CLASTIC TRANSPORT AND FLUID FLOW CHAPTER 3

Chapter-3 Clastic transport and fluid flow Weathered rock and minerals fragments are transported from source areas to depositional sites (where they are subject to additional transport and redeposition) by three kinds of processes: 1- dry (non-fluid assisted), gravity-driven mass wasting processes such as rock fall and rock slides;

2- wet (fluid assisted), gravity-driven mass wasting processes (sediment gravity flows) such as grain flows, mudflows, debris flows, and some slumps; and 3- processes that involve direct fluid flows of air, water, and ice.

Mass wasting Fluid flow Reynolds Number Froud Number Entrainment, transport and deposition of clasts Transport

Mass Wasting Mass wasting processes are important mechanisms of sediemnt transport. Although they move the soil and rock debris only short distances downslope , these processes play a crucial role in sediment transport by getting the products of weathering into the longer-distance sediment transport system.

Mass Wasting In dry mass-wasting processes, fluid plays either a minor role or no role at all. In rock or talus falls, clasts of any size simply fall freely. Downslope movement of bodies of rocks or sediment in slumps or slides glide downslope en masse without significant internal folding or faulting. Fluids near the base provides lubrication and promotes failure along slippage surface.

Reynolds Number Re= 2rVp/ Sir Osborne Reynolds addressed the problem of how laminar flow changes to turbulent flow. The transition from laminar to turbulent flow occurs as velocity increases, viscosity decrease, the roughness of the flow boundary increases, and/or the flow becomes less narrowly confined.

Froud Number The Froud Number is the ratio between fluid inertial forces and fluid gravitational forces. It compares the tendency of a moving fluid (and a particle borne by that fluid) to continue moving with the gravitational forces that act to stop that motion. The force of inertia express the distance traveled by a discrete portion of the fluid before it comes to rest. Like reynolds Numbers, Froud numbers are dimensionless.

Froud Number Fr = fluid inertial forces . gravitational forces in flow

Deposition: What forces control the settling of particles? As soon as a particle is lifted above the surface of a bed, it begins to sink back again. The distance that it travels depend on the drag force of the current, and the settling velocity of the Particle. The velocity at which a clast settles througha fluid is calculated using STOKES’ LAW of settling

Stokes’ Law of settling The gravitational force pulling the particle down versus the drag force of the fluid resisting this sinking. The particle will be initially accelerate due to gravity, but soon the gravitational and drag forces reach equilibrium, resulting in a constant “Terminal Fall Velocity”.

The drag force exerted by a fluid on a falling grain is proportional to the fluid density (F), the diameter (d) of the grain (in centimeters), the drag coefficient (CD) and the fall velocity (V). Drag force= CD π (d2/4) (F V2/2)

Drag force Upward force due to buoyancy of the fluid is: Fupward = 4/3 π (d/2)3 Fg Downward forces due to gravity: Fg = 4/3 π (d/2)3  sg, where ps is the density of the particle. Drag force= Fg - Fupward

Drag force= Fg - Fupward CD π (d2/4) (pF V2/2)= 4/3 π (d/2)3 sg - 4/3 π (d/2)3 Fg V2= 4gd (s- F) 3 CD F For low laminar flow at low concentrations of particle and low Reynolds numbers, CD is equal to 24/Re. V = 1/18 ([ s-  F] g d2 /u) - Stokes’ Law of settling V-velocity, g-gravity, u-viscocity, even the differnce in density (s-  F) is constant for a given situation. It can be substitute for C C = ( s-  F)g 18u Stokes law reduces to V=CD2

Stokes law reduces to V=CD2 When density and viscosity are constant, settling velocity increases with the diameter of the particle. Larger grains fall faster. Settling velocity decreases with higher viscosities and increases with denser particles. C = (s- F)g 18u

Implications of the Stokes Law High density minerals settle more rapidly than low density minerals. Slow-moving, highly viscous fluids such as mudflows and density currents can transport coarser-grained materials than less viscuos fluids such as rivers and the wind, despite the normally higher velocity of these less viscous fluids.

Lower temperatures will increas viscosity decreasing the fall velocity. Because of turbulence, coarser particles fall more slowly than predicted. Non-spherical flakes such as mica will settle more slowly than spheres with the same density. Angular grains will generate small turbulent eddies that retard settling velocity.

Hydraulic equivalency The term refers to clasts that settle at identical velocities despite substantial differences in size, shape, angularity, and density. ie. Sediment mixes of fine grained, silt-size magnetite, fine sand-size biotite flakes, and medium sand-size quartz.