Coastline changes and sedimentation related with the opening of an artificial channel: the Valo Grande Delta, SE Brazil

Abstract

The role played by human activity in coastline changes indicates a general tendency of retreating coasts, especially deltaic environments, as a result of the recent trend of sea level rise as well as the blockage of the transfer of sediments towards the coast, especially due to the construction of dams. This is particularly important in deltaic environments which have been suffering a dramatic loss of area in the last decades. In contrast, in this paper, we report the origin and evolution of an anthropogenic delta, the Valo Grande delta, on the south-eastern Brazilian coast, whose origin is related to the opening of an artificial channel and the diversion of the main flow of the Ribeira de Iguape River. The methodology included the analysis of coastline changes, bathymetry and coring, which were used to determine the sedimentation rates and grain-size changes over time. The results allowed us to recognize the different facies of the anthropogenic delta and establish its lateral and vertical depositional trends. Despite not being very frequent, anthropogenic deltas represent a favorable environment for the record of natural and anthropogenic changes in historical times and, thus, deserve more attention from researchers of different subjects.

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

“…In the warm season (September–April), rainfall is related to the activity of the South American Summer Monsoon (SASM), which is responsible for more than 80% of the annual mean precipitation in its activity centre (23°S) and for 50% of the total summer precipitation on the coast south of 25°S (Vera et al ., ; Nagai et al ., ). As a result of the typical variations in the rainfall pattern, the outflow in the lower course of the Ribeira de Iguape River ranges from 300 to more than 1200 m 3 /s (Mahiques et al ., ).…”

“…Tidal currents were not strong enough to distribute the sediment causing accumulation at the Porto Grande Harbour, which became inaccessible and therefore is no longer functional. Moreover, a series of sand banks emerged in the inner part of the lagoon system (Tessler and Furtado, 1983;Mahiques et al, 2014). The partial diversion of the river course by the Valo Grande artificial channel was temporarily interrupted in 1978, when a dam was built, but in 1983, intense rains and floods related to an El Niño event allowed the dam to open again (Mahiques et al, 2009).…”

From sambaquis (shell mounds) to plastic debris: a summary of the geological imprint of human occupation in the coast of São Paulo (Southeast Brazil)

“…Originally named Valo do Rocio, it aimed to directly connect the Ribeira River to the harbor of Iguape, at that time an important place for the export of agricultural goods, and located in the interior of a lagoon system, the Cananeia-Iguape Lagoon (De Mahiques et al, 2014).…”

“…From the original dimensions of 4.4 meters wide per 2 meters the channel presently shows widths of approximately 250 meters per depths of about 7 meters. Nowadays, about 60% of the freshwater and suspended sediments carried by the Ribeira River, the most important coastal river in São Paulo, reach the lagoonal system, with a consequent modification of environmental conditions (Martinez et al, 2013) and intense siltation (De Mahiques et al, 2014), forming several islands at the mouth of the artificial channel (figures 4 and 5).…”

“…From the original dimensions of 4.4 meters wide per 2 meters the channel presently shows widths of approximately 250 meters per depths of about 7 meters. Nowadays, about 60% of the freshwater and suspended sediments carried by the Ribeira River, the most important coastal river in São Paulo, reach the lagoonal system, with a consequent modification of environmental conditions (Martinez et al, 2013) and intense siltation (De Mahiques et al, 2014), forming several islands at the mouth of the artificial channel (figures 4 and 5).Furthermore, due to a Pb-mining activity that lasted from 1945 to 1995 in the upstream area of the Ribeira River, the lagoon system received contaminated sediments, with enrichment factors as high as 20 times the natural regional background (de Mahiques et al, 2012) (figure 6). Nowadays, despite the cessation of the mining activities, the system still receives significant quantities of metalenriched sediment since the mining debris are still deposited in the margin of the river (Tramonte et al, 2015).…”