Journal of Coastal Research SI 56 pg - pg ICS2009 (Proceedings) Portugal ISSN Morphological Changes in a Low-energy Backbarrier A.R. Carrasco†, Ó. Ferreira‡, P. Freire∞ and J.A. Dias‡ †CIACOMAR/CIMA University of Algarve, Olhão 8700-311, Portugal azarcos@ualg.pt ‡ FCMA/CIACOMAR/CIMA University of Algarve, Faro 8005-139, Portugal oferreir@ualg.pt ; jdias@ualg.pt ∞ Núcleo de Estuários e Zonas Costeiras National Laboratory of Civil Engineering 1700-066, Portugal pfreire@lnec.pt ABSTRACT CARRASCO, A.R., FERREIRA, Ó., FREIRE, P., and DIAS, J.A., 2009. Morphological Changes in a Low-energy Backbarrier. Journal of Coastal Research, SI 56 (Proceedings of the 10th International Coastal Symposium), pg – pg. Lisbon, Portugal, ISBN Profile characteristics of low energy sandy beaches include narrow foreshores that are often steep, with reflective swash zones. Seaward of the foreshore a low gradient terrace is generally present, acting as a wave energy filter. Low magnitude changes are usually associated to this type of environments, either cross- or longshore. To examine the short- (between months) and medium-term (from months to years) morphological changes at Ancão backbarrier (Ria Formosa), data from monthly cross-shore surveys was used. The degree of morphological mobility was given by the volumetric variability within each morphology (foreshore, tidal flat and sand spit) and through the analysis of three cross-shore sections. Results show low medium-term variability, not seasonally distributed, and without a direct connection to changes on average wind intensity. The three morphologies are not interdependent and do not show any correlation on sediment exchanges between them. Low intensity short- term changes are however present, which seem to be related with net sediment adjustments. Although the profile and beach slope changes do not indicate a linear pattern of onshore-offshore exchanges, cross-shore transport seems to dominate the small scale short-term process, mainly at the shoreface, being related with changes in wind direction (erosion associated to S-SE directions). Variability in the sediment distribution can also be attributed to changes in wind conditions. ADDITIONAL INDEX WORDS: backbarrier, variability, wind climate INTRODUCTION The term low energy has been applied to beaches in environments ranging from narrow, shallow lagoons, with non- storm significant wave heights smaller than 0.25 m (NORDSTROM et al., 1996), to beaches in the lee of reefs, where the significant breaker height is up to 1.0 m (HEGGE et al., 1996). Locally generated waves are found in fetch-limited conditions (e.g. enclosed lagoons) while non-locally generated waves are characteristic of sheltered environments (e.g. lee of islands, JACKSON et al., 2002). In fetch-limited environments the wave climate reflects the wind conditions affecting the surrounding coastal area. Due to the short-period waves, fetch-limited sandy beaches tend to be morphodynamically reflective with low wave amplitudes and steep foreshores (WRIGHT and SHORT, 1984; SHERMAN et al., 1994; JACKSON et al., 1999). Locally waves are less affected by wave refraction and may approach the shoreline at relatively larger angles, increasing the potential for strong longshore currents for a given wave height (JACKSON et al., 2002). At these environments, beach morphology includes narrow foreshores (JACKSON and NORDSTROM, 1992), planar (NORDSTROM, 1980; HEGGE et al., 1996) and without backshore (NORDSTROM et al., 1996), with little evidence of bar forms seaward of low still-water levels (JACKSON et al., 2002). Extreme dissipative conditions prevail on the low tide terrace, and beach profile change is usually restricted to the foreshore (JACKSON and NORDSTROM, 1992). Survey profiles generally reveal little change in morphology, either alongshore or cross-shore (NORDSTROM, 1980). The magnitude of beach mobility is a function of the controls that increase or decrease susceptibility to erosion (JACKSON and NORDSTROM, 1992). Sand availability and wind conditions are important factors in beach morphotype modulation (ANTHONY et al., 2006). Moreover, tidal range, coupled with changes in wind wave energy will affect the location of the beach forms. Tidal range affects the vertical distribution of the wave- energy over the profile, and consequently determines the width of the beach and the duration that waves break at any elevation. Tidal currents may also be an important process on estuarine sites, controlling patterns of beaches evolution (NORDSTROM, 1992; CARRASCO et al., 2008). Several models of beach change identify the variability of beach stage through time as a result of seasonal and short-term changes in wave height, associated with storm cycles (NORDSTROM, 1980). Magnitude of morphological changes will be dependent on the energy/duration of these events. Modal conditions following a storm event may not be sufficient to return the beach to its original state (TRAVERS, 2007). Therefore, the variability inherent to the beach response will be mostly dependent on the natural resistance of the beach profile and the magnitude of the seasonal changes. The aim of this paper is to discuss the short- (between months) and medium term evolution (months to years) of a sandy beach located in a backbarrier system. Beach volumetric mobility, in terms of cross- and longshore transport dominance, is discussed within the prevailing wind conditions. Journal of Coastal Research, Special Issue 56, 2009