Deadly Landslide and Debris Avalanche in Abikar Village, Farsan City, Chaharmahal and Bakhtiari Province, Iran.

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Deadly Landslide and Debris Avalanche in Abikar Village, Farsan City, Chaharmahal and Bakhtiari Province, Iran.

https://doi.org/10.21203/rs.3.rs-919112/v1

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On the evening of April 1, 1998, near Farsan city, Chaharmahal and Bakhtiari province, Iran, along the Labad river, the southern edge of the valley of Mt. Kino slid down into the Labad river and buried Abikar village under several meters of debris on the opposite flank of the river valley. The overall casualties and damages claimed the life of 54 people (20 men, 30 women, and 4 infants), the death of 1300 livestock, and the destruction of 40 hectares of farmlands and orchards. The observation of several pieces of extraordinary evidence in the landslide area, such as the lack of a landslide dam or barrier across the river after the slope sliding, evidence of high velocity debris flow, the transformation of some debris material by jumping into the far end of debris flow, and the effects of a severe storm in front of debris flow mass, have made this phenomenon a concerning complex landslide event for further research.

The causes for long debris-flow run out have been discussed in some works. Most of these studies have pointed out the effect of air cushion formation under the debris collapsed into the river valley that facilitates the long-distance transport of detrital flows. Another rare mechanism for interpreting the long-distance travel of rock boulders is blocks and boulders displacement in the form of jumping, which might be due to the energy from collisions between heavy rocks and the hard bottom of the river, breaking them down into smaller pieces and making them jump into the far distant points.

Field evidence observed in Abikar landslide indicates that the occurrence of this landslide and the consequent events were a combination of these two mechanisms. Part of the debris has moved in a rapid flow, and the other part has jumped to the opposite flank of the river over the debris flow.

The deadly landslide in Abikar followed an exceptional rainfall that reached 190 mm during a week prior to the landslide event. In this event, the southern flank of the Labad river valley that composed of limestone, shale, and marlstone layers with a slope angle of 75 degrees and the highest point of 1200 m from the bottom of the river, fell down into the valley and mounted the opposite bank of the river with the deposit height exceeding 100 m. Some observed evidence of the field study showed that the toe erosion by the river as well as intensive rainfall could be the main triggering factor for this landslide. After the fall of southern block (1200 m height, 600m width and average thickness of 30 m), most detrital material moved toward the opposite flank by following the ground surface with a high shear force that cut buildings, gardens, and other people properties to the depth of approximately 6 m. Observing a transported walnut tree with the root and trunks at the end part of the debris confirms the high shearing energy of the debris. The uprooting of shrubs and clear-cutting of the entire vegetation cover to the distance of 100-200 m in a vast area in front of the moving debris flow confirmed the generation of a severe typhoon in front of the debris as well as the generation of high-pressure air cushion under the debris. Another interesting observed evidence confirming the movement of some material by jumping is the presence of large masses of source rock on the top of the deposit and at the end of the debris without any sign of mud contamination of the debris matrix.

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Farsan city

high shearing energy

river valley

extraordinary evidence

In a landslide study, identifying the type of landslide is one of the main responsibilities of the research team. This issue has long been considered by researchers. The first coherent classification was proposed by Varnes in 1954, which is based on the type of moving mass material and the mechanism of the primary failure initiation. In recent years, different types of classification have been presented by other researchers, each of which has examined this issue from a certain perspective (Varnes, 1954, 1978; Emelianova, 1972; Zolotariov, 1964, 1983; Zolotariev et al., 1969; WP/WLI, 1993 a, b, 1994; Cruden and Varnes, 1996; Cruden and VanDine, 2013). Most of these classifications emphasize the initial conditions of motion and slope instability formation procedure. However, the majority of them did not address the kinetic behavior of the material or its displacement after the main slip. Unlike most previous classifications, the latest classification presented by Hunger et al., 2012, and 2014, discusses both the formation of instability and the movement of materials, especially in debris flow materials. This classification also indicates the types of slope instabilities and mode of movement as follows: Fall, Topple (block and flexural), Slide (rotational/planar/wedge/compound/irregular), Spread, and Flow (slope deformation). Moreover, in landslide risk assessment and landslide-induced damage estimation, especially in large-scale landslides, estimating the debris flow velocities and travel distances is one of the main findings contributing to the management of this natural disaster. Rapid landslides, debris flows, debris avalanches, rockslide avalanches, and large-scale liquefaction-induced landslides entail higher risk and usually lead to more severe injuries (Hunger, 1995).

1.1 Long-distance travel and high-speed debris flow

The original morphology of landslide areas such as slope angle, mass volume, and debris flow path are controlling the landslide and its consequent behavior. The type of mass material, as well as the shape of landslide surface, are also effective in the post-failure behavior of moving masses and debris. For instance, in a clayey landslide, a creep-type movement at a low speed of about a few centimeters per year can continue for decades up to reach an equilibrium state (Shoaei et al., 2001). In small-scale low-to-moderate velocity landslides, the amount of the materials displacement is approximately twice the length of the displaced block, while in some large-scale landslides, this ratio may be as much as 20 times (Hunger et al. 2005). However, a special combination of original morphology and landslide mass materials, such as Ontake Landslide in Japan, can lead to high-velocity (about 10 m/s) landslides and debris flows over long distances (13 km far from the detached area), (Shoaei, Z., K. Sassa, 1994). Numerous studies have been carried out using laboratory models and field studies on the causes and the mechanism of debris-flow initiation.

1-2 Air Cushion and Internal Fluid Pressure

A variety of mechanisms have been proposed to explain the main causes of long travel distance of debris flow in different type of landslides (Bessette et al., 2020; Busslinger, M., 2010). According to a review of some works, when the volume of a landslide exceeds about 1 million cubic meters, rock particles and boulders bouncing and colliding with one other has caused significant changes in the detrital material. Thus, in some of these slips, the pressure inside the stream increased, therefore, the debris flow is transferred to a greater distance than expected. Despite numerous research efforts and models, a reliable method for accurate forecasting of the area covered by detrital materials to manage the crisis in susceptible areas has not been presented yet (Giardino, 2013).

The hypothesis of the influence of fluid pressure between debris materials in several landslide-initiated and debris-flow events has been investigated. In some of these researches, the hypothesis of debris formation above a water-saturated layer or on a cushion of high-pressure air has been proposed (Yueping Y., Aiguo, X. 2012). In investigating how the fastest types of landslides and the subsequent debris flows occur, perhaps the role of the mixed air pressure in the body of the debris and the formation of air cushions beneath the masses is the only mechanism that can justify the mechanism of these events. In Seimareh landslide (Figure 1- A) as one of the largest in the world that occurred in western Iran (Shoaei, 2014), a large-scale debris avalanche with a volume of 32 million cubic meters detached from the southern edge of Kabirkuh Mountain and flow along Pol-e Dokhtar plain, with a slope of lower than 5 degrees, up to a distance of 24 km. In this event, the only possible hypothesis to justify this long-range displacement of dry debris is the formation mechanism of cushion of air entrapped beneath the debris and the increase pressure of trapped air between the particles of debris. Another example of a long travel distance debris avalanche is the 1970 debris avalanche in (Figure 1-B) Huascaran, Peru, in which 100 million cubic meters of a mixture of rock, ice, snow, and mud entered the city in less than 3 minutes after passing a distance of 16 km. A densely populated area with an area of 8 square kilometers was buried under the debris. The average thickness of the material left was about 5 meters. A typhoon of a strong wave of compressed air happened at the forehead of the material with a thickness of about 80 meters (Martin, 2018). The proposed theories of long and unexpected travel distance of Elm and Blackhawk landslides (Figure 1-C) make the hypothesis of the generation of dense air mass under the detrital flow and the increase in fluid pressure more attractive and acceptable (Shreve, 1968; Nature, 1881).

In the mentioned examples of landslides and their relatively long travel distance debris, high-velocity flow, the condition of debris and size distribution and non-saturated condition could not be justified by the core principles of soil mechanics. Only the hypothesis of the formation of compressed air cushions underneath the debris as well as the increase in fluid pressure trapped between the particles can explain the detrital behavior (Nicholas, 1995h).

In such landslides, huge masses of rocks are separated from the upstream source and fall down the valley which has a significant height to the valley floor. The high speed of fallen material will result in air condensation beneath the debris. Under such conditions, a blanket of compressed air, which is up to a few meters, facilitates debris flow mounted on the compressed air cushion. The debris mass will continue to travel long distances unless this compressed air is drained out of the space underneath the debris until the debris stopped. In such landslides and debris flows, the hypothesis of air and water lubrication has been proposed in facilitating the movement; however, no significant research work has been conducted in this field. Although the lubrication hypothesis can be effective in terms of general scientific principles, this process may not be responsive to all major landslides and may not justify all occurrences. The main question is whether the air mass can remain beneath the debris layer for a long time and facilitate the movement?

1-3 Rock Jump Avalanches

In the process during which a rock block hit the floor of the valley, before debris flow initiation, part of the rock fragment will be transferred over distances and on the moving debris by throwing or jumping.

Little is known about jumping rock avalanches because their formation requires a special combination of slope topography and rock mass structure. For a jumping rock avalanche to occur, the slope of the rock mass near a rocky valley must be steep so that enough kinetic energy is generated to climb to higher altitudes and move longer distances after the rock block has fallen and hit the valley floor (Strom and Abdrakhmatov, 2018). Furthermore, rapid separation and the sudden collapse of the large block initiates kinetic energy that can be amplified by colliding with any hard obstacle and barrier in its path as well as its movement along the concave sliding surface results in jumping rock boulders similar to a springboard in the front part of the material. Due to the large size of the falling block, the materials at the top of the ridge have higher potential energy. There is no accurate and precise information about the processes and the precedence/latency of the formation of mass flow and jumping parts. However, the collision between high-velocity sliding mass or the falling block and the valley floor seems to precede the formation of the detrital flow. The materials of the lower portion of the block move in the form of a debris flow with less energy.

Based on a phone call report on the evening of April 1, 1998, to the Landslide Research Group of Soil Conservation and Watershed Management Research Institute (SCWMRI) of Iran, part of Mt. Kino had fallen down into the valley. In the dark of the night, there was no light or trace of Abikar village on the opposite bank of the Labad River, and it seemed that the village had been vanished. The geographic coordination of Abikar village was Lat. 32.6560 and Lon. 49.5637 longitude and 32º: 42' North latitudes. The next day (April 2), during an aerial survey by a helicopter, it was observed that the northern ridge on which the village of Abikar was located was completely buried under the debris of the landslide separated from the southern ridge. This is while the river path was not blocked by landslide debris, and there was a small flow of water at the bottom of the river.

A field research team from the Landslide Group of SCWMRI was immediately dispatched to the area. This team could access the landslide area two days after the incident (April 4). The first report from the research team indicated that on the evening of April 1, 1998, when the weather was relatively clear, the southern edge of the Kino Valley fell into the Labad River, causing a deadly landslide in which the debris composed of limestone, marl, and shale rapidly dumped into the river, and then, due to its high velocity, crossed the valley, along with part of the river sediments, and moved to the opposite bank to Abikar village (with geographical coordinates of 49º:34' east longitude and 32º: 42 North latitudes) that had been completely buried (Figure 2).

Chaharmahal and Bakhtiari province, with an area of 16,533 km², is a highland located in the west of the Iranian plateau. This province, one of the main national sources of water, is situated in the foothills of the Middle Zagros Range, western Iran. Based on the De Martonne (1941) climate classification, this province has a typical Mediterranean climate of humid/very humid, cold/very cold, and in some parts, moderately humid/very cold alpine winters. The type of precipitation in the province varies according to geographical location and altitude. In highlands, half of the annual precipitation is snow. The snowfall in the north of the province in different years has varied between 34 and 59 percent. Late melting of the snow cover in spring and the early onset of cold autumn are the characteristics of the region's climate. Prolonged rainfall resulted in advanced weathering, increased weight of the rock block, increased driving force, and reduced shear strength of interbedded shale and marlstone in mountainous regions. These are some of the intrinsic characteristics that increase potential for landslide occurrence in such climate.

According to a study on precipitation in Chaharmahal and Bakhtiari province, winter is the rainiest season, and autumn and spring have moderate amounts of precipitation. Summer presents the least rainy season of the year. In some exceptional years, long heavy rains in the region and exceptional day and night temperature differences result in interlayer ice freezing and thawing, triggering landslide occurrence.

According to Iran Meteorological Organization, heavy rains and exceptional showers have occurred in the northwestern region of Chaharmahal and Bakhtiari province, in the Labad valley area, southeast of Mt. Kino in three consecutive days (March 29 to 31, 1998), lasted a total of 10 hours and amounted to 190 mm, of which 146 mm rained in the last day. The intensity of this rainfall has been unprecedented over the past 50 years. Abikar landslide occurred on April 2, two days after the rainfall (Figure 2).

The damage caused by the landslide included 54 fatalities (20 males, 30 females, and 4 infants) out of the total population of 65. Eleven survivors were working in surrounding villages and cities. Loss of 1300 head of livestock and the destruction of 40 hectares of orchards and farmlands must be added to the damage induced by this landslide. The topographic feature of the landslide area is illustrated in Figure 3.

In Abikar landslide, the detached block of the opposite outcrop consisted of limestone, shale, and marl layers with a slope of 75 degrees. The ridge of the highest point of the sliding block is 2750 meters in altitude. The elevation of the bottom of the valley is 1600 a.s.l.. Thus, the height of fall was about 1200 meters (Figure 4).

Using the graphic method, the height of the center of gravity of the falling block was approximately 600 m from the bottom of the block. The falling block on the southern ridge has a remarkable slope difference with the ridge on which the Abikar village is located. Regarding the altitude, it is about 500 meters lower than the center of gravity of the sliding block. In this landslide, the volume of the separate mass was equal to 2.5 million m³. Considering of lithology of block that consisted of dolomite, lime and sandy argillaceous limestone the average bulk factor of 40% was considered for the total volume of the detached rock block (measured by Engineering Tool Box, 2009). Thus, the volume of the displaced rock was estimated to be around 3.5 million m³. The travel distance of the moving material was estimated to be 200 to 300 m. In addition, according to the estimated duration of the event, based on the interviews with residents and eye witnesses from surrounding villages, such landslides should be categorized as extremely high-speed landslides (Turner, and Schuster, 1996). The moving material crossed the river and ascended at an astonishing speed to a maximum height of 1700 meters over the opposite flank. The residents of upstream villages stated that it only took a few seconds from the time when they heard the loud sound of the block falling until the lights of the Abikar village went out. Based on such reports, the debris flow velocity is estimated to be 50 to 60 m/s, regarding the distance and a probable time (3 to 5 seconds), the area of about 70 hectares is under the influence of detrital materials. The thickness of the debris left in the village and the surrounding areas is about 15 meters.

The rescue team reported from helicopter, that there was no debris remained in the river, and no landslide dam or barrier was formed. Several landslides have been reported after any heavy rains in the area along the valleys of the Labad River. For example, a research in 1980 reported that several landslides occurred in Hayes and Atok valleys near MT. Kino, after long, heavy rains and damaged parts of the 400kV power transmission line in the region (Kheradmand, 2005).

2-1 Geology of the region

In general, a large part of western Iran is covered by the Zagros Mountain range. Labad valley and region are located in the High Zagros in the southwest of Zagros Range that is highly tectonically active (Tehrani, 2017), (Figure 5). Alternation of hard calcareous layers with soft and erodible shale and marl layers as well as active tectonic, the passage of several main faults in this region, are among inherent factors contributing to the occurrence of many landslides (Parsa et al., 2006).

Figure 6 illustrates a cross section of the landslide. The oldest geological formations in this section are thick limestones with Orbitolina (K) with locally evaporites interlayers in the lower part. In this formation, the K8 series with shale and marl with marly limestone interlayers containing ammonite and inoceramus fossils can be seen. Over this formation, radiolarite deposits, calcareous layers, and conglomerate (KP) are deposited. This formation has a less outcrop on the surface due to an unconformity resulting from a fault in the northern part of the valley. The green and gray colors of the Paleogene Formation are composed of Paleogene-Neogene limestones and dolomitic limestones (EO) of Paleogene Lime. This formation (E) consists of a red conglomerate, mainly composed of chert pebbles, sandstone, siltstone, and evaporates interlayers. In the northern part of the Labad Valley, the EO Formation of the Lower Paleogene-Neogene is repeated with white numulitic limestone and dolomitic limestone units. Figure 6 shows the K8 and K formations in this area are normally deposited, with no unconformities or faults. Moreover, in the southern part of the valley, the E and EO formations are in contact without any unconformities.

Conversely, due to the numerous faults that follow the fault trend of northwest-southeast, the K-series formation is located on EO, KP, and E with an unconformity. At the foot of Kino Mountain and under the hard limestone (K), the thin layer of shale and marl units of EO and KP) belonging to Pabdeh and Gurpi Formations (Upper Cretaceous-Paleocene), can be observed. The erosion of these soft layers can play the main role in the occurrence of various types of instabilities in the region. Furthermore, heavy rain and saturation of these soft formations can result in sliding surfaces between the hard and soft formations.

A characteristic of this region is the abundance of minor faults due to the activation of some main compression faults during several tectonic phases. The significant fault in the landslide zone is the Bazoft fault, which passes along Bazoft river. As mentioned, the Zagros region is known for its large number of low-intensity earthquakes. Naturally, the occurrence of numerous mild earthquakes during the rainy season can generally be considered an influential factor stimulating landslides (Khosrow Tehrani, 2008). However, seismic information studies showed that no significant earthquakes had occurred in the region in the week leading up to the landslide (IRSC, 1998).

In addition to the effect of heavy rainfall, the alternation of freezing and thawing of water inside the joint and other rock mass discontinuities at daily and seasonal intervals, the presence of karstic limestone, high weathering capacities of limestone shale and marlstone, and different erosion severities in hard and soft layers predispose this region to the occurrence of this type of landslide.

2-2 The mechanism of Abikar landslide occurrence

Landslides are one of the most common natural disasters through Zagros Range. A review of literature on Zagros region landslides indicates that the leading cause of relatively large landslides in this region is alternating eroded soft layers and hard thick layers and sliding surface occur along soft interlayered shale and marl. These soft layers have low shear strength in the saturated and partially saturated states. Therefore, rainfall can be considered the most effective triggering factor. Obviously, due to the seismicity of the Zagros region, in addition to rainfall, earthquakes can also be a factor that can trigger some landslides. The combination of rainfall and earthquake as landslide-triggering factors in landslides in Chaharmahal and Bakhtiari Province has also been reported (Zolnoor, 1994).

Seimareh historical landslide (Shoaei, 2014), western Iran, has an alternating thick layer of Asmari limestone of Upper Cretaceous-Eocene marls. However, some studies have considered heavy rains as the primary cause of landslides in Seimareh. In a newly designed model, given the landslide scale, the occurrence of this massive landslide without causing a severe earthquake has been reported as unlikely (Fomenko et al., 2021).

A study of the morphology of the Labad valley demonstrates that toe erosion provided suitable conditions for the fall of steep rock blocks due to its shale and marl bedrock. In addition to this effect, a saturation of marls and shales and a severe reduction in their shear strength can also lead to the occurrence of block falls. The same type of landslides in the basins adjacent to the Abikar landslide have been reported. A deadly landslide also occurred in Chelo village located in the same region following the persistent rainfall in the spring of 1993. This landslide occurred in a broad area, 2000 meters long and 1500 meters wide (Zolnoor, 1994). Other landslide events include dozens of landslides along the road under construction in Charmohal and Bakhtiari province during rainy seasons, have occurred due to the reduces the shear resistance of sand and marl geological formations (Khradmand, 2005).

Several reasons were proposed to interpret the mechanism of rapid landslides with high displacement. Among them, perhaps the most likely processes proposed for the occurrence of high-velocity landslides and debris flows are the presence of high-pressure air and water within the porosity of the debris flow and the fluid trapped between debris particles that facilitate the movement of unsaturated materials (Shoaei and Sassa, 1994).

Another case that can describe the mechanism of initiation and high velocity of debris flow is the formation of an air cushion underneath the falling debris. The compressed air mattress formed under the debris material prevents the material from colliding with the floor and the debris material mounted on the compressed air mattress travels long distances.

In such a process, given the height of the falling block, it is likely that some of the fragments gain height and jump to long distances by colliding with the hard floor or the upper surface of the compact air mass. An important feature of debris avalanche deposits is unsorted debris, high porosity, angular boulder, and rock fragments with scratch on their lateral surfaces and are usually impregnated and coated with muddy debris.

However, some part of the debris transported in the form of jumping is angular particles with fresh surfaces not contaminated with the muddy matrix of debris that fall on the surface of the displaced debris. A schematic presentation of the process of landslide occurrence based on field observations and evidence analysis is shown in Figure 7 and a close-up view of the Abikar landslide satellite image is shown in Figure 8.

Due to the extension of the river to erodible sediments EO and KP, erosion will likely intensify in flooding seasons. Surveys at the same elevation adjacent to the landslide area show ample evidence that toe erosion is a dominant morphology in the region. The toe erosion is more severe, especially in areas where the river is twisted. Therefore, toe erosion can be considered the first effective factor contributing to landslide potential in the Abikar area with an acceptable probability.

As shown in Figure 7-1, the presence of marl and shale layers under the hard limestone block and the tendency for landslides to occur at the point of contact due to rainfall and saturation can be a prelude to large slips in the area. Therefore, continuous heavy rains, alternations of limestone, shale, and marl layers, and high fragmentation of hard layers that provide sliding surfaces, can be a plausible hypothesis for the occurrence of the Abikar landslide.

Field evidence in the Labad area well demonstrates the accuracy of this mechanism. Due to the considerable base flow in the Labad River and the intense rainfall before the landslide, the accumulated bed sediments of the river should likely be fully saturated at the time of the landslide.

As mentioned in the previous section, the high velocity and field evidence observed in the landslide area and the area affected by the detrital flow, as well as the absence of rock debris in the river, indicate the formation of compact air mattress under the moving debris. The sudden fall of the rock block and the lack of sufficient time to drain the air trapped beneath the debris, as well as the increased fluid pressure inside the debris, can explain the long travel distance of debris flow and burial of the Abikar village under several meters of debris. A schematic presentation of the proposed mechanism is illustrated in Figure 7-2. Examining the debris flow texture and composition shows that most of the flow (as a high-velocity flow of high shear strength) is made of soft layers of EO and KP layers left in the village. Due to its strong shear force, this extraordinarily energetic rubble has scoured the village buildings and part of the thickness of the soil, on which the village was built, and carried them upstream. During field surveys, we found a complete 50-year-old walnut tree thrown to the end of the debris material (location is shown in Figure 8), indicating that the ground surface has been shaved to a depth of about 6 meters.

Another interesting piece of evidence for high-pressure air mattress formation is a storm surge caused by typhoons at the forehead of the moving debris, especially at the end of the displacement pathway, which has been cited in some studies (Yue Ping and Aiguo, 2012). As shown in Figure 9, the storm has exerted an impact due to the sudden discharge of air trapped under the debris up to several hundred meters above the moving debris, while no trace of debris flows has been observed in this area. The intensity of this storm is such that the total vegetation cover in the affected area was completely clear-cut to the extent of a few hundred meters from the end debris zone and bended without any traces of debris.

An examination of field information indicates that a significant part of the debris material has been transferred by jumping from the point of formation to the end of the debris flow zone. As mentioned in the previous section, the mechanism of debris avalanche transport is based on the energetic impact of the falling block to the bottom of the valley and the subsequent ascent with high altitude and displacement in the form of a jump. Evidence that confirms the occurrence of this mechanism is the presence of relatively large boulders at the end of the debris flow material on which there is no contamination of the muddy matrix of the debris flow on their surfaces.

This indicates their displacement by several hundred meters in a jump and then fall on the material that had displaced a few seconds before in the form of creep. The images in Figures 10 and 11 illustrate two examples of these boulders. The positions of these images are depicted in Figure 8. Some of these boulders have fallen after the debris flow has stopped completely (Figure 10), and others have fallen on it in the last seconds of the debris flow, implying the accumulation of the debris material behind these boulders (Figure 11).

Based on field studies and collected evidence, it can be concluded that the mechanism of landslide occurrence and the subsequent massive debris flow was an integrated mechanism of the occurrence of several events. First, due to heavy rainfall, the weight of the mass increased and perhaps the instability of the underlying marl layers caused a huge block to fall into the valley. The weight of the mass, the velocity of the fall, and the collision with the hard bottom of the river facilitated an extremely high-speed movement. This is why the compressed air under the mass did not have a chance to drain, and the air cushion facilitated the movement of materials to Abikar village on the opposite bank of the river. In initial stages, before the flow forms, the crown part of the block collided with the river floor with a different altitude of about 1000 meters. Due to intense kinetic energy, part of the material jumped on the Abikar area and the village and buried the village under a layer with a thickness of about 8 to 15 meters. The Abikar landslide event could not be named based on the conventional classification methods. In this paper, the Abikar landslide and its subsequent event was considered as an especial case and suggested to be named as "Rock Block Fall-Debris Flow-Debris Jumping Complex (Composite) Landslide."

Based on the lessons learned from Abikar landslides, it can be concluded that a similar event is expected to occur in areas with the alternation of soft and hard layers and large differences in height and slope angle of both flanks of the valleys. Any development plan should be avoided in such areas. Installation of any lifelines and any passage should be planned with special caution. We also need to formulate a plan to relocate the villages to safer places.

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Deadly Landslide and Debris Avalanche in Abikar Village, Farsan City, Chaharmahal and Bakhtiari Province, Iran.

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1- Introduction

2- Abikar Landslide And Debris Avalanche

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