Two new conceptual models for the formation and degradation of baymouth spits by longshore drift and fluvial discharge (Iguape, SE Brazil)

Abstract

This study describes the formation of two successive baymouth spits systems on the south‐eastern Brazilian coast and the degradation of the first system. The study area includes the Jureia Beach spit, the deflected Ribeira de Iguape River mouth, the central Iguape sandy headland, the Icapara Inlet of the Mar Pequeno Lagoon and the northern end of the Comprida Island barrier spit. The wave and river flow patterns were combined with the coastline evolution and the alongshore migration rates deduced from satellite images. Initially, both spits showed convergent alongshore migration rates equal to or less than 83 m/yr. However, the extreme river flow due to high rainfall during a very strong El Niño event in 1983 eroded the inland side of the Jureia Beach spit, which finally retreated due to wave erosion. In 1989, a sand bank emerged in the river mouth, which attached to the central headland forming a recurved northeastward spit. In 1994, the high fluvial discharge associated with another very strong El Niño event caused the landward migration of the new spit and emersion of a second sand bank. This second sand bank merged with the Jureia Beach spit in 1997 at an alongshore migration rate of 1795.6 m/yr. Wave erosion of the central headland continued and the attached spit disappeared in 2000. In 2009, the headland erosion merged the river mouth and the Icapara Inlet, which resulted in flanking baymouth spits in a configuration that remains today. Therefore, two models for the formation of baymouth spits have been documented for wave‐dominated microtidal coasts in humid tropical regions with intense fluvial discharge. The convergent longshore migration of the spits is controlled by both the bidirectional longshore drift and the fluvial discharge, the latter eroding the fronting spit, supplying sediments and acting as a hydraulic blockage for longshore drift. Copyright © 2017 John Wiley & Sons, Ltd.

Active mechanisms controlling morphodynamics of a coastal barrier: Ilha Comprida, Brazil

“…This period is related to the moment of greatest erosion in the channel, between 1875 and 1890 (Young, 1903;Geobrás 1966;Furtado et al, 1981;Nascimento et al 2008), which elevated the volume of sediments carried to the lagoon and the beach, and may be determinant for the high rates of island migration. This fact corroborates with Nascimento et al (2008) and Alcántara-Carrió et al (2017), regarding the acceleration in migration rates during the channel's enlargement. Indeed, the island's accretion rate has increased from 35x10³ m² per year (from 2.0 to 0.2 ka B.P., Guedes et al, 2011a) to 90x10³ m² per year, after the end of the blockade by the hills near Iguape and the opening of the Valo Grande (Guedes et al, 2011a).…”

“…Thus, the acceleration of dynamics in the IC is also influenced by climate, besides anthropogenic influences, altering the interaction between these agents. Guedes et al (2011) flow of the Ribeira de Iguape and Valo Grande, the potential longshore drift, and the wave and wind regime (Geobrás, 1966;Nascimento et al, 2008;Kawakubo, 2009;Silva et al, 2016;Alcántara-Carrió et al, 2017). Northeastward migration rates from the IC obtained in this study ranged from 15.5 m/y, in the GNSS surveys, to 134 m/y, calculated from OSL rates.…”

The Valo Grande Channel in the Cananéia-Iguape Estuary-Lagoon Complex (Sp, Brazil): Environmental History, Ecology, and Future Perspectives

“…thus, a stretch of Juréia beach northeast of the Icapara inlet had the longshore drift inverted to the southwesterly direction. this local inversion of the drift progressively pushed the mouth of the Ribeira southwest, with the simultaneous growing of a 3-km long sand spit (alCántaRa-CaRRIó et al, 2018). thus, since the opening of VG, Icapara inlet and Ribeira River mouth display a tendency to move in convergent directions (Figures 1 and 2), until they met in 2009 (aRaGOn, 2017).…”

Global Distribution and Morphodynamic Patterns of Paired Spits Developed at the Mouths of Interdistributary Bays of Deltas and within Coastal Channels

“…On exceptional occasions, the bays, estuaries, and deltas are confined by systems of paired spits (PS), i.e., two spits with a converging longshore drift that partially or almost completely enclose the bay or river mouth. The formation of paired spits has been described by five morphogenetic models: (i) coastal barrier break [23,24]; (ii) convergent longshore drift in a narrow bay [25]; (iii) bidirectional longshore drift with hydraulic blockage [26]; (iv) cutting a detached spit due to ebb-tidal currents [27] or by high energy events such as storms [28], hurricanes [29] or high fluvial discharge [30]; and (v) the convergence of two estuary mouths and the associated spits [30]. In previous studies about paired spits, they are also referred to as baymouth spits, double spits or baymouth barriers.…”

“…On the other hand, on open coasts, the paired convergent spits are related to a seasonal alternation of bidirectional waves. At each time of the year, the waves favor the development of one of the spits, while the hydraulic blockage, generated by tidal currents [29,37] or intense fluvial discharge [27,30,38], favors the net progradation of the countercurrent spit, by preventing or at least minimizing its erosion by the dominant waves. Usually, paired spits have been described at the mouth of bays and rivers [39].…”