Flooding, as a major natural disaster, affects many parts of the world including developed countries, and has severe impacts on populations and causes socioeconomic damage. Due to this kind of natural disaster, billions of dollars in infrastructure and property damage, as well as hundreds of human lives, are lost each year (Demir and Kisi, 2016; Elnazer et al., 2017). It is understood that flood risks and hazards will not subside in the future and, with the onset of climate change, flood intensity and frequency will threaten many regions of the world (IPCC, 2014).

These hazards can be prevented and reduced by providing reliable information to the public about the flood hazard through flood inundation maps (Alaghmand et al., 2010; Demir and Kisi, 2016). This information can, for example, assist urban management or even help the rescue operations during these events (Cook and Merwade, 2009), thus helping the communities directly to avoid or mitigate such future losses and damage (Arrighi et al., 2013; Savage et al., 2014; Speckhann et al., 2017).

Flood hazard maps therefore need to be created as they provide a basis for the development of flood risk management plans. These plans need to be effectively communicated to various target groups, including decision makers, emergency response units and the public, as a measure to reduce flood risk by integrating different interests, and potential conflicts over space and land use in a city (Ouma and Tateishi, 2014). However, creating flood maps is a complex process which is affected by the input data, flow design, and consistent topographic information (APFM, 2013). A major aid in the construction of flood maps is use of geoprocessing (Chen et al., 2009; Sarhadi et al., 2012; Demir and Kisi, 2016; Ovando et al., 2016; Liu and Yamazaki, 2018). Geoprocessing is a set of techniques based on the study of spatially distributed information in order to describe the characteristics of the phenomenon under investigation in the whole area of interest (Costa and Lourenço, 2011). The construction process of flood maps requires an understanding of flow dynamics over the floodplain, topographic relationships and sound judgments of the modeler (Noman et al., 2001; Sinnakaudan et al., 2003; Sanders, 2007; Alagmand et al., 2010; Sarhadi et al., 2012). Frequently, applications for flood hazard mapping found in the literature are based on low-resolution models, mainly using free digital elevation models, which are not vertically calibrated to a local geodetic reference system (Komi et al., 2017; Patel et al., 2017; Speckhann et al., 2017; Dhiman et al., 2019).

Thus, special attention is required for topographic data because a robust flood model needs high vertical accuracy linked to a geodetic reference system, since in many cases, flood-prone areas are found in marginal regions of low-slope floodplain and may embrace large areas, such as this case study (Uruguay River basin, Itaqui) (Gupta, 2009; Mistry, 2009). The modeling of a flood hazard map is linked to the local geodetic reference system, corroborated with the identification and monitoring of flood situations, and construction of realistic predictive scenarios (operational level) for risk management and adaptation (governmental agencies, such as the local civil defense) (Joshi et al., 2012; Gallien et al., 2018; Jongman, 2018). It is extremely important to do the calibration of the digital elevation model (DEM) using high-accuracy ground control point (GCP) data, aimed at improving the vertical accuracy, and applied to regional or local studies about floods (Araújo et al., 2018). Therefore, this study aims to conduct a robust mapping of flood hazard delimitation areas influenced by the Uruguay River, using the case study of Itaqui in the state of Rio Grande do Sul, southern Brazil, through a calibration of DEM from Shuttle Radar Topography Mission (SRTM) images, historical fluviometric level data collected at Itaqui station, and geoprocessing techniques.

The Uruguay River basin is one of the most important hydrographic basins of Brazil; it is located in the south of the country and extends throughout the neighboring countries of Argentina and Uruguay. Therefore, this basin marks the division between the Brazilian states of Rio Grande do Sul and Santa Catarina, and also between Brazil and Argentina. The Uruguay River basin occupies a total extension area of 385 000 km2, where approximately 274 300 km2 is located in Brazil, corresponding to 3 % of national territory. This hydrographic region has a prominent economic sector, mainly focused on the improvement of agricultural and industrial activities. The Uruguay River hydrographic region has great hydroelectric potential with a total production capacity of 40.5 kW km2, and when considering both Brazilian and Argentinian sides, one of the biggest energy per square kilometer relations in the world (ANA, 2015).

The Uruguay River basin area has a temperate climate, with a regular intra-annual rainfall distribution, and some highs during May to September, coinciding with the winter season in the Southern Hemisphere. The natural hydric availability of the Uruguay hydrographic basin is largely influenced by the significant spatial and temporal variables of a few climatic parameters, such as the pluviometric regime, which reflects directly on the economic activities developed in the region, which are largely in the agricultural sector (BID, 2008). Many areas were deforested due to the growth of agro-industrial activities in the region, which led to an environmental imbalance, from river siltation to water pollution with pesticides and their residues. Therefore, this region shows the original and remaining native vegetation cover of the Brazilian Atlantic forest biome and Araucaria moist forest biome. Geomorphology is dominated by rugged relief in the upper Uruguay River basin, followed by a flatter patch in the gaucho campaign region with shallow soil, which is the reason why the Uruguay River flows through a rocky substrate. This characteristic implies a flow regime that follows the rainfall trend: when periods of intense precipitation occur, they cause floods in riverside areas and, in the same way, when periods of drought occur, the flow regime is abruptly reduced, sometimes even to guarantee the hydric requirements of human and socioeconomic activities. In Brazil, the Uruguay hydrographic region is composed of 11 hydrographic sub-basins: (1) Apuae–Inhandava, (2) Passo Fundo, (3) Turvo–Santa Rosa–Santo Cristo, (4) Piratinim, (5) Ibicui, (6) Quarai, (7) Santa Maria, (8) Negro, (9) Ijui, (10) Varzea and (11) Butui–Icamaqua (ANA, 2015).

The study area comprises the urban area of Itaqui and is located on the banks of the Uruguay River on the western border of the state of Rio Grande do Sul, southern Brazil (Fig. 1). It corresponds to Ibicui sub-basin, the largest sub-basin of the Uruguay River, which has the Uruguay River and also the Cambai and Sanga das Olarias streams as its main water bodies. The study area has a territorial area of 34 066.06 km2 and an estimated population of 39 049 inhabitants according to the Brazilian official 2016 Census by the National Institute of Geography and Statistics (IBGE, in Portuguese). This area plays an important role in regional but also national economic scope due to rice cultivation, as well as Hereford and Braford bovine breeding, and commercial pig breeding for slaughter. These economic activities influence the region's environmental quality because they are directly related to the land use, occupation of the area, and increases in water consumption (Bariani and Bariani, 2013).

Itaqui was settled on the edge of the Uruguay River. Currently, this riverside location is occupied by communities, some of very low economic class, that depend on the river for their livelihoods. These dwellers are often displaced to higher regions to move away from the constant flood. Itaqui survives under the same conditions as most Brazilian small cities, with a lack of resources and of specialized professionals that can act in the projection and the orderly urban development. This fact is reflected in the lack of studies and information that aid the aim of urban management to minimize negative impacts on society.

This undue occupation causes various problems that have a direct impact on society, including floods within the urban area. One of the reasons is the disorderly urban sprawl without adequate infrastructure, along with the non-observance of the natural characteristics of the occupied environment that, as a consequence, means that it is impossible for areas near rivers and creeks to absorb floods (Silva et al., 2017). It is important to highlight that, in addition to the lack of financial resources to be invested in the municipality, there the almost constant expense of the damages caused by the increase in river level. Most riverside families live in so-called “volantes houses” (Fig. 2); these are wooden houses and can be transported from one place to another. But in some extreme events such as the one that occurred in 2017, some families lost all of their belongings and in some cases even their homes, leaving it to the local government to help with the expenses of these communities (Fig. 3).

The flooding process of the Uruguay River in Itaqui is a natural phenomenon that has afflicted the riverside population for decades. It is practically intrinsic to the city's history. However, even though it is a relevant problem to the local population, only after 2011 were papers published emphasizing the local hazards and risks of this natural phenomenon. Among the published papers, we emphasize Saueressig (2012), Saueressig and Robaina (2015), Silva et al. (2017), and Silva (2017). All of these are results of Masters dissertations in postgraduate programs in Brazil.

Saueressig (2012) emphasized a zoning of flood risk areas of the urban marginal area of the Municipality of Itaqui. The author organized an inventory of floods that happened between 1980 and 2010, and focused on a socio-environmental discussion, developing models with a few consistent criteria in relation to altimetry.

Silva (2017) concentrated on the usage of geodetic methods to elaborate a DEM of integrated elevation with hydrologic data for monitoring affected areas lined by the Uruguay River. However, Silva (2017) focused his efforts on the marginal urban areas of Itaqui.

Thus, this work intends to fill an information gap, covering the entire urban area of the city, with the hazard in focus, priming a methodological application that uses high-accuracy altimetric data to model flood hazard maps. Furthermore, a statistical analysis using a temporal series of maximum annual level records of Uruguay River data was developed, which will provide innovative information about the return period of the flood phenomenon in study region.

The temporal series of annual maximum fluviometric records of the Uruguay River, from 1942 to 2017 (76 years of data), linked to Brazilian Geodetic System (Sistema Geodésico Brasileiro – SGB) and using geometric leveling, was submitted for statistical analysis. Then, calibration of a digital elevation model (DEM), obtained free of charge from the Shuttle Radar Topography Mission (SRTM), was conducted using ground control points (GCPs) of high vertical accuracy. The results of the temporal series statistical analysis served as the basis for the spatialization of floods on the calibrated DEM. Finally, a visual comparison between a modeled flood and a digitally processed image from the flood area satellite image was performed, both having been registered concomitantly on the same day in study region.

In summary, the flood simulation model is based on the fill of the DEM, calibrated at river level orthometric heights, and linked to a common geodetic reference system. The workflow of the proposed methodology is shown in Fig. 4.

In this work the use of geoprocessing techniques was extremely important to reach the results. We use the following techniques:

Geographic information system (GIS) was the technique most used throughout the work. All data were used to implement a robust GIS.

A large part of the city of Itaqui is in floodplain area of the Uruguay River, as are many of the riverside cities of the study area (Saueressig, 2012). The floods in these regions are intimately linked to the rise of the river level. Therefore, in the case under study, the river level altimetry is the main driver of the flood hazard.

Flood hazard maps are crucial for planning and intervention in flood-prone areas. The results of the Itaqui case study can mainly be applied to characterizing hazards and supporting the implementation of flood risk management plans and flood risk maps for river basins and coastal areas, thus improving the overall availability of such risk management tools. An equivalent methodological approach could be helpful, for instance, in preparing urban charts to identify areas more suitable to occupation in the municipality. In cases where the areas are already occupied, like in some locations of the Itaqui city center, the result could be useful for defining and implementing the necessary measures that address potentially damaging events.

In Brazil, only a few studies involve mapping to assess potential flood damage. Spatial details of the hazard indicators are a valuable tool for flood risk management since the map provides a more direct and fast assessment than other methods. The methodology and materials applied to Itaqui proved effective in identifying flood hazard areas using free DEM from SRTM images that were calibrated based on a post-processed high-precision GNSS database, historic river level gauge data, and geoprocessing techniques, as basic information.

It is worth mentioning that the methodological approach applied to the Itaqui is adequate to be replicated to other municipalities, in particular those of riparian and coastal communities. The flood hazard mapping methodology could be particularly useful for regions with a good historic time series of fluviometric level gauge data. In general, the most limiting factors to be considered for adopting these methodological approaches are currently the availability of high-accuracy altimetry points, the GCPs, for calibrating the free DEM from the SRTM scene in regions impacted by a series of flood events. Such a procedure involves a large amount of time, both for the collection of local GCP data and also for post-processing data treatment. However, depending on the geographic location, IBGE level references (RRNN) can be used as GCPs.

This flood hazard mapping in digital format can be used as a database to assist governmental stakeholders, e.g., civil defense, in partnership with other sectors of civil society for implementation of flood risk management plans that are more adaptable to local restrictive environmental constraints. Thus, composing these plans can be more flexible, easily modifiable, and updatable in accordance with the distinguishing physiographic and socioeconomic setting of the river basin.

In this distinct case study, the scenario modeling for the 1983 and 2017 flooding events were compared with a real flood of 12 June 2017 and high visual similarity was established between them. The study area generally covers residential areas. Considering the results of this case study, it is crucial to advise the Itaqui policy makers and politicians that a planned public policy should be implemented to relocate part of the urban population that occupy areas of high flood hazard. However, the removal process of inhabitants to other areas without hazard, or even less risk, is complex and requires a multidisciplinary strategy. It is already known that the population will suffer historical, social, and cultural impacts that will make the planning more difficult to be entirely applied. Therefore, it is necessary that people have decent housing, with adequate infrastructure and basic sanitation, reducing their exposure and vulnerability to risk adversity. Furthermore, it is necessary for communities to understand the importance of these changes, not only for the population's health but also for the Itaqui economy and sustainability.

This paper also demonstrates that well-applied technical measures based on geotechnologies, such as remote sensing, GIS, and high-accuracy geodesy, give results in return that can be very effective in urban and rural management with low-cost investments, highlighting the unique features of a given locality, especially floodplains and flat low-lying areas. This methodological approach is very effective for mitigating flooding damage in coastal and riparian areas. It can be valuable in reducing strategic monitoring costs and the operational expenses of providing assistance to population affected by severe flooding events and their consequences.