Page 90 - Soil Degradation, Conservation and Remediation

3.3 Types of Water Erosion 77

3.3.5 Tillage Erosion

The Soil Science Society of America defines tillage as the mechanical manipulation
of the soil for any purpose (SSSA 2008 ). The objectives of tillage include seedbed
preparation, weed control, and incorporation of agricultural amendments. Sometimes
tillage becomes intensive and continuous, drastically alters soil functions, and

causes soil erosion. Blanco and Lal ( 2008 ) emphasize the significance of a new type
of erosion “tillage erosion,” which refers to the gradual soil translocation or dis-
placement downhill caused by tillage operations (Lindstrom et al. 1990 ). The
mechanical manipulation of tillage implements transports soil and does not involve
the water action. The net soil translocation by tillage is expressed in units of vol-
ume, mass, or depth per unit of tillage width. In recent years, particularly with the
advent of mechanized agriculture, tillage erosion has become an important compo-
nent of total soil erosion in hilly croplands. Tillage erosion can represent as much as
70 % of total soil erosion (Lobb et al. 1999 ). Tillage erosion is a serious soil degra-
dation process in sloping cultivated soils worldwide. Tillage erosion modifi es the
landscape geomorphology by progressively removing topsoil layers from convex

field positions (summits, crests, and shoulder slopes) and redistributing the removed
materials along concave landscape positions (foot- and toe-slopes).

3.3.6 Landslide/Landslip/Mudfl ow

The downward and outward movement of a large block of soil and regolith caused by
gravity are called landslides (Fig. 3.6 ) and landslips. Landslides are deep-seated mass

movement, and soil slip is a shallow and rapid sliding or flowing movement of the soil.
There are different forms of landslides, including mudflows, mudslides, debris fl ows,

rock falls, and rockslides. Slides move in contact with the underlying surface. Flows
are plastic or liquid movements in which land mass breaks up in water and fl ows during
movement. Landslides are caused by unstable geological conditions, steep slopes,
intense rainfall, weak soils, earthquakes, and human- induced changes of landforms.
Human-induced causes are excavation, loading, deforestation, irrigation, mining,
vibrations, and water impoundment. The volume of soil detached by landslides depends
on the geology and the mechanism of landslide. For example, landslides initiated by
rainfall are smaller, while those initiated by earthquakes are very large. There are two
categories of landslide: (1) rapidly moving and (2) slow moving. Rapidly moving land-

slides (debris flows and earth flows) present the greatest risk to human life.

3.3.7 Riverbank/Stream Bank Erosion

Stream/riverbank erosion occurs due to bank scour and mass failure. The direct
removal of bank materials by the physical action of flowing water is called bank scour.