ANALYSIS AND RESULTS
i. Geomorphological recognition
Seven landslides were found in the geomorphological field reconnaissance, and there are some differences between the landslides as predict in the methodology.
Figure 6. Landslides in Tenerife (Data source: GRAFCAN).
Güímar and La Orotava were very easy to identify because these landslides have a clear break in the slope in the top and the sides of the landslide area like the figure 7 shows. High backscar and cliffs are the main evidence of these landslides.
Icod and Roques de García are overlapping in a big part of the top and in the limits and also, insensitive volcanic activity after the landslides has covered the landslides areas. Anyway, the cliffs in the top are clear (fig. 8) and demonstrate landslides activity in the area.
Teno and Anaga landslides have an intensive fluvial erosive process (fig. 9) with a lot of small valleys and apparently with a complex hydrographical network, but landslide edges still can be located in the area. Also, Teno landslide area has a post-landslide volcanic activity in the area which has created a new land (fig. 10).
Figure 7. Landslide of Güímar. Coordinates: 28º 29’ 45” N, -16º 40’ 43” W. Distance: 15 km across. Figure 8. Cliffs on the top of the landslides of the North. Coordinates: 28º 25’ 76” N, -16º 61’ 07” W. Distance: 5.8 km across. Figure 9. West cliffs of Anaga landslide. Coordinates: 28º 55’ 01” N, -16º 20’ 56” W. Distance: 2.6 km across. Figure 10. New land in Teno landslide area. Coordinates: 28º 36’ 18” N, -16º 80’ 26” W. Distance: 6.2 km across.
Finally, the landslide Bandas del Sur does not have clear boundaries but some cliffs and mountains to the limits to the west and east were found (fig. 11) which could be evidence of landslides. In this case we have to contrast the information of this step with the next to confirm the landslide area.
Figure 11. Bandas del Sur landslide area. Coordinates: 28º 11’ 38” N, -16º 65’ 89” W
Figure 12. Data sheet of the routes in Tenerife.
ii. Location and definition of the landslides
Landslides in Tenerife have been located and defined in this part. Bandas del Sur landslide area has been done as well according to the information of the different authors and the information obtained from the last step and the analysis of the geological map, orthophotos and 3D vision in GRAFCAN but this landslide still isn’t clear.
Location and definition of the landslide in El Hierro has been done with geological map, orthophotos and 3D vision and the rest of the islands expect Fuerteventura and Lanzarote because there are not geomorphological evidence in the relief (Cendrero, 1996)., but there were landslides in the past according with the barometry researches of the sea floor (Canals, 2000 and Masson et al., 2000). The erosion processes in Fuerteventura and Lanzarote have deleted the backscar in the land so located or definite the area of the landslides is not possible.
iii. Physical variables. Slope and hydrology network
Physical variables such as slope and hydrology network could be the key to understand and describe the landslides processes in Tenerife and El Hierro, and land use could be conditioned by these physical variables.
a. Slope
High slopes in the relief could be evidence of the landslides in Tenerife. In fact, in the geomorphological field reconnaissance, backscar and high slopes have been found and slope maps should show this. Therefore, one objective of this research is analysis the slopes as important physical variable in the landslides.
Tenerife has a high slope around the island but the main slopes are in the landslide areas and volcanos (fig. 13).
Figure 13. Slope map of Tenerife (Data source: USGS).
In the slope map of Tenerife intensive erosion processes affect the slope in two areas of the island and clear evidence of landslides as backscar and high slopes can be seen in the rest of the landslide areas expect Bandas del Sur where boundaries cannot be defined.
The landslides can be classified in four groups according with the slope characteristics:
- Landslides with intensive erosion process: Teno and Anaga
- Landslides with borders and clear landslide evidence: La Orotava and Güímar
- Landslides with overlapping: Icod and Roques de García
- Landslides without clear landslide evidence: Bandas del Sur
Teno and Anaga
Teno and Anaga have an intensive erosion processes with similar characteristic in age and geological materials, this is evident in the slope maps of both with high variations in the slope.
In Teno, post-landslide volcanic activity is evident in the map where the volcano is in the middle of a big plain (fig. 14) (Amalia, 1998).
In both landslides areas of Teno and Anaga, middle volcanic axis determinates the top of the landslide. In Teno this axis is NW – SE (fig. 14) and in Anaga is NE – SW (fig. 16). Both landslides present similar characteristics but the different is that Teno landslide area has been affected by volcanic activity.
Figure 14. Slope map of Teno. (Data source: USGS).
Figure 15. 3D map of Teno (Data source: USGS).
Figure 16. Slope map of Anaga (Data source: USGS).
La Orotava and Güímar
La Orotava and Güímar landslides have formed valleys (fig. 17) and edges of landslides can be recognized clearly on the most of the area. In the landslide of Güímar, there are three important geomorphological elements: historic volcanic activity, depressions and backscars. The historic volcanic activity is located in the SE of the landslide area, and new land has been created. The depressions are in the south of the map landslide area. Finally, backscars are in the boundaries across south and north mainly of the landslide (fig. 18).
Slope in both landslides is very weak inside of the landslide areas and very high in the limits of the landslides. This permits a good location and defination of the landslide area.
In the landslide of La Orotava, the top part of the landslide has been affected by overlapping with the Icod landslide. This is a problem when the limits of the landslide have to be determined (fig. 20).
Finally, the central land in the landslide area is very plain and it could be favourable for the development of urban centers, agriculture and industrial areas which will be analysed in the land use section.
Figure 17. Slope map of Güímar (Data source:: USGS).
Dissertation in Geography
University of Plymouth
2014
Figure 18. 3D map of South-east part of the Güímar landslide (Data source:: USGS).
Figure 19. Depressions in the landslide of Güímar. Coordinates: 28º 29’ 45” N, -16º 40’ 43” W. Distance: 10.2 Km across
Figure 20. Slope map of La Orotava. (Data source: USGS).
Icod and Roques de García
The landslides of Icod and Roques de García are the biggest in the area affected (fig. 38). Both landslides are situated in a special area with a high volcanic activity where El Teide is with an elevation of 3.718 m. This volcanic activity has been a post-landslide event and the landslides area overlapping (fig. 21). Therefore, evidences of landslides are very difficult to find (fig. 21).
Figure 21. Slope map of Icod and Roques de García (Data source: USGS).
However, the limits in the top of the landslides are very clear as shows the figure 8 and it allows us to see that in this area of the island have been significant landslides. Also, according to Masson et al. (2002), evidence of the landslides can be seen in the ocean topography especially in Tenerife, El Hierro and La Palma and defined the limits of the affected area of the landslides.
The new volcanic activity in Icod and Roques de García landslides have modified the slope of this area after the landslide episodes, also the hydrological network is being affected for the new volcanic activity. El Teide is playing a very important role in these landslide areas.
Lastly; in the coastline of Los Roques de García landslide there is a high slope affecting the landslide area (fig. 22). Ocean erosion in the coast is playing an important role but some researches have explained the uplift in Tenerife (Maurer, 2008) and in the Canary Islands in general (reference) and it is contributing to the elevation of the coastline. According to Maurer et al. (2008), the elevation in Playa de Gordejuela which is close to the cliffs that we are studying, has increased 18.5 metres above the present sea-level since last 0.6 ma, and the uplift is highest in the south of the island with 35 m in Montaña Pelada. Also, Playa de Gordejuela was 20 m up to the present sea level between <0.54 – 0.69 ma and the landslide of Los Roques de García was 0.6 ma. This uplift processes could be affected the area after the island lost the materials due to the less weight and therefore, the elevation of this part of the island increase. Uplift processes could be affecting to the other landslides as well. Additionally, the uplift in the island could be 8 m over the present sea-level according with the Mio-Pliocene marine deposits although this elevation is being produced for a regional uplift between all islands instead to the local uplift agreeing with Meco et al. (2007).
Figure 22. Slope map of cliffs in the coast of Los Roques de García landslide (Data source: USGS).
Other important element, that can explain this cliffs, is the erosive processes in the gap-stage between the formation stage and volcanism post-erosive stage. Gap – stage is when the island has not volcanic activity and the erosive processes can be more effectives.Therefore, the uplift and the erosive processes in this part of the island have produced these unusual cliffs in the coastline of the landslide area (Carracedo, 1998).
Bandas del Sur
Bandas del Sur landslide has not clear limits, and only small cliffs and elevations have been observed . This is matching with the mountains that in the geomorphological recognition were found (fig. 11). This cliffs and mountains could be other geomorphological features so probably this landslide has incorrect boundaries.
In figures 23 and 24, boundaries cannot be defined clearly but some volcanic activity has been found as other landslides. In the different landslides around the island, volcanic activity is important in the origin of the landslides, but also in the evolution of the landslides geomorphology and in the influence over the landscape.
Figure 23. Slope map of Bandas del Sur (Data source: USGS).
Figure 24. 3D map of Bandas del Sur landslide area (Data source: USGS).
The age of this landslide is the same that Güímar landslide consequently the erosion processes are not the cause of the footprints of the landslides. This landslide could be other main factor or it has been very weak in comparison with the other landslides in the island.
Bandas del Sur and Güímar have the same age consequently, if there are clear backscar in the landslide area of Güímar, Bandas del Sur should shows backscar as well but this is not happen. Therefore, this landslide could be weak in comparison with the rest of the landslides in the island and backscar cannot be found because the erosive processes have deleted the landslide evidence.
In brief; Slope presents relevant information about the backscar and boundaries of the different landslides. Also, volcanos have been observed and they play an important role in the influence of the landslides and in the consequences that landslides have over the landscape in the island. Bandas del Sur landslide is still not clearly defined and the cliffs in Los Roques de García and the depressions in Güímar are very important points to analysed in the next sections.
b. Hydrological network
Hydrological network is the second element in the physical factor analysed. Landslides effects can be affecting the distribution of streams, channels and basins. Analyse this factor can definite the landslides areas more exactly and generate information of the landslides areas.
The hydrography of Tenerife can be divided in two orientations mainly. These orientations are controlled by the main fracturing axes NE – SW and NW – SE (CIA, 2007) as figure 25 shows. Therefore the volcanic axes are playing a crucial character in the distribution of the streams in the island, but landslides are modifying the fluvial network. In the top limits of Los Roques de García and Icod landslides and in the southern boundary of Güímar landslide, figure 26 displays how the landslides have modified the hydrological network.
Bandas del Sur landslide has not differences with the rest of the hydrological network how in Güímar have been observed and Anaga and Teno are controlled by the fracturing axis in the top of the both landslides.
Figure 25. Hydrological network of Tenerife (Data source: GRAFCAN).
Figure 26. Hydrology network modified by landslides (Data source: GRAFCAN).
Therefore, hydrological network physical variable has been very useful to get information and confirm the landslides in Güímar and in the North of the island (Icod, Los Roques de García and La Orotava) but this variable also demonstrations the importance and impact that the volcanic activity has in the islands as predominate factor in the hydrographical network in some areas like El Teide (fig. 27) where the streams have been modified completely and this volcanic activity have modified again the impacts of the landslides in the landscape of the areas of Icod and Los Roques de García.
Figure 27. Impacts of El Teide in the hydrology network in the Icod and Roques de Garcia landslide area (Data source: USGS).
iv. Land use
Physical variables (slope and hydrological network) have been done and both are presenting a crucial role of the post-landslide volcanic activity in the impacts of the landslides in the landscape. Now, land use will be done and a comparison between physical elements and human elements could be done, especially, in the questions about the depressions in Güímar landslide area, and the cliffs on the north of the Los Roques de García landslide. Also, distribution and characteristics of the land use will be analysed in Bandas del Sur landslide where limits have been very difficult to definite.
With the new land use map with five types the occupation: With vegetation, without vegetation, Agriculture and livestock, urban and industrial; have obtained the different areas in the diverse landslides around the island with the attribute table in ArcGIS. These areas have been compared in the figure 28.
The classification of the landslides in the island is the same that before (physical variables) because this makes easier understand and link with the physical variables particularly the slope.
Figure 28. Land uses (%) in the landslides areas (Data source: GRAFCAN).
a. Teno y Anaga
The land use in the affected landslide areas of Teno and Anaga has some similar points such as the characteristics in the physical variables expect in the new land created by historic volcanic activity in Teno landslide area. This also affect to the land use that will be different in this area.
Figure 29. Land occupation in Teno landslide area (Data source: GRAFCAN).
Anaga and Teno have high percentage of vegetation (86.73 % of the land) due to influence of trade winds in this area mainly cause to the condensation of the rain or “horizontal rain”. The vegetation in the landslide area of Anaga is biggest than in Teno due to urban, agriculture and livestock are high (fig. 28).
This new land is mainly plain and very useful for the agriculture and the urban expansion (fig. 29), and the urban societies have taken advantage in this new part created by the volcanic activity. The urban areas are mainly in the new land area while agriculture is very important in the west valley of the landslide that 3D map of slope showed in the last section (fig. 15).
Industrial occupation is non-existent in Anaga and in Teno the percentage is very low.
b. La Orotava and Güímar
La Orotava and Güímar landslide areas present same characteristics in the vegetation, agriculture and livestock activities. Güímar landslide has a major percentage of areas without vegetation due to the recent lava flows in the middle of the landslide area (fig. 30). This eruption was Arafo and it was in 1705 with duration of 52 days (AGSystems, 2006).
Figure 30. Land occupation in Güímar landslide area (Data source: GRAFCAN).
In regard to the urban and industrial occupation, both areas have around 8 %, but in Güímar area industrial and urban use have the same valour while in La Orotava the urban use is very important and industrial is only a 0.5 %. This is because, in Güímar landslide area, there is an important industrial park which is considered the most important in the economics of the island.
In Güímar and La Orotava, the vegetation is mainly in high slopes and backscars. The middle zone has a low slope and human activities are concentrated in this place. The vegetation area in the east of Güímar is in a historic volcanic activity that has been colonized by vegetal communities.
The depressions in the SE of the landslide are matching with industrial land and in this case, this is mining. Mining land use can explain why there are high depressions in the landslide area different to the rest of landslides. Also the depression areas are in the bottom of the high slopes in the east limit of the landslide, and this area is rich in sediments caused by the erosion in the cliffs after the landslide.
c. Icod and Roques de García
In Icod and Roques de García, the occupation without vegetation is very important (20 – 25 %) while in the rest of the landslides do not overcome 4 %. This is because the volcanic activity of El Teide is the main factor in this area and lava flows have not been colonized by the vegetation significantly. In this case, the overlapping is not relevant, but the high volcanic activity in both landslides is characteristic similar these landslides. Human activities are not more that 13 % and the vegetation areas are in the middle of the ranking.
Once more, El Teide is the protagonist in the land occupation as in the slope and hydrological network analysis (fig. 31) and it is a determinant factor in the distribution of the human societies.
The vegetation is concentred in the high altitude with more slope and the anthropogenic activities are concentred in the North of the landslides in the plain areas.
Also, the cliffs area analysed in the slope section cause by the uplift and erosion processes have a different distribution, and the vegetation occupies this area instead urban uses or agriculture. Consequently, uplift and erosion can also play an important role in the landscape of the island.
Figure 31. Land occupation in Icod and Roques de García landslide areas (Data source: GRAFCAN).
Figure 32 shows how the agriculture and livestock uses end radically in the top of the cliffs. Moreover, areas without vegetation are in this figure because there is lava flows areas where the cliffs have not been an impediment to the volcanic activity.
Figure 32. Slope and land occupation map of cliffs in the coast of Roques de García landslide (Data source: USGS and GRAFCAN).
d. Bandas del Sur
Bandas del Sur land use is very different to the rest of the landslides and is completely different to the rest landslide areas.
Agriculture and livestock have the double use that in the other landslides and the urban and industrial land is the highest of all landslides. Consequently, the vegetation area is around 20 % less in comparison with the other landslides.
The low slope can be the reason why the anthropogenic activity is bigger in the south of the island although, tourism areas are concentrated in this part of the island and it will be the key to understanding the land use in this area (fig. 33).
Figure 33. Land occupation in Bandas del Sur landslide area (Data source: GRAFCAN).
In summary, land use is linked with the slope in the most of the areas, and there is a relationship between vegetation, land use and high slope. Also, the volcanic activity is important again and the main factor in some landslides.
v. Differences between areas with landslides and areas without landslides
In this part, Güímar town area will be used to do a comparison between areas with landslides and areas without landslides. Güímar town is sited in the east of Tenerife and it has half town in the area affected by landslide of Güímar and the other half without effects of the landslides.
Slope and land occupation are the two variables analysed.
a. Slope
The slope is both areas is very similar although the landslide area (Güímar) has a volcano, depressions and high slopes in the top however, the area without landslide (Agache) is more similar and only have some lines which could be streams.
The important thing in the slope map is the line in the middle that separates both areas. This line is the backscars of Güímar landslide in the south (fig. 34).
Figure 34. Slope in Güímar town (Data source: USGS and GRAFCAN).
b. Land use
The land use in Güímar town is a bit different between the area with landslide and the area without landslide.
Figure 35 shows a comparison between the land use in area with landslide effects and the land use in the area without landslide.
The vegetation area is similar and also the industrial area, but there are three important differences.
- In Güímar, the area without vegetation is the historic volcanic activity and in Agache there is not a recent activity.
- The agriculture area in use is biggest in Güímar area while in Agache is the abandoned crops.
- Mining activity is in Güímar area due to the deposit of the intensive erosion processes in the cliffs of the landslide.
- Urban land use is biggest in Güímar.
These are the main differences in the land use, but the distribution of these areas is also important, and differences are clearer in figure 36 in the distribution of the land use.
Figure 36. Land occupation in Güímar town (Data source: GRAFCAN).
This map shows a big different in the distribution in the land use of the Güímar town area. Urban use area is concentred in the middle of the landslide area while in Agache (without landslide) the urban use is follow the coastline and some in the medium between the agriculture land areas. However, agriculture land area is abandoned mainly in the top of the part without landslide while in the landslide area, agriculture is still very high, it could be due to the medium slope in the area without landslide where is more difficult the water distribution while, in the valley is very easy because the slope is really low.
In brief, landslides are modifying the distribution of the land in Güímar town but the percentage of occupation is approximately the same with the differences explained before. Therefore, landslides are affecting to the land use but mainly the distribution.
vi. El Hierro. Other example in the Canary Islands
El Hierro is the other island studied. This island has been selected because El Hierro also has three fracturing axes, and the most recent landslides have succeeded here.
Four landslides have been defined in El Hierro (Fig. 37) and between them, there are three fracturing axes.
Figure 38. Characteristics of the landslides in Tenerife and El Hierro (Data source from Masson et al., 2006; Canals et al., 2002; Carracedo et al., 2009; Whelan et al., 2003; Alonso et al., 2005; Seisdedos et al., 2012 and Masson et al., 2002).
El Golfo, El Julán and Las Playas I and II are located around the island and in general they are forming valleys. Figure 38 is a brief of the landslides in Tenerife and El Hierro.
The most recent landslides have been in El Hierro and the area affected by landslides is biggest in Tenerife because the island is bigger than El Hierro. Also, the volume of some landslides is similar in both islands except Los Roques de García.
Landslides in El Hierro can be classified in three groups:
- Landslides with clear boundaries: El Golfo and Las Playas II
- Landslides with overlapping: Las Playas I
- Landslides without clear landslide evidence: El Julán
a. Slope
El Golfo and Las Playas II are landslides with high cliffs, and with clear evidence of landslides such as La Orotava and Güímar in Tenerife while, El Julán has not clear boundaries or geomorphological evidences. Also, Las Playas I is being affected by Las Playas II because this landslide is over the other one (fig. 39).
Figure 39. Slope map of El Hierro with landslides references (Data source: USGS).
All landslides are affected by volcanic activity over the landslides areas, and the influence of the landslides in the landscape is very high due to the island is very small and the landslides area very high. However, the volcanic activity is playing a crucial role in this island with a lot of footprint in the relief of volcanos include in the landslide areas such as Güímar and Teno landslides in Tenerife (fig. 40).
Figure 40. Detail of slope map (Data source: USGS).
b. Land use
In this point, there are differences with Tenerife, but also similar things mainly in the landslides with clear evidences and backscars.
Figure 41. Comparison of land use between Tenerife and El Hierro (Data source: GRAFCAN).
The land use is very similar in percentages with a bit differences in the areas with anthropogenic influence (agriculture and livestock, industrial and urban). This island has less population and the economy is lower than in Tenerife then, the urban areas and economic activities are less.
This part of the area is gained for the vegetation land and the areas without vegetation are similar.
In El Golfo the land use is very similar to Güímar landslide in Tenerife, but the part without vegetation is not only the new volcanic land, the cliffs have not vegetation enough because these slopes are high that in Güímar and Tenerife landslides (fig. 42). The urban area is in the zone with low slope such as Güímar landslide, and also the agriculture and livestock.
Figure 42. Land occupation in El Golfo landslide area (Data source: GRAFCAN).
Figure 43. Land occupation in Las Playas I and Las Playas II landslide area (Data source: GRAFCAN).
El Julán and Las Playas I have a different with the landslides in Tenerife. In the landslides of Tenerife, area with vegetation should be in the top of the landslides and in the boundaries, but in El Hierro vegetation is mainly in the coast and the agriculture and urban areas are in the top (fig. 43). it is because the boundaries in the top of these landslides aren’t clear and this area is mainly plain.
Lastly; the volcanic footprints are very clear in this island in the land affected by the landslides and in the land without these processes.
In summary; slope and land use in El Hierro have a very important influence of the volcanic activity. Landslides have an important protagonist in the landscape but again, volcanic activity is modifying the landslides areas such as in Tenerife.
