Bed Erosion Research Articles

Numerical investigation of nonlinear interaction between salinity- and sediment-induced stratification in highly turbid estuaries and coastal seas

ABSTRACT In highly turbid estuaries and coastal seas where freshwater rivers meet the salty ocean, both salinity-induced stratification (SaIS) and sediment-induced stratification (SeIS) are crucial in shaping total estuarine stratification. This study investigates the nonlinearity emerging from the interaction between SaIS and SeIS using a one-dimensional water column model over an erodible bed, driven by a longitudinal salinity gradient and oscillating tidal currents. The results reveal that total stratification deviates significantly from a simple addition of SaIS under clearwater conditions and SeIS under freshwater conditions, indicating a highly nonlinear interaction. The complex spatial and temporal patterns of the nonlinearity arise from the differing source-sink dynamics of salt and sediment. SaIS reduces SeIS during flood but intensifies it during ebb, whereas SeIS consistently strengthens SaIS. This nonlinearity, which exhibits notable flood-ebb asymmetry, is relatively insensitive to changes in tidal forcing compared to salinity gradient forcing, due to the conflicting effects of tidal currents on salinity straining, sediment diffusion, and turbulent mixing. These findings highlight the necessity of incorporating sediment's modulation of classical strain-induced periodic stratification due to salinity gradients in numerical models to improve predictions of buoyancy and stability in highly turbid estuarine and coastal systems.

The blockage and erosion characteristics of woody debris flow on an erodible gully bed: Insight from a small-scale model experiment

During the transportation process of debris flow with large wood (LW), phenomena such as channel blockage and collapse frequently occurs, resulting in increased discharge surges, heightened erosion intensity, and amplified damage. Accurately predicted the blockage performance is the basis of evaluating the damage and disaster mitigation of woody debris flow. In this study, we conducted a series of laboratory experiments of woody debris flow on erodible gully bed. The experiment results show that the final blockage types can be divided into three types: non-blockage, blockage, and semi-blockage. Temporary blockage will cause abundant sediment deposited temporarily and then released instantaneously, resulting in destructive surges and eventually lead to semi-blockage and non-blockage. The blockage degree is positively correlated with the relative length, relative content of LW, and bulk density of debris flow, but negatively correlated with slope. Channel blockage is often accompanied by significant local erosion effect, and the erosion depth of downstream channel increases with the increase in blockage degree. The blockage and collapse mechanism of woody debris flow was analyzed, and the results emphasized that channel erosion promoted the outbreak of blockage collapse. Based on the analysis of blockage performance, we propose an improved blockage criterion F to evaluate the blockage degree, and the high probability range of temporary blockage is determined as 1.5–5.0. The results can provide reference for the risk assessment and mitigation of woody debris flow.

Performance and sensitivity analysis of the Morpho2DH model in extreme events in southern Brazil

Debris flows are a mixture of water and sediment with a continuous fluid behavior driven mainly by gravity. Computational modeling can be used to predict areas susceptible to these phenomena. When using computational modeling, sensitivity analysis is a crucial step to evaluate and interpret the model results. Thus, the objective of this study was to evaluate the performance of the Morpho2DH debris flow model, perform a sensitivity analysis of its input parameters, recommend application criteria, and increase the use of such a model in South America and Brazil. The model was applied in two representative areas with occurrences of debris flows in southern Brazil. The methodology consisted of two steps: (i) model calibration; (ii) one-at-a-time (OAT) sensitivity analysis (SA) for selected input parameters. The sensitivities of input parameters were evaluated for total area affected, distance traveled, mean velocity, and deposited volume. The results of the SA demonstrate that the most sensitive parameters were maximum bed erosion depth, resistance coefficient, minimum flow depth, and mean particle diameter. Meanwhile, static bed sediment concentration, vegetation, and internal friction angle showed moderate sensitivity. Parameters such as liquid density, sediment concentration, and sediment density showed the lowest sensitivities. Finally, it is recommended to use the model for situations where input volume and terrain data (mainly the erosion thicknesses or soil depth) are well-defined. Additionally, the computational cost of this model should be considered, concerning the magnitude of the event to be simulated.

Grain-size distribution in suspension through open channel turbulent flow using space-fractional ADE

The main aim of the present study is to develop a mathematical formulation that can describe the grain-size distribution (GSD) of non-uniform sediments in suspension over erodible sediment beds in a wide open channel under non-equilibrium conditions. The two important characteristics of sediment particles, such as non-local mixing and the hiding-hindering effect during settling, has been considered in the current study. The traditional advection–diffusion equation (ADE) is taken in the modified form as fractional ADE which is capable of capturing non-local movement of particles unlike the traditional diffusion theory where particles jump within a restricted distance along the vertical direction over a small time interval. The equation is further modified for any kth class of non-uniform sediment and the effect of non-local movement is included in the expression of depth averaged sediment diffusivity also. The settling velocity of a particle is taken in a form that contains the effect of hiding and hindering due to the presence of non-uniform particles in the flow. The space-fractional derivative is taken in the Caputo sense where the order α of the fractional derivative varies from 1<α≤2. The non-local effect is considered in the bottom and top boundary conditions also and the governing equation with boundary and initial condition have been solved by Chebyshev collocation method along with Euler’s backward method. The temporal variation of concentration profiles for different size particles is studied by varying α and it is observed that the magnitude of concentration decreases for each size as α increases. Non-local effect on bottom concentration for different size particle is also studied and it is seen that overshooting of bottom concentration gradually decreases as α increases for a particular size. Sensitivity analysis of α on GSD at different heights also has been done. The model has been validated for GSD under equilibrium conditions with available experimental data and it is found that the model aligns well when the non-local mixing effect is taken into account. At the end, an error analysis has been conducted to confirm the accuracy of the presented model.

Two-layer multi-state SPH modelling of momentum growth and its feedback in viscous debris flow on wet bed sediment

Flow-momentum growth and resultant feedback from bed-sediment entrainment significantly influence the mobility of debris flows and valley topography. Existing models inadequately capture the mass and momentum growth regulated by scale-sensitive, time-dependent pore water pressure in erodible beds, which exhibit pronounced anisotropy and nonlinearity. In this study, we propose a three-dimensional, two-layer, multi-state smooth particle hydrodynamics (TLMS-SPH) model to assess the flow-momentum growth of debris flows on wet beds. Herein, an enhanced Drucker-Prager (DP) yield criterion is integrated into the Herschel-Bulkley-Papanastasiou (HBP) rheology to mitigate particle collapse induced by gravitational perturbations in steep terrains. Particularly, an efficient hydro-poro-mechanics-based pore water pressure algorithm is presented and embedded in the SPH scheme to simulate the pore pressure response. To address the size effect, we rigorously tested the method through a series of full-scale, well-documented entrainment experiments. The results demonstrate that the model effectively captures and reproduces the complex dynamics of flow-momentum growth, manifesting in erosion-induced excessive volume and mobility changes. Our method establishes a clear link between volumetric water content and pore pressure response, further underscoring its capability to delineate levee-channel and deposition morphology. This study represents the first attempt to model debris flow on beds of varying wetness from a particulate perspective.

Evaluation of the applicability of three sediment transport capacity equations on steep colluvial slopes and their modifications

AbstractAccurate estimations of the sediment transport capacity (Tc) are essential for soil erosion modelling. However, the applicability of existing Tc equations to colluvial soils with high steep slopes and high gravel content is limited. In this study, the variation of Tc with shear stress (τ), unit stream power (P) and stream power (ω) is investigated for different slopes and flow discharge. We also evaluate the applicability of the Yalin, Govers and GUEST equations (based on τ, P and ω, respectively) for estimating Tc on steep–slope colluvial deposits. Experiments were conducted using colluvial soil in a non‐erodible rill flume. The results reveal that Tc follows a power function with τ and ω and a linear function with P. The regression results of the three hydrodynamic parameters and Tc agree with the Tc equation forms of the corresponding equations. The Yalin equation, developed based on gently sloping erodible bed conditions, simulates overall low Tc values for steep sloping non‐erodible bed conditions (P.O.0.5–2 = 42.8%). The accuracy is significantly improved by correcting the sediment transport coefficient Kt (P.O.0.5–2 = 100%). The accuracy of the Govers‐simulated Tc values under steep slope conditions, based on gentle slope conditions, decreases with increasing slope gradient (P.O.0.5–2 = 37.14%), which is attributed to the large amount of coarse‐grained sediment present in this study. Thus, we retained the original form of the equation and further improved its accuracy by adjusting the coefficients (P.O.0.5–2 = 94.29%). As Tc increases, the GUEST equation can accurately simulate Tc. The accuracy is improved by calibrating the F‐value (P.O.0.5–2 = 100%). Using dimensional analysis, the equations built based on hydraulic conditions (excess current power, shear stress, flow velocity, etc.) and median particle size can accurately simulate Tc values (P.O.0.5–2 = 100%). These findings provide a basis for the development of erosion models for avalanche deposits on steep slopes.

Particle-scale kinematic model for the surface erosion of granular beds

Particle-scale kinematic model for the surface erosion of granular beds

Sediment transport on rippled beds

We conduct an Euler-Lagrange, direct numerical simulation of a turbulent channel flow at a shear Reynolds number of Reτ=180 over an erodible particle bed. The particle bed consists of approximately 1.3 × 106 monodisperse particles, resulting in a bed thickness of around 12–13 particles. The particle density and size are chosen to achieve a ratio of 4 for the Shields stress to the critical Shields stress necessary for incipient motion such that particle transport occurs primarily as bedload. The simulation is run long enough for ripples to form. We track the temporal evolution of the particle flux and excess Shields stress for the entire bed as well as for the four regions of a ripple, namely, the crest, trough, lee side, and stoss side. We find that the particle flux and excess Shields stress closely match the Wong and Parker correlation when the particle bed is featureless at early time but diverge from the correlation when ripples form. This deviation primarily arises from particle transport in the trough and lee side regions. Conversely, particle transport in the crest and stoss side regions remains largely consistent with the Wong and Parker correlation. A root mean square-based correction for the bed is proposed to be used in conjunction with the Wong and Parker correlation. Additionally, ripples attain a self-similar profile in the shape and near-bed shear stress when they are sufficiently distant from their upstream neighbor. Any departure from self-similarity occurs when the upstream neighbor gets within close proximity.

Impact of emergent vegetation on three-dimensional turbulent flow properties and bed morphology in a partially vegetated channel

The study aimed to explore three-dimensional turbulent flow properties and bed morphology in a partially vegetated channel with sand bed conditions. Presence of flexible vegetation in the river and its interaction with the flow are of great significance in understanding the momentum and mass transport in the flow. Experiments were conducted in a straight, tilting rectangular flume with staggered emergent vegetation covering half of the channel width. The results show that the presence of vegetation diverts streamwise velocity from the vegetated side to the non-vegetated side. The study reveals that the presence of vegetation leads to an increase in turbulent intensity, turbulent kinetic energy, and Reynolds shear stress at the transition area between the vegetated and non-vegetated sides of the channel. This increase is attributed to higher transverse flow and momentum exchange in the transition area between the vegetated and non-vegetated sides. In the vegetated side, the vegetation serves as an obstruction, reducing turbulent intensity, turbulent kinetic energy, and Reynolds shear stress compared to the transition area between the vegetated and non-vegetated sides. This reduction in turbulence supports the stability of bed materials and promotes sediment deposition. The presence of vegetation significantly alters the secondary current in the channel. Scour depth along the non-vegetated side was higher than the vegetated side, mainly because the flow concentrated in the centre and non-vegetated side of the channel. The investigation determines that the existence of vegetation on the vegetated side effectively protects against bed erosion and sediment transport. Understanding the impact of emergent flexible vegetation on flow properties and sediment transport can inform decisions about vegetation layouts in river ecosystems.

Lateral Shear Stress Calculation Model Based on Flow Velocity Field Distribution from Experimental Debris Flows

Debris flows continuously erode the channel downward and sideways during formation and development, which changes channel topography, enlarges debris flow extent, and increases the potential for downstream damage. Previous studies have focused on debris flow channel bed erosion, with relatively little research on lateral erosion, which greatly limits understanding of flow generation mechanisms and compromises calibration of engineering parameters for prevention and control. Sidewall resistance and sidewall shear stress are key to the study of lateral erosion, and the distribution of the flow field directly reflects sidewall resistance characteristics. Therefore, this study has focused on three aspects: flow field distribution, sidewall resistance, and sidewall shear stress. First, the flow velocity distribution and sidewall resistance were characterized using laboratory debris flow experiments, then a debris flow velocity distribution model was established, and a method for calculating sidewall resistance was developed based on models of flow velocity distribution and rheology. A calculation method for the sidewall shear stress of debris flow was then developed using the quantitative relationship between sidewall shear stress and sidewall resistance. Finally, the experiment was validated and supplemented through numerical simulations, enhancing the reliability and scientific validity of the research results. The study provides a theoretical basis for the calculation of the lateral erosion rate of debris flows.

Impact and erosion dynamics of inclusion-enriched dry granular flows on rigid barriers with basal clearance: Numerical insights from hybrid MP-DEM simulations

Rigid barriers with basal openings are often installed along predicted flow paths to protect downstream facilities from dry granular flows containing large boulders. However, current design practices do not consider the effects of these inclusions during impact, discharge, overflow, and bed erosion. We conducted hybrid MP-DEM simulations to study the dynamics of dry sand flow with inclusions impacting a rigid barrier with a basal opening. Our findings show that accounting for large inclusions is crucial for barrier design, as they alter dry granular flow regimes, delay dead zone formation, and affect overflow dynamics. Current analytical solutions may overestimate velocity attenuation by up to 60% if inclusions are not considered. A 10% increase in inclusion volume fraction leads to 6%-28% increases in internal energy dissipation during overflow, resulting in an overflow distance 20% lower than existing predictions. Additionally, channel bed erosion due to landing flow with inclusions creates a flow depth up to 3.5 times thicker than at the first barrier location. This suggests that using the initial flow depth for impact force assessment on a second barrier may not be conservative for erodible channel beds and inclusion-enriched flows.

Data-based models to investigate protective piles effects on the scour depth about oblong-shaped bridge pier

The primary objective of this study is to examine the suitability of employing a machine learning models (MLMs) comprising Support Vector Machine (SVM), Gene Expression Programming (GEP), and Artificial Neural Network (ANN) algorithms to emulate the influence of geometric and geotechnical parameters of the piles on the scour depth reduction about the oblong-shaped bridge pier. A dataset consisting of 90 laboratory observations was utilized, allocating 70 % of the data for the training and 30 % for the testing the MLMs. An oblong-shaped bridge pier of diameter D, was positioned and subjected to a rectangular flume featuring an erodible bed comprising sand sediment particles of D50 = 1.7 mm under clear water and subcritical flow conditions. The spacing between the piles, LD, the angle of pile orientation with respect to the flow, θ, and the Froude number, Fr, were identified as independent parameters through the dimensional analysis and Buckingham-Π theory. The MLMs’ performance were assessed employing root mean square error (RMSE), mean absolute error (MAE), coefficient of determination (R2), and developed discrepancy ratio (DDR). The outcomes, affirming the predictive capacity of all MLMs included, revealed that the GEP model employing a three-gene configuration exhibited superior accuracy compared to the other two MLMs. Specifically, the performance indicators (RMSE, MAE, R2, DDR) during the training and testing phases were (0.0433, 0.033, 0.981, 6.8) and (0.054, 0.045, 0.958, 4.81) respectively. The sensitivity analysis elucidated the hierarchy of influential variables as follows: Fr, LD and θ.

Cascading consequences of structural interventions in a tropical wandering gravel‐bed river

AbstractRiverscapes are often modified and managed through ‘command and control’ structural engineering approaches. These structures not only alter flow and sediment transport patterns but also negatively affect riverscape biodiversity. These impacts are particularly acute for rivers that are characterised by high mobility and wide flow distribution during extreme events. We used hydrodynamic and geomorphological change assessment to appraise the impacts of existing flood structures and their potential for removal in a tropical wandering gravel‐bed river in the Philippines. Our approach informs a systematic decision‐making process to screen opportunities for channel reconnection. Two‐dimensional hydraulic modelling showed flood structures confine flood water within the active channel for more frequent flood events, but banks are overtopped during larger magnitude events (&gt;50‐year recurrence interval). Incorporating modelling results, a multi‐criteria analysis was developed to appraise opportunities for river reconnection based on structure purpose, functionality, and local land‐use. We differentiate among four actions for each structure: removal, redesign, set back, or retention. One structure is recommended for removal because it cuts off an anabranch, reduces channel capacity, and increases main anabranch bed shear stress. Increased potential for bed and bank erosion due to the structure's presence had a cascading effect, resulting in further interventions downstream and on the opposite bank. We show how the use of system‐scale two‐dimensional hydraulic modelling alongside high‐resolution topographic mapping can support management of interventions to reconnect a river to its wider anabranch‐bar morphology. Such actions form part of proactive, adaptive, and nature‐based flood management plans with local and non‐local benefits.

Changes in the suspended floc properties by bed erosion in a macrotidal muddy flat: Case of Asan Bay, Korea

Intertidal flats have closely connected sediment transport systems with tidal channels. The properties of suspended sediments derived by horizontal advection and local erosion were examined under various current and wave. A field campaign was conducted over two weeks (October 17–31, 2021) in the Asan intertidal flat. Suspended sediments were in the form of flocs, which are clumps of particles, with an average floc size (davg) of 70–275 μm. The maximum current velocity and water level at each tidal cycle continuously decreased from 0.19 to 0.04 m s−1 and from 5 to 2 m, respectively. In accordance with the hydrodynamic conditions, continuous decrease in davg from spring to neap tides was superimposed by flood-ebb variations in davg. Geological heterogeneity between channels and flats can be significantly influenced by tide and wind conditions. Under tide-dominant conditions, the spring tidal current was strong enough to transport macroflocs with davg of 200–500 μm into the intertidal flats. The grown macroflocs up to 1.6 times of davg at early flood tide were deposited in the flats during the slack tide and transported back to the channels by ebb tidal current. Under wind-dominant conditions, the suspended flocs originated primarily from the bed by erosion. The deposited macroflocs or microflocs were entrained into water column and transported to adjacent tidal channel as microflocs or flocculi. The structure of the advected flocs had more varied density in the range of 42–147 kg m−3, compared to the eroded flocs. Under severe erosion, the structure of suspended flocs was similar to that of advected flocs despite their smaller size. This suggests that bed erosion supplied more porous flocs into the water column than those transported by advection. Consequently, the intertidal flats are at an increased risk of erosion due to the diverse structure of flocs.

Response of suspended sediment dynamics to human activities in the transitional zone between Changjiang Estuary and Hangzhou Bay

The Changjiang Estuary and Hangzhou Bay system has experienced river damming and estuarine engineering in the last decades. However, few studies focused on the shifts in its sediment dynamics due to such human activities. In this study multi-decadal development of sediment dynamics in the transitional zone of the two large estuaries was analyzed, based on the synchronous hydrographic data in the winter of 2023, 2014 and 1983. The results revealed significant changes in regional hydrodynamics and suspended sediment transport, despite the continuous good correlations between the current velocity, suspended sediment concentration (SSC), water/sediment fluxes and tidal range. Specifically, the current velocity has been decreased by 8 - 21% after 2014, mainly due to the land reclamation (implemented around 2016) with several groins stretching into deep water and altering alongshore hydrodynamics. The SSC has decreased further by 29 - 38% in addition to the significant decrease during 1983 - 2014. The SSC changes are related to the combination of river damming which induced sediment load reduction and land reclamation which enclosed a large amount of sediment. Furthermore, the sediment transport from Changjiang Estuary to Hangzhou Bay decreased by 36% - 53%, explaining the observed bed erosion in the northern bay mouth in recent years. The findings are also relevant for studies on sediment dynamics in other large estuaries worldwide.

Soil Quality Assessment and Influencing Factors of Different Land Use Types in Red Bed Desertification Regions: A Case Study of Nanxiong, China

Soil environmental issues in the red bed region are increasingly conspicuous, underscoring the critical importance of assessing soil quality for the region’s sustainable development and ecosystem security. This study examines six distinct land use types of soils—agricultural land (AL), woodland (WL), shrubland (SL), grassland (GL), bare rock land (BRL), and red bed erosion land (REL)—in the Nanxiong Basin of northern Guangdong Province. This area typifies red bed desertification in South China. Principal component analysis (PCA) was employed to establish a minimum data set (MDS) for calculating the soil quality index (SQI), evaluating soil quality, analyzing influencing factors, and providing suggestions for ecological restoration in desertification areas. The study findings indicate that a minimal data set comprising soil organic matter (SOM), pH, available phosphorus (AP), exchangeable calcium (Ca2+), and available copper (A-Cu) is most suitable for evaluating soil quality in the red bed desertification areas of the humid region in South China. Additionally, we emphasize that exchangeable salt ions and available trace elements should be pivotal considerations in assessing soil quality within desertification areas. Regarding comprehensive soil quality indicators across various land use types, the red bed erosion soils exhibited the lowest quality, followed by those in bare rock areas and forest land. Within the minimal data set, Ca2+ and pH contributed the most to overall soil quality, underscoring the significance of parent rock mineral composition in the red bed desertification areas. Moreover, the combined effects of SOM, A-Cu, and AP on soil quality indicate that anthropogenic land management and use, including fertilization methods and vegetation types, are crucial factors influencing soil quality. Our research holds significant implications for the scientific assessment, application, and enhancement of soil quality in desertification areas.

Forecast of Stabilized Channel Transformations in Prismatic River Channels

The perfection of nature in its simplicity and harmony. It is revealed in the process of her cognition. A striking example of harmony is the relationship between a river flow and its bed. In one area the stream begins to wash away the river bed, in another the washed away soil is deposited. In both cases, over time, the processes practically die out and a long-term equilibrium is restored. The work considers the problem of predicting river bed transformations at the stage of their stabilization. The goal is to determine the slope of the transformed bed, which has a prismatic shape. It is noted that between the transit zone and the sediment deposition zone there is a “critical section” of the river, where the turbidity of the flow reaches its maximum value and there is no erosion of the river bed or sedimentation. The parameters of this section have an important influence on the process of stabilization occurring in the river. The slope of the new bed of the channel after stabilization of the process was determined by two analytical methods. In the first case, the sediment balance equation uses a number of formulas for sediment consumption. In the second case, this slope is established by analyzing the equation describing the period of stabilization river bed transformations. The research results show that after stabilization of river bed transformations in the prismatic section of the river, a new slope is established that is quite close to the slope of the “critical section”.

Effect of typhoon on suspended sediment concentration, bed erosion and sediment transport in the Yangtze Estuary

Annually 5–6 typhoons strike the Yangtze Estuary (YE) as extreme events. However, their high energy and importance for sediment transportation and geomorphic changes are still not fully understood. In this study, high-resolution observations of wind, wave, flow velocity, and suspended sediment concentration (SSC) at two in-situ stations were carried out during the 2022 Hinnamnor typhoon. Additionally, we simulated the change in SSC, estuarine bed erosion/deposition, and flow and sediment transport with and without a typhoon in the YE using MIKE3 numerical model. The findings revealed that the Hinnamnor typhoon-induced waves increased the SSC of the turbidity maximum zone (TMZ) by a factor of 5.6 times (maximum is 2.8 kg/m3). The TMZ area also extended by 2.68 times (maximum is 7880km2, 70.4% of YE) in the YE. Moreover, the typhoon caused a dramatic change in sediment transport and bed erosion/deposition in the YE. First, in the delta front area where the mean water depth is >5 m, the typhoon significantly increased the southward flux of residual flow and sediment, causing sediment transport into Hangzhou Bay to abruptly increase 26.3 times (increase of 52 million tons, accounting for 1/3 of the present annual flux of the Yangtze River (150 million tons)) during a single spring-neap period. The net erosional area and volume extended to 6770km2 (60.4% of YE) and 91.18 × 106 m3. Second, in the delta shoals (where the mean water depth is <5 m, including east Chongming Shoal, Hengsha Shoal, Jiuduansha Shoal, and east-south Nanhui Shoal), residual flow and sediment flux decreased northward from the typhoon and resulted in the erosion of the shoal. Third, in channels with trumpet-shaped mouths (North Branch (NB), North Channel (NC) and South Passage (SP), except for North Passage (NP)), the upward flux of residual flow and sediment increased due to the typhoon, resulting in bed deposition in these channels (NB, NC and SP). This study highlights the important influence of typhoons on flow and sediment transport and bed erosion in estuarine areas.

Numerical Simulation of River Channel Change in the Suspended Sediment-Dominated Downstream Reach of the Sangu River

This study aims to clarify the characteristics of the riverbed deformation, bank erosion, and channel changes in the lower Sangu River basin using a depth-averaged 2-D flow model where suspended sediment transport is dominant and its flow characteristics are influenced by active tides. A 45 km long area including the river mouth is computed using the 2-D model, and the results are compared with the observed channel changes. The computation results show that bed deformation and channel change are mainly caused during the ebb tide period of the spring tide particularly in an area of about 10 km from the river mouth. The detail study domain calculation results show that the flow concentration at the bend area causes the bed erosion there, and the eddy separated from the main stream causes the sediment deposition at the inner bank, while this eddy is not developed at the outer bank, resulting in the bank erosion there. Through these investigations, the characteristics and mechanisms of the morphodynamics in the lower Sangu River reach, particularly the potential of the combination of tides and floods to enhance riverbed deformations and the associated bank shifts, have been clarified.

MODERN PROCESSES IN THE BEADED CHANNELS OF STEPPE RIVERS OF THE KHOPER-BUZULUK PLAIN

Beaded-shaped channels (or chain-of-ponds), characterized by alternating lake-like extensions and narrow runs, are widespread on small rivers of the steppe zone of Russia. Extensions of channels - beads (pools or ponds), with a depth of up to 5-6 m, are of great importance for cattle breeding, providing a watering place during the dry periods of the year, when the flow of small rivers stops. However, the mechanisms of their specific channel shape formation are still debatable: it is explained by uneven siltation of channels, formation of whirlpools, groundwater discharge, karst processes or relict thermokarst. The aim of the study was to assess the role of water flow in the formation and support of the beaded shape of river channels. The tasks were to reveal the hydrological regime of the Kardail and Kupava rivers, determine the hydrodynamics of the flow in beads and runs, describe the characteristics of erosion and accumulation processes and the sediment transport, as well as to identify relict elements of the channel and floodplain structure that do not correspond to modern water state. Water flow of the studied rivers shows a long-term decrease in snowmelt flood, as evidenced by the rare flooding of the high-level floodplain, low rates of bank dynamics, and the discrepancy between the size of meanders and the width of the channel. The annual sediment load of the studied beaded rivers is low, estimated at 6,05 t/km2 in 2023. The reason is a low supply of sediments from the catchment area and low rates of erosion of channel bed and banks built of clay and silt. The present-day dynamics of water flow do not help the deepening and expansion of the beads. The flow reaches maximum speed (up to 1-1,3 m/s) in channel runs, while in channel extensions flow speeds drop to 0-0,3 m/s, supporting the sediment accumulation, despite the presence of whirlpools. A significant part of channel extensions is located on sites of former pools of the ancient meandering channel, inherited by the modern watercourse. On the former riffles the channel narrowed and a low-level floodplain was formed. Deep channel extensions were preserved due to the low sediment load and its fine composition, which prevents its rapid settling from the suspension. The results of the study suggest possible contribution of other factors to the formation of original deep pools, including the cryogenic ones.