Page 76 - Fundamentals of Geomorphology

GEOMORPHIC MATERIALS AND PROCESSES 59

(its components) can be worked out. Take the case of Reaction to
a boulder on a hillslope (Figure 3.3). The weight of Normal weight, R
the boulder acts vertically in the direction of gravity, reaction, R n
Boulder
but the reaction with the ground surface prevents the
Frictional
boulder from moving in that direction. Nonetheless, Frictional
force,
Downslope force, force, F f F f
movement downslope is possible because the weight of (= W sin Height
F 1
Normal force,
the boulder is resolvable into two forces – a force nor- Normal force,
F 2 (=
(= W cosW
cos
mal to the slope, which tends to hold the boulder in Weight, W F 2
W
eight,
W
place, and a force parallel to the slope, which tends to Ground surface
angle,
Slope
move the boulder downhill. Normal and parallel reac- Slope angle,
tion forces balance these. Now, the boulder will not Distance
move unless the downslope force can overcome the Figure 3.3 Forces acting upon a boulder lying on a
resistance to movement (friction) to counter the par- hillside.
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 the water, owing to differential velocity of flow and tur-
boulder’s accelerating down the slope. This means that bulent eddies. In the case of a boulder at rest on a flat
a smaller downslope force component is required to surface, if no lateral force is applied to the boulder, then
continue the motion at constant velocity, in the same the frictional resistance is zero as there is no force to
way that it is easier to pull a sledge once it is moving resist. If a lateral force, F, is applied, then the frictional
than it is to start it moving. force, F f , increases to balance the force system. At a
Resistance is fundamental to transport processes. critical value for F, the frictional resistance, generated
Without resistance, Earth surface materials would between the boulder and the surface, will be unable to
move under the force of gravity until the landscape balance the applied force and the boulder will start to
was all but flat. Many factors affect resistance, but accelerate. For any given surface contact
none so much as friction. Friction exists between bod-
F critical /R = a constant = m s
ies and the surface over which they move. It occurs
between where matter in any state (solid, liquid, gas) As the ratio is constant, the force required to move
come into contact, as in solids on solids, solids on flu- the boulder increases in proportion with R (the normal
ids, fluids on fluids, and gases on solids or fluids. In a reaction, which, on a flat surface, is equal to the weight
river, friction occurs at the fluid bed contact and within of the boulder).

own weight, which is a gravitational force. Moving 3 Water pressure forces. Water in soil and sedi-
the water uses only part of the downslope force, ment creates various forces that can affect sediment
and the portion left after overcoming various resis- movement. The forces in saturated (all the pores
tances to flow may carry material in the flow or along filled) and unsaturated (some of the pores filled)
the water–ground contact. The water also carries differ. First, under saturated conditions with the
dissolved material that travels at the same velocity soil or sediment immersed in a body of water
as the water and essentially behaves as part of the (for example, below the water table), an upward
fluid itself. buoyancy or water pressure force equal to the