{"id":569099,"date":"2025-11-21T19:07:48","date_gmt":"2025-11-21T22:07:48","guid":{"rendered":"https:\/\/revistapesquisa.fapesp.br\/?p=569099"},"modified":"2025-11-21T19:07:48","modified_gmt":"2025-11-21T22:07:48","slug":"landslide-scars-and-shallower-rivers-increase-rio-grande-do-suls-vulnerability-to-rainfall","status":"publish","type":"post","link":"https:\/\/revistapesquisa.fapesp.br\/en\/landslide-scars-and-shallower-rivers-increase-rio-grande-do-suls-vulnerability-to-rainfall\/","title":{"rendered":"Landslide scars and shallower rivers increase Rio Grande do Sul\u2019s vulnerability to rainfall"},"content":{"rendered":"

At the beginning of May, newspaper, website, and television reports portrayed the recovery of major cities in the state of Rio Grande do Sul that were hit by the colossal flood one year ago. However, little attention was given to the consequences of the landscape changes in rural areas and in less populated municipalities. Open fissures on the hillsides resulting from landslides, and shallower rivers due to the accumulation of soil and sediments carried by rainfall, have left Brazil\u2019s southernmost state even more susceptible to erosion and flooding, even during lighter rainfall than occurred in 2024. The region is already naturally vulnerable to the climate, due to its geographical position, terrain, and shallow soil (see<\/em> Pesquisa FAPESP issue n\u00b0 340<\/em><\/a>). In the second half of June, heavy rains returned and around 170 cities reported flood damage, forcing more than 9,000 people to leave their homes and seek temporary shelter.<\/p>\n

The major flood in April and May of last year affected 478 of the 497 municipalities in the state of Rio Grande do Sul, flooding 15,000 square kilometers (km2<\/sup>) and directly impacting 2.4 million people. The episode \u201chighlighted existing vulnerabilities in urban planning, water resource management, and communicating risks to the population,\u201d noted the book As enchentes de Rio Grande do Sul<\/em> (the floods of Rio Grande do Sul), coordinated by Brazil\u2019s National Water and Sanitation Agency (ANA), published at the end of April.<\/p>\n

One year later, there are still reasons for concern. \u201cThe areas that suffered landslides remain vulnerable to rainfall and erosion until the vegetation cover returns,\u201d explains geographer Harideva Egas, from the Brazilian Center for Natural Disaster Monitoring (CEMADEN). \u201cMore heavy rain could cause new landslides in the already impacted areas.\u201d<\/p>\n

He and fellow geographer Rodrigo Stabile coordinated a mapping exercise using high-resolution (3 meter) satellite imagery, which identified 15,087 landslides between April 30 and May 6, 2024. Triggered by the heavy rains, the landslides left marks on the terrain\u2014landslide scars\u2014concentrated in hillside areas of the central-northeastern portion of Rio Grande do Sul, along the basin of the Rio Gua\u00edba, according to a study published in November in the scientific journal Landslides<\/em>.<\/p>\n

The landslides spread across an area of 63,000 km2<\/sup>, affecting 130 municipalities, leaving scars up to 2 km long, and directly contributing to the death of at least 67 people.<\/p>\n

\u201cThe water that swept down the hills dragged soil and tree trunks, covered streams, and formed natural barriers that temporarily dammed the flow,\u201d says Egas. \u201cWhen the barriers broke, they released waves of mud and debris that caused violent flash floods, which in some places reached up to 4 m in height, leaving towns such as Roca Sales, in the Taquari Valley, 142 km from Porto Alegre, covered in mud.\u201d He visited the region for the first time in September 2024 to verify, in the field, the conclusions about the landslides drawn from satellite imagery.<\/p>\n

\u201cResidents of rural areas were more concerned about flooding, since landslides were not common,\u201d comments Stabile, who participated in the expedition. \u201cThey said nothing like that had ever happened before, but, as we saw, the properties sat on layers of sediment likely the result of past landslides over the course of thousands of years.\u201d<\/p>\n<\/div>

\n \n \n \n <\/picture>Alexandre Affonso \/ Pesquisa FAPESP<\/span><\/div>
\n

Egas returned to the Bento Gon\u00e7alves region in March this year and observed that the vegetation had already began to cover some scars left by the landslides. Others, however, still display patches of red soil exposed by heavy rainfall amid the forested hills.<\/p>\n

\u201cThe ground movement continues in Bento Gon\u00e7alves, Gramado, and Lageado, for example,\u201d observes forestry engineer Masato Kobiyama, coordinator of the Natural Disasters Research Group of the Hydraulic Research Institute at the Federal University of Rio Grande do Sul (GPDEN-IPH-UFRGS). \u201cThere is still a lot of water in the soil, that has penetrated the fissures in the underground rocks and has not completely evaporated.\u201d<\/p>\n

Kobiyama has been visiting the region hit by the rains. He accompanied Egas and other researchers in March and, together with colleagues from UFRGS, produced a synthesis of the 2024 tragedy in an article published that same month in Revista Brasileira de Recursos H\u00eddricos<\/em>. \u201cIn my opinion, there are many more than 15,000 landslides, maybe twice as many, because the satellite doesn\u2019t detect the smaller ones, between 1 and 2 m, which I regularly see on the hillsides,\u201d he observes.<\/p>\n

At least 35% of the volume of sediment displaced by the landslides reached the rivers, especially those near springs and canyons in the mountainous region, according to calculations by geographer Jo\u00e3o Paulo Ara\u00fajo, who took part in the CEMADEN survey. Stabile estimates that this volume corresponds to at least 10 million tons (t) of sediment, enough to fill almost seven Maracan\u00e2 stadiums, the largest in Brazil, located in Rio de Janeiro.<\/p>\n

Environmental engineer Hugo Fagundes, from the University of Campinas (UNICAMP), estimated\u2014based on a mathematical model representing water flow in large-scale river basins\u2014that the Rio Gua\u00edba alone, which crosses the Metropolitan Region of Porto Alegre, would have received around 5 million tons of sediment between April 27 and June 17, 2024. \u201cPart of this volume of sediment, which we are unable to quantify, remained in the rivers, while another part was carried by the rivers towards the sea.\u201d<\/p>\n

While lighter sediment is transported by the water, heavier particles settle on the riverbed, making the river shallower and reducing its capacity to allow the flow of rainwater. This phenomenon increases the size of river islands where sediment accumulates, complicates navigation, and creates the need for dredging to remove excess sediment.<\/p>\n

\"\"

Cleberton Bianchini\u2009\/\u2009FREITAS, E. M. de et al<\/em>. Marcas na paisagem<\/strong>. 2025 | Elisete Freitas\u2009\/\u2009FREITAS, E. M. de et al<\/em>. Marcas na paisagem<\/strong>. 2025 <\/span>The banks of the Rio Forqueta, a tributary of the Taquari, in November 2022 (left<\/em>) and in August 2024Cleberton Bianchini\u2009\/\u2009FREITAS, E. M. de et al<\/em>. Marcas na paisagem<\/strong>. 2025 | Elisete Freitas\u2009\/\u2009FREITAS, E. M. de et al<\/em>. Marcas na paisagem<\/strong>. 2025 <\/span><\/p><\/div>\n

\u201cThe rivers are clogged with silt, organic matter, furniture, and branches,\u201d commented the mayor of Sapucaia do Sul (Rio Grande do Sul), Volmir Rodrigues, during a debate held in the Brazilian House of Representatives in Bras\u00edlia in July 2024, \u201cThe municipalities do not have the financial resources to carry out dredging.\u201d<\/p>\n

In December 2024, the state government of Rio Grande do Sul announced investments amounting to R$691 million for dredging the state\u2019s waterways, as part of the environmental recovery program for areas impacted by the floods. \u201cDredging has already removed millions of cubic meters of sediment from the waterway channels between the ports of Porto Alegre and Rio Grande,\u201d reported the Press Office of the Department of Environment and Infrastructure (SEMA) of Rio Grande do Sul.<\/p>\n

It is unknown, however, whether these and other measures, such as the construction of dikes, will be sufficient to withstand the heavy rains, which are expected to be five times more frequent in southern Brazil over the coming decades, according to an article published in February in Geophysical Research Letters<\/em>. \u201cIf it is not done well,\u201d warns Kobiyama, \u201cdredging provides a localized solution, restricted to the area where it is carried out, and can create a channel along the riverbed that causes water to flow more quickly, increasing flooding in areas close to the river\u2019s mouth.\u201d<\/p>\n

The book As enchentes no Rio Grande do Sul<\/em> emphasizes the preservation of green areas to hinder the construction of homes in areas of risk and block the advance of floodwaters. \u201cIn locations without vegetation cover, erosive processes tend to worsen,\u201d stresses Fagundes.\u00a0However, in a reverse movement, the area of agricultural land grew 35% from 1985 to 2022, increasing the vulnerability to severe climatic events, according to an article published in April in the journal Environmental Research Letters<\/em>.<\/p>\n

\u201cThe banks and the vegetation along the river banks should not be occupied, because of the dangers they pose to people in the event of flash floods,\u201d adds Kobiyama. On June 4 and 5, at the invitation of a town councilor, he gave lectures to farmers, teachers, students, and other residents of Rolante, a municipality 90 km from Porto Alegre next to a river that has caused violent floods. \u201cWe all need to learn to observe the movements of rivers and the soil during heavy rains, so we can prepare for another climate attack,\u201d he commented.<\/p>\n

In Rolante, Kobiyama proposed the formation of community hubs, made up of volunteers, to organize the warnings and move residents to shelters in secure locations already stocked with food supplies, in the event of heavy rains. \u201cIn one city here in Rio Grande do Sul,\u201d he says, \u201cpeople took shelter in a church at the foot of a hillside, but had to evacuate quickly when they realized that the church could be hit by a landslide.\u201d<\/p>\n

The story above was published with the title “Lands in Rio Grande do Sul even more fragile<\/strong>” in issue 353 of July\/2025.<\/p>\n

Scientific articles<\/strong>
\nCOLLISCHONN, W. et al<\/em>. The exceptional hydrological disaster of april-may 2,024 in southern Brazil<\/a>. Revista Brasileira de Recursos H\u00eddricos<\/strong>. Vol. 30, e1. Mar. 19, 2025.
\nEGAS, H. M. et al.<\/em> Comprehensive inventory and initial assessment of landslides triggered by autumn 2,024 rainfall in Rio Grande do Sul, Brazil<\/a>. Landslide<\/strong>. Vol. 22, pp. 579\u201389. Nov. 26, 2024.
\nPETRY, I. et al<\/em>. Changes in flood magnitude and frequency projected for vulnerable regions and major wetlands of South America<\/a>. Geophysical Research Letters<\/strong>. Vol. 52, no. 5. e2024GL112436. Mar. 16, 2025.
\nTRANCOSO, R. et al.<\/em> Deadlier natural disasters\u2014a warning from Brazil\u2019s 2024 floods<\/a>. Environmental Research Letters<\/strong>. Vol. 20, no. 4. 41,001. Apr. 2025.<\/p>\n

Books<\/strong>
\nAG\u00caNCIA NACIONAL DE \u00c1GUAS E SANEAMENTO B\u00c1SICO. As enchentes no Rio Grande do Sul: Li\u00e7\u00f5es, desafios e caminhos para um futuro resiliente<\/strong><\/a>. Bras\u00edlia: ANA, 2025.
\nFREITAS, E. M. de et al.<\/em> Marcas na paisagem: Mem\u00f3rias para construir a resili\u00eancia no vale do Taquari (RS)<\/strong><\/a>. Lajeado, RS. Editora Univates, 2025.<\/p>\n","protected":false},"excerpt":{"rendered":"Millions of tons of sediment were carried from the state\u2019s hills into its waterways ","protected":false},"author":17,"featured_media":569108,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"_exactmetrics_skip_tracking":false,"_exactmetrics_sitenote_active":false,"_exactmetrics_sitenote_note":"","_exactmetrics_sitenote_category":0,"footnotes":""},"categories":[159],"tags":[224,200,2413],"coauthors":[5968],"class_list":["post-569099","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-science","tag-ecology","tag-environment","tag-technology","position_at_home-sumario"],"acf":[],"_links":{"self":[{"href":"https:\/\/revistapesquisa.fapesp.br\/en\/wp-json\/wp\/v2\/posts\/569099","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/revistapesquisa.fapesp.br\/en\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/revistapesquisa.fapesp.br\/en\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/revistapesquisa.fapesp.br\/en\/wp-json\/wp\/v2\/users\/17"}],"replies":[{"embeddable":true,"href":"https:\/\/revistapesquisa.fapesp.br\/en\/wp-json\/wp\/v2\/comments?post=569099"}],"version-history":[{"count":4,"href":"https:\/\/revistapesquisa.fapesp.br\/en\/wp-json\/wp\/v2\/posts\/569099\/revisions"}],"predecessor-version":[{"id":569826,"href":"https:\/\/revistapesquisa.fapesp.br\/en\/wp-json\/wp\/v2\/posts\/569099\/revisions\/569826"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/revistapesquisa.fapesp.br\/en\/wp-json\/wp\/v2\/media\/569108"}],"wp:attachment":[{"href":"https:\/\/revistapesquisa.fapesp.br\/en\/wp-json\/wp\/v2\/media?parent=569099"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/revistapesquisa.fapesp.br\/en\/wp-json\/wp\/v2\/categories?post=569099"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/revistapesquisa.fapesp.br\/en\/wp-json\/wp\/v2\/tags?post=569099"},{"taxonomy":"author","embeddable":true,"href":"https:\/\/revistapesquisa.fapesp.br\/en\/wp-json\/wp\/v2\/coauthors?post=569099"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}