Sediment Erosion,Transport, Deposition, and Sedimentary Structures

1. Sediment Erosion,Transport, Deposition, and Sedimentary Structures An Introduction To Physical Processes of Sedimentation

2. Sediment transport • Fluid Dynamics • COMPLICATED • Focus on basics • Foundation • NOT comprehensive

3. Sedimentary Cycle • Weathering • Make particle • Erosion • Put particle in motion • Transport • Move particle • Deposition • Stop particle motion • Not necessarily continuous (rest stops)

4. Definitions • Fluid flow (Hydraulics) • Fluid • Substance that changes shape easily and continuosly • Negligeable resistance to shear • Deforms readily by flow • Apply minimal stress • Moves particles • Agents • Water • Water containing various amounts of sediment • Air • Volcanic gasses/ particles

5. Definitions • Fundamental Properties • Density (Rho (r)) • Mass/unit volume • Water ~ 700x air • = 0.998 g/ml @ 20°C • Density decreases with increased temperature • Impact on fluid dynamics • Ability of force to impact particle within fluid and on bed • Rate of settling of particles • Rate of occurrence of gravity -driven downslope movement of particles • H20 >  air

6. Definitions • Fundamental Properties • Viscosity • Mu (m) • Water ~ 50 x air •  = measure of ability of fluids to flow (resistance of substance to change shape) • High viscosity = sluggish (molasses, ice) • Low viscosity = flows readily (air, water) • Changes with temperature (Viscosity decreases with temperature) • Sed load and viscosity covary • Not always uniform throughout body • Changes with depth

7. Types of Fluids:Strain (deformational) Response to Stress (external forces) • Newtonian fluids • normal fluids; no yield stress • strain (deformation); proportional to stress, (water) • Non-Newtonian • no yield stress; • variable strain response to stress (high stress generally induces greater strain rates {flow}) • examples: mayonnaise, water saturated mud

8. Types of Fluids:Strain (deformational) Response to Stress (external forces) • Bingham Plastics: • have a yield stress (don't flow at infinitesimal stress) • example: pre-set concrete; water saturated, clay-rich surficial material such as mud/debris flows • Thixotropic fluids: • plastics with variable stress/strain relationships • quicksand??

9. Why do particles move? • Entrainment • Transport/ Flow

10. Entrainment • Basic forces acting on particle • Gravity, drag force, lift force • Gravity: • Drag force: measure of friction between water and bottom of water (channel)/ particles • Lift force: caused by Bernouli effect

11. Bernouli Force • (rgh) + (1/2 rm2)+P+Eloss = constant Static P + dynamic P • Potential energy= rgh • Kinetic energy= 1/2 rm2 • Pressure energy= P • Thus pressure on grain decreases, creates lift force Faster current increases likelihood that gravity, lift and drag will be positive, and grain will be picked up, ready to be carried away Why it’s not so simple: grain size, friction, sorting, bed roughness, electrostatic attraction/ cohesion

13. Flow • Types of flow • Laminar • Orderly, ~ parallel flow lines • Turbulent • Particles everywhere! Flow lines change constantly • Eddies • Swirls • Why are they different? • Flow velocity • Bed roughness • Type of fluid

14. Geologically SignificantFluid Flow Types (Processes) • Laminar Flows: • straight or boundary parallel flow lines • Turbulent flows: • constantly changing flow lines. Net mass transport in the flow direction

15. Flow: fight between inertial and viscous forces • Intertial F • Object in motion tends to remain in motion • Slight perturbations in path can have huge effect • Perfectly straight flow lines are rare • Viscous F • Object flows in a laminar fashion • Viscosity: resistance to flow (high = molasses) • High viscosity fluid: uses so much energy to move it’s more efficient to resist, so flow is generally straight • Low viscosity (air): very easy to flow, harder to resist, so flow is turbulent • Reynolds # (ratio inertial to viscous forces)

16. Reynold’s # Re = Vl/(r/m)dimensionless # • V= current velocity • l= depth of flow-diameter of pipe • r= density • m= viscosity u=(r/m)- kinematic viscosity • Fluids with low u (air) are turbulent • Change to turbulent determined experimentally • Low Re = laminar <500 (glaciers; some mud flows) • High Re = turbulent > 2000 (nearly all flow)

17. Geologically SignificantFluid Flow Types (Processes) • Laminar Flows: • straight or boundary parallel flow lines • Turbulent flows: • constantly changing flow lines. Net mass transport in the flow direction

18. Geologically Significant Fluids and Flow Processes Debris flow (laminated flow) • These distinct flow mechanisms generate sedimentary deposits with distinct textures and structures • The textures and structures can be interpreted in terms of hydrodynamic conditions during deposition • Most Geologically significant flow processes are Turbulent Traction deposits (turbulent flow)

19. What else impacts Fluid Flow? • Channels • Water depth • Smoothness of Channel Surfaces • Viscous Sublayer

20. 1. Channel • Greater slope = greater velocity • Higher velocity = greater lift force • More erosive • Higher velocity = greater intertial forces • Higher numerator = higher Re • More turbulent

21. 2. Water depth • Water flowing over the bottom creates shear stress (retards flow; exerted // to surface) • Shear stress: highest AT surface, decreases up • Velocity: lowest AT surface, increases up • Boundary Layer: depth over which friction creates a velocity gradient • Shallow water: Entire flow can fall within this interval • Deep water: Only flow within B.L. is retarded • Consider velocity in broad shallow stream vs deep river

22. 2. Water Depth • Boundary Shear stress (o)-stress that opposes the motion of a fluid at the bed surface (o) = gRhS • = density of fluid (specific gravity) • Rh = hydraulic radius • (X-sectional area divided by wetted perimeter) • S = slope (gradient) • the resistance to fluid flow across bed (ability of fluid to erode/ transport sediment) • Boundary shear stress increases directly with increase in specific gravity of fluid, increasing diameter and depth of channel and slope of bed (e.g. greater ability to erode & transport in larger channels)

23. 2. Water depth • Turbulence • Moves higher velocity particles closer to stream bed/ channel sides • Increases drag and list, thus erosion • Flow applies to stream channel walls (not just bed)

24. 3. Smoothness • Add obstructions • decrease velocity around object (friction) • increase turbulence • May focus higher velocity flow on channel sides or bottom • May get increased local erosion, with decreased overall velocity

25. 4. Viscous Sublayer • At the surface, there is a molecular attraction that causes flow to slow down • Thin layer of high effective viscosity • Reduce flow velocity • May even see laminar flow in the sublayer • Result? Protective “coating” for fine grains on bottom • Smallest grains are within the layer • (larger grains can poke up through it, causing turbulence and scour of larger particles)

26. Forces acting on particles during fluid flow Inertial forces, FI, inducing grain immobility FI= gravity + friction + electrostatics Forces, Fm, inducing grain mobility Fm= fluiddrag force + Bernoulli force + buoyancy Flow/Grain Interaction: Particle Entrainment and Transport

27. Deposition • Occurs when system can no longer support grain • Particle Settling • Particles settle due to interaction of upwardly directed forces (bouyancy of fluid and drag) and downwardly directed forces (gravity). • Generally, coarsest grains settle out first • Stokes Law quantifies settling velocity • Turbulence plays a large role in keeping grains aloft

28. Particle SettlingForces opposing entrainment and transport • VS  =  [(ρg - ρf)g/18 m]d2 • VS : settling velocity • ρg = grain density • ρf = fluid density • m = fluid viscosity • d = grain diameter Stoke’s “law” of settling

29. Theory vs application • Increase velocity, increase turbulence and entrainment • Material plays a role • Hjülstrom’s curve • Empirical measure of minimum Velocity required to move particles of different sizes

30. Hjülstrom’s curve • EMPIRICAL • Series of grain sizes in straight sided channel • Increased velocity until grains moved • Threshold velocity (min. V) to entrain particles • Transition zone (specifics like packing • Intuitive except for clays • Cohesion (consolidated fines) • Electrostatic attraction (unconsolidated fines) • Viscous sublayer

31. Fm     >    Fi Hjulstrom Diagram Empirical relationship between grain size (quartz grains) and current velocity (standard temperature, clear water) Defines critical flow velocity threshold for entrainment As grain size increases entrainment velocity increases (sand size and > particles) For clay size particles electrostatics requires increased flow velocity for entrainment (gray area is experimental variation) Critical Threshold for Particle Entrainment

32. Grains in Motion (Transport) • Once the object is set in motion, it will stay in motion • Transport paths • Traction (grains rolling or sliding across bottom) • Saltation (grains hop/ bounce along bottom) • Bedload (combined traction and saltation) • Suspended load (grains carried without settling) • upward forces > downward, particles uplifted stay aloft through turbulent eddies • Clays and silts usually; can be larger, e.g., sands in floods • Washload: fine grains (clays) in continuous suspension derived from river bank or upstream • Grains can shift pathway depending on conditions

33. Transport Modes and Particle Entrainment • With a grain at rest, as flow velocity increases Fm     >    Fi ; initiates particle motion • Grain Suspension(for small particle sizes, fine silt; <0.01mm) • When Fm  >  Fi • U (flow velocity)>>> VS (settling velocity) • Constant grain Suspension at relatively low U (flow velocity) • Wash loadTransport Mode

34. Transport Modes and Particle Entrainment • With a grain at rest, as flow velocity increases Fm     >    Fi ; initiates particle motion • Grain Saltation: for larger grains (sand size and larger) • When Fm  >  Fi • U   > VS  but through time/space U < VS • Intermittent Suspension • Bedload Transport Mode

35. Transport Modes and Particle Entrainment • With a grain at rest, as flow velocity increases Fm     <    Fi ,but fluid drag causes grain rolling • Grain Traction: for large grains (typically pebble size and larger) • Normal surface (water) currents have too low a U for grain entrainment • Bedload Transport Mode

36. Velocity/Particle Size Fields and Entrainment, Transport Mode, and Deposition Model • Entrainment/Transportation • Suspension • Saltation • Traction • Settling/Deposition

37. Depositional structures indicate flow regime of formation • Traction Currents • Air and Water • Bed is never perfectly flat • Slight irregularies cause flow to lift off bottom slightly • Leads to pocket of lower velocity where sediments pushed along bottom can accumulate • Bump creates turbulence, advances process • Bedform height and wavelength controlled by: • Current velocity • Grain Size • Water depth

38. Theoretical Basis for Hydrodynamic Interpretation of Sedimentary Facies • Beds defined by • Surfaces (scour, non-deposition) and/or • Variation in Texture, Grain Size, and/or Composition For example: • Vertical accretion bedding (suspension settling) • Occurs where long lived quiet water exists • Internal bedding structures (cross bedding) • defined by alternating erosion and deposition due to spatial/temporal variation in flow conditions • Graded bedding • in which gradual decrease in fluid flow velocity results in sequential accumulation of finer-grained sedimentary particles through time

39. Grain size and Water Depth-Bedform • Grain size impacts bedform formation • coarse grains, no ripples are formed • fines (clays), no dunes form • Water depth affects bedform • Increase w.d., increase velocity at which change from low to upper flow regime occurs

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

42. Sedimentary structures • Sedimentary structures occur at very different scales, from less than a mm (thin section) to 100s–1000s of meters (large outcrops); most attention is traditionally focused on the bedform-scale • Microforms (e.g., ripples) • Mesoforms (e.g., dunes) • Macroforms (e.g., bars)

43. Sedimentary structures • Laminae and beds are the basic sedimentary units that produce stratification; the transition between the two is arbitrarily set at 10 mm • Normal grading is an upward decreasing grain size within a single lamina or bed (associated with a decrease in flow velocity), as opposed to reverse grading • Fining-upward successions and coarsening-upward successions are the products of vertically stacked individual beds

45. Sedimentary structures Cross stratification • Cross lamination (small-scale cross stratification) is produced by ripples • Cross bedding (large-scale cross stratification) is produced by dunes • Cross-stratified deposits can only be preserved when a bedform is not entirely eroded by the subsequent bedform (i.e., sediment input > sediment output) • Straight-crested bedforms lead to planar cross stratification; sinuous or linguoid bedforms produce trough cross stratification

46. 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 Bed Response to Water (fluid) Flow

47. 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.

48. 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

49. 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

50. 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)