Geology Reference
In-Depth Information
(its components) can be worked out. Take the case of
a boulder on a hillslope (Figure 3.3). The weight of
the boulder acts vertically in the direction of gravity,
but the reaction with the ground surface prevents the
boulder from moving in that direction. Nonetheless,
movement downslope is possible because the weight of
the boulder is resolvable into two forces - a force nor-
mal to the slope, which tends to hold the boulder in
place, and a force parallel to the slope, which tends to
move the boulder downhill. Normal and parallel reac-
tion forces balance these. Now, the boulder will not
move unless the downslope force can overcome the
resistance to movement (friction) to counter the par-
allel reaction force. Once the downslope force exceeds
the surface resistance, the boulder will accelerate, and
its reaction then involves an inertia force due to the
boulder's accelerating down the slope. This means that
a smaller downslope force component is required to
continue the motion at constant velocity, in the same
way that it is easier to pull a sledge once it is moving
than it is to start it moving.
Resistance
is fundamental to transport processes.
Without resistance, Earth surface materials would
move under the force of gravity until the landscape
was all but flat. Many factors affect resistance, but
none so much as friction. Friction exists between bod-
ies and the surface over which they move. It occurs
between where matter in any state (solid, liquid, gas)
come into contact, as in solids on solids, solids on flu-
ids, fluids on fluids, and gases on solids or fluids. In a
river, friction occurs at the fluid bed contact and within
Reaction to
weight,
R
Normal
reaction,
R
n
Boulder
Frictional
force,
F
f
Downslope force,
(=
F W
1
sin
Normal force,
(=
Normal force,
(=
F W
2
F W
2
cos
cos
Weight,
W
Slope angle,
Distance
Figure 3.3
Forces acting upon a boulder lying on a
hillside.
the water, owing to differential velocity of flow and tur-
bulent eddies. In the case of a boulder at rest on a flat
surface, if no lateral force is applied to the boulder, then
the frictional resistance is zero as there is no force to
resist. If a lateral force,
F
, is applied, then the frictional
force,
F
f
, increases to balance the force system. At a
critical value for
F
, the frictional resistance, generated
between the boulder and the surface, will be unable to
balance the applied force and the boulder will start to
accelerate. For any given surface contact
F
critical
/
R
=
a constant
=
m
s
As the ratio is constant, the force required to move
the boulder increases in proportion with
R
(the normal
reaction, which, on a flat surface, is equal to the weight
of the boulder).
own weight, which is a gravitational force. Moving
the water uses only part of the downslope force,
and the portion left after overcoming various resis-
tances to flow may carry material in the flow or along
the water-ground contact. The water also carries
dissolved material that travels at the same velocity
as the water and essentially behaves as part of the
fluid itself.
3
Water pressure forces
. Water in soil and sedi-
ment creates various forces that can affect sediment
movement. The forces in saturated (all the pores
filled) and unsaturated (some of the pores filled)
differ. First, under saturated conditions with the
soil or sediment immersed in a body of water
(for example, below the water table), an upward
buoyancy or
water pressure force
equal to the
