Sheet Erosion Research Articles

Effects of polysaccharides on the hydrodynamic parameters of sheet erosion on loessial slopes.

The variations in hydrodynamic parameters at different polysaccharides rates and the relationships between sheet erosion modulus and hydrodynamic parameters were analyzed to reveal the hydrodynamic mechanism of sheet erosion on loessial slopes. Artificially simulated rainfall experiments were carried out under three slope gradients (10°, 15°, and 20°), three rainfall intensities (1.0, 1.5, and 2.0 mm·min-1), and four dry-spreading rates of polysaccharides (0, 1, 3, and 5 g·m-2). The results showed that (1) four hydrodynamic parameters (flow velocity, shear stress, stream power, and unit stream power) all increased with both rainfall intensities and slope gradients at four rates of polysaccharides. (2) Polysaccharides could effectively reduce hydrodynamic parameters. In contrast to the bare slope, the average flow velocity, shear stress, stream power, and unit stream power diminished by 27.11~41.18%, 9.53~18.67%, 31.82~50.24%, and 27.11~41.18%, respectively. (3) Polysaccharides could effectively reduce the growth rate of the sheet erosion modulus with hydrodynamic parameters, and there were few differences among the different rates (1, 3, and 5 g·m-2). The increasing rates of the sheet erosion modulus with flow velocity, shear stress, stream power, and unit stream power were 14.0~65.7%, 14.8~33.9%, 7.8~23.7%, and 9.7~29.5%, respectively. (4) At different polysaccharides rates, the relationships between sheet erosion modulus and hydrodynamic parameters were all in logarithmic functions. Moreover, flow velocity (R2 ≥ 0.920) and stream power (R2 ≥ 0.876) were better hydrodynamic parameters than shear stress (R2 ≥ 0.598) or unit stream power (R2 ≥ 0.537). Polysaccharides decreased the hydrodynamic parameters and the response rates of sheet erosion to hydrodynamics.

Pliocene-Pleistocene ash-fall tuff deposits in the intermountain Humahuaca and Casa Grande basins, northwestern Argentina: tracers in chronostratigraphic reconstructions and key to identify their volcanic sources

Ash-fall tuffs of the Pliocene-Pleistocene deposits of Humahuaca and Casa Grande intermountain basins, northwestern Argentina, have been differentiated into two groups based on new geochemical and geochronological data which correspond to the tuffs of the Pliocene-Lower Pleistocene alluvial fan deposits dominated by debris flow, deep sandy gravel braided, and shallow ephemeral lake deposits (Uquia and Mal Paso formations), and those recorded in Pleistocene alluvial fans sheet flood deposits. The two clusters of ages recognized: 4.3 to 2.6 Ma, and 2.2-pre 0.8 Ma, corresponding to these tuff groups, are in agreement with pulses of ignimbrite eruptions in the Altiplano Puna Volcanic Complex (APVC), and Southerm Puna calderas located west of the study region. The ash-fall tuffs of both groups are mainly vitreous to phenocryst-poor of rhyodacite-dacite composition with minor andesites to trachyandesites, characterized by 58-69% SiO2 contents, A/CNK 1-1.4, FeO/MgO (0.8-2.8), which plot in the calc-alkaline range. They can be differentiated based on its immobile trace elements ratios as indicated by a slight enrichment in LREE, higher arc affiliation and somewhat higher Sm/Yb ratios in the case of the younger group, although in both Sm/Yb ratios are indicative of sequestration of HREE in residual hornblende. The new geochemical and geochronological data of those ash fall tuffs point to these as chrono-stratigraphic tracers of the Humahuaca and Casa Grande intermountain basins stratigraphy, during the Pliocene-Pleistocene, also as the key to identify their volcanic sources.

Optimizing the Dryland Sheet Erosion equation in South China

Optimisation of models applied in sheet erosion equations could facilitate effective management of sheet erosion in the field, and sustainable agricultural production. To optimise the characterisation of sheet erosion on slope farmland in South China, the present study conducted field simulation rainfall experiments with vegetated and fallow soils. According to the results, sheet erosion rate first increased with an increase in rainfall duration and then stabilised. Exclusive P. vulgaris planting and P. vulgaris in combination with earthworms could reduce sheet erosion by 10–60%, and the combined method could better control sheet erosion. There were significant differences in erosion rate between mild and steep slopes, and light and heavy rain conditions. The influence of rain intensity on sheet erosion was greater than that of slope. Soil organic matter (SOM), rain intensity, and slope can be used to optimise sheet erosion equations of exposed slopes, and SOM and hydraulic parameters can be used to optimise sheet erosion equations in vegetated slopes. The results of the present study could facilitate the reduction of the time and space variability errors in the establishment of sheet erosion models for vegetated slopes.

Linking rill development characteristics to sediment production on different coarse-textured granite topsoils

The sheet and rill erosion phases dominate the erosion process of the catchment area and up-slope of Benggang (a special gully) area, and its evolution has a vital impact on erosion characteristics in the erosion feedback loop. Here, the rill development processes, morphological characteristics and corresponding soil loss were investigated by laser scanning on the four coarse-textured granite topsoils defined as WH, TC, GX, and CT for the counties of Wuhua, Tongcheng, Ganxian, and Changting in China, respectively. A soil flume (3 × 0.8 m) at a slope gradient of 16.7% was treated with rainfall-inflow at the upslope inflow rate of 2 L min−1 and rainfall intensity of 90 mm h−1. Results showed that soil erosion rate increased with erosion evolution (sheet erosion, rill advance, and rill maturity phases). A comparison of the four different soils exhibited that the soil erosion rate showed a downtrend with the increase of soil clay and gravel contents. Specifically, the CT soil with high clay and gravel content showed the minimal soil erosion rate (0.024 kg m−2 min−1), rill density (0.95 m m−2), headward erosion rate (0.41 cm min−1), and surface evolution rate. The maximal soil erosion rate and rill depth were observed on the WH slope, with the rill headward erosion rate reaching the maximum of 20.59 cm min−1. Despite no significant difference in the geomorphologic comentropy of the four soils, their variation trend and rate could reflect dynamic changes in erosion. The total sediment transport efficiency was improved by rill formation and development, with rill erosion contributing to about 70% of total soil loss for the four soils. This study can enrich our understanding of the rill erosion process and surface morphology of coarse-textured topsoils.

Las2DoD: Change Detection Based on Digital Elevation Models Derived from Dense Point Clouds with Spatially Varied Uncertainty

The advances of remote sensing techniques allow for the generation of dense point clouds to detect detailed surface changes up to centimeter/millimeter levels. However, there is still a need for an easy method to derive such surface changes based on digital elevation models generated from dense point clouds while taking into consideration spatial varied uncertainty. We present a straightforward method, Las2DoD, to quantify surface change directly from point clouds with spatially varied uncertainty. This method uses a cell-based Welch’s t-test to determine whether each cell of a surface experienced a significant elevation change based on the points measured within the cell. Las2DoD is coded in Python with a simple graphic user interface. It was applied in a case study to quantify hillslope erosion on two plots: one dominated by rill erosion, and the other by sheet erosion, in southeastern United States. The results from the rilled plot indicate that Las2DoD can estimate 90% of the total measured sediment, in comparison to 58% and 70% from two other commonly used methods. The Las2DOD-derived result is less accurate (65%) but still outperforms the other two methods (30% and 48%) for the plot dominated by sheet erosion. Las2DoD captures more low-magnitude changes and is particularly useful where surface changes are small but contribute significantly to the total surface change when summed.

History of slope instability in the Oldina plantation, Tasmania

Background: This paper describes a landslide swarm generated by exceptionally high two-day rainfall (c. 300 mm) associated with a stationary cut-off low pressure system over northern Tasmania in early June 2016 and investigates evidence for previous slope instability. The landslide swarm occurred in a recently harvested plantation in the Inglis River catchment at Oldina, south of Wynyard in north-west Tasmania. Within a relatively small area of plantation underlain by weathered Permian tillite and minor siltstones more than thirty rapid earthflows, rotational and translational landslides occurred. Many landslides also occurred in the nearby Forth and Mersey River catchments. Methods: Field observations combined with a digital elevation model produced from high-resolution drone imagery were used to describe the morphology of the Oldina landslides, and to calculate the mass of soil, sediment, and woody debris displaced. Radiocarbon dating of charcoal exposed in landslide backwalls enabled palaeo-landslides and periglacial activity at Oldina to be dated. Results: An estimated 48,400–72,310 t of soil, sediment and woody debris was carried downslope by the major landslides but has been retained within the plantation area. The total sediment loss from the affected upper catchments is likely to be greater than the above estimate as the contribution from small riparian landslides, sheet, gully, and rill erosion has not been accounted for, nor has streambank erosion and sediment transported off the study site been measured. Radiocarbon dating of charcoal in sediments indicated that two landslides had evidence of previous instability in the Holocene. Most ages indicated that previous instability dated to 35–15 cal ka BP, i.e., to a time when the climate was cold and dry and freeze-thaw processes in a sparsely vegetated landscape were active. Conclusions: During planning for harvest the soils developed on Permian tillite were correctly described as having a low to moderate risk of landslide erosion. This study concludes that the landslides initiated in June 2016 resulted from exceptionally heavy rain falling on harvested steep and hilly land coupled with the decline in root strength of the harvested trees. The frequency of such a combination of circumstances may increase if high-intensity rainfall increases in Tasmania as the result of climate change. To improve the long-term stability of this terrain and the overall sustainability of plantation forestry it is recommended that landslides and riparian areas are seeded with native vegetation, and that the current assessment of landslide risk for this terrain is re-evaluated.

Effects of tillage-induced soil surface roughness on the generation of surface–subsurface flow and soil loss in the red soil sloping farmland of southern China

The role of tillage-induced soil surface roughness (SSR) in altering surface–subsurface flow and soil loss needs to be investigated in the red soil region of southern China. In this study, two different roughness conditions (smooth surface and rough surface), produced by plowing with and without flattening, were employed to assess the effects of surface roughness on rainfall redistribution, surface and subsurface flow generation, and soil loss. In-situ rainfall simulations with three different rainfall intensities (30, 60, and 90 mm h−1) during 180 min of rainfall were tested for a steep farmland sloped at 15°. The results showed that the rough surface had a higher capability for improving subsurface flow, whereby 34% of the rainfall changed into surface flow, and 24% changed into subsurface flow; whereas the proportions of surface and subsurface flow on the smooth surface were 40% and 16%, respectively. The rough surface exhibited a faster initial generation of surface flow and subsurface flow, and the progress of erosion from sheet erosion to rill erosion was more rapid than on the smooth surface, leading to a higher sediment yield from the rough surface under stronger rainfall events. However, the rough surface demonstrated enhanced infiltration behavior and prevention of soil loss under low-intensity rainfall events. The difference in soil crusts on smooth and rough surfaces was the dominant factor affecting surface–subsurface flow generation and soil loss from the red soil sloping farmland. The roughness decreased on the rough surface and increased on the smooth surface during rainfall events. The interactions between surface and subsurface flow should be considered to evaluate soil loss; hence, this research provides a comprehensive understanding of the impact of tillage activities on soil erosion.

Soil Erosion and Deposition in a Taiwanese Watershed Using USPED

Soil erosion is a global problem that has been exacerbated in recent decades by global warming and the increased frequency of extreme weather events. It is also a global issue addressed by the United Nations’ Sustainable Development Goal #15 that seeks to recover degraded land and create a world free of land degradation by 2030. In this study, we used the Unit Stream Power-based Erosion Deposition (USPED) model to investigate the distribution of soil erosion and deposition in an important reservoir watershed in Taiwan, which is known to have a high risk of sediment hazard. We found the average soil erosion rate to be 136.4 Mg/ha/year using the model’s recommended m = 1.3 and n = 1.2 empirical coefficients for a combined occurrence of sheet and rill erosion. Additionally, we selected the Sule sub-watershed and the Kala area as examples to illustrate the pattern of soil erosion and deposition and their relationship to rivers, roadways, and anthropogenic activity, and 3D terrain was employed to further enhance visualization of the model output. It was estimated that 12.6% of eroded soil was deposited with a 200 m buffer of the rivers in the watershed and might be swept into the river system by the next typhoon, torrential rain, landslide, or earthquake. In comparison to previous USLE- and RUSLE-based soil erosion modeling in the same research area, our USPED modeling is unique in that it included the amount and distribution of soil deposition. This successful implementation of USPED in Taiwan establishes a new modeling alternative in addition to the widely used USLE and RUSLE models. The findings can be used to direct future erosion pin placement in the research area, allowing for improved monitoring of sediment movement and avoiding sediment hazards.

Particle size characteristics of sediment by sheet erosion and their responses to related parameters on a Loess hillslope: a plot-scale study

Abstract Particle size distribution (PSD) of sediment by sheet erosion not only indicates erosion behaviors, but also provides a basis for potential nutrient losses. PSDs are significantly influenced by topographic, rainfall, and associate runoff characteristics, such as rainfall intensity (I), rainfall kinetic energy (KE), slope gradient (S), runoff velocity (v), and stream power (Ω), while their effects have not been fully understood so far. To fill this research gap, in this study, simulated rainfall experiments were conducted to investigate the characteristics of effective and ultimate PSDs of sediments, as well as their responses to related parameters, under five levels of I and S. The results showed that (1) for effective PSDs, there was a significant enrichment of clay-sized and silt-sized fractions (P<2μm and P2–50μm) in sediment (percentage range: 12.11–20.64% and 57.09–65.94%), whereas there was a depletion of sand-sized fractions (P50–250μm and P250–2000μm). The ultimate PSDs of sediment were, however, similar to the soil matrix under experimental conditions. (2) With an increase in I and S, P<2μm and P2–50μm decreased exponentially or logarithmically, and P50–250μm and P>250μm increased correspondingly. The contribution rates (PCs) of the effect of I on clay-, silt-, fine-sand-, and coarse-sand-sized fractions in sediment were 23.44, 35.45, 29.09, and 56.82%, respectively, whereas the PCs of that of S were 56.54, 24.42, 59.89, and 31.47%, respectively, indicating that S plays a more pivotal role in influencing P<2μm and P50–250μm, whereas P2–50μm and P>250μm were more sensitive to I than to S. (3) The aggregation ratio (AR) for the fractions of different sizes indicated that clay-sized particles tend to be transported as aggregates, while silt-sized particles tend to be transported as primary particles. A higher I or steeper S leads to a larger proportion of particles being transported as aggregates. (4) KE, Ω, and unit stream power (U) were the most correlated parameters influencing effective PSDs. Equations were finally established using KE, Ω, and U to predict the effective PSDs of sediment by sheet erosion. This study can further the understanding of the sheet erosion process and provide a scientific basis for the establishment of a sheet erosion model.

Modelling sheet erosion on steep slopes of clay loess soil using a rainfall simulator

The process of sheet erosion has a dramatic influence on the development of soil erosion. To successfully model sheet erosion it is necessary to establish a suitable erosion prediction model for regions with clay loess soil. This study strives to achieve the following: 1) understanding and quantifying sheet erosion patterns caused by different variations of the rainfall intensity, slope steepness, and flow velocity, and 2) evaluate the influence of hydrodynamic parameters on sheet erosion for clay loess soil. A series of simulation experiments of rainfall on clay loess soil were carried out using a range of values of rainfall intensity (54, 78, 102, 126 and 150 mm h −1 ) and gradient (8.7, 17.6, 26.8, 36.4 and 46.6%). The simulation outcomes demonstrated that the rate of sheet erosion followed a power function of rainfall intensity and also gradient with a coefficient of determination R 2 of 0.98 and a Nash–Sutcliffe efficiency (NSE) of 0.96. The sensitivity of the sheet erosion rate to rainfall intensity was higher than its sensitivity to slope steepness. Furthermore, the sheet erosion rate could be predicted using its power-law dependence on flow velocity with R 2 of 0.88 and NSE of 0.82. The sheet erosion rate was optimally and linearly fitted with the shear stress with R 2 of 0.95 and NSE of 0.95, it was optimally fitted through power functions with either the stream power (with metrics of R 2 of 0.94 and NSE of 0.93) or the unit stream power (with R 2 of 0.74 and NSE of 0.68). Thus stream power represented a reliable predictor of the sheet erosion rate, and the shear stress was the better predictor. By contrast, the unit stream power was found to be a poor predictor of the sheet erosion rate. The findings of this study can help to accurately predict sheet erosion for clay loess soil on steep slopes. • Response of sheet erosion to rainfall intensity and slope gradient. • Response of sheet erosion to hydraulic parameters. • Shear stress is a preferred hydraulic parameter for estimating sheet erosion. • Some reliable models were obtained to predict sheet erosion for clay loess region.

Assessment of Spatial Soil Erosion Using RUSLE Model Integration with GIS and RS Tools: The Case of Gilgel Gibe-I Catchment, South West Ethiopia

Water-induced soil erosion is one of the serious environmental, agricultural, and socioeconomic problems in Ethiopian highlands. Accurate information on the rates of soil erosion helps environment protection and socio-economic development efforts of the nation. The objective of this research was to estimate annual soil loss, sediment yield, and map erosion risk areas of Gilgel Gibe-I (GG-I) catchment via integrating Revised Universal Soil Loss Equation (RULSE) model with Geographical Information System (GIS) and Remote Sensing (RS) technologies. The model inputs variables; rainfall erosivity (R), soil erodibility (K), topographic (LS), land cover (C) and land management (P) were derived from meteorological stations, Ethio-soil map, and satellite image of the catchment. The annual soil loss (t^-1ha^-1yr) was estimated using pixel-by-pixel ArcGIS map overlays to ensure the accuracy of RULSE output. The model output revealed on average 12.52 (t^-1ha^-1yr) soils was lost from GG-I catchment through sheet and rill erosion. The rates of soil loss were varying in the catchment, 59.8% of the catchment exposed to low rate (<5 t^-1ha^-1yr), 12.2% to moderate rate (5-12 t^-1ha^-1yr), 11.7% to high rate (12-30 t^-1ha^-1yr), and 6.6% to severe (>30 t^-1ha^-1yr). The annual sediment yield capacity of the catchment was 2.54 t^-1ha and delivery ration estimated 0.203% transported to outlet of the catchment-GGI hydropower dam. To combat the problems of GG-I hydropower dam siltation, land degradation, and low agricultural productivity an integrated natural resource management intervention is required throughout the catchment particularly in high and severe erosion risk areas.

Responses of Soil C and N Losses to Aggregate Stability and Erodibility Under Sheet Erosion Condition in South China

Responses of Soil C and N Losses to Aggregate Stability and Erodibility Under Sheet Erosion Condition in South China

Hillslope erosion improvement targets: Towards sustainable land management across New South Wales, Australia

Hillslope erosion, including sheet and rill erosion, is the dominant form of erosion in Australia and many parts of the world. Hillslope erosion improvement (HEI) targets are necessary for the evaluation of natural resource condition and sustainable land use and land management practices. The tolerable soil loss concept has been widely applied to agricultural lands for setting soil erosion targets. It is a static target that is difficult to apply to diverse land uses and soil conditions. In this study, we developed a new approach for setting HEI resource condition targets by considering seasonal erosion levels over a large climate gradient and range of land management practices across New South Wales (NSW), Australia. A 20-year fractional vegetation cover and rainfall erosivity time-series have been used to estimate hillslope erosion rates and assign monthly and annual HEI targets across NSW. To assist with land management action target, monthly vegetation cover levels have been set to meet the HEI targets on a pixel-by-pixel (100 m) basis. We further assessed the HEI targets against land and soil capability classes and their spatial and temporal variation across local land service regions. The findings can help identify the locations and periods of time with hillslope erosion exceeding a desirable threshold level and indicate management actions that might be required to improve the soil condition over areas of concern. The processes are fully automated in a geographic information system (GIS) environment, thus providing a useful tool to determine HEI targets for sustainable soil and land management at any location and period. The methods are globally portable as the input datasets are widely available.

Sediment Budgets for Small Salinized Agricultural Catchments in Southwest Australia and Implications for Phosphorus Transport

Examples of sediment budgets are needed to document the range of budget types and their controls. Sediment budgets for three small agricultural catchments (7.6 to 15.6 km2) in southwestern Australia are dominated by channel and gully erosion, with sheet and rill erosion playing a subordinate role. Erosion was increased by clearing naturally swampy valley floors and hillslopes for agriculture and grazing, and episodic intense rainstorms. The proportion of sediment from channel and gully erosion in the sediment budget appears to be determined by the depth of alluvial fills. Dryland salinization caused by clearing native vegetation has connected hillslopes to channels across narrow floodplains, increasing the Sediment Delivery Ratio (SDR). Yield and SDR are found to be insensitive to major in-catchment changes of vegetation cover after initial clearing, the ratio of sheet and rill erosion/channel and gully erosion, and sediment storage masses. This supports the idea that yield alone is often a poor indicator of the impact of land use and land management change. Riparian vegetation would reduce sediment yield but not phosphorus yield. This study demonstrates the value of mixed methods where field observations and chemical analysis are combined with information from local people.

Conglomerate to mudstone lacustrine cycles revealed in Junggar Basin, Northwest China: Middle Permian Lucaogou and Jingjingzigou formations

The Junggar Basin is one of the very large, long-lived, Late Paleozoic lacustrine basins of China, developed by the closure of the early Paleozoic Junggar Ocean. The novelty of the reported research is outcrop measurements through the mid-Permian Jingzingzigou and Lucaogou formations, allowing reconstruction of interacting very coarse and very fine-grained alluvial-to-lacustrine depositional cycles within a km-thick succession of organic-rich mudstones. The deposits are interpreted as coarse grained fan deltas that extend subaqueously into Junggar lacustrine basin, depositing fine-grained muddy, sediment gravity flow deposits. The coarseness and frequency of both alluvial and subaqueous sediment gravity flows that thin and pinch out over short distances suggests that the margin of the basin was likely steep and fringed by relatively deep lake waters, but was not the main margin of sediment supply. The proposed depositional model highlights the process response to climate-driven lake-level changes around the steep lake margin. However, the coarse-grained alluvial to fan-delta cycles and the muddy lacustrine cycles occur at different thickness scales. The former (5–13 m thick) contain internal erosion surfaces and are dominated by river sheet flood and debris flow deposits and are likely incomplete due to frequent shifting, erosion and reworking of the sediments. The lacustrine cycles (few meters thick) are dominated by current-driven, very fine sandstone, siltstone and organic-rich mudstone beds with ripple lamination and graded beds, as well as dolomites during times or places where subaqueous currents were lacking. The age dating control is not detailed but we propose that depositional cycles were likely a response to Milankovitch orbital cyclicity, also suggested by previous Permian climate modeling, that cause alternating wet and dry periods and trigger tens of meters lake level changes with abrupt grainsize changes in the shallow proximal areas and more subtle changes in sediments of distal/deep lacustrine regions. The climate changes therefore cause both sediment supply changes subaerially and lake accommodation changes subaqueously, though these would also be modulated to a lesser extent in the resultant stratigraphy by autogenic responses and tectonic forcing of sediment yield.

Soil C, N, P stocks and stoichiometry as related to land use types and erosion conditions in lateritic red soil region, south China

Irrational land-use can accelerate soil erosion by deteriorating soil structure and depleting soil nutrients, especially profoundly alter soil organic carbon (C), total nitrogen (N), total phosphorus (P) contents and stocks. However, it remains unknown how soil C, N, P stoichiometry patterns are affected by land-use type and soil erosion. This study was carried out to examine the soil C:N:P stoichiometry patterns in woodland (WL), upland (UL), and paddy (PD) at soil depths of 0–10, 10–20, and 20–30 cm under collapsed gully erosion (CGE) and sheet erosion (SE) conditions in lateritic red soil region. Compared to WL, C content and stock decreased by 19.1–31.9 g kg−1 and 12.8–31.7 Mg ha−1, but P content and stock increased by 4.9–7.4 g kg−1 and 6.7–9.9 Mg ha−1 in UL soil at 0–30 cm, resulting in a decrease in C:N, C:P and N:P ratios by 8.6–14.8, 23.0–28.6 and 0.7–1.0, respectively. In addition, N, P contents and stocks increased by −0.04–1.8 g kg−1, 4.1–10.5 g kg−1 and 1.8–1.1 Mg ha−1, 7.0–8.2 Mg ha−1 in PD soil at 0–30 cm, resulting in a decrease in the ratio of C:N, C:P and N:P by 8.1–10.6, 20.7–26.0 and 0.6–0.8, respectively. Furthermore, C, N, P contents, stocks and C:N ratio in UL and PD soils were more significant under SE than CGE condition with, while it was opposite trend in WL at 0–30 cm depth. Soil C, N, P contents and stocks significantly decreased with soil depth, while no significant changes were detected on their stoichiometric ratio. Soil porosity and texture had significant contribution to C, N and P stoichiometry by PCA and RDA analysis. Our results demonstrate that the intensive agricultural practices changing soil porosity and texture under SE condition led to more pronounced differences of soil C, N, P and their stoichiometric ratios in lateritic red soil region.

Rehabilitation of the Doce River Basin after the Fundão dam collapse: What has been done, what can be done and what should be done?

AbstractFive years after the failure of the Fundão Dam in Brazil, no scientific report has been published regarding the rehabilitation interventions performed by the Renova Foundation, which is solely responsible for the rehabilitation of the Doce River Basin. This study aimed to assess the fluvial and riparian rehabilitation efforts performed by Renova, as well as to test and propose alternatives, when appropriate. Before the dam rupture, the fluvial system showed a single‐thread morphology that was sinuous and meandering. After the rupture, changes occurred in the river dynamics with strong sediment deposition in riparian areas, resulting in the loss of several meandering streams, oxbow‐lakes, lagoons, and fluvial islands. There was also a loss of the riparian vegetation, from 61% to 6%, as estimated by the Normalized Difference Vegetation Index (NDVI). Renova subjected nearly 61% of the study area to fluvial rehabilitation techniques, particularly using rip‐rap or larssen sheet piling (37%) and erosion control fibre blankets (17%). However, 50% of these interventions have been unsuccessful because the rip‐rap was poorly positioned and the biodegradable fibre blankets decomposed rapidly. We recommend changing the position of the rip‐rap from convex to concave banks and to avoid using biodegradable fibre blankets. Instead, we encourage the planting of riparian forests with native species to form buffer zone systems. Renova' riparian rehabilitation attempts were found to have low success rates, which was attributed to the unaddressed presence of toxic compounds, such as sodium and ether amine. However, our results proved that riparian rehabilitation using native species is feasible, if physical, chemical, biological, and phyto‐remediation strategies are adopted, to alleviate the ether amine and sodium toxicity.

Coastal Alluvial Fans of the Raghama Formation, Northern East Red Sea, Saudi Arabia

Tawfik, M.; Al-Hashim, M.; El-Sorogy, A.; Alharbi, T., and Wadani, M., 2021. Coastal alluvial fans of the Raghama Formation, Northern East Red Sea, Saudi Arabia. Journal of Coastal Research, 37(6), 1193–1203. Coconut Creek (Florida), ISSN 0749-0208. Four Miocene outcrops of the Raghama Formation have been investigated for the first time in NW Saudi Arabia to identify facies types, facies associations, and sequence stratigraphy. These sections have been controlled by fans that flow into the sea from higher altitudes. The fans can be divided into alluvial fans and a braided fluvial environment that pass seaward into lagoons and fringing reefs. The continental deposits of the fan are characterized by the fan apex, which passes downslope into alluvial plains. These alluvial deposits are located beside high hinterlands and contain dwindling gravel pavement and local dunes. The transitional zone between the continental and marine deposits comprises mainly erosional beaches and sheet flood sediments. The fan in the marine environment is regulated by longshore drift and wave action. The fringing reef is located offshore from the fan and close to the shore with no lagoons. The fringing reefs are locally affected by the force of the water rushing down the active streams, as indicated by the occurrence of irregularities, remnants, and gravels in the reef cervices. One third-order depositional sequence is well preserved within the studied sections. This sequence is bounded by unconformities and is controlled by the ratio between accommodation and sediment supply (A/S ratio). Variations in the A/S ratio are related to eustatic sea level, climate, and tectonics related to the collision between the Eurasian and Arabian plates, the latter which is responsible for the uplift of the studied areas and the widening of the Red Sea, leading ultimately to subsidence during the Middle Miocene.

Soil erosion and sediment transport in Tanzania: Part II – sedimentological evidence of phased land degradation

AbstractSoil resources in parts of Tanzania are rapidly being depleted by increased rates of soil erosion and downstream sediment transport, threatening ecosystem health, water and livelihood security in the region. However, incomplete understanding to what effect the dynamics of soil erosion and sediment transport are responding to land‐use changes and climatic variability are hindering the actions needed to future‐proof Tanzanian land‐use practices. Complementary environmental diagnostic tools were applied to reconstruct the rates and sources of sedimentation over time in three Tanzanian river systems that have experienced changing land use and climatic conditions. Detailed historical analysis of sediment deposits revealed drastic changes in sediment yield and source contributions. Quantitative sedimentation reconstruction using radionuclide dating showed a 20‐fold increase in sediment yield over the past 120 years. The observed dramatic increase in sediment yield is most likely driven by increasing land‐use pressures. Deforestation, cropland expansion and increasing grazing pressures resulted into accelerating rates of sheet erosion. A regime shift after years of progressive soil degradation and convergence of surface flows resulted into a highly incised landscape, where high amounts of eroded soil from throughout the catchment are rapidly transported downstream by strongly connected ephemeral drainage networks. By integrating complementary spatial and temporal evidence bases, this study demonstrated links between land‐use change, increased soil erosion and downstream sedimentation. Such evidence can guide stakeholders and policy makers in the design of targeted management interventions to safeguard future soil health and water quality.

Sediment transport mechanisms and selective removal of soil particles under unsteady-state conditions in a sheet erosion system

Selective removal of particles and nutrients by water erosion is a key factor in soil erosion studies. Most agricultural soils are located on gentle slopes where fertility is high; however, until now, the main attention on sediment transport mechanisms was paid to high-slope gradients, where soil erosion is intense, but soils are less productive. Despite the importance of sediment size distribution (SSD) and transport mechanisms under unsteady-state conditions, few studies have been done on this issue. Higher sediment concentrations in the early stages of the runoff indicate the need to deal with unsteady-state conditions. To address this issue, sheet erosion experiments were done using a 0.2 m by 1 m tilting flume under controlled laboratory conditions. Six inflow rates (75, 100, 125, 150, 175, and 200 mL/s) were applied on two contrasting soils, including an adjacent agricultural soil and a not cultivated soil (namely Cropland soil and Control soil, respectively) at two low slope gradients (1.5% and 2%). No rill formation was observed during the experiments. The sediment-laden runoff was sampled during unsteady-state flow to determine the SSDs and sediment transport mechanisms of eroded particles for unsteady-state conditions. The results indicated different trends in the selective removal of sediment particles depending on hydraulic conditions and soil aggregate size. The contribution of suspension-saltation (SS) to the total sediment load for the Cropland and Control soils varied from 22% to 68% and from 35% to 59%, respectively, while up to 78% and 65% of soil particles were transported by bed load (BL), respectively. However, SS and BL indicated a reverse trend with stream power. The Cropland soil showed a single peak in the SSD at low stream powers because of the selective removal of fine particles (0.042 mm), whereas the SSD was shifted to a bimodal distribution at higher stream powers with the selective depletion of both fine (≤ 0.084 mm) and coarse (1.5 mm) particles. The Control soil experienced a unimodal SSD in a range of sizes between 0.109 and 0.175 mm for all stream powers. The findings of the current study highlight the need for further study on the erosion of low-slope agricultural soils, where small stream powers can remove fine and fertile soil particles.