Page 85 - Soil Degradation, Conservation and Remediation

72 3 Soil Erosion by Water

Fig. 3.2 Severe sheet erosion has exposed tree roots in a teak forest (Photo courtesy of
Dr. Animesh Biswas)

3.3.2 Sheet Erosion

When a thin layer of soil is removed by raindrop impact and shallow surface fl ow
from the whole slope, it is called sheet erosion. It removes the fi nest fertile topsoil
with plenty of nutrients and organic matter. It is the most dangerous type of soil ero-
sion because it occurs gradually and almost silently leaving little or no signs of soil
removal.
Sheet erosion involves the removal of a more or less uniform layer of soil over
the whole slope of the land (Fig. 3.2 ). Soil particles are detached primarily by
raindrops and secondarily by frost, hooves of farm animals, tillage, and mechanical
action of farm machines. Detached particles are transported by runoff water as over-

land flow. Sheet erosion is more uniform and gradual, as the surface becomes
smoother. However, water may still accumulate even on the smoothest slope. The
intensity of accumulation of runoff water depends on the height of the water stream,
the coarseness of the surface, vegetation, or crop distribution. Sheet erosion removes
deeper layer of soil gradually, if allowed to proceed unhindered and the subsoil is
exposed over a large area. The subsoil is usually of different texture and color and
is more compacted. However, slopes are often not so uniform over the whole area,
and water accumulates in tiny channels, so that the surface is crisscrossed by
discontinuous rillets. It is then known as the interrill erosion (Fig. 3.3 ).