Environmental Engineering Reference
In-Depth Information
This general approach is adopted from Marsh (2010). Landscape formation processes are
complex, and this reality creates the need for rigorous investigation, or the inputs of matter
and energy from humans will render landscapes unsustainable.
15.3.1 How Landscapes Form and Change
Natural landscapes result from Earth's crust being hammered from below and above. From
below, tectonic plate motion (Chapter 2) provides the tremendous force necessary to create
and modify natural landscapes. At broad geographic scales including global, continental,
and regional, various forms of tectonic activity (e.g., orogeny and volcanism) have created
mountain ranges, ridges, valleys, trenches, and mixed terrains called mélange (Table 15.1).
After uplift, the crust is hammered by the forces of water and wind. Now, other forma-
tive processes of the landscape including waves, wind, glaciers, and runoff fed by over-
land flow, streamflow, soil water, and groundwater get their shot.
15.3.2 The Special Role of Water in Landscape Change
Even in glacial environments and sand deserts, the landscape is influenced heavily by
running water. Flowing water is responsible for more landscape change than all other
processes combined (Marsh 2010). As noted in previous chapters, special attention to water
flows in all media and directions must be given during site investigations, and this prin-
ciple holds true for any study of landscapes at larger geographic scales.
The properties of water account for its prominent role in landscape change. Water is
heavy and relatively viscous, two properties allowing it to function as an efficient agent of
erosion. Water also expands when frozen by 9%, and this increase in volume helps it break
apart rock and soil. Most of the depositional processes creating sedimentary environments
involve water, and these form the physical framework for urban watersheds. In addition to
these properties, water exists everywhere on earth, although in varying quantities.
As noted in Chapter 12, human modifications of water resources have also had a pro-
found impact on urban watersheds. Two of the major environmental impacts in urban
TABLE 15.1
Landforms Associated with Different Plate Boundaries
Plate Boundary Type
Associated Landforms
Convergent
Ocean to ocean
Trench, accretionary wedge, forearc basin, magmatic mountains
Ocean to continental
Trench, mélange, magmatic mountains
Continent to continent
Fold mountains
Divergent
Ocean
Mid-oceanic ridges
Continental
Rift valleys, shield volcanoes
Transform
Ocean
Ridges
Continental
Varies, depends on the erosion processes operating on each
side of the boundary
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