Emergency forecasts for water levels in the Guaíba River during the 2024 unprecedented flood in South Brazil

Medeiros, Matheus Sampaio; Fan, Fernando Mainardi; Paiva, Rodrigo Cauduro Dias de

doi:10.1590/2318-0331.302520250025

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Scientific/Technical Article • RBRH 30 • 2025 • https://doi.org/10.1590/2318-0331.302520250025 linkcopy

Emergency forecasts for water levels in the Guaíba River during the 2024 unprecedented flood in South Brazil

Previsões emergenciais de níveis d’água no rio Guaíba durante a cheia sem precedentes de 2024 no Sul do Brasil

Authorship SCIMAGO INSTITUTIONS RANKINGS

ABSTRACT

In May 2024, the state of Rio Grande do Sul, in southern Brazil, experienced an unprecedented flood that severely impacted Porto Alegre and its metropolitan region. Water levels in the Guaíba River exceeded all historical records, reaching 5.37 meters on May 5, 2024, surpassing the previous peak of 4.76 meters recorded in 1941. In response to this extreme event, a team from the Hydraulic Research Institute at the Federal University of Rio Grande do Sul (IPH/UFRGS), including the authors of this study, mobilized efforts to perform daily operational forecasts of Guaíba water levels during the event. Despite the challenges, the forecasting system successfully identified critical moments of the flooding: flood levels exceeding 5 meters with a 3-day operational lead time; non-occurrence of levels above the 6 meters levees top; second peak above 5 meters with 8-day lead time; prolonged duration of the flooding around 30 days. The accuracy of the forecasts were crucial for mitigation measures such as preventive evacuations and emergency actions. This study highlights technical and operational aspects of the forecasts and discusses the importance of an integrated approach to managing extreme events.

Keywords:
Hydrological forecasting; Unprecedented flood forecasting; Extreme events

Date	Main Message	Occurred	Lead Time	Comment
May 1	Could exceed the November 2023 event and approach the 1941 flood level	Yes	~72h (3 days)	Surpassed the 1941 flood level on May 3
May 2	Many scenarios suggest levels equal to or higher than the 1941 mark	Yes	24h	Surpassed the 1941 flood level on May 3
May 3	Recommendation for evacuation preparedness measures in regions potentially affected in Porto Alegre	Yes	-	Evacuation recommendations were well followed
May 5	Forecasts indicate a prolonged flood event	Yes	Entire horizon
May 6	Could reach 5 m if rainfall forecasts in the Guaíba river basin are confirmed	Yes	~192h (8 days)	First warning of a potential resurgence for the week of May 14
May 7	Possibility of resurgence due to forecasted rainfall starting at the weekend, potentially reaching the 5 m mark	Yes	~168h (7 days)	Second warning about the resurgence for May 14 (1 week prior)
May 8	Elevated levels above 5 m expected in the coming days	Yes	~144h (6 days)	Third warning about the resurgence for May 14 (1 week prior)
May 9	Possibility of resurgence due to forecasted rainfall starting at the weekend, potentially reaching the 5 m mark	Yes	~120h ( 5 days)	Fourth warning about the resurgence for May 14 (1 week prior)
May 12	Confirmation of the flood resurgence with new levels above 5 m	Yes	~48h (2 days)	Warning about the resurgence for May 14
May 13	Maximum level expected between Monday and Tuesday	Yes	~24h	Reached 5.23 m on Tuesday, May 14
May 14	Forecast scenarios indicate stabilization at levels above 5 m after the resurgence	Yes	~48h (2 days)
May 14	Slow recession expected over the next few days, remaining above 4 m throughout the week	Yes	~168h (7 days)	Levels stayed above 4 m for at least 1 week
May 16	Slow reduction of levels below 5 m	Yes	~48h (2 days)
May 16	Expected to stay above 4 m until the start of next week	Yes	~120h (5 days)
May 16	Remaining above the flood level of 3 m until the end of the month or beyond	Yes	Entire horizon
May 20	Possible Guaíba river impoundment due to strong south winds expected on Friday, causing a new rise to around 4 m	Yes	~96h (4 days)	Warning about impoundment due to wind effects, raising the Guaíba to 4.3 m on May 24
May 23	Rainfall during the week could also contribute to an increase in levels, prolonging the flood above 3 m into June	Yes	Entire horizon
May 24	Guaíba river levels expected to rise due to forecasted winds, maintaining high levels	Yes	-	Confirmed impoundment due to wind effects, raising the Guaíba to 4.3 m on May 24
May 28	The Guaíba river level is expected to reach the flood level defined for the Usina do Gasômetro station in the next few days	Yes	~72h (3 days)	3 days later, the flood level was reached
May 29	The Guaíba river level is expected to reach the flood level defined for the Usina do Gasômetro station in the next few days	Yes	~48h (2 days)	2 days later, the flood level was reached
May 31	The Guaíba river level is expected to reach the flood level defined for the Usina do Gasômetro station in the next few days	Yes	~24h	1 day later, the flood level was reached on June 1
June 3	The Guaíba river is expected to reach the flood level defined for the Usina do Gasômetro station starting Tuesday	Yes	-	The level was reached on June 3 at 5:00 PM, confirming the forecast
June 3	May approach the alert level by the end of the week	Yes	~96h (4 days)	On June 7 (Saturday), the alert level of 3.15 m was reached
June 4	The Guaíba river is expected to reach the alert level defined for the Usina do Gasômetro station by the end of the week	Yes	~72h (3 days)	On June 7 (Saturday), the alert level of 3.15 m was reached
June 5	The Guaíba river is expected to fall below the alert level defined for the Usina do Gasômetro station in the next few days	Yes	~48h (2 days)	On June 7 (Saturday), the alert level of 3.15 m was reached
June 6	The Guaíba river is expected to fall below the alert level defined for the Usina do Gasômetro station between today and tomorrow	Yes	~24h	On June 7 (Saturday), the alert level of 3.15 m was reached
June 6	It is expected to remain below the alert level for the next 10 days	Yes	Entire horizon	Guaíba returned to the alert level in the early hours of June 19-20
June 12	South winds from the weekend cause impoundment	Yes	~48h (2 days)	Impoundment action and slight increase in levels observed
June 14	Forecast scenarios indicate stabilization/increase in Guaíba river levels	Yes	Entire horizon	Increase caused by south wind effects and precipitation volumes
June 14	Increase in Guaíba river levels expected throughout the week	Yes	~120h (5 days)	Increase caused by south wind effects and precipitation volumes
June 14	Forecasts do not indicate levels above the alert level	No	-	Levels surpassed the alert level on June 19
June 16	New forecast scenarios indicate a possible increase in Guaíba river levels throughout the week	Yes	~96h (4 days)	Increase caused by south wind effects and precipitation volumes
June 16	Could exceed the alert level in the worst-case scenario	Yes	~96h (4 days)	Levels surpassed the alert level on June 19
June 17	Forecast scenarios indicate an increase in Guaíba river levels in the coming days	Yes	~48h (2 days)	Increase caused by south wind effects and precipitation volumes
June 17	Could exceed the alert level in the worst-case scenario	Yes	~48h (2 days)	Levels surpassed the alert level on June 19
June 18	Expected to reach values around the alert level	Yes	~24h	Levels surpassed the alert level on June 19
June 19	Forecast scenarios indicate maintenance or further increase in Guaíba river levels in the coming days	Yes	~12h
June 19	Expected to remain around or above the alert level	Yes	~12h	Guaíba returned to the alert level in the early hours of June 19-20
June 20	Followed by a reduction below the alert level by the weekend	No	-	Levels slightly exceeded the alert level over the weekend of June 22-23, reaching 3.22 m
June 21	Forecast scenarios indicate a reduction in Guaíba river levels below the alert level by the weekend	No	-	Levels slightly exceeded the alert level over the weekend of June 22-23, reaching 3.22 m
June 21	South wind event between Sunday and Tuesday, causing another rise	Yes	~72h (3 days)	Impoundment action observed

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