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SSS2.4

Soil erosion has been traditionally divided into surface (sheet, rill, and gully erosion) and subsurface erosion (soil piping). Rills and gullies concentrate overland flow, whereas soil pipes concentrate subsurface flow, leading to a significantly increased flow erosivity. These forms of concentrated flow erosion, both above and below ground, represent an important sediment source within watersheds and produce sizeable economic losses (e.g. reduced crop yields, reservoir sedimentation, mass failures including landslides and embankment failures). These processes occur in almost all climatic zones, soil types, and land use conditions suggesting a great variability of controlling factors. Moreover, soil pipes, rills and gullies are effective links for transferring water, sediment and pollutants. Despite their relevance, the physical mechanisms that constitute concentrated flow erosion remain poorly understood.
This session aims to address this research gap and will focus on recent studies aiming to better understand the process of rill, piping and gully erosion, with the ultimate aim of developing predictive tools and effective management strategies. As such we welcome contributions on: monitoring and measurement techniques; the factors and processes controlling rill, piping and gully erosion; modelling approaches; prevention, restoration and control measuress; and the role of soil pipes, rills and gullies in hydrological and sediment connectivity.

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Co-organized by HS9
Convener: Javier Casalí | Co-conveners: Henrique Momm, Anita Bernatek-JakielECSECS, Estela Nadal Romero, Glenn V. Wilson, Małgorzata Mazurek
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| Attendance Wed, 06 May, 14:00–15:45 (CEST)

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Chat time: Wednesday, 6 May 2020, 14:00–15:45

Chairperson: Glenn Wilson
D1953 |
EGU2020-10940
Ronald Bingner, Robert Wells, and Henrique Momm

Concentrated runoff increases erosion and moves fine sediment and associated agrichemicals from upland areas to stream channels. Ephemeral gully erosion on croplands in the U.S. may contribute more of the sediment delivered to the edge of the field then from sheet and rill erosion. Typically, conservation practices developed for sheet and rill erosion are also expected to treat ephemeral gully erosion, but science and technology are needed to account for the separate benefits and effects of practices on each of the various sediment sources.

Watershed modeling technology has been widely developed to aid in evaluating conservation practices implemented as part of a management plan, but typically lacks the capability to identify how a source, such as sheet and rill erosion, ephemeral gully erosion, or channel erosion, is specifically controlled by a practice or integrated practices. The U.S. Department of Agriculture’s Annualized Agricultural Non-Point Source pollutant loading model, AnnAGNPS, has been developed to determine the effects of conservation management plans on erosion and provide sediment tracking from all sources within the watershed, including sheet and rill, ephemeral gully, and channel erosion. 

This study describes the ephemeral gully erosion capabilities within the AnnAGNPS model and discusses research needs to further improve these components for integrated conservation management planning.  Conservation management planning by agencies within the U.S. and by international organizations requires a systematic approach when determining the extent of ephemeral gully erosion impacts on a field, watershed, or national basis, and/or to predict recurring or new locations of ephemeral gullies prior to their development.  This technology provides the capability to separate the impact of ephemeral gullies on erosion from other sources and then evaluate the impact of targeted practices to control erosion at the source and subsequent downstream resources.

How to cite: Bingner, R., Wells, R., and Momm, H.: Ephemeral Gully Erosion Advancements within the AnnAGNPS Watershed Simulation Model , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10940, https://doi.org/10.5194/egusphere-egu2020-10940, 2020

D1954 |
EGU2020-1701
Pedro Henrique Lima Alencar, José Carlos de Araújo, and Adunias dos Santos Teixeira

Gullies are key drivers of land degradation, are important sources of sediment and increase sediment and pollutant connectivity in the catchment. They also play an important role in desertification areas, changing the water-table height and in farmlands, reducing productive areas. In this study, we attempted to model small permanent gullies, common in the Brazilian Semiarid Region, where the shallow soils limit the size of gullies cross-sections to a depth of no more than one meter. To model this process, we coupled the models of Foster and Lane (1983) and Sidorchuk (1999), in order to consider the effect of permanent gullies not considered in the first. Both models need as input the discharge peak and its duration, however, these data are frequently not available. We tested four different rain intensities (average, 60-minute, 30-minute and 15-minute), finding that the most intense 30 minutes represent the best the effects of the storms over gully erosion. The coupling of the two models is defined by a threshold that indicates when the equations for sidewall erosion proposed by Sidorchuk should be applied. To validate the model, we measured three gullies in the Brazilian Semiarid Region. The gullies were initiated in 1958 after the construction of a country road and have drainage area below 1 ha. The model yielded a Nash-Sutcliffe coefficient of 0.85.

How to cite: Lima Alencar, P. H., de Araújo, J. C., and dos Santos Teixeira, A.: Small permanent gullies: modelling and application to a semiarid region, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1701, https://doi.org/10.5194/egusphere-egu2020-1701, 2019

D1955 |
EGU2020-11469
Robert Wells, Yafei Jia, Henrique Momm, Carlos Castillo, Dalmo Vieira, Ronald Bingner, Sean Bennett, and Martin Locke

Soil erosion due to rainfall and overland flow can be detrimental to agricultural management and long-term agricultural sustainability. Although numerous conservation measures and planning strategies have greatly reduced the amount of sediment moving within the landscape, there are still unresolved questions concerning initiation of particle motion, susceptibility to erosion, total soil loss, sediment transport and general measurement theory. Within agricultural fields, ephemeral erosion is particularly harmful because these sources can accelerate sediment transport, often yield more sediment than interrill sources and are more challenging to mitigate. In this study, terrain data were collected by aerial photogrammetry using an unmanned aerial system (UAS) following planting and approximately one month later, while climate variables during the period were collected using NexRad radar. Imagery was captured within seven agricultural fields (six in Iowa and one in Minnesota), ranging in size from 0.6 to 3.6 hectare (1.6 to 8.8 acre). Considering the small scale in topographic variation between two surveys, extreme efforts were applied to image processing and geospatial registration. Advanced models for camera calibration utilizing Micmac open-source photogrammetry software package were used to account for complex distortion patterns in the raw image data set. The undistorted images were then processed using Agisoft Photoscan for camera alignment, model georeferencing and dense point cloud generation (millions to billions of points per survey), from which digital elevation models (DEMs; 10 to 57 million cells) were produced. A physically-based finite element hydrodynamic and sediment transport model (CCHE2D, developed at the National Center for Computational Hydroscience and Engineering) was applied to simulate hydrological (runoff), sediment detachment (raindrop splash, sheet flow, and concentrated flow erosion) and sediment transport/deposition landscape evolution processes. Simulated geomorphological and sediment budget results over time were compared to field observations for model input parameter adjustment and consequently quantification of estimates. Integration of high-resolution spatial and temporal topographic measurements with physically-based numerical models support the development and validation of dynamic landscape evolution models needed for accurate prediction and quantification of gully initiation, evolution and impact on total soil loss and effective conservation management planning.

How to cite: Wells, R., Jia, Y., Momm, H., Castillo, C., Vieira, D., Bingner, R., Bennett, S., and Locke, M.: Evaluation of CCHE2D hydrodynamic and sediment transport model to simulate erosional sources in row crop agriculture in Midwest US, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11469, https://doi.org/10.5194/egusphere-egu2020-11469, 2020

D1956 |
EGU2020-4380
Lanhua Luo and Fayuan Li

Gully development is an important topic in the evolution of modern geomorphology. The study of the development process of gullies is key to explain the genesis, mechanism and spatial differentiation of loess geomorphology. Geographic cellular automata (Geo-CA) can simulate complex geographical phenomena by expanding and elements of Cellular automata (CA). This study explores the mechanism of the development process of loess gullies while taking into account the dynamic factors of head-cut erosion. Based on geographic cellular automata (Geo-CA), the transition rules for gully evolution are designed, including the rules of gully head region, the rules of water infiltration, flow direction rule, flow rules, and sediment transport rules. Based on the small simulated Loess watershed under artificial rainfall, the simulation model of loess gully evolution is constructed and implemented. In order to evaluate the accuracy of the simulation results, the negative terrain, Hypsometric Integral (HI) and a gully head confusion matrix of the simulated results and the measured data are compared. The evaluation produces encouraging results in terms of numeric accuracy and spatial distribution, in agreement with the evolution of the loess gully. In addition, the simulation model of loess gully evolution this study proposed is applied to the evolution of a natural watershed, the Madigou watershed located in Jingbian County, Yulin City, Shaanxi Province. The comparison between the simulated results of the model and the measured data is used to verify the validity of the model. All the results show that the evolution model of loess gully based on Geo-CA is satisfactory in simulating the process of loess gully evolution, which provides a new research method and ideas for in-depth study of the process of gully evolution. 

How to cite: Luo, L. and Li, F.: Simulation of Loess Gully Evolution Based on Geographic Cellular Automata, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4380, https://doi.org/10.5194/egusphere-egu2020-4380, 2020

D1957 |
EGU2020-8661
Sofie De Geeter, Matthias Vanmaercke, Gert Verstraeten, and Jean Poesen

Gully erosion is an important land degradation process, threatening soil and water resources worldwide. However, contrary to sheet and rill erosion, our ability to simulate and predict gully erosion remains limited, especially at the continental scale. Nevertheless, such models are essential for the development of suitable land management strategies, but also to better quantify the role of gully erosion in continental sediment budgets. We aim to bridge this gap by developing a first spatially explicit and process-oriented model that simulates average gully erosion rates at the continental scale of Africa.

We are currently developing a spatially explicit model that (i) allows to simulate the spatial patterns of gully density at high resolution (30-90 m); (ii) is based on the physical principles that control the gully erosion process; (iii) uses GIS and data sources that are available at the continental scale. Our model structure is based on the threshold-dependent character of the gully-initiation process where a proxy of flow shear stress is weighted against a proxy of local shear resistance at the pixel scale. To calibrate and validate this model, we make use of an extensive database of 44 000 gully heads mapped over 1680 sites that are randomly distributed across Africa. The exact location of all gully heads was manually mapped by trained experts, using high resolution optical imagery available in Google Earth. This allows to extract very detailed information at the level of the gully head, such as the local slope and the area draining to the gully head. Based on these variables, we simulate indices for peak runoff (based on the Curve Number method), the shear stress of the concentrated runoff and the critical shear stress of the soil. The combination of these indices reflects the process leading to gully initiation and therefore provides an accurate indication of the susceptibility of that location to gully initiation.

Preliminary results indicate that it is feasible to model gully head locations and densities using this process-oriented approach. However, important trade-offs exist between an accurate description of the (threshold-dependent) gully initiation process and the uncertainties on the GIS data used to describe this process. One important issue is the resolution of the digital elevation model (DEM) used to extract local slopes (S) and to delineate contributing areas (A).  Comparing S- and A-values obtained from 30m SRTM-data with those obtained from higher resolution DEMs (5-12m) showed that SRTM data allows to obtain reasonable proxies of S and A but that uncertainties can be significant and correction factors are needed to avoid biases.

Overall, our results indicate that modelling gully densities using a process-oriented and spatially explicit approach has (conceptual and pragmatic) advantages as compared to a purely empirical ‘black-box’ modelling approach and offers opportunities to better quantify this important land degradation process at the global scale.

How to cite: De Geeter, S., Vanmaercke, M., Verstraeten, G., and Poesen, J.: Process-oriented gully density modelling at the continental scale of Africa: first insights, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8661, https://doi.org/10.5194/egusphere-egu2020-8661, 2020

D1958 |
EGU2020-147
Yixian Chen, Juying Jiao, Matthias Vanmaercke, Xiqin Yan, and Jianjun Li

Gully erosion is a major cause of land degradation in many regions worldwide. Recent research shows that the challenges posed by gully erosion are likely to further increase as a result of climate change and increasing land use pressure. Nonetheless, our understanding of this process remains limited in many ways. While numerous studies have focused on the occurrence and morphology of gullies at local (catchment) scale, relatively little research has explored their spatial variations at regional to continental scales. As a result, the factors controlling the density, size and morphology of gullies at such scales remain poorly understood. This is especially the case for the role of climate/weather conditions. Here we aim to advance our understanding on this topic by studying gully densities and gully morphology in the Chinese Loess Plateau (CLP), a region severely affected by gully erosion. 
We selected five representative catchments in the CLP that are relatively similar in size (7-30 km²), topographic context, soil characteristics and land use but represent a large gradient in rainfall conditions. We mapped 2511 gullies in these catchments, using Pleiades-1B (panchromatic resolution at 0.5 m) and WorldView-3 images (panchromatic resolution at 0.31 m). For each of the gullies, we calculated a range of morphological parameters including the gully length, width, surface area, length-width ratio and shape index. Next, we explored to what extent differences in gully density and morphology are correlated to contrasts in rainfall and other environmental factors.
Overall, the gullies showed large variations in gully length (2.1-308 m, average 38.1 m), width (1.3-87 m, average 11.5 m) and density (0-4.8 km/km², average 2.3 km/km²). Gully densities showed a negative correlation with rainfall amounts. This is likely partly attributable to feedbacks between rainfall amounts and vegetation cover. However, also contrasts in rainfall intensity and regime likely play an important role. Also variations in gully width appear strongly correlated with rainfall patterns (with more humid catchments resulting in overall wider gullies). Surprisingly, gully lengths (a first indicator of gully headcut retreat) showed no clear correlation with rainfall patterns. Overall, our results indicate that contrasts in rainfall regime are crucial to understand gully erosion dynamics at regional to continental scales. This is true for their initiation but also for their subsequent expansion (and especially gully widening). These findings have important implications for the development of models aiming to predict gully erosion at regional to continental scales.

How to cite: Chen, Y., Jiao, J., Vanmaercke, M., Yan, X., and Li, J.: Spatial variation of gully density and morphology and their controlling factors at a regional scale: a case study for the Chinese Loess Plateau, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-147, https://doi.org/10.5194/egusphere-egu2020-147, 2019

D1959 |
EGU2020-14026
Nick Krekelbergh, Amaury Frankl, and Stefaan Dondeyne

Soil erosion constitutes a major problem in the European loess belt. From England to Eastern Europe, loess-derived soils are particularly susceptible to water and tillage erosion. This is certainly the case for the Aa River Basin (Nord-Pas-de-Calais, northern France), where a relatively thin Pleistocene loess cover is present on top of a substrate of clay-with-flints and Cretaceous chalk. This research aimed at quantifying the amount of soil eroded since its initiation. Making a gross balance of the soil erosion and sedimentation processes intends to study the evolution of the soil surface and the effects of different types of erosion over longer periods of time, and quantify erosion rates in agricultural areas.

The extent and amount of eroded soil was mapped in the Lauwerdal, a 63 ha large catchment in the headwaters of the Aa River Basin (Northern France). Based on four soil profiles, described and sampled along a topographic transect, and 256 augerings spaced along a grid, the original soil surface level was reconstructed. The current topographic surface was analysed based on a Digital Terrain Model obtained from UAV aerial photographs. The organic matter present in the filling of a former erosion channel, observed in one of the soil profiles, was dated by 14C as an indication of the onset of the erosion and sedimentation process.

Water and tillage erosion are the main processes characterizing the study area: eroded soils (Nudiargic Luvisols) dominate the upper reaches of the study area with colluvium at the footslopes (Colluvic Regosols). The sediment budget reveals that the bulk of the sediments are discharged from the headwater catchment as the quantity of eroded soil (0.87 × 106 tonnes) is more than a ten-fold higher than the deposition (0.068 × 106 tonnes). The 14C dating indicates that the erosion channels started filling up between the Early Iron Age and the Roman period, ca. 1200 years BP. The historical erosion rates are estimated at 491.4 t/km2 per year, and deposition rates at 91.8 t/km2 per year.

Our findings illustrate how the amount of soil eroded over a long time span can be estimated from soil morphologic features in combination with a detailed Digital Terrain Model. Indeed, human induced soil erosion dates back at least to Early Iron Age, when forest clearing for agricultural expanded. Surely, the mechanization and upscaling of agriculture in the 20th century will have exacerbated this process. The results also show that sediments are evacuated from headwater catchments and, consequently, must accumulate in the lower alluvial plains. Our findings corroborate research findings from the silt-loess belt of central Belgium where it was shown that soil erosion started in the same period and also led to the formation of wide alluvial valleys.

How to cite: Krekelbergh, N., Frankl, A., and Dondeyne, S.: Understanding soil profiles and sediment redistribution over long time scales in an agrarian setting: the case of Lauwerdal (Northern France), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-14026, https://doi.org/10.5194/egusphere-egu2020-14026, 2020

D1960 |
EGU2020-10866
Youssef Chahor, Javier Casalí, and Rafae Giménez

Ephemeral gullies (EG) are linear erosion features located in swales where runoff concentrates during or immediately after rainfall events. EG are temporary because they are easily filled by conventional machinery and cause important soil losses in cultivated areas. Casalí et al. (1999) distinguished three types of EG: “classical”, formed by concentrated runoff flows within the same field where runoff started; “drainage”, created by concentrated flows draining areas upstream from the field; “discontinuity”, found in places where management practices create a sudden change in slope. There is still a great lack of knowledge about the true extent and importance of this EG. In this sense, the information obtained from aerial photographs can be of great value. The main objective of this work is to evaluate the possibility of making an exhaustive characterization of the space-time evolution of ephemeral gullies in a relatively large area from color aerial photographs. The effect of precipitation on the EG will be also analyzed.

The 570 ha study area is almost completely cultivated with winter cereals and located in the Pitillas district (Navarre). Climate is Continental Mediterranean (on average 550  mm yr-1). Soil (upper horizons) are loam–silty loam in texture.

EG within cultivated fields were located, classified and digitized using GIS interfaces over seven colour orthophotos (1:5000 with 0.5mx0.5m resolution) taken between 2003 and 2014. Gully length was determined after locating EG down and upstream ends. EG drainage areas and slopes were determined using a 2 m resolution DEM.

To determine EG volumes, an empirical power model for the study area defining the relationship between EG lengths and volumes was first obtained from previous field measurement, and then used for the EG lengths from this study. The corresponding erosion rates were also calculated.

57 small watersheds affected by EGs were identified, being 39 of them classified as drainage EGs, and the remaining 18 EGs as classic. 70% of the small watersheds were affected by EG only once. In remaining watersheds EG reappeared from twice to seven times. Therefore, it seems that the repeatability is not as high as thought.

The average erosion rate in classical EG is about 1.1 Kg m-2 year-1. Previous assessments using accurate direct methods reported an average value of 0.8 Kg m-2 year-1 for very similar watersheds in the same area. Although it is not a conclusive proof, this findings indicate that both methods provide similar results.

A very high correlation (r2= 0.84) has been found between the length of the gullies formed in the study area and the total annual precipitation. It would follow that EG erosion would also be controlled by the overall amount of rainfall also in Mediterranean climates, and not only by high intensity-low frequency events.

References

  1. Casalí, J. J. López, J. V. Giráldez, 1999. Ephemeral gully erosion in Southern Navarra (Spain). CATENA 36: 65-84.

How to cite: Chahor, Y., Casalí, J., and Giménez, R.: Analysis and assessment of ephemeral gully erosion in wide areas of Navarre (Spain) from routinely obtained ortophotographs, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10866, https://doi.org/10.5194/egusphere-egu2020-10866, 2020

D1961 |
EGU2020-19925
Ionut - Costel Codru and Lilian Niacsu

Gully erosion represents one of the major environmental problems within the agricultural hilly areas of Moldavian Plateau, Romania. Deforestations, made in the last two centuries, and unsustainable agricultural techniques made the Moldavian Plateau a very susceptible area especially to soil erosion, gullying and landsliding. Beside the human influence that accelerated the process, gully erosion is a natural hazard that occurred from heavy rain falls and water concentrations in catchment areas. The most important natural induced factors that influenced gully head retreat in the Moldavian Plateau are the landform features, favorable lithology, land cover changes under improper human impact on the background of specific climacteric conditions such as heavy rain falls, snow melt and or freeze-thaw phenomenon. The Moldavian Plateau is the area that has one of the biggest densities of gullies in Europe, and, because of its large population of gullies and lack of materials, techniques and time no comprehensive inventory was made. Due to the appearance of high resolution LIDAR images and the evolution of GIS software in recent years, a lot of researchers, from all around the world, tried to identify gullies and quantify them using these modern techniques. For this research, morphometric features (e.g. profile curvature and a 360⁰ hillshading grid with eight points of lightening distributed at a distance of 45 azimuth degrees) derived from LIDAR were used for a more facile mapping of gullies. These morphometric features were classified using different classification method. To evaluate the quality of the results, a shapefile with gully contours had been created by digitizing the banks of the gully. The results of the manually digitized shapefile were confronted with the results of the automatic morphometric features obtained in this research. The combination of hillshading grids that were used in this research covered a very big part of the surfaces occupied by gullies, excepting the gullies affected by landslides that were hardly recognized. Also, the derived profile curvature identified the banks of the gully in a very accurate way.

How to cite: Codru, I.-C. and Niacsu, L.: Using GIS techniques for automatic mapping of gullies in the Moldavian Plateau, Romania, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19925, https://doi.org/10.5194/egusphere-egu2020-19925, 2020

D1962 |
EGU2020-168
Ikenna Osumgborogwu, John Wainwright, and Laura Turnbull-Lloyd

Gully erosion was unknown in the Orlu area of Southeast Nigeria before the Nigerian civil war (from 1967 to 1970) but has now become endemic and continues to present day.  Human activities are central to this acceleration of erosion due to their intervention with ecogeomorphic processes. This paper aims to improve understanding of ecogeomorphic drivers of gully erosion using case studies from the Orlu area of southeast Nigeria, and to achieve this aim, focus-group meetings and analyses of remotely sensed data were adopted. High-resolution (0.61 – 5 m) satellite imagery for 2009 and 2018 were acquired from different platforms and used for gully mapping and monitoring while ASTAR DEM was used to estimate topological parameters. Upslope contributing areas were produced for two gullies; A and U, while gully evolutions between 2009 and 2018 were related to changes in contributing areas during same time span. Ecogeomorphic and climatic drivers such as vegetation-cover change, slope angle, elevation, rainfall, and nearness to roads and rivers were studied and their associations with gullying established. Vegetation cover was classified into three: non-vegetated, open vegetation and trees while daily surface runoff between 2009 and 2018 was estimated for these vegetation classes using the Curve Number approach. Results from focus-group meetings show that both gullies started in 1969 during the civil war as a result of increase in population density arising from the influx of refugees as well as other military activities. Gully growth was sustained after the civil war was a result of land use changes. Average gully headcut retreat rate between 2009 and 2018 was 64 m yr-1 and 12.2 m yr-1 for gully A and U respectively, while a positive correlation was recorded between change in vegetation cover in contributing areas and increase in gullied area with Pearson’s correlation of 0.6 and r2 of 0.4. The runoff model predicted runoff for only the non-vegetated areas with runoff coefficients ranging from 11.5 % to 22 %. Slope angle, profile and plan curvature had positive associations with gullies while elevation, nearness to rivers and nearness to roads recorded negative correlations with gullies. In conclusion, while geomorphic drivers such as slope angle are preparatory factors, human activities including civil wars and land-use changes are forcing factors of gully erosion. This study has implications for gully remediation especially as regards land use management of upslope contributing areas.

Keywords: Gully erosion, Ecogeomorphology, upslope contributing area, south east Nigeria

How to cite: Osumgborogwu, I., Wainwright, J., and Turnbull-Lloyd, L.: The gullies of southeast Nigeria: an ecogeomorphic investigation, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-168, https://doi.org/10.5194/egusphere-egu2020-168, 2019

D1963 |
EGU2020-10791
Guy Ilombe Mawe, Eric Lutete Landu, Fils Makanzu Imwangana, Charles Nzolang, Robert Wazi Nandefo, Jean Poesen, Charles Bielders, Olivier Dewitte, and Matthias Vanmaercke

Urban gullies cause major infrastructural damages and often claim casualties in many tropical cities of the Global South. Nonetheless, our understanding of this hazard is currently limited to some case studies while the overall impacts remain poorly quantified. Here, we aim to bridge this gap by making a first quantification of the number of persons and buildings affected by urban gullies at the scale of the Democratic Republic of Congo (DRC). We used Google Earth imagery to identify and map urban gullies for cities in the DRC and evaluate their expansion rate and the resulting damages where possible. In total, more than one thousand urban gullies were mapped across 22 affected cities. Over 80% of these gullies were active and, by analyzing their expansion, we identified 1463 houses and 386 roads destroyed. Nonetheless, the actual impacts are likely much larger since the limited amount of imagery available does not allow to quantify all impacts.

We therefore also made an estimate of the total number of persons directly affected by urban gullies (i.e. displaced due to the destruction of their house). For this, we calculated the areal fraction of urban gullies in affected cities (which ranged from 0.12% to 4.66%) and combined these fractions with the urban population density. From this, we estimate that a total of 212 000 people have been affected. The problem is especially acute in the cities of Kinshasa, Mbujimayi, Tshikapa, Kananga, Kabinda, and Kikwit. Given that these gullies are linked to recent urban growth and typically less than 30 years old, we estimate that at least 7000 people/year lose their house as a result of urban gullies in DRC. This is likely an underestimation since (i) not all urban gullies are detectable; (ii) urban gullies may disappear and reappear over time; and (iii) many of these gullies are likely more recent than 30 years. Furthermore, this assessment does not take into account numerous other indirect impacts of urban gullies (e.g. impacts on traffic and sanitation, increased flood risks, real estate value loss and intangible impacts like fear or stress). 

Overall, this research shows that urban gullying is a serious problem in DRC, but likely also in many other tropical countries. More research is needed to better understand this processes and, ultimately, to prevent and mitigate its impacts. The results and the database of this study provide an important first step in this direction.

How to cite: Ilombe Mawe, G., Lutete Landu, E., Makanzu Imwangana, F., Nzolang, C., Wazi Nandefo, R., Poesen, J., Bielders, C., Dewitte, O., and Vanmaercke, M.: Assessing the impacts of urban gullying in the Democratic Republic of Congo , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10791, https://doi.org/10.5194/egusphere-egu2020-10791, 2020

D1964 |
EGU2020-10706
Eduardo Luquin, Richard M. Cruse, Karl R. Gesch, Matthew J. Helmers, Henrique G. Momm, Robert R. Wells, Miguel A. Campo, and Javier Casalí

Ephemeral gullies (EG) are linear erosion features located in swales where surface and/or subsurface runoff concentrate during or immediately after rainfall events. As its name states, EGs are temporary because they are easily filled by conventional machinery, but when filled they reform if the area is not appropriately managed. Downstream water quality issues and decreased soil productivity are the main environmental impacts. EGs are frequently identified as (the most) relevant sediment sources in agricultural areas but their dimensions and particular contribution to the total erosion under different temporal, spatial, climate and land use condition is still unknown. Therefore, the objective of this study is to obtain ephemeral gully erosion rates and estimate the main morphological characteristics of the ephemeral gullies (width, length and depth) and their evolution both in relation to time and position on the landscape.

The studied EGs, B6 with a 0.94 ha watershed and I3 with a 0.95 ha watershed formed in two fields located in the Walnut Creek watershed, Iowa (US). The field-sized watersheds are less than 1.5 Km apart and have similar topography and soils. The cropping system consists of a two-year corn-soybean rotation managed by one farmer using no-till and other standard management practices. EG were measured using close range photogrammetry techniques. In order to achieve a suitable characterization of the EG evolution over time and space, EGs were divided in three sections (bottom, middle and top) of equal length. Photographs were taken at least once in 2013, 2014 and 2018 (a total of five in I3 and three in B6). Cross section profiles along the EG perpendicular to the flow path direction were selected and their width, area and depth were determined from a graphical representation of the cross sections. EG volumes were estimated by the sum of interpolating sequential cross-section areas and multiplying by the distance between them.

Average EG erosion rates during 2013-2014 were 3.19 Mg ha-1 year-1 for B6 and 3.63 Mg ha-1 year-1 for I3. Values in agreement with rates estimated by the United States Department of Agriculture (USDA) Natural Resources Conservation Service (NRCS) of 0.49 to 5.18 Mg ha-1 year-1 across the USA and other simulated values of 4.00 ± 1.76 Mg ha-1 year-1 for no till systems in the state of Iowa. The current study shows evidences that EG in no till systems may not stabilize after their formation. EG dimensions (depth, width and length, thus volume) varied over time and space during the continuously monitored period. In general, volumes tend to increase in the middle position while depths decrease in the bottom position. When the EG was filled, it reformed again in approximately the same location showing similar dimensions to that which existed prior to filling.

How to cite: Luquin, E., Cruse, R. M., Gesch, K. R., Helmers, M. J., Momm, H. G., Wells, R. R., Campo, M. A., and Casalí, J.: Analysis of ephemeral gully erosion in small agricultural watersheds in Iowa (USA), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10706, https://doi.org/10.5194/egusphere-egu2020-10706, 2020

D1965 |
EGU2020-4034
Chao Qin, Fenli Zheng, and Robert Wells

Channel widening constitutes about 80% of total soil loss, especially in the presence of a plow pan which manifests a less or nonerodible soil layer. Channel bank erosion quantification is prerequisite to couple effectively the bank sediment supply system with fluvial sediment transport fluxes. The objectives of this study were to: 1) describe and evaluate methods for monitoring and data post-analysis of channel widening and 2) investigate how inflow rate, slope gradient and initial channel width affect channel widening processes in the presence of a non-erodible layer. Technology was developed to capture 5-cm spaced cross-sections along a soil flume at 3-s time intervals. Two off-the-shelf digital cameras were positioned 3-m above the soil bed and controlled by a program to trigger simultaneously and download images to the computer. Methods utilizing color differences in images and elevation differences in DEMs were applied to detect discontinuities between channel walls and the soil bed. Channel widths were calculated by differentiating the coordinates of these surface discontinuities. A volumetric method was used to calculate flow velocity with measurements of flow depths obtained from ultrasonic depth sensors. Sediment concentration was determined by manual sampling.

The results showed that different channel width calculation methods exhibited comparable outcomes and achieved satisfactory accuracy. Sediment discharge showed a significant positive linear correlation with channel widening rate, while exhibiting a 5 to 25-s time lag compared to the peak of channel widening rate. Total sediment discharge calculated by photogrammetry was 3.1% lower than that calculated by manual sampling. Flow velocity decreased with time and showed a significant negative power correlation with channel width. Sediment delivery and channel width increased with the increase of inflow rate, bed slope and the decrease of initial channel width. Exponential equations were used to predict the channel width time series. Toe scour, crack development, sidewall failure and block detachment and transport, in sequence, were the four main processes of channel widening. Basal scour arc length, tension crack length and width decreased with initial channel width and increased with time, flow discharge and bed slope. Basal scour arcs were divided into three patterns according to different shapes in comparison to the failure arcs. Sediment delivery equations based on the disaggregation of concentrated flow entrainment and mass failure were also fitted. Advantages of the described methodology include automated high spatial and temporal monitoring resolution, semi-automated data post-processing, and the potential to be generalized to large scale river/reservoir bank failure monitoring. This study provides new insight on improving channel widening measurements and prediction technology.

How to cite: Qin, C., Zheng, F., and Wells, R.: Channel widening quantification under laboratory conditions, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4034, https://doi.org/10.5194/egusphere-egu2020-4034, 2020

D1966 |
EGU2020-2199
Elena Zubieta, Juan Larrasoaña, Rafael Giménez, Alaitz Aldaz, and Javier Casalí

In gully erosion, the soil detached by the action of the erosive flow can be transported over long distances along the drainage network of the watershed. In this long way, the eroded material can be redistributed and/or deposited on the soil surface, and then eventually buried by eroded material from subsequent erosion events. Likewise, the variability of the soil (i.e., in texture and moisture content) over which this material moves can be considerable. The presence of the eroded material could be detected through magnetic tracers attached/mixed with the eroded soil. In this experiment, the degree to which the magnetic signal of the magnetite is conditioned by (i) the burying tracer depth, (ii) the texture and moisture content of the soil covering the tracer and (iii) the tracer concentration was evaluated.

The study was carried out in the lab in different containers (0.5 x 0.5 x 0.3 m3). Each container was filled with a given soil. In the filling process, a 0.5-cm layer of a soil-magnetite mixture of a certain concentration was interspersed in the soil profile at a certain depth. Overall, 3 different soil:tracer concentrations (1000:1, 200:1, 100:1), 4 tracer burying depths (0 cm, 3 cm, 5 cm and 10 cm from soil surface), and  2 contrasting soils (silty clay and sandy clay loam) were used. In each case, the magnetic susceptibility was measured with a magnetometer (MS3 by Bartington Instruments). Experiments were repeated with different soil moisture contents (from field capacity to dry soil).

If the tracer is located under the soil surface a minimum soil:tracer concentration of 200:1 is required for its correct  detection from the surface using a magnetometer. The intensity of the magnetic signal decreases dramatically with the vertical distance  of the tracer from the soil  surface (burying depth). The maximum detection depth of the tracer magnetic signal is strongly dependent on the natural magnetic susceptibility of the soil which hides the own tracer signal. Variation in soil moisture content does not significantly affect the magnetic signal. For extensive field studies the soil-tracer volume to be handled would be very high. Therefore, it is necessary to explore new tracer application techniques.

How to cite: Zubieta, E., Larrasoaña, J., Giménez, R., Aldaz, A., and Casalí, J.: Evaluation of the magnetite as a magnetic tracer of eroded sediment from ephemeral gullies: conditioning factors of magnetic susceptibility, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2199, https://doi.org/10.5194/egusphere-egu2020-2199, 2020

D1967 |
EGU2020-13446
Romana Kubínová, Petr Kavka, Martin Neumann, and Tomáš Laburda

In this contribution the grain size distributions of the soil sediment obtained from soil erosion experiments were analysed. All the tests were done on arable topsoil’s, separately the size distribution of the soil aggregates and individual soil particles were evaluated. Soil erosion was initiated under the controlled conditions in the laboratories. The rainfall was artificially generated with use of a nozzle type rainfall simulator. The sediment transported due to the surface runoff and rill erosion was collected from the discharge of the inclined soil erosion plots (slopes 20 – 34°, slope length 4 m).

The soil sediment was collected in four sampling times. The first and the second were collected in fifteen and thirty minutes from the beginning of the simulation, then followed fifteen minutes long pause without raining and then the simulation continued and soil samples were collected again in fifteen and thirty minutes from the beginning of the rain. After ten days long pause whole process were repeated at the same experimental plot contains rills from previous simulation. Experimental plots were vertically divided into two parts. On one part was an eel and on the second part were different types of rolled erosion control products (RECPs) – Enkamat 7010, Biomac-C, coir fibres K700 and K400, jute, Macmat 8.1 with soil, mulch, hay and nonwoven. The influence of RECPs to the grain size distribution was investigated.

Laser diffraction has been selected as a method to determine grain size distribution and device Mastersizer 3000 was used. By the comparison of the grain size distribution, of more than five hundreds samples, the different response to the soil erosion mechanism and the influence of external factors (experimental plot slope, sampling time from the surface runoff and presence of RECPs) on the grain size distribution and soil aggregates content in eroded sediment were investigated. It has been found that both the particle size and aggregates size distribution of the eroded sediment changes considerably in time.

This research is funded by the TH02030428 - „Design of technical measures for slopes stabilization and soil erosion prevention”.

How to cite: Kubínová, R., Kavka, P., Neumann, M., and Laburda, T.: Particle and aggregates size distribution of soil transported due to surface runoff and rill erosion , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13446, https://doi.org/10.5194/egusphere-egu2020-13446, 2020

D1968 |
EGU2020-22509
Corina Lees, Robert Simmons, and Jane Rickson

Grassed waterways reduce water runoff, prevent scouring and encourage sediment deposition from erosion prone land. The aim of this study was to assess the efficacy of conventional and novel grass species (as monocultures and mixtures) to control erosion, at an early establishment stage (6 weeks), within grassed waterways. The experimental treatments included bare soil (B), a conventional mix of Lolium perenne and Festuca rubra (C), Festulolium cv prior (F1), Festulolium cv prior and Festulolium bx511 (F1+F2), and all grass species combined (F1+F2+C). F1 is adapted to flooded conditions, whilst F2 is adapted to drought conditions. With climate change in the UK likely to result in drier summers and wetter winters these Festulolium species will be adapted to future climatic conditions. However, little is known about their efficacy within grassed waterways. The grasses were established in 1.2 x 1 x 0.5m macrocosms in a sandy clay loam soil during June-Aug, 2019. A sub sample of each experimental treatment was taken (0.3 x 0.1 x 0.1m) from the macrocosms within a stainless steel box. Tests were replicated in quadruplicate.
The following above ground trait (Stem area density) and the following below ground traits (Total root length of fine roots <0.25mm, root diameter and root surface area) were determined for each experimental replicate. Prior to testing, the grass was cut to circa 3.0 cm height to represent a mowed grass sward before being placed into a fully instrumented hydraulic flume. The hydraulic flume simulated a concentrated flow event and treatment performance was assessed in terms of turbidity, sediment concentration, soil loss and flow velocity.
The effects of roots+shoots and of roots only on performance indicators were determined to quantify the relative contribution of above ground vs below ground traits in controlling erosion. One set of replicates was tested only with roots whilst another set of replicates was tested with roots+shoots and then with roots only. This was done to isolate the effect of below ground and above ground traits.
All replicates were subjected to a concentrated flow event with increasing incremental flow velocities from 0.2-0.6l s-1 for bare soil, 0.2-0.8l s-1 for roots+shoots treatments and 0.2-1.4l s-1 for roots only treatments. Each flow rate velocity was run for 60 seconds. For each flow rate, duplicate water samples were taken downslope of the treatment and water depth was measured, upstream of the treatment, in the centre of the treatment and downstream of the treatment.  The water samples were used to determine sediment concentrations. The water depth measurements were used to determine runoff velocity. Furthermore, a turbidity meter continuously measured turbidity during the concentrated flow event. Soil detachment and transport rates were significantly reduced for all experimental treatments as compared to the bare soil (p<0.05). Final treatment efficacy will be assessed based on a ranking of the key performance indicators. The knowledge gained from this research can be used and applied to other grassed soil erosion mitigation features such as in field and riparian buffer strips, swales as well as grassed waterways.

 

How to cite: Lees, C., Simmons, R., and Rickson, J.: Grass species selection to control for concentrated flow erosion in grassed waterways., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22509, https://doi.org/10.5194/egusphere-egu2020-22509, 2020

D1969 |
EGU2020-1497
Glenn Wilson, Anita Bernatek-Jakiel, John L. Nieber, and Garey A. Fox

Internal erosion of soil pipes can be a very important process in gully erosion as well as other mass failure events such as sinkholes, landslides and levee/dam breaching. Flow through preferential flow paths such as macropores can be rapid enough to exceed the soil critical shear stress and cause detachment of particles from the walls of the flow path, i.e. internal erosion. Development of a soil pipe from enlargement of a macropore results in more rapid flow and thus greater internal erosion, particularly mass failure of aggregates from pipe walls and roofs. If the sediment transport capacity of the pipe is exceeded, the pipe will plug causing back-pressure to build up within the soil pipe, which can foster hillslope instability. However, limited research has been conducted on particle and aggregate detachment within soil pipes as well as transport of sediment through soil pipes. The objective of this paper is to present observations of little known and poorly described processes involved in pipeflow and the resulting internal erosion of soil pipes. Many of the processes involved in internal erosion of soil pipes are assumed based upon processes observed in surface and stream erosion studies but are so poorly quantified for soil pipes that they are yet to be transferable. For example, the role of solution chemistry on sediment detachment from pipe walls has been quantified to a limited extent but little has been done on the effects of seepage forces on particle detachment in pipes and even less done on sediment transport capacity of soil pipes. Recent advances have included: development of suspended sediment and bedload rating curves for soil pipeflows but the results are crude and warrant further study. Quantification of the interactive effects of surface flow in channels with flow through soil pipes below channels on headcut migration and gully widening is in its infancy. Other processes, such as air-entrapment in creating or temporarily plugging pipes, have been suggested as important but lack quantification. These processes and others combine to result in internal erosion of soil pipes but they must be better understood and quantified in order to develop the next generation of soil erosion models and landscape morphology prediction technologies.

How to cite: Wilson, G., Bernatek-Jakiel, A., Nieber, J. L., and Fox, G. A.: Internal Erosion of Soil Pipes: Still More Questions Than Answers, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1497, https://doi.org/10.5194/egusphere-egu2020-1497, 2019

D1970 |
EGU2020-665
Taco Regensburg, Joseph Holden, Pippa Chapman, Michael Pilkington, and Martin Evans

As part of the EU-funded MoorLIFE2020 project, which examines strategies to restore degraded blanket bog in the Peak District of northern England, we investigated natural soil pipes. These pipes are a cause of concern to peatland restoration practitioners who are unsure whether to block them to reduce erosion and flood risk when conducting restoration work. Soil pipes often occur in complex networks with varying channel sizes, undulating through the soil profile. Their prevalence is often linked to controls such as topographic location, slope, aspect, vegetation cover, climate, and properties of the surrounding soil. Such relationships are poorly understood for degraded blanket bog. A before-after-control treatment study was designed to examine the effects of pipe blocking on fluvial carbon removal and streamflow in Upper North Grain (UNG), a small headwater catchment located between 490 m and 541 m above sea level. The catchment has a blanket peat cover up to four meters thick at places, with a branching network of deep gullies that incise into the bedrock. This experimental design was envisaged to address the following hypotheses: (i) the severity of degradation of UNG is a dominant control on pipe density; (ii) blocking of pipe outlets impairs pipe-to-stream connectivity. Our results point towards a rejection of both hypotheses. An initial field survey used to locate and characterize pipe outlets, resulted in 353 individual outlet recordings with a density of 13.79 per km of surveyed gully bank. Southeast, south, southwest and west-facing gully banks accounted for more than 75% of identified pipe outlets. The experimental design compares water and aquatic carbon fluxes in two streams - in one catchment the active pipe outlets (n=25) were blocked by closing off the void behind the pipe outlet with peat and stones, wooden screens or plastic pilling, while in the other catchment the pipes were left open. Areas on the gully bank around original outlets were photographed every two weeks. This analysis showed that within the first month after blocking, all treated pipes had formed bypass routes around the block and initiated new pipe outlets. New outlets were found both above and below the original pipe outlet at distances up to 1 meter from the original pipe outlet regardless of bank aspect, suggesting the networks behind a pipe outlet to be a porous system that connects in both vertical and horizontal directions when issuing onto gully banks. Further results will be presented from the ongoing monitoring showing effects of pipe blocking on streamflow storm responses and the export of particulate and dissolved organic carbon from pipes and streams.

How to cite: Regensburg, T., Holden, J., Chapman, P., Pilkington, M., and Evans, M.: New insights on controls of piping distribution in degraded blanket bog, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-665, https://doi.org/10.5194/egusphere-egu2020-665, 2019

D1971 |
EGU2020-2017
Felipe Jesus, Wellington Silva, Jorge Porsani, and Marcelo Stangari

A Soil Pipe System (SPS) were identified in Candói region, located in the Paraná state, southern Brazil. This region is constituted of intensive agriculture and cattle raising. SPS correspond to a structure associated to an erosive processes stage in downhill slope that tend to increase over time. The growth of the SPS results in instability to the terrain and the possibility of collapse, in this case the collapse can be accelerated by external factors, such as the overload of agricultural machinery and animals that circulate around the site, may leading to the machinery loss, animal’s death or even risk to worker safety. Frequently, the SPS are identified by surface methods that don’t provide parameters such as shape, distribution and depth. In this research, Electrical Resistivity Tomography (ERT) and Ground Penetrating Radar (GPR) were used to obtain a 3D characterization of the SPS identified in a farm in Candói region to estimate the soil cover over the structure, the subsurface channels distribution and identify potential collapse risk portions. Seven ERT profiles using dipole-dipole array and twenty-one GPR profiles of 200 MHz antenna were acquired, covering an area of 900 m². The results were combined in a block diagram, which enabled: i) identify the subsurface channels distribution and direction, ii) estimate the average soil cover thickness, with 1.5 m over the whole structure. The possible connection between subsurface secondary and primary channels has also been suggested in results interpretation through of identification of a channel parallel to acquisition profiles direction. It was verified that in structure portions closer to the river next to the slope, the SPS ceiling has larger dimensions than the walls, suggesting areas with increased vertical tension, which was classified as potential collapse risk areas.

How to cite: Jesus, F., Silva, W., Porsani, J., and Stangari, M.: ERT and GPR Characterization from Soil Pipe System in Brazilian Tropical Soil, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2017, https://doi.org/10.5194/egusphere-egu2020-2017, 2020

D1972 |
EGU2020-7410
Juan Pablo Aguilar-Lopez, Manuel Wewer, Thom Bogaard, and Matthijs Kok

Backward piping erosion (BEP) is a highly complex erosive process which occurs on granular soils when large head differences are exerted. This process represents a significant threat to dams and levees stability and therefore a large part of the design and reliability assessment of these water retaining structures is devoted to this single process. Several authors have achieved great accuracy in predicting the critical head difference that triggers the process but not so much has been studied regarding the time of occurrence and the duration of the erosive process.  In the present study we propose a 2D finite element model for which not only the critical head difference can be predicted but also the development of the erosive process in time. This was achieved by coupling the 2D Darcy partial differential equation with Exner’s 1D sediment transport mass conservation equation. Different laminar sediment transport rate empirical models were tested and used as inputs in the coupled model. To test the performance of the proposed model, the IJkdijk real scale experiment for piping erosion was simulated. The results show that the model is capable of predicting not only the critical head and its progression in time but also specific events of the process such as the instants of start of the erosion and the  complete seepage length development . An important conclusion of the study is that from several transport empirical formulas tested, the model from Yalin which is widely recognized by the sediment transport community performs the best.

How to cite: Aguilar-Lopez, J. P., Wewer, M., Bogaard, T., and Kok, M.: Time dependent backward piping erosion 2D modeling with laminar flow transport equations , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7410, https://doi.org/10.5194/egusphere-egu2020-7410, 2020

D1973 |
EGU2020-7465
Juan Pablo Aguilar-López, Andres Garcia-Ruiz, Thom Bogaard, and Miguel Gonzalez-Herraez

Backward piping erosion (BEP) is considered the most dangerous failure mode for levees due to its unpredictable nature. This erosive process happens most of the time underneath the impermeable layers on which levees are commonly founded. This makes it very difficult to detect as conventional geophysical methods are either too expensive or too imprecise for real time monitoring of longitudinal soil made structures such as Dams or levees. Fiber optic based distributed acoustic sensing (DAS) is an innovative technology which allows to retrieve information from an acoustic propagating medium in a spatially dense manner by using a fiber optic cable. The present study aimed to explore the potential of DAS for early detection of BEP  under levees based on the frictional emissions of the sand grains during the erosive process. The tests were performed in the lab under controlled ambient noise conditions. The technology was tested by embedding fiber optic based microphones underneath and outside a laboratory scaled aquifer set up capable of recreating BEP. The results show that indeed the process emits certain characteristic frequencies which may be located between 1200 to 1600 Hz and and that they can easily be captured by the fiber optic cables.

How to cite: Aguilar-López, J. P., Garcia-Ruiz, A., Bogaard, T., and Gonzalez-Herraez, M.: Backward piping erosion detection in levees by fiber optic acoustic sensing, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7465, https://doi.org/10.5194/egusphere-egu2020-7465, 2020

D1974 |
EGU2020-10628
Jan Vojtisek, Jiri Bruthans, and Martin Slavik

Piping is an erosion process in which cracks and macropores extend into channels with a diameter of cm or more. This process is important for the formation of highly permeable porosity, failure of the levees, formation of gullies and intense erosion of agricultural soil. In this research we studied the evolution of conduits in Střeleč locked sand (SLS). This material composes mainly of quartz and resembles friable sandstone. Study was done in Střeleč quarry (Czech Republic), where depression in the regional water table (decrease of water table by ~20 m) due to the water pumping causes fast flow (up to 40 cm/s) through fractures in the SLS body. Large conduit systems developed along fracture zones that divide the SLS body into subvertical blocks with a width of cm to tens of cm in each fracture zone. Erosion starts at water table and blocks bordered by fractures are eroded by the fast water flow, especially the parts that are in stress shadows due to inefficient loading from the surrounding sandstone mass. Blocks whose base is eroded tend to collapse, which leads to the creation of free space above the water table and also possibly destabilization of the sides. Empty space propagates upward mostly meters but sometimes tens of meters toward ground surface. Experiments showed that the SLS is prone to erosion when it is under low gravity induced stress. In addition to observation of the existing conduits, the experiments focused on the evolution of transversal section of conduits in SLS were performed. Experimental erosion was done on fracture systems exposed in quarry by the flow of water from the hose. Sequence of photos of fracture zones evolving into conduit during experiments was taken and the evolution of the transverse section of conduits was observed. By this method the blocks were eroded to a depth of several decimeters. Based on field experiments and time-laps photos two erosion mechanisms are responsible for conduits evolution. While the less thick blocks are eroded mainly by rapid water flow, thicker blocks are eroded by tension failures (gravity driven wasting). The tension failure dominates, forming about 65 % of total erosion.

Many thanks to the management of Střeleč Quarry for enabling of the field documentation and experiments. The research was supported by Charles University Grant Agency (GAUK #1292119).

How to cite: Vojtisek, J., Bruthans, J., and Slavik, M.: Combination of stress-controlled erosion and tensile failures during development of piping conduits in locked sand, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10628, https://doi.org/10.5194/egusphere-egu2020-10628, 2020

D1975 |
EGU2020-20163
Miguel Cooper, Renata Cristina Bovi, Cesar Augusto Moreira, Raquel Stucchi Boschi, Lucas Moreira Furlan, and Fernanda Teles Gomes Rosa

Piping is a type of subsurface erosion caused by subsurface water and is considered one of the most difficult erosive processes to study. The nature of this erosion process makes it very difficult to study and quantify.  The aim of this study was to characterize the surface and subsurface distribution of the pipes and to understand the network architecture of pipe systems in tropical forested areas.The study area is situated at the Experimental Station of Tupi, state of São Paulo, Brazil. We conducted a Digital Elevation Model allied to a superficial pipe mapping, and 2D and 3D geophysical surveys. The subsurface erosion identified by surface mapping and geophysical surveys appeared at two depths: one more superficial, in the upper part of the study area, and one at greater depth, in the lower part of the study area. The higher topographical positions presented the pipes at less developed stages (closed depressions and simple sinkholes), while the lower topographical positions showed the most advanced features (multiple sinkholes and blind gullies). The method of electroresistivity showed zones where low resistivity values correspond to water saturation (~ 70 omh m) and high values (> 4040 ohm m) that define the pipe; this method was efficient in detecting the presence of collapsed and non-collapsed pipes. We concluded that the use of different methods (superficial and subsuperficial) was essential for the characterization of pipe systems. The integrated analysis of the results obtained from the superficial and 2D subsurface mapping allowed us to infer the spatial continuity of the pipes. The 3D geophysical survey was efficient in mapping soil pipe and the connectivity in situ. The 3D modeling of the pipes revealed the connection and connectivity of the pipe network’s complexity and morphology.

How to cite: Cooper, M., Bovi, R. C., Moreira, C. A., Stucchi Boschi, R., Moreira Furlan, L., and Teles Gomes Rosa, F.: Spatial distribution and geophysical characterization of natural pipes in Ultisols, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20163, https://doi.org/10.5194/egusphere-egu2020-20163, 2020

D1976 |
EGU2020-1875
Ximeng Xu, Glenn V. Wilson, Fenli Zheng, and Qiuhong Tang

Headcut formation and migration is sometimes mistaken as the result of overland flow without realizing that the headcut was formed by or significantly influenced by flow through soil pipes into the headcut. To determine the effects of a soil pipe and flow through a soil pipe on headcut migration, laboratory experiments were conducted under free-drainage conditions and conditions of a shallow water table. Soil beds with a 3-cm deep initial headcut were formed in a flume with a 1.5-cm diameter soil pipe 15 cm below the bed surface. Overland flow and flow into the soil pipe was applied at a constant rate of 68 L/min and 1 L/min, respectively, at the upper end of the flume. The headcut migration rate and sediment concentrations in both surface (channel) and subsurface (soil pipe) flows were measured with time. The typical response without a soil pipe was the formation of a headcut that extended in depth until an equilibrium scour hole was established at which time the headcut migrated upslope. The presence of a soil pipe below the channel, and particularly the phenomena of flow through a soil pipe and into the headcut, whether by seepage from a shallow water table or upslope inflow, significantly impacted the headcut migration. Pipeflow caused erosion inside of the soil pipe at the same time that runoff was causing a scour hole to deepen and migrate. When the headcut extended to the depth of the soil pipe, surface runoff entering the scour hole interacted with flow from the soil pipe also entering the scour hole. This interaction dramatically altered the headcut processes, greatly accelerated the headcut migration rates and sediment concentrations. Conditions in which a perched water table provided seepage into the soil pipe in addition to pipeflow increased the sediment concentration by 42% and the headcut migration rate by 47% compared with pipeflow under free-drainage conditions. The time that overland flow converged with subsurface flow was advanced under seepage conditions by 2.3 and 5.0 minutes compared with free-drainage condition. This study confirmed that pipeflow dramatically accelerates headcut migration especially under conditions of shallow perched water tables and highlights the importance of understanding these processes in headcut migration processes.

How to cite: Xu, X., Wilson, G. V., Zheng, F., and Tang, Q.: The Role of Soil Pipes and Pipeflow in Headcut Migration Processes, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1875, https://doi.org/10.5194/egusphere-egu2020-1875, 2019