Page 129 - Biosystems Engineering

110 Cha pte r T h ree

3.6 Soil Erosion and Sediment Transport
Soil erosion is the process of the detachment of soil particles from the soil
due to the forces exerted by raindrops, flowing water, and wind. In the
United States, soil has been eroded at about 17 times the rate at which it
forms: about 90 percent of U.S. cropland is currently losing soil above the
sustainable rate. The situation is even worse in Asia, Africa, and South
America, where soil erosion rates are estimated to be about twice as high
as in the United States. In addition to the loss of fertile soils, soil erosion
also has important implications for water quality and water resources.
Often, eroded sediments carried by rivers to reservoirs results in a reduc-
tion in reservoir capacities. As a matter of fact, in highly erodible lands,
reservoir life is dictated by the rate of sediment transport to the reservoir.
Various pesticides, radioactive materials, and nutrients attach them-
selves to the sediments and are transported by the sediments. Therefore,
management and conservation practices geared toward sediment reduc-
tion also helps to mitigate some problems in water quality.
Sediment sources could be natural or artificial. Natural sources
are mainly upland areas where erosion is largely due to overland
flow or erosion of ephemeral gullies. Sheet erosion results in removal
of a fairly uniform layer of sediment from an area, whereas rill ero-
sion is restricted to concentrated channel flows. Large runoff events,
as in the case of floods, can lead to massive amounts of sediment
removal. Artificial sources are primarily created by human activities,
among which agricultural tillage has a strong influence on erosion.
Highway construction, timber cutting, mining, urbanization, land
development for recreational use, and animal grazing also contribute
to fresh sediment sources to varying degrees. Large channels within
a watershed not only transmit sediment but may also act as a sedi-
ment source because of erosion from streambeds or banks.
There are two important time scales associated with sediment
movement. Depending on geomorphologic properties, nature of the
sediment source, and size of the storm, sediment may move from the
source-region to the watershed outlet in a single event. In such
instances, the time scale is fairly short and limited to the duration of
the surface runoff over the watershed. The longer time-scale problem
arises when sediment travels more slowly over the watershed. Each
rainfall event moves the erodible sediment closer to the watershed
outlet. It may take several rainfall events before sediment that was
initially eroded several events ago finally exits the watershed.
In this section we talk only about rainfall and water erosion.
Although wind erosion could be significant in some areas, it will not
be covered here.
3.6.1 Mechanics of Soil Detachment and Deposition
Soil erosion and sediment transport is a complex process. It involves
detachment of soil particles, their transport, and sediment deposi-
tion. The complexity arises from the fact that these processes are not