Estimating the daily flooding probability by the compound effect of rainfall and tides in an Amazonian metropolis

Belém, the capital of the state of Pará, host of the 30th UN Climate Change Conference (COP30), which is located in the Brazilian Amazon, suffers from flooding caused by rainfall and tides. The objective was to estimate the daily flooding probability in Belém based on the two assumptions, i.e., the compound effect of rainfall and tides and rainfall only. In this case, rainfalls were analysed considering intensity and duration. Thus, it was possible to evaluate the flooding probability for each assumption and propose technological solutions to reduce this probability. Rainfall and tide height data for the 2003 to 2021 period were used. The minimum level causing flooding was defined analysing the compound effect (rainfall + tide height). The effects of rainfall and tides were also separately analysed with the aim of isolating the entry of tides into city canals. The daily flooding probability was estimated based on the study of the a priori probability derived from the ratio between the number of flooding events and the number of possible floods. The results showed that certain areas of Belém-PA could be flooded with a minimum value of rainfall plus tide height is 640.4 mm, if the rainfall intensity reaches at least 30 mm/h. Regarding the rainfall duration, the minimum value of rainfall plus tide height is 1398 mm for flooding to occur if rainfall lasts at least two hours. The flooding events were validated by examining local newspaper reports and photographic records describing and capturing flooding, respectively. The highest daily flooding probability occurred during the wettest period in the city, from January to May. The daily flooding probability was greatly reduced when only rainfall was considered. Thus, if city canals encompass floodgate systems, flood damage could be reduced. A floodgate systems project would be an important COP30 legacy for Belém. This system could help reduce the losses suffered by the poorest populations in Belém, who live in floodplain areas.