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• Hillslope erosion dynamic model integrating vegetation effect was established. • Unified baseline parameters were obtained across different land uses. • Effect of vegetation on soil detachment and sediment transport was revealed. • Contribution rates of vegetation to sediment reduction along slopes were quantified. Developing a process-based soil erosion model that comprehensively considers the effects of vegetation is complex but crucial for evaluating sediment reduction during vegetation restoration. Current understanding of the effect of vegetation on sediment reduction along hillslopes across different land uses is limited. In this paper, we developed a dynamic model of hillslope erosion that integrates the effects of vegetation on soil detachability and hydrodynamics (VED) based on the feedback mechanism between soil detachment and sediment transport. The VED was calibrated and validated with runoff plot data for woodland, grassland, and farmland in the Wuding, Yan, Jing and Wei Rivers of the Loess Plateau in China. Unified baseline parameters and decay coefficients of vegetation were obtained. The model validation results indicated that the coefficient of determination, Nash–Sutcliffe simulation efficiency and relative error ranged from 0.59 to 0.99, 0.56 to 0.90, and −45.99 % to 46.36 %, respectively. The decay coefficients for soil detachment capacity (Φ), sediment transport capacity (T c), and the sediment reduction effect of vegetation exhibited the following order: woodland > grassland > farmland. Compared with VED, existing process-based soil erosion models failed to effectively characterize the influence of vegetation on Φ and T c on the Loess Plateau, with differences of several orders of magnitude. The individual contributions of runoff, soil detachability, and hydrodynamics with vegetation impact on sediment yield were quantified by VED. The contribution rates of vegetation to sediment reduction decreased gradually with increasing slope length because the soil detachment capacity of bare slopes exceeded that of vegetated slopes, resulting in a faster increase in sediment yield. The contribution rates of vegetation to sediment reduction for woodland, grassland, and farmland with 20 %–60 % vegetation covers were 66.75 %–99.95 %, 57.43 %–99.63 %, and 27.91 %–88.63 % in the boundary conditions of this study, respectively. The results reveal the potential for integrating VED into other distributed watershed hydrological and sediment models to assess the effects of vegetation restoration on sediment reduction at a watershed scale. [ABSTRACT FROM AUTHOR]

• SY and Sr of the SL was the largest, but the sediment MWD was the smallest. • Change in the flow discharge did not affect the distribution pattern of sediment particles. • Dynamic coupling of runoff hydraulics and EG morphology drove soil loss. • SDR was the most direct influencing factor on the Sr in EGs. Ephemeral gully (EG) erosion represents the early stage of large-scale gully (locally named as "Benggang") development on Ultisols in the granite hilly area. However, the EG erosion process and the mechanism underlying sediment production have not been fully elucidated. In this study, a series of field scouring experiments were conducted under different flow discharge levels (10, 15, 20, 25, and 30 L·min−1), and the primary EGs in three typical soil layers with different weathering degrees (red soil layer (RL), transition layer (TL), and sandy layer (SL)) was dynamically monitored. The results indicated that the ratio of width to depth of the EGs rapidly decreased as the flow discharge increased. At 30 L·min−1, the average sediment yield and transport rate (Sr) of the SL were 4.85 and 5.39 times, greater than those of the RL, respectively. The sediment particles were concentrated within the 1–2 mm particle size range, accounting for over 35 %. In addition, the mean weight diameter of the sediment particles in the RL was the largest. The sediment distribution characteristics did not respond to flow changes but were affected by EG morphological changes. With increasing flow discharge, the flow shear stress and stream power increased. Further analysis revealed that the coupling effect of flow hydraulic characteristics and EG morphological evolution drove the EG erosion sediment production process, and the Sr of the SL was highly sensitive to flow discharge changes. The soil disintegration rate was an internal factor that directly affected soil loss in the EGs, with a path coefficient of 0.71. The bulk density, organic matter content, and sand content were all indirect influencing factors. The RL exhibited greater anti-erodibility than the other layers and could serve as an effective protective layer for granite hilly areas. These findings provide a theoretical basis for improving soil loss management systems. [ABSTRACT FROM AUTHOR]

• It is essential to utilize diverse sources of observational data for the validation of long-term morphological simulations. • The multiple underlayer settings of the Delft3D model enable its acquisition of decadal bed layer stratigraphy. • Wind and waves sort sediment based on grain size, smoothing out morphological changes induced by tides. • Sea level rise significantly influences regions characterized by intricate morphological features. • Radial sand ridges form under a radial regime with low tidal ellipticity, abundant sand, and minor cohesive sediment. The tidal sand ridges in the southwestern Yellow Sea are unique in the world characterized by radial morphology, significantly higher elevation above low water level, and fine-grained sediment deposits. Using a numerical model, we examined the morphological evolution of the radial sand ridge system from 1979 to 2059. The model was calibrated and validated using hydrodynamic and sediment transport data, seabed topography, and grain size distributions from sediment cores. Our findings reveal that tidal force is the primary driver of the ridges' evolution, and they will continue to expand in response to sediment supply from the abandoned Yellow River delta and the Yangtze delta, with the latter contributing more. The ridges are mainly composed of fine sand and mud, with finer sediment dominating in the north and coarser sediment dominating in the south. Wind and waves sort sediment, depositing sand on ridges and fine sediment in channels, leading to erosion of higher elevated lands and filling of deep channels. Sea-level rise increases regional erosion, which has a large impact on areas with complex morphology. The 500-year morphological evolution of sand ridge systems, simulated through an idealized model, illustrated that the formation of radial sand ridges is dependent upon specific conditions such as the radial tidal regime, sufficient sediment sand, and the presence of cohesive sediment. This study deepens our understanding of the long-term prediction of coastal morphology and interpretation of seabed stratigraphy, and offers insights for effective coastal management. [ABSTRACT FROM AUTHOR]

• Mass movement is an important erosion mode and sediment source in gully network. • Mass movement increased sediment concentration and led to peak sediment concentration. • Mass movement was prone to be triggered by rainfall with a medium-duration and moderate-intensity. • Rainfall amount and slope gradiet determined the trigger and total volume of mass movement. Mass movement erosion on the gully sidewalls is recognized as an important erosion mode and sediment source in hilly and gully watersheds. However, the trigger of mass movement erosion and transport of gravity-detached material remain poorly understood. A series of rainfall simulation experiments were conducted on six loess gully sidewall models, with a height (1 m), three rainfall (60 mm, 48 mm and 24 mm) and two slope gradients (70° and 80°), to quantitatively explore the sensitivity factor and transport mechanisms of mass movement. Results revealed that the accumulation volume and initiation of the mass failure were significantly influenced by rainfall amount. The accumulative volume of failure events was observed to increase in a linear pattern with increasing rainfall amount. The minimum cumulative rainfalls for the initiation of mass failure on the loess gully sidewalls were approximately 43.6, 28.2 and 65.6 mm for short-duration and high-intensity, medium-duration and moderate-intensity and short-duration and moderate-intensity rainfalls, respectively. This implied that the initiation of mass failure was more prone to rainfall events with a medium-duration and moderate-intensity than those with a high intensity and short duration. Rainfall amount and initial slope gradient were the most sensitive elements driving the number and volume of the mass movement erosion. Mass movement erosion raised sediment concentration, but the eroded material was unable to be transported by runoff in a timely manner. Average sediment concentrations at the model outlet increased by 1.9–11.9 times following the occurrence of mass failures compared to those before. A low instantaneous sediment delivery ratio of 0.032 for the mass movement erosion. The experimental findings help comprehend the influence of mass movement erosion on sediment supply on the gully sidewall in the hilly and gully network, i.e. the Loess Plateau. [ABSTRACT FROM AUTHOR]

• Relatively small changes in rainfall at the event scale were observed. • Anthropogenic activities play a leading role in hydro-erosive responses variation. • Saturation-excess overland flow is increasingly occurring. • Reduced sediment connectivity resulted from the increasing disconnected areas. China's Loess Plateau remains one of the most serious water erosion areas on Earth, while runoff and sediments of the Yellow River continue to decrease due to soil and water conservation (SWC) measures. Besides, climate conditions are dynamic. A total of 197 rainfall-runoff events from 1965 to 2020 in the Huangfuchuan catchment, which were divided into 3 sub-periods (P0: 1965 – 1982 limited measures; P1: 1983–1998 only dams and P2: 1999–2020 dams and revegetation), were selected to reveal the differences and mechanisms of runoff generation and sediment dynamics. The results show that: 1) anthropogenic activities played a leading role in water and sediment reduction; 2) infiltration-excess overland flow during the first two periods was dominant mechanism, but the results imply a potential shift towards saturation-excess overland flow occurring more often during P2; 3) the studied system, in a highly erodible region, is transport-limited; 4) sediment connectivity is reduced noticeably at the catchment scale, in which the decrease in index of connectivity (IC) values mainly resulted from the increase in proportion of the most disconnected areas of the catchment, especially in P2. These findings can help towards a better understanding of the mechanisms behind the changes in sediment dynamics, which can be applied to develop and implement sustainable water and soil conservation planning in the local or other similar areas. Besides this knowledge is very important for improving hydrological and sedimentological models in order to forecast the effects of human induced changes. [ABSTRACT FROM AUTHOR]

• Ridge slope erosion included inter-rill erosion, headward erosion, and rill erosion process. • Erosion rate reached the maximum during headward erosion process. • The dominated erosion process was detachment-limited after headward erosion occurring. • Re-detachment, transport, and deposition simultaneously occurred with the appearance of headward erosion. Extreme rainfall is becoming more frequent and intense, which no doubt increases the risks and uncertainty of water erosion in widely-used contour ridge systems. Ridge slope erosion can increase the probability of contour failure occurrence and then induce contour ridge losing anti-erosion properties. However, there is little understanding of ridge slope erosive process controlled by contour ridge system responding to extreme rainfall. In this study, five rare earth elements were applied in different ridge segments and soil layers to monitor the successive development of ridge slope erosion process under extreme rainfall in contour ridge system. Simulated rainfall experiments at high rainfall intensity (100 mm h−1) lasting for 60 min were conducted in runoff plots with ridge tillage practices characterized by microtopography indices and ridge geometry indices. Results showed that three sub-processes were observed for ridge slope erosion, including inter-rill erosion, headward erosion, and rill erosion. Headward erosion mainly resulted from the break-point at ridge surface and sidewall collapse at ridge toe, and the pouring of ponding water in furrow greatly promoted rill development. Erosion rate was the largest during headward erosion process, and rill erosion contributed the most to eroded source. The dominated erosion process was transport-limited during inter-rill erosion process, and detachment-limited during headward and rill erosion processes. Sediment transport and deposition processes as well as their transformation occurred simultaneously after the development of headward erosion. Eroded sediment was mainly from the low part of ridge, which was transported by raindrop-impacted sheet flow or overflow. These findings provide better responding of agricultural tillage to global climate change and references for the development of physically-based erosion models. [ABSTRACT FROM AUTHOR]

• The sediment transport capacity was developed for loessal slopes. • Two-dimensional erosion numerical model was developed for loessal slopes. • Erosion numerical model could accurately simulate slope runoff and sediment process. Simulated slope erosion sediment yield and its development process have great significance for quantitative evaluation of erosion and its sources. In this study, a loess slope erosion experiment was implemented indoors to establish a sediment transport capacity formula suitable for loess slope erosion. A two-dimensional slope erosion numerical model was developed with corrected sediment transport capacity formula, and the model was verified by a simulated indoor slope rainfall erosion experiment. The results showed that the corrected slope sediment transport capacity formula is suitable for loess slopes. The proposed two-dimensional slope erosion numerical model could simulate runoff and sediment processes in a relatively more accurate. The most severely eroded areas were the middle and lower parts on erosion spatial contribution based on the simulation erosion results. Good agreements between the simulated and observed results were found for erosion and its spatial distribution. The research results can further improve loessal slope erosion process simulation and prediction. [ABSTRACT FROM AUTHOR]

• A mathematical model considering the operation of 24 mega reservoirs was proposed. • The sedimentation mainly occurs in the mainstream during the flood season. • The cascade reservoirs operation leads to easier and prolonged sedimentation. The intensive construction of large dams in the upper Yangtze River Basin (UYRB) has formed large-scale cascade reservoirs and obviously altered the flow regime and sediment transport of the basin. However, few studies have quantitatively described the cumulative impact of these reservoirs on sediment interception. In this study, a flow and sediment transport model that fully considers the operation processes of 24 mega reservoirs in the UYRB was established, and the spatial and temporal variations in sedimentation from 2011 to 2080 were elucidated, assuming repeated series of daily input flow discharge and sediment concentration during 2011–2020 and monthly water operation levels of 24 reservoirs during 2018–2021 as boundary conditions. The results showed an annual average sedimentation of more than 300 Mt in the UYRB during 2011–2060, which falls to 270 Mt in the period 2061–2080, and more than 80% of the sedimentation occurs in the flood season. Nearly 80% of the total sedimentation during 2010–2080 was observed in the mainstream reservoirs, especially the four mega reservoirs in the lower Jinsha River and the Three Gorges Reservoir (TGR), which accounted for 31.9% and 22.7% of the total sedimentation, respectively. The proportion of sedimentation in mainstream reservoirs will further increase with increasing operation periods. Most reservoirs in the UYRB will not reach sedimentation equilibrium by 2080, except for reservoirs in the Min River Basin. The ratio of siltation capacity to average annual sediment yield determines the percentage of residual siltation capacity of each reservoir. The operation of the cascade reservoirs in 2013–2020 had no more immediate impact on the middle and lower Yangtze River than that in 2003–2012, when the TGR started impoundment. However, the joint operation of these cascade reservoirs has changed the spatial distribution of sedimentation in the UYRB, and will lead to prolonged impacts on downstream channel erosion. [ABSTRACT FROM AUTHOR]

• Effective sediment sizes were determined along a gradient of soil structural stability. • Sediment average size exhibited a unimodal variation with soil structural stability. • Suspension-saltation dominated sediment transport for low and high stable soils. • Suspension-saltation and rolling jointly dominated erosion for medium stable soils. • Soil structural stability determined rainfall intensity effect on sediment transport. Sediment transport patterns (suspension, saltation and rolling) not only depend on flow hydrodynamic characteristics, but also on sediment properties, both of which are largely influenced by soil structural stability. However, how sediment sizes and transport patterns response to original soils over a wide range of structural stability remains unclear. Herein, field plot rainfall simulation experiments on five soils along an increased gradient of structural stability were conducted at two rainfall intensities of 45 and 90 mm h−1 to investigate effective sediment size characteristics and associated transport patterns in sheet erosion. Differing from runoff and sediment yield, sediment sizes were most influenced by soil structural stability (F = 386.9) rather than rainfall intensity (F = 246.7, p < 0.001). Mean weight diameter of sediment (0.16–0.84 mm) and the proportion of coarse particles (>0.25 mm) exhibited a unimodal variation with increased soil structural stability. The effective sediment size distribution shifted from unimodal with the peak at < 0.10 mm for low and high stable soils to bimodal with two peaks at < 0.10 mm and 0.5–1 mm for medium stable soils. Correspondingly, suspension-saltation (>74 %) dominated sediment transport for low and high stable soils, which was determined by fine sized fragments from aggregate breakdown and small flow transport capacity, respectively; while both suspension-saltation (<0.10 mm) and rolling (>0.25 mm) jointly contributed to sediment transport for medium stable soils, which was controlled by the interaction between fragment size and flow transport capacity. Moreover, the positive effect of rainfall intensity on sediment sizes was more remarkable for the medium than for the low and high stable soils. A conceptual model of sediment transport mechanisms in response to soil structural stability is firstly proposed, which merits consideration in the future development of erosion modeling and soil conservation techniques. [ABSTRACT FROM AUTHOR]

• Demonstration of rise and fall of carbon discharge with time. • Demonstration of enrichment of sand in loose layer above soil. • Demonstration of no carbon enrichment is sediment at the steady state. Soil carbon losses from sheet and interrill erosion areas have an impact on the global carbon cycle. Sediment discharged in many experiments where artificial rainfall has been applied to sloping soil surfaces show greater proportions of carbon in the sediment than found in the cohesive soil source. In many cases, conclusions drawn from these experiments fail to recognise that the enrichment of carbon in the sediment discharge is a result of unsteady conditions that occur before the steady state develops. It is known that particles of sand of different sizes move at different rates when raindrop induced salation occurs with the result that initially the particle size distribution is finer than the source but tends towards that of the source as time progresses. The presence of carbon in aggregates influences the density of aggregates. Here a qualitative model of sediment transport of sand and coal particles by raindrop induced saltation is used to demonstrate time varying enrichment of coal resulting from the effect of carbon on particle settling velocity. The model also demonstrates how the layer of loose particles that develops on top of the cohesive soil surface influences the relative amounts of carbon and sand in the sediment discharges. The steady state is not achieved until the slowest moving particle detached at the farthest distance from the downslope boundary is discharged. That may not be achieved in many of the experiments reported in the literature. [ABSTRACT FROM AUTHOR]

[Display omitted] • Temporal evolution of fine sediment deposits on a gravel bar during a two-year period. • Simple physical-based models introduced to better understand fine sediment dynamics. • Flow-related events can yield thick deposits or erosion depending on the discharge-concentration time-series. • Strong wind-related events disperse fine sediments and yield erosion. • Limitation of the image-analysis due to the seasonal variability of the image quality. In this study, the temporal evolution of fine sediment deposits on a gravel bar is analysed during a two-year period. The time-series of fine deposit surface area is obtained using the methodology proposed in Part 1 of this work. By combining the hydro-meteorological data, the main evolutions of the fine deposits observed on the bar surface are found to be triggered by events such as natural floods, dam flushes, and strong wind valley breezes, and are quantified for each of these events. A simple processed-based flow model is proposed to investigate the sediment dynamics during the flood events, which balances the erosional/depositional processes of fine sediments. Variability in the fine deposit evolution after floods is mainly due to the different flow and sediment concentration conditions. For small events for which the gravel bar is not totally submerged, a slight erosion generally occurs. For larger events for which the gravel bar is totally submerged, a large net deposition is often observed. The fine deposit surface area time-series also shows sharp decreases of the surface deposit coverage after some strong valley breeze events, with an evolution of the same order of magnitude than floods. The application of a wind transport model shows that strong wind valley breezes can transport fine particles from the bar surface to the river channel, thus leading to a loss of fine deposits on the bar surface. Also, the deposit initial moisture is a crucial element having impact on the fine sediment transport on the bar surface. Dry deposits are much more likely to be transported than wet deposits. A discussion is also provided on the potential impact of wind and vegetation on the long-term dynamics of fine sediment deposits. [ABSTRACT FROM AUTHOR]

• Mudstone deposits behave like noncohesive sediments in terms of erosion resistance. • High erosion resistance of laterites cannot be predicted merely by particle size. • Shields' parameter predicted by water content, clay content, and plasticity index. Soil erosion and sedimentation are major factors in landscape evolution and river morphology. The erosion resistance of sediments plays an important role in landscape evolution processes because it determines the threshold of an acting force at the initiative sediment motion, and the amount of sediment transports. This study examined the sediment properties and their correlations to the erosion resistance of mudstone deposits and laterites, which are two representative natural cohesive soils in hill and terrace topography in Taiwan. Soil physical properties and pore water chemical properties, and the erosion resistance were investigated and determined using laboratory tests and hydraulic flume experiments, respectively. Investigations of sediment properties showed that the mudstone deposits and laterites had significant differences in median particle size, soil texture, level of plasticity, internal structure, and pH value and electrical conductivity of their aqueous slurry. Results from the hydraulic flume experiments showed that the laterites had stronger erosion resistance, indicating by larger values of critical shear stress and the Shields' parameter, than the mudstone deposits. Through the stepwise multiple regression analysis, a quantitative equation was proposed to estimate the Shields' parameter using fine sediment properties including water content, clay content, and the plasticity index expressed in decimal fractions, of the two soils. Despite more experiments of the two natural soils with different ranges of the sediment properties are needed to validate the empirical equation for predicting the Shields' parameter, the proposed methodology provides an essential reference in which erosion resistance is an important parameter. [ABSTRACT FROM AUTHOR]

[Display omitted] • Total volume and peak discharge are the most important drivers of sediment transport. • Antecedent moisture condition is correlated only with the initial discharge of floods. • Hysteresis loops between SSC and Q are analyzed with the hysteresis index (HI). • Negative HI are predominant indicating delayed SS transport and distant SS sources. • In the catchment soil erosion from hillslopes is the dominant process. This study aimed to evaluate the drivers of suspended sediment (SS) transport by analyzing the flood events recorded from July 2010 to June 2011 in the Celone River basin (Apulia, S-E Italy). High-resolution data of rainfall, streamflow (Q), and measurements of suspended sediment concentrations (SSC) were used to carry out the multivariate analysis. Among the initial 18 variables, the Principal Component Analysis revealed that total volume (m3), peak discharge (Qmax, m3/s), and the flood duration were the most important factors influencing SS transport. Total rainfall and total kinetic energy recorded in the gauging station located in the lowland were also important factors, whereas the same rainfall variables recorded in the remote mountainous area of the basin had a minor influence on SS transport. Antecedent moisture conditions were significantly correlated only with the initial discharge of the flood. Hysteresis loops between SSC and Q were analyzed for each flood event by using the hysteresis index (HI). Results showed that this catchment was dominated by anticlockwise loops (negative values of HI) representative of delayed SS transport and of distant SS source areas in terms of the gauging station. These results also suggested that soil erosion from hillslopes was the dominant process compared with channel and bank erosion and that there was no exhaustion of sediment availability in the basin. [ABSTRACT FROM AUTHOR]

[Display omitted] • The SWAT model supports better management of erosion processes. • Computational models can be used with methods of visual recording erosion. • Simple statistical models aligned to SWAT is adequate to analyze erosive processes. • The spatially distributed erosion identifies the susceptible areas to water erosion. Water-driven erosion associated with the transport and deposition of sediments in watersheds may lead to contamination problems causing significant damage to land and infrastructure. The key factors that influence erosive processes are well known and include the precipitation rate, terrain topography, soil type, and vegetation cover. The soil and water assessment tool (SWAT) considers these factors and may be used to identify areas susceptible to soil erosion and to estimate soil loss and sediment transport and deposition. To corroborate this assumption, real data on rill erosion and sediment transport from the Mogi Guaçu River Basin in southeastern Brazil were used to validate SWAT estimates. The study shows that the model is able to identify soil-eroded areas and replicate the scale and variation patterns of sediment production, although not with great precision. Hence, it can successfully be used for prioritizing areas that require action and for evaluating alternative soil management and conservation practices to mitigate erosive processes. [ABSTRACT FROM AUTHOR]

[Display omitted] • Coherent dataset on specific sediment yield for 30-year' period (1990–2019) • Strong variation in sediment yield along the Peruvian western Andes. • River discharge and river channel steepness explain 55% of the variance in sediment yield. • Spurious correlation between land cover, river discharge and topography. • Empirical data on sediment yield during extreme events are needed. The tropical Andes has been less studied in terms of erosion processes in comparison to other major mountain ranges in the world. Environmental gradients are steepest along the western flank of the Andes that is characterized by marked differences in vegetation, rough topography with deeply incised canyons, and highly variable and extreme precipitation patterns. Previous efforts mostly focused on sediment fluxes in large rivers draining to e.g. the Amazon basin while small to medium-sized rivers such as the ones flowing towards the Pacific Ocean have been relegated. They highlighted the link between sediment yield, anthropogenic and natural factors, e.g. climate, topography, river runoff, lithology and vegetation cover. In this study, we identified the spatial patterns of specific sediment yield along the western slopes of the Peruvian Andes between 3° and 13° S latitude for 21 catchments. We collected and analysed data from 22 environmental factors to elucidate their importance on spatially varying sediment yield. The sediment load was derived from gauging stations, reservoir sedimentation and water turbidity over a 30-yr period. The specific sediment yield varies strongly along the Peruvian western Andes as a consequence of the spatial variation in climate, topography and land cover controlling sediment production and transport. We reported higher-than-average specific sediment yields for the central part (6°-11°S) with values of 2130 and 2300 t km−2 yr−1 and low and uniform yields of 39 to 551 t km−2 yr−1 in the southern part (11° − 14.5° S). Given the scarcity of data on sediment yield, we included an uncertainty assessment based on bootstrapping approaches as to get a better grasp on the potential range of specific sediment yields in the study region. Using statistical techniques including Spearman correlation rank, univariate and multivariate regression analyses, we were able to determine the importance of the 22 environmental variables on the specific sediment yield. About 55 % of the observed variance can be explained by river discharge (Q 90) and river steepness index (ks 50). By adding an anthropogenic variable based on land cover, the explained variance in SSY increases up to 63 %, however, the effects of land cover on specific sediment yield are not clear because of spurious correlation between land cover, river discharge and topography. Our study therefore provides important new insights in the ongoing scientific debate on sediment yield variability in the western Andes. [ABSTRACT FROM AUTHOR]

• • Thorough analysis of WSRS-induced impacts on the river channel and delta. • • The natural hydrological cycle process has been altered by the artificial floods. • • Reservoirs regulation scheme altered the erosion–deposition regime of the riverbed. • • New light is shed on saving the global sinking deltas. The water and sediment regulation schemes (WSRS) orientated by dams have profoundly adjusted the hydrological process and geomorphological evolution of the lower Yellow River and delta. Two decades since the implementation of WSRS, the observed multidisciplinary datasets allowed us to systematically evaluate the impacts of this unprecedented engineering effort on the river-coastal system. The sediment fluxes transported to the sea showed a notable rebounded and coarsened process due to the erosion of the riverbed during the WSRS periods. The average median grain size of suspended sediment increased from 17.7 μm in the 1990 s to 26 μm in 2002–2009. The natural rhythm of hydrological process is fundamentally altered by WSRS, about 50% and 30% of annual sediments and water delivered to the sea for only 20 days. The water discharge system is more temporally dynamic and complex with higher dimensionality after the operation of WSRS. The erosion pattern has been a dominant feature in all the channel segments downstream the Xiaolangdi Reservoir, with an average total erosion volume of 1.02 × 108 m3/a after the WSRS. However, the erosion efficiency of the lower river showed a decreased trend, due to the riverbed had formed a coarse armored substrate for the continued scouring since 2002. The YRD has changed from the pattern of erosion to progradation for the increase of sediment supply, and the average growth rate of subaerial delta was about 6.89 km2/a during 2003–2014. The lessons learned from the operation of WSRS may provide a valuable reference for the management of global large river systems. [ABSTRACT FROM AUTHOR]

• Soil erosion process on frozen-stony slope is experimentally investigated. • Sediment concentration on frozen-stony slope follows linear-exponent curves. • The impacts of thawed soil depth and rock fragment content on sediment transport process are quantified. Rill erosion on frozen-stony slope in high-altitude or high-latitude areas commonly undergo freeze–thaw cycle, which is significantly influenced by partially thawed soil with rock fragment. This research aims to investigate the influences of thawed soil depth (TSD) and rock fragment content (RFC) on rill erosion and quantifying the mathematical relationships among them. The laboratory experiment is conducted to evaluate the influences of TSD (1, 2, 5, and 10 cm) and RFC (0%, 10%, 20%, and 50%) on sediment delivery processes by concentrated meltwater flow under three slope gradients of 5°, 10°, and 15°, and three flow rates of 1, 2, and 4 L min−1 at nine rill lengths from 0.5 m to 8.0 m. Five runoff samples are obtained from each experiment run and used to calculate sediment concentration (SC). The SC increases linearly and then exponentially with rill length to approximate the extremum and fits well with piecewise functions. Restrictive effect of TSD and RFC on sediment supply is evident during linear stage induced by shallow TSD, which keeps a long distance under shallow TSD and low RFC to collect sufficient sediment for exponential period. However, TSD and RFC impacts become apparent during the exponential period and lead to higher sediment yield with RFC of 10% and 20% than that on totally thawed soil without rock fragment. In this research, the positive effect on sediment yield increases with RFC increases from 10% to 20%, which may need to be eliminated for efficiency in erosion control. In contrast to low RFC, the protection impact of rock fragment always exerts with RFC of 50%. The experimental data and the functional relationship can provide the basic knowledge for evaluating the model parameters such as soil detachment rate and erodibility of rill erosion on frozen-stony slope. [ABSTRACT FROM AUTHOR]

[Display omitted] • Dominant sediment source in the MYR changed from upstream input to fluvial erosion and tributary input. • Sediment supply amount accounted for 21% of sediment transport capacity in the MYR. • Bank erosion contributed to 9–18% of the sediment amount transported in the MYR. Sediment transport in alluvial rivers is critical to reshape channel morphology and to the global geochemical cycle. This study reports a detailed investigation into the contributions of different sources to sediment transport in the Middle Yangtze River (MYR). It is found that: (i) after the operation of the Three Gorges Project (TGP), the dominant sediment source in the MYR changed from the upstream input to the fluvial erosion and tributary input, with the upstream input providing 38% of the outlet sediment amount in the MYR and the fluvial erosion and tributary input providing around 39% and 23%, respectively; (ii) river bed erosion contributed to 21% of the transported sediment after the TGP, whereas bank erosion contributed to around 18%, which was estimated to be 2 times larger if bank protection works were not constructed after the TGP; (iii) in terms of the sediment transport processes of different fractions after the TGP, the fraction of fine sediment (diameter d ≤ 0.31 mm) in the MYR mainly came from the upstream and tributary input, and the fraction of medium sediment (0.31 mm < d ≤ 0.125 mm) almost equally came from each source, whereas the fraction of coarse sediment (d > 0.125 mm) was primarily eroded from the bed and banks. Besides, the MYR was transformed from a transport-limited river system into a supply-limited one after the TGP operation, with the total amount of sediment supply accounting for 21% of its transport capacity under current flow and boundary conditions. The development of bed armoring in the main stream and the construction of various river regulation works, would result in a further decrease in the sediment supply rate of the MYR in the future. [ABSTRACT FROM AUTHOR]

• An integrated 1D morphodynamic model was proposed covering three different modules. • Bank erosion and accretion processes in the braided reach of the LYR were simulated. • Large-scale bank deformation played an important role in the channel evolution. The braided reach of the Lower Yellow River (LYR) often experienced dramatic channel evolution during flood seasons, with both bed and bank deformations occurring simultaneously, which led to several negative effects such as land loss, channel migration and overbank flooding. A coupled one-dimensional model was proposed to simulate both bed evolution and bank deformation, including three modules of flow-sediment transport, bed deformation, and bank erosion and accretion. The proposed model was applied respectively to simulate the longitudinal and lateral channel deformations in the braided reach in the years of 2018 and 2006, in order to conduct detailed processes of model calibration and verification. The results indicate that: the proposed model can accurately simulate the flow and sediment transport processes, and the majority of the Nash-Sutcliffe efficiency coefficients (NSEs) were larger than 0.60 between the calculated and measured hydrographs of water discharge, water level and sediment concentration at different sections, with the maximum value up to 0.96; the calculated erosion and accretion widths of the riverbanks closely agreed with the measured data, and the absolute errors between the calculated and measured post-flood reach-scale bankfull widths were 18 m in 2018 and 14 m in 2006, with the relative errors of 1.7% and 1.3%; the effects of riverbank deformation on sediment discharge, flow velocity and channel deformation were also analyzed, which indicate that the module of riverbank erosion and accretion can play a significant role in predicting hydrodynamic and morphodynamic processes in the braided reach of the LYR. [ABSTRACT FROM AUTHOR]

• Differential travel rates cause particle size distributions to vary with time. • Sediment discharges peak at drop size dependent flow depths. • Sediment concentrations increase before declining as flow depth tends to zero. In laboratory experiments undertaken using sand on a sloping surface in the 1970s, it was observed that while fine material moved as suspended load, fine to medium sand was induced to saltate as the result of raindrop impacts whereas coarse sand and gravel were induced to roll. The particle size distribution of the sediment discharge was initially much finer than the original bed and coarsened with time to approach that of the bed as time passed. At the time the exact nature of the raindrop induced transport mechanisms was not well known. Later, experiments using an apparatus that enabled the depth and velocity of rain-impacted flow to be controlled over an eroding surface, showed that raindrops induced some particles to travel limited distances that depended on a number of factors including flow velocity and particle size and density. The resultant differential travel rates of particles of different size and density provided an explanation as to why particle size distributions of sediments discharged by rain-impacted flows vary temporally, and why a non-cohesive layer develops on top of the surface of the cohesive matrix soil. The formation of the non-cohesive layer is indicative of erosion being transport limited when raindrop induced saltation occurs. The experiments also showed that sediment discharges produced by rain-impacted flows peaked at a drop size dependent flow depth so that a function that accounts for the interaction between raindrop size and flow depth could be developed for drops impacting at close to their terminal velocities. One of the implications of the observed responses to variations in rainfall and flow conditions is that the assumption that rain has certain erosive power that is defined simply by the product of flow discharge and rainfall factors such as rainfall intensity or the kinetic energy flux is a false assumption. [ABSTRACT FROM AUTHOR]

• Under the action of concentrated flow, the change of surface runoff of karst gently sloping farmland is mainly controlled by the magnitude of concentrated flow. • Under the action of concentrated flow, the underground runoff of karst gentle slope farmland is significantly negatively correlated with the slope. • Concentrated flow can lead to severe soil erosion in karst gently sloping farmland. Exposed bedrock in karst areas considerably alters runoff paths causing partial runoff to converge into a concentrated flow, which is a potential source of soil erosion risk. However, research is lacking on how concentrated flow affects regional soil erosion. In this study, the surface and underground dual structure in karst areas was simulated in a steel tank. Combined with runoff plot monitoring, the runoff conversion process and erosion characteristics of gently sloping farmland in karst areas under different water discharge rates were examined through indoor concentrated flow discharge scouring tests, and the influence of concentrated flow changes on the runoff spatial distribution and sediment yield in gently sloping farmland was clarified. The results demonstrated the following: (i) there existed a significant positive correlation between surface runoff and the water discharge rate (p < 0.01); the partial correlation coefficient value reached 0.87. The higher the water discharge rate, the more surface runoff formation was facilitated. (ii) When the slope remained the same, the underground runoff was relatively high at 3 L/min. Given the same discharge, a significant negative correlation between underground runoff and slope was found (p < 0.05); the partial correlation coefficient value reached −0.71. (iii) In response to a water discharge rate of 5 L/min, surface soil erosion was serious, and the maximum sediment transport modulus was 174.088 g/min∙m2. A significant cubic function relationship existed between the scouring time and surface sediment yield. Overall, a critical flow rate occurred between 3 and 5 L/min, which controlled changes in surface runoff, subsurface runoff and surface sediment yield. Under highly concentrated flow, drastic soil erosion could occur. These results can provide a scientific basis for the control and prevention of concentrated flow-induced erosion in karst areas. [ABSTRACT FROM AUTHOR]

• Rill erosions impacted by saturation & subsurface water flow are compared. • Method is suggested to quantify rill erosion affected by subsurface water flow. • A method is evaluated to determine rill erosion function with few data points. • Rill erosion models are applicable to slopes with subsurface water flow. Understanding rill erosion processes on purple soil slopes with subsurface water flow at different depths is of great significance to determine rill erosion model parameters. The purposes of this study were to quantify the influences of subsurface water flow at different depths on the rill erosion process. Laboratory experiments were carried out to measure sediment delivery under 5°, 10°, 15°, and 20° slope gradients, 2, 4, and 8 L min−1 flow rates and 0, 5, 10, and 15 cm subsurface water flow depths (SWFDs) below the soil surface at eight slope lengths of 1, 2, 3, 4, 5, 6, 7, and 8 m. The results showed that the rill erosion prediction model is applicable to rills on purple soil slopes with subsurface water flow at different depths. The effect of the slope gradient on the sediment concentrations is greater than that of the flow rate under various SWFDs. Slopes with shallower subsurface water flow depths from the soil surface produced higher soil erosion and sediment delivery, and the sediment concentrations of rill erosion under the impacts of SWFDs of 5, 10, and 15 cm from the soil surface were 61%, 50%, and 41% of that of the saturated soil slope, respectively. Moreover, the rill erosion process can be well simulated with sediment concentration data from 1 to 4 or 1–8 m rill lengths when the maximum sediment concentration at the sediment transport capacity level is determined. Our results provide a simplified and accurate method for predicting erosion processes with few data and reveal rill erosion processes under the effect of subsurface water flow. [ABSTRACT FROM AUTHOR]

• Changes in spatial variation of rock fragment contents (SVRFC) due to tillage altered hydrological and erosional processes. • Yields of subsurface flow and deep percolation increased with decreasing SVRFC. • The decreasing SVRFC caused a decrease in surface erosion, but an increase in subsurface erosion. • Effects of rainfall intensity on hydrological and erosional processes were associated with SVRFC. Cultivation activities play an important role in shaping the spatial distribution of rock fragments in hilly agricultural landscapes. The spatial variation in rock fragment contents (SVRFC) can exert strong effects on the spatial heterogeneity of soil properties with great implications for hydrological fluxes and soil erosion. However, previous research always assumed that rock fragments were homogenously distributed, but little has been known concerning the SVRFC over the hillslope and its influence on hydrological and erosional processes. In this study, simulated rock fragment contents (RFCs) in soil layers at the upper (0.6 kg kg−1) and lower slope positions (0, 0.2, 0.4 and 0.6 kg kg−1) were established in combination with artificial rainfall experiments, to investigate the influence of RFC difference between the upper and lower slope positions on hydrological and erosional processes. Results show that the decreasing RFC difference reduced the yield of surface flow (5–100%), but increased yields of subsurface flow (58–792%) and deep percolation (19–130%). Additionally, the decreasing RFC difference reduced the runoff velocity of surface flow and surface erosion rate, but aggravated subsurface erosion. Especially under the 0 kg kg−1 RFC difference, the intense soil loss occurred at the subsurface which may be due to the highly developed preferential flow. With increasing rainfall intensity, great increments in yields of subsurface flow, deep percolation and subsurface sediments were found under the small RFC difference which had small increments in yields of surface flow and surface sediments. These results suggest that the changing SVRFC related to tillage erosion can exert strong influence on behaviors of hydrological and erosional processes. Our study provides a better understanding of the interaction between agricultural cultivation and soil–water loss in hilly agricultural landscapes. [ABSTRACT FROM AUTHOR]

• Upslope sediment-laden flow caused additional erosion in ephemeral gully zone. • Transport-dominated sediment regime was found under filed and laboratory conditions. • An increase in sediment concentration of inflow decreased downslope soil erosion rates. • Laboratory simulations verified the field monitoring results for extreme rainfall events. Ephemeral gully (EG) is an important linear erosion feature, often occurring at mid-slope position, that can be greatly influenced by upslope inflow and its sediment transport capacity. How sediment concentration in upslope inflow influences downslope EG erosion is still unclear. Thus, field monitoring on a natural hillslope and indoor simulations on a dual-box system were conducted to better understand their relationship. The results showed that both field monitoring and indoor simulation displayed the additional sediment delivery caused by upslope sediment-laden flow, verifying the transport-dominated sediment regime in EG systems. In field observations, the sediment increment coefficient (SIC , ratio of net sediment delivery caused by upslope sediment-laden flow to the total sediment delivery) on an event scale varied from 4.6% to 88.6%. In indoor simulations, the SIC changed from 24.9% to 87.5%. Indoor simulation experiments with a range of 0–378.2 g L−1 sediment concentration supplied by a feeder box, showed that SIC linearly decreased as sediment concentration of upslope inflow increased. Field monitoring showed complicated phenomenon because of natural random variations. For rainfall events with PI 30 (the product of precipitation and maximum 30-minute rainfall intensity) larger than 400 mm2 h−1 (similar to the designed extreme rainfall in laboratory simulations), SIC also generally decreased as sediment concentration of upslope inflow increased, especially when sediment concentration of upslope inflow was smaller than 400 g L−1. Thus, laboratory simulations verified the field monitoring results of extreme rainfall events with large rainfall amounts and intensities. This study illustrated the necessity of in-situ field monitoring and indicated that design of indoor simulation experiments needs more comprehensive considerations on complicated hydrological and sediment conditions. [ABSTRACT FROM AUTHOR]

[Display omitted] • A theoretical framework was used to explain dynamic scale effect on soil erosion. • Scale effect on soil erosion varies with erosion types, rainfall, and soil properties. • Slope length factor of USLE/RUSLE performs better on rill erosion dominant slopes. The variation of soil erosion across scales remains a controversial issue. A theoretical framework, coupling the normalized Green-Ampt equation for infiltration, 1D kinematic wave model for overland flow, and WEPP erosion modeling approaches for soil erosion, was used to explain and quantify the direct effect of scale on the soil erosion process. The results show that the relation between interrill erosion and slope length accords with a power-law decreasing trend, while the relation of rill erosion versus slope length shows a power-law increasing trend. Moreover, the power-law scaling of interrill erosion becomes more prominent with an increase of rainfall duration and intensity but is nearly independent of the interrill erodibility factor. The rainfall duration, soil type, soil critical shear stress, sediment transport coefficient, and rill erodibility factor substantially influence the magnitude of scale effect on rill erosion, while rainfall intensity, slope gradient, and Manning's roughness have minor effects. It suggests that the scale effect on soil erosion is variable and dynamic for different erosion types, rainfall and soil characteristics, which can explain why the controversial scaling relationships of soil erosion exist in previous studies. The physically based soil erosion model WEPP is compared to the empirical models USLE and RUSLE at various scales. Only when rill erosion plays a dominant role in soil loss do all these models exhibit consistent scale effect on erosion. These results provide insights into the essence of scaling in the hillslope soil erosion process, which are expected to benefit the development of upscaling techniques for the large-scale hydrological, geomorphological, and ecological processes. [ABSTRACT FROM AUTHOR]