In this application the suitability for urban expansion in the city of Villavicencio, Colombia will be analysed. The selection of suitable sites is based upon a specific set of local criteria. The characteristics of a site (e.g. present land use, slopes, water availability, distance to employment, development costs) influence its suitability for a specific land use type. To assess the overall suitability a scoring and weighting system is applied to the various aspects of suitability. By applying a risky scenario and a safe scenario a point in time is calculated where there is no more non-urban land available in the area of Villavicencio for urban expansion.
One of the main concerns of physical planning is the proper designation of suitable sites for appropriate land uses. Certain characteristics like steep slopes, religious values (old graveyard), or designation as government-reserved land may lead to non-suitability of a site for a specific type of land use. Moreover, a site may be rated highly suitable for several land uses, which then compete for the same site. Suitability ratings unavoidably contain an element of subjectivity and caution should be observed to their application.
A natural hazard is defined as the probability of occurrence within a specific period of time in a given area of a potentially damaging natural phenomenon (Varnes 1984). These phenomena can be earthquakes, mass movements, floods, droughts, hurricanes, etc. All of them occur with different intensities and frequencies, producing different levels of environmental impact. In the Villavicencio area the natural hazards flooding, landslide and land subsidence are present.
Hazard map of Villavicencio
A natural risk can be defined as the vulnerability of the area in terms of expected number of lives lost, persons injured, damage to property and disruption of economic activity due to a natural hazard. In other words, a natural hazard becomes a natural risk when population and property might be affected.
Modelling land requirements for urban expansion
To evaluate urban expansion, a time series of land use information was examined. Land uses were reclassified in a new table on the basis of a new domain and used to create an attribute map.
Landuse map Villavicencio
From the histogram of the attribute map the areas occupied by different land uses can be expressed in hectares. Urban expansion is estimated by using the exponential growth formula:
Af = Ab * (1+%/100)(f-b)
where: A is the area, f is the future, b is the base year and % is the growth rate per year. In this application the basic assumption is that all non-urban land in the study area in 1991 is available for urban expansion and that the annual increase in urban land uses in the years after 1991 is 6.0%. Since the growth formula must be repeated several times, it could be convenient to express it as a function. Optionally urban expansion in time can be visualized in a graph.
Urban expansion graph
For simplification a number of factors, such as municipal plans, road and bridge construction projects, reserved land, ownership, environmental constraints, socio-economic classes, political and commercial factors, urban developments in proximity, but outside the study area, are not considered in this application.
Risky and safe scenarios
Urban expansion may be limited by natural hazards and the corresponding risks within certain areas. In this exercise two urban expansion scenarios for the period 1991-2006 have been considered:
To analyse the relation between hazard zones and urban expansion, a cross operation is performed on the hazard map and the landuse map. After that the resulting cross table is evaluated on the basis of the two above mentioned scenarios.
Scoring, weighting and classifying suitability factors
To assess the overall suitability a scoring and a weighting system is applied to the suitability factors:
These suitability factors are available in maps. To assign proper suitability scores to each class in each map new tables have been created with a score column and these tables have been used to create attribute maps. The three suitability maps are combined into an overall suitability map with a weighting system using a MapCalc operation. Weighting should be applied when not all aspects have an equal importance. Creation of a graph from the overall suitability map can help in defining score boundaries to group the values into the suitability classes unsuitable, marginally suitable, moderately suitable and highly suitable. Reclassification is performed on the basis of this new group domain.
Suitability classes for urban expansion
Applying the suitability models
Parts of the existing urban area are located in unsuitable or marginally suitable zones, because of the factors distance to city center (far), slopes (steep) or hazards (high flooding risk or high land slide risk). The urban land uses in those areas in principle should be relocated to more suitable zones.
Urban area located on unsuitable and marginally suitable land
With a MapCalc statement it is also possible to quantify the non-urban moderately and highly suitable area available for urban expansion in the future.
Suitability for future expansionFrom the analysis of the information the years are calculated when all highly and moderately suitable non-urban areas have become urban area and when all highly, moderately, and marginally suitable non-urban areas will be built-up area.
For more information on this case study, contact:
Drs. P. Hofstee and Ir. M. Brussel