A Influência da Inflexão Costeira na Variabilidade da Linha de Praia em Mostardas – RS

Abstract

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.

The strike-fed sandy coast of Southern Brazil

“…The large volume of sediment consisting of transgressive dunes was formed during shoreline erosion (transgressive phases) due to destabilization of the foredune or dune system, which allowed significant landward transport of sand by strong onshore winds . In two locations in this sector, at Mostardas and Dunas Altas, a large influx of sediment produced by subtle changes in coastline orientation is responsible for the local positive sediment budget (Toldo Jr et al 2004;Absalonsen & Toldo Jr 2007). This sector is considered the transition between a predominantly transgressive sector to the south (Estreito -Lagamarzinho) and a predominantly regressive sector to the north (Tramandaí-Cape Santa Marta).…”

Monitoramento mensal da linha de costa no Balneário Mostardense - RS entre 2016/2017 utilizando dados de VANT

“…A orientação da linha de costa na área, somada a fatores morfodinâmicos, podem ser os responsáveis pela mobilidade praial (Absalonsen & Toldo Jr. 2007), uma vez que praias intermediárias do litoral médio apresentam mobilidade sedimentar moderada a alta se comparada a outros segmentos da costa do RS, sendo que as flutuações anuais da linha de costa podem apresentar um padrão ondulatório, com áreas em erosão e acreção adjacentes (Esteves 2006).…”

Variação do limite praia duna nos Balneários Mostardense, Mar Grosso, Cassino e Barra do Chuí – RS no intervalo entre 2003-2015

“…Dillenburg et al (2005a) afirmam que o balneário se encontra em um trecho da Barreira IV (Holocênica) marcada pela transgressão eólica. Absalonsen & Toldo Jr. (2007) discutem que os sedimentos da praia são de granulometria fina e quartzosa de alta maturidade, o que favorece um comportamento morfodinâmico de praia intermediária a dissipativa.…”

“…As correntes, a maré, as ondas, a deriva litorânea e os ventos são fatores crucias no desenvolvimento da morfologia das praias. As variações do nível relativo do mar, a dispersão de sedimentos e as tempestades também estão entre os fatores que influenciam naturalmente a dinâmica costeira (Absalonsen & Toldo Jr. 2007). Ainda em 26 praias que sofrem influência de estuários e rios, pode-se observar uma massa de água flutuante com densidade inferior a das águas costeiras, chamada de pluma de rio ou estuarina.…”

“…predomina certa estabilidade entre erosão e acresção (Absalonsen & Toldo Jr. 2007). Os dados obtidos no LSP mostram que o balneário apresenta uma linha de costa erosiva, embora se tenha apenas dois anos de análise.…”