley, Chichester, pp. 43–88. Costa, J.E., Schuster, R.L., 1988. The formation and failure of natural dams. Geological Society of America Bulletin 100, 1054–1068. Costa, J.E., Schuster, R.L., 1991. Documented historical landslide dams from around the world. U.S. Geological Survey Open-File Report, 91– 239. 486 pp. Chai, H.J., Liu, H.C., Zhang, Z.Y., Xu, Z.W., 2000. The distribution causes and effects of damming landslides in China. Journal of the Chengdu Institute of Technology 27, 302–307. Clague, J.J., Evans, S.G., 1994. Formation and failure of natural dams in the Canadian Cordillera. Geological Survey of Canada Bulletin 464 35 pp. Cruden, D.M., Varnes, D.J., 1996. Landslide types and processes. In: Turner, K.A., Schuster, R.L. (Eds.), Landslides: Investigation and Mitigation, Transport Research Board Special Report, vol. 247. National Academy Press, Washington DC, pp. 36–75. Davies, T.R., McSaveney, M.J., 2004. Dynamic fragmentation in landslides: application to natural dam stability. In: Stedile, S. (Ed.), Special Issue on Security of Natural and Artificial Rockslide Dams NATO ARW, Bishkek (Kyrgyzstan), June 2004. Italian Journal of Engineering Geology and Environment, special issue, vol. 1. University of Rome La Sapienza, Research Center Ceri, Valmontone-Rome, Italy, pp. 123–126. DMI (General Directorate of State Meteorological Affairs), 2005. Temperature and Precipitation Records of Uzundere Station. DMI, Ankara. Duman, T.Y., Çan, T., Emre, Ö., Keçer, M., Doğan, A., Ateş, Ş., Durmaz, S., 2005. Landslide inventory of north western Anatolia, Turkey. Engineering Geology 77, 99–114. Duman, T.Y., Nefeslioğlu, H.A., Çan, T., Olgun, Ş., Durmaz, S., Hamzaçebi, S., Çörekçioğlu, Ş., 2007. Landslide inventory map of Turkey-1/500 000 scaled Trabzon quadrangle, MTA Special Publication Series-9, 25 pp. Ankara [in Turkish].

Dunning, S.A., 2004. The grain size distribution of rock-avalanche deposits in valleyconfined settings. In: Stedile, S. (Ed.), Special Issue on Security of Natural and Artificial Rockslide Dams NATO ARW, Bishkek (Kyrgyzstan), June 2004. Italian Journal of Engineering Geology and Environment, special issue, vol. 1. University of Rome La Sapienza, Research Center Ceri, Valmontone-Rome, Italy, pp. 117–122. Ergin, K., Güçlü, U., Uz, Z., 1967. An Earthquake Catalogue for Turkey and Surrounding Regions (MS 11-1964) (in Turkish); Publ. 24. Earth Phys. Inst., Istanbul Tech. Univ., Min. Fac., Istanbul. 189 pp. Erguvanlı, K., Tarhan, F., 1982. Engineering geological evaluation of mass movements along the eastern Black Sea coastal zone. KTÜ. Yer Bilimleri Dergisi 100 yıl Özel Sayısı. [in Turkish]. Ermini, L., Casagli, N., 2002. Criteria for a preliminary assessment of landslide dam evolution. In: Rybar, J., Stemberk, J., Wagner, P. (Eds.), Landslides. Proceedings 1st European Conference on Landslides 24–26 June 2002, Prague, Balkema, pp. 157–162. Gasiev, E., 1984. Study of the Usoy landslide in Pamir. Proceedings of IVth International Symposium on Landslides, 16–21 September, Toronto, pp. 511–515. Glinken, H., 1998. Rock slide and debris avalanche of May 18, 1980, Mount St. Helens Volcano, Washington. Bulletin of the Geological Survey of Japan 49 (2/3), 55–106. Hancox, G.T., Perrin, N.D., 1994. Green Lake landslide: a very large ancient rock slide in Fiordland, New Zealand. In: Oliveira, R., Rodrigues, L.F., Ceolho, A.G., Cunha, A.P. (Eds.), Proceedings Seventh International Congress International Association of Engineering Geology, pp. 5–9. Hancox, G.T., McSaveney, M.J., Davies, T.R., Hodgson, K., 1999. Mt Adams Rock Avalanche of 6 October 1999 and Subsequent Formation of a Landslide Dam in Poerua River, Westland, New Zealand. Institute of Geological and Nuclear Sciences, Lower Hutt. Report 99/19, 22 pp. Hanisch, J., Söder, C.O., 2000. Geotechnical assessment of the Usoi landslide dam and the right bank of Lake Sarez. In: Alford, D., Schuster, R.L. (Eds.), Usoi Landslide Dam and Lake Sarez. An Assessment of Hazard and Risk in the Pamir Mountains, Tajikistan International Strategy for Disaster Reduction Prevention Series No. 1. United Nations, Geneva, pp. 23–42. Hejun, C., Hanchao, L., Zhuoyuan, Z., 1998. Study on the categories of landslidedamming of rivers and their characteristics. Journal of the Chengdu Institute of Technology 25, 411–416. Hewitt, K., 1998. Catastrophic landslides and their effects on the Upper Indus streams, Karakoram Himalaya, northern Pakistan. Geomorphology 26, 47–80. IAEG (International Associations of Engineering) Commission on Landslide, 1990. Suggested nomenclature for landslide. Bulletin of the International Associations of Engineering Geology 41, 13–16. ICIMOD (International Centre for Integrated Mountain Development), 2000. http:// www.icimod.org.sg/focus/risks_hazards/lslide_tibet_2000.htm. King, J., Loveday, I., Schuster, R.L., 1987. Failure of a massive earthquake-induced landslide dam in Papua New Guinea. Earthquakes and Volcanoes 19, 40–47. Konak, N., Hakyemez, H.Y., Bilgiç, T., Bilgin, Z.A., Hepşen, N., Ercan, T., 2001. Geology of the Northeastern Pontides (Oltu-Olur-Şenkaya-Narman-Tortum-Uzundere-Yusufeli). General Directorate of Mineral Research and Exploration (MTA), MTA Report no: 10489, Ankara. 369 pp. [in Turkish]. Korup, O., 2002. Recent research on landslide dams — a literature review with special attention to New Zealand. Progress in Physical Geography 26, 206–235. Korup, O., 2004. Geomorphometric characteristics of New Zealand landslide dams. Engineering Geology 73, 13–35. Kovari, K., Fritz, P., 1984. Recent developments in the analysis and monitoring of rock slopes. Proc., Forth International Symposium on landslides, Canadian Geotechnical Society, Toronto, vol. 1, pp. 1–16. Mason, K., 1929. Indus floods and Shyok glaciers. Himalayan Journal 1, 10–29.Miller, A.A., 1953. The Skin of the Earth. Methuen & Co, London. 198 pp. Pamir, H.N., 1930. Geology in the vicinity of the Of-Sürmene and the 1929 landslide and flooding. Fen. Fak. Mec. 7, 1–2 İstanbul Darülfünunu. [in Turkish]. Read, S.A.L., Beetham, R.D., Riley, P.B., 1992. Lake Waikaremoana barrier: a large landslide dam in New Zealand. In: Bell, D.H. (Ed.), Landslides. Glissements de terrain. Proceedings of the Sixth International Symposium, 10–14 February 1992, Christchurch, pp. 1481–1488. Reneau, S.L., Dethier, D.P., 1996. Late Pleistocene landslide-dam lakes along the Rio Grande, White Rock Canyon, New Mexico. Geological Society of America Bulletin 108,1492–1507. Schuster, R.L., 1993. Landslide dams — a worldwide phenomenon. Proceedings Annual Symposium of The Japanese Landslide Society, Kansai Branch, 27 April, Osaka, pp.1–23. Shoaei, Z., Ghayoumian, J., 1997. Landslide dam in Iran, mechanism and history. In: Marinos, P.G. (Ed.), Engineering geology and the environment. Proceedings Symposium Athens, vol. 1, pp. 1043–1047. Shroder Jr., J.F., 1998. Slope failure and denudation in the western Himalaya. Geomorphology 26, 81–105. Stuiver, M., Reimer, P.J., 1993. Extended 14C database and revised CALIB radiocarbon calibration program. Radiocarbon 35, 215–230. Stuiver, M., Reimer, P.J., Bard, E., Beck, J.W., Burr, G.S., Hughen, K.A., Kromer, B., McCormac, G., van der Plicht, J., Spurk, M., 1998. INTCAL98 radiocarbon age calibration, 24000–0 cal BP. Radiocarbon 40 (3), 1041–1083. Swanson, F.J., Oyagi, N., Tominaga, M., 1986. Landslide dams in Japan. In: Schuster, R.L. (Ed.), Landslide dams: process, risk, and mitigation. Special Publication, vol. 3. American Society of Civil Engineers, New York, pp. 273–378. Tarhan, F., 1991. A general view of the landslide in the Eastern Black Sea region. Türkiye I. Heyelan Sempozyumu Bildiriler Kitabı s. Karadeniz Teknik Üniversitesi, Trabzon, pp. 38–63 [in Turkish]. Varnes, D.J., 1978. Landslides types and processes. In: Eckel, E.B. (Ed.), Landslides and Engineering Practice. Highway Research Board Spec. Rep., vol. 29, pp. 20–47. Wayne, W.J., 1999. The Alemania rockfall dam: a record of a mid-Holocene earthquake and catastrophic flood in north-western Argentina. Geomorphology 27, 295–306.

T.Y. Duman / Engineering Geology 104 (2009) 66–79 Weidinger, J.T., 1998. Case history and hazard analysis of two lake-damming landslides in the Himalayas. Journal of Asian Earth Sciences 16, 323–331. Weidinger, J.T., 2004. Landslide dams in the high mountains of India, Nepal and China — stability and life span of their dammed lakes. In: Stedile, S. (Ed.), Special Issue on Security of Natural and Artificial Rockslide Dams NATO ARW, Bishkek (Kyrgyzstan), June 2004. Italian Journal of Engineering Geology and Environment, special issue, vol. 1. University of Rome La Sapienza, Research Center Ceri, Valmontone-Rome, Italy, pp. 67–78. WP/WLI (International Geotechnical Societies UNESCO Working Party on World Landslide Inventory), 1990. A suggested method for reporting a landslide. Bulletin International Association of Engineering Geologists 41, 5–12.

79

WP/WLI (International Geotechnical Societies UNESCO Working Party on World Landslide Inventory), 1993. A suggested method for describing the activity of a landslide. Bulletin of the International Association for Engineering Geology 47, 53–57. Yılmaz, Y., Tüysüz, O., Yiğitbaş, E., Genç, Ş.C., Şengör, A.M.C., 1997. Geology and tectonic evolution of the Pontides. In: Robinson, A.G. (Ed.), Regional and petroleum geology of the Black Sea and surrounding region. AAPG Memoir, vol. 68, pp. 183–226.