Incipient Motion Research Articles

Impact of Fenton aging on the incipient motion of microplastic particles in open-channel flow

Upon entering the environment, Microplastics (MPs) experience aging processes that modify their properties and integrity. Previous methods for predicting the incipient motion of MPs have been validated using pristine plastics, which do not account for the effects of aging. This can lead to uncertainties in both quantification and characterization. This study investigates the effect of aging on the incipient motion of MPs with different bed roughness (smooth and rough beds) and MP properties (e.g., grain sizes and densities) in an open-channel flow. Five types of MPs were subjected to four different degrees of aging using the Fenton reagent, and their incipient velocities were tested on beds with two distinct roughness. The results suggest that the incipient velocity of MPs increases linearly with aging. However, this increase is not uniform across different particles and bed roughness. Upon comparing various commonly employed sediment incipient velocity equations, experimental results are in agreement with Ruijin Zhang's equation as the most precise. The parameters in Ruijin Zhang's equation are modified to enhance its applicability for predicting the incipient velocity of aged MPs. This study provides novel insights into the incipient motion of aged MPs in an open-channel flow, highlighting the intricate interaction between aged MP characteristics and bed roughness.

Defining and characterizing the phenomenon of river-bottom tearing scour (RBTS): A case study of the Middle Yellow River

River-bottom tearing scour (RBTS) in natural rivers refers to the incipient motion and transport processes of clay blocks formed by fine cohesive sediment after deposition and consolidation in riverbeds. The RBTS phenomenon can lead to significant channel erosion and changes in river planview morphology, and has, thus, attracted the attention of hydrologists and engineers. In the study, a new determination measure (K) for the occurrence of a RBTS event is derived based on the theoretical expression for the critical incipient velocity of the clay block, K=α2(CL+2λ22), which is a combination of the coefficients of the velocity (i.e., α is a local velocity coefficient), lift force (i.e., CL is the lift coefficient), and clay block size (i.e., λ2 is the ratio of block thickness to length). Furthermore, to explain river behavior during a RBTS event, the Fugu, Xiaobeiganliu, and Weihe river reaches in the Middle Yellow River (MYR) were selected as study areas. Analysis of hydrological data from 1950 to 2023 in the examined reaches implies that a single discharge or sediment threshold cannot predict the occurrence of RBTS. The cross-sectional erosion and deposition volume (CEDV) and the migration intensity of the channel thalweg (MI) also were calculated at the section and reach scales for the examined reaches during the RBTS events. It was observed that RBTS typically causes significant channel erosion originating from the most upstream portion of the study reach with CEDV values ranging from 61 to 6034 m2, while MI values during the RBTS events were close to the multi-year average for the study reach. Finally, a flume experiment simulating RBTS was done to discuss and verify the threshold value of K. Field survey and experimental results indicated that RBTS occurs if K > 0.5, thus, providing theoretical support for the prediction and prevention of RBTS.

Wetting characterisation on complex surfaces by an automatic open-source tool: DropenVideo

HypothesisInvestigating solid–liquid interactions to determine advancing and receding contact angles, and consequently contact angle hysteresis, is crucial for understanding material wetting properties. A reliable, automated, and possibly open-source tool is desirable, to standardize and automatize the measurement and make it user-independent. ExperimentsThis study introduces an open-source software, DropenVideo, as an extension of Dropen. DropenVideo automates frame-by-frame video analysis for the advancing and receding contact angle determination, by considering needle presence, contrast tuning, and compensating for missing drop edge data. Contact angles are calculated using convolution mask, circle, and polynomial fittings. An innovative feature in DropenVideo is the automatic protocol for identifying advancing and receding contact angles: (i) the advancing contact angle is determined as the average value during drop inflation; and (ii) the receding contact angle is determined from the frame of incipient motion during drop deflation. FindingsExploring the application of DropenVideo across a range of complex surfaces as representative test cases, we highlight existing challenges in interpreting wetting measurements by addressing different wetting scenarios. Our study demonstrates that employing frame-by-frame automatic analysis of contact angle measurement videos using DropenVideo significantly mitigates the potential risks of subjective bias associated with manual interpretation and enhances the precision of identified wetting characteristics.

Investigations into dynamic variation characteristics of near-bottom boundary flows over a long term at strait areas

Dynamic processes at the bottom boundary layers are crucial for various marine applications such as marine energy exploration and environmental protection. This paper explored hydrodynamic characteristics of near-bottom boundary currents based on the continuous measured velocity data at a site near the bottom boundary layer in the Strait of Georgia. Previous studies on the bottom boundary layer encompasses various aspects, including physical, chemical, and biological dimensions. However, the mechanism underlying the hierarchical structure of flow velocity remain poorly understood. Thus, the power spectrum and wavelet analysis were used to analyze the significant period of the averaged velocity and tidal characteristics, and to calculate the thickness of near-bottom boundary layer. The results indicate that the current velocities at the boundary layer tend to flow in south-eastern and north-western directions. There is a significant period of 15 days in August and September. At a depth of 301 m, the vertical velocities are stratified, indicating different velocity layers. The boundary layer thickness varies from 5.23 to 14.74 m, as indicated by the vertical structural characteristics. The structural characteristics of the seabed boundary layer are conducive to deepening our understanding of sediment's incipient motion, transport, and sedimentation process. The findings provide a theoretical foundation for future studies on sediment movement and the numerical simulation analysis of the bottom boundary layer. It is of great significance for the study of wave flow interaction and marine dynamic structure, and can provide scientific support for the development and utilization of the strait area.

Estimating Grain Stress and Distinguishing Between Mobility and Transportability Improves Bedload Transport Estimates in Coarse‐Bedded Mountain Rivers

AbstractEstimating sediment transport in mountain rivers is challenging because of sediment supply limitation, broad grain size distributions, complex flow hydraulics, and large form drag. Consequently, sediment transport equations are lacking for application in rivers where the bed is coarse and largely immobile, but small fractions of finer, transportable sized material contribute disproportionately to bedload transport. We introduce a framework for estimating sediment transport in mountain rivers that addresses two limitations: estimating the shear stress acting on mobile grains, and accounting for the difference between mobility of size fractions, that is, whether or not a specific grain size can move at a given flow, and transportability, which we define as how much of that size present in the bed will be recruited into transport. We use two bedload data sets to develop equations for predicting incipient motion and transport rates of each grain size fraction present in the bed. We tested the new equations against incipient motion and sediment transport data we collected from streams in the Rocky Mountains, USA, and against published regional sediment yield data. Using this method results in transport estimates where the finer fractions, despite being a small fraction of the bed surface, make up a large part of the total yield. Fractions greater than the median bed grain size are mobile only during peak flood flows, consistent with the existing mountain river bedload data sets. The approach is parsimonious, requiring only data that are often readily available or obtainable: a bed grain size distribution, hydraulic geometry measurements, and discharge.

Influence of a Meandering Channel on the Threshold of Sediment

River meanders and channel curvatures play a significant role in sediment motion, making it crucial to predict incipient sediment motion for effective river restoration projects. This study utilized an artificial intelligence method, multiple linear regression (MLR), to investigate the impact of channel curvature on sediment incipient motion at a 180-degree bend. We analyzed 42 velocity profiles for flow depths of 13, 15, and 17 cm in a laboratory flume. The results indicate that the velocity distribution was influenced by the sediment movement threshold conditions due to channel curvature, creating a distinct convex shape based on the bend’s position and flow characteristics. Reynolds stress distribution was concave in the upstream bend and convex in the downstream bend, underscoring the bend’s impact on incipient motion. Bed Reynolds stress was highest in the first half of the bend (0 to 90 degrees) and lowest in the second half (90 to 180 degrees). The critical Shields parameter at the bend was approximately 8–61% lower than the values suggested by the Shields diagram, decreasing from 0.042 at the beginning to 0.016 at the end of the bend. Furthermore, our findings suggest that the MLR method does not significantly enhance the understanding of sediment movement, highlighting the need for a more comprehensive physical rationale and an expanded dataset for studying sediment dynamics in curved channels.

Influence of cnoidal wave-induced seepage force on shields number in shallow water

Sediment incipient motion in shallow water is a major concern for coastal engineers. Wave-induced seepage force has reportedly had an important impact on the sediment incipience. The effect of wave asymmetry and skewness is important for defining nearshore wave motions and the resulting sediment movement, but it has been discounted in most previous studies considering seepage. Using analytical and numerical models, this study examines the influence of two-dimensional nonlinear wave-induced seepage forces on sediment incipient motions in shallow water. A new modified Shields number was derived, which better addresses the problem of seepage. The results indicate that seepage force could stabilize the sand particles by lowering the maximum Shields number by up to 38.0%; in the linear wave cases, the opposite is true. This discrepancy is subject to the nonlinearity and asymmetry of shallow wave motions, as well as the decreasing “suction” force and increasing “injection” force. Besides, disregarding horizontal seepage would greatly underestimate the contribution of seepage to sediment incipient motions, especially in situations with steeper waves or an unsaturated and less permeable seabed. A lower Young's modulus, larger saturation, permeability, and water depth would all considerably decrease the maximum Shields number.

Modelling of Granular Sediment Transport in Steady Flow over a Mobile Sloped Bed

This paper introduces a three-layer system, proposing a comprehensive model of granular mixture transport over a mobile sloped bed in a steady flow. This system, consisting of the bottom, contact, and upper zones, provides complete, continuous sediment velocity and concentration vertical profiles. The aim of this study is to develop and experimentally verify this model for sediment transport over a bottom locally sloping in line with or opposite the direction of sediment flow. The model considers gravity’s effect on sediment transport in the bottom (dense) layer when the component of gravity parallel to the bottom acts together with shear stresses associated with water flow. This is a crucial factor often overlooked in previous studies. This effect causes an increase in velocity in the mobile sublayer of the dense layer and significantly affects the vertical distributions of velocity and concentration above this layer. The proposed shear variation due to the interaction between fractions and an intensive sediment mixing and sorting process over a mobile sloped bed adds to the novelty of our approach. The data sets used for the model’s validation cover various conditions, including slopes, grain diameters, densities, and grain mobility conditions, from incipient motion to a fully mobilized bed. This extensive validation process instils confidence in the theoretical description and its applicability to real-world scenarios in the design of hydraulic infrastructure, such as dams, barrages, bridges, and irrigation, and flood control systems.

Exploring the Behaviors of Initiated Progressive Failure and Slow‐Moving Landslides in Los Angeles Using Satellite InSAR and Pixel Offset Tracking

AbstractCatastrophic landslides are often preceded by slow, progressive, accelerating deformation that differs from the persistent motion of slow‐moving landslides. Here, we investigate the motion of a landslide that damaged 12 homes in Rolling Hills Estates (RHE), Los Angeles, California on 8 July 2023, using satellite‐based synthetic aperture radar interferometry (InSAR) and pixel tracking of satellite‐based optical images. To better understand the precursory motion of the RHE landslide, we compared its behavior with local precipitation and with several slow‐moving landslides nearby. Unlike the slow‐moving landslides, we found that RHE was a first‐time progressive failure that failed after one of the wettest years on record. We then applied a progressive failure model to interpret the failure mechanisms and further predict the failure time from the pre‐failure movement of RHE. Our work highlights the importance of monitoring incipient slow motion of landslides, particularly where no discernible historical displacement has been observed.

Effects of bag characteristics and channel side slope on incipient motion of a single Geobag under river current loading

Geobag stability has not been extensively studied under river current loading. In this study, the impacts of geobag characteristics (shape, bag material, and fill ratio of sand) and channel side slope (flat and 1V:2H) on a single geobag's stability were systematically investigated in a physical model to form the solid foundation for the research of group geobags. Overall, a geobag with a higher fill ratio, combined with the more flexible cloth material, was found to be the most stable. Critical Shields parameters were estimated between 0.0018 and 0.019, and the cross-sectional averaged flow velocity at incipient motion ranged from 0.49 m/s to 1.08 m/s. A shape factor was introduced to better describe the relationship between geobag characteristics and their stability on both riverbed configurations. Both the fill ratio and the bed side slope had higher importance on the geobag's stability compared to the relative depth, bag shape, and angle of flexibility.

Incipient Motion of Single Shells under Currents in Flume Experiments

Understanding the motion thresholds of shells is important, as shell motion allows the analysis of beach profiles, prevents excessive erosion of the coastline, and helps to resource the use of discarded shells, providing new ideas for the protection of beaches. In this study, the orientational motions and motion thresholds of two types of typical molluscan shells, bivalve and gastropod shells, were investigated by means of flume experiments. The final orientations with the statistically highest number of occurrences during the orientational motions of each shell were used as the initial orientations for the respective threshold flow velocity measurements. The critical Shields parameter and the incipient mean velocity of the flow were used to represent the critical threshold of the motion. The critical Shields parameters for bivalve shells in the convex upward position were overall higher on average than those for gastropod shells. The experimental data showed that the incipient mean flow velocities of bivalve shells in the convex upward position were about 1.4–2.8 times larger than those in the convex downward position. The incipient mean velocity data were regressed to obtain the motion threshold equations applicable to bivalve shells in the convex upward and convex downward positions as well as gastropod shells under different final orientations.

The Role of Coherent Airflow Structures on the Incipient Aeolian Entrainment of Coarse Particles

AbstractThe role of coherent airflow structures capable of setting gravel‐sized particles in motion is studied theoretically and experimentally. Specifically, a micromechanical model based on energy conservation is proposed to describe the incipient motion of large‐particles ranging from rocking (incomplete entrainment) to incipient rolling (full entrainment). Wind tunnel experiments were conducted on an aerodynamically rough bed surface under near‐threshold airflow conditions. Synchronous signals of airflow velocities upwind of the test particles and particle displacement are measured using a hot film anemometer and a laser distance sensor, respectively, from which coherent airflow structures (extracted via quadrant analysis) and particle movements are interlinked. It is suggested that the incipient motion of gravel‐sized particles (rocking and rolling) may result from sufficiently energetic sweep events corresponding to aerodynamic drag forces in excess of the local micro‐topography resistance. However, full entrainment in rolling mode should satisfy the presented work‐based criterion. Furthermore, using an appropriate probabilistic frame, the proposed criterion may be suitable for describing processes of energy transfer from the wind to the granular soil surface, ranging from the creep transport of gravels to the “mechanical sieving” of mega‐ripples, as well as the transport of light anthropogenic debris (such as plastics).

An analytical study for predicting incipient motion velocity of sediments in ecological open channel flows

Sediment movements on the riverbeds are crucial in fluvial processes. Therefore, studying incipient sediment motion is valuable for predicting changes in sediment behavior. In this study, the influence of aquatic vegetation is introduced into the momentum balance equation under the condition of a bare bed. The presented model for incipient sediment motion in open channels covered by vegetation is derived, in consideration of vegetation density and relative submergence. This study simplifies the model format. The model and its coefficients demonstrate good applicability to various working conditions without requiring numerous changes. The predicted velocity determined by the proposed model is validated against experimental data. Results show excellent agreement between the two in various scenarios, including combinations of rigid or flexible conditions and submerged or emergent states. Additionally, the velocity for incipient sediment motion is observed to decrease with increasing vegetation density. However, a further increase in vegetation density may amplify vegetation resistance, hindering incipient sediment motion. Meanwhile, sediment movement becomes challenging as water depth increases, assuming that flow discharge and vegetation conditions remain constant.

Features and formulae of sediment incipient motion in vegetated flow environment

Features and formulae of sediment incipient motion in vegetated flow environment

Cross-Shore Sediment Transport in the Coastal Zone: A Review

This paper presents a review of cross-shore sediment transport for non-cohesive sediments in the coastal zone. The principles of sediment incipient motion are introduced. Formulations for the estimation of bedload transport are presented, for currents and combined waves and current flows. A method to consider the effect of sediment heterogeneity on transport, using the hiding–exposure coefficient and hindrance factor, is depicted. Total transport resulting from bedload and transport by suspension is also addressed. New research is encouraged to fill the knowledge gap on this topic.

Characteristics of a Particle’s Incipient Motion from a Rough Wall in Shear Flow of Herschel–Bulkley Fluid

Characteristics of a Particle’s Incipient Motion from a Rough Wall in Shear Flow of Herschel–Bulkley Fluid

The incipient motion safety factor of sediment considers the coupling between the surface and subsurface flows under multi-motion modes

The incipient movement of sediment particles in natural rivers under the action of water scour has remained a key issue in river sediment movement research. In this study, we establish a mathematical coupling model between surface and subsurface flows for investigating the interface velocity. The Navier-Stokes equation and Darcy's law are employed to describe surface and subsurface water flows, respectively. The interface velocity U between surface and subsurface flows is derived by introducing the Beavers–Joseph boundary conditions. Furthermore, based on the force equilibrium of a single particle, combined with three incipient modes, namely, sliding, rolling, and saltation, analytical solutions are derived for the critical instantaneous bottom velocity u0, and mean incoming velocity. The derived analytical solutions exhibit a higher computational accuracy than existing flume test data and classical flow velocity model results, with relative errors less than 12 %. Furthermore, the incipient motion state of sediment particles can be directly assessed by defining the critical incipient safety factor for particle motion (i.e., k = U/u0). Sensitivity analysis of the safety factor for sediment particle motion reveals a nonlinear positive correlation with the slope angle, riverbed permeability, riverbed porosity, and hydraulic gradient, while indicating a linear positive correlation with the water depth. However, a nonlinear negative correlation is observed with the B-J coefficient, relative exposure degree, and sediment particle dry bulk density. The safety factor shows a rapid decrease followed by a stabilizing trend with increasing B-J coefficient. Additionally, the safety factor first increases and then decreases with increasing in sediment particle size.

Under cover ice transport processes under equilibrium and non-equilibrium conditions

The transport of ice particles beneath a stable ice cover (referred to as cover load transport), and the processes leading to ice deposition and erosion are highly dynamic and complex. Based on the strong analogous relationship between bed load and cover load transport processes, knowledge from bed load transport research can be used when developing experiments for cover load transport processes. This paper presents a detailed experimental study of cover load transport rate in a laboratory flume with simulated ice particles and a simulated ice cover. The effect of the particle shape and size on cover load transport rate is quantified using the buoyant velocity and its velocity coefficient. The critical dimensionless flow strength is identified as an important parameter which determines the incipient motion of the under cover ice particles, and was calculated using hydraulic measurements obtained through laboratory experiments. A new cover load transport formula is introduced based on regression analysis of experimental data for a wide range of dimensionless flow strength conditions. The impact of incoming ice supply rate on the under cover ice transport rate was also investigated by altering the ice supply rate to the flume. A control volume analysis of ice discharge underneath the ice cover is used as the approach to quantify the dimensionless ice transport rate at different sections of the flume. The concept of equilibrium ice transport rate is introduced based on the experimental observations with variable incoming ice supply rates. The experimental observations on the impact of incoming ice supply were supported by an analysis of field data from Hequ Reach, China (Sun et al., 1986; Wang, 1992). The introduction of a cover load transport formula developed exclusively with simulated ice particles that have similar physical properties to ice forming in a natural environment, and the discussion on equilibrium ice transport rate are important contributions to advance the present understanding on under cover ice transport processes.

Sediment mobility over a dispersive inner shelf from combined wave and tide bed shear stress

The sediment transfers that take place between the beach and the depth of closure on the inner continental shelf play a major role in the evolution of sandy shores at different time scales. The depth of closure of the foreshore sedimentary prism, which is generally dependent on wave conditions, remains poorly constrained in the context of an internal macrotidal platform. This work aims to define and evaluate this theoretical depth, which delimits the extension of sedimentary exchanges, in particular by including the constraints applied on the sea bottom by the tidal circulation, which are very strong on the inner continental shelf of western Brittany. The use of wave (WAVEWATCH III®), tidal (MARS3D), and bottom sediment (EMODnet) databases allows us to follow an original cartographic approach to study closure depths, based on the formulations of Hallermeier (1978 and 1981) and Soulsby (1997). This cartographic approach is adapted to a spatial analysis of sediment mobility on a regional scale. First, the depth of closure shows a regional spatialisation of the seaward extension of the sediment mobility zone according to exposure to different wave climates and tidal ranges. Secondly, the method for calculating the depth of closure with the combined shear stresses (wave and tide), applied at the scale of the internal platform of western Brittany, allows the identification of three areas of movement of sedimentary particles according to critical mobility thresholds: 1) Area of no motion; 2) Area of transport and deposition; 3) Area of transport without deposition. The main contribution of this work is to propose at distinction between 2 offshore sediment motion limits, the transition to the upper plane bed (DoTupb), and the incipient motion of particle (DoTmotion). In addition, this study confirms the potential of transport and deposition of sand particles at depths >100 m in the most hydrodynamically intense areas, which implies the possibility of interconnections between hydro-sedimentary cells, through bypassing of headlands and crossing over rocky outcrops.

Incipient motion of sands near the pipe tip

Incipient motion of sands near the pipe tip