1 Introduction

Ravines and gullies are advanced forms of soil erosion, whose evolution can be divided into three stages: (1) surface water runoff concentration or groundwater contribution through liquefaction; (2) V-shaped linear channel development; and (3) stabilization of the downcut after reaching the water table, development of the channel and U-shape resulting from the joint action of surface and underground runoff leading to a morphological and hydropedological imbalance, which can branch out into several channels (Carvalho Junior et al. 2010; Abdo et al. 2013).

Human influence is usually the main trigger, while factors such as soil type, relief and hydrogeological properties determine the susceptibility to erosion and the consequent magnitude and speed of the process (Castillo and Gomez 2016). The development of gullies and ravines following agricultural land use change is well documented in many tropical developing countries (Mirghaed et al. 2018). However, a literature review indicated that only 3.1% of the studies address ravines in urban areas, while 13.2% address ravines in forest areas and just over 40% address ravines in pasture and plantation areas (Castillo and Gomez 2016).

Although scarcely studied, ravines and gullies impact cities worldwide. Balzerek et al. (2003) report the destruction of residences, streets and bridges in Nigeria. Hundreds of ravines in Kinshasa, Democratic Republic of the Congo, caused the destruction of housing and urban infrastructure (Ozer 2014; Imwangana et al. 2015; Landu et al. 2023; Mawe et al. 2024). In Tanzania, in the city of Babati, Strömquist (1992) reported the destruction of urban infrastructure by gullies. In Argentina, in the city Comodoro, more than 400 houses were destroyed or damaged, in addition to various urban infrastructures, as a result of ravines or gullies (Paredes et al. 2020).

Although the risk of disasters in cities cannot be totally eliminated, it can be reduced (Lall and Deichmann 2012). Market price analyses, which allow the estimation of the economic cost of impacts, can contribute to creating incentives or mechanisms for transferring the risk of losses, combined with economic policy and urban management, make it possible to develop an assertive response (Logar and van den Bergh 2013).

The low number of fatalities related to ravine and gully normally does not make international headlines (Balzerek et al. 2003). Likewise, studies estimating the cost of ravines and gullies are rare. However, Yitbarek et al. (2012) performed a cost‒benefit analysis of ravine and gully rehabilitation on farmland in Ethiopia. In the same country, Ayele et al. (2015) quantified the cost of erosion, considering topsoil nutrient losses, time costs due to disrupted travel networks, and the value of lost animals and trees. Additionally, in the Fitzroy region of Australia, Rust and Star (2018) discussed strategies to reduce the amount of sediment export from ravines and gullies, considering the costs of the measures applied.

In Brazil, the impacts of ravines and gullies in urban areas have been studied by several authors. Guerra et al. (2018) analysed a case of ravines in the city of São Luís in the state of Maranhão. Rotta and Zuquette (2014) developed methodologies to study erosion based on the municipalities of São Pedro, Franca, São Carlos and Casa Branca in the state of São Paulo and in the municipality of Fortaleza in the state of Ceará. Gullies have caused economic impacts in dozens of municipalities, especially in the Cerrado (Brazilian Savanna) and Amazon Rainforest biomes, related to changes in land use (Kuhn et al. 2023a, b). One of the Brazilian cities with the highest number of cases of erosion is Bauru, in the state of São Paulo, where Almeida Filho (2000) described more than 25 gullies or ravines in the late 1990s, which represented a loss of almost 2 million m3 of sediment. However, in none of these cases, an assessment of the impacts was carried out, leaving open the question: What are the economic impacts caused by a ravine or gully?

Considering the replacement price of different items, this work aims to estimate the economic impacts of ravines and gullies in the municipality of Bauru, Brazil. For this purpose, Google Earth timelapse, as well as previous studies conducted in the municipality and field campaigns conducted herein, were applied to reconstruct the history of the three regions analysed in this study.

1.1 Study area

Founded in 1896, the emergence of the city of Bauru (Fig. 1) was due to the establishment of coffee plantation areas in the second half of the nineteenth century. The construction of the railroad in the early twentieth century gave the city a privileged position related to logistics (Alves 2009), which boosted population growth. The appearance of ravines and gullies worsened in the second half of the twentieth century with the creation of subdivisions without considering geotechnical criteria (Da Silva and Barbieri 2004; Ide et al. 2009). The sandy lateritic soils, which are slightly cohesive and permeable with depths of more than 13 m, in a region where wide hills predominate and which annually experiences tropical rains that exceed 100 mm daily, combined with land occupation without proper planning, created a scenario that favoured the development of ravines and gullies that can exceed 1500 m in length (Salomão 1994; Almeida Filho 2000; Da Silva et al. 2004; Rocha and Giacheti 2018). Ravines and gullies cause several impacts in the municipality, including loss of land and property values and silting of rivers and consequent flooding (Santos 2008).

Fig. 1
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Location map of the municipality of Bauru

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2 Materials and methods

The work was carried out in two main stages: (a) analysis of the evolution and impacts of ravines and gullies and (b) assessment of impacts (Fig. 2).

Fig. 2
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Summary of the methodological approach adopted in this study

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2.1 Analysis of the evolutions and impacts of ravines and gullies

Theses, dissertations, scientific, peer-reviewed articles, technical reports and other publications were used in the identification of erosional events and estimation of their impacts in Bauru. The work by de Almeida Filho (2000) was considered the most complete and was used as a basis for obtaining descriptions of ravines and gullies. Then, Google Earth images taken between 2004 and 2021 were analysed to (a) identify changes in the geometry of the channel, (b) remeasure the basin for containment or restoration of the area, (c) monitor the change in land use and land cover and (d) determine the environmental impacts to the region. In addition to the information obtained from the analysis of the images, two field studies were carried out, in December 2020 and in March 2022 to analyse the areas.

The ravines and gullies analysed were classified as follows: (a) recovered: the erosion scar area was restored through recovery measures and the social and economic use was resumed; (b) stable: erosion was stabilized due to measures aimed at recovering the area, but without fully restoring the use of the area; (c) in development: ongoing, gradual increase in the size of the eroded area over time.

The watershed limits of the areas chosen for the case studies were extracted using tools available in ArcGis, using Shuttle Radar Topography Mission (SRTM) images available on the Topodata portal of the National Institute for Space Research of Brazil (INPE).

2.2 Impact valuation

To identify the impacts of urban ravines and gullies, the classification proposed by Kuhn et al. (2023a, b) was employed. Impacts were classified into (a) environmental, considering the cost of replacing soil, nutrients, carbon and pasture; (b) costs for recovery and mitigation of erosion in the watershed area; (c) destruction of infrastructure, impacts on streets, storm sewers, dams, residences, among others; (d) estimated economic losses, which were calculated considering the surface area directly affected by the channels in ravines and gullies, the annual loss of gross domestic product (GDP) per square metre in the affected area in allotments; (e) loss of collection of the Property and Urban Land Tax (IPTU); and (f) positive impacts after the recovery of areas affected by ravines and gullies.

The analysis time of the estimated economic impacts and tax collection losses was limited to between 2000 and 2021 based on the information sources described in Sect. 2.1. The cost of the environmental impacts, public infrastructure destruction, and erosion containment and recovery associated with urban gullies and ravines were estimated by considering identified infrastructure close to erosions and descriptions of the channels affected by erosions, that is, all impacts that could be identified from the onset of erosion to the date of the current analysis.

The valuation of the impacts was carried out considering the replacement cost, which is the amount currently paid for the affected items and products (Lillo et al. 2014). The values obtained allow identifying the magnitude of the impacts.

Because current replacement cost values and impact dimensioning were used, inflationary updates were not performed, as it is understood that these values are already considered in the current cost. To convert the value from Brazilian real to U.S. dollar, the reference value of 5.30 reais to 1 U.S. dollar (as of November 14, 2022) was used.

2.2.1 Environmental impacts

For soil-related environmental costs, such as loss of vegetation, information from the Anuário da Pecuária Brasileira [Brazilian Livestock Yearbook] (Anualpec 2022) was used. The previous standard vegetation was considered pasture and semi-intensive. According to the yearbook, pasture is worth US$ 756.68/ha.

The loss of nutrients—nitrogen (N), phosphorus (P) and potassium (K)—and of organic carbon (OC) in the soil caused by ravines and gullies were monetized considering the replacement cost, as previously done by Yitbarek et al. (2012). For the analysis of replacement of soil nutrients (NPK), data from Radam Brasil (Brasil 2018) were considered for up to 1 m of soil depth, according to analyses carried out on the Pederneiras-Bauru Road. A reference value of 1.4 t/m3 of soil was adopted. The total value of nutrient loss per square metre (Table 1) was calculated according to the market price of the fertilizer sector statistical yearbook (Anda 2022) for 2021.

Table 1 Amount of nutrients per square metre and replacement cost
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For the analysis of the carbon stock, data from Sobral et al. (2014), who reported that the carbon stock in the region ranges from 49.2 to 221.0 Mg ha−1, with 96.5 Mg ha−1 of C being stored in undisturbed vegetation types; thus 9.65 kg/m2 was adopted as a reference value for the region. According to the Power BI platform (2022), the average price paid for carbon stocks is $32.37 per ton (22/10/2022). Thus, the value of 9.65 kg/m2 proposed by Sobral et al. (2014) represents a corresponding stock of $0.31 of carbon per square metre.

2.2.2 Erosion recovery and containment and public infrastructure destruction

To estimate the costs of containment works and recovery of the affected areas and destruction of homes and infrastructure, the National Research System of Civil Construction Costs and Indices (SINAPI), the System of Reference Costs of Works (SICRO) and Civil Construction Union (Sinduscon) were used for cost analysis of engineering works in Brazil (Sinapi 2021; Sicro 2021; Sinduscon 2021) according to the compositions elaborated in Table 2:

Table 2 Compositions used to calculate impacts, according to data from SINAPI (2021), SICRO (2021) and SINDUSCON (2021)
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2.2.3 Estimated economic impacts

To estimate the economic losses caused by the impacts of urban ravines and gullies, an analysis of the GDP per square metre was also carried out, according to official information available from the Brazilian Institute of Geography and Statistics (IBGE 2019, 2022). The value of the municipal GDP is US$ 2,891,432,319, divided by an area of 667,684,000 m2, which represents a GDP/m2 of US$ 4.33 per year. Estimates of economic impacts were carried out considering a time interval of 20 years between 2000 and 2020.

The loss of useful area of urban space was calculated by the average of the existing values in Municipal Law No. 7,510/2,021 (Bauru 2021), which corresponds to US$ 72.88/m2. The costs of the affected area were calculated based on the market price available on online sales sites. The following average values were used: (a) US$ 72.57 per square metre for unused lots on paved streets; (b) US$ 36.28 per square metre for unused lots on unpaved streets.

2.2.4 Urban property and land tax (IPTU)

Municipal losses or gains from tax collection were calculated using the information available in Municipal Law No. 7510/2021. The calculation considered the following equation as the Urban Land Tax (IPTU) value: 1% of the value resulting from the multiplication of the average land value per square metre (US$ 72.88) by the size of the affected area and the number of years that erosion has affected the site (Bauru 2021, 2022). Estimates related to gains or losses in IPTU collection performed considering a maximum time interval of 20 years.

The calculation was carried out considering the lots indicated in the Google Earth images or estimates made based on the existing streets in the area; that is, areas used for streets, sidewalks, green areas and other infrastructure were not considered.

2.2.5 Data presentation

The data obtained in the analysis are presented in spreadsheets, considering the values obtained in the impact analysis considering the number of units and the total value of the impact. For impacts that are continuous, such as Urban Property and Land Tax (IPTU) and estimated economic impacts, the value over 20 years was considered.

3 Results and discussion

3.1 History of erosion processes in Bauru

Several studies, such as de Salomão (1994), de Almeida Filho (2000), Da Silva and Barbieri (2004); Corghi (2008), Santos (2008), Ide et al. (2009) and Thomazini and da Cunha (2012) have been carried out to understand the evolution of erosion in the municipality of Bauru.

de Salomão (1994) carried out an analysis of aerial photographs from 1972 and identified a total of 516 ravines and 409 gullies in the region, most of which formed in drainage headwaters after deforestation due to changes in hydrological conditions related to increased surface runoff. Some were also formed due to the impacts of deepening and widening of river channels.

The most impressive case recorded in the municipality occurred between November 1992 and April 1993. The month of February 1993 was atypical, with the monthly rainfall exceeding 450 mm, approximately 250 mm above the usual amount. The gully formed was 800 m long, with an average width of 26 m, a maximum width over 50 m and a depth ranging from 15.5 to 28 m, removing a volume of approximately 3,60,000 m3 of sediment (de Almeida Filho 2000). According to Santos (2008), there was one death related to the development of the gully.

de Almeida Filho (2000) described in detail the 25 areas affected by ravines and gullies in the late 1990s (Table 3). The survey of erosion evolution in the municipality of Bauru between 2004 and 2021 identified 7 features in the recovered class, 7 in the stable class and 11 in the in-development class. In some regions, such as Quinta da Bela Olinda, although the main gully has not shown reactivations in the last 20 years, new large-scale erosion has appeared, increasing the impact on the basin.

Table 3 Current situation of erosions described by de Almeida Filho (2000)
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In the municipality of Bauru, ravines and gullies caused the following impacts: silting of watercourses and reservoirs (Fig. 3a); destruction or clogging of the storm sewer network (Fig. 3b); soil loss, destruction and breakdowns in the water and sewer system; expansion of floods; dispersion of pollutants; isolation of housing or neighbourhoods; social and economic damage caused by flooding; increase in the cost of maintaining streets, canals and sewers; environmental changes caused by siltation (Fig. 3c and d); depreciation of real estate value; discouragement of new investments; decrease in water potential; loss of agricultural areas; increased cost of production and increased cost of water treatment (de Almeida Filho 2000; Da Silva and Barbieri 2004; Thomazini and Da Cunha 2012). Another type of impact cited is decreased urban mobility (Cunha 2020). For Santos (2008), among the problems triggered by erosion is the fragmentation of the city and the creation of urban voids.

Fig. 3
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a Erosion process at the valley bottom, where the lowering of the channel upstream and the silting and drying of wetlands downstream occur; b ravine at Quinta da Bela Olinda, developed due to problems in stormwater sewers; c silted-up area in the region of Quinta da Bela Olinda; d soil profile, indicating lowering of the channel and drying of wetlands

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Bauru is located in the Western Plateau of São Paulo, located within the Tiete watershed. The Bauru and Ribeirão do Campo Novo River subbasins are composed of wide hills. The Ribeirão da Água Parada area is composed of 85% wide hills and 15% medium hills. In the Batalha River subbasin, 66% of the area are wide hills, 9% are medium hills, 8% are elongated hills and spires and 7% are slopes furrowed by subparallel valleys (de Almeida Filho 2000). According to de Salomão (1994), the local relief favours the concentration of rainwater.

In addition to the relief in Bauru being composed of wide hills, and the thick, loose and sandy soils favour soil erosion. According to de Salomão (1994), the changes in water conductivity and porosity between different soil horizons and between different soil types, added to ruptures in the topography, favour the development of the piping phenomenon. This occurs because in the higher positions of the slope, the more developed soil facilitates water infiltration, while in the lower positions, in addition to the natural tendency of the water table to be closer to the surface, the less developed soil contains more clay and the few, generally non-communicating macropores increase pressure and percolation forces (de Salomão 1994).

Some erosions have affected large areas in the urban perimeter since the 1970s, as is the case for erosions located in the Jardim Grama region, which have been reactivated more than three times over the last 50 years, mainly related to years with atypical rainfall events or due to changes in land use in the watershed in areas close to the erosion process. Another problem is that the adopted structural measures are often inefficient or may lose efficiency due to the advancement of erosion or to the lack of periodic maintenance (Fig. 4a and b). The solutions adopted are, in general, palliative and do not resolve the causative factors, with inefficient measures that prioritize the channel of the formed feature, without considering the processes within the watershed as a whole.

Fig. 4
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Erosion containment structures damaged by lack of maintenance or sizing errors: a Jardim Grama; and b Quinta da Bela Olinda

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Analyses of the evolution of land use and occupation between 2004 and 2021 also illustrate the dispute for urban space. The urban voids caused by ravines and gullies in development were the result of occupation by low-income communities in search of sites to build homes. Conversely, other areas, such as the erosion of Jardim América, were fully recovered, returning to the conventional social and economic use, because of its good location in the urban perimeter and appreciation of the area, which enabled the construction of a high-end condominium.

Although Brazilian law currently prohibits it, de Almeida Filho (2000), Da Silva and Barbieri (2004), Corghi (2008) and Ide et al. (2009) cite the use of areas affected by erosion, such as ravine and gully channels for the disposal of urban waste and even hospital waste, as is the case of Vila Garcia, Pousada da Esperança II, Jardim Paulista, Jardim Colonial, Vila Jussara, Vila Santista, Cohab 16 and Quinta da Bela Olinda. In places such as Parque Bauru (Fig. 5a) and in the gully of Vila Ipiranga (Fig. 5b), garbage disposal, including of construction and organic waste, was observed during the field stage. These residues can contaminate the channel with leachate and pollute the environment through the release of gases, transforming these areas into areas of disease outbreaks and creating other types of problems related to soil and water contamination. According to Santos (2008), the use of technogenic materials, without performing technical analyses, aims at reducing costs for companies and may increase environmental impacts due to contamination of the soil and aquifers.

Fig. 5
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a Use of the area affected by erosion for urban waste disposal (pruning, organic and construction waste); b waste disposal in the gully channel in Vila Ipiranga

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Some erosions have been fully or partially recovered. In the CESP gully, part of the area was recovered for the construction of an avenue. The erosion of the industrial district was recovered for the construction of industrial warehouses. In Jardim Ouro Verde, the erosion area was recovered for the construction of a soccer field. In Jardim América, the area was transformed into a high-end condominium.

In Núcleo Mary Dota, an allotment built over a buried gully, several houses have cracks, demonstrating the technical difficulty of recovering the affected areas (Santos 2008). The problems can be aggravated because in some cases, companies do not follow the technical suggestions for works in areas affected by ravines and gullies. In some cases, there are changes in the drainage regime and migration of watercourses (Santos 2008).

The lack of an official technical record in areas affected by erosion made it difficult to build a detailed history of actions taken to analyse the related costs and technical solutions adopted.

3.2 Valuation of impacts

Three areas were selected to assess the economic and environmental impacts (Table 3). The areas were chosen for the following reasons: (i) Quinta da Bela Olinda, an area where there are ravines and stable and developing pits; (ii) Jardim América, as it represents a place where social and economic use has been fully recovered; (iii) Parque Bauru, w one of the erosions that is partially recovered.

3.2.1 Quinta da Bela Olinda

The area of Quinta da Bela Olinda is located in the urban expansion zone. In this location, 5 linear erosions were identified (Table 4 and Fig. 6), four of which (1, 2, 3 and 4) remained stable between 2004 and 2021, but in this period, ravine number 5 appeared due to problems in a rainwater pipeline. Despite attempts to stabilize the process, erosion has been growing at an accelerated rate (Fig. 7a and b). The largest gully in the area is stable, with vegetation in the channel and its surroundings and the development of a water source inside it (Fig. 7c). Other erosions, although stable, predominate the pasture, with few trees (Fig. 7d).

Table 4 Existing erosions in the Quinta da Bela Olinda area
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Fig. 6
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Quinta da Bela Olinda area and identified impacts

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Fig. 7
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a Top view of ravine 5, the implemented works already have some of their functionality damaged, b top view of ravine 5, where it is possible to observe grooves demonstrating the concentration of surface runoff into the ravine channel; c gully 1, which shows water flow in the channel and vegetation composed of trees in the surroundings; d ravine 2, with pasture and small trees inside and around the channel, indicating stability of the feature

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Despite several attempts to stabilize ravine 5 (Fig. 6) by carrying out engineering works in the channel, including the implementation of barriers at contour lines, placement of rainwater drainage pipes and construction of gabion walls, erosion advanced in an accelerated manner after November 2017, causing a risk to homes and infrastructure located upstream. The works implemented thus far have proven to be palliative and ineffective in containing the advance of erosion.

Da Silva et al. (2004) note that the development of gullies made unfeasible a housing complex that already had a water distribution structure, sewage collection network, and asphalt and rainwater drainage system ready. According to the authors, part of this structure was destroyed. In the analysis of aerial images, it is possible to identify elements of the affected urban infrastructure, such as isolated and abandoned paved streets due to the development of erosion processes.

Table 5 and Fig. 6 show the impacts identified in the Quinta da Bela Olinda area, namely, soil erosion, loss of carbon stock, loss of pasture, loss of soil nutrients (NPK), destruction of storm drain pipelines, silting of the dam, destruction of streets, isolation of paved streets, unfeasibility of lot construction due to lack of access, unfeasibility of social and economic use of urban areas, loss of municipal tax collection, economic losses due to non-social use and economic impact of the areas surrounding the erosions. The following measures taken to control and stabilize erosion were identified: construction of contour lines, infiltration basins, hydraulic ladders, energy sinks and canalization of part of the erosion channel.

Table 5 Estimate of the impacts of erosion identified in the Quinta da Bela Olinda area
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The impacts of erosion at Quinta da Bela Olinda are estimated at US$ 76.4 million, including US$ 17,300 in erosion recovery and containment, US$ 1.65 million in environmental impacts, US$ 72.5 million in estimated economic losses, US$ 1.23 million in lost municipal tax collection, and US$ 949,600 in infrastructure destruction.

3.2.2 Parque Bauru

Erosion in Parque Bauru developed mainly in February 1993 (Fig. 8a). To contain erosion in Parque Bauru, in February 1993, four works were carried out in the area: dams and infiltration basins upstream and works to divert and conduct rainwater downstream, in addition to the implementation of drainage systems in streets parallel and transverse to the gully. In October 1993, three more works were carried out: earth dikes with underground drainage with sewer networks (Fig. 8b), mixed underground drain and erosion embankment with slope abatement (Almeida Filho 2000). Due to the large volume of eroded soil, the gorge impacted the downstream drainage basin, causing the silting of the river and dams such as the one at Bauru Country Club (Fig. 8c and d).

Fig. 8
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a Gully of Parque Bauru in 1993; b emergency works to mitigate erosion; c Bauru Country Club dam before the development of erosion; d silting of the Bauru Country Club dam due to the development of erosion upstream in 1993 (Photos: Nariaqui Cavaguti)

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Despite these emergency and subsequent works, in 2000, an accelerated erosion process from downstream to upstream was observed, compromising the works already carried out, with the destruction of 100 m of drainage pipes (de Almeida Filho 2000).

In 2010, reports mention that erosion continued to cause problems. Because of this, in 2011, emergency works were carried out in Parque Bauru to recover the streets. To contain erosion on Maria de Lourdes Almeida Street, 65 trucks filled with dirt and debris were used. Other works, such as the construction of new sewers and paving, were carried out around the area affected by erosion in 1993. In 2016, a hole 3 m long and 4 m deep caused the partial collapse of a house. City Hall carried out new recovery works for sewers in the streets of Parque Bauru neighbourhood (Mello 2018).

Although it is not possible to identify all the impacts caused due to changes made in the area and the lack of adequate records, the erosion in Parque Bauru has caused recurrent problems in the area since its formation in 1993. The image analysis and field work allowed us to identify the following impacts (Fig. 9): loss of soil, loss of nutrients (NPK) and carbon stock, silting of the dam of the former Bauru Country Club, destruction of houses and public structures, construction of erosion channels, making social use unfeasible, economic impact to the urban area, loss of pasture, loss of municipal tax collection, and economic losses due to the social and economic non-use of the areas surrounding the erosions (Table 6). The case of the gully in Parque Bauru demonstrates the partial recovery of an area affected by ravines and gullies, but part of the impacted area has continued to have no social use since 1993, in addition to causing damage to infrastructure and homes in the region.

Fig. 9
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Parque Bauru area and identified impacts

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Table 6 Estimate of the impacts of erosion identified in the Parque Bauru area
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The impacts of the sinkhole in Parque Bauru are estimated at US$ 72.98 million, with US$ 1.65 million related to erosion recovery and containment, US$ 7.18 million to environmental impacts, $ 63.3 million to estimated economic losses, US$ 704,700 to lost municipal tax collection, and US$ 81,800 to infrastructure destruction. The damage caused by this erosion was not greater because the works carried out in 1993 allowed the control of erosion and the occupation of the area around and even under the old channel in the areas close to the head of the erosion.

The impacts caused by the erosion of Parque Bauru could be even greater, but the works carried out in 1993 allowed an area of 51,000 m2 affected by erosion to be reinserted into the urban space in 1993; currently, this area houses more than 77 buildings (Fig. 9).

3.2.3 Jardim América

The Jardim América gully emerged between 1987 and 1988 and, since then, has been used for garbage disposal (de Almeida Filho 2000). In 1993, City Hall implemented a drainage system inside the erosion with a network of sewers and a landfill with rubble. Erosion was completely recovered between 2004 and 2010, and a gated community with high-standard housing was constructed on the site. This case demonstrates that areas affected by ravines and gullies, if recovered with appropriate techniques, can be reinserted into the urban space, returning to social and economic use.

Among the impacts identified in the area until its recovery were soil erosion, loss of nutrients (NPK) and carbon stock, unfeasibility of social and economic use of the urban area, loss of municipal tax collection, economic losses due to the non-social and economic use of areas surrounding the erosion, and loss of pasture (Fig. 10). In the areas, the containment and recovery measures identified were construction of contour lines, infiltration basins, drainage systems and sewer networks and landfill in the affected area (Table 7). After the recovery of the area, the value of a square metre of land increased to more than US$ 188, demonstrating a significant appreciation of the area. This change also increases the municipal tax collection.

Fig. 10
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Jardim América area and identified impacts

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Table 7 Estimate of the impacts of erosion identified in the Jardim América area
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The negative impacts of the Jardim América sinkhole are estimated at US$ 23.2 million, of which US$ 110,000 is related to erosion recovery and containment, US$ 4.67 million to environmental impacts, US$ 17.32 million to estimated economic losses, and US$ 1.16 million to lost municipal tax collection. In contrast, after the recovery and occupation of this area, the minimum estimated gains are US$ 19 million, with US$ 3 million from municipal tax collection after recovery of the area and US$ 16 million in estimated economic gains based on GDP per square metre.

4 Discussion

The impacts of the three areas represent significant amounts spent by the government. Regarding costs with recovery and construction of containment structures, the highest estimated value was estimated for Parque Bauru, which exceeded US$ 1.65 million. With regard to environmental impacts, analysed by replacement cost, the highest value was estimated for soil replacement, which, due to the large volume of eroded material, can represent values of US$ 7.1 million in cases such as Parque Bauru if a complete replacement of the area affected by erosion were to be carried out with soil extracted from another location. Considering the three areas analysed, the cost of replacing the eroded soil is estimated at US$ 13.3 million.

The loss of soil nutrients, although having lower values, also exceeded US$ 27,800 in the case of Parque Bauru and US$ 17,000 in Quinta da Bela Olinda. These high values are the result of the large surface area affected by erosion channels.

The loss of carbon in the soil represented US$ 17,300 when adding the three areas analysed. According to Somasundaram et al. (2018), ravines are fragile ecosystems, and the recovery of carbon stocks is a slow and long-term process that requires continuous input of biomass into the soil. Thus, the recovery of these areas requires adequate conditions for a long period.

Quinta da Bela Olinda was the area where the value of infrastructure destruction was more relevant, as the damage exceeded US$ 949,000. The main cause for this value is the destruction of public infrastructure built for the development of the subdivision, such as paved streets, sidewalks and rainwater drainage systems.

The highest values found correspond to estimated economic impacts and losses municipal land tax collection.

Ravines and gullies in Bauru have created large urban voids. When calculated, the annual economic losses exceed values US$ 4.4 million in Quinta da Bela Olinda. This value is explained by the large unused area due to the development of erosion and the non-construction of the lots planned for the area, which was made unusable by erosion, despite the entire urban structure built. Likewise, the loss with municipal land tax collections between 2000 and 2010, considering the average IPTU value, exceeds US$ 113,000 per year in areas such as Jardim América. In this particular case, a comparative analysis of tax values can be performed for before and after recovery of the area affected by erosion. After 2010, a high-standard condominium was built on the site, where the value per square metre land exceeds US$ 188 dollars; thus, the IPTU value in the area, currently collected by the City Hall, is at least approximately US$ 300,000 a year. This change in value in the area after recovery is yet another strong argument for calculating losses in municipal taxes and the average value of GDP per square metre in analyses of impacts of erosion in urban areas.

If the impact from 2000 to 2021 is considered, Quinta da Bela Olinda had a municipal loss of US$ 76.4 million, Parque Bauru had a municipal loss of US$ 73.2 million, and Jardim América had a municipal loss of US$ 23.3 million up to the beginning of recovery, while after recovery the economic gains, including with municipal taxes, were at least US$ 15.8 million in Jardim América. Of this total, more than US$ 3.5 million was lost with IPTU collection in the three areas. In the Jardim América area, in the period between 2010 and 2021 alone, the gain in revenue after the recovery of the site exceeded US$ 3 million.

Among all the erosions described by Almeida Filho (2000), the recovery of areas affected by ravines and gullies occurred mainly for the development of public works, transport or leisure, or private undertakings related to subdivisions and condominiums. This is explained by the ease with which a large enterprise manages to dilute the cost of recovery among the gains provided by the reintroduction of social use to the area. The difference in recovery measures also results from the position of the erosion in the urban perimeter. In Jardim América, which is located in an upscale neighbourhood, the surroundings were occupied while leaving the feature area as a vegetated area with public squares and other spaces. In Parque Bauru, which is a popular neighbourhood, houses are being built on top of the channelled and buried feature, increasing the risk in this case.

The emergence of new ravines and gullies can increase the impact of these areas. This is the case for ravine 5 in Quinta da Bela Olinda, which, due to rapid growth, is getting closer every year to houses and other public infrastructure located upstream. The reactivation of erosion in periods of atypical rainfall or due to changes in land use is another factor that requires attention from City Hall and owners in areas affected by erosion or people who live in the surroundings.

Both the development of containment works and the recovery of areas affected by erosion need to be well documented to build a history of the measures taken in the area, so that it is possible to carry out cost analyses and more easily solve any future geotechnical problems.

Valuations carried out in rural areas by Yitbarek et al. (2012) and Ayele et al. (2015) calculated the values for variables similar to those analysed in this work, such as the cost of recovering the area, loss of nutrients, and cost of missing opportunities.

Yitbarek et al. (2012) quantified the costs of ravine erosion and the costs and benefits of rehabilitation in an area in Ethiopia. The authors used as a reference the cultivated area yield lost by farmers and calculations of rehabilitation costs, considering the cost of labour, materials and equipment at each level of rehabilitation or maintenance, in addition to the monetization of soil nutrients (NPK) and organic carbon (OC). The study indicated that ravine rehabilitation may be economically viable in some cases. The average cost of investment in gully rehabilitation per hectare was calculated at US$ 24,412. In the analyses carried out by the authors, the rehabilitation nutrient benefit generally exceeds the rehabilitation establishment cost by up to 25% more, as is the case for the Eshim Wofena gully, where the cost of erosion was estimated at approximately € 7000, the rehabilitation nutrient benefit at € 57,100 and rehabilitation establishment cost at € 41,400.

Ayele et al. (2015) quantified the cost of erosion in Ethiopia in a watershed for two years and concluded that the cost of erosion was at least US$ 18,313, with US$ 7848 related to loss of soil nutrients, US$ 7204 to missed job opportunity cost, US$ 1208 to animal losses, US$ 243 to eucalyptus wood losses, and US$ 1810 to Rhamnus prinoides losses. According to the authors, the cost per ha per year was US$ 22, or US$ 17 per person per year, a figure that represents 19% of per capita income. The loss of nutrients in the soil and the cost of lost opportunities were the main impacts identified.

In rural areas, where the function of the land in general is related to agricultural production, the cost of ownership is much lower per square metre, and the ease of replacement of the area is greater. In urban areas, however, competition for land and the value per square metre are greater due to the proximity to numerous economic activities and public structures. The use of variables such as GDP per square metre was a simplified way of considering the economic complexity involved in urban space.

According to Lall and Deichmann (2012), the risk posed by ravines and gullies in cities should increase in the coming decades due to population growth and land scarcity. According to these authors, in cities such as Caracas, Venezuela or Rio de Janeiro, Brazil, poor families occupy land in risk areas to enter the urban labour market. In Bauru, the analysis of images taken between 2004 and 2020 demonstrates that areas affected by erosion are a frequent focus of irregular occupations for the creation of slums.

In cases of the use of engineering works for containment and control of ravines or gullies, long-term impact analyses need to account for the resources used to restore the functionality of infrastructure. In Bauru, at Quinta da Bela Olinda and other places such as Jardim Grama and Parque Bauru, several infrastructures such as hydraulic ladders have been partially or completely destroyed, suggesting that the works may have been poorly sized or that the solution was insufficient because it focused on the consequence, not the process. For Bartley et al. (2020) and Frankl et al. (2021), the key to recovering areas affected by erosion is vegetation, as engineering works have a high failure rate. In this sense, Romero-Díaz et al. (2019) suggest that works built to stabilize ravines and gullies be checked annually after the season with the highest rainfall.

The use of urban waste to fill in gully areas can further aggravate the impacts of these processes. In areas of ravines and gullies, where garbage has been disposed of without any type of waterproofing, it is likely that in addition to the problem caused by erosion, there will also be problems with soil and groundwater contamination. The disposal of garbage in erosions also occurs in other cities of São Paulo. Rotta and Zuquette (2014) cite this practice in an erosion area in São Carlos. This suggests that urban ravines and gullies can also become contaminated areas and a source of contamination in the basin.

For Santos (2008), in addition to the problems in the area directly affected by erosion, ravines and gullies cause the devaluation of the land in the surroundings. According to the author, it is difficult to sell lots in closed subdivisions such as Chácara Odette and Jardim Tavano, and price dropped in areas close to the gully in Jardim Jussara.

Rotta and Zuquette (2014), when analysing areas affected by ravines and gullies in several cities in Brazil, presented examples of erosion that affected land development for decades. In São Pedro, gullies started in 1972 and were recovered with the construction of open channels after the advance of urbanization between 1995 and 2000. This allowed reinserting this area into the urban space with the construction of homes and public structures.

The problems and impacts caused by ravines and gullies in urban areas have many similarities with contaminated areas classified as brownfields. According to Cabernet (2006) brownfields are sites affected by previous use which are abandoned or underutilized due to real or perceived problems of contamination, occur mainly in urban areas, and require intervention so that the area can be used again for beneficial use. Ravines and gullies favour the creation of urban voids, where recovery measures aiming at the reinsertion of these areas into the urban space have a high cost. Contamination caused by garbage disposal in erosion channels brings the concept of brownfields even closer to the scenario created by large erosions in the urban perimeter.

Studies carried out for the recovery of brownfields demonstrate methodological paths that may be applied to the recovery and analysis of areas affected by ravines and gullies. The recovery of brownfield areas requires analysis of environmental, economic, social, time, uncertainty aspects and user friendliness of a sustainable site remediation (Huysegoms and Cappuyns 2017). Thus, the recovery of these areas involves risk assessment, land use planning and the selection of the best remediation methods and technologies (Hammond et al. 2021). Instruments, such as the Timbre Brownfield Prioritization Tool, that use multicriteria tools can help identify priority areas for remediation (Pizzol et al. 2016). If tools such as this are adapted, they can also be used for the analysis of ravines and gullies.

However, ravines and gullies are also a form of natural disaster. Thus, governmental and land planning aimed at reducing the risk of danger is essential. Gully erosion susceptibility maps are an important instrument in land planning and in the development of erosion control and mitigation measures (Arabameri et al. 2019). Monitoring the temporal evolution of erosion can be an important geoindicator (Busnelli et al. 2006), as urban development can affect hydrological connectivity, change natural flow routes (Gudino-Elizondo et al. 2018), and promote erosion in areas with lower natural susceptibility.

The use of remote sensing can help in valuation analyses, as was the case with the use of Google Earth images in this study (Mahamba et al 2023). The analysis of spectral images allows the identification of land cover changes (Pani 2017). Automatic identification through the use of remote sensing is an alternative to area monitoring (Vrieling et al. 2007; Wang et al. 2023; Mawe et al. 2024). Municipalities with a high susceptibility to ravines and gullies can use this type of technology as an initial diagnostic tool. The early identification of areas with ravines and gullies at an early stage of development can facilitate a quick response to contain the process, thus avoiding greater environmental and economic damage.

Unmanned aerial vehicles (UAVs) and terrestrial laser scanners (TLSs) are other tools that can be used in the precision analysis of erosion (Julian and Nunes 2020). These methods can provide better results when monitoring the evolution of erosion processes, since two images from different dates are needed to carry out an impact analysis.

In places such as Erosion 1 at Quinta da Bela Olinda, where there is already a recovered erosion with dense vegetation and a watercourse in the interior, the development of a geotourism itinerary or educational itineraries aimed at explaining to students the processes that operate in the formation of ravines and gullies, as well as their economic and environmental impacts, can be an alternative use for these areas. Ravines are used for this purpose in other countries such as Poland (Zgłobicki et al. 2015).

Although natural conditions influence susceptibility and City Hall has an important role in land planning, the development of ravines and gullies is driven by human action. In this sense, the valuation process is also important in identifying those responsible for triggering accelerated erosion. The impacts of ravines and gullies can cause environmental, social and economic damage to several points of the watershed, including upstream, downstream and surrounding areas.

For other types of disasters, an increasing number of studies address their disasters and the value of remediation strategies. For example, Logar and van den Bergh (2013) examined the methods available to assess all types of drought costs, considering direct, indirect and nonmarket costs. Montaña et al. (2016) calculated the natural capital value of forests in Saldaña related to mitigating the risk of landslides. Additionally, Semenova et al. (2013) calculated the cost of wildfires in California. Valuing the impact of erosion is a way of following the same trend, whether to measure the impact for management purposes or for accountability purposes or to trigger any insurance that covers losses.

The method proposed in this study made it possible to carry out an analysis of the short- and long-term costs of urban erosion, considering important factors for understanding the problem, the cost of erosion recovery and containment, environmental impacts, estimated economic losses, loss of revenue from municipal taxes, and losses with infrastructure destruction, in addition to also demonstrating the positive impacts of remediation of these areas. Although the values of losses from erosion and gains due to the reinsertion of these areas can be calculated using the same variables, they need to be calculated separately. The different types of economic and environmental impacts can be identified following the proposal by Kuhn et al. (2023a, b), while the loss of opportunity in the urban space can be determined considering the municipal GDP per square metre. The methodology used in this study can be replicated in other countries around the world, as long as local values are used to determine reference values for replacement or infrastructure costs.

The method proposed in this study allows the valuation of the impacts of ravines and gullies, considering the short-, medium- and long-term losses. The analysis of losses in urban areas, based on the calculation of GDP per square metre, is a simplified way to consider the impact of making areas unfeasible for use in cities, thus creating a way to account for the complexity of the urban environment. The results showed that the application of the replacement cost of items affected by ravines and gullies allows the valuation of items related to damage to infrastructure and ecosystem services performed by the soil. The method to be applied in other countries needs to use local reference values due to variations in the cost of products and services.

5 Conclusions

This study estimated the impacts of ravines and gullies in the medium and long term for the city of Bauru, Brazil. This estimation demonstrated that in the urban perimeter, the main impacts are related to the abandonment of the area and the creation of urban voids. Among the environmental impacts, the most significant is the mobilization of sediments. The data indicated that the recovery of areas affected by ravines and gullies is the best option in urban areas, although it can be a costly process for municipalities and governments. Gains from tax collection, risk reduction and economic gains are some of the main arguments for creating strategies aimed at reinserting these areas into the urban environment.

The valuation and remediation of areas affected by ravines and gullies can be facilitated by the creation of municipal databases of the actions and occurrences recorded in each erosion to allow reconstructing the history of costs and technical choices. For municipalities such as Bauru, the creation of a permanent erosion identification and infrastructure maintenance program can favour the early identification of ravines and gullies so that containment measures can be carried out quickly, avoiding the expansion of damage.

The use of GDP per square metre is a way of valuing impacts in the urban environment that simplifies the complexity of the city in an economic analysis of land use. The calculation of impacts, positive or negative, related to urban land taxes can be an important argument in convincing local municipal authorities.

Strategies for reinsertion of urban voids created by ravines and gullies need to be developed, similar to those that already exist for brownfields. Economic valuation and accountability analyses should also be carried out to identify the companies and individuals responsible for the development of ravines and gullies due to poor soil use. Similar other processes, such as forest fires, erosions can occur naturally, but when caused by human action, penalties must be imposed on those responsible and economic reparations must be made for the affected people.