4b_G435.pps 1 Flow Regime and Sedimentary Structures An Introduction To Physical Processes of...
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Transcript of 4b_G435.pps 1 Flow Regime and Sedimentary Structures An Introduction To Physical Processes of...
4b_G435.pps
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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