Gully erosion is an important sediment source and causes severe environmental degradation, particularly in the drought-prone regions of Ethiopia. We investigated the morphological characteristics of ...gullies, the topographic thresholds of gully formation, and estimated headcut retreat rates in three agro-ecologies of the Upper Blue Nile basin, Ethiopia: highland (Guder), midland (Aba Gerima), and lowland (Dibatie). Gully morphological characteristics were analyzed using data measured in the field, whereas the topographic thresholds were estimated using slope and upslope drainage area. Average annual linear (Rl) and volumetric (Ve) headcut retreat rates were estimated by visual interpretation of very high resolution (0.5–1.5 m) satellite images (QuickBird, IKONOS, SPOT-7, and Pleiades) in a GIS environment. A significant power relationship, fitted between gully volume (V) and length (L) for the three sites, is V = 8.097 L1.032 (R2 = 0.902, n = 94). The average annual Rl, Ve, and soil loss rates were estimated at 0.76 m yr−1, 6.77 m3 yr−1, and 8.73 t ha−1 yr−1 in Guder, 2.09 m yr−1, 19.58 m3 yr−1, and 20.76 t ha−1 yr−1 in Aba Gerima, and 3.42 m yr−1, 42.16 m3 yr−1, and 49.33 t ha−1 yr−1 in Dibatie. The higher gully headcut retreat rate in Dibatie is mainly related to the highly erodible nature of Vertisols. The coefficients of the slope (S)-drainage area (A) relationship S = aA−b were a = 0.219 and b = −0.139 in Guder, a = 0.133 and b = −0.234 in Aba Gerima, and a = 0.113 and b = −0.216 in Dibatie, indicating that topographic thresholds for gully initiation varied among the agro-ecologies. The results of this study can be used to estimate gully erosion rates and identify areas for gully initiation, thereby supporting the planning of appropriate gully control measures in the study sites and other areas with similar environmental settings.
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•Gully erosion is highly dynamic erosion process in northwest Ethiopia.•We characterized 94 gullies for 10 morphological parameters in three environments.•Gully length is the most dynamic parameter in all agro-ecologies.•A higher gully initiation threshold was found in the highland agro-ecology.•High gully head retreat rate was observed in the Vertisol-dominated lowland site.
Yeon, Y.J. and Lee, J.L., 2021. Cost comparison between hard and soft approaches adapted as preventive methods of beach erosion. In: Lee, J.L.; Suh, K.-S.; Lee, B.; Shin, S., and Lee, J. (eds.), ...Crisis and Integrated Management for Coastal and Marine Safety. Journal of Coastal Research, Special Issue No. 114, pp. 519–523. Coconut Creek (Florida), ISSN 0749-0208. To protect the properties behind a beach, submerged breakwater, which is a representative hard approach, or beach nourishment, which is a representative soft approach, has been performed. The submerged breakwater protects the facilities behind the beach by controlling waves to mitigate erosion. This study estimates the cost required to secure a buffer section for the wave inflow of 30-year return period using submerged breakwater and beach nourishment methods for Sokcho Beach (Sokcho-si, Gangwon-do), one of the coastal maintenance project areas, and conducts economic assessment of the two methods. For the submerged breakwater method, the construction cost was calculated using the shoreline observation data, which were observed four times a year for about 8 years. For the beach nourishment method, the construction cost was calculated using the sand loss half-life (shoreline retreat over time) considering that the nourished sands cannot be maintained. The result showed that beach nourishment could obtain a higher economic feasibility. By enhancing sand maintenance, beach nourishment was evaluated as more economical than the hard approach when the sand loss rate (k), which determines the half-life of beach nourishment, was lower than 0.167 yr–1. Furthermore, beach nourishment is highly evaluated because it is an eco-friendly construction method, and it will be more effective when combined with other soft approaches.
Riverbank migration has historically been seen as a risk to infrastructure that can be combated through channelization, bank stabilization, and sediment trapping. The physical processes involved with ...riverbank erosion and deposition are well defined, yet the solutions to equations that describe these processes are computationally and data intensive over large domains. While current understanding of large‐scale river channel mobility largely comes from reach‐ and watershed‐scale observations, we need global observations of riverbank erosion and accretion to broaden our understanding of sediment processes within and across river basins. In this work, we create the first global data set of riverbank erosion for >370,000 km of large rivers using up to 20 years of water classifications from Landsat imagery. We estimate uncertainty by propagating water classification errors through our methods. Globally, we find riverbank erosion for rivers wider than 150 m to have an approximately log‐normal distribution with a median value of 1.52 m/yr. Comparing our data set to 25 similar estimates of riverbank migration, we found a normalized mean absolute error of 42% and a bias of 5.8%. We show that river width is the best first‐order predictor of riverbank erosion, in agreement with existing literature. We also show that the relationship between width and bank erosion is substantially different among a sample of global river basins and suggest that this is due to second‐order influences of geology, hydrology, and human influence. These data will help improve models of sediment transport, support models of bank erosion, and improve our understanding of human modification of rivers.
Plain Language Summary
Riverbank erosion presents a serious risk to people and infrastructure. These risks are becoming increasingly difficult to predict because of direct modification of the rivers by damming and bank stabilization, as well as indirect modification by climate change and land use change. Geomorphologists have developed scaling relationships based on watershed characteristics; however, these relationships are very coarse and limited by the available data. This research uses 20 years of satellite imagery to develop the first global data set of riverbank erosion, which both confirm existing knowledge and opens new avenues of research.
Key Points
We developed the first global data set of riverbank erosion and accretion for rivers wider than 150 m
We confirm that regional first‐order relationships between river width and riverbank erosion apply globally
We show large interbasin differences in riverbank erosion that arise from the unique geography of each river basin
The erosion of engineered slope surfaces has become an urgent problem in loess regions in China, which has a serious impact on the safety and stability of the slope projects. In this paper, to study ...the effect of the confining pressure, dry density of the slope surface, and water flow velocity during wetting procedure on the erosion characteristics of compacted loess, we developed a laterally confined uniaxial compression-erosion apparatus, which was characterized by the fact that it can change the confining pressure of the being eroded soil sample to analyze the loess erosion rates and particle size distributions of the eroded sediments. The fractal dimensions of the eroded sediments were calculated by fractal theory and their variations were analyzed. Results show that the erosion rate is approximately linearly related to the confining pressure. The higher pressure has a tendency to move towards the free top of soil sample, the erosion rate increases. Compared with the water flow velocity, the higher confining pressure plays a greater role in the erosion rate, the influence of water flow velocity on the erosion rate decreases. With the increase of dry density, the erosion rate decreases, the higher dry density enhances the ability of soil sample to resist water erosion. The confining pressure changes the particle size distribution of the eroded sediments of loess samples, the median diameter
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increases. However, the fractal dimension decreases with the increase in confining pressure. The confining pressure of slope surface has a negative effect on the loess anti-erosion characteristics. The results in this paper would provide detailed information on the mechanisms of loess erosion, which could contribute toward prevention of surface erosion of high fill slopes in loess regions.
The native property of weakly conductive Boron-doped polycrystalline diamond (B-doped PCD) as a high-melting point, resistant, super-hard material makes it particularly difficult to machine. This ...paper focuses on the relative merits of eroding B-doped PCD by spark-erosion based on different power sources: commercial transistorized, Resistance-Capacitance (R-C), and designed high-frequency microspark-erosion power. Experimental results show B-doped PCD endures spark-erosion longer under commercial transistorized power although its Material-Erosion-Rate (MER) is higher. This greatly facilitates surrounding air scurries into the PCD matrix during melting and cooling. A poorer material-erosion-rate presents under R-C power due to its lower duty cycle. In contrast, dense eroded microcraters realizing a solid and regular distribution on the PCD matrix occur under high-frequency microspark-erosion power. The erosion-energy beam is supplied by a current train of high-frequency, high-peak and short-pulse-time, resulting relatively more diamond being vaporized than melted. The amount of eroded diamond is so little that debris is exceedingly slight and swiftly cleared away between each pulse-on-time. The extensive solid erosion craters resulting from the process are very useful as chip-pockets on the PCD wheel-tool for disposal of ground chips during microgrinding. Additionally, aspects relating to the merits of B-doped PCD are evaluated in detail: spark-erosion-ability (SEA) of B-doped PCD, surface roughness on B-doped PCD, depositional amounts of cobalt, and graphitization of diamond.
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•The two metrics in evaluating an appropriate power for machining B-doped PCD are ‘pulse on-time’ and ‘work frequency’.•Microspark-erosion creates a current train of high-frequency, ensuring PCD microtools have highly neat and sharp edges.•Spark-erosion-ability of 0.01805 mm3/Coulomb implys a unit coulomb of electric discharge erodes very little material.•Cobalt could have been dissolved into diamond lattice ensuring the depositional amounts of cobalt are very low.
Land degradation by erosion is especially important in drylands, which are among the most vulnerable to disturbance by human activity or climate change. Biocrusts are an essential surface component ...of these ecosystems and one of the most important contributors to surface resistance and stability, and therefore, keeping soil fertile in these nutrient-limited-environments. Loss of biocrusts can result in increased sediment losses and subsequent loss of soil organic carbon (SOC) and other soil resources, which are vital for vegetation and general ecosystem functioning. Despite their importance, the consequences of biocrust loss on sediment and SOC losses in drylands have been poorly analysed. In this study, we examined the influence of two biocrust types (cyanobacteria and lichen-dominated biocrusts) and biocrust removal on runoff and sediment yield, from natural rainfall during one hydrological year in a semiarid badlands catchment (Tabernas, SE Spain). The influence of biocrust type and biocrust removal on SOC losses from water erosion (dissolved and sediment organic carbon losses, DOC and SdOC, respectively) was also analysed. Our results show that sediment yield significantly increased after biocrust removal, especially during the first rain after biocrust removal, when particles were left directly exposed to raindrop impact and easily washed away by runoff. Annual sediment yield was 465, 75 and 24gm−2 in biocrust-removed, cyanobacteria-covered and lichen-covered soil, respectively, and the first event represented 87% of annual sediment losses on biocrust-removed plots. Biocrust removal was accompanied by a significant increase in both DOC and SdOC mobilisation. Total organic carbon (TOC) mobilisation was the highest in soils where the biocrust had been removed and decreased as the biocrust was more developed. Annual TOC mobilisation was 10.2, 3.0 and 1.4gm−2 in biocrust-removed, cyanobacteria-covered and lichen-covered soil, respectively. TOC mobilisation was mainly driven by sediments and thus, 89% of annual TOC mobilisation occurred during the first rain after biocrust removal. The high sediment and SOC losses recorded after biocrust removal emphasize the importance of biocrust conservation for avoiding the loss of soil resources and maintaining fertility in interplant soils in drylands.
•Biocrusts strongly protect soil from water erosion and reduce TOC losses.•Well-developed biocrusts are more effective in decreasing sediment yield and TOC loss.•Sediment yield increased up to 20 times after biocrust removal.•89% of annual TOC mobilization occurred during the first rain after biocrust removal.•Rainfall properties greatly influence sediment and TOC losses in biocrusted soils.
Glacial cirques are armchair-shaped erosional hollows, typified by steep headwalls and, often, overdeepened floors. They reflect former regions of glacier initiation, and their distribution is, ...therefore, linked to palaeoclimate. Because of this association, cirques can be analysed for the information they provide about past environments, an approach that has a strong heritage, and has seen resurgence over recent years. This paper provides a critical assessment of what cirques can tell us about past environments, and considers their reliability as palaeoenvironmental proxies. Specific focus is placed on information that can be obtained from consideration of cirque distribution, aspect, altitude, and morphometry. The paper highlights the fact that cirques potentially provide information about the style, duration and intensity of former glaciation, as well as information about past temperatures, precipitation gradients, cloud-cover and wind directions. In all, cirques are considered a valuable source of palaeoenvironmental information (if used judiciously), particularly as they are ubiquitous within formerly glaciated mountain ranges globally, thus making regional or even global scale studies possible. Furthermore, cirques often occupy remote and inaccessible regions where other palaeoenvironmental proxies may be limited or lacking.
The benefits of conservation tillage on water erosion control have been extensively tested in China, while few studies have involved their regional differences. We synthesized 81 recent publications ...about the water erosion control by conservation tillage in China, to explore the regional difference of conservation tillage and identify the optimal tillage methods of each region. The benefits of conservation tillage for runoff retention and sediment reduction are different in different regions due to the special environmental condition of each region. Conservation tillage demonstrated the lowest efficiency in the Northwest China Loess Plateau region and highest in the North China mountainous region, while no significant differences were found among other 4 regions. In the Northeast China black soil region, micro-basins tillage (MBT) is the optimal tillage method due to its greatest benefits in reducing runoff and sediment. In the North China mountainous region, the values of regional efficiency evaluation index (REEI) of runoff and sediment were both larger than 1 only by contour tillage with hedgerow (CTH), thus CTH will be the optimal tillage method in this region. In the Northwest China Loess Plateau region, some engineering measures should be the priority due to the limited benefits of conservation tillage. In the South China red soil region, no tillage with mulch (NTM) presented the greatest benefits on water erosion control and largest values of runoff and sediment REEI, thus CTH is the priority in this region. In the Southwest China purple soil region, though the values of runoff and sediment REEI by collecting soil to form ridges with no tillage (CSNT) are less than those by MBT, CSNT is the most suitable tillage method in this region. The small sample size leads to the unreliable results in the Southwest China karst region, thus more researches are needed in this region.
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•Conservation tillage showed least efficiency in Northwest Loess Plateau region and highest in North China mountainous region•No significant differences were found in other 4 regions•The most suitable tillage methods are different in different regions•More studies are need in the Southwest China karst region
Most field erosion studies in agricultural areas provide little information on the probable errors involved. Here, for the first time, we compare the accuracy, time and cost of conventional and new ...methodologies for gully surveying, and provide a model to estimate the effort required to achieve a specified accuracy. Using a terrestrial LiDAR survey of a 7.1‐m‐long gully reach as a benchmark data set, the accuracies of different measurement methods (a new 3D photo‐reconstruction technique, total station, laser profilemeter, and pole) are assessed for estimating gully erosion at a reach scale. Based on further field measurements performed over nine gullies (>100 m long), a simulation approach is derived to model the expected volume errors when 2D methods are used at the gully scale. All gullies considered were located near Cordoba, Spain. At the reach scale, the field measurements using 3D photo‐reconstruction and total station techniques produced cross‐sectional area error values smaller than 4%, with other 2D methods exceeding 10%. For volume estimation, photo‐reconstruction proved similar to LiDAR data, but 2D methods generated large negative volume error (EV) values (<–13% for laser profilemeter and pole). We show that the proposed error expressions derived from the model are in line with the reach‐scale field results. A measurement distance factor (MDF) is defined that represents the ratio between cross‐section distance and the gully length, and thus reflects relative survey effort. We calculate the required MDF for specified values of EV, illustrating how MDF decreases with increasing gully length and sinuosity.
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