Ravines and gullies, a review about impact valuation

Gullies and ravines affect human lives and the environment in many countries worldwide, however, few studies have assessed the impacts of the damage caused by these large linear erosions and clarified their influencing parameters. This work systematizes available information about the variables relevant to analyze the impacts of ravines and gullies on socioeconomic conditions and ecosystem services. Based on keyword searches performed in the Scopus, Web of Science and Dimensions databases, more than 120 articles from 27 countries were identified that addressed and valuated these different types of impacts. Although many studies discuss the impacts of ravines and gullies, few studies have performed valuations of these impacts. We compiled the impacts in urban and rural areas, considering the changes caused by ravines and gullies throughout a hydrographic basin. The results allowed for the elaboration of a theoretical model of the possible impacts of gullies and ravines and how they can be valued. The results demonstrate that the most significant impacts, according to the literature, are in South America, Africa, China and India, which are related to disordered land occupation. In some cases, the lack of capacity to respond to environmental and social problems aggravates the scenario and leads to significant losses. Implementing the evaluation is challenging due to the high number of different forms of possible impacts, in the short, medium and long term, related to environmental, economic and social changes.


Introduction
Gullies develop via a combination of extreme rainfall events and socioeconomic changes, such as the conversion of forests to cultivated areas, which has been described for the prehistory and the Roman period in Belgium, Poland and Germany by Poesen (2011) and Dagar (2018). In North Carolina, the USA, Spell and Jonson (2019) related the formation of ravines to the initial deforestation and agriculture activities carried out by settlers in the 18th century. Bacellar et al. (2005) list erosions in the state of Minas Gerais, Brazil, as a consequence of inadequate land use practices in the late 17th century, such as the construction of boundary ditches.
Several authors carried out reviews addressing different aspects related to gullies and ravines: Valentin et al. (2005) addressed erosion impacts, causes and control; Castillo and Gómez (2016) assessed the evolution of erosion; Vanmaercke et al. (2016) studied the mapping of temporal and spatial evolution; Poesen (2018) identified that there are still gaps in studies on gullies related to understanding the spatial and temporal patterns of soil erosion rates and landscape evolution and to the development of innovative techniques and strategies to prevent or reduce soil erosion; Dagar (2018) evaluated the history and trends of gullies and ravines in recent studies, particularly regarding the formation process and methods of recovery and containment of linear erosion; Romero-Díaz et al. (2019) carried out an analysis on methodologies for the rehabilitation of affected areas; and Dube et al. (2020) carried out an analysis on the evolution and morphology of erosion in different climates, among others.
Only a few studies, including those by Yitbarek et al. (2012) and Ayele et al. (2015), have focused on the valuation of the impacts of linear erosion. Therefore, there is still no widely accepted method in the literature for assessing damage caused by ravine and gully development. Economic losses and impacts on ecosystem services are mentioned in several works in the literature but mostly without an impact assessment (Balzerek et al. 2003;Argüello et al. 2006;Abdo et al. 2013;Rowntree 2014;Somasundaram et al. 2018;Kouidri 2018;Paredes et al. 2020). Environmental and socioeconomic impacts are easily observed in many places in the world, such as the city of Kinshasa and the Democratic Republic of Congo ( Fig. 1). Accounting for the impacts of ravines and gullies is important to justify the use of financial resources in the prevention and recovery of areas affected by erosion.
Among the factors that influence gully erosion/gullied area, recovery and control measures are (a) the characteristics and dimensions of erosion at the site; (b) geomorphological conditions; (c) land use characteristics; (d) the availability of alternative areas; d) access to water; (e) distance from residences; (f) history of erosion at the site; and (g) cost of recovery with works (Pani 2016;Romero-Díaz et al. 2019) cite among the consequences of erosion control an increase in the production of food, organic matter and wood, an increase in the cultivable area and water retention, and an improvement in water quality.
To contribute to a better understanding of the impacts of linear erosion, this article presents a systematic bibliographic review using a state-of-the-art evaluation of the socioeconomic impacts and ecosystem services caused by erosion in ravines and gullies. In addition, a discussion is carried out regarding the conditions that aggravate these impacts and the recovery processes of areas affected by erosion, considering the spatial dimensions of the hydrographic basin and the temporal evolution of erosion. An overview on the current methodologies/available technologies to measure/quantify erosion over time is given. A special focus on erosion processes in gullies and ravines in Brazil supports the development of an evaluation method for an area in the city of Bauru that has a long history of erosion.

Bibliographic methods
The bibliographic search was carried out in the Web of Science, Dimensions and Scopus databases. Several combinations of the following keywords were used: ravines, gully, valuation, natural hazards, invest, soil loss, urban, city, disaster, ecosystem services and linear erosion. After reading abstracts, 124 works from 27 countries were analyzed in full (Fig. 2). The criterion for selecting the articles was the existence of results that help to understand the temporal evolution and impacts caused by ravines and gullies and the recovery measures implemented. In addition, the articles published related to Brazil are overrepresented in relation to other countries because the chosen sample is related to research on the valuation of linear erosions in Brazil (additional keyword "Brazil").
This search had limitations because the searches were performed with only English keywords. Gray literature, such as reports and technical studies or articles that were not indexed in the three cited databases, are underrepresented in the analysis. However, when some relevant information from any of these bibliographic sources was identified in the references of some work, the reading and analysis of the contents were performed.
Ecosystem services and natural capital are fundamental for supporting life and essential for human beings (Costanza et al. 2003). They must be accounted for as part of the overall economic value, along with other commercial values. Ecosystem services cited include gas regulation, climate regulation, disturbance regulation, water regulation, water supply, erosion control and sediment retention, soil formation, nutrient cycling, waste treatment, Fig. 1 Comparison of two images of the same area, the first from 2014, where the erosive process development of the ravines is clearly visible between 2014 and 2021, in an area with high population density in Kinshasa, the Republic Democratic Republic of Congo. Images extracted from Google Earth (Lat. 4° 30′ 6.50″ S-Log. 15°13′ 3.67″ L) 1 3 pollination, biological control, refugia, food production, raw materials, genetic resources, recreation and cultural (Costanza et al. 2003).
From the analyzed bibliography, the anthropic and biophysical factors that influence the development of ravines and gullies, methods of prevention and analysis of temporal evolution, methodologies for the recovery of areas affected by linear erosion and the socioeconomic and ecosystem services impacts are discussed. When data regarding the costs of impacts or other recovery measures were presented in currencies other than US dollars, they were converted to US dollars at the rate for 04/26/2021.

Factors that influence the intensity of impacts
Erosion occurs due to changes in land use and occupation caused by anthropogenic interventions, followed by rainfall events.
Among the anthropogenic factors that contribute to the development of erosion are vegetation removal, overgrazing, deforestation, forest burning, infrastructure construction such as roads, arable land obstacles that promote the accumulation of surface runoff, hydrological changes in rural and urban areas, trails used by animals or humans, retention basin overflows, mining activities or other interventions that concentrate surface flow water (Saksa and Minár 2012;Valentin et al. 2005;Dagar 2018;Wouters and Wolff 2010). When analyzing the development of ravines in Kinshasa (Congo), Imwangana et al. (2015) observed that between the initial construction of buildings and streets and the appearance of the first ravines, there was a time span of 5 to 10 years, suggesting that a critical density of houses and roads in the area was needed for the erosion process to develop.
In urban centers, according to Kayembe and Wolff (2015), the impact of erosion tends to be greater in neighborhoods with low-income populations. According to the authors, among the factors that contribute to erosion are the type of roads and houses constructed without appropriate drainage system and surface runoff planning and the lack of maintenance of the erosion control system.
The development of linear erosions is related to the concentration of rainwater on the surface; thus, the greater the intensity of rainfall is, the greater the possibility of developing gullies and ravines. According to Wischmeier (1959), erosivity by water is calculated based on kinetic energy and rain intensity (mm per hour) (Wischmeier 1959). Soil erodibility is calculated by the ratio between soil losses and rainfall erosivity (Wischmeier and Smith 1978). That is, erosivity is related to the intensity of rainfall that triggers erosion, and erodibility is related to the susceptibility of the soil to erosion.
The percentages of soil erosion and water erosivity demonstrate a correlation between erosive dynamics and rainfall regimes (Busnelli et al. 2006). There is a relationship between the deposition of sediments in the hydrographic basin and the rate of rain intensity, that is, the amount of kinetic energy and runoff, and both factors contribute to greater erosion via rain (Onyelowe et al. 2018). Prolonged drought periods followed by short rainy periods favor runoff and erosion (Arabameri et al. 2019).
Based on the calculation of the kinetic energy of rain, Klik et al. (2016) found that highintensity storms combined with fragile ground cover in steep slope regions created perfect conditions for laminar erosion and gully formation in the Ethiopian highlands. Argüello et al. (2006) described the occurrence of extreme hydrometeorological events as the main cause for the development of erosions in Corralito, Argentina, in 1978/79, when the precipitation of just one month almost reached the precipitation of the driest year (1971/72). In an exceptionally humid hydrological year (1978/79), rainfall accumulated to 1348 mm, much higher than the average in the region, and according to analyses carried out by the authors, the 230 mm precipitation observed in September 1978 would have a return period of 500 years and the 180 mm from February 1979 would have a recurrence of 200 years.
In Seirós, Portugal, an extreme rainfall event, with precipitation of close to 160 mm in the head of the basin and precipitation of 80 and 120 mm at the site on the 14th and 15th in December 2015, caused the development of erosion with a 49 m long channel, 0.70 m wide and 0.75 m deep (Martins et al. 2019).
Soil saturation due to recurrent rains and the consequent increase in runoff is an important factor in erosion development, as it can lead to significant environmental and socioeconomic impacts. One case in Comodoro Rivadavia documented by Paredes et al. (2020), which considered decade-and century-long climatic cycles, demonstrated the destructive potential of extreme rain events associated with changes in the physical environment for which appropriate planning was not done. The authors noted that between March 29 and April 8, 2017, precipitation of 399.4 mm and 232 mm occurred only on March 30. This event modified the landscape and created several ravines and gullies, the largest of which had a V-shaped cross section, exceeding 15 m in depth and 870 m in length (Paredes et al. 2020). According to the same authors, area photographs from the 1970s revealed that ditches in the area were filled during the construction of the city, modifying old drainage 1 3 routes. A slope of 20° and the depth of the resistant substrate at its foot were also factors that influenced the development of erosion. Rowntree (2014) reported the development of erosions related to extreme rainfall events of 90 mm in September 1891 in Wellwood and 81 mm in September 15 and 16 in Cranemere, South Africa, followed by other high-intensity events in 1900, 1902 and 1903. Ravines and gullies occur in all climates, except for locations close to the poles with permanent ice (Castillo and Gómez 2016). Dube et al. (2020) analyzed length and depth in ravines under different climatic conditions. The length tends to increase in a temperate climate when compared to the existing features in subtropical and/or tropical climates, which have similar sizes. On the other hand, gullies tend to be narrower in temperate climates than in subtropical climates. The shallowest gullies have been found in tropical climates. Vanmaercke et al. (2016) carried out a literature review related to the progression rates of 933 ravines in more than 70 study areas and assumed that the main variables influencing erosion headcut advances are the upstream drainage area and the normal rainy day (average rainfall depth on a rainy day). The authors found a relationship between the rate of linear erosion and the affected volumetric area (r² = 0.83). In the studies analyzed, the retreat rate varied between 0.01 and 135 m y − 1 (median: 0.89 m y − 1 ), and the affected area varied between 0.01 and 3628 m 2 y − 1 (median: 3.12 m y − 1 ). The width varied between 0.4 and 104 m (mean: 9 m, median: 3.2 m), and the depth varied between 0.2 and 35 m (mean: 2.1 m, median: 1.3 m). The normal rainy day and the upstream drainage area were the main factors that influenced the rate of erosion growth.
In Brazilian classifications, an aquifer connection differentiates ravines, which are linear features with a depth of more than 50 centimeters, generally V-shaped and rarely branched; gullies, the next stage in the evolution of erosion, are created when an aquifer intersects a ravine, usually have a U-shape, and, as erosion evolves, tend to branch out into several channels (Salomão 1994). In tropical regions, due to the high volume of annual rainfall, the connection between linear erosion and the aquifer becomes clearer; when intercepted, the lateral evolution of erosion is favored, which explains why in these climatic conditions, erosions tend to be shallower than in colder climates with less annual rainfall.
For Real et al. (2020), fault and/or shear zones favor the development of soil erosion. Da Silva et al. (2003) related the development of ravines and gullies in Paty do Alferes, Rio de Janeiro, Brazil, to the low resistance to weathering and high permeability of the mineralogical and textural characteristics of the rocks. For the authors, the presence of structural lineaments favors the development of erosion. Likewise, Akgün and Türk (2011) noted that in Turkey, most gullies occurred in areas with a high density of lineaments; thus, the distribution of lineaments and lithological variations can affect the location of erosive processes. In addition, the type of rocks and susceptibility to weathering influence the formation of the soil.
Geology can also be related to uplift in tectonically active regions such as the Loess Plateau in China, with a rise in elevation of 150-200 m during the Quaternary, which increased the kinetic energy of the area and thus favored erosion (Hui and Mingan 2000).
Vegetation also influences the development of erosion; when present, it protects the soil from the impact of rain, helps to maintain the soil structure, reduces the speed of surface runoff and favors the infiltration of water into the soil due to root channels (Dagar 1986(Dagar , 2018. Therefore, in arid and semiarid regions, the low presence of vegetation favors the formation of ravines. Dasgupta et al. (2013) considered vegetation cover to be the most important factor for the protection of soil fertility. The removal of vegetation can favor the loss of nutrients. Selkimäki and González-Olabarria (2017) reported that forest domains reduce the probability of erosion development. However, forest fires, especially in regions with trails, tend to increase water connectivity and favor erosion (Martínez-Murillo and López-Vicente 2018). According to the authors, after vegetation has recovered, water connectivity also returns to its initial state.
Vegetation plays a very important role in the recovery of areas affected by erosion. For Gardziel et al. (1998), biological construction, through the introduction of different types of plants in an affected area, is an alternative to reduce the evolution of erosion. Vegetation is important to both prevent erosion and recover areas affected by erosion. Vegetation cover reduces the velocity and volume of surface runoff, favors the infiltration of water into the soil and helps retain nutrients and sediments.
Topographic or physiographic characteristics influence the development of ravines and gullies, according to Valentin et al. (2005), Dagar (2018), Gayen et al. (2019) and Dube et al. (2020). For example, the size, shape and drainage density of the catchment and the curvature, length and gradient of the slopes control the flow speed and erosive forces. Vanmaercke et al. (2016) reported a relationship between the size of the upstream area that drains into the gully headcut, the amount of runoff, and the rate of erosion. For Valentin et al. (2005), the impacts of soil erosion must be analyzed at the basin scale. If, on the one hand, the larger the size of the watershed area is, the greater the potential for rainwater concentration during extreme events, then the shape of the slope determines how the runoff will occur; for example, in concave slopes, the concentration is greater, and the slope helps to determine the erosion potential of runoff.
Soil properties such as texture, moisture, bulk density, water retention, and inflation capacity also influence the development of erosion and influence the formation of ravines and gullies (Stocking 1972;Morgan 1984, Dagar 2018. For Real et al. (2020), the boundary between different soil types may favor the development of gullies due to changes in soil properties.
Although the infiltration rate increases from clayey to sandy soil, the resistance to erosion decreases; thus, erosion in sandy soils is higher (Hillel 1982;Dagar 2018). The type of soil can favor the development of linear erosions. For example, lateritic soils are highly weathered and leached and are fragile due to characteristics related to aridity, loss of nutrients, chemical deficiency, crust formation, water erosion and poor water retention capacity, thus favoring the development of erosive processes (Jha 2008;Jha and Kapat 2009). According to Hudec et al. (2005), ravines advance faster when the erosion reaches the lateritic horizon, with lateral advance widening the transversal size of the erosion.
Several studies done in different locations throughout the world report different types of soils in regions prone to the development of ravines and gullies. In Seirós, northern Portugal, erosion features reported by Martins et al. (2019) showed depths limited to the thickness of the colluvium. Nosko et al. (2019) reported that accelerated runoff causes more gullies. Sandy materials are more permeable but have little cohesion (Lucía et al. 2011;Vanmaercke et al. 2016).
Several studies have related erosion to soil characteristics in Brazil. For Guerra et al. (2018), erosion is more intense in friable and unconsolidated soils. De Lima and Guerra (2019) described soil degradation and the development of erosive processes in latosols and clayey soil following the implementation of the Dourados National Agricultural Colony in the 1950s. Souza et al. (2017), in an analysis carried out along the Malha Paulista railway line, identified the predominance of gullies in latosols, which are deeper soils with high maturation that facilitate erosion, but the authors also identified the development of erosions in Argisols, Nitosols and Neossols. In the hydrographic basin of Maracujá, Minas Gerais, gullies were found in areas of thicker saprolites and in places with natural concentrations of surface or groundwater (Bacellar et al. 2005;Wantzen et al. 2006) noted the proximity to footpaths and springs in gullies located in the plateau region in the cerrado biome. For the authors, porous and slightly cohesive soils favor the development of ravines. Erosion in sandy soils was found in the Ibicuí basin (Corbonnois et al. 2011). For Rotta and Zuquette (2014), erosion in Casa Branca occurred in response to reworked, unconsolidated, thick and porous sandy soils. Table 1 shows how each of the factors described above can aggravate the impacts caused by linear erosion based on the information described in the previous paragraphs.
Soil characteristics such as permeability and porosity influence water infiltration. Although porous and permeable rocks and soils favor infiltration, once saturated, surface runoff triggers linear erosion. If the ravine created intercepts the groundwater table, internal erosion by piping starts in addition to tail erosion by runoff.
The cases of extreme events in places of low cohesive soils in Africa, Argentina, Portugal, among others, indicate that the combination of land use on vulnerable soils and/or friable rocks and large areas of drainage and extreme rainfall events have the potential to form gullies and ravines hundreds of meters long and tens of meters wide and deep in a few days.
In other regions, due to climatic conditions that hinder consolidated vegetation cover, e.g., semiarid regions, significant impacts from ravines and gullies must be expected. In some specific cases, such as the Loess Plateau, the recent uplifting of old surfaces may be important to the existing impacts of erosion on the site.
Although linear erosion occurs in many soil types (clayey, sandy, or loosely cohesive soils), oxisols, colluvium, sandy soils and friable soils, or thick soils of tens of meters, are developed in regions where the climate favors the development of a thick intemperate blanket (Guerra et al. 2018;Dagar 2018;De Lima and Guerra 2019;Martins et al. 2019). The faster the linear erosions form and/or evolve, the more complex the erosion control process is due to the need for quick response and the difficulty in carrying out sediment retention.

The evolution of erosion
Linear erosions, although they can have moments of rapid evolution due to storms, when not stabilized, have variable annual growth. Understanding how gullies and ravines evolve is important to analyze the impact of these processes in the medium and long term.
The quantification of the erosion rate is performed by several methods, such as the dendrochronological method, which uses the roots exposed in ravine areas to estimate the erosion rate (Vandekerckhove et al. 2001); orbital image analysis (Busnelli et al. 2006); use of isotopic tracers of elements such as carbon and nitrogen to analyze the fingerprint of sediments (Valentin et al. 2005); topographic field surveys (El Khalili et al. 2013); and use of images of drones and unmanned aerial vehicles Julian and Nunes 2020).
However, according to the review carried out by Poesen et al. (2011), there are still no reliable and validated models that allow foreseeing all factors related to the evolution of ravines and gullies, such as infiltration, time scales, sediment production, and evolution of the landscape. Guerra et al. (2017) considered that there is a need to monitor soil erosion using experimental stations, but although these data are important for understanding erosion, even when monitoring is carried out for decades, they generally will not record extreme events such as high magnitude precipitation.

Aquifers
The characteristics of the rocks influence the characteristics of the local aquifer. The relationship between the local aquifer and erosion is important in tropical regions.
Uplift of old surfaces Changes kinetic energy and favors the development of gully systems with a regional dimension. Calculating the future evolution of linear erosion is a topic that still requires attention via scientific research, but the current state-of-the-art research on the evolution of erosion allows us to establish some assumptions about the development of this process, which can be used to analyze the valuation of its impact. The first assumption is that it is possible to identify the past evolution of erosion through the analysis of satellite images and aerial photographs, in which the erosion scar dimensions can be identified. The data on the evolution of erosion allow us to create scenarios about the possible impacted area in the coming years or decades. The rate of erosion tends to be regressive, that is, to grow at an everslower pace, but the natural growth of erosion tends to impact a growing area for decades or centuries. Some of the methods described are best applied on a regional scale, while others are best applied on a local scale (Table 2).

Recovery and containment
Among the measures that can slow erosion, remediate affected areas, and slow the development of ravines and gullies are dams, drainage energy dissipators, spillways, accumulation basins, protection dikes, artificial channels and water capture structures, watercourse and lake desilting, earthworks, pasture area revegetation, gabions with vegetation, grass planting, biodegradable mats, conservation practices in agriculture such as no-tillage, fire control, crop rotation, soil fertilization, permanent protection strips, furrow terracing, level spreaders, and infiltration piers, among others (Rotta and Zuquette 2014;Dagar and Singh 2018). Rotta and Zuquette (2014) divided recovery measures into five categories: ecological, agricultural, mechanical, structural macrodrainage and structural microdrainage. Romero-Díaz et al. (2019) classified gully control measures as (1) structural, (2) vegetative and (3) structural/vegetative. The gully headcut area and the head of the erosive process are important places to stop erosion; vegetative measures within the channel and containment of impacts in the basin, such as sediment retention dams in the river channel, are valuable practices (Frankl et al. 2021). Dagar (2018) considers the following steps to be important for the control of ravines and gullies: (a) improvement of the hydrographic basin by reducing the peak flow rate; (b) stabilization of the head of the ravines; (c) development of structural measures and revegetation; (d) reduction in the slope of the sidewalls; and c) treatment of the area affected by erosion.
In a review carried out by Bartley et al. (2020), the author divided the recovery measures into three categories according to location: (a) treat the watershed above the erosion, Table 2 Main methods used to analyze erosion evolution and the recommended work scale for each method

Method Scale
Satellite image analysis Regional or local Aerial photograph and image analysis using drones, unmanned aerial vehicle (UAV) or Digital Terrain Model (DTM) analysis (LiDAR data)

Regional or local
Terrestrial laser scanner (TLS) Local Chronological analysis (e.g., isotopes, dendrochronology) Local Isotopic tracers and sediment analysis Local or Regional Topography Local aiming to reduce runoff discharge; (b) carry out measurements within the channel, with the objective of interrupting the expansion of the ravine; and (c) a combination of both approaches. According to the review carried out by the author, after stabilizing erosion, there is a reduction in sediment production between 12 and 94%. Remediation in only the area upstream of erosion has an average response value of approximately 28 years; when the treatment is carried out only in the area directly affected by erosion, the response occurs in approximately 25 years, and when several methodologies are integrated, the average recovery value of the area drops to approximately 19 years (Bartley et al. 2020).
The data demonstrate that containment and recovery measures can be implemented considering the biophysical and social conditions of the region. The greater the erosion, the more complex the containment and recovery of the area. To contain erosion, actions in the hydrographic basin and in the canal are necessary, but measures can also be taken downstream, such as dam construction, to reduce the impacts related to the released sediments. Vegetation has the most impact and the best results in the long run, but structural measures also have an important effect on containing erosion. The need to maintain the installed structures must also be accounted for in any analyses.
All measures adopted have costs, whether in relation to the implementation itself, shutdown of activities, labor costs, and maintenance costs, among others. Some works have analyzed the costs that exist in the recovery of ravines and gullies. The cost of actions to recover areas affected by erosion depends on the biophysical characteristics of the site and the technique applied (De Brito Galvão et al. 2011;Romero-Díaz et al. 2019). Balzerek et al. (2003) reported that unsuccessful erosion control measures cause more damage, and to carry out appropriate measures, it is necessary to plan, coordinate and organize their implementation. According to Rotta and Zuquette (2014), in some cases, problems are related to the adoption of inadequate recovery measures, which are chosen due to popularity or ease and do not always result in the best recovery of the area, or in some cases, the measures are poorly executed or scaled. Guerra et al. (2017) stated that the integrated management of watersheds for soil conservation and the use of bioengineering can offer alternatives for erosion control.
Containment and analysis of erosion impacts cannot be carried out in a generic way, and the actions taken need to be implemented and monitored by qualified professionals, aiming to apply resources and measures that are truly effective. In this way, an understanding about the dimensions of the impacts of linear erosion by civil society and managers becomes even more necessary. The recovery of affected areas demands time, financial resources and qualified work.
Among the existing problems for the containment of ravines and gullies is the cost of recovery actions. Valentin et al. (2005) have discussed the difficulty for farmers in adopting techniques for the prevention and recovery of ravines due to the small short-term return, but in the long term, the returns are more significant. Dagar and Singh (2018) reported that gully recovery techniques are rarely adopted by farmers due to deficiencies in extension services, policy implementation and return on investments.
For Romero-Díaz et al. (2019), increased productivity is the best argument to convince farmers to develop soil and water conservation measures, and the possibility of increasing the productivity of the area, in addition to reducing soil loss, reducing flooding and increasing soil moisture and vegetation cover, can be noted as some ecological benefits.
The development of a valuation methodology that allows the identification of the dimensions of the impacts of ravines and gullies can contribute to the construction of policies or lines of financing that help to make it possible to contain erosion.
Thus, this work classifies the recovery and containment measures into four different types: (a) structural, (b) watershed management, (c) vegetative and (d) educational or/and qualification (Table 3). Both measures can be implemented to contain the same erosion.

Economic impacts
Among the economic impacts of ravines and gullies are reduced agricultural productivity due to a loss of planting areas and change in soil characteristics; a loss of crops and available land; increases in agricultural costs; increases in agricultural activities workloads due to limitations or difficulties imposed by erosion and the cost of production agriculture and livestock in rural areas; the siltation of reservoirs and their consequential loss of functionality; increases in flooding risks and the subsequent damage to infrastructures situated in areas located near rivers; damage to infrastructure and increases in costs of drains or channel maintenance; and the rapid drainage of aquifers, which lowers the water table and subsequently dries up wells (Balzerek et al. 2003;Valentin et al. 2005;Romero-Díaz et al. 2019).
In rural areas, several authors have described impacts related to the occurrence of ravines and gullies. Zgłobicki et al. (2015a) analyzed the impacts of ravines in eastern Poland. For the authors in some districts where gullies covered more than 2% of the area, where there is a high density of ravines, special management must be carried out to reduce negative economic impacts. According to the authors, the landscape forms developed by ravines are unfavorable to modern agriculture and can become an obstacle to the development of efficient agriculture or to the consolidation of agricultural land.
According to Daba et al. (2003), gullies are expanding in areas of agricultural land in Ethiopia at an accelerated rate. They are considered to be the main cause of silting of lakes (such as Lake Alemaya) and other sources of drinking water and irrigation; in addition, they increase the risk of flooding.
Gullies and ravines reduce the cultivated area, reduce production, hamper the movement of people and livestock, and have other environmental impacts (Belayneh et al. 2020). Ravine fields cause a permanent loss of agricultural land if recovery measures are not carried out based on government funding (Olson and Morton 2012). Abdo et al. (2013) noted that, in Australia, progressive erosion made it impossible to continue farming in areas previously used for agriculture; however, these areas could still Introduction of vegetation in the watershed area and at the site of erosion Educational and qualifying Development of actions to guide the population of the affected area or train people on how to manage areas affected by erosion be used for fishing, ecotourism and other economic activities. The first step in restoration is to interrupt the evolution of the erosion process and recover the area through engineering works, restrict livestock use, and recover the natural vegetation. Morokong and Blignaut (2019) analyzed the costs and benefits of reducing soil erosion in the municipality of Mutale, South Africa. The authors identified that an area of 1470 hectares (ha) was affected by soil erosion; in some places, the observed impact was related to the loss of pasture productivity, siltation of rivers and loss of water bodies, as well as the consequent increase in the cost of water, loss in land production capacity, destruction of residential areas and interruption of roads. If considering the opportunity cost lost by unrestored land, the value is estimated at US$10 per hectare and US$56 per hectare for crops. To control erosion, US$ 0.5 million was spent using rock structures for the period 2010-2016, and with this investment, a benefit between US$ 140 thousand and US$ 200 thousand was generated, in other words, a cost-benefit between 0.29 and 0.41. Pani (2017) determined that of 766 villages in India, 343 are affected by ravines, among which the main effects are the loss of agricultural land, reductions in production, changes in villages, and impacts on infrastructure and subsistence economic activities. Ravines affect access to land and can contribute to changes in the drainage pattern, in addition to affecting the supply of public goods and basic human development structures such as access to health and education.
According to Pani (2016), in the Chambal Valley, India, ravines affected 26.5% of the total agricultural land, but the percentage varied by village. According to the author, 71 families reported the loss of land due to the development of ravines; of this total, 11% were unable to recover the land, 76% partially recovered it and 13% moved to new land. Busnelli et al. (2006) described the silting of an artificial lake in La Angostura, Argentina, where there was a reduction in the depth and loss of surface area of the reservoir, from 23% to 1977 and 2000. They demonstrated the impact of erosion on the useful life of reservoirs.
In urban areas worldwide, the impacts of ravines and gullies are felt. Balzerek et al. (2003) noted the destruction of houses, streets and bridges in the State of Gombe, Nigeria. Hudec et al. (2005) mentioned that in Nigeria, according to official data, between 1981 and 1994, erosion and other forms of soil degradation caused the country to lose 3.7 million hectares of forests and agricultural land. The authors noted an expenditure of approximately US $ 7.7 million in the state of Imo, with the objective of reducing gullies in the southeast of the country.
The city of Kinshasa (Congo) has the best documented processes of erosion in the literature. Accumulated erosion in the last 50 years in this municipality totaled 308 linear features totaling 94.7 km, that is, an average advance of 2 km annually. The 10 largest gullies had an average width of 57 m and an average depth of 20 m; however, when considering the general average of all gullies, the numbers dropped to 17 m and 6 m, respectively (Imwangana et al. 2015). The annual economic impact caused by the destruction of a home is US $ 1.5 million per year, in addition to other expenses such as US $ 10 million for the reconstruction of the Drève de Selembao in 2004 and US $ 7.8 million in 2006 for the recovery of areas affected by gullies in the Mataba district (Imwangana et al. 2015). Another author who described erosion processes in Kinshasa was Ozer (2014), who, through the observation of images, identified the destruction of more than 60 houses due to the development of a ravine between 2006 and 2011.
Erosions in some cases evolve suddenly due to large storms and occupational use that has not been properly planned, as in the case of Comodoro Rivadavia in Argentina in 2017. After heavy rains, a large number of V-shaped gullies were generated with lengths 1 3 of up to 870 m and depths of up to 15 m; the event transported over 400,000 m 3 of fine to very fine sand and damaged or destroyed more than 400 houses, in addition to urban and industrial infrastructure (Paredes et al. 2020).
The increase in the cost of maintaining roads and transport routes, or costs with the paralysis of roads, is also another type of existing economic impact of ravines and gullies. Souza et al. (2017) stated that their possible impacts on the Malha Paulista railroad in Brazil included the halting of cargo and passenger transportation, the occurrence of accidents involving employees and passengers or even the spillage of flammable cargo, which contaminate soil and water resources. In Algeria, Kouidri (2018) described the impacts of ravines formed after civil works on roads, which included erosion in the fields and the consequent abandonment of production in the area, an increase in the cost of maintaining roads and railways, and impacts on bridges, dikes and water pipes.
The use of gullies and ravines areas after the development process is only possible once the area has recovered. In countries such as Poland, there are geotourism initiatives inside old ravines, but even after stabilization, intensive use can reactivate erosive processes (Zgłobicki et al. 2015b).
Despite being cited in several studies, the socioeconomic impacts of Gullies and ravines are not usually detailed. Perhaps this lack of detail influences the focus of the works on typology, spatial evolution of erosion, efficiency of containment processes, among others. However, including a brief description of their impacts, mainly financial ones, can be an important step to demonstrate the losses caused by ravines and gullies and, in this way, demonstrate to society why it is so important to take measures to prevent and contain linear erosions.
Socioeconomic impacts occur in three different areas of the hydrographic basin (Table 4). The first of these is damage caused to the place where erosion occurs, which can mean from a permanent loss of an area that previously could be used for economic activities or a partial or temporary loss, depending on the recovery measures that are established. Among the damages caused are also economic losses in productivity, the destruction of infrastructures and homes, losses of production or animals, the loss of water sources and drying of wells, an increase in production costs and reduction in productivity, and an expansion of the risk of flooding, among others. The second is the downstream impact on the basin, such as an increase in the cost of water treatment and silting up of reservoirs, among others. Finally, upstream impacts may include, for example, the drying of wells or the loss of access routes (Fig. 3).

Impacts on ecosystem services
In the literature, impacts on ecosystem services are mentioned, such as loss of nutrients from surface layers, reduction in soil productivity, drainage of aquifers, changes in river drainage patterns, loss of soil, loss of moisture and soil cover, increases in surface runoff, increases in adverse events, reductions in aquifer recharge, reductions in carbon sequestration capacity and soil organic matter, changes in the biodiversity of plants and animals, reductions in soil moisture and vegetation cover, increases in floods, contamination of soils and surface and groundwater, wetland dryness, changes in sediment load and impacts on life in rivers, lakes and seas (Balzerek et al. 2003;Abdo et al. 2013;Rowntree 2014;Rust and Star 2018;Somasundaram et al. 2018;Romero-Díaz et al. 2019;Paredes et al. 2020).
The effects of ravines and gullies impact several fundamental ecosystem services, for example, (a) provisioning services: production of food, fibers, fuel and water; (b) Table 4 Main economic impacts identified in the literature, classified as follows: impact (0) does not occur; (1) may occur but is not common; (2) occurs but is not predominant; and (3)  In cases such as in the Peak District (United Kingdom), peat erosion can release leadcontaminated sediment (Rothwell et al. 2010). According to Shuttleworth et al. (2015), truffles are an important carbon stock but can also assimilate heavy metals; thus, truffle erosion in the UK contributes to carbon and pollutant storage. If these systems are affected by erosion, their functions will be modified.
According to Valentin et al. (2005), gullies tend to increase drainage and increase the speed of the aridification process in semiarid areas. Morokong and Blignaut (2019) noted among the impacts caused by ravines the compromise of ecosystem water services, in addition to the reduction in carbon sequestration. Somasundaram et al. (2018) considered ravines to be an extreme form of soil degradation and mentioned that in India, this kind of erosion process occupies an area of approximately 10.37 million hectares. For the authors, these areas, if restored, could contribute to carbon sequestration through the adoption of restoration practices such as leveling land, planting perennial trees and sowing legume strips, all of which aim to improve soil quality. Didoné et al. (2014) indicated that erosion problems include changes in the amount of sediment in the water, the loss of fertility, loss of useful area, reduction in water storage in the soil, and remobilization of nutrients and pesticides to water courses, which can increase the cost of water treatment for society.
The impacts on ecosystem services also occur in three different positions in the hydrographic basin (Table 5). Where erosion has developed, loss of soil and removal of the most fertile horizons, loss of vegetation cover, changes in the landscape, reduction in soil biomass, reduction in carbon sequestration, drying of wetlands and reduction in biodiversity have occurred. Downstream from erosion, contamination of watercourses, silting Schematic spatial distribution of the socioeconomic impacts of ravines and gullies in the hydrographic basin. 1-reduced productivity; 2-increased workload; 3-silting of reservoirs; 4-increased risk and impacts of floods; 5-damage to infrastructure and homes; 6-increase in the cost of road maintenance; 7-loss of access to water; 8-loss of areas with economic and social use; 9-changes in the use of the area; 10-abandonment of farmland; 11-home abandonment; 12-loss of access to land; 13-increase in the cost of maintaining drains and channels; 14-use of reclaimed areas for geotourism; 15-development of new economic activities in reclaimed areas (e.g., fishing); 16-expansion of agricultural land in areas of sited dams and changes in river drainage patterns can occur, increasing the sediment load, which can impact biodiversity in rivers and coastal regions. Upstream, aquifer drainage can occur (Fig. 4).

Impact valuation
Few studies have evaluated the impacts caused by ravines and gullies; those that have include Yitbarek et al. (2012), Ayele et al. (2015) and Rust and Star (2018). Yitbarek et al. (2012) developed a technique for valuing the financial impacts caused by ravines related mainly to soil components. The authors analyzed the cost of four erosion events in Ethiopia by analyzing yield loss, the cost-benefit of soil rehabilitation, rehabilitation and personnel costs and the monetization of soil nutrients, based on the price of fertilizers. For the rehabilitation of ravines, structural measures were used, such as loose stones, gabion boxes and other structures of control dams, to reduce the speed of the water and vegetative measures, with the instruction of trees and grasses in the affected area. The soil loss estimate was performed according to dimensions such as mouth width, depth, width of the erosion bed, and length of the ravine. The cost of erosion was obtained based on the loss of income in the area, adjusted by the interest rate of the Central Bank of Ethiopia. Rehabilitation expenses considered from the initial surveys to the final maintenance costs, that is, all expenses with labor, materials, training, equipment in the stages of rehabilitation or maintenance. The cost of nutrient loss considered the conversion of the percentage of lost values of nitrogen (N), phosphorus (P) and potassium (K) and the level of organic carbon (OC) and the replacement cost based on the values of the fertilizers. To calculate the loss of productivity in the area, the period of time since the ravine was formed and the reduction in the cultivated area due to erosion were considered. Werken Gashajagrie, Werken Adura, Eshim Wofena and Tsegur Eyesus, the areas directly affected by each Fig. 4 Schematic spatial distribution of impacts caused by linear erosion on ecosystem services in the watershed. 1-aquifer drawdown; 2-loss of soil nutrients; 3-changes in the drainage pattern; 4increased runoff; 5-reduction in organic material in the soil; 6-impact on biodiversity fauna and flora; 7-Reduction in vegetation cover; 8-soil erosion; 9-reduction in soil moisture; 10-drying of wet areas; 11-increases in sedimentary load; 12-impact on the life of lakes, rivers and seas; 13-contamination of water Table 5 Main ecosystem impacts, where they occur in the hydrographic basin and to what extent they occur. Impact (0)  of the ravines in hectares were 2.60, 1.87, 1.51 and 0.95, respectively. For the four erosion events, the damage caused was US$ 4,896, the rehabilitation cost was US$ 24,480 and the replacement of nutrients was US$ 28,320. According to the calculation developed by the authors, the cost of rehabilitation tends to be greater than the future gains from recovery, which makes it difficult for poor farmers to implement these actions. For Yitbarek et al. (2012), if the study had accounted for the monetary value of wood production, grass fodder, carbon sequestration and sedimentation in bodies of water, the benefits of recovery would probably outweigh the costs.
Other authors, such as Ayele et al. (2015), also accounted for the cost of erosion in Ethiopia's highlands by counting the loss of soil nutrients (N and P) and the cost of missed opportunities; the value of the daily wage was used to account for the loss of opportunities related to increased travel time due to road interruptions and the value of animal deaths and tree replacements. In the hydrographic basin studied, the cost over two years was US$18,000, which corresponds to US$22 per ha per year, or a value of US$17 per person per year, which represents 19% of the per capita income. According to the authors, although the estimated costs are high compared to family income, they may still be underestimated because the nutrient replacement cost may not reflect the total replacement value. In addition, the estimated values did not consider costs such as the silting of reservoirs, reduction in hydroelectric power generation, impacts on irrigation, and the reduction in water quality, among other types of ecosystem services.
Due to the increase in sediment load in Fitzroy, Australia, and the subsequent changes in the Great Barrier Reef, Rust and Star (2018) carried out a study to analyze the cost of remediation of six gullies, considering measures to decrease stocking rates in pasture, increase revegetation and increase infrastructure construction to contain erosion and earthworks. The calculation of the cost of remediation was carried out by the authors considering the cost of construction, the cost of annual maintenance and the opportunity cost requested, considering a period of 10 years and a discount of 7% per year, in addition to the exclusion of cattle from the site for a period of 18 months. Among the six properties analyzed, the value to prevent erosion of one cubic meter of soil ranged between US$ 78.43 and US$ 604.96 m³ per year, and the general average was US$ 127.42 per m³ per year of sediment. Rust and Star (2018) considered that the study presented a view on the complexity related to the cost of gully remediation measures, as the specific characteristics of each erosion event impact the final cost of recovery. Hence, the construction of policies becomes more difficult.
The works analyzed above in this topic were all carried out in rural regions. The case of changes caused by the increase in sediment load on Australia's reefs is a clear example of impacts outside the area directly affected by erosion. To analyze the socioeconomic impacts and ecosystem services impacts, the three different areas of the hydrographic basin must be considered.
The greater the erosion and/or the longer it takes for measures to contain erosion, the greater the impacts. The estimates proposed by Bartley et al. (2020) of the reduction in sediment load downstream of erosion in a period between 19 and 28 years after the adoption of recovery measures indicate that linear erosion is a problem that requires planning and analysis that consider the results in the medium and long term. The review carried out by Romero-Díaz et al. (2019) indicated that after adopting recovery measures, the positive results increase over the following years.
Considering the economic and environmental impacts described in the previous items, there are complex cases in cities such as the city of Kinshasa in Congo, where in the literature, there is a report of destruction of dozens of houses due to the evolution of only one erosion area (Ozer 2014;Imwangana et al. 2015), demonstrating that in urban areas, the costs can be higher. The case of the Comodoro Rivera in Argentina is another example of significant damage in urban areas (Paredes et al. 2020). The waterproofing of areas, which changes the natural dynamics of rainwater runoff, the construction of linear structures and the exposure of the soil can favor the rapid development of large erosions. However, none of the analyzed studies assessed socioeconomic impacts or ecosystem services in urban areas.
An analysis of the impacts of erosion must consider the temporal and spatial dimensions of the damage caused, including the direct economic losses at the erosion site, downstream and upstream; the value of the containment or recovery measures; the partial or permanent losses of the productive area, according to the period in which the erosion developed; and the cost of maintaining any containment measures, in addition to accounting for impacts on ecosystem services along the hydrographic basin (Table 6).
Three possible future evolution scenarios may be considered: (a) total recovery, that is, where through recovery measures, the affected area can recover all environmental, social and economic functions; (b) stabilization, in this case, the recovery measures stabilize the erosion; however, some of the socioeconomic or environmental functions are not recovered or are partially recovered; and (c) natural evolution, where no recovery measures are taken, and in this way, erosion continues to grow until is stabilizes naturally.
The calculation of the impact of erosion depends on the variables measured in the affected area. In all cases, the damage caused and/or recovery measures will be carried out in different locations in the watershed area. Some variables will always be linked to the time the impact, damage or cost persisted, such as, for example, the stoppage of economic activities, loss of cultivated areas, and cost of annual maintenance. Other variables, however, generate direct damage that may persist until the item is replaced or rebuilt, such as animal death and infrastructure destruction. Additional variables are linked to local socioeconomic changes, such as increased workloads due to erosion containment and route changes to address road interdiction. Ecosystem services can be monetized by analyses such as replacement cost, as performed by Yitbarek et al. (2012), or by other methodologies that allow stipulating the value related to each variable already used for monetization of impacts such as the Contingent Valuation Method (CVM), Willingness to Pay (WTP) and Net Present Value (NPV) (Getzner et al. 2017;Maghsood et al. 2019;Moos et al. 2019), which have been used by the authors to value forests, manage floods under climate change and reduce risks based on the ecosystem provided by the forest. The final calculation of the impacts of erosion will be the result of the sum of the total variables and expenditures made at the site considering the time, the recovery measures, the impacts on ecosystem services and socioeconomic impacts.

Conclusion
The analysis and valuation of the impacts of ravines and gullies is an important step in quantifying their actual social, economic and environmental dimensions. This review summarizes and evaluates the different dimensions of their impacts and the costs and achievements of their associated recovery measures. The impact assessment should consider the spatial and temporal dimensions of the impacts, such as damage caused, recovery time and the cost of the measures taken downstream, at the erosion site and upstream. Cost of aquifer recovery (when possible) With worldwide demographic growth, a growing demand for food, and a greater concern about human impacts on the environment, the control of linear erosion and the recovery of affected areas have become a fundamental measure to optimize the use of natural resources.
The recovery of affected areas means introducing, or reintroducing, valuable portions of land into the economic and social scenario and making it possible to maximize the available ecosystem resources. Erosion recovery and control play an important role in preventing environmental, social and economic damage, especially in cities where there is a large population concentration. In rural areas, impacts can make economic activity on properties unfeasible.
The recovery of affected areas and the containment of evolving ravines and gullies should be considered in addition to the individual benefits to the landowner, as the impacts affect the entire watershed and can modify the way the site can be used for decades or centuries.
An analysis of erosion evolution and impacts must consider the appropriate scales and limitations of each method. The list of impacts listed in this work and the different forms of analysis for each variable allow valuations to be carried out anywhere on the planet. However, for the analysis to be accurate, it is necessary to identify which variables are applicable in the area under study and what information is available to support the valuation process.
The lack of basic sanitation infrastructures in cities, the use of inadequate agricultural techniques, the lack of response capacity, the delay in the response to control small erosions, and the existence of roads, especially unpaved, unplanned and incorrectly executed engineering ones, contribute to the rapid evolution of erosions in developing countries.
The best measure for areas susceptible to the formation of gullies and ravines is the correct planning of territorial occupation and the rapid recovery of any small erosion that appears. However, for other areas where erosion has become a problem in recent decades, understanding the environmental and economic impacts is important to know how to prioritize the application of financial resources in the recovery and containment of linear erosion.