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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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Consider Variation in Grain Size & Flow Velocity for sand 0.8: No ripples nor lower plane bed

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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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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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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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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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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.
http://volcanoes.usgs.gov/Hazards/What/Lahars/RainLahar.html

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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

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