Research highlights the importance of monitoring intense rainfall

A team of researchers from Embrapa, the Geological Survey of Brazil and postgraduate students in Environmental Sciences from the State University of Rio de Janeiro (UERJ), carried out a lifting of extreme events that caused flooding in the city of Petrópolis, during the 2010s, with historical data from four pluviometric stations from the State Environmental Institute (INEA), collected from 2011 to 2020. The analysis of precipitation data shows information that in conjunction with data on the spatial variability of rainfall, it can allow the implementation of strategic solutions to the problems of flooding, floods and landslides.

"We can highlight that rain monitoring must be done on a reduced time scale, at least one hour. Daily data, which are very useful in various uses, for example in agricultural zoning, irrigation management, are not sufficient for these studies of "production of floods", flooding, landslides and erosion, which cause victims, destruction of property and urban and rural soil", explains Embrapa Solos researcher Wenceslau Teixeira.

Features

Located in the mountainous region of the state of Rio de Janeiro, with an area of ​​791 thousand km², population of just over 295 thousand people, demographic density of 371,85 inhabitants/km², in the Atlantic Forest biome, the city presents relief varying from flat areas to steep and with variable slopes. Its location is at the top of Serra da Estrela. The municipality's area is equivalent to 1,8% of the area of ​​the state of Rio de Janeiro and 11,5% of the Mountain Region, with its total area divided into five districts: Petrópolis, Cascatinha, Itaipava, Pedro do Rio and Posse.

Understanding the factors that affect precipitation in built cities located on high terrain and steep slopes must be extremely important, as certain extreme events, such as rain, can cause flooding, flooding and landslides, which can be intensified depending on their location. "As an example of extreme events, we cite the accumulation of rain far from a pattern of historical series analyzed for a given location, exceeding its resistance capacity and thus causing impacts in different areas", explains Petropolitan Ana Cláudia Nascimento, master's student in Environmental Engineering at UERJ, one of the authors of the study.

The variability of precipitation is associated with the general dynamics of the atmosphere and the relief of the region, and the distribution of rainfall affects agricultural cycles, sanitation actions and other different social and economic activities.

Precipitation 

Petrópolis has an average annual rainfall of over 2.000 mm, but differences can be noted in these values ​​throughout the municipality. In order to analyze this precipitation, four INEA rainfall stations were selected: Alto da Serra, Araras, Barão do Rio Branco and Bingen.

It was evident that extreme events in Petrópolis were rare as rainfall with more than 20mm per hour represented less than 2% of the total. In the analysis of total daily precipitation data, in the four rainfall seasons, it was observed that the percentage of rainfall with a daily total in the range of 1 to 5 mm per hour was approximately 50%, explain Embrapa researchers Wenceslau Teixeira and Alexandre Ortega.

Events of high intensity and volume occur in these intervals with rainfall totaling 30 mm in an interval of 15 minutes, which would correspond to 120 mm/hour. Events of this nature will certainly cause flooding problems. Extreme events show high totals that were not necessarily continuous, but that totaled a large volume of water in 24 hours (>100 mm). This large volume, depending on past soil moisture conditions, can, in addition to causing floods, also contribute to slips.

From this research it was found that mainly Coronel Veiga, Washington Luís, Caldas Viana and Imperador streets are frequently flooded with extreme rain, these roads are busy and are part of the central region of the city.

The areas most at risk from flooding are: the neighborhoods of Quitandinha, Bingen, Corrêas, Nogueira, Itaipava, Pedro do Rio and Posse. The Quitandinha river is one of the city's main outlets and drainage, it runs through a very urbanized area on its way to the center, developing along Rua Coronel Veiga, with numerous bridges and crossings, an intensely reduced drainage section in some sections, certain of them further narrowing the channel section. Your gutter is constantly exposed to overflow in high rainfall events.

According to the dates of extreme rainfall events covered in the news at the time and the five maximum events recorded at rainfall stations, they show that the months suitable for flooding are January, February, March, October, November and December. A study carried out with historical data on daily precipitation from the National Water Agency (ANA) rain gauge stations in Petrópolis, showed that the months with the highest rainfall are January, February and March, with average monthly totals exceeding 200mm, and present great variability.

Solos

The municipality's dominant soil classes are Haplic Cambisols with a clayey or medium texture, which make up around 40% of the city's area.

Haplic Cambisols are typically poorly developed soils, however they can have good drainage in this region. The second most common class of soils are the Red Yellow Oxisols with a clayey or very clayey texture, followed by the Red Oxisols, which are often associated with hilltops and areas with flatter relief, and the Litholic Neosols, the latter generally have a small thickness and many of the occurrences are in steep areas, associated with rockiness and rock outcrops.

An interesting aspect is the large percentage of the municipality covered by rock outcrops, around 20% of its area, totaling around 16.000 hectares. The bodies of water total approximately 112 hectares of the municipality's area. Urban areas total around 2.800 hectares and these should be grouped with areas with low or no impermeability to paved roads.

According to the scientists, the next step will be to evaluate and estimate the water infiltration, conduction and retention capacity in the different soils of Petrópolis. It is also intended to make estimates of the volume of rain intercepted by civil works and estimates of infiltration in urban areas. With these data it will be possible to estimate the volume of water that needs to be ordered in high intensity rainfall events, aiming at proposals to reduce flooding and flooding events.

*This work was presented at the XXIV Brazilian Symposium on Water Resources, authors Ana Cláudia Nascimento, master's student in Environmental Engineering at the State University of Rio de Janeiro – UERJ, Wenceslau Teixeira, Embrapa Solos, Alexandre Ortega Gonçalves, Embrapa Solos, Embrapa Meio Ambiente and Edgar Shinzato, Geological Survey of Brazil – CPRM