Large Gullies Research Articles

Adaptation of the TIMBRE methodology for brownfields gully erosion analysis in urban areas

Soil erosion is a concern in many parts of the world, causing environmental and social impacts. Aiming at obtaining indicators of the recovery of brownfields created by gullies in urban areas, this study adapts the Tailored Improvement of Brownfield Regeneration in Europe (TIMBRE) for the analysis and classification of areas affected by gullies in the city of Bauru, Brazil. The TIMBRE methodology assists in the decision-making of priority areas for remediation and their reinsertion in urban spaces. The inventory of areas affected by gullies was compiled based on the analysis of two image sets (2004 and 2020) available on Google Earth. For the classification of brownfields, three classes were considered: Class 1 - local potential for business development, Class 2 - attractiveness and marketing, and Class 3 – environmental risks. These results demonstrate a correlation between gullies and urban expansion. The inventory identified 175 gullies in the municipality's urban perimeter in 2004, which affected an area of over 64 ha. In 2020, the number of gullies increased to 189, but the affected area decreased to 62 ha due to the recovery of some large gullies. The proposed methodology identified the area of Quinta da Bela Olinda as the one with the highest scores in all three classifications. Quinta da Bela Olinda is the location that has a local potential for business development, as it is the most attractive brownfield, as well as the area with the highest environmental risk. This area should, thus, be prioritized by public management for remediation. In conclusion, the proposed method of analysis can be transferred to other areas with adaptations in the criteria used and, therefore, may facilitate the management of urban areas affected by gullies in other places around the world.

Gullying and landscape evolution: Lavaka in Lac Alaotra, Madagascar shed light on rates of change and non-anthropogenic controls.

Gully evolution remains poorly understood, largely because multidecadal analysis is lacking. Large gullies (called lavaka) that pepper Madagascar's highlands are generally attributed to human impact; but longitudinal data are few, and anthropogenic causation is inferred not verified. We focus on Lac Alaotra, Madagascar's largest lake and wetland, its major rice-growing region, and an ecological hotspot surrounded by fault-controlled steep hills with abundant lavaka. Analysis of historical aerial photographs and recent orthoimagery reveals that the proportion of highly active lavaka has decreased since mid-20th century. At the same time, human population, farming intensity, and livestock density have increased exponentially. This suggests that background factors, including seismicity, are primary drivers of lavaka formation. Although human activities may contribute to erosion overall, land management policies that overemphasize human causation of gullying and neglect background forcing factors (in Madagascar and elsewhere) are unlikely to be effective in erosion mitigation.

Sediment Sources, Erosion Processes, and Interactions with Climate Dynamics in the Vakhsh River Basin, Tajikistan

The Vakhsh River is tributary to the Amu Dayra, supporting numerous hydropower facilities as well as irrigation and community water supplies. High sediment loads are major concerns for these uses, yet little is known about the spatial distribution of the dominant sediment sources or their connectivity to fluvial systems. Here, we address this gap by combining findings from a series of field expeditions, remotely sensed climate and vegetation assessments, systematic sediment sampling, hydrograph analysis, and a review of local literature. Our preliminary findings show that various mass wasting processes (e.g., landslides, debris flows, rockfall, dry ravel, bank failures) constitute the major connected sources of sediment, particularly in the mid- to downriver reaches, many of which are unaffected by land use. Surface erosion, including the large gullies in loess deposits of the lower basin, are more affected by poor agricultural practices and road runoff, and can supply large loads of fine sediment into the river. Climate trends detected through remote sensing show an increase in rainfall in the lower half of the basin from spring to early summer while solid precipitation has increased in the eastern half in March. These trends may lead to more runoff and increases in sedimentation if they continue.

Lavaka (erosional gullies) provide productive patch environments for flora and farming in Madagascar's grassy highlands

Societal Impact StatementLavaka, large gullies in Madagascar, can cause problems for farmers and for infrastructure, but these features also have beneficial aspects that have generally been overlooked. They provide plant refugia, and commonly host species that would otherwise not thrive on the grassy uplands. Farmers sometimes make use of lavaka to plant crops that require more protection or soil moisture. Colonial narratives blame lavaka erosion on poor land management but ignore their pre‐human‐settlement existence on the landscape and the ecological functions they serve. The knowledge and wisdom of Malagasy farmers are key to better understanding the complex roles that lavaka plays in the landscape.SummaryThis paper combines a review of lavaka—erosional gullies in Madagascar's grass‐covered highlands—with new observations of their importance as ecological patch environments for a wide range of plants not seen elsewhere within the grassy biome. Lavaka play a role as a natural refugia for flora that would otherwise not thrive on the exposed and infertile hillslopes, and local farmers exploit their topography and sediment deposits to grow a variety of crops. We provide a classification scheme for lavaka based on the extent to which they have been colonised by vegetation. Our analysis is based on observations made over many field seasons in the highlands as well as interviews with Malagasy farmers living with lavaka on their land. We emphasise the importance of local knowledge and environmental wisdom in the analysis of landscape evolution, and we conclude that lavaka, while certainly an erosional problem in many places, can nonetheless provide valuable ecosystem services and agricultural opportunities that should not be overlooked.

Evolution of gully erosion and susceptibility factors in the urban watershed of the Kimemi (Butembo/DR Congo)

Gully erosion is one of the most intense landscape degradation mechanisms in areas with varying environmental characteristics. Both natural and anthropogenic factors affect the gullying process. Reliable documentation of these processes in tropical African cities is scarce. This study assessed the gully erosion dynamic and the susceptibility factors in the urban watershed of Kimemi in eastern Democratic Republic of Congo (DRC). Data were obtained through a combination of fieldwork and digitization of very high spatial resolution images from Google Earth (from 2011 to 2021). The length, width, and area of large gullies (width ≥ 5 m) were measured for each year of the study. A logistic regression model (LRM) was also used to investigate the influence of both physical and anthropogenic factors on gully susceptibility. The results revealed that the number of gullies has increased from 36 to 61 during the last decade. The gully mean length of 63.9 ± 61.1 m, 129.3 ± 104.9 m, and 174.7 ± 153.8 m were obtained for the years 2011, 2015, and 2021 respectively. The average density of gully network for the study period was 0.12 km/km², while the degraded land was ∼1.3 and ∼1.1 ha/year for 2011–2015 and 2015–2021 for the entire watershed. The significant changes in morphometric parameters (length, width, area) were found only in the bare land and building land uses. A strong and positive relationship between the length (m) and the area (ha) was found. Furthermore, the susceptibility of gullying was significantly influenced by the slope, stream power index (SPI), distance to roads and rivers, land use and land cover (LULC), and normalized difference vegetation index (NDVI). This means the areas located in the bare land and building or close to roads and/or streams are more likely to be gullied. The findings emphasize the impact of urbanization on gully erosion in the Kimemi watershed, highlighting the importance of informed land management decisions with a close attention to anthropogenic factors.

The abnormally large “hushes” of Teesdale, North Pennines, England: Differentiating mining legacy and natural landforms in glaciated Carboniferous bedrock terrain

The unusually large, steep-sided gullies and gorges incised into bedrock at Coldberry Gutter and Pike Law in Teesdale in the North Pennines are traditionally associated with ‘hushing’, a hydraulic mining process designed to gain access to lead-bearing veins. However, it is difficult to reconcile the size and form of these features, as well as the bedrock geology, entirely with a hushing origin. Instead, it is proposed that these landforms originated naturally on valley sides and interfluves through a combination of karstification, spring sapping, steephead evolution, fluvial slope channel enlargement, landsliding and meltwater channel erosion, which preferentially developed in fault and mineralised zones. The practices of hushing and quarrying then targeted the natural bedrock exposures for the extraction of visible mineralised materials, which were small and locally workable orebodies that would likely have led the way to the discovery of much more substantial mineralised ground and then subsequent adit excavation in adjoining areas. Large numbers of leats and hushing dams were constructed above and around Coldberry Gutter and Pike Law and were used to enlarge some engineered trenches/gullies (hushes), but were mostly too small to account for the excavation of the substantial volumes of bedrock necessary to create the largest incisions. Volume calculations on the alluvium located at the base of the incisions, likely to contain all the waste rock from the hushing process, are also incompatible with the total sizes of the proposed excavations; additional rock waste contained in spoil tips are composed of material excavated directly from adits within the gulleys and gorges. It appears that engineered reservoirs and leats were employed mostly to clear out waste material, wash processed rock and contribute to hydro-power activities in the larger gullies and gorges. Some of the most intensive hushing activity is in the surficial (Late Pleistocene and Holocene) deposits laid down by glaciation and postglacial fluvial processes, suggesting that it was employed also as a placer mining technique.

Strategies for Gully Stabilization and Highland Protection in Chinese Loess Plateau

Loess tablelands are large-scale platforms with flat top surfaces in the center of Chinese Loess Plateau, such as Dongzhiyuan, which is a ∼900 km2 loess stratum, with the thickness larger than 200 m. The break of loess tablelands is widely reported due to the retreat of gully systems, resulting in serious social problems and economic losses. The retreating rate of large gullies has been 0.5–6.6 m/year over the past 2,000 years, estimated by the calculation of erosion modulus, literature archaeology, and field monitoring. The retreating rate of a small gully can reach 7.5–27.0 m/year after a rainstorm. To protect the farmland, buildings, roads, and pipelines near to the gully head areas, a project named Gully Stabilization and Highland Protection (GSHP) was launched by the local government, including the small watershed management (SWM) around loess slopes and gully channels and sponge city construction (SCC) achieved by improving the drainage system of tableland surfaces. These efforts improve the soil and water conservations, although they contribute less to controlling gully expansion. From the perspective of mutual promotion of gravity erosion and hydraulic erosion, this paper proposes a technical framework for GSHP, focusing on the avoidance of geological disaster. This case study reports on an illustrative GSHP project, the restoration of Huoxianggou gully, including the backfilled loess embankment across the gully for convenient urban transportation, and the stabilization of unstable loess slopes. After the implementation, the retreat of the gully head has been stopped, and the soil erosion rate has been reduced by 90%.

The impact of the development of negative soil processes on the land fund of the Russian Federation

One of the reasons of the reduction of agricultural land is the degradation of soil processes. The main negative processes occurring in soils and leading to their degradation are erosion (water and wind), reduction of humus content in the soil. A significant spread of negative processes developing due to the action of natural and anthropogenic factors leads to a critical decrease in soil fertility. By washing away the upper fertile soil layer and the formation of large gullies due to improper tillage, soil erosion is formed. In order to prevent soil erosion, it is necessary to take measures to prevent an increase in the area of degraded land. The article considers the impact of negative processes, such as water and wind erosion, salinization, overwetting, flooding and waterlogging of soils, etc. on the land fund.

A case‐study on history and rates of gully erosion in Northeast China

AbstractMollisols are of major importance for food security worldwide but are increasingly degraded by soil erosion. Mollisols in Northeast China have been converted into agricultural use only since the 19th century, but gullies are widely distributed. Gully erosion history, rates, and causes in this region remained unclear. We chose a study area with landforms and land‐use history typical for the central Mollisol region of Northeast China to estimate the initiation years and rates of gully erosion from 1968 to 2018 using aerial and satellite imageries. The outlet fan deposits of a large gully system were dated by Cesium‐137 (137Cs) and artifacts. Local farmers were interviewed to verify the results. Gully volumes were measured by a structure‐from‐motion technique using photos taken from an unmanned aerial vehicle. Our results showed that gully systems had already appeared on the steep slopes and along unpaved roads in 1968. They had become larger and more complex in 2018 by the upslope retreat of the main gullies and side gully formation. Gully incision started in the 1950s and 1960s when the original grassland and forest were completely converted into arable land. From 1968 to 2018, the gully density increased from 1.2 to 2.3 km km−2 and the gully heads retreated at speeds from 1.5 to 2.5 m yr−1. The soil loss from gully erosion ranged from 25.7 to 44.7 Mg yr−1 ha−1. These data demonstrate the severity of gully erosion in the Mollisol region of Northeast China and underline the importance of appropriate countermeasures.

ТИПЫ ВЗАИМОСВЯЗИ ИНФРАСТРУКТУРЫ НАСЕЛЕННЫХ ПУНКТОВ С ЭРОЗИОННО-РУСЛОВЫМИ СИСТЕМАМИ

The results of studies of relationship between populated areas infrastructure (rural and urban) and erosional relief, gully erosion and channel processes are generalized. The formation of infrastructure and living conditions of the population depend on the degree of territory dissection by ravine-gully and river network, location of settlements on river banks – from small to the largest. Most of the settlements are confined to rivers and large gullies with a permanent watercourse, because these landforms supply water, ideal for industry development, energy production and transportation. At the same time, gully erosion and channel migration create certain risks for disfunction or destruction of operating facilities. With the growth of settlements and development of their technical and economic potential, the impact on certain elements and on the entire erosional relief is expanding, and the protection from hazardous gully erosion and channel processes is necessary. At different stages of their existence, settlements may completely depend on the relief, adjust to or transform erosion and channel processes, neutralizing their negative impact on human activities. Sustainable development of settlements depends on adequate choice of the territory, rational use of the erosional relief features, and accounting for probability of changes in the course of channels and gully formation. The study present the assessment and general classification of relationship of urban and rural settlements infrastructure (from small rural to the largest cities) with erosional relief, gully erosion and channel processes. Cities and rural settlements, subordinated to or suppressing erosional relief, rivers and channel processes are distinguished. The larger the city, the more complex these relationships are. Within the large city, different areas may refer to different types of interaction.

Anurofauna in an Impacted Area in the Municipality of Ituverava in the São Paulo State of Brazil

There is great concern about the world amphibian populations reduction resulting from six major ecological factors cited as causes of modern amphibian declines and extinctions. Habitat loss resulting from deforestation and pollution of water reservoirs are usually events in agricultural production processes in Brazil and are important examples of these factors. In addition, there are many gaps in the knowledge of amphibian communities across the Brazilian territory. We carried out an anurofauna inventory for an impacted rural area, consisting of a pond located in the middle of an agricultural soybean plantation, in the municipality of Ituverava, State of São Paulo, in 2009. The inventory showed 15 species of anurans in 4 families. The most abundant families were Hylidae (six species) and Leptodactylidae (six species), one of them belonging to the subfamily Leiuperinae. The families Bufonidae (two species) and Microhylidae (one species) were also observed. Two species identified could be interesting finds, since the registration of <i>Pseudis bolbodactyla</i> (Lutz, 1925) in the São Paulo state until the present moment has no mentioned, and <i>Rhinella mirandaribeiroi </i>(Gallardo, 1965), was first described recently to São Paulo state in the municipality of São Joaquim da Barra, 30 km distant for the study area. The species was well adapted to their microhabitats, even with high ecological degradation, showing themselves to be opportunistic and tolerant to anthropic areas. Unexpectedly the pond started to dry, and it dried up totally in the second half of 2010. Then a new inventory was done in the study area for raining season in 2010, showing 6 species. The species were in 4 families (one for Hylidae, one for Leptodactylidae, one for Bufonidae, and three for Microhylidae), all showing explosive reproduction behavior in temporary pools formed by heavy rain. The study data stayed archived by twelve years. To conclude this work, the environmental conditions of the study area were analyzed in the year 2021. The pond region was substituted by a pasture for cattle confinement. We couldn’t observe even tadpoles in temporary puddles in periods of torrential rain. The soil is compacted with large gullies. Unfortunately, all species inventoried in 2009 and 2010 could not be found. In that way, this study highlights the worrying reality of environmental degradation involving agriculture and its relationship with anurofauna in Brazil.

Modern innovative and unconventional methods to combat soil erosion

One of the reasons for the decline in agricultural land is soil degradation processes. The main negative processes occurring in soils and leading to their degradation are erosion (water and wind) and a decrease in the humus content in the soil. A significant spread of negative processes developing as a result of the action of natural and anthropogenic factors leads to a critical decrease in soil fertility. The washout of the upper fertile soil layer and the formation of large gullies due to improper soil treatment results in soil erosion. In order to prevent soil erosion, it is necessary to take measures to prevent an increase in the area of degraded land. The paper discusses modern innovative methods of combating water and wind soil erosion. Various examples of the application of modern innovative methods in different countries are presented.

Gully mapping using geographic object-based image analysis: A case study at catchment scale in the Brazilian Cerrado

Water erosion is one of the main factors of soil degradation, causing several environmental damages. The most severe stage of water erosion culminates in the emergence of gullies, which increases soil loss and sediment production. The Cerrado biome, a global diversity hotspot, has been affected by gullies in many regions of Brazil. This study investigates the use of Geographic Object-Based Image Analysis (GEOBIA) to detect large gullies from RapidEye and SRTM data in anthropized Brazilian Cerrado. For the first time, a two-sided 50% overlap criteria was used to assess gully segmentation by applying Segmentation Evaluation Index (SEI). The results were checked against manually digitized reference data. The results of gully mapping indicated a user's accuracy of 69.78% in area 1 and 90.24% in area 2; a producer's accuracy of 52.10% in area 1 and 55.42% in area 2. The model can be considered robust since it was possible to detect gullies and generate few false positives in a heterogeneous scene, even using medium resolution data. This approach has the potential of application on a regional scale and can provide valuable information for land use management.

Using very high resolution (VHR) imagery within a GEOBIA framework for gully mapping: an application to the Calhoun Critical Zone Observatory

AbstractGully erosion is a form of accelerated erosion that may affect soil productivity, restrict land use, and lead to an increase of risk to infrastructure. An accurate mapping of these landforms can be difficult because of the presence of dense canopy and/or the wide spatial extent of some gullies. Even where possible, mapping of gullies through conventional field surveying can be an intensive and expensive activity. The recent widespread availability of very high resolution (VHR) imagery has led to a remarkable growth in the availability of terrain information, thus providing a basis for the development of new methodologies for analyzing Earth's surfaces. This work aims to develop a geographic object-based image analysis to detect and map gullies based on VHR imagery. A 1-meter resolution LIDAR DEM is used to identify gullies. The tool has been calibrated for two relatively large gullies surveyed in the Calhoun Critical Zone Observatory (CCZO) area in the southeastern United States. The developed procedure has been applied and tested on a greater area, corresponding to the Holcombe's Branch watershed within the CCZO. Results have been compared to previous works conducted over the same area, demonstrating the consistency of the developed procedure.

Comparison of Three Algorithms for the Evaluation of TanDEM-X Data for Gully Detection in Krumhuk Farm (Namibia)

Namibia is a dry and low populated country highly dependent on agriculture, with many areas experiencing land degradation accelerated by climate change. One of the most obvious and damaging manifestations of these degradation processes are gullies, which lead to great economic losses while accelerating desertification. The development of standardized methods to detect and monitor the evolution of gully-affected areas is crucial to plan prevention and remediation strategies. With the aim of developing solutions applicable at a regional or even national scale, fully automated satellite-based remote sensing methods are explored in this research. For this purpose, three different algorithms are applied to a Digital Elevation Model (DEM) generated from the TanDEM-X satellite mission to extract gullies from their geomorphological characteristics: (i) Inverted Morphological Reconstruction (IMR), (ii) Smoothing Moving Polynomial Fitting (SMPF) and (iii) Multi Profile Curvature Analysis (MPCA). These algorithms are adapted or newly developed to identify gullies at the pixel level (12 m) in our study site in the Krumhuk Farm. The results of the three methods are benchmarked with ground truth; specific scenarios are observed to better understand the performance of each method. Results show that MPCA is the most reliable method to identify gullies, achieving an overall accuracy of approximately 0.80 with values of Cohen Kappa close to 0.35. The performance of these parameters improves when detecting large gullies (&gt;30 m width and &gt;3 m depth) achieving Total Accuracies (TA) near to 0.90, Cohen Kappa above 0.5, and User Accuracy (UA) and Producer Accuracy (PA) over 0.50 for the gully class. Small gullies (&lt;12 m wide and &lt;2 m deep) are usually neglected in the classification results due to spatial resolution constraints within the input DEM. In addition, IMR generates accurate results for UA in the gully class (0.94). The MPCA method developed here is a promising tool for the identification of large gullies considering extensive study areas. Nevertheless, further development is needed to improve the accuracy of the algorithms, as well as to derive geomorphological gully parameters (e.g., perimeter and volume) instead of pixel-level classification.

Degradação do Solo em Municípios do Sul do Estado de Mato Grosso do Sul Decorrente da Implantação da Colônia Agrícola Nacional de Dourados - CAND

Soil degradation in municipalities in the south of the state of Mato Grosso do Sul resulting from the implantation of the National Agricultural Colony of Dourados – CAND. The National Agricultural Colony of Dourados (CAND), implemented since 1950 in the southern state of Mato Grosso caused profound changes in the local landscape, mainly environmental changes. The original vegetation was removed, crops took its place. With the degradation of the soils the crop was replaced, in great part of the area, by artificial pastures. This article analyzes the colonization process in the municipalities of Fatima do Sul, Gloria de Dourados and Deodapolis, aiming to show that soil degradation occurred with intensity, especially in the area covered by dystrophic Red Latosols (LVd) and Red Argisol (PV) Was a consequence of improper land use, in addition to the loss of soil fertility, there was also an intense erosion process, to demonstrate this process, we take the basin of the corrego Lagoa Bonita, where we highlight two large gullies, phenomena common in the area.

Accuracy Assessment of the Planar Morphology of Valley Bank Gullies Extracted with High Resolution Remote Sensing Imagery on the Loess Plateau, China.

Gully erosion is a serious environmental problem worldwide, causing soil loss, land degradation, silting up of reservoirs and even catastrophic flooding. Mapping gully features from remote sensing imagery is crucial for assisting in understanding gully erosion mechanisms, predicting its development processes and assessing its environmental and socio-economic effects over large areas, especially under the increasing global climate extremes and intensive human activities. However, the potential of using increasingly available high-resolution remote sensing imagery to detect and delineate gullies has been less evaluated. Hence, 130 gullies occurred along a transect were selected from a typical watershed in the hilly and gully region of the Chinese Loess Plateau, and visually interpreted from a Pleiades-1B satellite image (panchromatic-sharpened image at 0.5 m resolution fused with 2.0 m multi-spectral bands). The interpreted gullies were compared with their measured data obtained in the field using a differential global positioning system (GPS). Results showed that gullies could generally be accurately interpreted from the image, with an average relative error of gully area and gully perimeter being 11.1% and 8.9%, respectively, and 74.2% and 82.3% of the relative errors for gully area and gully perimeter were within 15%. But involving field measurements of gullies in present imagery-based gully studies is still recommended. To judge whether gullies were mapped accurately further, a standard adopting one-pixel tolerance along the mapped gully edges was proposed and proved to be practical. Correlation analysis indicated that larger gullies could be interpreted more accurately but increasing gully shape complexity would decrease interpreting accuracy. Overall lower vegetation coverage in winter due to the withering and falling of vegetation rarely affected gully interpreting. Furthermore, gully detectability on remote sensing imagery in this region was lower than the other places of the world, due to the overall broken topography in the Loess Plateau, thus images with higher resolution than normally perceived are needed when mapping erosion features here. Taking these influencing factors (gully dimension and shape complexity, vegetation coverage, topography) into account will be favorable to select appropriate imagery and gullies (as study objects) in future imagery-based gully studies. Finally, two linear regression models were built to correct gully area (Aip, m2) and gully perimeter (Pip, m) visually extracted, by connecting them with the measured area (Ams, m2) and perimeter (Pms, m). The correction models were and , respectively. These models could be helpful for improving the accuracy of interpreting results, and further accurately estimating gully development and developing more effective automated gully extraction methods on the Loess Plateau of China.

Quantification by numerical simulation of the impact of gullies on the water budget of a basement area, southeastern Brazil

ABSTRACTMany regions of the world are affected by high density of large gullies that can impact on water resources. As these impacts have not been properly evaluated, this study presents a quantification model by numerical simulation of the effect of gullying on the water budget. An active gully of a very eroded region (Bação Complex) was selected and systematically monitored during a hydrological year. The simulation of the gully area by finite element method enabled the quantification of impacts, such as baseflow reduction and groundwater drawdown in the vicinity of this erosional feature. The baseflow reduction, when extrapolated to the whole complex, showed a significant baseflow loss during the dry seasons and an increase of stream flows during the rainy seasons, favouring floods. The numerical simulation results indicate that these modifications are relevant and must be considered when managing aquifers in intensely gullied areas with problems related to water availability.

Gully erosion as a mechanism for wetland formation: An examination of two contrasting landscapes

AbstractWetlands occupy broad, gently sloping valleys characterised by net deposition. Consequently, gully incision is viewed as an agent of wetland destruction. However, little attention has been given to the role of gullies in natural wetland formation. This paper determines if gully cutting and filling cycles have led to wetland formation in both an extant eroding wetland and a semi‐arid environment that once supported wetlands. At both sites, the valley fill was investigated to identify infilled gullies, and a 14C date was obtained from the gully fill. The longitudinal slopes of the valley floor prior to incision, the valley bedrock floor, and the gully fill were also measured. In the Krom wetland, four infilled gullies (5–8.2 m deep) were found adjacent to alluvial fans and dated to 470–7060 bp. Ten infilled gullies were found at Zuurplaats (Karoo), which were more recent, although one gully started filling 1260 bp. At both sites, large gullies were grounded on bedrock. Alluvial fans deposited by tributaries cause a reduction in mainstem valley width and locally increase longitudinal slope, leading to gully initiation. The slope of the gully beds was less than the surface prior to erosion, and the slope of the gully fill was comparable with the valleys' regional slope. The bedrock on both valley floors had a near‐horizontal cross‐sectional profile, suggesting that repeated gully incisions had led to valley floor planing. A new model of wetland formation is proposed where gully cut‐and‐fill cycles laterally plane valley bottoms and contribute to valley widening and longitudinal slope reduction.

Assessment of Practices for Controlling Shallow Valley-Bottom Gullies in the Sub-Humid Ethiopian Highlands

Rehabilitation of large valley bottom gullies in developing countries is hampered by high cost. Stopping head cuts at the time of initiation will prevent large gullies from forming and is affordable. However, research on practices to control shallow gully heads with local materials is limited. The objective of this research was therefore to identify cost-effective shallow gully head stabilization practices. The four-year study was conducted on 14 shallow gullies (&lt;3 m deep) in the central Ethiopian highlands. Six gullies were used as a control. Heads in the remaining eight gullies were regraded to a 1:1 slope. Additional practices implemented were adding either riprap or vegetation or both on the regraded heads and stabilizing the gully bed downstream. Gully heads were enclosed by fencing to prohibit cattle access to the planted vegetation. The median yearly head retreat of the control gullies was 3.6 m a−1 with a maximum of 23 m a−1. Vegetative treatments without riprap prevented gully incision by trapping sediments but did not stop the upslope retreat. The gully heads protected by riprap did not erode. Regrading the slope and adding riprap was most effective in controlling gully head retreat, and with hay grown on the fenced-in areas around the practice, it was profitable for farmers.