Non-cohesive Sediment Research Articles

Modelling the impact of sediment composition on long-term estuarine morphodynamics

Sediment composition, characterized by different contents of cohesive and non-cohesive sediments, is known to play a role on long-term estuarine and deltaic morphodynamics, but the exact impact is poorly understood. We establish a two-dimensional morphodynamic model to investigate the influence of different sediment compositions on the development of a schematic fluvio-deltaic system driven by river and tides. Though excluding the density effects, results suggest that the model captures the development of distributary channels and elongated sand bars with resemblance to that in the Yangtze Estuary. Sensitivity simulations show fundamentally different channel-shoal patterns take shape under different sediment compositions. Ebb dominance and associated seaward sediment flushing lead to faster morphodynamic development and more prograded delta under larger river discharge and sediment supply. We detect a positive correlation between the content of cohesive sediment and the speed of development, particularly cohesive sediment content is <50%. However, when the proportion of mud is larger (i.e., 50–75%), a deceleration of the morphological development occurs after 200 years. A sand-dominated environment exhibits the largest channel numbers and fast channel formation near the mouth within the first 300 morphodynamic years. Spatial distribution of bottom sediments changes with morphology, exhibiting increasing mud deposits near the mouth, whilst the sand remains inside the estuary. This study indicates the importance and need for a more realistic representation of bed compositions in long-term estuarine morphodynamic simulations.

Mountain permafrost landslides: Experimental study investigating molard formation processes

Landslides triggered by degrading mountain permafrost pose an increasing risk to life and infrastructure in mountain regions due to current climate change trends. Because discontinous mountain permafrost is not directly detectable by remote sensing, permafrost molards offer a way of indirectly detecting its location and state. Permafrost molards are cones of loose debris that are associated with landslide deposits. They originate from ice-cemented blocks of sediment that are transported downslope with the landslide. These ice-cemented blocks are fragmented parts of the permafrost initially located in the landslide source area. Over time, these blocks degrade into conical mounds with diameters typically ranging from decimeters up to 40 m. They indicate the presence of an area of degrading permafrost at the level of the detachment zone. The physical processes that lead to the formation of molards from the ice-cemented blocks have not yet been studied in detail. Therefore, we perform an experimental study to recreate molards in a controlled laboratory environment and investigate the key formation processes. We downscale the molards to an initial cube side length of ~30 cm, and reduced the model's complexity by using two different types of gravel with a defined granulometry and lithology. We quantified the degradation phase by using a novel photogrammetric time-lapse system to detect changes in the digital elevation model between half-hourly time-steps. For the first time, we successfully recreated morphologies resembling molards under controlled laboratory conditions on a decimeter scale. We find the main processes to produce the final molard shape are cascades of grainfall for slightly cohesive sediment, and individual grainfall for non-cohesive sediment. Our experiments reveal three possible cross-section shapes (bell-shaped, triangular, trapezoidal) that correspond to molards that can be found in the field. Along with these field observations, we suggest that it may be possible to identify the cohesion of a molard's sediment based on its morphology. Together with future field data and experiments investigating the granulometry, ice and clay contents of the initial ice-cemented block, we aim to use molards as a record to better understand the landslide dynamics and state of the mountain permafrost.

The Application of Sand Transport with Cohesive Admixtures Model for Predicting Flushing Flows in Channels

The feature of self-cleansing in sewer pipes is a standard requirement in the design of drainage systems, as sediments deposited on the channel bottom cause changes in channel geometric properties and in hydrodynamic parameters, including the friction caused by the cohesive forces of sediment fractions. Here, it is shown that the content of cohesive fractions significantly inhibits the transport of non-cohesive sediments. This paper presents an advanced calculation procedure for estimating flushing flows in channels. This procedure is based on innovative predictive models developed for non-cohesive and granulometrically heterogeneous sediment transport with additional cohesive fraction content to estimate the magnitude of increased flow necessary to ensure self-cleansing of channels. The computations according to the proposed procedure were carried out for a wide range of hydrodynamic conditions, two grain diameters, six cohesive (clay) fraction additive contents and two critical stress values. The trend lines of calculations were composed with the results of experimental studies in hydraulic flumes.

Numerical modeling of local scour around a subsea pipeline on cohesive seabed under steady currents

The present study introduces a numerical model for simulating local scour around a subsea pipeline on a cohesive seabed, representing a pioneering endeavor in the development of such a numerical model. The flow dynamics around the pipeline are solved using Reynolds-Averaged Navier–Stokes (RANS) equations with the SST k–ω turbulent model. Sediment movement on cohesive seabed is assessed through erosion rates, determined by an empirical formula derived from experimental data, rather than sediment transport rates typically used for non-cohesive sediment. Three different erosion rate formulas—linear, power law, and stochastic—are investigated in the simulation of the scour process. The numerical simulations are validated using laboratory data with artificial and marine sediments, demonstrating a satisfactory level of agreement in predicting scour bed profiles. Nevertheless, it is recommended to utilize the erosion rate formula that is calibrated to the specific sediment type based on experimental data for precise numerical prediction of scour process. The study reveals that the equilibrium scour depth is approximately a linear function of the bed shear stress, and the nondimensional time scale is inversely proportional to the bed shear stress within the specified range. Subsequently, an examination of the scale effect on the scour process with cohesive sediment around a pipeline is conducted. The results indicate that the nondimensional equilibrium scour depth marginally increases with larger pipeline diameter on cohesive seabed, contrary to the observed trend for non-cohesive sediments.

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.

Experimental study of reservoir flushing through a bottom tunnel initially covered by cohesive sediment

Reservoir sediment flushing, one of the most effective strategies for alleviating reservoir sedimentation, involves discharging sediment-laden flows downstream through bottom tunnels. However, whether flushing can be accomplished if the intake of a bottom tunnel is initially covered by cohesive sediment remains poorly understood. Here, flume experiments were done to investigate cohesive sediment flushing in a reservoir. It is demonstrated that cohesive sediment in a reservoir is harder to flush than non-cohesive sediment. A higher water level in the reservoir, initially smaller cover layer thickness, and lower dry density of the sediment favor the occurrence of sediment flushing. The flushing process of cohesive sediment is significantly affected by seepage. Under the combined action of gravity erosion and water erosion, the scour hole upstream of the dam is characterized by angular and broken edges. The threshold conditions for flushing of non-cohesive and cohesive sediments are evaluated. Empirical formulas applicable to both non-cohesive and cohesive sediment are proposed to estimate the equilibrium scour depth immediately upstream of the bottom tunnel intake. Also, empirical models are proposed for the time variation of sediment position in the bottom tunnel. The current findings are significant for informing the design and operation of reservoirs on rivers carrying fine-grained cohesive sediment in support of reservoir benefits and capacity preservation.

Experimental study on the effects of sediment size gradation and suspended sediment concentration on the settling velocity, ws

The settling velocity of non-uniform sediment is of great importance for the sediment transport on coasts and estuaries. Although existing settling velocity formulas have already considered factors such as the Suspended Sediment Concentration (SSC), temperature, salinity, turbulence, and the effect of size gradation on the settling velocity, more studies are needed. In this study, the fiber optic sensors and Particle Track Velocimetry (PTV) for high SSC were used to measure the settling velocity of two size gradations with different SSCs in a settling column in laboratory. The size gradation curves and settling velocities of these two non-cohesive sediment with SSCs ranged from 0.1 kg/m3 to 50 kg/m3 were obtained. When the SSC is less than or equal to 0.1 kg/m3, the settling velocity is close to the grain size distribution weighted average settling velocity. With the increase of SSC, the distance between sediment particles decreases, and the mutual influence between different components become important, resulting in a decrease in the settling velocity of large-sized particles and an increase in the settling velocity of small-sized particles. This phenomenon is more pronounced for natural sediments. Based on the experimental results, a particle size distribution correction coefficient was proposed to fit the fine-grained settling velocity in quiescent water considering size gradation and sediment concentration.

Influence of Cumulative Effective Stream Power on Scour Depth Prediction Around Bridge Piers in Cohesive Bed Sediments

Streambed scour in cohesive sediment is complex because erosion processes depend on the physical, geochemical, and biological properties of the sediment. The scouring processes can also be characterized as a slow fatigue phenomenon. Therefore, repetitive hydraulic loadings from multiple stormflow events are likely necessary for equilibrium scour depths to develop in cohesive sediment compared with non-cohesive sediment. Cumulative effective stream power, which is a surrogate measure of effective stream power duration, showed a significant relation with scour development and propagation in cohesive sediments around bridge piers, where results from this study identified a statistically significant correlation between cumulative effective stream power and the observed scour depths around different bridge piers ( R2 = 0.56, p &lt; 0.001). However, some localized and site-specific variations were observed. It was also observed that scour depth development in cohesive soil appeared to be dependent on effective shear duration, rather than the number of flow events above erosion threshold values. In addition, the relationship between an erodibility index ( K) and critical stream power showed a significant statistical correlation ( R2 = 0.61, p = 0.017). Results from this study deviated from the Annandale empirical relationship for sediments when K &lt; 0.1. This finding supports that site-specific critical stream power should be measured using an empirical relationship for cohesive bed sediments to predict scour depths.

Assessment of sediment accumulation inside the harbour basin in the development plan due to longshore sediment transport (LST) rate. A case study of Genaveh port

ABSTRACT In this paper, the hydrographic data, empirical formulas and numerical modelling were used to investigate the longshore sediment transport (LST) rate in the Genaveh port to calculate the sediment accumulation in the harbour basin under the development plan. The Genaveh port is located on the northern coast of the Persian Gulf. Based on the field sediment collection, the type of sediment in this area is fine sand and silty sand. The empirical formulas, LITPACK, and MIKE21-FM were utilised to calculate the LST rate in the region, bathymetric changes in front of the port, and the sediment accumulation in the harbour basin, respectively. The results indicated that the main direction of non-cohesive sediment is from the northwest to the southeast in the region. Moreover, due to the existence of Dareh Gap River, the location of the harbour basin’s mouth is important in reducing sediment accumulation in the harbour basin.

Sediment Grain Size Affects Vegetation Patterns in River-Dominated Deltas

River deltas contain some of the most densely populated areas in the world and are characterized by a rich biodiversity and highly productive ecosystems. Although previous studies have revealed that sediment grain size plays a key role in determining delta morphology, its subsequent effects on deltaic vegetation distribution patterns remain elusive. We used a theoretical templet to facilitate our study by conducting numerical experiments to simulate deltaic morphological evolution in response to various sediment grain sizes and explored the vegetation distribution determined by the elevation profile of a typical river-dominated delta. Our results showed that a fine-grained, cohesive delta would develop vegetation patterns transitioning from high- to low-elevation adapted vegetation species with increasing distance, whereas the same vegetation transformation trend could occur with decreasing distance to the center of a large-grained, noncohesive delta. The modeled vegetation distribution pattern of fine-grained, cohesive and large-grained, noncohesive deltas could be well demonstrated in the natural deltas. In addition, vegetation distribution patterns are related to the grain size-driven morphological processes of deltas, and the leading role of cohesive and noncohesive sediments on constructed delta is critical in determining the resulting vegetation distribution pattern. The dominance of each vegetation type would gradually become stable as delta matures. The interspecies competitions could influence distribution patterns of each vegetation type by enhancing the fragmentation of mid-elevation adapted vegetation. The effects of sediment grain size on the deltaic vegetation patterns associated with a delta’s morphological processes have potential implications for the conservation and restoration of deltaic habitats.

Small-scale measurement of the transition in fracture behavior of marine sediments

Bubbles grow and burrows extend through cohesive, muddy marine sediments by fracture. In contrast, sands are non-cohesive, granular materials. Natural sediments comprised of heterogeneous mixtures of muds and sands are common in coastal areas and provide important habitat for infaunal animals. To explore the transition from cohesive to non-cohesive mechanical behavior of natural sediments, we modified a probe designed for measuring fracture toughness (KIc). The helical probe is rotated and translated into sediment to grip a plug of sediment, then translated upward to break off the plug while force is measured. Fracture toughness is calculated from the peak net force. The probe shows clearly distinct results in muddier sediments, in which fracture occurs, and in sandier sediments, in which no fracture occurs. The modified probe is limited to near-surface sediments, but it provides a novel method for distinguishing cohesive sediments with tensile strength from non-cohesive sediments on scales relevant for burrowing animals or bubble growth. This measurement allows for comparison of surface and subsurface cohesion and for assessing how tensile strength depends on other properties of sediments.

Field based inventory of river bank erosion susceptibility model (BESI) of Raidak-II river in the Himalayan foreland basin

ABSTRACT River bank erosion is a fluvio-hydrological hazard, and sometimes, it turns into disaster in the human-encroached river bank and flood plain. The principal objective of this study is to detect the river bank erosion potential zone using new Bank Erosion Susceptibility Index (BESI) model and spatiotemporal shifting of river Raidak-II (1980–2020). Sedimentary bank facies (SBF) analysis was conducted to identify the nature of cohesiveness of bank materials. The result showed that the maximum average lateral shifting (213.20 m) was recorded in the year 1990 (right bank), whereas the minimum average shifting (77.32 m) was measured in the year 2020 (right bank). The result also showed that the right bank of Raidak-II was mostly oscillated due to poorly sorted quaternary non-cohesive bank materials. The Bank Erosion Hazard Index (BEHI) model showed that 46.15% and 50% bank erosion sites are fallen in high-to-extreme bank erosion susceptibility zones in the years 2020 and 2022. In the case of BESI model, 69.23% and77.77% bank erosion sites are confined in high-to-extreme bank erosion susceptibility zones in the years 2020 and 2022. Therefore, BSEI model gives more precise results compared with BEHI due to three additional factors. The poorly sorted non-cohesive quaternary sediments stimulate high rate of bank erosion within Himalayan foreland basin.

On sediment incipient motion of marine cohesive sediments with different bulk densities

This paper aims to methodically examine the incipient characteristics of natural marine fine sediment with particle size less than 0.012 mm for a variety range of bulk densities. Theoretical derivation and experimental data analysis are appropriately combined to explore the effect of bulk density on the inter-particle cohesive force and to deduce the incipient shear stress (ISS) of fine sediment. Under the same particle size condition, the achieved results indicate that the ISS of sediment intensifies with the growth of the bulk density. In addition, the larger the bulk density, the faster the increase rate, revealing a power exponential growth trend. The ISS of sediment also magnifies with the reduction of the particle size under the same bulk density condition; however, as the particle size decreases to 0.007 mm, the ISS of sediment would no longer vary. The influential factors on the cohesive force, including intermolecular gravitational force, film water pressure, and bulk density, are thoroughly analyzed, and a rational expression for the cohesive force is also proposed. The modified shields parameter represents the ratio between the sediment driving force and the stabilization force (e.g., gravitational and cohesive forces) and is still a function of the sediment Reynolds number. The modified shields curve still follows the shape of the original shields curve and satisfies both cohesive and non-cohesive sediment conditions but with differences in the form of shields parameter.

Influence of different skew angles of the submerged vane on pressure flushing performance

Siltation significantly threatens a reservoir’s original storage capacity and lifespan. Pressure flushing is an effective measure against siltation through the partial drawdown of the reservoir water level with limited flushed cone volumes in front of the bottom outlet. In this study, a novel configuration with submerged vanes has been proposed and tested experimentally to increase the flushed sediment volume during pressure flushing. In the new configuration, submerged vanes aligned with ten skew angles (θ) of 10°, 20°, 30°, 45°, 70°, 110°, 135°, 150°, 160°, and 170° to the flow direction in noncohesive sediment bed materials were used. The results showed that increasing the skew angle at 45° ≤ θ ≤ 160° increased the flushing cone geometry. The minimum and maximum flushing cone dimensions and volume occurred at skew angles of θ = 45° and 135° ≤ θ ≤ 160° around the bottom outlet. The sediment flushing volume in the presence of submerged vanes at 135° ≤ θ ≤ 160° increased 25 times compared to tests without submerged vanes. Eventually, nonlinear regression analysis yielded an equation for estimating flushing cone volumes. The developed equation was tested in real case studies of a target reservoir, and an acceptable correlation between the calculated and experimental results was obtained.

The effectiveness parameters analysis for piers scour calculation

AbstractA computational fluid dynamics numerical model addressed the problem of local scouring and deposition calculation for non-cohesive sediment and clear water conditions near single and double cylindrical piers. The numerical results of single cylindrical piers correlate very well with the physical model's results while are higher than the case of the double pier, especially when the large-eddy turbulence model, the van Rijn bed-load transport equation, and fine mesh size are considered. Additionally, the final numerical predictions are compared to experimental data after parameters effectiveness explores the range of results based on projected user inputs like the bed-load equation, mesh cell size, and turbulence model.

A model for predicting the grain size distribution of an armor layer under clear water scouring

Clear water scouring often occurs in the channel downstream of a dam and changes the grain size distribution of the channel bed, which impacts the bed resistance and flood conveyance capability. Based on the proposed spatial-mean rolling probability of the non-cohesive sediment and the effects of the constantly decreasing sand content during riverbed armoring, a model was established to predict the grain size distribution of an armor layer under clear water scouring. Based on iterative calculation and the new spatial-mean rolling probability, the predicted grain size distribution of an armored bed was found to be in good agreement with measurements from laboratory experiments and field studies. This indicates that the new model is capable of accurately predicting the grain size distribution of an armor layer. Finally, the model was applied to predict the grain size distribution of the bed load and the depth of bed erosion in combination with an existing estimator of the thickness of the active layer in a channel bed. A high friction velocity transported more coarse sediment downstream and resulted in more significant scouring.

Development of a Two-Dimensional Hybrid Sediment-Transport Model

This paper presents the development of a two-dimensional hydrodynamic sediment transport model using the finite volume method based on a collocated unstructured hybrid-mesh system consisting of triangular and quadrilateral cells. The model is a single-phase nonequilibrium sediment-transport model for nonuniform and noncohesive sediments in unsteady turbulent flows that considers multiple sediment-transport processes such as deposition, erosion, transport, and bed sorting. This model features a hybrid unstructured mesh system for easy mesh generation in complex domains. To avoid interpolation from vertices in conventional unstructured models, this model adopted a second-order accurate edge-gradient evaluation method to consider the mesh irregularities based on Taylor’s series expansion. In addition, the multipoint momentum interpolation corrections were integrated to avoid possible nonphysical oscillations during the wetting-and-drying process, common in unsteady sediment transport problems, to ensure both numerical stability and numerical accuracy. The developed sediment transport model was validated by a benchmark degradation case for the erosion process with armoring effects, a benchmark aggradation case for the deposition process, and a naturally meandering river for long-term unsteady sediment-transport processes. Finally, the model was successfully applied to simulate sediment transport in a reservoir that was significantly affected by typhoon events.

Lithofacies and Depositional Environments of the Garubathan Alluvial Fan, North Bengal, India

Abstract The sedimentology of the Garubathan alluvial fan of north Bengal foothills is a product of Himalayan neotectonics and monsoon climate. The active neo-tectonics of the Main Central Thrust (MCT), Ramgarh Thrust (RT) and Ghis Transverse Fault (GTF) produce a huge amount of angular clasts in the upper reaches of the river Chel and Mal. During the wet season, either sediment flow by gravity or turbulent stream flow or both, move the clasts to the downhill. The tectonic zone of Main Boundary Thrust (MBT) and Main Frontal Thrust (MFT) are relative passive thrusts than the MCT, RT and GTF in the study area. A very low angle slope is observed in the area of MBT and MFT of the study area. So, the downhill deposition occurred in MBT and MFT zones of the study area. The river Chel and Mal are perinial rivers. During dry periods (pre-monsoon and post-monsoon), in the absence of sediment flow by gravity or turbulent stream flow, the low stream competency deposit the fine sediments. To identify the depositional environments three, seven and six lithofacies varieties were identified in the upper, middle and lower fan surfaces, respectively. The non-cohesive sediment flows by gravity deposited large angular boulders, cobbles and pebbles in the upper fan area. These deposits are either chaotic, unsorted, un-imbricated or graded and imbricated. Deposition of sand is scanty. Both, sediment flow by gravity and fluvial processes deposited angular to round clasts with matrix in the middle fan area. In this area the deposits are reverse graded, reverse to normally graded, laminated and cross-bedded. The high facies diversity of this area indicates highly variable nature of the depositional environments. In the lower fan area, presence of both clast-supported chaotic and matrix-supported laminated or cross-bedded layers suggests a complex nature of the depositional environment of sediment flow by gravity and fluvial processes.

Multistate Overflow Fragility Models for Homogenous Inland Levees with Noncohesive Sediments

Existing efforts for analyzing the overflow fragility of inland levees have focused primarily on breach and incorporated simplified definitions of breach for that purpose. The intermediate events that are precursors of a breach have not been analyzed, and several key performance measures including the duration of overflow, discharge rate, and time to the intermediate and breach events, which can inform decisions on evacuation and recovery planning, have not been investigated. This study, for the first time, provides a set of two-dimensional fragility models with an initial water level before surge and peak surge elevation as intensity measures. Fragility models are developed for the class of homogenous levees with noncohesive sediments. Breach formation is modeled through the Dam/Levee Breach model, and failure probabilities are derived using Monte Carlo Simulation (MCS). Unlike existing fragility models, the new class of fragility models accurately represents key performance failure events in levees including initiation of a breach, local geotechnical instabilities, and breach development. These models can be used in risk analysis frameworks to probabilistically account for local geotechnical failures and breach scenarios. Furthermore, probabilistic models for the time of peak discharge rate, breach peak outflow, and volume of water outflow are developed based on stochastic simulations of the flood performance of levees. A power model is investigated for representing breach peak outflow and is compared with previous deterministic models. The proposed model for breach peak outflow can be integrated with hydraulic inundation modeling to probabilistically estimate the inundated area downstream of levees.

Local scour of cohesive sediment bed at the pile subjected to lateral vibration

In this study, the local scour of cohesive sediment bed at the pile subjected to lateral vibration was investigated experimentally. The experiments were carried out in a unidirectional free-surface flow channel in the laboratory. The findings of the experiments indicate that lateral vibrations produce significant effects on local scour of the bed at the pile, and the effects are completely different for non-cohesive and cohesive sediment bed. For non-cohesive sediments, the equilibrium scour depth decreases with the increase of vibration intensity. On the contrary, for cohesive sediments, the equilibrium scour depth increases with the increase of vibration intensity, and there appears a peak and plateau, which may be attributed to the shear-thinning behavior of cohesive sediments near the pile under vibration loadings. Cohesive sediments around the pile are fluidized by vibration, so that the cohesive force among sediment particles decreases, resulting in larger local scour. An empirical formula for estimating equilibrium local scour depth at pile foundation subjected to lateral vibration is proposed and tested using reported data and data obtained in this study.