2. Sediment Transport and Deposition
Media:
1. Water
Overland flow, channel flow
Waves, tides, ocean currents
2. Air (small particle)
3. Ice
4. Gravity
Rock falls (no transport medium
involved)
Debris flows, turbidity currents
(water involved)
10. Laminar flow vs Turbulent Flow
Two modes of flow dependent upon:
1. Velocity
2. Fluid viscosity
3. Bed roughness
• Laminar Flow: streamlined, uniform current.
Requires: Low fluid velocity or High viscosity or Smooth
beds
• Turbulent Flow: discontinuous, distorted, flow
w/considerable motion perpendicular to primary flow direction.
e.g: Eddies - highly turbulent flow (water and air)
Eddy Viscosity - internal friction at a larger scale
11. Laminar flow vs Turbulent Flow
Laminar flow occurs at low flow velocities
Turbulent flow occurs at higher velocities
Reynolds number (Re) is used to determine whether
flow is laminar or turbulent
Re =ULρ
μ
When μ dominates, Re is small (<500), flow is laminar
e.g., low velocity, shallow, or ice flows
At high U and/or L, flow becomes turbulent (>2000)
U = mean velocity, L = water depth, ρ=density, μ = fluid viscosity
12. Surface Waves & Froude Number
Fluid Flow variables:
– Viscosity (ignore for water)
– Inertial forces
– Gravity & depth (surface wave velocity)
Froude # (Fr) - ratio of inertial forces to gravity,
Fr= U , U= Mean velocity, g = gravity, L= water depth
√gL
Fr= 100 cm/s
√9.8m / s2 * 50cm
Froude #
< 1, max wave velocity exceeds current, tranquil (sub-critical)
>1, current velocity exceeds max wave velocity (supercritical)
14. Critical velocities are different for sediment
entrainment and deposition, especially in the finer
fractions
Fluid density and viscosity play a key role in
determining which particle sizes can be transported
The amount of sediment transport is not only related to
flow velocity (or bed shear stress) and grain size, but
also to:
i. Grain density
ii. Grain shape
15. Sediment transport and deposition
Stokes’ Law (settling velocity in a static fluid)
( ) 2
18
g f
vg= settling velocity; D=grain diameter; ρg = grain
density; ρf= fluid density; μ=dynamic viscosity
Stokes’ Law only applies to fine (<100 μm), quartz-density
grains in water
g
gD
V
17. Transport modes in a turbulent fluid
Traction (rolling over the bed surface)
Saltation (jumping over the bed surface)
Suspension (permanent transport within the fluid)
Solution (Chemical transport)
20. Sedimentology Concepts
Fluid flow and bedforms
Unidirectional flow leads predominantly to
asymmetric bed forms (two-or-three dimensional) or
plane beds
Current ripples
Dunes
Plane beds
Anti-dunes
Oscillatory flow due to waves causes predominantly
symmetric bedforms (wave ripples)
Combined flow involves both modes of sediment
transport and causes low relief mounds and swales.
21.
22.
23. Bed forms and sedimentary structures for different flow regimes.
(after Harms and Fahnestock, 1965 and Simons et al., 1965)
26. Current Ripples
Once movement of sand grains (<0.7 mm) occurs, current
ripples are formed as a result of boundary layer
separation, commonly accompanied by a separation
vortex
Current ripples have a stoss side (erosion and transport)
and lee side (deposition), the latter with a slope of ~ 30°
(angle of repose)
Current ripples only form under moderate flow velocities,
with a grain size < 0.7 mm
Height: 0.5-3 cm; wavelength: 5-40 cm
27. Dunes
Dunes are distinctly larger than current ripples
There is a relationship between boundary layer
thickness (≈ flow depth in rivers) and the dimension of
dunes
Dunes only form in grain sizes > 0.2 mm
Low flow velocities (bed shear stresses) yield straight-crested
bedforms (valid for both dunes and current
ripples); higher shear stresses result in sinuous to
linguoid crest lines.
Sand waves constitute the largest category of
subaqueous dunes
28. Plane beds and anti-dunes
In coarse sands (> 0.7 mm) lower-stage plane beds develop
instead of current ripples
At high (but still subcritical) flow velocities upper-stage
place beds are formed in all sand grain sizes
Supercritical flow conditions (Fr≈1 or higher) enable the
formation of anti-dunes, characterized by bedform
accretion in an upstream direction
29. Waves
Waves are wind generated oscillatory motions of water
Wave height is dependent on wind strength and fetch
The depth to which the oscillatory motion due to wave
action extends is known as the wave base; shallow water
leads to breaking waves
Wave ripples are distinct from current ripples due to their
symmetry, and include low energy rolling grain ripples
and high energy ‘vortex ripples’
30. Tides
Tides are formed by the gravitational attraction of
the moon and sun on the earth, combined with the
centrifugal force caused by the movement of the
earth around the center of mass of the earth moon
system
Semi-diurnal or diurnal tidal cycles
Neap-spring tidal cycles
Annual tidal cycles
31. Ocean currents
The circulation of sea water in the world’s oceans is
driven by wind and contrasts in density due to
variable temperature and salinity (thermo-haline
circulation), combined with the Coriolis effect
Ocean currents transport clay and silt in
suspension, and sand as bed load, and their effects
are especially important in deep waters, where
storms and tides are less important.
32. Gravity flows
Debris flows have a high (> 50%) proportion of sediment to water can
be both subaerial and subaqueous low Reynolds numbers
Turbidity currents have a higher proportion of water, are always
subaqueous and move due to density contrast higher Reynolds
numbers
Re = 휌vL = vL
μ υ
where:
v is the mean velocity of the object relative to the fluid (SI units: m/s)
L is a characteristic linear dimension, (travelled length of the fluid;
hydraulic diameter when dealing with river systems) (m)
μ is the dynamic viscosity of the fluid (Pa·s or N·s/m² or kg/(m·s))
υ is the kinematic viscosity (υ = μ / 휌 ) (m²/s)
휌 is the density of the fluid (kg/m³).
33.
34. Ripple marks
Fine silt to fine sand
Slow to fast flow velocity
35. Dunes
Fine to coarse sand
Medium to high flow velocities
Sand loves to make dunes
Modern Ancient
36. Lower plane beds
Fine to coarse sand, low flow velocity
Sand grains like to make ripples
Really slow velocity
37. Upper planar beds (laminae)
Fine silt
High flow velocity
Hard to tell apart from lower planar beds
(Use grain size)
38. Anti-dunes
Fine silt to coarse sand
High flow velocity
Rarely seen in rocks
39. Flow direction
From looking at the rock structures, flow direction can
be determined even if the rock is millions of years old
When the flow has changed direction,
the structure known as crossbeds
40.
41. Conclusion
Planar stratification is primarily the product of aggrading plane
beds
Cross stratification is formed by aggrading bedforms
Planar and trough cross stratification are the result of straight
crested (2D) and linguoid (3D) bedforms, respectively
- Small scale cross stratification (current ripples)
- Large scale cross stratification (dunes)
-Wave cross stratification (wave ripples)
- Hummocky cross stratification [combined flow] (mounds and
swales)
A single unit of cross stratified material is known as a set;
multiple stacked sets of similar nature form co-sets.