Geography Reference
In-Depth Information
3.5. Conclusions
Environmental biophysics requires knowledge of the resources and processes affecting
ecological systems conservation, as well as planning and land management. In completion
of the erosion cartography it was possible to develop topographic models (slope, orientation
of slopes and distance to the coastline), climate (rainfall, hillshading, Modified Fournier
index) and lithofaces. These models represent quantitatively the environmental variables
that affect the process of erosion.
Moderate erosive status is the most frequent class in the study area. Highlands, where the
soil material have moderate resistance (flysch formation) and the precipitation is higher, and
the coastline, essentially composed of cliffs with low cohesive material (sand gravel), are the
most sensitive areas to erosion. However, using this erosive model it is difficult to justify the
risk of water erosion at the coastline. In coastal areas infiltration prevails (sand dunes) and
the wind action is the most important factor in the erosion process. Therefore, in future
studies, the model should be capable of including wind erosion.
The largest amount of precipitation is falling in December and January and the lowest in
July and August. Through the mapping of the rainfall erosivity, it was found that the
aggressiveness of rain in the coastline and in the highlands is higher in these months.
4. Flood delimitation mapping of the Tavira urban area
4.1. Introduction and objective
This case study illustrates the use of GIS as a tool to establish hydrologic regional
parameters for urban flood mapping purposes. Cartographic elements, hydrological and
hydraulic models, and boundary conditions used to establish the maximum flood levels of a
10- and a 100-years return period flood and a 100-years climate change scenario are
described. Cartographic information was completed by
in-situ
measurements and
observations.
Hydrologic regional parameters are extensively used in flood simulation, since drainage
basins are characterized by natural variability in land-surface features (
e.g.
, Wooldridge and
Kalma, 2001). Prasad (1997) refers that the improved accuracy of GIS-based hydrologic
simulation comes from the capability that these models have to integrate hydrologic
regional parameters; updating or modifying GIS data to study the impact of changes in a
drainage basin (
e.g.
, land use) becomes a relatively easy task.
This application is focused on the simulation of fluvial-originated urban flooding. The area
selected for this study is the town of Tavira. This town is situated in the southernmost
region of Portugal - Algarve. The Séqua/Gilão River crosses throughout the Tavira urban
area until it flows into the Ria Formosa coastal lagoon. As the Séqua/Gilão River is
intrinsically connected with the urban fabric, an overtopping of the margins always has
negative consequences to people and assets. An example of a severe flood event was the 3
rd
December 1989 flood which caused extensive damage in the city.
