Page 75 - Fundamentals of Geomorphology

58 INTRODUCING LANDFORMS AND LANDSCAPES

the continuous transition from dry moving material to the force makes the agency move (water for instance) and
muddy water. in turn the moving agency exerts a force on the sediment
and tends to move it, as in sediment transport in rivers.
The chief forces that act upon geomorphic materials are
Transport mechanics gravitational forces, fluid forces, water pressure forces,
Geomorphic forces expansion forces, global fluid movements, and biological
forces.
The transport of all materials, from solid parti-
cles to dissolved ions, needs a force to start and 1 Gravitational forces. Gravity is the largest force for
maintain motion. Such forces make boulders fall from driving geomorphic processes. It acts directly on bod-
cliffs, soils and sediment move hillslopes, and water and ies of rock, sediment, water, and ice, tending to make
ice flow along channels. For this reason, the mechanical them move. Moreover, it acts the world over at a
principles controlling movement underpin the under- nearly uniform magnitude of 9.81 metres per second
2
standing of transport processes (Box 3.3). per second (m/s ), with slight variations resulting
The forces that drive sediment movement largely from distance from the Earth’s centre and latitude.
derive from gravity, from climatic effects (heating and 2 Fluid forces. Water flows over sloping land surfaces.
cooling, freezing and thawing, winds), and from the It does so as a subdivided or uniform sheet or as
action of animals and plants. They may act directly, as in channel flows in streams and rivers. Water is a fluid
the case of gravity, or indirectly through such agencies as so that it moves in the direction of any force that
water and wind. In the first case, the force makes the sed- is applied to it, and no critical force is necessary.
iment move, as in landslides; while, in the second case, So water flows downhill under the influence of its

Box 3.3

FORCE AND RESISTANCE

A body will not move unless a force is applied, and the acceleration due to gravity and exert a force in the
its movement will not continue without the sustained direction of gravity, which is approximately vertically.
exertion of a force. Likewise, forces act on a body at The magnitude of this force is generally the weight
rest that are in balance while the body remains station- of the body in a static condition (but, if the body is
ary. For this reason, forces are immensely important in moving, the force alters).
determining if the transport of sediments takes place. Forces have direction and magnitude. If two or
A force is an action in a specified direction that more forces are acting on a body, then the magnitude
tends to alter the state of motion of a body. An equal and direction of a resultant force is determinable. For
and opposite force called the reaction always balances example, a sediment grain entrained in flowing water is
it. A boulder resting on the ground exerts a vertical subject to several forces: a vertical force pushing it ver-
force on the ground due to its weight; the ground tically upwards in the flow, the force of its own weight
exerts a force of the same magnitude in the opposite dragging it down vertically, and the downstream force
direction on the boulder; and, if it did not do so, the of the flowing water carrying it along the river channel.
boulder would sink into the ground. Forces result from The magnitude and direction of all these forces dictate
the acceleration of a body. If a body is not subject to an the net direction in which the grain will travel and so
acceleration, then it cannot exert a force in any direc- whether it will stay suspended or sink to the riverbed. If
tion. At the Earth’s surface, most bodies are subject to a single force is known, its effects in different directions