Geography Reference
In-Depth Information
In this chapter three GIS-based models were used, relying on different methodologies and
techniques, to illustrate visualization and quantification of the geomorphic processes. This
valuable input for decision-makers is attained through the versatility of GIS. Geo-form
mapping in a coastal lagoon catchment, rainfall erosivity on a mountain ridge and urban
flood delimitation show the potential usefulness of DEM/DTM based cartographic models
for helping to solve environmental issues. All case studies presented are from the south of
mainland Portugal.
The first case study is used to illustrate the main geo-form classes of the catchment
including the Ria Formosa. The Ria Formosa is a shallow coastal lagoon covering an area of
about 16,000 ha in the south of Portugal. It is protected by EU and national Laws, and is
classified as a Wetland of International Importance under the RAMSAR convention
(PORTUGAL Ramsar Site 212). This study aims to establish a method based on the
Hammond hierarchical criteria and geographical information related to soft-slopes, local
topography and terrain profiles, to locate and classify the geo-forms present in the Ria
Formosa catchment.
The second case study focuses on the use of DEM/DTM based on climate models to obtain
and analyze isohyetal maps, and to identify how rainfall distribution influences water
erosion. Rainfall distribution, which is highly variable in space and time, is difficult to
study, due to the lack of good quality data (
e.g.
, insufficient or poorly-distributed gauges in
the study areas; non-homogeneous rainfall data series; dubious readings from non-
automated gauges; lack of radar coverage). This issue may be partially addressed by
geographic models (
e.g.
, DEM/DTM) and climate data related to rainfall. Parameters such as
curvature, slope and orientation of hillslopes, which influence local climate, can also be
obtained from these geographic models. Isohyetal maps, through multilinear regression
analysis, can then be created using the DEM/DTM and their derivative models, the
hillshading potential and some specific data (
e.g.
distance to coastline). From these elements
and using the Modified Fournier Index (MFI), rainfall erosivity can finally be quantified.
This technique was used to assess the soil erosion risks in Serra de Grândola, which is a
north-south oriented mountain ridge with an altitude of 383 m, located in southwest
mainland Portugal.
The third case study demonstrates the use of cartographic information to produce flood
delimitation maps. The city of Tavira (10,600 inhabitants) in the south of Portugal embraces
the outfall of the Séqua/Gilão River into the Atlantic Ocean. A GIS-based hydrologic model
of the 221 km
2
Séqua/Gilão river catchment was first created to obtain soil use and type
regional parameters. Afterwards, and to identify the maximum water heights for those
return periods, a hydraulic model of the rivers' last 9.5 km was produced. These maximum
water heights were compared with the observed values (flood level marks, photographs and
video records) of the 3
rd
December 1989 flood and used to validate the model. Mean sea
level changes due to climate change were also considered. With this procedure, it was
finally possible to produce flood delimitation maps for the Tavira urban area. This type of
modelling may provide a useful tool for urban planners and city authorities.
