Page 88 - Soil Degradation, Conservation and Remediation

3.3 Types of Water Erosion 75

proportion of precipitation water making up the surface flow and the numbers of
particles separated from the soil by raindrops both increase.
Greater predominance of rill erosion may be found on steeper slopes with imper-
meable soil material consisting of younger sediments which are susceptible to
erosion. As is seen usually, rill erosion prevails and affects the whole length of the
slope, which means that precipitation water, as soon as it reaches the soil, fl ows
away through the dense network of rills, virtually cutting the slope into thin plates.
Occurrence of splash erosion or sheet erosion is not seen, but there was some erosion
of these forms too. Similar phenomena occur on steep slopes, even on impermeable
loamy clay material. On impermeable or still heavier and more resistant material,
rill erosion forms ridges which are separated by sharply cut rillets and gullies. The
rillets are occasionally so narrow that they resemble cracks, and it could be termed
crack erosion. On steep slopes composed of material of varying resistance, vertical
openings are formed, and these soon develop into tunnel erosion or hollow erosion,
separating the washed forms into isolated pipes, etc. Where the material is more
homogeneous and the incline less steep, rilling prevails. If the material is more
coarse grained and less resistant, flowing water carries the soil along rapidly and

creates triangular or trough-shaped forms with respect to the cross section of the
channel. In such cases, the lengths of the rills are greater, but the interrill lamellae
are thinner, and the edges sharper. The more coarsely grained and more permeable

the material, the less pronounced is the channeling, until finally the rills are widely
shaped, and resemble more the form of moderately undulating depressions, even on
very steep parts of the eroded slope. On permeable, coarse-grained, non-resistant

fluvioglacial deposits, on the other hand, shallow, rapidly growing rills develop with

an immense production of silt. Here, the action of flowing water is the predominant
force. These forms are transitional toward gully erosion. Rill erosion usually begins
to appear in the lower part of the slope. This is true especially when the source of
the water is thawing snow or precipitation of low intensity. As soon as the intensity
of the rainfall increases, the intensity and velocity of surface runoff both increase
also, and consequently the proportion of the total erosion due to rills becomes
greater, depending on the permeability of the soil.

3.3.4 Gully Erosion

Gullies are large channels deeper than 30 cm. Gullies develop when large quantities
of water accumulate and run through a single channel with high speed in relatively
steep slopes (Fig. 3.5 ). Gullies may also develop by the gradual deepening of rills.
There are two types of gullies: ephemeral and permanent. Ephemeral gullies form
shallow channels that can be readily corrected by routine tillage operations. On the
other hand, permanent gullies are very large and cannot be smoothed by regular
tillage (Blanco and Lal 2008 ). Gullies of various size and form develop by the grad-
ual deepening of rills. A number of forms may be distinguished in gully erosion.

The first form includes gully with a depth between 30 cm and 2–3 m. In this form,