1. Flow Regime and Sedimentary Structures
An Introduction To
Physical Processes of Sedimentation

2. 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

3. 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)

4. 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.

5. 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

6. 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

7. 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

8. 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)

9. Climbing Ripples
Angle of climb and decreasing flow capacity (downwards on figure)

10. 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

11. 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

12. 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)

13. Flow regime Concept (summary)

14. Application of Flow Regime Concept to Other Flow Types

15. 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

16. 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

17. 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

18. 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.

19. 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

20. 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

21. 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

22. 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

23. 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

24. 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

25. Debrites Debris flow deposits
See TurbiditesTurbidity current deposits