The flood of Lake Guaíba is expected to continue reaching the metropolitan region of Porto Alegre in the coming weeks and keep the water level in the range close to five meters next week. The forecast is from the Hydraulic Research Institute (IPH) of the Federal University of Rio Grande do Sul (UFRGS).
The relief and hydrography of the metropolitan region make it difficult for water to drain, which delays a solution even without new rain.
The water flow also depends on the low tide and should cause problems in Rio Grande, where the delta that flows into the Patos lagoon, to which the Guaíba is connected, is located. All the water accumulated in the lagoon needs to pass through there to reach the ocean. With new rains and rising tides, the solution may take time.
The researchers remember that, during the flood of 1941, it took 32 days for the lake to drop below three meters again, meaning the end of the overflow.
If weather forecasts are confirmed, the rain should keep the lake level above five meters. Further rainfall is forecast over the weekend. If this is not confirmed, the tendency is for the level to be a little lower, in the four meter range. The projection is that, in any scenario, the flood will remain beyond the 18th.
Professor Rualdo Menegat, from the UFRGS Institute of Geosciences, states that the persistence of flooding is due to the geographical position of the city. Porto Alegre is located at sea level, on the slope of the Southern Plateau. “To compare with São Paulo, it is the location where the municipality of Cubatão is located, between the sea and the mountains. The capital [paulista] It’s at the top of the plateau”, he explains.
He highlights, however, climate change and the lack of prevention policies as determining factors for the current situation.
Porto Alegre’s situation is aggravated by the municipality being on the banks of the Guaíba, where the main rivers that originate in the interior of the state flow into the plateau area. The Jacuí, Taquari/Antas, Sinos, Caí and Gravataí rivers flooded due to the rains in the interior and the water quickly reached the capital because most of them are in narrow valleys, like canyons. This causes the water to descend at a greater speed and carry more clay.
In some of them, it is normal for river levels to rise up to 14 meters during the rainy season. Now, at these points, the elevation was 30 meters. “They’re like slides over 800 meters high”, he compares.
Gravity helps with the accumulation of water in the metropolitan region of Porto Alegre. And its flow also depends on the tide. With storms, the tide remains high, which prevents water from the lake and lagoon from reaching the sea. Lagoa dos Patos has only one outlet point, a small mouth into the Atlantic Ocean in the delta located in the Rio Grande region. “When the tide goes down, it is expected that the water will pass there at speed, which can cause erosion and more destruction”, says Menegat.
The professor says that the geographical position was fundamental for Porto Alegre to become the capital of Rio Grande do Sul in the 18th century. According to him, the Azoreans had settled in the region precisely because of the logistical ease of being close to the ocean and connected to the rivers that connected with inside.
The proximity that marked the city’s beginnings has been forgotten over the years. “With the flood of 1941, a protection system was created, but there was no awareness of the sensitive place where 4 million inhabitants live today”, he says.
For him, urban populations do not understand the landscape where they live, like the residents of the Roman city of Pompeii, hit by the eruption of the Vesuvius Volcano in the first century. “Man did not dominate the world. There are limits to human occupation,” he says.
Due to the relief, he does not believe that the flood threatens neighboring countries. “Their concern should be greater about the increase in extreme weather events than about this situation itself,” he believes. Menegat recalls that Cyclone Catarina also hit Rio Grande do Sul in 2004. “Problems like this were already common in the northern hemisphere and are now increasingly affecting regions below the Equator”, he says. The high temperature records, for him, are another sign of the problem. According to him, cold fronts encounter a warmer atmosphere, which causes more intense storms.
