LITERATURE REVIEW

 

According to Keller and Blodgett (2004), landslides are an important hazard around the world and they are often connected with other hazards such as volcanic activity or earthquakes. The majority of landslides are small and slow but some are fast and big with catastrophic consequences (Keller and Blodgett, 2004).

 

Turner and Schuster (1996) define five types of movement in landslides; fall, topple, slide, spread and flow.

 

a) Fall is a vertical movement extremely or very rapid and can be in rocks or debris. This type has low moisture (Summerfield, 1991).

b) Topple is a rotation out of a mass of soil or rock about a point or axis below the center or gravity of the displaced mass.

c) Slide is a downslope movement of soil or rock mass where there is a rupture of the surface. There are two types of slides, rotational or translational and the moisture is low or moderate (Summerfield, 1991).

d) Spread is a movement of cohesive soil or rock mass over a material unstable. The moisture is moderate to high and this movement is very complex due to the different materials and the water interaction.

e) Finally, Flow is a movement of no consolidate materials and this movement can be fast or slow depends of the materials and the moisture (Summerfield, 1991).

 

In line with Keller and Blodgett (2004), there are five factors in the landslides; materials, slope, climate, weather, vegetation and water. These factors may act together or separately and they have different consequences in the landslides.

 

Geological materials may affect the type and frequency of the landslides. The composition and consolidation of the materials are two factors very important in the stability of the soil mass and rocks. Slides should be controlled by the geological materials (Keller and Blodgett, 2004).

 

Inclination of the slope is important as well because in general, when the slope is bigger the driving force is higher. If there is a high slope, fall is the more usual type and when the slope is low flows are the most frequent (Keller and Blodgett, 2004).

 

Climate is different around the world and it has an important role in superficial landslides mainly. In arid climates where vegetation is sparse, landslides are frequent fall, debris flow and superficial landslides because soil mass and rocks are exposed to high erosion however, humid and sub humid areas have abundant vegetation and soil mass cover the most part of the slopes and the landslides are more complex (Keller and Blodgett, 2004).

 

Weather can be a complex problem in the different types of landslides. Extreme weather events such as high intensity rainfall or long periods of rain may produce landslides dangerous (Keller and Blodgett, 2004).

 

Landslides cannot be explained without the water role because the most landslides are linked with this element directly or indirectly with a relevant role in landslides. Water can affect landslides in many ways: rain events, infiltration in the slopes or erosion in the slope especially in the base (Keller and Blodgett, 2004).

 

The role of vegetation has two opposite effects in landsliding. On the one hand, the vegetation covers the soil and produces less erosion with the roots providing stability and cohesion in the materials of the soil mass but, on the other hand, vegetation adds weight in the slope and may result in loss of stability and produce a landslide (Keller and Blodgett, 2004).

 

The factors above are crucial and the main cause of the landslides but other hazards play an important role over the landslides. Storms, forest fires, earthquakes or volcanos are the cause of dangerous landslides around the world (Keller and Blodgett, 2004), in fact in some places such as Hawaii and the Canary Islands the landslides related to volcanic activity and earthquakes have produced mega-landslides1 or submarine mass movements (Whelan and Kelletat, 2003). Also, the mega-landslides may produce tsunamis, which can affect areas away from the landslide area. Güímar landslide in Tenerife (Spain) produced a big tsunami which affected to Gran Canaria Island (Giachetti et al., 2011).

 

Landslides have an impact over the landscape of the islands affecting to human population distribution, vegetation or stream network (Villalba, 1996) although the human activity may produce landslides caused by deforestation or urban expansion (Keller and Blodgett, 2004). Also, there is a relation between the evolution of the landscape and the landslides caused by the erosion of human activity and natural factors (Korup, 2009).

 

Cendrero and Dramis (1996) explain that in Europe the contribution of the landslides in the landscape has been important in the actual relief. In the Alps different types of landslides such as rock and debris falls and avalanches, debris and earth flow and slides have created important slopes and a singular mountain relief. Landslide triggering factors have been affecting this area (climate, rain events, erosion…). In Calabria (Italy); slope, extreme meteorology and geological materials on the one hand, and type and size of the landslides in the other hand, have created a singular relief as well.

 

In Spain, there are two important areas where landslides have a relevant role in the landscape; the Cantabrian Cordillera and the Canary Islands. In the Cantabrian Cordillera the climate and geological materials are the most important factors in landslides while in the Canary Islands, volcanic activity is in some cases, the main factor of the origin of landslides which produce landscape evolution and relief degradation. Tenerife, La Palma and El Hierro have large valleys and slopes created during the Quaternary which have created a new landscape. These geomorphological elements affect the vegetation distribution and the land occupation in these areas (Cendrero and Dramis, 1996).

 

The affected area for landslides in the Canary islands is 1762.31 Km2 and 750.2 Km2 is in Tenerife (Carracedo et al, 2009 and Canals et al, 2000) and several authors have described that landslides can create a new landscape and produce changes in the area affecting the vegetation distribution, stream networks and land use.