Vegetation-Erosion Dynamics - River Dynamics and Integrated River Management - page 52

Wind has a greater frictional component when the wind carries sand, every grain of which is like a
cutting tool. In some desert regions the bases of rocks or cliffs have been sandblasted, leaving a
mushroom-shaped formation. Figure 2.3 shows mushroom-shaped rocks resulting from wind erosion in
the Arches National Park of the U.S.
Fig. 2.3 Mushroom-shaped rocks resulting from wind erosion in the Arches National Park of the U.S.
Cultural erosion —Human activities also cause or intensify soil erosion, which is termed cultural erosion.
When land is disturbed by construction activities, soil erosion increases from 2 to 40,000 times the
pre-construction erosion rate (Wang and Wang, 1999). Erosion rates from construction sites are typically
10 to 20 times those from agricultural lands and they can be 100 times as high. Wolman and Schick (1967)
found sediment yields on an open construction site in Maryland of 49,200 t/km 2 compared to 380 t/km 2
in a stable urban area nearby. Goldman et al. (1986) found that in the San Francisco Bay Area the average
rate of erosion for non-construction land uses (grazing, agriculture, forests, etc.) was about l,780 t/(km 2 yr),
whereas the erosion rate from construction sites was 11,600 to 15,700 t/(km 2 yr) and sometimes higher.
Erosion rates from construction sites typically were 20 times the average erosion rates of non-construction
land uses. Although a wide variation in erosion rates is reported in the literature, it is clear that construction
causes a large increase in erosion.
Erosion removes the smaller and less dense constituents of topsoil. These constituents, clay and fine
silt particles and organic material, hold nutrients that plants require. The remaining subsoil is often hard,
rocky, infertile, and dry. Thus, reestablishment of vegetation is difficult and the eroded soil produces less
growth. In the management of rivers, people can control erosion by planting vegetation that holds the soil,
by carefully managing and controlling land usage, and by lessening the slope angle in places where
gravity tends to erode the soil.
2.1.2 Water Erosion
2.1.2.1 Types of Water Erosion
Flowing water is the main agent causing erosion. Water erosion is the most important erosion for river
management. Water erosion is essentially a two-part process. One part is the loosening of soil particles