Sediment Erosion,Transport , Deposition, and Sedimentary Structures

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

2. PREFACE • UNESCO’s International Hydrological Programme (IHP) launched the International Sediment Initiative (ISI) in 2002, taking into consideration that sediment production and transport processes are not sufficiently understood for practical uses in sediment management. Since information on ongoing research is an important support to sediment management, and bearing in mind the unequal level of scientific knowledge about various aspects of erosion and sediment phenomena at the global scale, a major mission of the ISI is to review erosion and sedimentation-related research. The two papers below were prepared in conformity with this important task of the ISI, following the decision of the ISI Steering Committee at its session in March 2004.

3. Sediment Dynamics

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

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

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

7. 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 down slope movement of particles • H20 >  air

8. 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) • Sediment load and viscosity co-vary • Not always uniform throughout body • Changes with depth

9. 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

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

11. 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

12. 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

14. 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

15. 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

16. Flow: fight between inertial and viscous forces • Inertial 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)

17. 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)

18. 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

19. 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)

20. What else impacts Fluid Flow? • Channels • Water depth • Smoothness of Channel Surfaces • Viscous Sub-layer

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

22. 2. Water depth • Water flowing over the bottom creates shear stress (retards flow; exerted parallel 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 boundary layer is retarded • Consider velocity in broad shallow stream vs deep river

23. 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)

24. 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)

25. 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

26. Flow/Grain Interaction: Particle Entrainment and Transport • 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

27. Deposition • Occurs when system can no longer support grain • Particle Settling • Particles settle due to interaction of upwardly directed forces (buoyancy 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. 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

29. 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

30. 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

31. 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

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

34. 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)

35. 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

37. 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

39. Sedimentary structures Cross stratification • The angle of climb of cross-stratified deposits increases with deposition rate, resulting in ‘climbing ripple cross lamination’ • Antidunes form cross strata that dip upstream, but these are not commonly preserved • A single unit of cross-stratified material is known as a set; a succession of sets forms a co-set

41. 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)

45. Question?

46. Test • In which year UNESCO launched International Sediment Initiative? • Write the Sedimentary Cycle. • Write the Bernouli’s Force equation. • What is Laminar & Turbulent flow? • Write the equation of Renold’s Equation.