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 slowmoving 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 500-2000 range.
 Lower
(below the critical 500-2000 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 5002000 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.