Geology of Urban Watersheds - Urban Watersheds: Geology, Contamination, and Sustainable Development - page 31

rock in landslides and rockfalls; (3) frost wedging, caused by wetting and freezing cycles;
and (4) thermal expansion, from the contraction of rock by heating and cooling.
Chemical weathering is the breakdown of rocks and minerals through reactions between
the rocks and minerals and atmospheric constituents including water, oxygen, and carbon
dioxide. This process alters the chemical composition of the rocks involved. The most com-
mon types of reactions include (1) solution, where molecules and elements in rocks and
minerals dissolve directly into water; (2) oxidation and hydration, consisting of reactions
between oxygen, water, and iron-bearing minerals, which assist in weakening and break-
ing down rocks; and (3) hydrolysis, a complex weathering reaction forming clays. A more
detailed treatment of chemical weathering is presented later in this section and Section
2.3.2 within the context of sediment composition and karst topography.
The processes of weathering are the first step in soil formation.
2.3.1.2 Mass Wasting
Once weathering occurs and a portion of the bedrock is separated from itself and becomes
regolith, it becomes prey for other forces that move the freed materials downslope under
the influence of gravity. The several types of movement acting on regolith to transport
the material downslope are collectively termed mass wasting. Mass wasting (process #2,
Figure 2.8) is the movement of rock or soil downslope by gravity without the aid of mov-
ing water, glaciers, or wind (Flint and Skinner 1974). On any slope, mass wasting processes
depend on the interplay between gravity, slope angle, moisture content, cohesion, and fric-
tion. When the gravitational force acting on a slope exceeds its resisting force, slope failure
or mass wasting occurs. The slope materials' strength and cohesion and the amount of
internal friction between the materials contribute to maintaining slope stability and are
collectively known as the slope's shear strength. The steepest angle that a cohensionless
slope can maintain without losing its stability is known as its angle of repose (Pudasaini
and Hutter 2007).
Mass wasting can affect large bodies of regolith or just one small rock particle. In addi-
tion, mass wasting is not confined to continental regions; it also takes place in marine envi-
ronments where submarine landslides occur on steep slopes and material subsequently
spreads out onto the seafloor. In areas of low relief, mass wasting may still take place but
at a very slow rate. By contrast, in mountainous areas, mass wasting processes can be very
fast, as in a rockfall, rockslide, or debris flow.
Factors affecting the potential or rate of mass wasting include (1) a change in slope angle,
(2) weakening of material through weathering, (3) increased water content, (4) changes in
vegetation cover, and (5) overloading (Wicander 2005).
Anthropogenic sources increase the rate of mass wasting commonly through the build-
ing of roads requiring the construction of road cuts through uneven terrain. This prac-
tice creates situations where the cutback slope is constructed beyond the angle of repose.
Overloading commonly occurs from anthropogenic sources such as heavy loading on
roadways and building structures. In addition, irrigation may significantly increase the
rate of mass wasting in urban areas of varied terrain.
Types of mass wasting include
• Rockfall or debris fall—the rapid descent of a rock mass, vertically from a cliff or
by leaps down a very steep slope (Figure 2.9).
• Rockslide or debris slide—the rapid, sliding descent of a rock mass down a slope
(Figure 2.9).