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Flow Regime andSedimentary Structures
An Introduction ToPhysical Processes of Sedimentation
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Bed Response to Water (fluid) Flow Common bed forms (shape of the unconsolidated bed)
due to fluid flow in Unidirectional (one direction) flow
Flow transverse, asymmetric bed forms 2D&3D ripples and dunes
Bi-directional (oscillatory)
Straight crested symmetric ripples Combined Flow
Hummocks and swales
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Bed Response to Steady-state,
Unidirectional, Water Flow Hydrodynamic variables
Grain Size | Most Important
Flow Depth |-->Variables in Natural Fluid Flow
Flow velocity | Systems
Fluid Viscosity Fluid Density
Particle Density
g (gravity)
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Bed Response to Steady-state,Unidirectional, Water Flow
FLOW REGIME CONCEPT Consider variation in: Flow Velocity only
Flume Experiments (med sand & 20 cm flowdepth)
A particular flow velocity (after critical
velocity of entrainment) produces a particular bed configuration (Bed form)
which in turn
produces a particular internal sedimentary
structure.
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Bed Response to Steady-state,Unidirectional, Water Flow
Consider Variation in Grain Size & Flow Velocity for sand 0.8: No ripples nor lower plane bed
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Bed Response to Steady-state,Unidirectional, Water Flow
Lower Flow Regime No Movement: flow velocity below critical entrainment
velocity
Ripples: straight crested (2d) to sinuous and linguoid
crested (3d) ripples (< ~1m) with increasing flow velocity Dunes: (2d) sand waves with straight crests to (3d) dunes
(>~1.5m) with sinuous crests and troughs
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Dynamics of Flow TransverseSedimentary Structures
Flow separation and planar vs. tangential fore sets Increased flow velocity/decrease in grain size produces greater
flow separation and more vertical accretion bedding component inturbulent flows
Lateral Accretion from bed load
angle of repose, fore-set bedding Vertical Accretion from suspended load
tangential to draped stratification
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Bed Response to Steady-state,Unidirectional, Water Flow
Lower Flow Regime No Movement: flow velocity
below critical entrainmentvelocity
Ripples: straight crested (2d)
to sinuous and linguoid crested(3d) ripples (< ~1m) withincreasing flow velocity
Dunes: (2d) sand waves withstraight crests to (3d) dunes(>~1.5m) with sinuous crestsand troughs
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Dynamics of Flow TransverseSedimentary Structures
Flow separation and planar vs. tangential fore sets Aggradation (lateral and vertical) and Erosion in space and
time Due to flow velocity variation
Capacity (how much sediment in transport) variation
Competence (largest size particle in transport) variation Angle of climb and the extent of bed form preservation(erosion vs. aggradation-dominated bedding surface)
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Climbing
Ripples Angle of climb and
decreasing flow
capacity(downwardson figure)
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Bed Response to Steady-state,Unidirectional, Water Flow
Lower Flow Regime No Movement: flow velocity
below critical entrainmentvelocity
Ripples: straight crested (2d)
to sinuous and linguoid crested(3d) ripples (< ~1m) withincreasing flow velocity
Dunes: (2d) sand waves withstraight crests to (3d) dunes(>~1.5m) with sinuous crests
and troughs
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Bed Response to Steady-state,Unidirectional, Water Flow
Lower Flow Regime No Movement: flow velocity below critical entrainment velocity Ripples: straight crested (2d) to sinuous and linguoid crested (3d)
ripples (< ~1m) with increasing flow velocity
Dunes: (2d) sand waves with straight crests to (3d) dunes
(>~1.5m) with sinuous crests and troughs
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Bed Response to Steady-state,Unidirectional, Water Flow
Upper Flow Regime Flat Beds: particles move continuously with no relief on the bed
surface
Antidunes: low relief bed forms with constant grain motion; bedform moves up- or down-current (laminations dip upstream)
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(summary)
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Application of Flow Regime Concept toOther Flow Types
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Application of Flow RegimeConcept to Other Flow Types
Deposits formed byturbulent sedimentgravity flow mechanism
turbidites Decreasing flow
regime in concertwith grain sizedecrease
Indicates decreasingflow velocity throughtime during deposition
S di G i Fl
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Sediment Gravity FlowMechanisms
Sediment Gravity Flows: 20%-70% suspended sediment High density/viscosity fluids
suspended sediment charged fluid within a lower density, ambientfluid
mass of suspended particles results in the potential energy forinitiation of flow in a the lower density fluid (clear water or air)
mgh = PE
M = mass G = force of gravity H = height PE= Potential energy
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Distinction of Sediment Gravity FlowMechanisms otbo
Fluid Flow and Grain Support Mechanisms Newtonian Fluids (fluidal flows)
turbidity currents; grain supportturbulence Plastics with a yield stress, or finite strength
High concentration sediment gravity flows: debris flows; grain support fluid strength & buoyancy
X X
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Sediment Gravity Flows
Not distinct in nature Different properties within different
portions of a flow
Leading edge of a debris flowtriggered by heavy rain crashes downthe Jiangjia Gully in China. The flowfront is about 5 m tall. Such debrisflows are common here because thereis plenty of easily erodible rock andsediment upstream and intenserainstorms are common during thesummer monsoon season.
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Fluidal Flows
Turbidity Currents Re (Reynolds #) is large due to (relatively) lowviscosity
turbulence is the grain support mechanism
initial scour due to turbulent entrainment ofunconsolidated substrate at high currentvelocity
Scour base is common
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Fluidal Flows
Turbidity Currents deposition from bedload & suspended load when
Fi>Fm (Fm = mobility forces; Fi = grain inertia)
initial deposits are coarsest transported particles
deposited (ideally) under upper (plane bed) flowregime
Fl d l Fl
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Fluidal Flows
Turbidity Currents as flow velocity decreases (due to loss of minimum
mgh) finer particles are deposited under lower flowregime conditions
high sediment concentration commonly results in climbingripples
final deposition occurs under suspension settlingmode with hemipelagic layers
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Fluidal Flows The final (idealized) deposit: Turbidite
graded in particle size with regular vertical transition in sedimentary structures
Bouma Sequence andfacies tract in a
submarine fandepositionalenvironment
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High Concentration SedimentGravity Flows
Grain Support Matrix strength (yield stress)
Matrix density causing grain buoyancy in excess of clear water fluids
Laminar flow mechanisms due to very high fluid viscosity (Re is
low) Occur in both subaqueous (clear water is ambient fluid) and air
Cessation of flow is by "freezing" (gravity stress < yield stress)
X X
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High Concentration
Sediment Gravity Flows Indicate generally unstable
slopes (moderate to high relief)
Internal sedimentary structures little scour at base
very poor sorting, massive bedding
large particle sizes may betransported, matrix support
inverse to symmetric size grading clast alignment parallel to flow
surface
X
X
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Debrites Debris flow deposits See TurbiditesTurbidity
current deposits