Braided rivers commonly exhibit frequent channel shifting and bar migration, which remains a major challenge to predict. Numerical modeling has evolved into a proactive approach for resolving fluvial processes for over half a century. To date, however, it has remained poorly understood how flow–sediment–morphology interactions dictate braided river evolution. Here, a fully coupled two-dimensional shallow-water hydrosediment-morphodynamic model is developed, and a timescale-based dynamic braiding index is proposed to characterize channel evolution. To unravel the roles of flow–sediment–morphology interactions, secondary flow effects, slope collapse, and sediment heterogeneity in braided river evolution, the model is applied to flume experiments and a typical braided reach of the Tuotuo River in the source region of the Yangtze River. Braided channel development follows a three-stage pathway, i.e., bar initiation with flow division, bifurcation with active channel formation, and channel reworking toward dynamic equilibrium. Fully coupled flow–sediment–morphology modeling produces higher braiding intensity during multithread network development. Secondary flow enhances lateral sediment transport and channel asymmetry, whereas slope collapse limits excessive local incision and regulates steep bar margins. Critically, sediment heterogeneity and mobility modify braided channel evolution through bed surface sorting, leading to different bar-channel sorting patterns under partial and full mobility conditions. In the Tuotuo River flood case, nonuniform sediment amplifies sediment transport peaks and directly affects channel evolution. Overall, this work clarifies the distinct roles of key physical mechanisms in braided river evolution and provides a basis for informing the effective management of sediment and cross-river infrastructure safety.
The Upper Permian Longtan Formation in the Upper Yangtze Platform represents a complex paralic succession of siliciclastic rocks and interbedded coals. To evaluate its hydrocarbon resource potential and elucidate organic matter (OM) enrichment mechanisms, this study integrates petrographic, mineralogical, and organic and inorganic geochemical analyses. The Longtan shale and coal exhibit high total organic carbon (TOC) contents, averaging 7.2 and 47.2 wt%, respectively. Rock-Eval pyrolysis, gas chromatography, and petrographic data indicate that the OM is dominated by type III kerogen, characterized by hydrogen index values of 33–131 mg HC/g TOC and Tmax values of 455–476 °C. These parameters, alongside the vitrinite reflectance values (1.23%–1.72%), indicate that the strata have reached thermal maturities corresponding to the late oil window to wet gas window. Consequently, shale acts as a fair-to-very-good source rock, while coal is an excellent gas-prone source rock. Geochemical proxies (specifically the Fe–S–TOC ternary diagram) indicate deposition under predominantly dysoxic to oxic bottom-water conditions. Additionally, relatively low TS/TOC and Sr/Ba ratios suggest a freshwater, lower delta-plain setting punctuated by rare marine incursions. Elevated TiO2 concentrations and intermediate chondrite-normalized rare earth element patterns indicate a mixed provenance of background terrigenous sediment diluted by mafic volcaniclastic influx. Furthermore, near-unity (La/Yb)N ratios, normalized to post-Archean Australian shale values, indicate rapid sedimentation rates. Ultimately, OM accumulation in this volcanically influenced setting was driven by tectonic-sedimentological coupling. Under a warm and humid paleoclimate, intense chemical weathering of the Emeishan basalts delivered abundant nutrients that sustained extensive gymnosperm forests, providing an abundant supply of refractory type III kerogen. In the shale facies, rapid clastic influx ensured the immediate burial and preservation of this OM despite dysoxic-to-oxic bottom waters. In the coal mires, the organic purity was influenced by fluctuations in the sediment supply rate, where accelerated clastic and volcaniclastic pulses physically diluted and suppressed peat accumulation.
Local scour around bridge piers involves complex three-dimensional (3D) flow–sediment interactions poorly documented for non-circular geometries. This study investigates the 3D velocity field and Reynolds shear stress (RSS) around an oblong bridge pier under clear-water (CW) and live-bed (LB) conditions using full-circumference Acoustic Doppler Velocimetry (ADV) at eight azimuthal positions over fixed reconstructed equilibrium scour beds.The oblong pier generates a vertically compact vortex system, shallow near-bed downflow upstream, lateral sweeping along the shoulders, and wake-induced upflow downstream confined to the lower flow depth (y/H ≲ 0.2). The pier-influence zone is strongly asymmetric: upstream disturbance is confined to r/D ≤ 2.4, while the downstream wake sustains elevated velocity deficits (δU > 5%) to r/D ≈ 4.4, with far-field recovery (δU ≤ 5%) first confirmed at r/D = 4.6 under both regimes.Under CW conditions, peak normalized bed shear stress reaches 1.1–1.7 at the upstream shoulders (θ ≈ 45°–75°), producing deep, focused scour. Under LB conditions, the shear field is more diffuse, yielding wider but shallower scour (∼11% shallower). In the far-field, log-law and RSS-based shear estimates agree within ± 20%; near the pier, the log-law fails under strongly 3D flow. These findings support a mechanism-based framework for scour assessment at oblong bridge piers under the tested conditions. Although maximum local scour occurs immediately adjacent to the pier, the hydrodynamic influence zone extends to r/D ≤ 2.4 upstream and r/D ≤ 4.6 downstream, and countermeasures should target this broader zone. RSS-based estimation provides the more robust near-pier shear method.
In alluvial rivers, sediment particles exhibit a strong affinity to phosphorus (P), with the adsorption capacity varying across different particle sizes, and the experimentally derived adsorption parameters can be used in water quality models that consider the effect of sediment adsorption processes. However, there exist limited systematic experimental studies on how particle size influences P adsorption by sediment. To investigate the P adsorption characteristics of different-sized sediments, surface bed sediment samples were collected from two sections in the Middle Yangtze River (MYR), with particle size (D) ranging between 2 and 500 μm. These sediment mixtures were then separated into 7 size fractions (down to D < 8 μm), with the corresponding mean particle size () ranging from 5.9 to 424.2 μm. The native adsorbed P (NAP) amounts for different-sized sediment fractions were measured, and P adsorption experiments were conducted for the 7 sediment fractions under 3 different sediment concentrations (S) with a similar environment of pH and temperature when sampled from the MYR. Experimental results show that: (i) NAP values for sediment particles with D < 500 μm ranged from 0.15 to 0.47 mg·g−1, exhibiting a negative exponential relation with ; (ii) P adsorption by sediment primarily occurred within the first 4 h, with the highest adsorption rate, and both the amount and rate of P adsorption were negatively correlated with S and ; (iii) key adsorption parameters in the Langmuir isotherm and kinetic equations were calibrated using the experimental data, achieving high fitting accuracy. The maximum P adsorption amount (Qmax) obtained in isothermal experiments was well described by the equation considering the combined effects of S and : Qmax = 1.6189S-0.228 -0.283, with particle size having a greater influence than sediment concentration; and (iv) in addition, the kinetic adsorption data for most sediment fractions (D < 250 μm) were closely distributed and displayed a consistent linear trend, leading to the determination of unified adsorption kinetic parameters for nonuniform sediment under different S values: k1 = 0.257 L·mg−1·h−1 and k2 = 0.057 h−1.
Turbidity currents frequently occur in rivers, reservoirs, and estuaries, where they exhibit distinctive hydrodynamic and sediment transport characteristics. In the current study, the motion characteristics of turbidity currents in a non-uniform sloped flume are systematically investigated, focusing on the effects of inflow discharge, sediment concentration, ambient water depth, and bed slope, as well as the variations in motion characteristics at the slope transition. The experimental results show that the plunge point shifts downstream with increasing inflow discharge or bed slope and shifts upstream with increasing sediment concentration or ambient water depth. For low-sediment-concentration turbidity currents, the Froude number (Fr) at the plunge point remains relatively stable at approximately 0.78. For medium-sediment-concentration turbidity currents, the plunge point Fr is jointly influenced by inflow discharge, sediment concentration, ambient water depth, and bed slope. For high-sediment-concentration turbidity currents, the plunge point Fr is primarily governed by the inflow sediment concentration. The head velocity of the turbidity current increases with increasing inflow discharge and sediment concentration but is not significantly influenced by ambient water depth. The head thickness of the turbidity current increases with increasing ambient water depth and inflow discharge but decreases with increasing sediment concentration and is most strongly influenced by ambient water depth. At the slope–horizontal transition of the flume, intensified mixing leads to a decrease in head velocity and a pronounced increase in head thickness. Empirical formulas are developed for turbidity current head motion, and the contributions of key factors to head travel distance and head thickness are quantified using a data-driven approach. The main significance of the current study is to provide a clearer understanding of the motion characteristics of turbidity currents under different influencing factors. The results can serve as an initial reference for river regulation and reservoir sediment flushing.
This study investigates Campano-Maastrichtian sediments from the northern Anambra Basin, Nigeria, to refine existing paleoenvironmental models through a multi-proxy integration of particle size analysis, scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDX), and principal component analysis (PCA). While previous studies relied on first-order textural data, the current integrated approach provides new insight into the interplay of provenance, episodic energy fluctuations, and diagenetic overprinting. Seventeen samples of sandstone, siltstone, and ironstone were collected from the Ajali and Mamu formations at Odagbo, Egume, and Iyale. Granulometric analysis reveals predominantly fine to medium-grained, unimodal, and poorly to moderately sorted sediments. Linear discriminant functions suggest deposition in a mixed fluvial and shallow marine setting, with potential episodic influence of high-energy density flows or storm-induced turbulence. SEM imaging shows grain morphologies ranging from angular to rounded, with microtextures such as: conchoidal fractures, dissolution pits, and quartz overgrowth, reflecting variable transport distances and diagenetic modifications. EDX spectra confirm quartz-dominated mineralogy with variable aluminosilicate, iron oxide, and traces of heavy mineral contributions, while the PCA identified three compositional groups: (1) quartz-rich, texturally mature assemblages; (2) feldspathic and clay-rich sediments of proximal origin; and (3) organic and iron (Fe)-bearing sediment reflecting redox variations. These results demonstrate that sediment supply was derived from cratonic interior sources, modified by fluvial input and reworked under shallow marine conditions with subsequent diagenetic overprinting. By integrating textural and geochemical data, this study highlights the interplay of provenance, episodic hydrodynamic fluctuations, and diagenesis, providing new insight into the paleoenvironmental evolution of the Anambra Basin.
The Pliocene, a pivotal period in Earth's climatic evolution, holds immense significance. It marks the transition from a notably warmer period, with no permanent ice in the northern hemisphere, to the colder phases associated with the Pleistocene glacial periods. Several geological events were significant during the Neogene, but one of the most crucial was the closure of the Isthmus of Panama. This land bridge, connecting the North and South American continents, formed primarily through the convergence of the Cocos plate, the Caribbean plate, and the Panama microplate, resulting in widespread orogeny and intense volcanism. This study is based on textural, mineralogical, and geochemical proxies derived from samples collected in the Caribbean Sea basin at site 999A of the ocean drilling program (ODP). It aims to understand the influence of the closure of the Isthmus of Panama and climate change on sedimentation in the study area (Colombian Basin). The geochemical and mineralogical data suggest that orbital (Milankovitch) cycles influenced sedimentation patterns in the Caribbean Sea during the Pliocene due to substantial changes in regional precipitation. However, after the shoaling of the Central American seaway, approximately 4 million years ago, the expression of these cycles in sedimentation in the study area decreased. The impact of these orbital cycles on precipitation began to diminish as the North Atlantic and the Caribbean Sea warmed. This shift had profound implications for climate dynamics, causing the Intertropical Convergence zone to migrate northward. With the onset of the Pleistocene, approximately 2.58 million years ago, the relevance of orbital forcing reemerged, fundamentally influencing sedimentation in the Caribbean Sea and specifically affecting precipitation patterns in the equatorial region and the South American monsoon system. Understanding this complex interaction is crucial, as it highlights how geological and climatic factors have shaped our modern environment.
Suspended sediment load (SSL) modeling is crucial in hydrology and river engineering for infrastructure design, river basin management, flood mitigation, and channel change studies. Monitoring SSL often is costly and labor-intensive, especially in the Brahmaputra-Jamuna River, a complex braided river with a nonlinear relation between discharge and sediment. In this study, the potential of six standalone machine learning (ML) models—Random Forest (RF), Support Vector Regression (SVR), Adaptive Boosting (AdaBoost), Elastic Network (EN) Regression, Gradient Boosting Regression Tree (GBRT), and eXtreme Gradient Boosting (XGB)—is investigated for estimating SSL by integrating antecedent hydro-sediment information and utilizing the Isolation Forest data outlier detection technique, which influences model robustness and accuracy. Monthly observed SSL, discharge (Q), and ERA5 reanalysis rainfall (R) data from 1976 to 2023 were used to calibrate and validate the model. Various input scenarios were developed by combining SSL, discharge, and rainfall with different temporal lags. The results reveal that discharge is the best predictor for modeling SSL (Pearson correlation coefficient, r = 0.72), and the most effective input combination is the discharge and rainfall in the current month and two previous months, and the SSL in the two previous months (Qt, Qt-1, Qt-2, Rt, Rt-1, Rt-2, SSLt-1, SSLt-2). All applied models yield good performance (Nash-Sutcliffe Efficiency, NSE ranging from 0.71 to 0.82), except SVR (NSE = 0.68), which showed relatively weak performance, while XGB outperformed the other methods (NSE = 0.82). A Taylor diagram and reliability analyses also confirmed XGB’s superiority in estimating SSL. Using outlier detection, the results show performance improvements of 9.76% for XGB, 7.59% for GBRT, 7.41% for RF, 6.67% for AdaBoost, 5.48% for EN, and 4.41% for SVR, indicating that outliers significantly impacted the models. The current study identified the best ML model for SSL estimation at Badadurabad, Bangladesh, and showed how to use antecedent hydro-sediment data to estimate the current sediment load more easily and affordably in braided rivers with weak hydro-sediment coupling, supporting sustainable sediment management in similar rivers.
Estimating bed shear stress (BSS) is essential to determine the critical shear stress under incipient motion conditions of sediments which typically occur within transitionally rough regime for sandy beds. Considering most of the studies have focused on fully rough regime, this study aims to cover knowledge gaps of the velocity-based methods of BSS estimation in transitionally rough regime of shallow and wide flows over sand particles of 0.43–1.94 mm, at three different water depths of 99-140 mm. In this specific conditions, the existing research gaps in applicability of applying quadratic stress law, possibility of using boundary layer characteristics method (BLCM), determination the values of the virtual bed level (Δz) and roughness length (z0) for the log-fit method and innovatively development of a water surface velocity approach were explored. The velocity profiles were best described by logarithmic trendlines, outperforming the power law and validated using the parabolic law. Water surface and average flow velocity approaches yielded average absolute percentage change relative to the Reynolds method (AAPC-RSS) of 13% and 17%, which improved to 4% when grouped by shear Reynolds number. Applying the Darcy-Weisbach equation gave 11% AAPC-RSS, reduced to 9% with sidewall correction for average flow velocity approach. Log-fit methods applied to the wall-shear layer or to the combined with intermediate layer yielded ∼30% AAPC-RSS with Δz = 0 and von Kármán constant of κ = 0.4. The one-point log-fit method, with z0 = 0.034D50, achieved 8% AAPC-RSS, and 11% for z0 = 0.11ν/u∗+ 0.03ks. For the BLCM, empirical constant of C = 4.3 (range 4.2–4.6) was identified, providing an AAPC-RSS of 21%. Overall, water surface and average flow velocity approaches were practical and accurate. The log-fit method though underestimated, remained robust, while the log-fit one-point approach was effective for limited data but was measurement-error-sensitive, and BLCM recommended for full available profiles.
Water pollution caused by synthetic dyes (including azo, cationic, and anthraquinone dyes) and heavy metals (Pb, Cd, Cr, Cu, Ni, and Zn) presents a significant environmental challenge, necessitating effective and sustainable remediation strategies. This review explores the potential utilization of marine sediments from the neritic zone as an alternative adsorbent for the removal of these contaminants. This review focuses on neritic (shelf) marine sediments. These materials are abundant, accessible, and mineralogically reactive. When responsibly sourced and governed, they offer a practical pathway for dye and heavy metal removal in saline and brackish waters. The study examines the physicochemical properties of neritic sediments, their adsorption mechanisms, and modification techniques to enhance their pollutant removal efficiency. Furthermore, the review evaluates the feasibility of integrating marine sediments into wastewater treatment applications, emphasizing their alignment with Sustainable Development Goals (SDGs), particularly in achieving clean water and environmental sustainability. The findings indicate that marine sediments can serve as a cost-effective and environmentally friendly adsorbent, with modification strategies such as acid/base treatment, thermal activation, and biofunctionalization further improving their performance. Challenges such as pollutant desorption, variability in sediment composition, and potential ecological impacts are also discussed, providing insights into future research directions and the scalability of sediment-based treatment systems. Overall, this review highlights the promising role of marine sediments from the neritic zone in water pollution control, offering an innovative approach to addressing global environmental concerns.
Shallow coastal lagoons are among the ecosystems most vulnerable to eutrophication; their sediments, which act as nutrient reservoirs under oxic conditions, become diffuse sources of nitrogen and phosphorus in hypoxic conditions, prolonging the eutrophic state despite reduced external inputs. This systematic review, conducted according to PRISMA-ScR, searched Web of Science, Scopus and Google Scholar; 82 publications were selected after double screening, and quantitative data from 26 lagoon/estuary ecosystems across four continents were analyzed. The meta-synthesis of 327 benthic flux measurements shows a median of 1.9 mmol·m-2·d-1 for phosphorus in lagoons (approximately ×2 of shallow lakes) and 3.8 mmol·m-2·d-1 in tropical environments, revealing a high thermal and redox sensitivity of P-Fe locks. For nitrogen, the median net flux reaches 11 mmol·m-2·d-1; 85% of positive fluxes come from NH4+ efflux (DNRA dominant in eutrophic contexts), while 34% of cases indicate a sink via N2 production (denitrification/anammox). Analysis of the controlling factors identifies background oxygen as the main lever, followed by organic load, NO3- availability, bioturbation, salinity and temperature, which together explain 82% of the variance in PO43- and NH4+ fluxes. On this basis, effective management of eutrophic lagoons rests on two inseparable pillars: (i) sustainable reduction of external inputs (a prerequisite) and (ii) treatment of internal stocks (targeted dredging, reactive capping, oxygenation/mixing) in a diagnostic-dependent and adaptive framework, complemented by nature-based solutions to stabilize gains. This summary highlights the operational importance of sediments in eutrophication trajectories and provides orders of magnitude and levers to guide the restoration of coastal lagoons.
In alluvial river systems, riparian vegetation extensively colonizes unstable bars, substantially influencing hydrodynamic characteristics and morphodynamic processes. Nonetheless, its effects on water level variation, velocity distribution, and flow diversion ratios in compound anabranching-confined river systems remain unclear. This study presents a numerical sensitivity analysis of a compound channel segment in the middle Yarlung Tsangpo River in the Tibetan Plateau. Using TELEMAC-2D, we simulated hydrodynamic processes under different vegetation scenarios (2.5-10.0 m heights, 1.5-6.0 m planting spacings), based on field observations and remote sensing images. Main results are summarized: (i) Riparian vegetation significantly elevates water levels, with the effect attenuating downstream. During a 100-yr flood, vegetated bars increased the upstream-downstream water level difference 1.71 m (unvegetated condition) to 2.44 m. (ii) Vegetation alters the cross-sectional velocity distribution and may drive cross-sectional evolution toward narrow-deep "V/W" forms. This modulation is positively correlated with upstream inflow. In a 100-yr flood, main channel velocity rose by up to 1.4 m/s, while bar velocities dropped by up to 1.3 m/s. (iii) Flow resistance controls shift from spacing-dominated under low discharges to height-dominated at high discharges. (iv) The bar flow diversion ratio shows sigmoidal growth with discharge. This study clarifies riparian vegetation's role in affecting transitional high-altitude river hydraulics and provides a foundation for balancing ecological restoration with alpine river management.
This study analyses the response of a Mediterranean agroforestry catchment to Storm Juliette, an episode of prolonged rainfall (208 mm in 5 days). Hydrological monitoring combined with sediment fingerprinting using colour tracers and a Bayesian mixing model was applied to assess sediment dynamics. The results show a peak discharge of 2.4 m 3 & centerdot;s -1 and a sediment yield of 3.9 t & centerdot;km -2 , markedly higher than the 2012-2018 average (0.8 m 3 & centerdot;s -1 ; 0.7 t & centerdot;km -2 ). Channel banks and cropland were identified as the main sediment sources during the event, in line with long-term observations suggesting a dominant role of the most functionally connected catchment compartments. Although this less frequent, high-magnitude event markedly increased the effective catchment area and overall sediment mobilisation, sediment contributions remained strongly conditioned by the spatial organisation of buffers, barriers, and source areas within the catchment. The analysis of such infrequent events provides critical insight for improving soil and water management strategies in highly anthropized Mediterranean catchments under increasing climate variability. (c) 2026 International Research and Training Center on Erosion and Sedimentation, China Institute of Water Resources and Hydropower Research and Tsinghua University. Publishing services by Elsevier B.V. on behalf of KeAi Communications Co. Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
Watershed management and engineering, emphasizing soil erosion and sediment yield management, typically employs two primary structural and biological approaches. Despite indications that the biological approach may be more favorable in many scenarios, there has been a notable global tendency toward the structural approach. This disparity in preference has not been thoroughly studied or systematically documented. Accordingly, this research quantifies and compares the economic benefits of the structural and biological approaches in the form of the construction of 578 check dams, and the scenario of implementation of enclosure and pit-seeding, respectively, within the Taham watershed of Zanjan Province, Iran, over a 40-year period (2011–2051). To that end, the cost-benefit ratio of both approaches was calculated based on the costs of implementing those measures and the benefits derived from sediment and nutrient trapping (organic carbon, nitrogen, phosphorus, and potassium) in check dams, as well as the soil erosion control and sediment prevention in the biological measures scenario. Results revealed that, even excluding numerous indirect benefits of the biological approach and the costs of the structural approach, the biological approach indicates nearly sixteen times the economic efficiency of the structural approach. These findings emphasize the importance of watershed management practices that prioritize soil conservation and runoff prevention through maximum rainfall infiltration, highlighting the critical role of biological measures and the need to reduce overreliance on structural solutions.
Understanding the mechanisms by which vegetation influences phosphorus transport in agricultural drainage ditches is crucial for accurately assessing non-point source pollution loads and ecosystem restoration in irrigation areas. This study conducted experiments in outdoor artificial ditches to comprehensively analyze the effects of vegetation density, hydraulic characteristics, and suspended sediment properties (with three settings for each) on suspended sediment and phosphorus transformation and transport. Particular emphasis was placed on sediment-water interface dynamics. Building on the adsorption-desorption properties of phosphorus and particles, this study examined suspended sediment movement and phosphorus adsorption capacities across varying sizes within ditches. As vegetation density increased from 0 to 1800 stems/m2, the phosphorus interception rate increased from 0.11 to 0.358, particularly through the capture of fine particles with higher adsorption capacities. Vegetation was found to intercept suspended sediment with particle sizes smaller than 73 μm more effectively than gravitational settling, and this size threshold increases to 124 μm with increasing flow velocity. Under high flow velocity conditions, more than 90 % of phosphorus is intercepted by plant interception. The initial concentration of suspended sediment influences the distribution of phosphorus and sedimentation. As the initial concentration increases, dissolved phosphorus concentration decreases from 0.86 mg/L to 0.757 mg/L. This study provides a theoretical basis for managing phosphorus emissions in agricultural ditches under diverse environmental conditions.
Rill erosion is a hydrological-geomorphic process occurring on hillslopes across small spatial extents. Despite advances in soil erosion research, the dynamic feedbacks among the rill erosion process, morphological adaptation, and hydraulic characteristics remain poorly quantified. The current study introduces a high-resolution photogrammetric technology to resolve real-time coupling of rill network evolution with hydraulic characteristics and sediment yield. Four sets of successive rainfall simulation experiments were done with four rainfall intensities of 30, 60, 90, and 120 mm/h. A photogrammetric observation system was applied to dynamically capture the landscape evolutions of rill erosion during ongoing rainfall. The results showed that rainfall of 120 mm/h caused the most damage to soil surfaces with the soil loss being about 9 times higher than that at 30 mm/h. The key morphological indicators of rill density, splitting degree (ω), fractal dimension, and geomorphological comentropy (H) increased with rainfall duration and intensity, while the width-depth ratio decreased. H and ω were the parameters most closely related to sediment yield, contributing 61 % and 39 %, respectively. Based on H and ω, a comprehensive quantitative indicator (CQI) was established by regression analysis. The CQI increased exponentially with the rainfall duration. Among all hydraulic variables, the stream power was the best parameter to characterize the dynamic mechanism of soil erosion and the morphology. These findings are useful for understanding the rill erosion process on a slope and assessing landscape degradation by erosion.
Gravel-bed microtopographies are closely connected with many aspects of river dynamics, including incipient sediment motion, sediment transport, flow structure, and flow resistance. Grain arrangement and grain size are two important parameters for quantitatively characterizing gravel-bed microtopographies. However, how grain arrangement and grain size affect gravel-bed microtopographies is still unknown. For this research, fifteen groups of uniform gravel samples were obtained by screening natural river sediment. Then, these uniform gravel samples were separately used to artificially pave gravel beds with six typical types of grain arrangements in a laboratory, and the gravel-bed elevations were measured. On this basis, the effects of grain arrangement and grain size on gravel-bed microtopographies were analyzed using statistical parameters and variograms. The experimental results showed that the elevation frequency distributions for the stacked-layer type of gravel beds established negatively skewed, leptokurtic, and unimodal shapes, but those for the other bed types exhibited bimodal shapes; therein, the main peaks for the one-layer and imbricate bed types were close to a normal distribution. The stacked-layer and one-layer types of gravel beds are approximately isotropic, while the imbricate and striped types are anisotropic. The spherical variogram model can be used as a good theoretical model to quantify the elevation variabilities for the stacked-layer, one-layer, and imbricate types of gravel beds. The gravel nuggets of elevation variograms for these three types of gravel beds are insensitive to the grain arrangement and decrease when the grain size increases, but the sills and correlation lengths linearly increase with increasing grain size.
Exploring the flocculation characteristics and mechanisms of suspended sediment under artificial intervention is important for understanding the flocculation process, so as to achieve rapid, ultra-clean removal of particles. To investigate flocculation characteristics and mechanisms under the coupled action of an electric field and acoustic field, an experimental platform was established. Through continuous electric intervention and intermittent ultrasonic intervention (with a duration of 5 min), flocculation tests were done targeting three sediment concentrations (500, 1500, and 2500 mg/L), and a total of 24,570 sets of particle size data and 1287 sets of turbidity data were obtained. The results showed that acoustic–electric coupling intervention can significantly increase the peak particle size of suspended sediment and reduce the turbidity. The ultrasonic cavitation effect promotes the agglomeration of sediment particles by overcoming electrostatic repulsion, while the flocculants generated by electrolysis can further enhance floc growth, forming spherical (lump-shaped) and dense floc structures. In general, under a high sediment concentration, a combination of high current density and low ultrasonic power results in the optimal turbidity removal effect; whereas under a low sediment concentration, a combination of high current density and high ultrasonic power yields a better turbidity removal effect. Additionally, the critical response particle size for flocculation (CRPSF) in the case of high sediment concentration is larger than that in the case of low sediment concentration. Overall, current density, ultrasonic power, and ultrasonic intervention duration are the dominant factors regulating the CRPSF. Therefore, real-time adjustment of current density and ultrasonic parameters serves as a key approach to further improve flocculation efficiency.
Plant roots, soil properties, and hydraulic characteristics can significantly influence soil detachment by overland flow. However, research quantifying the effects of those various factors on soil detachment capacity (Dc) of naturally restored grasslands on the Loess Plateau of China is limited. Moreover, the impact of soil surface electrochemical properties (EPS) on Dc has received little attention. Therefore, 135 soil samples were collected from three typical experimental areas on the Loess Plateau and subjected to flow scouring in a hydraulic flume under five shear stresses (combinations of a 15° slope and flow discharges of 0.1, 0.2, 0.3, 0.4, and 0.6 L/s). The measurements were taken for plant roots and soil properties, including basic soil physicochemical properties (BPS) and EPS. The results indicated that the mean Dc at different sampling sites ranged from 1.11 to 203.25 g/(m2·s). Dc had a significant exponential correlation with root length density (RLD), root surface area density, the sand content, the content of water-stable aggregate > 0.25 mm, and soil surface potential (φ0) (coefficient of determination (R2) ranged from 0.315 to 0.771; p < 0.01); Dc was significantly related via a power function to the clay content (Clay), soil organic matter (OM), the contents of aluminum oxide (Al2O3) and iron (Ⅲ) oxide (Fe2O3), soil cation exchange capacity, soil specific surface area, flow charge, shear stress, stream power (w), and flow velocity (R2 ranged from 0.636 to 0.986; p < 0.01). Dc demonstrated significant linear relations with soil bulk density, soil exchangeable sodium percentage, soil surface charge density (σ0), and soil surface electric field (R2 ranged from 0.476 to 0.706; p < 0.01); and Dc was significantly logarithmically related to the silt content (R2 = 0.598; p < 0.01). In addition, the ranking of the influence degrees of different factors on the Dc variation was as follows: hydraulic characteristics > EPS > basic soil physical properties > basic soil chemical properties > plant roots. Among them, RLD, Clay, OM, σ0, and w were the indicators with the highest explanatory rates among the aforementioned different influencing factors in sequence. The path analysis further indicated that EPS and hydraulic characteristics were the significant factors affecting Dc, with the direct effect playing a dominant role. These findings enhance the understanding of soil erosion mechanisms and contribute to the development of soil erosion models for the Loess Plateau.
River runoff and suspended sediments exhibit high dynamic and multiscale variability, particularly during rainstorm-driven floods in arid and semiarid regions. Despite the widespread use of the hysteresis loop model, flood-event-scale hysteresis patterns and their controls remain poorly quantified in these environments. This study investigated the hysteresis patterns between runoff and sediment and identified the dominant factors affecting the hysteresis index (HI) in a typical semiarid catchment in northern China. Forty-eight flood events were classified by K-means clustering using the runoff depth (H), flood duration (T), and peak flood flow (Qp) into three types: A (low-intensity, medium-duration floods), B (medium-variability, long-duration floods), and C (high-intensity, short-duration floods). The monthly runoff and sediment load were decomposed via multivariate empirical mode decomposition (MEMD) to extract scale-specific dynamics, whereas event-scale hysteresis patterns were quantified separately to assess sediment transport mechanisms during floods. Figure-eight loops occurred most frequently (31.3%), followed by counterclockwise (29.2%), complex (22.9%), and clockwise (16.7%) loops. HI is governed by distinct hydrological factors for each loop type: flood peak timing skewness (FT) for clockwise loops; Qp, suspended sediment yield (SSY), mean suspended sediment concentration (Sm) and peak suspended sediment concentration (Sp) for counterclockwise loops; T and high traffic duration (HT) for figure-eight loops; and flood variability (FV) for complex loops. By combining MEMD-based scale decomposition with objective K-means event classification and HI quantification, we provide a transferable, scale-aware framework for diagnosing sediment transport regimes in semiarid basins.
