GIS-BASED VULNERABILITY ANALYSIS AND MULTI-CRITERIA
EVALUATION FOR AN URBAN PLANNING CASE: THE STUDY OF CAMPUS
DO VALE – UFRGS, SOUTHERN BRAZIL
Tatiana S. da Silva
tatiana.silva@ufrgs.br
Flávia C. Farina
flavia.farina@ufrgs.br
Ricardo N. Ayup-Zouain
ricardo.ayup@ufrgs.br
Institute of Geosciences, Federal University of Rio Grande do Sul – UFRGS, Av. Bento Gonçalves,
9500, Agronomia, Porto Alegre, CEP 91509-900, Rio Grande do Sul, Brazil
Abstract
The pace, magnitude and spatial reach of human activities make the society increasingly dependent on
environmental and urban planning solutions, in order to minimize its vulnerability to natural disasters
and extreme events. Urbanization and population migration toward urban centers are among the most
drastic drivers of human-induced alterations of the ecological systems. Porto Alegre, the capital of the
Rio Grande do Sul State, Southern Brazil, and neighboring cities have experienced an intense
urbanization process, leading to drainage problems, congested traffic, unsuitable housing, and a lack of
water treatment facilities and other amenities. Therefore, the environmental vulnerability of this area is
highly controlled by rainfall and urban flood risk. The social and economic consequences of
environmental incidents are particularly damaging in the area, due to the great amount of inhabitants
and the concentration of economic outputs. Then, the goal of the present study is to build integrated
spatial decision support tools based on geographic information systems (GIS) and multi-criteria
evaluation (MCE) to objectively identify priority locations for flood risk management and urban
planning in the surrounding basin of the Campus do Vale (Federal University of Rio Grande do Sul)
and related micro watersheds. The GIS-based hydrological model used to spatially define the relative
flood risk incorporates a digital elevation model and also rainfall and soil infiltration potential data to
calculate flow direction and accumulation. The flood risk image joins flow accumulation and distance
from watercourses data. The urban suitability, in the other hand, is based on a multi-criteria evaluation.
Urban suitability is considered inversely proportional to flood risk, in a linear fashion. Absolute
constraints to urban development comprise the occurrence of environmental preservation areas, water
bodies, urban areas, and slopes higher than 30%. The resulting image is a map showing the relative
suitability to urban development, excluding the areas legally forbidden or impossible to develop. The
flood risk model gives us a hint about what areas should be the focus of concern in the case of extreme
rainfall events. It also can be used as a scenario generation tool, prospecting the effect of land use and
climate changes on hydrological vulnerability. The urban suitability model merges flood risk elements
with urban constraints. Additional criteria, if necessary, can be included to the model. The resulting
maps of urban suitability are intended to support environmental management and development
planning of the surrounding micro basin of the Campus do Vale and related micro watersheds.
Introduction
The pace, magnitude and spatial reach of human activities make the society increasingly dependent on
environmental and urban planning solutions, in order to minimize its vulnerability to natural disasters
and extreme events. Urbanization and population migration toward urban centers are a global
phenomenon (Jain e Subbaiah, 2007). They are one of the most drastic drivers of human-induced
alterations of the ecological systems. Porto Alegre, the capital of the Rio Grande do Sul State, Southern
Brazil, comprises almost one million and a half inhabitants, corresponding to 13% of the state
population. We find the same concentration process regarding production: the Gross Domestic Product
(GDP) of Porto Alegre was about 37% billion of Brazilian Real in 2008, corresponding to 18% of the
state production in the same period (FEE, 2011). Porto Alegre lies by the Guaíba Lake. The
geological/geomorphologic evolution of the region resulted in two different landscapes: one is
characterized by a rough relief, and the other consists in a coastal plain, resulting from the sea level
variations during the Holocene.
Drainage problems, caused by the intense urbanization together with unsuitable land use and extreme
meteorological events, increase the environmental vulnerability in certain locations and, consequently,
the flood risk and incident rate. Therefore, the environmental vulnerability of Porto Alegre and
surrounding cities is highly controlled by rainfall and urban flood risk. The social and economic
consequences of environmental incidents are particularly damaging in the area, due to the great amount
of inhabitants and the concentration of economic outputs.
Thus, the definition of high-risk areas in these cities is fundamental in adapting to global changes and
planning development. The Campus do Vale (Federal University of Rio Grande do Sul) and associated
micro watersheds is a diverse area in terms of geomorphology, soils, and land use (figure 1). As a
result, different levels of hydrological risk and urban suitability are expected. The rapid population
increase and associated economic changes apply an ever-increasing pressure to the urban environment,
leading to drainage problems, congested traffic, unsuitable housing, and a lack of water treatment
facilities and other amenities.
In spite of a number of efforts made by hydrological modelers in the region, there is a lack of a
decision support framework, integrating multi-source spatial data, providing visual tools for
stakeholders to better understand how flood risk and urban suitability can affect decision policy. In this
context, Geographic Information Systems (GIS) has excelled in the development of hydrological
models. Beyond the conventional models of flow direction and accumulation, GIS is capable of
generate vulnerability and suitability indexes, based on map algebra and spatial modeling, point areas
at high-risk of flooding or highly suitable for development. GIS, remote sensing and numerical
modeling techniques have been proved to be efficient tools to assess urban development suitability
(Dong et al., 2008). Whereas Remote Sensing is an unique data source, GIS spatial modeling allows to
establish multiple analytical approaches to assess local suitability. Thus, the goal of the present work is
to assess flood vulnerability and urban suitability of the Campus do Vale and associated micro
watersheds based on GIS and Remote Sensing in order to support the decision making process of the
University.
Figure 1. Study area.
Methodology
A flood risk and urban suitability models were built to support the planning process of the UFRGS
Campus do Vale and surrounding areas. The GIS hydrologic model (Idrisi Taiga) used to define areas
at flood risk was based in the algorithm proposed by Jenson and Domingue (1988). Unlike other GIS
hydrologic models, based only on elevation data, water infiltration rates are also included to calculate
flow direction and accumulation.
Hydrologic soil groups, or HSGs, along with land use, management practices, and hydrologic
conditions, determine a soil's associated runoff curve number. A HSG is a group of soils having the
same runoff potential under similar storm and cover conditions. The A group consists in soils with low
runoff potential, having high infiltration rates (higher than 7.6 mm/hour) even when thoroughly wetted
and consisting chiefly of deep, well drained to excessively well-drained sands or gravels. The B group
has moderate infiltration rates (between 3.8 to 7.6 mm/hour) even when thoroughly wetted and
consisting chiefly of moderately deep to deep, moderately well drained to well drained soils with
moderately fine to moderately coarse textures. The C group has slow infiltration rates (between 1.3 to
3.8 mm/hour) and consists in chiefly of soils with a layer that impedes downward movement of water,
or soils with moderately fine to fine textures. The D group has a high runoff potential, very slow
infiltration rates (lower than 1.3 mm/hour) and consists in chiefly of clay soils with a high swelling
potential, soils with a permanent high water table, soils with a claypan or clay layer at or near the
surface, and shallow soils over nearly impervious material. The average infiltration rates were used to
build the flood risk model, in combination to the soil map (produced by EMBRAPA, 1999). The initial
absorption values were based on the runoff curve numbers (adapted from Tucci, 2007) and the land use
and cover map built by Buffon et al. (2011). Urban areas were assumed to be impermeable, that means,
a null infiltration rate. The same is assumed for water bodies. The maximum 24-h rainfall value of
149,6 mm was used as precipitation input, based on historical data recorded by INMET – 8° DISME.
The resulting flow accumulation image was reclassified by equal intervals in five classes (very low to
very high flow accumulation). Each class was isolated in a Boolean image and the Euclidian distance
from the related water courses were calculated. The flood risk due to flow accumulation was
considered inversely related to the multiplication between the distance images of each flow
accumulation class and the digital elevation model (obtained from SRTM images), based on a linear
function. Weights from 5 to 1 were assigned to the flow accumulation classes proportionally to the
amount of water accumulation.
The urban suitability, on the other hand, depends on multiple criteria following different mathematical
relationships with the object of analysis. A geographic information system (GIS) is capable of
modeling and analyzing various types of spatial data and provides options to assess site suitability of
multi-criteria nature for developmental purposes (Aly, 2005). The flood risk and the preservation areas
occurrence are among the most important factors that enhance or detract urban suitability. Porto Alegre
master plan also poses legal constraints on urban development. A multi-criteria evaluation of these
factors together was undertaken to assess the urban suitability of the Campus do Vale and surrounding
basin.
Flood risk is a fuzzy factor in our decision support frame. Urban suitability is inversely related to flood
risk based on a linear function. The occurrence of preservation areas, on the other hand, is a constraint,
so these areas are excluded from consideration. Besides, the Porto Alegre master plan states that no
development may proceed on slopes exceeding a 30% gradient. Thus the water bodies, urban areas,
slopes higher than 30%, and preservation areas area not available for development. They are constraints
in our decision support frame.
Therefore, the urban suitability model is based on one factor (flood risk) and four constraints (high
slopes, preservation areas, urban areas, and water bodies). The slope image was created from a digital
elevation model (DEM) using a context operator. DEM was derived from a SRTM image (Suttle Radar
Topography Mission), obtained from the Global Land Cover Facility (www.landcover.org). The source
of the preservation areas, water bodies, and urban areas data sets is the mapping work of Buffon et.al.
(2011).
The decision rule was defined by an inverse linear relationship to flood risk, and the exclusion of areas
designed as constraints. The lasts, excluded from consideration, are coded with a zero. Those areas
opened for consideration are coded with a suitability score. The result is a map of relative suitability of
an area for urban development, excluded the areas legally forbidden or impossible to develop.
Results
The flood risk image (figure 2) is the result of a 24h simulation, considering the spatial variation of
water infiltration rates and the worst scenario of rainfall (maximum 24-h rainfall value of historical
records). Note that higher values of risk are found in built-up areas, which decreased soil permeability.
The urban suitability is a continuous index, varying from 0 to 1. However, the urban suitability image
was classified in very low, moderate, high and very high risk for a better understanding of the results
(Figure 3). A considerable part of the study area has a high urban suitability. Low suitability values are
restricted to the west part of the study area, highly urbanized, and to those near the entrance of the
Campus do Vale. The rest of the area, without overlaid maps, is unsuitable to urban development. It is
worthy to mention that the constraints do not include the occurrence of native woods, which cover
significantly suitable areas for development according to our urban suitability model. Native wood
suppression is only admitted with the environmental agency authorization, if necessary to provide
public facilities or to develop projects of social interest.
Figure 2. Flood risk map.
Figure 3. Urban suitability map.
Conclusion
The control and prediction of flooding is one of the greatest challenges of humanity today, given the
current scenario of global changes and human intervention on environment. The surrounding basin of
Campus do Vale is increasingly urbanized, altering the hydrological patterns. In this scenario, flood
incidents are expected to become oftener and more severe. Urban areas comprise about 2900 ha of the
study area. Only 100 ha of these belong to the Campus do Vale. As new land changes or development
plans take place, or even if historical rainfall values change, the flood risk model can be updated so that
the hydrological impact of new land use patterns can be assessed.
The urban suitability model combines flood susceptibility and urban development constraints. New
criteria can be included and the inputs updated, if necessary. The model was built to help the decisionmaking process of the Campus do Vale and its areas of environmental influence, defined by micro
watersheds. Besides, both GIS-based models are highly applicable to the environmental policy in
Brazil. These are local examples that can be replicated on other scales and contribute to information
systems, environmental plans, and environmental zoning, among others instruments. Besides, they can
be useful in building and adapting master plans as well as municipal environmental plans.
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