Environmental Engineering Reference
In-Depth Information
Fig. 2.7 Soil erosion caused by freeze and thaw in the Xizhao Gully on the Loess Plateau of China
very infrequent in deserts; but when it does occur, it is typically very intense. Erosion rates often are high
because there is little ground cover to protect the soil. In cold climates the growing season is very short.
Plant reestablishment is a difficult and costly process. Even with the best of efforts, success cannot be
assured (White and Franks, 1978).
Soil Characteristics— The following four soil characteristics are important in determining soil erodibility:
ķ texture; ĸ organic matter content; Ĺ structure; and ĺ permeability. Soil texture refers to the sizes
and proportions of the particles making up a soil. Sand, silt, and clay are the three major classes of soil
particles. Soils high with sand content are said to be coarse-textured. Because water readily infiltrates
into sandy soils, the runoff, and consequently the erosion potential, is relatively low. Soils with a high
content of silts and clays are said to be fine textured or heavy. Clay, because of its stickiness, binds soil
particles together and makes a soil resistant to erosion. However, once the fine particles are eroded by
heavy rain or fast-flowing water, they will travel great distances before settling. Typically, clay and fine
silt particles will settle in a large, calm water body, such as a bay, lake, or reservoir, at the down stream
end of a watershed. Thus, silty and clayey soils are frequently the worst water polluters. Soils that are
high in silt and fine sand and low in clay and organic matter are generally the most erodible (Mills and
Clar, 1976; VSWCC, 1980).
Organic matter consists of plant and animal litter in various stages of decomposition. Organic matter
improves soil structure and increases permeability, water-holding capacity, and soil fertility. Organic matter
in an undisturbed soil or in mulch covering a disturbed site reduces runoff and, consequently, erosion
potential. Mulch on the surface also reduces the erosive impact of raindrops. Mulching can accelerates
re-establishment of grass vegetation. Figure 2.8(a) shows straw mulch placed on an area disturbed in the
construction of a rain garden in the Marquette University campus. Two weeks late the grass re-established
itself through the straw, as shown in Fig. 2.8(b).
Soil structure is the arrangement of soil particles into aggregates. A granular structure is the most
desirable one. Soil structure affects the soil's ability to absorb water. When the soil surface is compacted
or crusted, water tends to run off rather than infiltrate. Erosion hazard increases with increased runoff.
Loose, granular soils absorb and retain water, which reduces runoff and encourages plant growth. Soil
permeability refers to the ability of the soil to allow air and water to move through the soil. Soil texture
and structure and organic matter all contribute to permeability. Soils with high permeability produce less
runoff at a lower rate than soils with low permeability, which minimizes erosion potential. The higher
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