Page 77 - Fundamentals of Geomorphology

60 INTRODUCING LANDFORMS AND LANDSCAPES

weight of water displaces and relieves some of the Shear stress, friction, cohesion, and
downward force created by the weight of the sed- shear strength
iment. Second, under unsaturated conditions, a
negative pore pressure or suction force tends to A handful of key mechanisms explain much about
hold the water within the pores and even draw transport processes – force, stress, friction, and shear
it up from the water table by capillary rise. Such strength. The case of soil resting on a slope demon-
negative pore pressure increases the normal force strates these mechanisms. The force of gravity acts upon
between sediment grains and increases their resis- the sediment, creating stresses. The normal stress (acting
tance to movement. This capillary cohesion force perpendicular to the slope) tends to hold the sediment
keeps sandcastles from collapsing. Falling raindrops in place. The shear stress acts in a downslope direction
also create a force when they strike the ground. and, if large enough, will move the soil downhill.
Depending on their size and terminal velocity, they Three factors resist this downhill movement – friction,
may create a force strong enough to move sediment cohesion, and shear strength. Friction resists sliding.
grains. Many factors affect it, the most important being:
4 Expansion forces. Sediments, soils, and even solid
rock may expand and contract in response to changes friction between the sediment and the underlying
of temperature (heating and cooling, freezing and rock
thawing) or moisture content (wetting and dry- internal friction of grains within the sediment
ing), and sometimes in response to chemical changes (whichdependsupontheirsize,shape,arrangement,
in minerals. Expansion tends to act equally in all resistance to crushing, and the number of contacts
directions, and so any movement that occurs is per unit volume)
reversible. However, on slopes, the action of grav- normal stress (the larger this is, the greater the degree
ity means that expansion in a downslope direction of friction)
greater than contraction in an upslope direc- smoothness of the plane of contact between the sed-
tion produces an overall downslope movement of iment and the rock, which influences the angle of
material. friction.
5 Global fluid movements. Wind carries water sedi-
ment in much the same way as water does – along A soil mass on a slope needs no externally applied force
the ‘bed’ or in suspension. But, as air is far less dense for it to move. If the slope angle is steep enough, the
a fluid than water, for the same flow velocity it carries downslope component of the soil’s weight will provide
sediment of smaller grain size. sufficient downslope force to cause movement. When
6 Biological forces. Animals and plants create forces the slope angle reaches a critical value, the soil will start
that influence sediment movement. Plant root to slide. This critical angle is the static angle of slid-
systems push material aside, and if this occurs ing friction, j m , the tangent of which is equal to the
on a slope an overall downslope movement may coefficient of static friction. The effective normal stress,
result. Burrowing animals mine soils and sedi- which allows for the pore water pressure in the soil,
ment, redistributing it across the land surface (see also influences sliding. In dry material, the effective nor-
Butler 1995). Where animals burrow into slopes, mal stress is the same as the normal stress, but in wet
a tendency for an overall downslope movement but unsaturated soils, where pore water pressure is nega-
occurs. tive, the effective shear stress is less than the shear stress.
Cohesion of the soil (the degree to which the individual
In summary, most movements of sediment require a grains are held together) also affects sliding, cohesive sed-
downslope force resulting from action of gravity, but iment resisting sliding more than non-cohesive sediment.
climatic, meteorological, and biotic factors may also Finally, shear strength, which is the resistance of the soil
play an important role in moving materials. to shear stress, affects movement. Mohr–Coulomb’s law