Antecedentes internacionales

The Ria Formosa, that is part of a natural park, shows some atypical charac- teristics for a barrier island system including: (1) it is not located on a coastal- plain coast (Pilkey et al., 1989); (2) the maximum spring tidal range (around 3.5 m) is close to the upper limit of mesotidal coasts, which represents the upper limit for the occurrence of barrier islands according to the classification of Hayes (1979) and due to these relatively high tidal ranges, the lagoon side of the islands are modified by the erosive effect of spring tides (Pilkey et al., 1989): and, finally, (3) it is one of the few barrier island systems in the world with a triangular shape (Bettencourt, 1994). The origin of the Ria Formosa is not clear, however, Pilkey

et al. (1989) established the holocenic landform migration to be in agreement

with the classical shoreface transgression model. According to the same author, the islands probably originated as spits connected to a cape; these spits were detached when the shoreline retreat accelerated in response to the sea level rise over the lip of the platform. According to Salles (2001) the general shape and morphologic evolution of the barrier island results from (1) the geological origin and evolution of the system, and its response to relative sea-level rise (thousands of years), (2) the exposure to different wave and current conditions on either side of the system (years), and (3) from the effects of storms (weeks/days).

The cuspate shape of the Ria Formosa system produces two areas differenti- ated in terms of exposure to wave action. The west flank is more energetic, being under the direct influence of the dominant wave conditions, while the east flank

is only directly exposed to the ‘Levante’ conditions. A brief resume of the charac- teristics of the natural inlets (Anc˜ao, Armona, Fuzeta and Lac´em, see Figure 3.1) in the Ria Formosa is given in this section.

The west flank typically has only one tidal inlet although at various times in the past has had up to three tidal inlets (Weinholtz, 1964; Esaguy, 1986a). Anc˜ao Inlet (Figure 3.2) is a small migrating inlet that has an average width of 300 m (Vila et al., 1999). Several authors have described the behaviour of this inlet as having a cyclic eastward migration (Weinholtz, 1964; Esaguy, 1986a; Dias, 1988; Pilkey et al., 1989; Vila et al., 1999), although other authors have interpreted its behaviour as erratic movements with no defined directions or cyclic behaviour (Andrade, 1990; Salles, 2001). Eastward migration rates found in the literature are very variable, ranging from tens of metres per year (Pilkey et al., 1989; Bettencourt, 1994) to an extreme value of 170 m/year (Granja et al., 1984). Average historical values for inlet width were also examined by several authors and were found not to have very strong variations. Values obtained by Esaguy (1986a) ranged from 200 m to 250 m, which are comparable to the average values found by Salles (2001). According to Andrade (1990), the inlet is ebb-dominated. Anc˜ao Peninsula and Barreta Island are narrow (in the vicinity of Anc˜ao Inlet), consisting of one single dune ridge that can reach heights of 7 m (above MSL).

The east flank presently has five inlets, although the number of inlets has varied over time. Two of the inlets on this flank, Faro-Olh˜ao Inlet and Tavira Inlet (see Figure 3.1) were artificially opened and stabilised with jetties in 1929- 1955 (Esaguy, 1984) and 1927-1985 (Esaguy, 1987), respectively.

Armona Inlet (Figure 3.2) is considered to be the only naturally stable (in terms of location) inlet of the system (i.e., Weinholtz, 1964; Dias, 1988; Pilkey et

al., 1989). Bettencourt (1994) proposed that the locational stability of Armona

Inlet was related to the existence of a paleochannel in the same position, however, this hypothesis has never been validated through corer sampling in the area. Armona Inlet has occupied the same position through recent centuries although its width has varied significantly (Weinholtz, 1964; Andrade, 1990; Salles, 2001). According to Esaguy (1984) and Dias (1988), during the last century, its width has narrowed by approximately 2,500 m, from 4,300 m in 1873, to 1,850 m in 1983. The same authors explained that narrowing of the inlet was mostly related to the growth of the eastern tip of Culatra Island, since its growth rates were similar to the inlet narrowing rates. As a consequence of the rapid growth, the eastern part of Culatra Island contains low dune areas with high overwash susceptibility

Figure 3.2: Vertical aerial photos showing a view of the natural inlets of the Ria Formosa barrier island system in 1996.

(Andrade et al., 1998; Garcia et al., 2002) that under high energy conditions could lead to barrier breaching. Due to its width, this inlet has a very complex morphology and often consists of two or more channels.

Fuzeta Inlet (Figure 3.2) shows a clear pattern of cyclic easterly migration (Weinholtz, 1964; Esaguy, 1985; Dias, 1988; Pilkey et al., 1989; Andrade, 1990; Salles, 2001). Eastward migration rates were variable, ranging from 30 m/year (Andrade, 1990) to 344 m/year (Esaguy, 1984). The typical width of this inlet varies from a hundred to two thousand metres, consisting, on occasions, of two channels. According to Andrade (1990), Fuzeta Inlet is ebb-dominated. Armona and Tavira islands, in the vicinity of the inlet, do not have very high dune ridges thus, overwash susceptibility is quite high (Andrade et al., 1998), especially at the western tip of Armona Island.

Lac´em Inlet (Figure 3.2) typically opens over a wide area during storms be- cause Cabanas Island is easily inundated. Dias (1988) explains that Cabanas Island corresponds to the eastern part of Tavira Island, and that it was separated with the artificial opening of Tavira Inlet. According to the same author, this island was completely destroyed by major storms in 1941 and in 1961, however

in 1962 it had already recovered, having a length of 100 m. Therefore, accretion processes on Cabanas Island are fast and cause eastward inlet migration as well as inlet narrowing (Dias, 1988; Pilkey et al., 1989). Typical widths for this inlet range from thousands to hundreds of metres and a double channel can exist.