11.1 Sediment transport

Sediment transport is a crucial process in multiphase flow modeling, involving the movement of solid particles through fluids like water or air. It's essential for predicting sediment-laden flow behavior and environmental impacts. Key concepts include erosion, entrainment, deposition, and particle characteristics.

Understanding sediment transport mechanisms is vital for developing accurate models and predicting morphological changes in water bodies. These mechanisms include bedload, suspended load, and wash load transport, each influenced by flow conditions and sediment properties. Proper modeling approaches are crucial for effective sediment management.

Sediment transport fundamentals

• Sediment transport is a critical process in multiphase flow modeling that involves the movement of solid particles through a fluid medium, such as water or air
• Understanding the fundamentals of sediment transport is essential for accurately predicting the behavior of sediment-laden flows and their impacts on the environment
• Key concepts in sediment transport include erosion, entrainment, deposition, particle characteristics, and the distinction between cohesive and non-cohesive sediments

Erosion, entrainment, and deposition

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• Erosion is the detachment of sediment particles from the bed or banks of a water body due to the action of flowing water or wind
• Entrainment refers to the lifting of sediment particles into the flow, where they are transported as suspended load or bedload
• Deposition occurs when the flow conditions (velocity, turbulence) are insufficient to keep the sediment particles in suspension, causing them to settle back onto the bed
• The balance between erosion, entrainment, and deposition determines the net transport of sediment in a system

Sediment size, shape, and density

• Sediment size is typically characterized by the particle diameter, which can range from clay-sized particles (<0.002 mm) to boulders (>256 mm)
• Particle shape influences the hydrodynamic behavior of sediment, with more angular particles experiencing greater resistance to flow than rounded particles
• Sediment density, which is a function of the mineral composition, affects the settling velocity and the threshold for entrainment
• The combination of size, shape, and density determines the mobility of sediment particles under given flow conditions

Cohesive vs non-cohesive sediments

• Cohesive sediments, such as clays and silts, exhibit strong inter-particle forces due to their small size and high surface area-to-volume ratio
• Non-cohesive sediments, such as sand and gravel, have weak inter-particle forces and their behavior is dominated by the individual particle properties
• Cohesive sediments tend to form aggregates or flocs, which can significantly alter their transport characteristics compared to individual particles
• The distinction between cohesive and non-cohesive sediments is important for selecting appropriate transport equations and modeling approaches

Sediment transport mechanisms

• Sediment transport occurs through three main mechanisms: bedload, suspended load, and wash load
• The dominant transport mechanism depends on the flow conditions, sediment properties, and the availability of sediment in the system
• Understanding these mechanisms is crucial for developing accurate models of sediment transport and predicting the morphological evolution of water bodies

Bedload transport

• Bedload transport refers to the movement of sediment particles along the bed of a water body, through rolling, sliding, or saltation (hopping)
• Bedload transport typically involves coarser sediment particles (sand and gravel) that remain in contact with the bed
• The rate of bedload transport is a function of the bed shear stress, which is determined by the flow velocity and bed roughness
• Bedload transport plays a significant role in shaping the morphology of rivers, streams, and coastal areas

Suspended load transport

• Suspended load transport involves the movement of sediment particles that are lifted into the water column by turbulence and transported downstream
• Suspended load typically consists of finer sediment particles (silt and clay) that can remain in suspension for extended periods
• The concentration of suspended sediment varies with depth, with higher concentrations near the bed and lower concentrations towards the surface
• Suspended load transport is a major contributor to the total sediment load in many water bodies and can significantly impact water quality and aquatic ecosystems

Wash load transport

• Wash load refers to the finest fraction of sediment (typically clay and fine silt) that is transported in suspension and is not found in significant quantities in the bed material
• Wash load is supplied to the water body from upstream sources, such as hillslope erosion or tributary inflows
• The transport of wash load is largely dependent on the supply of fine sediment rather than the local flow conditions
• Wash load can have significant impacts on water quality, nutrient dynamics, and contaminant transport in aquatic systems

Sediment transport equations

• Sediment transport equations are used to quantify the rate of sediment movement based on the flow conditions and sediment properties
• There are two main categories of sediment transport equations: empirical equations and physics-based equations
• The selection of an appropriate equation depends on the available data, the desired level of accuracy, and the specific application

Empirical equations and limitations

• Empirical sediment transport equations are based on field or laboratory observations and relate the sediment transport rate to flow parameters, such as velocity or shear stress
• Examples of empirical equations include the Meyer-Peter and Müller equation for bedload transport and the Engelund-Hansen equation for total load transport
• Empirical equations are relatively simple to apply but have limited transferability to conditions outside the range of data used in their development
• These equations often have site-specific coefficients that need to be calibrated using local data

Physics-based equations and assumptions

• Physics-based sediment transport equations are derived from fundamental principles of fluid mechanics and sediment dynamics
• These equations, such as the Exner equation for bed evolution or the advection-diffusion equation for suspended sediment transport, provide a more mechanistic description of the transport processes
• Physics-based equations often involve simplifying assumptions, such as steady and uniform flow, or the use of depth-averaged variables
• While more complex than empirical equations, physics-based equations offer greater flexibility and can be applied to a wider range of conditions

Sediment transport modeling approaches

• Sediment transport modeling involves the application of mathematical equations and numerical methods to simulate the movement of sediment in water bodies
• There are various approaches to sediment transport modeling, each with its own advantages and limitations
• The choice of modeling approach depends on the spatial and temporal scales of interest, the available data, and the computational resources

Eulerian vs Lagrangian methods

• Eulerian methods describe the sediment transport processes at fixed points in space, using a grid or mesh to discretize the domain
• Lagrangian methods track the movement of individual sediment particles or particle groups through the domain
• Eulerian methods are well-suited for modeling large-scale, long-term sediment transport processes, such as the evolution of river morphology
• Lagrangian methods are more appropriate for simulating the transport of discrete particles, such as the dispersion of dredged material or the movement of contaminants attached to sediment

Decoupled vs coupled flow and sediment transport

• Decoupled modeling approaches simulate the flow and sediment transport processes separately, with the output from the flow model serving as input to the sediment transport model
• Coupled modeling approaches solve the flow and sediment transport equations simultaneously, allowing for dynamic feedback between the two processes
• Decoupled approaches are computationally more efficient but may not capture the full complexity of the interactions between flow and sediment transport
• Coupled approaches provide a more realistic representation of the system dynamics but require more computational resources and can be more challenging to implement

1D, 2D, and 3D modeling considerations

• One-dimensional (1D) models simulate sediment transport along a single spatial dimension, such as the longitudinal profile of a river or a cross-shore transect in a coastal area
• Two-dimensional (2D) models consider sediment transport processes in two spatial dimensions, typically in the horizontal plane (e.g., depth-averaged models)
• Three-dimensional (3D) models simulate sediment transport processes in all three spatial dimensions, allowing for the representation of complex flow structures and vertical gradients in sediment concentration
• The choice of model dimensionality depends on the spatial scales of interest, the required level of detail, and the available computational resources

Sediment transport in different environments

• Sediment transport processes vary significantly across different environments, such as rivers, coasts, and lakes
• The characteristics of the sediment, the flow conditions, and the geomorphology of the system all influence the nature and magnitude of sediment transport
• Understanding the specific sediment transport processes in each environment is essential for effective management and decision-making

Rivers and streams

• In rivers and streams, sediment transport is driven by the interaction between the flowing water and the bed material
• The transport capacity of a river depends on factors such as the flow velocity, depth, and slope, as well as the size and availability of sediment
• Rivers can exhibit a range of sediment transport regimes, from supply-limited conditions, where the transport capacity exceeds the sediment supply, to transport-limited conditions, where the sediment supply exceeds the transport capacity
• Sediment transport in rivers plays a crucial role in shaping the channel morphology, creating and maintaining habitats, and controlling the downstream delivery of sediment to coastal areas

Coastal areas and estuaries

• Coastal sediment transport is influenced by a complex interplay of waves, tides, and currents
• In the nearshore zone, wave action is the dominant driver of sediment transport, with breaking waves mobilizing sediment and generating longshore and cross-shore currents
• In estuaries, sediment transport is affected by the interaction between riverine and marine processes, with tidal currents and salinity gradients playing a significant role
• Coastal sediment transport is critical for the stability and resilience of beaches, dunes, and wetlands, as well as for the maintenance of navigation channels and the protection of coastal infrastructure

Reservoirs and lakes

• Sediment transport in reservoirs and lakes is primarily driven by the inflow of sediment-laden water from tributaries and the settling of sediment particles in the quiescent water body
• The transport and deposition of sediment in reservoirs can lead to the loss of storage capacity, the clogging of outlet structures, and the alteration of downstream flow and sediment regimes
• In lakes, sediment transport is influenced by factors such as wind-driven circulation, thermal stratification, and biological processes (e.g., bioturbation)
• Understanding sediment transport in reservoirs and lakes is important for the management of water resources, the assessment of water quality, and the preservation of aquatic ecosystems

Impacts of sediment transport

• Sediment transport has wide-ranging impacts on the physical, ecological, and socio-economic aspects of water bodies and their surrounding environments
• These impacts can be both positive and negative, depending on the context and the management objectives
• Recognizing and quantifying the impacts of sediment transport is crucial for informing management decisions and developing sustainable strategies

Channel morphology and stability

• Sediment transport plays a key role in shaping the morphology of rivers, streams, and coastal areas
• The balance between sediment supply and transport capacity determines the stability of channels, with excess sediment leading to aggradation (deposition) and insufficient sediment leading to degradation (erosion)
• Changes in sediment transport can result in the modification of channel geometry, the formation or destruction of geomorphic features (e.g., bars, islands), and the alteration of flow patterns
• Maintaining a stable channel morphology is important for flood control, navigation, and the integrity of infrastructure such as bridges and pipelines

Ecosystem health and habitat

• Sediment transport processes have a significant influence on the health and functioning of aquatic ecosystems
• Suspended sediment can affect water clarity, light penetration, and primary productivity, while deposited sediment can smother benthic organisms and alter substrate composition
• Sediment transport is essential for the creation and maintenance of habitats, such as gravel beds for fish spawning, sandy beaches for turtle nesting, and mudflats for shorebird foraging
• Changes in sediment transport regimes, due to factors such as dam construction or land use change, can disrupt the balance of ecosystems and lead to the loss of biodiversity

Infrastructure and navigation

• Sediment transport can have significant impacts on the performance and longevity of infrastructure in water bodies, such as dams, bridges, and harbor facilities
• Sedimentation in reservoirs can reduce storage capacity, impair water quality, and damage turbines and other mechanical components
• In navigable waterways, sediment deposition can create shoals and reduce channel depth, hindering the passage of vessels and increasing the need for dredging
• Coastal infrastructure, such as breakwaters and seawalls, can be undermined by sediment scour or overtopped by sediment accumulation, compromising their structural integrity and effectiveness

Sediment management strategies

• Sediment management involves the development and implementation of strategies to address the impacts of sediment transport on water bodies and their associated resources
• Effective sediment management requires a holistic approach that considers the physical, ecological, and socio-economic aspects of the system
• Management strategies can range from sediment control measures to dredging and disposal techniques, as well as ecological restoration and enhancement

Sediment control measures

• Sediment control measures aim to reduce the amount of sediment entering water bodies from upstream sources or to minimize the mobilization of sediment within the system
• Examples of sediment control measures include:
  • Erosion control practices, such as vegetated buffer strips, terracing, and sediment basins, to reduce soil erosion and sediment delivery from land surfaces
  • In-stream structures, such as check dams and grade control structures, to stabilize channel beds and banks and trap sediment
  • Flow regulation through dam operations to modify the timing and magnitude of sediment transport and deposition
• The selection and implementation of sediment control measures should be based on a thorough understanding of the sediment sources, pathways, and impacts in the system

Dredging and disposal techniques

• Dredging involves the removal of accumulated sediment from water bodies to maintain or restore channel depth, improve water quality, or recover valuable resources
• Dredging techniques can be classified as mechanical (e.g., clamshell, dragline) or hydraulic (e.g., cutterhead, hopper), depending on the method of sediment removal and transport
• The disposal of dredged material can be a significant challenge, with options including open-water disposal, confined disposal facilities, and beneficial use (e.g., beach nourishment, habitat creation)
• The environmental impacts of dredging and disposal, such as the resuspension of contaminated sediments or the alteration of benthic habitats, must be carefully assessed and mitigated

Ecological restoration and enhancement

• Ecological restoration and enhancement strategies aim to improve the health and resilience of aquatic ecosystems by managing sediment transport processes
• Examples of ecological restoration and enhancement approaches include:
  • Reconnecting floodplains and restoring natural sediment dynamics to promote the creation and maintenance of diverse habitats
  • Constructing artificial reefs or oyster beds to stabilize sediments, attenuate waves, and provide substrate for aquatic organisms
  • Planting riparian vegetation to stabilize banks, filter sediment, and provide shade and organic matter inputs to the system
• The success of ecological restoration and enhancement projects depends on a sound understanding of the sediment transport processes and the integration of ecological principles into the design and implementation of management actions

Case studies and applications

• Case studies and applications demonstrate the real-world implications of sediment transport processes and the effectiveness of different management strategies
• These examples highlight the challenges and opportunities associated with sediment management in various contexts and provide valuable lessons for future projects
• Analyzing case studies and applications helps to bridge the gap between theoretical concepts and practical implementation, fostering a more comprehensive understanding of sediment transport and its impacts

Sediment transport in dam removal projects

• Dam removal is an increasingly common strategy for restoring river connectivity, improving water quality, and enhancing aquatic habitats
• Sediment transport is a critical consideration in dam removal projects, as the release of accumulated sediment can have significant downstream impacts
• Case studies, such as the removal of the Elwha River dams in Washington, USA, demonstrate the importance of careful planning, monitoring, and adaptive management to minimize negative impacts and maximize ecological benefits
• Lessons learned from dam removal projects can inform the design and implementation of future projects, as well as the development of sediment management strategies in other contexts

Beach nourishment and coastal protection

• Beach nourishment involves the placement of sand on eroding beaches to restore and maintain their width, elevation, and recreational value
• Coastal protection structures, such as groins and breakwaters, are designed to stabilize shorelines and reduce the impact of waves and currents on coastal infrastructure
• Case studies, such as the Miami Beach nourishment project in Florida, USA, or the Sand Motor in the Netherlands, illustrate the effectiveness and challenges of these approaches in managing coastal sediment transport
• The long-term performance and sustainability of beach nourishment and coastal protection measures depend on a thorough understanding of the sediment transport processes, the availability of suitable sediment sources, and the integration of ecological and socio-economic considerations

Contaminant transport in sediments

• Sediments can act as a sink and a source of contaminants, such as heavy metals, pesticides, and persistent organic pollutants
• The transport and fate of contaminants in sediments are influenced by factors such as sediment composition, hydrodynamics, and biogeochemical processes
• Case studies, such as the Hudson River PCB contamination in New York, USA, or the Minamata Bay mercury pollution in Japan, demonstrate the complexity and challenges of managing contaminated sediments
• Effective management of contaminant transport in sediments requires a multidisciplinary approach that integrates knowledge from fields such as environmental chemistry, toxicology, and sediment transport modeling, as well as stakeholder engagement and risk communication