Sediment Transport Mechanisms Research Articles

Development of an Interdisciplinary Prediction System Combining Sediment Transport Simulation and Ensemble Method

The change in movable beds is related to the mechanisms of sediment transport and hydrodynamics. Numerical modelling with empirical equations and the simplified momentum equation is the common means to analyze the complicated sediment transport processing in river channels. The optimization of parameters is essential to obtain the proper results. Inadequate parameters would cause errors during the simulation process and accumulate the errors with long-time simulation. The optimized parameter combination for numerical modelling, however, is rarely discussed. This study adopted the ensemble method to simulate the change in the river channel, with a single model combined with multiple parameters. The optimized parameter combinations for a given river reach are investigated. Two river basins, located in Taiwan, were used as study cases, to simulate river morphology through the SRH-2D, which was developed by the U.S. Bureau of Reclamation. The input parameters related to the sediment transport module were randomly selected within a reasonable range. The parameter sets with proper results were selected as ensemble members. The concentration of sedimentation and bathymetry elevation was used to conduct the calibration. Both study cases show that 20 ensemble members were good enough to capture the results and save simulation time. However, when the ensemble members increased to 100, there was no significant improvement, but a longer simulation time. The result showed that the peak concentration and the occurrence of time could be predicted by the ensemble size of 20. Moreover, with consideration of the bed elevation as the target, the result showed that this method could quantitatively simulate the change in bed elevation. With both cases, this study showed that the ensemble method is a suitable approach for river morphology numerical modelling. The ensemble size of 20 can effectively obtain the result and reduce the uncertainty for sediment transport simulation.

Hydraulics and Turbidity Generation in the Milandre Cave (Switzerland)

AbstractKarst aquifers may convey significant sediment fluxes, as displayed by the intense turbidity peaks commonly observed at karst springs. The understanding of the origin of the suspended solids discharged at springs is key in assessing spring vulnerability and securing drinking water quality. The mechanisms for turbidity generation and sediment transport in karst are however difficult to investigate because of the general lack of access to the karst conduits. These processes have been examined in the Milandre Cave, which hosts a karst drain of regional importance, for more than 10 years by means of turbidity monitoring both inside and at the outlets of this karst system. Additionally, the composition of the suspended load (particle‐size distribution and Escherichia coli content) has been monitored over the course of a flood event. These data are compared against a numerical simulation of the mean boundary shear stress inside the conduit network. The following conceptual model for sediment transport through the system is derived: during minor flood events, most of the turbidity comes from underground sediment remobilization, while during medium to intense flood events, soil‐derived turbidity also reaches the spring. Hydraulics in the epiphreatic zone is tightly linked with autochthonous turbidity generation (mostly during the flooding and the flushing of conduits). In comparison, allochthonous turbidity is associated with finer particles, higher E. coli, and higher UV fluorescence. This improves the overall understanding of turbidity generation and could help the monitoring and forecast of pollution events at drinking water supplies.

3D Numerical Simulation of Gravity-Driven Motion of Fine-Grained Sediment Deposits in Large Reservoirs

Deposits in dam areas of large reservoirs, which are commonly composed of fine-grained sediment, are important for reservoir operation. Since the impoundment of the Three Gorges Reservoir (TGR), the sedimentation pattern in the dam area has been unexpected. An integrated dynamic model for fine-grained sediment, which consists of both sediment transport with water flow and gravity-driven fluid mud at the bottom, was proposed. The incipient motion driven by gravity in the form of fluid mud was determined by the critical slope. Shallow flow equations were simplified to simulate the gravity-driven mass transport. The gravity-driven flow model was combined with a 3D Reynolds-averaged water flow and sediment transport model. Solution routines were developed for both models, which were then used to simulate the integral movement of the fine-grained sediment. The simulated sedimentation pattern agreed well with observations in the dam area of the TGR. Most of the deposits were found at the bottom of the main channel, whereas only a few deposits remained on the bank slopes. Due to the gravity-driven flow of fluid mud, the deposits that gathered in the deep channel formed a nearly horizontal surface. By considering the gravity-driven flow, the averaged error of deposition thickness along the thalweg decreased from −13.9 to 2.2 m. This study improved our understanding of the mechanisms of fine-grained sediment transport in large reservoirs and can be used to optimize dam operations.

Grain Size Analysis as Paleo-environment Indicator of Lower Pliocene Sediments (Bai-Hassan Formation), Garmian Area, Kurdistan Region, NE Iraq

Grain size distribution has utilized to determine the paleoenvironment sediments. The study is primarily dependent on the analysis and description of plots and interrelation factors between parameters. Five samples have been collected in the five different facies along (90) m thickness in the outcrop. The grain size is ranging from (Ø -8 to 14), (Boulder to Clay) which has given good attention to such research. Grain size analysis indicated the major of the particles are Gravel and the minor proportion is (Sand and Mud), which shows that the samples are more Gravely (> 80%) except for the sample (5) is less (< 50%). Sediment transport mechanisms indicated that bimodal sources and were moved associated with the bed (rolling, sliding, and salting) based on the force of the flow. The statistical parameters pointed out the obvious results of the sediment environment. The values of Median and Mean were negative which inferred the coarse pebble size, sediments were very poorly sorted, finely skewed, and leptokurtic. Interrelationship factors between the parameters showed a clear interpretation of the Paleoenvironment and streamflow. The indications have shown that the sediment deposits in a river environment, high current energy, accompanied by strong turbulence currents.

The hydrology of glacier‐bed overdeepenings: Sediment transport mechanics, drainage system morphology, and geomorphological implications

AbstractEvacuation of basal sediment by subglacial drainage is an important mediator of rates of glacial erosion and glacier flow. Glacial erosion patterns can produce closed basins (i.e., overdeepenings) in glacier beds, thereby introducing adverse bed gradients that are hypothesized to reduce drainage system efficiency and thus favour basal sediment accumulation. To establish how the presence of a terminal overdeepening might mediate seasonal drainage system evolution and glacial sediment export, we measured suspended sediment transport from Findelengletscher, Switzerland during late August and early September 2016. Analyses of these data demonstrate poor hydraulic efficiency of drainage pathways in the terminus region but high sediment availability. Specifically, the rate of increase of sediment concentration with discharge was found to be significantly lower than that anticipated if channelized flow paths were present. Sediment availability to these flow paths was also higher than would be anticipated for discrete bedrock‐floored subglacial channels. Our findings indicate that subglacial drainage in the terminal region of Findelengletscher is dominated by distributed flow where entrainment capacity increases only marginally with discharge, but flow has extensive access to an abundant sediment store. This high availability maintains sediment connectivity between the glacial and proglacial realm and means daily sediment yield is unusually high relative to yields exhibited by similar Alpine glaciers. We present a conceptual model illustrating the potential influence of ice‐bed morphology on subglacial drainage evolution and sediment evacuation mechanics, patterns and yields, and recommend that bed morphology should be an explicit consideration when monitoring and evaluating glaciated basin sediment export rates.

Soil Erosion and Controls in the Slope-Gully System of the Loess Plateau of China: A Review

The Loess Plateau has long been suffering from serious soil erosion of which erosion from the slope-gully system is now dominant. The slope-gully system is characterized with distinctive erosion distribution zones consisting of inner and inter gully areas wherein erosion patterns spatially vary, acting as both sediment source and the dominant sediment and water transport mechanism. In this paper, a substantial body of research is reviewed concentrating on the soil erosion processes and control practices in the slope-gully system. The inner gully area is identified as the main sediment source while runoff and sediment from the inter-gully upland is found to significantly affect down slope erosion processes. Correspondingly, the protective vegetation pattern and coverage should be strategically designed for different erosion zones with an emphasis on the critical vegetation cover and pattern to reduce sediment yield of the whole slope-gully system. Check-dam could change the base level of erosion and reduce the slope length of the gully side, which will further decrease the possibility and magnitude of gravity erosion. We concluded that understanding the erosion processes and implementing erosion practices for the slope-gully system are of importance and require more research efforts that emphasize: 1) the influence of upland runoff on erosion processes at downslope; 2) the relationship between hydraulic characteristics of overland flow and erosion process at a slope-gully system scale; 3) physical mechanisms of different vegetation patterns on the slope-gully erosion process.

Three decades of ephemeral gully erosion studies

Although concern for ephemeral gully (EG) erosion has been growing within the research community, knowledge gaps still need to be identified for a state-of-the-art review. This review is based on an extensive search of published literature in the Web of Science database that specifically addressed EG erosion. This review included 173 papers published in 56 scientific journals over the last 30 years. The results indicated that the main EG erosion topics were related to gully erosion processes, factors, methodology, models, and the detrimental effects of EGs. The definition of EG was identical worldwide. There is an urgency to define critical morphological values under different conditions for identifying EG channel locations in fields in order to map and model their development quickly and accurately. Topographic factors have been much more studied than other contributing factors, including climate, land management, soil, and hydraulics. Fifteen different methodological approaches or technologies, which were categorized into four kinds of methods, including field monitoring, modelling, simulation experiments, and review, have been employed to assess and quantify EG erosion. The average soil erosion rate of EGs in 36 publications increased and decreased with increase of slope and catchment areas, respectively, and the number of EGs linearly increased with catchment area. The complexity of the development of EG channels and the driving force mechanisms should also be studied intentionally, e.g., the quantification of soil detachment and sediment transport mechanisms for both surface and subsurface flow during different sub-processes, including gully head advance, sidewall collapse and bed incision. It was concluded that the interactive effects of different contributing factors on EG erosion across various spatio-temporal scales should be further researched. The comprehensive utilization or application of new methods and technologies, e.g., automatic monitoring technologies, artificial intelligence, machine learning algorithms, would be helpful to improve study efficiency and accuracy. The scientific basis of prediction models still needs further development and refinement. This review provides recommendations and suggestions to improve knowledge, prediction, and control of EG erosion.

Sediment Particle Velocity and Activity During Dune Migration

AbstractThe processes underlying sediment transport mechanics are still debated. Here, we present novel ensemble particle bedload measurements, obtained with high‐frequency imaging of particle velocities and activity over various bedform migration stages. We show that the bedload particle flux quickly responds to the unsteady non‐uniform flow over the bed. Mean particle velocity linearly increases over the dune slopes towards the crest. In contrast, particle activity exhibits a power law increase over the dune slopes, with the top layer almost saturating in the saltation regime. Furthermore, the spatial and temporal variability of particle motion over dune slopes is high. For lower flow, spatial and temporal variability from small‐scale topography, such as superposed bedforms, spurs, and irregular crestlines, are shown to be important for quantifying sediment dynamics. With increasing flow, however, the cross‐stream particle velocities increase significantly, influenced by the large‐scale dune topography (transverse slopes). Overall, variability in the particle flux increases with both flow strength and frequency of turbulent events. For the mean particle flux over the dune slopes, however, differences between neighboring dunes are large, which are attributed to dune topography variations and flow acceleration. The presented insights are essential to validate numerical simulations of sediment dynamics in three‐dimensional flow conditions over dune beds.

Numerical study on two dimensional distribution of streamwise velocity in open channel turbulent flows with secondary current effect

For studying mechanism of sediment transport in river flows, open channel flow is a prototype. Flow has always three components of velocity for all types of channel geometry and for a time independent uniform flow along streamwise or main flow direction, all the components of velocity are functions of lateral and vertical coordinates. The present study investigates the two dimensional distribution of streamwise (or longitudinal) velocity starting from the Reynolds averaged Navier–Stokes equation for a turbulent open channel flow which is steady and uniform along the main flow direction. Secondary flows both along the vertically upward direction and along the lateral direction are considered which are also taken as functions of lateral and vertical coordinates. Inclusion of the secondary current brings the effect of dip phenomenon in the model. The resulting second order partial differential equation is solved numerically. The model is validated for all the cross-sectional, transverse and centreline velocity distribution by comparing with existing relevant set of experimental data and also with an existing model. Comparison results show good agreement with data as well as with the previous model proving the efficiency of the model. It is found that the transverse velocity distribution depends on the formation of circular vortex in the cross-sectional plane and becomes periodic as the number of circular vortex increases for increasing aspect ratios.

Complex sedimentary processes in large coastal embayments and their potential for coastal morphological and paleo tropical cyclone studies: A case study from Choctawhatchee Bay Western Florida, U.S.A

Storminess and sea-level can both have a significant impact on landforms in cyclone-prone coastal regions, although much of our understanding comes from short-timescale modern observations. This study aims to understand the variability of sediment transport and deposition in the Choctawhatchee Bay/Santa Rosa Island in the northern Gulf of Mexico, establishing the dominant sediment transport processes and morphological response of the barrier system to long-term variations in storminess and rising sea-levels.Here, we study the spatial and temporal changes in physicochemical properties of the sedimentary record of Choctawhatchee Bay to examine the character and fidelity of records of storm impacts spanning the Holocene. Proxies for marine and terrestrial conditions in the cores situated closer to the present barrier (proximal) show that sedimentation in coastal areas and marine influence of the bay during the last ~8000 yrs. were mainly determined by barrier response to the Holocene transgression and changes in storminess. In contrast, sedimentation close to the landward shore was governed by terrigenous input. The correlation of grain size and terrigenous proxies with regional hurricane records indicates that hinterland erosion by the rainfall during hurricane events is likely the dominant terrigenous sediment transport mechanism in areas close to the landward shore of the bay. These results suggest that sediment archives in large coastal deposition environments are equally suitable for sea level and cyclone modulated coastal morphological studies and paleo tropical cyclone studies, depending on the location, selected with an understanding of sedimentation processes in the vicinity.

Cross-shelf sediment transport in the Yangtze Delta frontal zone: Insights from field observations

Abstract The Yangtze River, which has the largest runoff in China, discharges massive amounts of sediment into the East China Sea every year, which has an important influence on the biogeochemical processes of estuaries and adjacent seas. However, how much Yangtze River sediment is transported to the East China Sea shelf and the dynamic mechanisms of cross-shelf sediment transport remain unclear, especially given the lack of sediment dynamics studies based on field observations. In this study, we performed 25 h field observations of cross-shelf dynamics in the frontal zone of the Yangtze River (122.74°E, 31.04°N, water depth: 28 m) in the summer and winter of 2011. The observations show that the strong tidal current with the maximum current speed of 0.8 m/s causes bottom shear stress up to 0.4 Pa, which is the main force for seabed sediment erosion and suspension, while wind waves play a negligible role. Significant stratification appears at the observation site with a maximum buoyancy frequency of 0.01 s−2 in winter and 0.2 s−2 in summer, which inhibits the high turbidity water within 10 m from the seabed. The cross-shelf sediment transport at the mooring site is mainly driven by the baroclinic force in the middle and bottom layers, and is influenced by wind in the surface layer. During the observations, the sediment from the Yangtze Delta was transported to the southeast in winter and to the northwest in summer, with sediment transport fluxes of 2.8 and 2.3 g/s/m2, respectively.

Regional classification of total suspended matter in coastal areas of South Korea

Total suspended matter (TSM) in coastal areas is affected by various sediment transport mechanisms. Thus, identifying systematically and robustly the regional pattern of TSM data is of great use for the management of coastal areas. The TSM sampling stations have been regionally clustered using an iterative toroidal SOM–k–means approach, which overcomes the cluster number determination and local optima problems. The iterative toroidal SOM–k–means is a two-step clustering approach that finds an optimal clustering result based on DBI by repeating the toroidal SOM and k-means clustering. Six variables (longitude, latitude, surface TSM in February and August, bottom TSM in February and August) were used as clustering variables, and cluster analyses were conducted while varying combinations of variables. All clustering results were analyzed using the sediment distribution, ocean currents, and tidal currents around South Korean coastal areas. Additionally, tidal flats around the Yeomha Channel, Hampyeong Bay, and Doam Bay, which are associated with sediment dispersion of turbid water, bay erosion, and tidal forcing, respectively, were identified during clustering analyses. This study's outcome provides the basis of regional sediment transport analysis in South Korea and an example of physical analysis based on the data-driven approach.

Investigation of a Gravel-Bed River’s Pattern Changes: Insights from Satellite Images

Changes in river pattern have been documented in the literature. The recognition of islands and vegetated patches, which is indicative of the evolution processes, requires simultaneous information at different points and at different times and field studies are still difficult and scarce. The present work, focusing the attention on a gravel-bed river, explores the possibility to conduct a first-step pattern changes analysis by identifying the evolution of the morphological features and taking information of islands and vegetated patches from satellite images easily available from Google Earth. The study is conducted by combining the information taken from both the satellite images and the field photos and the hydrological data. Here analysis concerns a reach of the Tagliamento River, in Italy, in a decadal timescale characterized by evident stream pattern changes. Results confirm that a combination of different factors determine favorable conditions for the river’s evolution, but the frequency of occurrence of high-magnitude events is the most important factor to consider in a first-step analysis. This is because the frequency of occurrence of high-magnitude events affects, in turn, some of the other influencing factors. In particular, the present study highlights that different morphological features form in time periods characterized by different frequency of occurrence of high-magnitude events. On one hand, this could be related to the fact that high-magnitude events could influence the sediment transport and deposition mechanisms, modifying the ratio coarse-to-fine sediments and the grain-to-grain interactions, with consequences in the sediment flux variation and in the migration of the morphological features. On the other hand, the frequency of occurrence of high-magnitude events could affect the vegetation distribution and growth, contributing to determine a variation of the typology of the vegetation characteristics depending on the tolerances of the vegetation species. A sort of interface between the vegetated-dominated area, where the flow resistance is significant, and the flood-disturbance-dominated area, where the action by flow is significant, is determined with a progressive reduction of the number of active channels and development of the meandering bend.

Accretion, retreat and transgression of coastal wetlands experiencing sea-level rise

Abstract. The vulnerability of coastal wetlands to future sea-level rise (SLR) has been extensively studied in recent years, and models of coastal wetland evolution have been developed to assess and quantify the expected impacts. Coastal wetlands respond to SLR by vertical accretion and landward migration. Wetlands accrete due to their capacity to trap sediments and to incorporate dead leaves, branches, stems and roots into the soil, and they migrate driven by the preferred inundation conditions in terms of salinity and oxygen availability. Accretion and migration strongly interact, and they both depend on water flow and sediment distribution within the wetland, so wetlands under the same external flow and sediment forcing but with different configurations will respond differently to SLR. Analyses of wetland response to SLR that do not incorporate realistic consideration of flow and sediment distribution, like the bathtub approach, are likely to result in poor estimates of wetland resilience. Here, we investigate how accretion and migration processes affect wetland response to SLR using a computational framework that includes all relevant hydrodynamic and sediment transport mechanisms that affect vegetation and landscape dynamics, and it is efficient enough computationally to allow the simulation of long time periods. Our framework incorporates two vegetation species, mangrove and saltmarsh, and accounts for the effects of natural and manmade features like inner channels, embankments and flow constrictions due to culverts. We apply our model to simplified domains that represent four different settings found in coastal wetlands, including a case of a tidal flat free from obstructions or drainage features and three other cases incorporating an inner channel, an embankment with a culvert, and a combination of inner channel, embankment and culvert. We use conditions typical of south-eastern Australia in terms of vegetation, tidal range and sediment load, but we also analyse situations with 3 times the sediment load to assess the potential of biophysical feedbacks to produce increased accretion rates. We find that all wetland settings are unable to cope with SLR and disappear by the end of the century, even for the case of increased sediment load. Wetlands with good drainage that improves tidal flushing are more resilient than wetlands with obstacles that result in tidal attenuation and can delay wetland submergence by 20 years. Results from a bathtub model reveal systematic overprediction of wetland resilience to SLR: by the end of the century, half of the wetland survives with a typical sediment load, while the entire wetland survives with increased sediment load.

The Response of Turbidity Maximum to Peak River Discharge in a Macrotidal Estuary

The Ou River, a medium-sized river in the southeastern China, is examined to study the estuarine turbidity maximum (ETM) response to rapidly varied river discharge, i.e., peak river discharge (PRD). This study analyzes the difference in ETM and sediment transport mechanisms between low-discharge and PRD during neap and spring tides by using the Finite-Volume Community Ocean Model. The three-dimensional model is validated by in-situ measurements from 23 April to 22 May 2007. In the Ou River Estuary (ORE), ETM is generally induced by the convergence between river runoff and density-driven flow. The position of ETM for neap and spring tides is similar, but the suspended sediment concentration during spring tide is stronger than that during neap tide. The sediment source of ETM is mainly derived from the resuspension of the seabed. PRD, compared with low-discharge, can dilute the ETM, but cause more sediment to be resuspended from the seabed. The ETM is more seaward during PRD. After PRD, the larger the peak discharge, the longer the recovery time will be. Moreover, the river sediment supply helps shorten ETM recovery time. Mechanisms for this ETM during a PRD can contribute to studies of morphological evolution and pollutant flushing.

Assessing Sediment Pulse during an Extreme Hydrological Event in the Alaknanda Basin, Northwestern Himalaya, India

ABSTRACT In the year 2013, the Northwestern Himalaya witnessed an extreme hydrological event that severely affected the upper Ganga Basin. The unexpected high sediment mobilisation in the basin affected the human population, infrastructure, and ecology in several reaches of the Ganga river and its tributaries. In an exceedingly affected reach, the Kedarnath valley, a considerable sediment volume (~ 26243 x 104 m3) was mobilised during the 2013 extreme hydrological event. In the Srinagar valley, up to 50 x 104 m3 of legacy sediment was washed away during the same event. This sediment mobilisation generated sediment pulses in the affected region, which is assessed through remote sensing, field observations and existing literature, in the Kedarnath and Srinagar valleys. Results show that the evolution of translative, dispersive, and their combined sediment pulse pattern in both selected reaches of the upper Ganga Basin. The mountainous region of the upper Ganga basin has very few monitoring stations observing sediment movement, and data from such stations are mostly unavailable due to which our understanding of the sediment movement in the Himalaya remains very poor. Therefore, it is suggested that there is a need for detailed studies of the sediment transport mechanism in the Himalayan river basins especially during such extreme hydrological events when large volume of sediment is mobilised.

Flow Characteristics and Bed Morphology in a Compound Channel between Two Single Channels

In mountainous areas, a river can widen from a single channel to a compound channel under the influence of geological conditions or human impacts, bringing about challenges in terms of flood control and channel regulation. This paper reports the results of tests conducted in a 26 m long flume with a uniform sediment bed (grain size = 0.5 mm), investigating the flow characteristics and bed morphology in a compound channel between two single channels. The stage‒discharge relationship in the compound channel and the longitudinal and cross-sectional bed profile in the compound channel between two single channels are presented and analyzed. The experimental results indicate that the flow characteristics and bed morphology in a compound channel between two single channels are significantly different from those in a normal compound channel. Based on the experimental data and observations, the mechanisms of flow and sediment transport in the compound channel between two single channels are illuminated.

Suspended sediment dynamics in a deltaic estuary controlled by subtidal motion and offshore river plumes

Two series of opportunistic observations captured the seasonal variability in the suspended sediment concentration (SSC) within the main inlet of Barataria Bay, a bar-built estuary within the Mississippi Delta, as the offshore environment transitioned from a period of low river discharge and high cold front activity to a period of high river discharge and low cold front activity. During winter, the SSC dynamics in Barataria Pass was mostly driven by resuspension in response to cold front passages, with correspondingly high SSC of up to 1.84 g L−1. In addition to cold fronts, diurnal variability associated with astronomic tides was also present in the SSC time series. The mean SSC during spring (0.23 g L−1) was significantly higher than during winter (0.15 g L−1). Higher SSC during spring, in spite of the less frequent and weaker cold fronts, was due to the strong offshore influence of the Mississippi River plume. The longitudinal salinity gradients during spring showed the existence of freshwater intrusion, which was associated with the Mississippi River plume advected north-west by the Louisiana Coastal Current. Intrusion of the river plume though the inlet into Barataria Bay was also observed in time-series of MODIS satellite imagery during March 2018. A decomposition of the sediment flux time series revealed that subtidal motion associated with cold fronts was the primary mechanism of sediment transport during both winter and spring. While cold fronts enhance sediment export during winter, they also increase sediment import when associated with the higher offshore SSC during spring. We conclude that the sediment discharged at the mouth of the Mississippi River is not completely lost to the coastal zone, but instead partially supplied to deltaic estuaries through their seaward boundaries.

Morphodynamics of intertidal dune field in a mixed wave-tide environment: Case of Baie de Somme in Eastern English Channel

Understanding the response of the intertidal dunes to the varying hydrodynamic conditions in mixed wave-tide environments is of high complexity. Field measurement is one of the most useful approaches to investigate the dune dynamics at different time (semi-diurnal and lunar cycles) and spatial (dunes and associated superimposed bedforms) scales. High-resolution laser scanner data for five successive surveys over a full neap-spring cycle (27 January – 9 February 2014), together with hydrodynamic measurements, have been explored to investigate the morphodynamics of dunes, their superimposed bedforms and their migration in a mixed wave-tide environment (case of the Baie de Somme, Eastern of the English Channel), by the use of a stochastic-physical coupling approach. The dunes are ~12 m wavelength, 0.4 height and oriented orthogonally to the shoreline with angles varying between - 45° and + 45° corresponding to clockwise and counter-clockwise changes, respectively.A stochastic approach, based on spectral analysis, exhibits a series of morphological defects modulating the dune changes following several scale ranges in response to various energy conditions. Such defects can be classified into four categories: (1) limited structures of ~1.6 m wavelengths locally developed along the dune during flood drainage; (2) small-scale superimposed bedforms of ~3.2 m wavelengths built-up across the dunes after high hydrodynamics and proliferated when the energy deceases; (3) large-scale superimposed bedforms of ~6.4 m resulting from the morphological growth of the smaller bedforms; (4) crestline sinuosity with ~25 m wavelength and induced by changes in the orientation of the dunes when they are exposed to high tide-wave activity. The evolving dune defects over the neap-spring cycle is strongly related to the physical mechanisms of sediment transport supplying the migrating dunes and favorable for their self-organization.The physical study of the migrating dunes with the wave-tide driven sediment transport highlights that the long-dune migration components are significantly important during high-energy episodes with a mean explained variance of 65% of the total migration while they decrease to 32% for moderate wave-tide conditions. Good matches of the cross- and long-migration components with the along-shore and the cross-shore sediment transport quantities are recorded with R2 coefficients higher than 70%. These coefficients increase for the offshore dunes where their morphological response to the time-averaged sediment transport is faster while onshore dunes respond at slower timescales resulting in lower R2 coefficients.

Analyzing and modeling sub-diffusive transport of bedload along a heterogeneous gravel bed using stochastic and statistical methods

Bedload transport is one of the main mechanisms for sediment transport in rivers. Bedload transport may exhibit anomalous dispersion behavior during the formation of clusters on the surface of a heterogeneous river bed, which cannot be quantified by the classical Fick’s law of diffusion. This study simplifies the complex bedload transport process to a “mass-spring-damper” system, which can describe the cluster formation process based on the observed cluster geometry. The simulation results are consistent with the morphology of clusters extracted from the experimental gravel bed in our flume. The level of spatial heterogeneity of individual clusters is characterized to distinguish the planform morphology of each cluster, based on estimation of the Hausdorff-Besicovitch dimension of the cluster area. Trapping of sediment particles during bedload transport on a heterogeneous gravel bed is quantified using a random walk approach and the birth-death emigration-immigration Markov process. Sub-diffusive dynamics of bedload transport is also modeled simultaneously using the Langevin equation that defines statistical properties of bedload sediments. Further analysis indicates that the microscopic stochastic Langevin equation relates to a macroscopic deterministic equation according to the continuous time random walk (CTRW) theory.