Storms in RS: understand how the relief of Porto Alegre and the ‘storm tides’ block flow | Environment

The relief of Porto Alegre is a key point for understanding the accumulation of water that causes flooding in the metropolitan region of the capital of Rio Grande do Sul. In a scenario of intense and constant rain 10 days agoLake Guaíba reached its historical maximum of 5.33 meters and is expected to remain above the flood level (3 m) until next week.

Along with the relief of the capital, they also contributed to the severity of the scenario caused by extreme rains:

1. the topographic formation of the surrounding regions of the metropolitan region,
2. the river basin of the area, and
3. the effect of storms in the Atlantic Ocean.

Porto Alegre is located on a plain, that is, in a flat territory. This type of relief is common in areas that are a few meters from sea levelwhich is exactly what happens there, since the city has an average altitude of 10 meters.

Gramado, for example, is 850 meters above sea level, and São José dos Ausentes, the highest municipality in RS, is almost 1,200 meters above sea level.

According to Rualdo Menegat, general coordinator of the Porto Alegre Environmental Atlas and professor at the UFRGS Institute of Geosciences, some parts of the city are even lower and are at the level of the Jacuí Delta (understand below).

In addition to having a low-lying territory, the state capital is surrounded on one side by 40 hills, and limited on the other side by the riverfront of Lake Guaíba.

Lake Guaíba, at the time the capital, is the confluence of five main rivers — Taquari-Antas, Gravataí, Sinos, Caí and Jacuí — which descend from higher points in the state. The waters of the rivers reach Lake Guaíba, go to Lagoa dos Patos and from there to the Atlantic Ocean

— Rualdo Menegat, professor at the UFRGS Institute of Geosciences.

In short, Porto Alegre is the last city on the route of a river that receives all the rain that falls in the central region of the state before it flows into the ocean.

The expert explains that the rivers that form the Jacuí Delta and originate the Guaíba flow at high speed due to the altitude of their courses. Normally, this helps the Guaíba flow into the lagoon and continue flowing to the sea..

However, with heavy rains and flooding of the region’s rivers, the lake is receiving more water than normal. This caused it to exceed its flood level — which is 3 meters — and reach a level of 5.26 meters in the second (6).

Strong winds form storm surges. This makes the sea ‘grow’, raising the level by up to 2 meters depending on the configuration of the beach. In the case of what happens here in Rio Grande do Sul, the sea is higher than the drainage level of Lagoa dos Patos, preventing excess water from being dumped into the sea.

— Rualdo Menegat, professor at the UFRGS Institute of Geosciences.

As a result, water from Lake Guaíba and the rivers that feed it continues to be trapped in Porto Alegre and neighboring low-lying cities. With constant rain and the speed of water flow, the level rises in minutes and leaves cities submerged in a matter of hours.

Satellite images show before and after the biggest flood in history in RS