5.1 River channel dynamics and sediment transport

River channels are dynamic systems shaped by water flow and sediment transport. They constantly evolve, influenced by factors like discharge, sediment supply, and bank resistance. Understanding these processes is crucial for grasping how rivers shape landscapes over time.

Sediment transport in rivers involves complex interactions between water flow and particles. From tiny clay particles to large boulders, rivers move sediment in various ways. This movement affects channel form, floodplain development, and even long-term landscape evolution.

River Channel Dynamics and Morphology

Factors Influencing Channel Dynamics

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• River channel dynamics result from interplay of water discharge, sediment supply, channel slope, and bed/bank resistance to erosion
• Channel patterns (straight, meandering, braided) determined by stream power, sediment load, and bank stability
  • Straight channels form in areas with low sediment load and resistant banks
  • Meandering channels develop in areas with moderate sediment load and erodible banks
  • Braided channels occur in areas with high sediment load and unstable banks
• Stream equilibrium (grade) involves balance between erosion and deposition, leading to stable longitudinal profile
  • Graded streams adjust their slope to efficiently transport their sediment load
  • Changes in base level can disrupt equilibrium and trigger channel adjustments
• Channel cross-sectional geometry shaped by relationship between discharge, sediment load, and bank material properties
  • Width-to-depth ratio increases with higher sediment loads and more erodible banks
  • Channels in cohesive materials tend to be narrower and deeper than those in non-cohesive sediments

Floodplain Development and Channel Migration

• Floodplain development driven by lateral erosion, point bar formation, and overbank deposition
  • Lateral erosion widens valley and creates space for floodplain development
  • Point bars form on inside of meander bends, gradually becoming part of the floodplain
  • Overbank deposition during floods adds fine-grained sediment to floodplain surface
• Channel migration occurs through processes of erosion and deposition
  • Cutoffs (neck and chute) can dramatically alter channel course
  • Avulsion involves abrupt abandonment of existing channel for new path across floodplain
• Anthropogenic activities significantly alter natural river channel dynamics and morphology
  • Dam construction reduces sediment supply and alters flow regime downstream
  • Channelization increases flow velocity and reduces habitat diversity
  • Levee construction disconnects river from floodplain, concentrating flow in main channel
• Climate change impacts on precipitation patterns and land use changes lead to long-term adjustments in river channel form and behavior
  • Increased frequency of extreme events may accelerate erosion and channel instability
  • Changes in vegetation cover affect runoff patterns and sediment supply to rivers

Sediment Transport in Fluvial Systems

Sediment Entrainment and Transport Modes

• Sediment entrainment occurs when fluid forces (lift and drag) overcome gravitational force and friction holding particles in place
  • Critical shear stress required for entrainment varies with particle size and density
  • Shields parameter used to determine critical shear stress based on particle characteristics
• Transport modes include rolling, sliding, saltation, and suspension, depending on particle size and flow conditions
  • Rolling and sliding occur for larger particles close to the bed
  • Saltation involves particles bouncing along the bed in short hops
  • Suspension keeps finer particles aloft in the water column through turbulence
• Sediment transport capacity influenced by flow velocity, turbulence, and channel geometry
  • Higher velocities and turbulence increase transport capacity
  • Channel constrictions can locally increase transport capacity due to flow acceleration

Deposition and Sediment Sorting

• Deposition occurs when flow velocity decreases below settling velocity of particles
  • Often happens in areas of reduced stream power (pools, channel expansions)
  • Settling velocity depends on particle size, shape, and density
• Hjulström curve illustrates relationship between particle size, flow velocity, and processes of erosion, transport, and deposition
  • Shows critical erosion velocity and settling velocity for different grain sizes
  • Demonstrates why clay particles are difficult to erode once deposited
• Sediment sorting during transport and deposition leads to characteristic grain size distributions in different fluvial environments
  • Downstream fining occurs as larger particles are deposited first
  • Vertical sorting in deposits can indicate changes in flow conditions over time

Stream Power and Sediment Transport

Stream Power Concepts and Calculations

• Stream power defined as rate of energy expenditure per unit length of channel
  • Calculated as product of discharge, slope, and specific weight of water
  • $\Omega = \gamma QS$, where $\Omega$ is stream power, $\gamma$ is specific weight of water, $Q$ is discharge, and $S$ is slope
• Relationship between stream power and sediment transport capacity generally positive and non-linear
  • Higher stream power enables transport of larger particles and greater sediment volumes
  • Critical stream power threshold exists for initiation of sediment motion
• Sediment size influences transport capacity through effect on particle entrainment thresholds and settling velocities
  • Larger particles require greater stream power for entrainment
  • Finer particles have lower settling velocities and can be transported at lower stream powers

Transport Capacity and Sediment Supply

• Competence describes maximum particle size a stream can transport at given flow condition
  • Related to stream power and local hydraulic conditions
  • Can be estimated using empirical equations or critical shear stress approaches
• Transport capacity varies spatially within channel due to variations in local hydraulics and bed roughness
  • Higher in areas of flow convergence or increased slope
  • Lower in areas of flow divergence or decreased slope
• Sediment transport formulae quantify relationship between hydraulic parameters and sediment transport rates
  • Meyer-Peter and Müller equation commonly used for bedload transport: $q_b = 8(\tau^* - \tau_c^*)^{1.5}\sqrt{(s-1)gD^3}$ where $q_b$ is bedload transport rate, $\tau^*$ is dimensionless shear stress, $\tau_c^*$ is critical dimensionless shear stress, $s$ is specific gravity of sediment, $g$ is acceleration due to gravity, and $D$ is particle diameter
• Balance between sediment supply and transport capacity determines whether reach experiences aggradation, degradation, or maintains equilibrium
  • Aggradation occurs when supply exceeds capacity
  • Degradation occurs when capacity exceeds supply
  • Equilibrium maintained when supply matches capacity

Fluvial Load Types

Suspended and Bed Load

• Suspended load consists of fine particles (typically silt and clay) carried within water column by turbulence
  • Concentration often increases with flow depth
  • Can constitute majority of total sediment load, especially during high flow events
• Bed load includes coarser particles (sand and gravel) that move along or near channel bed by rolling, sliding, or saltation
  • Generally makes up smaller proportion of total load compared to suspended load
  • Transport is more episodic and strongly dependent on flow conditions
• Ratio of suspended to bed load can influence channel morphology and sedimentary structures in depositional environments
  • High suspended load ratios associated with muddy floodplains and fine-grained channel deposits
  • High bed load ratios associated with gravel-bed rivers and coarse-grained deposits

Dissolved Load and Measurement Techniques

• Dissolved load comprises ions and molecules in solution, primarily derived from chemical weathering of rocks and soils
  • Major ions include calcium, magnesium, sodium, potassium, bicarbonate, sulfate, and chloride
  • Concentration can vary seasonally and with discharge
• Relative proportions of suspended, bed, and dissolved loads vary with lithology, climate, and watershed characteristics
  • Carbonate watersheds often have high dissolved loads
  • Arid regions may have higher proportion of bed load due to lack of vegetation and flashy runoff
• Measurement techniques differ for each load type
  • Water sampling used for suspended and dissolved loads
    • Depth-integrated samplers collect representative samples throughout water column
    • Automated samplers can capture temporal variations in load
  • Bedload samplers or tracers used for bed load
    • Helley-Smith sampler commonly used for sand and fine gravel
    • Painted or magnetic tracers can track movement of individual particles
• Bed load transport often follows power-law relationship with discharge
  • $Q_b = aQ^b$, where $Q_b$ is bed load transport rate, $Q$ is water discharge, and $a$ and $b$ are empirical coefficients