Coastal hazards studies on the Ria Formosa Barrier Island System (Southern Portugal)
Tiago Garcia
CIACOMAR, Olhão, (P)
Abstract
The Ria Formosa barrier island system is located
in the southern coast of Portugal, and is constituted by 5 islands, 2 peninsulas
and 6 tidal inlets. The islands show significant differences from each other
in what concerns its morphology and the role played by the several driving processes.
Among the later, shoreline retreat and overwashing are especially relevant to
management issues since they directly threat human development and have been
responsible for considerable damages in the last decades.
At CIACOMAR (www.ualg.pt/ciacomar), several studies are been carried out aiming
to provide scientific basis for a sustainable management of the Ria Formosa
barrier island system. Some of these studies make use of the analysis and comparison
of vertical aerial photography to quantify shoreline change evolutionary trends;
to identify different types of overwash structures and its evolution patterns;
and, finally, to represent coastal risk maps.
Aerial photography dating back from the 1940's up to the present was georeferenced
with ER Mapper software and exported into MapInfo Professional in order to digitalise
the shoreline positions and the existing overwash structures in each analysed
photography series.
The shoreline changes obtained were used to project future shoreline positions
by employing an integrated method for definition of set-back lines recently
developed by the CIACOMAR members. This method provides several set-back lines
for distinct coastal hazards which consist on a useful tool for decision makers.
A similar approach is being taken on the study of the overwashed areas evolution.
By understanding how these areas have evolved in the past it is possibly to
detail what are physical settings more suitable to the occurrence of overwashing
and take actions towards minimize the risk of flooding. By other hand, in the
areas where artificial dune recovery has been executed to prevent overwashing,
the evaluation of the success of these interventions is being performed in order
to benefit future actions.
Human activities are known to impress intense changes on the coastal zones.
Coastal sediment transport is often altered as a consequence of the construction
of jetties and groins, as well as dredging. The associated disturb registered
on the typical sediment flow along the coastal zone frequently generates a risk
situation on a previously safe area. The sediment volumetric changes derived
from human activities are also being quantified using analysis and comparison
of aerial photography.
The people involved in these studies are whiling to debate any aspects related
with methods and results obtained and encourage anyone who as interest in these
areas and/or has developed similar studies to do so.