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Geology

The lost beaches

Erosion and retreat of the sea redesign the Brazilian coastline

In July 1993, the Conceição Lighthouse succumbed to the action of the sea and collapsed on one of the beaches of São José do Norte, in the south of Rio Grande do Sul. The fall of the lighthouse is the result of a silent battle that has been unfolding for years in this municipality of 27,000 inhabitants, 350 kilometers away from Porto Alegre and a scenario of fighting in the Farroupilha Revolution, in the 18th century. It is where the waters of the Atlantic Ocean contend for space with the sands and advance slyly over a 30-kilometer range of deserted beaches of firm and dark sand. Built at the beginning of last century on a strip of land between the sea and the Patos Lake – the largest in the country – the Conceição Lighthouse, until the 40’s, remained 80 meters from the sea, guiding the vessels that would arrive in the port of Rio Grande. It only fell because of the process of natural erosion, a result of the vigorous waves that each year gobble up 3 meters of sand. It place is occupied today by another lighthouse, made of aluminum and set up on top of a dune.

Examples like the one of São José do Norte are more common than one imagines. About three quarters of the Brazilian coastline finds itself in a constant process of transformation, which is making the apparently stability of the silhouette of our 8,000 kilometers of coast a point of reference only on the school maps. 40% of its beaches are currently hit by some process of erosion and are losing ground to the sea, while in 10% of the Brazilian coast the opposite is happening – a phenomenon called progradation -, according to the conclusions of the first national survey about the alterations of the profile of the coast. According to this study, erosion is also corroding one quarter of the natural rock walls (cliffs) and invading the estuaries of 15% of the rivers that flow into the ocean. In another 15% of the cases, the river estuaries are gaining ground over the sea. Recently concluded, the Diagnosis of Coastal Erosion and Progradation is the result of a collective effort of 16 research groups and shows that the disappearance of the beaches at some points and the growth of the strips of sand at others is occurring with greater or lesser intensity in the 17 Brazilian states bathed by the Atlantic, depending on the local geography.

This survey indicates not only the stretches most affected and the points most susceptible to the force of the waves, like the northeastern region, where the beaches are gently sloped and facilitate the invasion of the sea. The diagnosis also does a detailed mapping of the areas in which human occupation has corroded the landscape and of those where the transformation of the coastline is a work of nature. This is the reason why it should provide guidance for the creation of a permanent network to monitor the stretches at greater risk and assist in drawing up standards that impose stricter limits for building seaside houses and condominiums. “Setting these limits depends on the speed at which the coast is retreating and on the monitoring of the areas of risk”, notes geographer Dieter Muehe, a researcher from the Federal University of Rio de Janeiro (UFRJ) and overall coordinator of the survey, financed by the Secretariat of the Interministerial Commission for Resources of the Sea (Secirm).

According to Muehe, the adoption of recommendations made by the Ministry of the Environment, such as not building at less than 50 meters from the beach in urban areas and at distances shorter than 200 meters in regions still not occupied – something that has not been observed to date in a good part of the coastal municipalities – would avoid problems like those faced in 2001 by the population of Barra de Maricá, on the north coast of Rio de Janeiro, where a storm brought down a group of houses built on the strip of sand. This stretch of the coastline of Rio de Janeiro State is to be found in an area where the thinning of the strip of sand is cyclical – in one year, the waters take the beach, which recovers itself in the next. Without respecting the limits imposed by the ocean, the owners rebuilt their houses and raised protective walls on the sand, mutilating the landscape.

Divided into 16 chapters, the survey is to be published by the end of this year in the form of an atlas, with maps of the stretches assessed in each state. Rio Grande do Sul, with a coast that is easily accessible to researchers, is the state that has best scrutinized its seashores and brought together information collected over the last 80 years on the 640 kilometers of the state’s coastline. Whereas in Maranhão, in the Northeast, and Pará, in the North, only more recent information is available about the conditions of the beaches, a dearth that makes it difficult to understand how these phenomena are evolving in time.

The game of advances and retreats of the ocean has been remodeling the Brazilian coast since South America began to separate itself from Africa, 130 million years ago. It is a natural phenomenon, determined by the energy of the waves and the geological characteristics of the beaches, which guide the marine currents and the transport of sand, like what is to be seen in São José do Norte. It is also like that on the north coast of Rio de Janeiro, in a 50-kilometer stretch between Cabo Frio and Saquarema, on the Massambaba coastal strip. It is there that an Atlantic with very blue waters – ice-cold too, even under intense sunshine – has eaten up 6 meters of beaches and dunes of fine white sand, between April 1996 and August 2002. As the region is practically deserted, the erosion was only identified because the researchers from UFRJ were already monitoring the region in this period.

Harmful intervention
The study details the impact of human intervention and is a reason for keeping alert. At several points of the coastline, erosion and its opposite, progradation, are arising or are aggravated by meddling with nature. Building a quay or a channel for navigation alters the cycle of the transport of sediments and makes sand build up on one side of this artificial barrier and be missing on the other. In the port of Rio Grande, in Rio Grande do Sul, two stone causeways (jetties) 3.5 kilometers in length, erected in the sea to protect shipping, have changed the profile of the beach at Cassino. Considered to be the longest beach on the planet, with an extension of 245 kilometers, Cassino beach has become even longer: it has put on 450 meters since 1947. But to the north of the artificial barrier, the Mar Grosso beach is living out the opposite fate and shrinking at the rate of 1.4 meters a year, over a stretch of four kilometers. It is something similar to what is happening in Ilhéus, in the south of Bahia, where the construction of a port has brought about severe erosion in the beaches to the north and an accumulation of sand in the beaches to the south.

The situation of alert does not mean, though, that the whole of the coast is threatened. The sunny beaches – and surrounded by coconut palms in the North and Northeast, or encircled by hills in the South and Southeast – should not disappear, although they may be transformed by the action of the sea. For the while, the effects of erosion are more serious in limited stretches, located above all in urbanized areas that have been the target of recent real estate speculation, as in the resort of Matinhos, located 111 kilometers to the south of Curitiba, in Paraná. Previously imperceptible to the population, the erosion that affects 6 kilometers of beach started to create incidents, with the advance of real estate speculation on the coast in recent years. In May 2001, rough sea destroyed 19 houses put up on the sand of the beach, on irregular building lots, leaving 50 families homeless.

The most effective – and obvious – solution is to respect nature and build at a safe distance from the high tide mark. Instead of this, half measures are taken, like the construction of walls, which can cause a disastrous result. “Walls transfer erosion to neighboring terrain, destroy the landscape, and devalue the properties”, comments geologist Rodolfo José Angulo, from the Coastal Studies Laboratory of the Federal University of Paraná, which is monitoring the erosion in Matinhos, a city nicknamed the Sweetheart of Paraná. In this Paraná bathing resort, the problem has been made worse because of badly planned urbanization, which removed the dunes from the beach and so a natural protection against rough seas – a beach has three fundamental parts: the zone where the waves break, the dry beach, and an accumulation of sand, known as a frontal dune. In Saquarema, on the north coast of Rio de Janeiro, the removal of the dunes to build a road was punished by the sea. The highway crumbled, after two decades of erosion. Muehe tells how the local town hall, “in an act of possibly innocuous insistence, filled in the place once again and rebuilt the road by the edge of the sea. Generally speaking, the lack of urban planning induces or even aggravates the damage from erosion.

The effects of human interference on the behavior of the sea are not just a result of works carried out on the beaches or close to the coast. Dams erected hundreds of kilometers from the coast, in rivers that flow out into the sea, interrupt the natural process of carrying sediments to the beaches. With the sudden shortage of sand in the rivers, the waves of the sea stop meeting natural barriers and invade the beaches with greater ease. An extreme example of this phenomenon took place in the village of Vila do Cabeço, 140 kilometers to the south of Aracaju, in Sergipe, in the estuary of the São Francisco River. In 1998, this village of 50 families was swept away by the waves. Previously on dry land, the lighthouse of the Navy built at the end of the 19th century finds itself today in the water, as a consequence of the process of erosion associated with the lack of sediments.

It is also known that the mouths of great rivers are naturally unstable regions. According to Muehe, these ecosystems suffer from erosion and progradation in equivalent proportions (15% show wastage and 15% addition), particularly on the coast of Paraná and Santa Catarina. The two phenomena may even live side by side in the same estuary. This is what is to be seen in Ilha Comprida, for example, on the south coast of São Paulo, where the mouth of the Ribeira de Iguape river is being slowly displaced to the south, by the force of the sands. “It is common for the collision between the energy of the waves and the force of the rivers to slowly change the configuration of the beaches”, says José Maria Landim Dominguez, from the Geosciences Institute of the Federal University of Bahia (UFBA), who is responsible for the chapters of the survey referring to the states of Bahia, Sergipe and Paraíba.

Threatening in a good part of the coast, the action of the ocean does not always cause damage. In some cases, the whims of the waves produce curious situations. On the north coast of São Paulo, three neighboring beaches in the municipality of Ubatuba show completely different characteristics. One of them, Sununga, receives violent waves coming from the south and is the victim of a process called beach rotation, in which erosion and deposition at the ends of the beach alternate, as shown in a study carried out by Michel Michaelovitch de Mahiques, a researcher from the Oceanographic Institute of the University of São Paulo (USP) and Cristina Célia Martins, who is studying for her doctorate. The group from the Institute, which includes Moysés Gonsalez Tessler, the author of the São Paulo chapter of the survey, which was able to count on support from FAPESP, is trying to understand exactly the processes of erosion and natural reconstitution of the beaches of the state of São Paulo. “The important thing isnot solely to detect the erosion, but to assess the balance of sediments that enter and exit the beach.” Next to Sununga, with its coarse sand and rough sea, one comes across the Lázaro beach, far calmer, protected from the direct impact of the waves by a bay and a rocky shore. Next, comes the Domingas Dias beach, placid and with fine sand.

The impact of the port
Of the entire coast, the areas most susceptible to erosion are in the northeastern region, where the beaches originally suffer from a scarcity of sand, caused by the lack of rivers capable of supplying the sea with sediments. In the state of Pernambuco, one of the states most affected by this problem, about six out of every ten beaches from the 187 kilometers of coastline are losing ground to the sea. In the capital of the state, the port of Recife, which extends for 3.4 kilometers, has displaced the effects of erosion to Olinda, a neighboring city 8 kilometers to the north. It is an interference that worsens the situation of the historically vulnerable coast of Pernambuco, affected by the scarcity of sediments and lacking natural dunes to hold back the advance of the sea.

The first records of erosion in the state go back to 1914 and were aggravated mainly by the works on the port, which altered the coastal currents that reach Olinda. In 1953, studies were commissioned from the Laboratoire Dauphinois d’Hydraulique Neyrpic, from Grenoble, in France, which recommended the construction of breakwaters close to the port, in an area where the natural reefs that are common in the region do not exist. Accordingly, the strips of sand in the city have practically disappeared, replaced by stone walls erected against the force of the waves. But all that was achieved was to transfer the erosion to the coast more to the north, reaching the city of Paulista, 16 kilometers from Recife, where the polluted Janga beach has slimmed 100 meters over the last ten years. A situation that is similar to the one to be seen in Fortaleza, in Ceará, after the 80’s, following the construction of a breakwater that blocked the flow of sediments and caused erosion in the beaches to the west of the city.

In the last few decades, erosion on the Pernambuco coast has just got worse, with the construction of buildings, roads and dikes on the beaches. At Boa Viagem beach, one of the best known of Recife, the sea has swallowed up five meters of the strip of sand in 20 years. “And the tendency is for things to get worse”, warns geologist Valdir Manso, from the Marine Geology and Geophysics Laboratory of the Federal University of Pernambuco (UFPE), which is monitoring continuously 15 spots on the Pernambuco coast. Human intervention in the landscape, with the construction of the port of Suape in the municipality of Ipojuca, 40 kilometers to the south of Recife, has interrupted the flow of sediments and magnified the problem. As a consequence, the sea has advanced into urbanized areas, such as the Toquinho beach, where the owners of seaside mansions opted to raise walls of rocks on the sand in an attempt to keep the sea at a distance. With the consultancy of UFPE, the city hall of Ipojuca drew up a R$ 5 million project with the intention of protecting the shore by the construction of a protective dike 200 meters into the sea.

Understanding why a beach loses or gains sand is not easy, even for researchers – what is clear is the effects of these phenomena. On the coastline of Rio Grande do Norte, the wear and tear is threatening oil wells that in the 80’s had been installed 800 meters from the beach in the Macau-Serra oilfield, close to Tubarão Point. As a preventive measure, these seaside wells have been encircled by contention walls. At high tide, the water reaches them. In Guamaré, a town that produces oil and natural gas, a little more than two hours from Natal by car, ducts and underwater outlets installed at a depth of 1.5 meters are now to be found on the surface of the land.

In a study financed by the Committee for Postgraduate Courses in Higher Education (Capes), by the Sectorial Fund for Oil and Natural Gas (CTPetro) and Petrobras, the team led by geologist Helenice Vital, from the Geology and Marine Geophysics and Environmental Monitoring Laboratory of the Federal University of Rio Grande do Norte, discovered that a convergence of factors causes erosion in this part of the state. On the one hand, the rivers that drain the region are small in size and do not supply sediments in significant quantities and, to worsen the situation, some of them have been dammed. On the other, the tectonic characteristics of the platform produce areas with a lowering of the terrain, creating the same effect as a raising of the level of the sea. In the municipalities of São Bento and Caiçara do Norte, the beaches have retreated 250 meters in the last 30 years.

The destruction of the Conceição Lighthouse, though, in Rio Grande do Sul, was only deciphered with an analysis of the movement of the waves. It was found that they are stronger and carry more energy than on the neighboring beaches. The bottom of the ocean works like a lens that magnifies the energy of the waves at the point of erosion. The syndrome is known as the focus effect. “As there had been no urbanization, the cause could only be a natural one”, says Lauro Calliari, a researcher from the Geological Oceanography Laboratory of the Federal University of Rio Grande.

International forecasts suggest that the level of the sea may rise 40 or 50 centimeters over the next hundred years. “Nobody knows yet whether this will really happen, but it would be just as well to treat the matter seriously, since the bottom of the sea close to the coast has low declivity”, says Muehe. A 50-centimeter rise in the level of the Atlantic could eat up 100 meters of beach, in regions like the North and Northeast. Even the south coast of São Paulo could suffer, in spite of being safeguarded by a plain that extends for as much as 25 kilometers between the sea and the mountains. To ignore this possibility is to disdain the dynamics of the sea. It is known that the level of the sea has fluctuated many times over the last 120,000 years, as a result of climatic phenomena. 17,000 years ago, the ocean started rising again, after having dropped more than 100 meters, due to the ice ages, making the coastline move to dozens of kilometers in front of where the coastline is today. 5,100 years, the sea rose, and its level stayed 4 meters above the current level. No one is betting on a new ice age, capable of affecting the level of the sea, in the next few generations, but it certainly works out less expensive to monitor the behavior of the coast and establish protective strips than to wait for the worst and later reconstruct the edges of the coastal cities.

The Projects
1. Atlas of Coastal Erosion and Progradation of the Brazilian Coastline; Modality Marine Geology and Geophysics Program; Coordinator
Dieter Muehe – UFRJ; Investment R$ 47,000.00 (Secirm)
2. Multithematic Assessment of the Transport of Sediments in a Beach Environment: Sununga Beach, Ubatuba, State of São Paulo  (nº 00/10298-7); Modality Research Line of Research Grants; Coordinator Michel Michaelovitch de Mahiques – Oceanographic Institute/USP; Investment R$ 163,920.67 (FAPESP)

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