Chapter 3: Clastic Transport and Fluid Flow

Weathered rock and mineral 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 falls (talus falls) and rockslides (avalanches);   (2) wet (fluid-assisted), gravity-driven mass wasting processes (sediment gravity flow) such as: grain flows mudflows debris flows, some slumps   (3) processes that involve direct fluid flow of air, water, and ice.

Mass Wasting   Mass wasting processes are important mechanism of sediment transport.   Although they moved soil and rocks short distances down slope from the site at which they originated, these processes play a crucial roll in sediment transport by getting the product of weathering into the longer-distance sediment transport system.

Dry mass-wasting processes   Fluid play a minor role or not a role at all.   In rocks or talus falls; clasts of any size simply fall freely.   Fluid at the base of this masses may provide lubrication

Ex. Swiss Village of Elm (1881) a crack in a quarry, almost 600 m high was undercut. Over 18 month a curving fissure grew slowly across the ridge about 350 m above the quarry. Runoff from heavy rains poured into fissure and saturated it. The entire mass started to slide, filling the quarry and falling freely into the valley. It reached the valley floor and run up the opposite site to a height of 100m. 115 people were killed. Ten million cubic meters of rock fell about 450 m, covered 3cubic Km to a depth of 10 to 20 m.

  The rocks traveled at 155 Km/hr (100 mph).   In this case the rock must have been in free fall through most of its descent, buoyed up by trapped air beneath it. (like in air hockey)

Mameyes

Fluid Flow, In Theory and in Nature   Fluid plays an important role in all other models of sediment transport, both in such wet, gravity-driven mass movement as debris flows and mudflows, and in mechanism that move weathering products long distances such as rivers, dust storms, and glaciers.   For that reason knowledge of hydraulics, the science of fluid flow, is essential to understanding sediment transport.   A fluid is any substance that is capable of flowing (liquid or gas).   Although fluids resist forces that tend to change their volume, they readily alter their shape in response to external forces.

The ability of a fluid to entrain (pick up), transport, and deposit sediments depends on many factors:   fluid density   viscosity   flow velocity

Fluid density   The density of a fluid is its mass per unit volume.   The density of seawater is 1.03 g/cm3 and fresh water density is 1.0 g.cm3   The density of a glacial ice is 0.9 g/cm3   The density of air is very low, less than 0.1 % that of water.

Viscosity   The VISCOSITY of a fluid is a measure of its resistance to shearing.   Air has a very low viscosity, the viscosity of ice is very high, and water has an intermediate viscosity between the two.   Many of the differences in clastic grain size in glacial, alluvial, and eolian sediments reflect the different fluid densities and viscosities of ice (coarse, poorly sorted), running water, and air (well sorted, very fine-grained sand and silt).

  Flow velocity Flow velocity determines the type of fluid flow: Laminar turbulent

In laminar flow (characteristic of water flowing at low velocities) individual particles move uniformly as sub parallel sheets. Streamlines (flow lines), visible when droplets of dye are injected into a slow- moving stream of water, do not cross one another. Cigarette smoke. Laminar fluid motion is basically parallel to the underlying surface and only down current or downwind.

In turbulent flow (characteristic of water flowing at high velocity), masses of material move in an apparently random pattern. Eddies of upwelling and subsidence develops. Particles move down current and parallel to the surface but also up and down in the fluid. Dye streamlines are intertwined and deteriorate rapidly downstream. Most natural fluid flow is turbulent.

There are several equations useful in understanding hydraulics and sediments deposits.   Reynolds Number   Froude Number   These numbers helps us to understand the relationship between fluid flow, the type of bedforms produced along the surface, and the mechanism by which entrained particles move.

Reynolds Number   In 1883, Sir Osborne Reynolds addressed the problem of how laminar flow changes to turbulent flow.   He found that 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.

Reynolds Number- 4 variables   velocity   geometry of flow (defined as depth of stream by hydrologist)   dynamic viscosity (resistance to flow)   density

Reynolds Number   The combined expression is called the Reynolds number: Reynolds number= Re= fluid inertial forces/fluid viscous forces   Re= 2rVp/    V=velocity, p=density,  =viscosity, and r=radius of the cylinder of moving fluid

Re= 2rVp/   This equation is a dimensionless number ie. no units-Reynolds.  It expresses the ratio of relative strength of the inertial and viscous forces in a moving fluid.  The numerator of the equation approximates the inertial forces; tendency of discrete parcels of fluid to resist changes in velocity and to continue to move uniformly in the same direction.

Re= 2rVp/   High inertial forces promote the preservation of laminar flow.  Fluid inertial forces increases with higher flow velocity and/or a denser, more voluminous fluid mass.  The denominator of the equation estimates the viscous forces.

What are some practical consequences of fluid inertial forces and fluid viscous forces for sediment transport?  Reynolds Number tells us if the flow is laminar or turbulent.  Turbulent flow show more potential to entrain and transport sediments.  Unconfined fluids moving across open surface (windstorm, surface runoff sheet flow, slow-moving streams, and continental ice) have Reynolds numbers below range.

 Lower (below the critical range) Reynolds numbers, indicating laminar flow, reflect viscous flow forces in excess of inertial forces.  Viscous fluids like maple syrup (pancake), slow-moving natural agents like ice and mudflows, exhibit laminar flow.

 Fluids with Reynolds numbers above range flow turbulently (fast moving streams and turbidity currents)  Turbulent flow … high inertial flow forces typify high-velocity windstorms and broad, deep, fast-moving rivers.