Geology of Urban Watersheds - Urban Watersheds: Geology, Contamination, and Sustainable Development - page 34

2.3.1.3.1 Erosion
Before the deposition of clastic sedimentary material occurs, the material must be eroded.
Erosion is defined as the complex group of related processes by which rock is broken
down physically and chemically and its products removed (Christopherson 2008). The rate
of erosion depends on many factors and includes
• Climatic factors
• Amount and intensity of precipitation
• Average temperature
• Temperature range
• Wind speed and changes in wind direction
• Storm intensity and frequency
• Geologic factors
• Sediment composition
• Sediment size and shape
• Sediment porosity and permeability
• Slope of surface
• Orientation of sediment surface (tilted, faulted, or folded)
• Biological factors
• Vegetative cover type and amount
• Animal habitation behaviors
• Land use
Estimates of the natural erosion rate in North America ranges between 1 and 3 cm every
1000 years (Flint and Skinner 1974). This estimate, however, does not include anthropogenic
sources of erosion that can double its rate (U.S. Soil Conservation Service 1992). In addition,
the rate of erosion is not uniform. Some areas may experience significant amounts of erosion
over a relatively short period of time, whereas in other areas, erosion may not be apparent.
Figure 2.12 shows the different types of water erosion on the land surface. Gravity is
responsible for the erosion associated with mass wasting; while precipitation initiates a
sequence of splash erosion (where a soil particle is dislodged), leading to sheet erosion
from overland flow, and in extreme cases, rills (small indentations) or gullies may form.
Strong downslope winds on unvegetated slopes can increase the potential for rills and
gully formation; this occurs in southern California during the Santa Ana wind season.
In urban environments, the creation of an impervious surface during development
activities increases the volume and velocity of surface runoff, and erosion is generally
accelerated (Figure 2.13a and b). Other types of anthropogenic processes that enhance or
accelerate erosion include
• Removal of vegetative cover through logging
• Overgrazing of livestock
• Changing native vegetation
• Overwatering of vegetation
• Slash and burn agriculture and forestry practices, especially on steep slopes