4º SIMPOSIO sobre el MARGEN IBÉRICO ATLÁNTICO / 4º SIMPOSIO sobre a MARGEM IBÉRICA ATLÂNTICA / 4 th SYMPOSIUM on the IBERIAN ATLANTIC MARGIN INLET RELOCATION AS A COASTAL MANAGEMENT TOOL Ana VILA-CONCEJO 1 , Óscar FERREIRA 2 , Ana MATIAS 1 , João A. DIAS 2 1 CIACOMAR/CIMA Univ. Algarve. Av. 16 de Junho s/n. 8700-311 Olhão. Portugal. 2 CIACOMAR/CIMA/FCMA Univ. Algarve. Campus Gambelas. 8000 Faro. Portugal. 1. INTRODUCTION Barrier islands are complex systems that due to influences such as sea level rise, human occupation or coastal engineering actions often have serious problems in maintaining dynamic equilibrium. Tidal inlets in barrier islands often undergo downdrift migration movements. Therefore, they can represent a threat to man made constructions (i.e., roads, houses) that are on their migration path. Traditional engineering solutions to tidal inlet migration are hard protection techniques such as the stabilisation of the inlet by the building of groynes. However, such structures have several problems (i.e., interrupting the sediment by-pass) and, aside from the artificial aspect of the area, usually imply high costs and need periodic maintenance. It has been only recently that coastal managers have started to use a soft protection engineering technique that involves the artificial opening of a new tidal inlet in a location inside the historic migrating path of the inlet. The old inlet is then artificially closed or it is left open and will eventually close when the new inlet captures the entire tidal prism. The objective of this contribution is to present two inlet relocation actions performed in Southern Portugal and to asses their performance in comparison with historical data. 2. STUDY AREA The Ria Formosa (Fig. 1) is a multi-inlet, barrier island system located in Southern Portugal. Its present configuration consists of two peninsulas and five islands. Connection between the ocean and the backbarrier area is made through six tidal inlets. The backbarrier area comprises 8.4x10 7 m 2 and has a complicated pattern of tidal channels and creeks, with an average depth of 4 m referenced to mean sea level (msl) (Andrade, 1990). Figure 1: Location of the study area. Tides in the area are semi-diurnal, average ranges are 2.8 for spring tides and 1.3 m during neap tides, however, maximum ranges of 3.5 m can be reached. Wave climate in the area is moderate to high (Ciavola et al., 1997). Incident waves are normally from the W-SW representing 68% of the total, although SE conditions represent 29% of the total (Costa, 1994). The cuspate shape of the system produces two areas differentiated 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 SE conditions (Fig. 1). The west flank has presently one tidal inlet (Ancão Inlet). The east flank has five inlets of which two (Faro- Olhão and Tavira inlets) were artificially opened and stabilised with groynes. The four remaining inlets show different patterns and degrees of eastward migration. Vila-Concejo et al. (2002) made a study of the recent evolution (~1945-1996) of the natural inlets in the Ria Formosa and defined two typical migration patterns, one for the west flank and one for the east flank. Since 1997, the Ria Formosa has undergone an engineering programme that includes inlet relocation, inner channel dredging and beach and dune renourishment (Dias et al., 2003). During this programme, Ancão and Fuzeta inlet, that were in a late stage of their migrating cycles, (Fig.2) were relocated in 1997 and 1999 respectively. Both old inlets showed similar problems such as infilling and meandering of the main channel. Figure 2: Vertical aerial photos showing Ancão and Fuzeta inlets in 1996. 3. METHODS Periodic topo-bathymetric surveys were performed at Ancão and Fuzeta inlets in order to monitor the evolution of the relocated inlets. The surveys were made using total station for positioning and echo sounder for water depth measurement. Each survey was made during spring tides over two consecutive days. After data quality control the data were used to make digital maps using topographic software. With these maps, the morphologic and volumetric evolution of each inlet was studied along with some key parameters such as inlet migration, channel width and cross- sectional area at inlet gorge.