Landscapes Forest and Global Change - ESA - Escola Superior ...

V.D. de Campos & S M. Carvalho 2010. Risk areas to flooding in the Hidrographical Basin of Arroio dos Pereiras
28
According to Cunha (2005), rivers and canals overflow their riverbeds at least once
every two years, in other words, when there is peak flow, triggered by concentrated rains and
the river begins to occupy its larger riverbed or floodplain. What makes these risk areas is
human occupation, that by ignoring or disregard the natural dynamics of the drainage network
provides housing in these areas and often the walls of buildings of the river banks (photo1).
When the occupation of the bottom of a city valley does not consider the natural
dynamic of the system, considering riverbed just the river watercourse and not respecting the
larger riverbed, which is its area of flight in the events of greater flow, creates what we call risk,
which is the sum of the natural susceptibility, more created susceptibility, more the vulnerability
that the population is exposed (EMPLASA / SNM, 1985).
Other factors that act as constraints or even aggravating of the flood events are the
infrastructure projects that increase according to the urban development such as soil sealing, the
reduction of green areas, changes in canals such as rectilinizations and canalizations, which,
when poorly scaled, ultimately aggravate the situation, since they reduce the area occupied by
the flow, blocking the water flow in the peak events, leading to leakage or even rupture of pipes
and galleries.
In the case studied, the hidrographical basin of Arroio dos Pereiras, the floodplain is the
area of the basin that has the more consolidated urban occupation, housing commercial activities
and services, in other words, it is precisely the central area of the city with a higher potential
risk of material losses.
2. Methodology
Figure 1 - Location of the Hidrographical Basin of Arroio dos Pereiras
Org: Campos, 2006.
The thematic maps were generated using the software Spring 4.3 (1996), from image of
QuickBird ® 2004 satellite (Irati 2006), topographic maps of the Army (SG 22-XCI-4) at
1:50000 scale, base-letters of the city 1:50.000 and cadastral urban letter cadastral at 1:10,000
scale.
To map the risk areas they were considered the subdivision and land use, the
characteristics of the drainage network (watercourse morphology and location of flood plains),
structural change and watercourse contention and historic of previous events in the local press,
brigade of firemen, residents and researchers of local history. These procedures allowed to
identify and map the most susceptible areas to flooding using the GIS tools.
For this work we established the period from 1983 to 2006 to identify the time of
recurrence of the phenomenon and they were considered only large and medium magnitude
events, which have records both in the local press and in government agencies as City Hall and
Fire Department . Because there is not a well established civil defense, the records are few and
Forest Landscapes and Global Change-New Frontiers in Management, Conservation and Restoration. Proceedings of the IUFRO Landscape Ecology
Working Group International Conference, September 21-27, 2010, Bragança, Portugal. J.C. Azevedo, M. Feliciano, J. Castro & M.A. Pinto (eds.)
2010, Instituto Politécnico de Bragança, Bragança, Portugal.