S EDIMENT E ROSION,T RANSPORT, D EPOSITION, AND S EDIMENTARY S TRUCTURES An Introduction To Physical Processes of Sedimentation

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.

S EDIMENT D YNAMICS

S EDIMENT TRANSPORT Fluid Dynamics COMPLICATED Focus on basics Foundation NOT comprehensive

S EDIMENTARY C YCLE Weathering Make particle Erosion Put particle in motion Transport Move particle Deposition Stop particle motion Not necessarily continuous (rest stops)

D EFINITIONS 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

D EFINITIONS Fundamental Properties Density (Rho (  )) Mass/unit volume Water ~ 700x air  = °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  H 2 0 >  air

D EFINITIONS Fundamental Properties Viscosity Mu (  ) 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

T YPES OF F LUIDS : S TRAIN ( DEFORMATIONAL ) R ESPONSE TO S TRESS ( 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

W HY DO PARTICLES MOVE ? Entrainment Transport/ Flow

E NTRAINMENT 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

B ERNOULI F ORCE  gh) + (1/2  2 )+P+E loss = constant Static P + dynamic P Potential energy=  gh Kinetic energy= 1/2  2 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

F LOW 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

G EOLOGICALLY S IGNIFICANT F LUID F LOW T YPES (P ROCESSES ) Laminar Flows: straight or boundary parallel flow lines Turbulent flows: constantly changing flow lines. Net mass transport in the flow direction

F LOW : 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)

R EYNOLD ’ S # R e = Vl/(  /  dimensionless # V= current velocity l= depth of flow-diameter of pipe  = density  = viscosity  /  kinematic viscosity Fluids with low  (air) are turbulent Change to turbulent determined experimentally Low Re = laminar <500 (glaciers; some mud flows) High Re = turbulent > 2000 (nearly all flow)

G EOLOGICALLY S IGNIFICANT F LUID F LOW T YPES (P ROCESSES ) Laminar Flows: straight or boundary parallel flow lines Turbulent flows: constantly changing flow lines. Net mass transport in the flow direction

G EOLOGICALLY S IGNIFICANT F LUIDS AND F LOW P ROCESSES 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 Debris flow (laminated flow) Traction deposits (turbulent flow)

W HAT ELSE IMPACTS F LUID F LOW ? Channels Water depth Smoothness of Channel Surfaces Viscous Sub-layer

1. C HANNEL Greater slope = greater velocity Higher velocity = greater lift force More erosive Higher velocity = greater inertial forces Higher numerator = higher R e More turbulent

2. W ATER 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

2. W ATER D EPTH Boundary Shear stress (  o )-stress that opposes the motion of a fluid at the bed surface (  o ) =  R h S  = 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)

2. W ATER 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)

3. S MOOTHNESS 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

F LOW /G RAIN I NTERACTION : P ARTICLE E NTRAINMENT AND T RANSPORT Forces acting on particles during fluid flow Inertial forces, F I, inducing grain immobility F I = gravity + friction + electrostatics Forces, F m, inducing grain mobility F m = fluid drag force + Bernoulli force + buoyancy

D EPOSITION 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

G RAINS IN M OTION (T RANSPORT ) 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

T RANSPORT M ODES AND P ARTICLE E NTRAINMENT With a grain at rest, as flow velocity increases F m > F i ; initiates particle motion Grain Suspension (for small particle sizes, fine silt; <0.01mm) When F m > F i U (flow velocity) >>> V S (settling velocity) Constant grain Suspension at relatively low U ( flow velocity) Wash load Transport Mode

T RANSPORT M ODES AND P ARTICLE E NTRAINMENT With a grain at rest, as flow velocity increases F m > F i ; initiates particle motion Grain Saltation : for larger grains (sand size and larger) When F m > F i U > V S but through time/space U < V S Intermittent Suspension Bedload Transport Mode

T HEORETICAL B ASIS FOR H YDRODYNAMIC I NTERPRETATION OF S EDIMENTARY F ACIES 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

F LOW R EGIME AND S EDIMENTARY S TRUCTURES An Introduction To Physical Processes of Sedimentation

S EDIMENTARY 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)

S EDIMENTARY 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

B ED R ESPONSE TO W ATER ( FLUID ) F LOW 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

S EDIMENTARY 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

B ED R ESPONSE TO S TEADY - STATE, U NIDIRECTIONAL, W ATER F LOW 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)

Q UESTION ?

T EST  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.