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Introduction
The instability of hillsides includes movements of soil, rocks, rubbles or their combination, descending downhill as rock falls , landslides and flows (CENAPRED, 2002), which break and disaggregate as they descend; reaching variable dimensions and velocities during its evolution, and depending on the deterioration state of the materials making the hillside up (Suárez, 1998).
A hillside is defined as a surface of land presenting a slope and its conformation is natural, it is said to be unstable when it weakens or loses balance as a result of the influence of triggering factors (CENAPRED, 2001), such as the presence of intense rainfalls and the influence of seismic (Anbalagan et al., 2008) and volcanic (Hinojosa-Corona et al., 2009) activities.
Hillsides instability is considered as the second geological phenomenon that provokes the greatest amount of human and material losses worldwide (Suárez, 1998), being developing countries and low-income populations living in mountainous systems the most affected (Highland & Bobrowsky, 2008), however, the literature reflects that when identified beforehand, difficulties could be avoided, if pertinent control and prevention measures are applied, reducing its occurrence up to 90 % (Suárez, 1998).
Factors influencing instability loss can be classified into inherent or conditioning factors, which are directly related to morphometric and landscape characteristics typical of the region, such as lithology, geology, topography and land use. These factors are not modified over time, contrary to external or triggering factors, which vary in occurrence, intensity and duration (Anbalagan et al., 2008).
According to the literature, all the mountainous systems can present some kind of instability, however, tropical mountainous systems present a higher susceptibility to difficulties related to hillsides instability and related phenomena, since four of the main elements are met in these regions for its occurrence, such as: topography, seismicity, presence of rainfalls and weathering (Suárez, 1998). Studies on the susceptibility to hillsides instability present high uncertainty indexes since the phenomenon encompasses a great variety of movements, velocities, failure modes, materials, geological restrictions and more variables that foster erroneous decision making, resulting in extraordinary operational costs and an imminent threat for human health (Suárez, 1998).
With the purpose of reducing the degree of uncertainty in the susceptibility analysis, the literature recommends classifying it into five categories according to its level of threat, into the following categories: very low, low, moderated, high and very high susceptibility (Suárez, 1998).
Non-susceptible or very low susceptibility zones encompass those regions where there is no natural propensity of occurrence of instability despite the presence of geologic failures or triggering activities (Cruden & Varnes, 1996), since they are located in low-lying areas and do not present high weathering indexes, as well as significant discontinuity (Suárez, 1998).
On the other hand, low susceptibility zones encompass those regions where there is the probability of occurrence of instability-related phenomena under the presence of strong modifications in their triggering factors (Cruden & Varnes, 1996), due to the presence of crevices and to soil erosion (Suárez, 1998).
Moderated susceptibility zones encompass those zones where lands have a natural propensity to present phenomena with significant magnitude related to hillsides instability even when there are no significant modifications in their parameters (Cruden & Varnes, 1996), due to the presence of intense erosion and the possible reactivation or incidence of new geological faults (Suárez, 1998).
For those zones comprised into the high susceptibility category, any parametric modification in triggering factors foster the occurrence of a considerable number of movements with significant magnitudes (Cruden & Varnes, 1996), since the land presents a natural propensity to instability, as a result of the high incidence of erosion and weathering, as well as a possible reactivation of movements, the presence of discontinuity and the possibility of occurrence of new movements (Suárez, 1998). For those zones classified into very high susceptibility of landslide, soil masses are highly degraded and saturated of unfavorable discontinuities (Suárez, 1998), where any change in magnitude or intensity of triggering factors results in the occurrence of a great amount of movements, due to the natural propensity presented by the land (Cruden & Varnes, 1996).
According to the National Risk Atlas and to the Mexican Geological Survey, the study area is in a convergence zone between the Sierra Madre Oriental, the Neovolcanic axis and the Sierra Madre del Sur, in addition to be influenced by the Circumpacífic belt, one of the zones with the highest seismic and volcanic activity in the world, and that is extended all over the eastern Pacific from Alaska costs to Argentine, going across the study area (Mexican Geological Survey, 2007).
Despite the above mentioned and although Mexico has a high susceptibility to phenomena associated with hillsides instability, since two thirds of its total extension is made up by mountainous systems of high complexity (SEDATU, 2014) and that the national territory is devastated by a high number of tropical storms and cyclones year after year (CENAPRED, 2002), most of the information related to threats, danger, vulnerability and risks in the country is scarce and heterogeneous, lacking of unified criteria allowing its compatibility and consistency (SEDATU, 2014).
Therefore, the application of the mathematical model under a SIG approach proposed by Anbalagan et al. (2008) was suggested, since it is used as a standardized method for urban planning in mountainous lands of the Himalayas, India. This model proposes a series of relative attributes for each one of the inherent and triggering factors impacting on the stability, analyzing individual and collective effects under an empiric approach, with the purpose of preventing risks related to hillsides instability, by generating information of quality that allows for decision making and the application of monitoring and accident prevention systems.
Material and Methods
Study area
The study area is located in the eastern Pacific in the geographical area known as Banderas Bay, made up by more than 40 hydrographic regions, distributed along 40 municipalities in the states of Jalisco, Nayarit and Zacatecas. The bay has a central valley surrounded by a mountainous system, mainly influenced by the flow of Ameca river, which has the most extensive and complex basin of the study area, with an area of more than 1 million of hectares. Thanks to its complexity, its geographical position and the diversity of touristic and recreational options offered by the region, a high popularity has been gained during the last decades both at national and international levels, ranking it as the second more important touristic site in Mexico (CEDESTUR-CEEB-AEBB, 2001).
The above mentioned has increased the demand and price of the territory, due to the stress induced by agricultural expansion and urban and touristic location, as a result of the development, increasing the susceptibility of the territory to phenomena associated with hillsides instability and fostering its degradation, reducing the resilience of the environment and affecting the development of human beings.
Zoning
Zoning the susceptibility to hillsides instability was performed by using the model proposed by Anbalagan et al. (2008), which is currently used as a normative method for managing the territory and urban location in mountainous regions of the Great Himalayas, India, by the Indian Standard showing a high efficiency and quality in its results.
The method proposes the individual and collective analysis of factors influencing on the stability, classifying and giving relative attributes for each one of them; these factors are classified into inherent, comprising lithology, morphometry and slope structure, relative relief, soil use and hydrological conditions and triggering factors, such as the seismic influence, the presence of rainfalls and the erosive state of the rock.
To this end, a series of digital cartographic data were used, which were classified according to the author, a process which is summarized in Figure 1.
Figure 1 Summary of the procedure applied for the mathematical calculation of susceptibility to hillside instability under a SIG approach.
The erosive degree of the rock was calculated by means of the development of five thematic maps, three representing the lithology and two representing the erosive state of the rock; for this, the lithological map proposed by Marín & Torres (1990) was used, which was classified into three categories, according to the relative attributes proposed by the author; resulting in three distinct thematic maps for each type of rock.
On the other hand, in order to determine the erosive degree of the rock, the Climatic Aggressiveness Index (CAI) was used, which expresses the propensity of the territory to erode according to rainfall volume (Lobo et al., 2010) and to the soil degradation map of the Mexican Republic proposed by SEMARNAT (2004). To this end, the CAI was classified according to Lobo et al (2010), for posteriorly complementing the product with polygons registering a high hydric erosion in the map proposed by SEMARNAT (2004), reclassifying it according to relative attributes proposed by Anbalagan et al. (2008), and resulting in two thematic maps belonging to types of rocks 1 and 2, which were multiplied by its corresponding lithological map and added to the map of type of soil 3, as shown by the author.
For the case of the sum of relations between the slope and its structure, Digital Elevation Models (DEM) were used, obtained by INEGI (s.f.), which were processed under a SIG approach, with the purpose of obtaining three thematic maps reflecting the slope structure, which were added with the purpose of obtaining a unique map.
The morphometric map of the slope and relative relief were also obtained by processing MDE and by the classification proposed by Anbalagan et al. (2008), as well as the Topographic Wetness Index (TWI) map, with which a map of hydrological conditions was made and complemented with a 10 m buffer for the main rivers of the study area.
For the cartography of soil and vegetation use, the map proposed by INEGI (2017) was used, which was classified according to the author and complemented with the map of urban and rural area (INEGI, 2010) and a 10 m buffer for communication pathways (INEGI, 2010) and geologic failures and fractures (INEGI, 2012) of the region, obtained through CONABIO (s.f.) and INEGI (s.f.).
Finally, for the maps corresponding to triggering factors, maps proposed by UAEM (s.f.) and SEGOB (s.f.) were used, which were classified according to Anbalagan et al. (2008). After that, all inputs were added as shown in Figure 1.
Results and Discussion
Results of the model showed that the study area presents susceptibility levels ranking from low to high, being the moderated susceptibility the most representative extension of all categories, with more than 1 million of hectares in extension, the equivalent to 80 % of the total extension, followed by the low and high susceptibility, with 13.3 % and 6.5 %, respectively (Table 1).
Table 1 Percentage and extension in hectares for each category of susceptibility.
| Susceptibility | Area (ha) | Percentage (%) |
|---|---|---|
| Very low | 27.62 | 0.0 % |
| Low | 204,929.40 | 13.3 % |
| Moderated | 1,238,080.50 | 80.2 % |
| High | 100,475.45 | 6.5 % |
| Very high | 1.08 | 0.0% |
| 1,543,514.04 |
According to Hernández & Ramírez (2016), when an instability phenomenon occurs in a zone categorized as a moderate susceptibility zone, homes with solid structural foundations are expected not to suffer from significant alterations, however, the damage could worsen for those homes with low quality structural foundations. In addition, significant damages in public infrastructure and communication pathways are expected, which could endanger the movement and the access to goods and services.
On the other hand, by facing a catastrophic event in zones classified with a high susceptibility, visible damaging effects are expected in structural foundations, such as cracks in the walls or breaks in walls and columns, which seriously endanger the structural integrity. Besides, severe damages are expected in communication pathways and public infrastructure (Hernández & Ramírez, 2016).
The study area presents 6.5 % of high susceptibility to hillsides instability, being equivalent to more than 100 thousand hectares in extension, indicating an imminent risk for the infrastructure and urban populations living in the zone (Figure 2).
The literature reflects that those regions with moderated susceptibility are considered as relatively safe zones for urban expansion, however these regions can present some unstable zones, and therefore highly detailed monitoring is recommended (Anbalagan et al., 2008).
Suárez (1998) assures that hillsides that remained stable for a long time can suddenly fail due to changes in topography and in wetness regimes, to seismic and volcanic influence, to changes in soil resistance. Therefore, it is recommended to strictly avoid urban location and modifications in the surroundings of zones categorized as high and very high, due the increase in the probability of occurrence of instability-related phenomena (Anbalagan et al., 2008).
Conclusions
Urban and rural communities of the study area present trends of accelerated population growth, thanks to job requirements needed by the touristic sector developed on the coasts, which in turn demand a high quantity of land and primary resource for its development.
For many years, trends of generation of goods and services in Mexico have generated a degradation of the resources that make them possible, fostering their damage; being soils one of the most affected resources.
Currently, the region already presents hillsides instability in the form of falls and slight flows, phenomena that could get worse as the damage develops and worsen deforestation, therefore the application of measures of monitoring, control and prevention of instability will be necessary, with the purpose of reducing the uncertainty in its occurrence.
The procedure proposed by Anbalagan et al. (2008) has proven to be highly efficient when determining areas susceptible to present hillside landslides-related events in different categories, however, the authors propose more detailed studies for high and very high susceptibility zones, with the purpose of reducing the uncertainty of the event and thus proposing control and prevention plans of the phenomenon.
