THE LANDSLIDE HAZARD MAP OF BOGOTA AN UPDATING
Keywords: Landslides, Bogota, GRASS GIS, Risk management, Modelling, Model Builder
Abstract. A land management plan (Plan de Ordenamiento Territorial, POT) is an instrument for planning the land-use of a territory considering physical, social and economic aspects. In the frame of the POT’s upgrading, the risk management plays an important role in conservation of human lives, prioritization of territory’s land-use as well as the formulation to reduce the fiscal vulnerability of the territorial institutions when a disaster occurs e.g. landslides. Landslides represent a rampant danger in Bogota’s hillside area because of accelerated urban growth, steep topography, degraded rocks, susceptible soils and abundant rain among other factors. It is calculated that more than 3 500 000 people live in such risky zones.
This paper presents the methodology followed to update the landslide hazard map – from the GIS techniques perspective – produced by an interdisciplinary team formed by geologists, geotechnical and GIS specialists.
The methodology used to obtain the map of landslide hazards consists of the combination of two partial results regarding to the evaluation and analysis of slopes’ stability in the studied area. The first one has to with a heuristic method that combines the variables prone to influence the occurrence of landslides. The following eight variables were included: relief, drainage, and material, land use as susceptibility factors and erosion, climate, earthquake, human factor as triggering factors. To combine these variable map algebra was used. While the second partial result involves stability of natural slopes.
Despite the fact that in Latin-American region and especially in public institutions proprietary software is widely used, the visualisation and edition of geographic information was performed with QGIS software. Additionally, the study area covered a considerable part of Bogota’s urban region which demanded a quite efficient and robust software that allow to execute a straightforward and complex processing. GRASS GIS software was used to carry out most of the geoprocessing tasks without splitting up the datasets. The Open Source GRASS yielded excellent results proving be an efficient tool when dealing with big datasets. Furthermore, in the study of some variables very specific geomorphology algorithms, such as Terrain Classification from SAGA GIS, were used. Meanwhile automation of some geoprocesses was implemented with proprietary software.
New techniques and procedures as well as detailed inputs with further level of detail provided a more accurate map in a shorter period of time. One of this techniques is the delimitation of areas for evaluating the stability of natural earth slopes which was analysed with the support of hydrological modelling in GIS. This helped to guarantee the homogeneity of these regions, yielding quite acceptable results when accuracy is proved as an important condition in the application of this method.
This map constitutes a valuable input in the risk management when dealing with land planning of the city. From this new hazard map, new suitable areas for housing can be defined. Also, it is the initial input for determining the necessity of new civil works for disaster mitigation. In some cases, resettlement of affected people will be recommended when the levels of landslide’s hazards are not capable of being mitigated.