Zones of erosion and accretion were delimited by comparing a DGPS shoreline mapping in 1997 and the beach line reproduced from the army chart collection of 1975. The results show extensive shore retreat along of Rio Grande do Sul central coast, while accretion was observed in Mostardas and Dunas Altas beach. Mathematical estimative of the regional longshore transport potential along the Rio Grande do Sul coast, a 630-km long holocenic fine sand barrier, resulted in a large net northward annual sand volume. Additionally, the estimated potential of sediment transport based on the CERC formula predicts a substantial variation of the energy flux into the surf zone, due to little changes in shoreline alignments and in the potential alongshore sediment transport. The reduction in the sediment flux due to changes in the shoreline alignment produce a jam in the longshore transport, meaning that part of the sediment arriving from the upstream stretch may be deposited or diverted offshore by coastal jet. Based on that, it is possible that changes in the net longshore sand transport are responsible for the increase in the shoreface width from less than 1 km to more than 3 km in Mostardas beach and Dunas Altas beach. Interesting to note that wider dune fields are associated to those beaches where shoreface is also wider. In this way, the volume of longshore sand transport and the sediment jam provide by changes on shoreline alignment in Mostardas and Dunas Altas beaches are important for both coastal dune fields and shoreface width.
Mostardas beach, with an approximate extension of 36 km, is located along the intermediate section of Rio Grande do Sul state's coastline; its north and south limits are São Simão beach and Mostardas lighthouse, respectively. At the northern portion of the beach, an anomalous depositional behavior is found, relative to most beaches along the state coast. The anomalous behavior observed at this beach strip is influenced by a nearly 11º change in beach orientation. Such inflexion makes the wave energy act differently in beach strips located north and south from the inflexion, as examined in both seasonal and annual scales. Within a given seasonal scale, the morphology will be shaped according to events that the beach was subjected to, leaving the beach surface even, low, and regular with the passage of storm events, and acquiring a greater sediment stock at its subaerial portion in low-sea conditions, thus marking this beach seasonality. Yearly variability, verified at an 8-year interval between monitoring conducted in 1997 and 2005, demonstrated that the southern half of this beach shows erosion in almost its totality. The pattern in the 17 km of the northern half is quite different, with a significant enhance of stability and depositional areas, 36.1 and 28.1 %, respectively, and a seaward progradation mean of 1.1 m/year. The northern half also shows progradation areas between 3 and 4 times longer than those found in previous researches made on the Rio Grande do Sul coast (Toldo et al., 1999; Esteves et al., 2002). Such behavior confirms that the coastal inflexion found at the place significantly contributes with the shoreline variability in Mostardas beach.
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