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Flow Regime and Sedimentary Structures

An Introduction To Physical 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 flow depth)

– 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 <~0.2mm: No Dunes

– for sand ~0.2 to 0.8mm Idealized Flow Regime Sequence of Bed forms

– 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 (< ~1mλ) with increasing flow velocity

– Dunes: (2d) sand waves with straight crests to (3d) dunes (>~1.5mλ) 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 (>~1.5mλ) with sinuous crests and troughs

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Dynamics of Flow Transverse Sedimentary 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 (downwards on figure)

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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 (< ~1mλ) with increasing flow velocity

– Dunes: (2d) sand waves with straight crests to (3d) dunes (>~1.5mλ) 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 (< ~1mλ) with increasing flow velocity

– Dunes: (2d) sand waves with straight crests to (3d) dunes (>~1.5mλ) 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; bed form moves up- or down-current (laminations dip upstream)

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Flow regime Concept (summary)

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Application of Flow Regime Concept to Other Flow Types

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Application of Flow Regime Concept to Other Flow Types

• Deposits formed by turbulent sediment gravity flow mechanism– “turbidites” – Decreasing flow regime

in concert with grain size decrease

• Indicates decreasing flow velocity through time during deposition

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Sediment Gravity Flow Mechanisms

• Sediment Gravity Flows: – 20%-70% suspended sediment– High density/viscosity fluids

• suspended sediment charged fluid within a lower density, ambient fluid• mass of suspended particles results in the potential energy for initiation 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 Flow Mechanisms otbo

• Fluid Flow and Grain Support Mechanisms • Newtonian Fluids (fluidal flows)

– turbidity currents; grain support turbulence

• 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 flow triggered by heavy rain crashes down the Jiangjia Gully in China. The flow front is about 5 m tall. Such debris flows are common here because there is plenty of easily erodible rock and sediment upstream and intense rainstorms are common during the summer monsoon season.

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Fluidal Flows• Turbidity Currents

– Re (Reynolds #) is large due to (relatively) low viscosity

– turbulence is the grain support mechanism– initial scour due to turbulent entrainment of

unconsolidated substrate at high current velocity• 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) flow regime

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Fluidal Flows• Turbidity Currents

– as flow velocity decreases (due to loss of minimum mgh) finer particles are deposited under lower flow regime conditions

• high sediment concentration commonly results in climbing ripples

– final deposition occurs under suspension settling mode 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 and “facies” tract in a submarine fan depositional environment

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High Concentration Sediment Gravity 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 be transported, 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