Metodología para elaborar mapas de susceptibilidad a procesos de remoción en masa, análisis del caso ladera sur de Tuxtla Gutiérrez, Chiapas

The city of Tuxtla Gutiérrez, Chiapas, has historically presented processes landslides in the southern part of the valley, specifically in deposits of slope whose genesis is determined from La Mesa karst of Copoya, geological forms subjected to intense fracturing processes dissolution and erosion, giving as a result the breaking into large blocks. These are distributed in the margins of La Mesa being altered and destroyed by mechanical and chemical weathering, which generated smaller particles that rest on siltstones, shale and sandstones. This condition determines that the slopes are unstable by nature.

A 1000 x 1000 m grid was constructed, corresponding to the canvass of the Mercator Transverse Universal Coordinate System (UTM) of the topographic map scale 1:50 000 (INEGI, 1984;emsp Instituto, 2004, Lugo-Hubp, 1988). In each cell a centroid was generated to apply the interpolation process and draw isolines. For numerical variables such as drainage density and unevenness, defined ranges (number of equal intervals) were used by ArcMap software (version 9.3). For the non-quantitative variables such as geology, edaphology and soil use and vegetation, the same AHP method was used, obtaining numerical values for the cartographic representation.

In all three cases, the normalized values and a Consistency Index (CI) and Consistency Ratio (CR) (the latter according to the dimension of the matrix) were obtained, below 10%, so that the weights are correct.

In order to obtain information on the changes in the use of the ground, images of Google Earth of 2006, 2008 and 2010 were revised. Subsequently, a very high resolution orthophoto was added (pixel of 0.2 m), obtained for cadastral purposes by Town Hall of Tuxtla Gutierrez between the end of 2012 and the beginning of 2013.

Finally, five layers of information are defined, one for each variable, and the final result overlaps with the events recorded in the last nine years (2006 to date), which shows that the areas classified as Very High Threat are the more susceptible to the occurrence of these events. It is to be expected that in time, the incidence of this type of phenomena is manifested in the levels of High and Very High Threat.

Applying the Analytic Hierarchy Process (AHP) developed by Saaty (1988), which consists of matrix analysis and involves value judgments. In this way the matrix of preference over the selected criteria was generated, obtaining the weighting of the five chosen variables. It was important the knowledge of the study area, the documentation and local studies generated to date, where the criteria of the specialists are taken up.

The process was done in an Excel spreadsheet (2007 version), applying the corresponding formulas. Because only five variables and the size of the area (250 km2) were handled, no specialized software was required.

With the data obtained a table was created in which a column with the name of “Threat” was created, which corresponds to the sum of the parameters of the six criteria mentioned above. This is reflected in the Map of Threats by Mass Removal Processes. Thus, in the one the extreme values are included in a range from 0 to 1.

The map of hazard by removing processes in mass or landslide (PRM), was developed from the heuristic combination of multi-criteria analysis method, and determined five levels of threat in the urban area, covering the following percentages: Very Low 5%, Low 27.1%, Middle 39.3%, High 15.3% and Very High 13.3%, the latter being distributed mostly in slope deposits around La Mesa of Copoya, confirming their status as maximum hazard.

For Tuxtla Gutierrez is estimated a population exposed about 62,500 inhabitants (11.6% of the total) (537.102 inhabitants in the urban area) who reside both in the southern part of the city and 30 rural towns settled on the flanks of La Mesa of Copoya; estimates about 28,000 dwellings exposed grouped into 850 blocks.

Current trends in the growth of the city, make evident the need to manage and regulate new buildings. Although the southern slope is determined unstable by nature, on the northern the housing complexes are changing the slope geometry sets, which can be a trigger for the occurrence of landslide in the short and medium term factor.