Reading Question
1. What are modes of sediment transport?Transport of Siliciclastic Sediment
• Modes of Transport
• Fluid Flows
– Wind
– Moving water
– Glacial ice
• Sediment Gravity Flows
– Debris & mud flows, turbidite currents
• Why study the properties of Flow?
– We only see the results…
Question
• Discuss the following with the people next to you:
• What factors are important to the transport of sediment?
Fluid Flow
• What is a fluid?
– Changes shape under own weight
– Responds to external forces
– Does not resist shear forces
• Important Fluid Forces
• Gravitational forces
• Viscous forces
• Inertial ForcesFluid Properties
• Important Physical Properties
• Density - r (rho) = mass/unit volume
• Increases with decreasing T
• Magnitude of forces
• Settling velocities
• Water = 0.998g/mL at 20º C - 700X r of air
• Particle size
Fluid Properties
• Viscosity - measure of ability of fluids to flow - inverse relationship
to temperature
• Dynamic viscosity- µ (mu) = t (tau)/du/dy =shear stress/velocity
gradient
– Tau = shear stress = shearing force/unit area
– du/dy = velocity gradient = change in local fluid velocity/ change in
distance from shearing surface
– Sediment size and sorting
• Shear stress along with shear velocity (U) control erosion and entrainmentFluid
Types
• Air and water
• Ice or water with sediment
• 3 types based on change in µ relative to shear or strain (deformation)
rates
• Newtonian fluids - no strength, no change in µ as t increases
– Clear water
• Non-newtonian fluids - no strength, but variable µ relative to
t
– Sands and mud, µ decreases as V increases
• Bingham plastic - initial strength must be overcome before µ change
with t change
– Very concentrated sediment loads - debris flows - mud with floating
grains
• Thixotropic substances - pseudoplastic - has strength until sheared
- t applied over time beforeµ change with t
– Liquifaction, turbidites, varies in muddy substrates
Laminar versus Turbulent Flow
• 2 end member flow types - (1) laminar, and (2) turbulent
• In between = transitional - characters of both
1. Laminar -water molecules move in straight lines parallel to current
• Very shallow, slow moving fluids, ice
2. Turbulent - complex - water molecules move in all directions
• Abundant mixing
• Most important flow type for sed. transport
Reynolds Number
• Re - experimental derived, dimensionless number that predicts degree
of turbulence
– Ratio of intertial forces that tend to cause turbulence to viscous forces
that suppress turbulence
• Re = ULr/µ = UL/v
– Re = (fluid inertial forces)/(fluid viscous forces)
– U = mean velocity, L = depth, v= kinematic viscosity
– Turbulent flow = Re > 2000
– Laminar flow - Re < 500
– What about air?The Boundary Layer
• Because of friction - the edges of the flow have different characteristics
• Thickness depends on Re and bed roughness
• Laminar flow = thick boundary layer
• Turbulent flow = thin boundary layer
• Viscous/laminar sublayer
– Muddy/smooth versus rough bottoms
– A control on sediment transport
– Determines how much shear applied to grains
– Bed Shear Stress - forces that result in grain movement and differences
in pressure above and below grains
Sediments and Flow
• Transport = entrainment
• Entrainment - critical threshold
• Grain factors - size, shape, and density
• Fluid factors - shear stress, viscosity, velocity
• Forces holding the grain in place = gravity and friction
• Forces that move the grain - drag and lift (fluid force)
• Lift - pressure gradient - Bernoulli effect
Entrainment
• Critical threshold - related to grain size
• Also include - shape, sorting, bed roughness
• Experimentally derived thresholds of grain movement
• Hjulström diagram
• Why is clay and silt so variable?
Settling Velocities
• Terminal Fall Velocity
• Controls = viscosity, size, shape, and density
• Opposing forces - buoyancy and drag
– gravity
Sediment Loads
• Bedload and suspended load
• Bedload transport
– Traction
– Saltation
• Suspended transport
– Intermittent
– Continuous
• Washload