Landslides, a recurrent phenomenon

by George D. BATHRELLOS & Hariklia D. SKILODIMOU

What is a Landslide?

Landslide is a general term used to describe the down-slope movement of soil, rock, and organic materials under the effects of gravity and also the landform that results from such a movement (Highland & Bobrowsky, 2008).

Gravity affects all objects on Earth. From the largest mountains to the smallest grains of sand, gravity pulls everything in a direction toward the center of the planet (Nagel, 2004). Mass movements are the result of shear stresses on slopes (caused by gravity, the weight of material and soil water) overcoming the inherent resistance of the materials (made up of the cohesive properties of slope particles and their internal friction) (Hart, 1986).

In general, the scientists use many terms worldwide for this phenomenon, such as "landslide", "landslide events", "mass movements", "gravity movements" and "slope movements" (Terzaghi, 1950; Coates, 1977; Varnes, 1978).

What does Landslides cause?

Gravity The main force which is responsible for mass movement is the gravity. Gravity is the force that acts everywhere on the Earth's surface, pulling everything toward the center of the Earth. On a flat surface the force of gravity acts downward. So long as the material remains on the flat surface (Fig. 1) it will not move under the force of gravity (Nelson, 2013).

On a slope, the force of gravity can be decomposed into two components (Fig. 2): a component acting parallel to the slope and a component acting tangential to the slope (Nelson, 2013).

Fig. 1: Influence of gravity on an object to a horizontal surface (modified from Nelson,

2013).

• The vertical component of gravity (gp) helps to hold the object in place on the slope. The parallel to the slope component of gravity (gt) causes a shear stress, which pulls the object in the down-slope direction parallel to the slope.

• On a steeper slope, the shear stress or parallel to the slope component of gravity, (gt) increases, and the vertical component of gravity (gp) decreases.

• The forces resisting movement down the slope are grouped under the term shear strength, which includes frictional resistance and cohesion among the particles of the object.

• When the shear stress becomes greater than the combination of forces holding the object on the slope, the object should move down-slope.

• Otherwise if the object consists of a collection of materials like soil, clay, sand, etc., if the shear stress becomes greater than the cohesion forces holding the particles together, the particles will separate and move or flow down-slope (Nelson, 2013).

Thus, down-slope movement is favored by steeper slope angles (BUT, be careful, not the very steep slope angles more than 60o), that increase the shear stress, and anything that reduces the shear strength, such as lowering the cohesion among the particles or lowering the frictional resistance. But this is often expressed as the safety factor (Fs) the ratio of shear strength to shear stress.

Fs = Shear Strength / Shear Stress

Shear strength consists of the forces holding the material on the slope and could include friction, and the cohesion forces that hold the rock or soil together. If the safety factor becomes less than 1.0, slope failure is expected (Nelson, 2013).

Fig. 2: Influence of gravity on an object on an inclined level (modified from Nelson,

2013)

Water Water becomes an important factor in mass movement processes, as the medium of transport, for several reasons:

1. Addition of water from rainfall or snow melt adds weight to the slope. Water can permeate into the soil or rock and replace the air in the pore space or fractures. Since water is heavier than air, this increases the weight of the soil. Weight is force, and force is stress divided by area (expanse), so the stress increases and this can lead to slope instability (Nelson, 2013).

2. Water has the ability to change the angle of repose, which is the stable angle for the slope (Nelson, 2013).

• Dry unconsolidated grains will form a pile with a slope angle determined by the angle of repose (Fig. 3). The angle of repose is the steepest angle at which a pile of unconsolidated grains remains stable, and is controlled by the frictional contact between the grains. In general, for dry materials the angle of repose increases with increasing grain size, but usually ranges about 30o - 45o.

• Slightly wet unconsolidated materials display a very high angle of repose (Fig. 4), because surface tension between the water and the solid grains tends to hold the grains in place.

Fig. 3: Dry unconsolidated grains (modified from Nelson, 2013)

• When the material becomes saturated with water (Fig. 5), the angle of repose is reduced to small values and the material tends to flow like a fluid. This happens because the water gets between the grains and eliminates grain to grain frictional contact.

3. Water can dissolve the mineral cements that hold grains together. If the cement is made of calcite, gypsum, or halite, all of which are very soluble in water, water

Fig. 4: Slightly wet unconsolidated materials (modified from Nelson, 2013)

Fig. 5: Slightly wet unconsolidated materials (modified from Nelson, 2013)

entering the soil can dissolve this cement and thus reduce the cohesion between the mineral grains (Nelson, 2013).

4. Liquefaction is occurred when loose sediment becomes oversaturated with water (Fig. 6) and individual grains loose grain to grain contact with one another as water gets between them. This can be occurred as a result of ground shaking, as in the case of earthquakes, or can be occurred when water is added as a result of heavy rainfall or melting of ice or snow. It can also be occurred gradually by slow infiltration of water into loose sediments and soils (Nelson, 2013).

5. Groundwater exists nearly everywhere beneath the surface of the earth. It is water that fills the pore spaces between grains in rock or soil or fills fractures in the rock. The water table is the surface that separates the saturated zone below, wherein all pore space is filled with water from the unsaturated zone above. Changes in the level of the water table is occurred due changes in rainfall. The water table tends to rise during wet seasons when more water infiltrates into the system, and falls during dry seasons when less water infiltrates (Fig. 7). Such changes in the level of the water table can have effects on the factors discussed above (Nelson, 2013).

Fig. 6: Liquefaction (modified from Nelson, 2013)

Other parameters Many other factors, that cause mass movements (landslides), are reported in the scientific literature (e.g. Bathrellos et al., 2009; Rozos et al., 2011). Some parameters are: • Lithology. Lithology is one of the most decisive parameters regarding the

landslide manifestation. In Greece the landslides density higher percentage is attributed to Cyclothematic formations (Plio- Pleistocene fine-grained sediments and Flysch formations).

• Land uses. The variation of the vegetation in an area is a parameter that seriously affects the slope failures, as slope stability is very sensitive in changes on vegetation.

• Tectonic elements. Tectonic activity increases landslide events by creating steep slopes and sheared, weakened rocks.

• Slope angle, • Slope aspect. The angle and the aspect of the slopes play a very important role in

the manifestation of the landslides because they express the result of the combined influence of many agents.

• Geometry of main discontinuities. The geometry of the main discontinuities in relation to slope geometry (aspect) is strongly related to the stability of hard soils, and soft rocks.

• Anthropogenic activities (e.g. roads). The artificial and natural parts of the slopes around a road are more sensitive in landslide manifestation.

• Hydrographic (drainage) network. The hydrographic axes continuously change the slopes of the rivers and can therefore be considered as one of the principal parameters in landslide manifestation.

Triggering factors The usual factors which trigger landslides are intense rainfall, earthquakes and human activities (Bathrellos et al., 2009; Rozos et al., 2011; Chousianitis et al., 2016).

Fig. 7: Changes in the level of the water table (modified from Nelson, 2013)

Which are the mass movements (landslides) types?

The term "landslide" describes a wide variety of processes that result in the downward and outward movement of slope-forming materials including rock, soil, artificial fill, or a combination of these. The materials may move by falling, toppling, sliding, spreading, or flowing (USGS, 2004). A great number of scientists have given the classification of landslides. The worldwide most used classification is that USGS (2004) and Varnes (1978) have proposed (Fig. 8).

Fig. 8: Types of landslides (modified from USGS, 2004 & Varnes, 1978)

According to Varnes (1978) the types of movements are: fall, topple, slide, spread and flow. Falls take place when rocks break off and material free-falls or bounds down to the base of a cliff. Topple is movement that is performed by the rotating of a unit or units around a point. In Slides cohesive blocks or material remain relatively intact, moving along a well-defined surface of sliding. Spread is lateral extension together with shear or tensile fractures. Flows are a fluid movement of loose earth material.

Which are the parts of a landslide? Landslide features are presented in figure 9, with the commonly accepted terminology, according to USGS (2004) and Varnes (1978).

Recorded landslide phenomena in Hellas

The affected geological formations from landslide phenomena in Hellas showed that flysch is the most critical prone geological formation while most landslide occurrences are mainly located in the western part of the country, where serious causing factors and triggering effects favor the manifestation of such phenomena on a broad scale (Koukis et al., 2005). Landslides are usual in Flysch formations (30.35%), Neogene sediments (28.20%) and loose Quaternary deposits (20.65%) (Koukis et al., 2005). In western Greece, landslides are common because of intense rainfall. Landslide events from Hellas are shown in the following Figures 10-12.

Fig. 9: A graphic illustration of a landslide, describing its features (http://www.oas.

org/dsd/publications/Unit/oea66e/p305.GIF).

Fig. 10: Disturbed - mingled material from a landslide near Pyli town (Koziakas Mt,

Central, Hellas).

Fig. 11: Debris falls in Koziakas Mt, Central Hellas.

Landslides are independent or related phenomena?

Generally, landslides are contemporary or related phenomena with other natural catastrophes (disasters). The landslides events usually accompany or / and follow other catastrophic events, as earthquakes, intense rainfall, erosion, etc (Fig. 13-16).

Fig. 12: Landslide in flysch formations in Koziakas Mt. Central Hellas.

Fig. 13 Erosion and landslide in Egnatia Odos (northern Hellas).

Fig. 14: Erosion and landslide near Agios Konstantinos town (Old Highway Athens-

Thessaloniki, Central Greece).

Fig. 15 An earthquake induced landslide in Kythira Island, southern Greece (photographs

by Efthymios Lekkas).

Is there continuity or repetition in landslide phenomena?

Landslide phenomena are usually characterized from continuity or repetition (Varnes, 1984; Lee, 2014; Bathrellos, 2014). Recurrence of landslides is controlled by the repetition of triggering factors (rainfall, earthquakes, etc). The characteristic phenomenon of landslide recurrence is recorded in Ropoto village (Trikala Prefecture, Central Greece). Landslides events have been catalogued in the winters of the years 1962–1963 (Dounas, 1963), 1975–1976, 1978–1979 (Κoukis, 1979), 1990–1991 (Aggelidis, 1991), 1993 – 1994 (Apostolidis, 1994), 2009-2010 (Bathrellos, 2014; Mpellas & Tsioumas, 2011), and 2011-2012 (Bathrellos, 2014). The reactivation of landslides is visible in the following figures 17-26.

Fig. 16: The 1999 multi-hazard event in Tanaguarena, in coastal Venezuela, South America. The floods and landslides were triggered by heavy rains. (photograph by

Matthew Larsen, U.S. Geological Survey).

Fig. 17: Landslide in Ropoto village (Trikala Prefecture, Central Greece) in winter of

1978–1979 (source: Κoukis, 1979).

Fig. 18: Landslide in Ropoto village in winter of 1978–1979 (source: Κoukis, 1979).

Fig. 19: Landslide in Ropoto village in winter of 1993 – 1994 (source: Apostolidis, 1994).

Fig. 20: Landslide in Ropoto village in winter of 2009 – 2010. In the small photograph we

can see the mitigation measures that the habitants took in the beginning of the event.

Fig. 21: Landslide in Ropoto village in winter of 2009 – 2010.

Fig. 22: Landslide in Ropoto village in winter of 2009 – 2010.

Fig. 23: Landslide in Ropoto village in winter of 2009 – 2010.

Fig. 24: Landslide in Ropoto village in winter of 2011 – 2012.

Landslide hazard and risk

Landslides, floods, drought, wildfire, storms, tsunamis, and earthquakes continue to take a heavy toll in lives and infrastructure. In the past decade, disasters have killed over 750,000 people and caused damage costing hundreds of billions of dollars (Centre for Research on the Epidemiology of Disasters - CRED, 2011). Most life losses were the result of earthquakes, tsunamis, and tropical storms; loss of lives from landslides was only a small percentage of the total. Landslides are one of the most damaging and deadly of natural hazards. Nevertheless, landslides are responsible for much primary and secondary economic damage – in the USA alone; landslides cause damage costing $1–2 billion and more than 25 fatalities each year (USGS, 2011). As the global population continues to increase, more people will be at risk from landslides. Even small slope failures are a threat to transportation infrastructure, as

Fig. 25: Landslide in Ropoto village in winter of 2011 – 2012.

Fig. 26: The evolution of landslide in Ropoto village from the winter 2009-2010 until the

winter 2011 – 2012.

they disrupt the movement of goods and are costly to clear. However, the economic costs of landslides are not limited to roads and railways. Underwater landslides have destroyed coastal infrastructure; catastrophic landslides may enter the sea and lakes, triggering destructive tsunamis; landslides have blocked rivers, producing upstream flooding and reservoirs, which are subject to sudden emptying and resulting downstream floods; and landslides may enter settled areas, causing death and injury (Clague & Stead, 2012). Each year, landslides are responsible for hundreds of millions of dollars’ worth of damage and, on average, claim more than 1,000 lives around the world (Clague & Roberts, 2012). Although most common in mountainous areas, landslides can occur anywhere with enough local relief to generate gravitational stresses capable of causing rock or soil to fail (Bathrellos, 2005; Bathrellos et al., 2009; Rozos et al., 2008; Rozos et al., 2011). Data from the Centre for Research on the Epidemiology of Disasters (CRED) suggest that landslides were responsible for over 10,000 deaths and left 2.5 million people homeless over the past decade (2001–2010) (CRED, 2011). However, the true loss of life and incidence of injury may be much larger, due to the under-reporting of small events in many parts of the world, the exclusion of events in the database that are below predefined loss thresholds, and the misattribution of some landslide events to the seismic or hydrologic events that triggered them. Therefore, assessment maps are developed in order to estimate the natural hazard. These maps are useful in land-use planning and mainly in urban design (Bathrellos et al., 2012; Bathrellos et al., 2013; Papadopoulou-Vrynioti et al., 2013; Youssef et al., 2015).

What are the effects and consequences of landslides?

Landslide effects occur in two basic environments: the built environment and the natural environment. Sometimes there is an intersection between the two; for example agricultural lands and forest lands that are logged (Highland & Bobrowsky, 2008). Landslides affect manmade structures (roads, settlements etc) whether they are directly on or near a landslide. Residential dwellings built on unstable slopes may experience partial damage to complete destruction as landslides destabilize or destroy foundations, walls, surrounding property, and above-ground and underground utilities (Highland & Bobrowsky, 2008). Landslides have effects on the natural environment: • The morphology of the Earth’s surface - mountain and valley systems, both on

the continents and beneath the oceans; mountain and valley morphologies are most significantly affected by downslope movement of large landslide masses;

• The forests and grasslands that cover much of the continents; and • The native wildlife that exists on the Earth’s surface and in its rivers, lakes, and

seas (Highland & Bobrowsky, 2008).

Exercises

1. Identify and draw the rock and debris falls on the satellite image in figure 27. The satellite image was taken from Google Earth. The study area is located in Koziakas Mt (Trikala Prefecture, Central Greece).

2. Evaluate and draw the possible initial (starting) position of rock and debris on the same satellite image.

3. In what type of geological formations the rock falls are usually manifested? 4. In the satellite images of Figure 28, an area is illustrated, before and after a big

landslide. The first image (Fig. 28a) was taken in 13-08-2007 and the second (Fig. 28b) in 27-09-2013. In photograph of Figure 29 this landslide is visible.

Fig. 27: Rock and debris falls from a satellite image (source Google Earth).

Fig. 28a: Rock Satellite image of an area before a large landslide - Ropoto, Trikala

Prefecture, Central Greece - (source Google Earth - 13-08-2007).

Fig. 28b: Satellite image of the same area after a large landslide (source Google Earth -

27-09-2013).

Fig. 29: Photograph of the landslide in Ropoto, Trikala Prefecture, Central Greece (taken

on 25-05-2014).

5. Draw the landslide outline on the satellite image of figure 28b. 6. On the same satellite image, draw

• the "surface of separation", • the "zone of depletion", and • the "zone of accumulation" of the landslide.

7. From the photograph of figure 29, evaluate how many stages of landslide evolution we can record.

8. Draw on this photograph the stages of landslide manifestation. 9. In the satellite image of figure 30, recognize and draw at least one active

landslide and one palaeo-landslide (inactive landslide). Use the differences in land uses (vegetation cover) and the characteristic shape of the landslide.

10. In the longitudinal section of figure 31, draw the shape of a possible future landslide. Mark the zones of depletion and accumulation.

Fig. 30: Satellite image in which an active landslide and an inactive landslide are visible

(source Google Earth - 18-07-2015).

11. Which should be the impacts to the settlement in the section of figure 31? 12. Calculate the slope of the hillside, if the altitude difference between the y and

y' is 235m and distance between the Δ and A is 400m. 13. In the following sketch of figure 32, determine (i) what potential mass

movement hazards exist, (ii) why these hazards exist, and (iii) what might be done to mitigate the hazard or remove the danger (Nelson, 2013).

Fig. 31: A longitudinal section with a possible future landslide.

Fig. 32: Potential mass movement (source Nelson, 2013)

14. In the following sketch of Figure 33, determine (i) what potential mass movement hazards exist, (ii) why these hazards exist, and (iii) what might be done to mitigate the hazard or remove the danger (Nelson, 2013).

15. In the following sketch of figure 34, determine (i) what potential mass movement hazards exist, (ii) why these hazards exist, and (iii) what might be done to mitigate the hazard or remove the danger (Nelson, 2013).

Fig. 33: Potential mass-wasting (source Nelson, 2013)

Fig. 34: Potential mass-wasting (source Nelson, 2013)

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Web pages

1. Landslide in La Conchita 2005 (source USGS) Quicktime Video of the La Conchita slope failure in 2005 (MOV 1.1MB)

2. Landslide videos (source AGU) http://blogs.agu.org/landslideblog/category/landslide-video/ http://blogs.agu.org/landslideblog/category/landslide-video/page/2/ http://blogs.agu.org/landslideblog/category/landslide-video/page/3/ http://blogs.agu.org/landslideblog/category/landslide-video/page/4/ http://blogs.agu.org/landslideblog/category/landslide-video/page/5/

http://blogs.agu.org/landslideblog/category/landslide-video/page/6/ http://blogs.agu.org/landslideblog/category/landslide-video/page/7/

3. Landslides - Natural Disasters and Hazards - Mudslides (source National Geographic) http://video.nationalgeographic.com/video/101-videos/landslides

4. Landslide in French Alps 19-04-2013 (source YouTube) https://www.youtube.com/watch?v=SlGTirtRP4c

5. Biggest landslides worldwide (source YouTube) https://www.youtube.com/watch?v=iciLigLgsWc

6. Three interesting new landslide videos from Colombia, China and Turkey (source AGU) http://blogs.agu.org/landslideblog/2016/06/05/hechi-1/

7. North Cliffs Failure - Amazing Cliff Collapse caught on Camera! (source YouTube) https://www.youtube.com/watch?v=ZVjr4mii3cE

8. Cliffs of Moher Collapse 17th Sept 09 (source YouTube) https://www.youtube.com/watch?v=Id3HFxs2A8w

10. Larger than Life Landslide (source YouTube) https://www.youtube.com/watch?v=K5pidzTZslo

11. Biggest landslides worldwide. Part 2 (source YouTube) https://www.youtube.com/watch?v=r4_yhypLuVQ

12. World's Most Killer Landslides ever Recorded (source YouTube) https://www.youtube.com/watch?v=VJ_ykqJuGI0

13. Massive Sink holes caught on camera (source YouTube) https://www.youtube.com/watch?v=ZEf6oGOuagE

14. Massive landslides caught on camera 2 (source YouTube) https://www.youtube.com/watch?v=H1Sq02QVEag

15. Massive Landslides caught on camera 4 (source YouTube) https://www.youtube.com/watch?v=aIB_Cd3JzcA